Sagaya Punithasegaran Arrshan...built-in parts (e.g. double-hull or stainless steel tanks, shaft strut connection, etc.), together with the surrounding shipbuilding structure with

Modular design of cross flow channel through structural optimization

Sagaya Punithasegaran Arrshan

Master Thesis

Presented in partial fulfillment of the requirements for the double degree:

“Advanced Master in Naval Architecture” conferred by University of Liege "Master of Sciences in Applied Mechanics, specialization in Hydrodynamics,

Energetics and Propulsion” conferred by École Centrale de Nantes

developed at University of Rostock in the framework of the

“EMSHIP” Erasmus Mundus Master Course

in “Integrated Advanced Ship Design”

Ref. 159652-1-2009-1-BE-ERA MUNDUS-EMMC

Supervisor: Prof. Robert Bronsart, University of Rostock, Rostock, Germany

Internship tutor: M.Sc. Tim Stockhausen, Lürssen Werft, Bremen, Germany

Reviewer: Prof. Pierre Ferrant,

École Centrale de Nantes, Nantes, France

Rostock, January 2019

Modular design of cross flow channel through structural optimization 1

“EMSHIP” Erasmus Mundus Master Course, period of study September 2017 – February 2019

Table of Contents DECLARATION OF AUTHORSHIP .................................................................................................... 3

ABSTRACT ............................................................................................................................................ 4

1. INTRODUCTION .......................................................................................................................... 5

1.1 Background & Motivation ...................................................................................................... 5

1.2 Tasks & Objectives ................................................................................................................. 6

2. CROSS FLOW CHANNEL............................................................................................................ 7

2.1 Literature Review: Sea chest & crossover .............................................................................. 7

2.2 Cross flow channel Structure .................................................................................................. 8

2.3 Present Production process ..................................................................................................... 9

2.3.1 Sub-Assembly ................................................................................................................. 9

2.3.2 Block-Assembly ............................................................................................................ 10

2.4 Problems encountered in present method ............................................................................. 11

2.5 Requirements of a Crossover ................................................................................................ 12

2.5.1 Functional Requirements .............................................................................................. 12

2.5.2 Operational Requirements ............................................................................................. 13

3. OPTIMIZATION .......................................................................................................................... 14

3.1 Different possible solutions .................................................................................................. 14

3.2 Integrated pipe in double-bottom .......................................................................................... 15

3.2.1 Selection of Pipe ........................................................................................................... 15

3.2.2 Crossover Pipe and fittings ........................................................................................... 17

3.3 Structural Modifications ....................................................................................................... 18

3.3.1 Girder openings & strength ........................................................................................... 18

3.3.2 Modification of the deck ............................................................................................... 20

3.3.3 Addition of floor plate and support ............................................................................... 21

3.3.4 Elimination of additional stiffening .............................................................................. 22

3.3.5 Other modifications....................................................................................................... 23

3.3.6 Final modified structure ................................................................................................ 23

3.4 Impact on general design ...................................................................................................... 25

3.4.1 Idea behind the FE model ............................................................................................. 25

3.4.2 Mesh .............................................................................................................................. 26

3.4.3 Constraints and Load condition .................................................................................... 27

3.4.4 Simulation and results ................................................................................................... 28

3.4.5 The necessity to increase the height of girders ............................................................. 30

3.4.6 Open-deck above the pipe ............................................................................................. 31

4. MODULAR DESIGN AND CONSTRUCTION ......................................................................... 32

4.1 Literature Review: Modular Construction ............................................................................ 32

2 Arrshan Sagaya Punithasegaran

Master Thesis developed at University of Rostock, Rostock

4.2 General design aspects .......................................................................................................... 33

4.2.1 Tolerances ..................................................................................................................... 33

4.2.2 Size standards ................................................................................................................ 33

4.2.3 Accessibility or Reachability ........................................................................................ 34

4.2.4 Working position & conditions ..................................................................................... 34

4.2.5 Number of parts ............................................................................................................ 34

4.2.6 Better utilization of the facility ..................................................................................... 34

4.2.7 Special parts/components .............................................................................................. 35

4.2.8 Possibility of subcontracting ......................................................................................... 35

4.3 Modular design of the cross-flow channel ............................................................................ 37

4.4 Selection of suitable substructures ........................................................................................ 38

4.4.1 Crossover pipe .............................................................................................................. 38

4.4.2 Transverse Frames and floor plates............................................................................... 38

4.4.3 Longitudinal Girders ..................................................................................................... 38

4.4.4 Deck and Shell plates .................................................................................................... 38

4.4.5 Stiffeners, brackets and docking profiles ...................................................................... 39

4.5 Sub-assemblies ...................................................................................................................... 39

4.5.1 Crossover pipe .............................................................................................................. 39

4.5.2 Longitudinals ................................................................................................................ 40

4.5.3 Transvers frames and floor plates ................................................................................. 40

4.6 Construction of the module ................................................................................................... 41

5. Review of benefits and drawbacks ................................................................................................ 46

5.1 Optimized structure ............................................................................................................... 46

5.1.1 Functional benefits ........................................................................................................ 46

5.1.2 Operational Benefits ..................................................................................................... 47

5.1.3 Structural & Production related benefits ....................................................................... 48

5.1.4 Drawbacks or issues not resolved ................................................................................. 50

5.1.5 Cost estimation for overall comparison of production .................................................. 51

5.2 Modular construction ............................................................................................................ 52

5.2.1 Benefits achieved .......................................................................................................... 52

5.2.2 Drawbacks ..................................................................................................................... 54

5.2.3 Cost estimation of modular construction ...................................................................... 55

6. CONCLUTION ............................................................................................................................. 56

ACKNOWLEDGEMENTS .................................................................................................................. 57

REFERENCES ..................................................................................................................................... 58



DECLARATION OF AUTHORSHIP

I declare that this thesis and the work presented in it are my own and have been generated by

me as the result of my own original research.

Where I have consulted the published work of others, this is always clearly attributed.

Where I have quoted from the work of others, the source is always given. With the exception

of such quotations, this thesis is entirely my own work.

I have acknowledged all main sources of help.

Where the thesis is based on work done by myself jointly with others, I have made clear exactly

what was done by others and what I have contributed myself.

This thesis contains no material that has been submitted previously, in whole or in part, for the

award of any other academic degree or diploma.

I cede copyright of the thesis in favour of the University of Rostock.

Date: Signature:



ABSTRACT

The master thesis work involves in studying the modular design of complex double bottom

structures in prefabrication as an alternative to working in section construction based on the

geometry of a cross-flow channel with its possible optimization.

Presently, the complex double bottom structures such as cross flow channel and sea chests are

built at the block assembly from separate single structural groups in most of the shipbuilding

industries. The goal of this study will be to look into the modular design and construction of

the cross flow channel. Which means that the entire construction is pre-fabricated and mounted

as one module in the block assembly. The background to this is the theme of producing such

built-in parts (e.g. double-hull or stainless steel tanks, shaft strut connection, etc.), together

with the surrounding shipbuilding structure with simple interfaces for subsequent assembly in

the section or on the ship as modules. This is to achieve a more favourable position &

accessibility, shorter distances and relocation of work into the prefabrication to save costs in

the preparation and construction time on the sectional building sites.

Taking advantage of the modular construction, there is a need of optimizing the cross flow

channel in order to achieve certain functional and operational advantages such as to

eliminate/reduce air bubbles & air cushions in flow, to have reduced flow resistance and

avoiding mud/sludge formation or marine growth and most importantly to have better

accessibility for inspection and maintenance. The cross-flow channel needs to be optimized

accordingly and should be designed to be constructed as a module since the complexity of the

structure might increase.

Nevertheless, in this study, we will review the basic principles of modular design and

construction, which will lead to better understanding of the method. In the company’s point of

view, the aim is to use recognized advantages of the optimized crossover and the modular

design for the future implementation in the upcoming projects.



1. INTRODUCTION

1.1 Background & Motivation

As the name suggests, ‘Complex’ double bottom structures have difficult and complex process

of construction along different building sites of a shipyard. Usually, such structures are built at

the block assembly as a complete block before it is moved to the ship erection site. During the

construction of certain components or parts, this process can be extremely difficult due to the

very limited space available between components especially in the double bottom. The biggest

challenge would be to reach the right location and to have a good quality welding.

The motivation of this thesis is to study the modular construction of such complex double

bottom structures specifically based on a cross flow channel and to identify the possible

modules with built-in parts (profiles, tanks & etc.) with simple interfaces for block assembly

or on board assembly. The main goal is to find an easy or favourable working position for a

good quality of job and studying the possibility of saving time and expenses by relocating the

work into the prefabrication. The results could be used to identify different other complex

geometries that could possibly be pre-fabricated as modules. Hence, the criteria for modular

design in general will also be studied.

Since the work is mainly concentrated on the cross flow channel, there are certain issues or

requirements with the crossover that needed to be sorted out. Briefly, they are mainly regarding

the flow and maintenance of the crossover. Hence, this study will take the advantage of modular

construction and begin with an optimization of the structure for the cross flow channel, that

can demonstrate certain improvements.



1.2 Tasks & Objectives

The main task of this study is to find out which assemblies or substructures of the cross flow

channel that would be suitable for modular prefabrication and the criteria for such a decision.

In terms of optimizing the structure of the cross flow channel, the main objective is to analyse

the requirements for a cross-flow channel and to optimize the structure to cope with those. It

can be studied that if the modular structures be used to meet these requirements as the

optimization might also increase the complexity as discussed earlier.

Considering on which possibilities of a modular design in prefabrication exist at the Lürssen

shipyard, it should be figured out if the existing infrastructure suitable for the construction of

modules of the selected size of the subassembly in terms of cranes, space, means of transport

and working conditions. Then a comparison should be done between the conventional building

process and the modular construction to see if it gives further advantages such as space,

capacities, costs, time and quality.



2. CROSS FLOW CHANNEL

2.1 Literature Review: Sea chest & crossover

Having a quick brief about Sea chests, a sea chest is a rectangular recess close to the bottom of

the vessel from which the piping systems draw water for cooling of the Engine, Generator, fire

pumps or other uses. A sea chest acts in much the same way as distilling basin or well, offsetting

the effects of the vessel speed and providing an intake reservoir.

In large vessels usually there will be two sea chests one on the side and the other on the bottom

of the double bottom, which are called high and low sea chest respectively. The purpose of

having high sea chest is to avoid the suction of mud or harbour silt at port or shallow waters

and the low sea chest is to get the cleanest and best seawater head at sea state. They also serve

as a redundancy measure in an event of failure or damage of one of the sea chests. A typical

representation of a sea chest is as shown below in fig. (1).

Figure 1. Sea chest. Available from: http://navy.memorieshop.com/Design-Details/Reservoir.html

[Accessed 10 June 2018]

Usually termed as Crossover, it is a means of structure or piping System that connects the Low

and High Sea Chests of the vessel. Water will be taken from the crossover with the help of

suction pipes connected to it for different uses such as cooling for the diesel generator, main

condensers or supply water for the forward diesel fire pump etc.



2.2 Cross flow channel Structure

On the yachts built at Lürssen shipyard, the crossover is designed to be a part of the structure

of double bottom between two water-tight bulkheads rather than just a piping system, hence

the name ‘Cross flow channel’.

This type of a channel is designed due to the limited space available in the machinery room

where it is required to place Engines, Generators and other equipment along with plenty of

pipes running along the floor while the conventional piping system would interrupt the

machinery room arrangements otherwise.

Typically the cross flow channel is designed between 3-5 frame spacing in such a way that,

either half of the channel is under fore and aft machinery rooms so that the water can be taken

out easily from each of the rooms without the need of extended piping. The main channel,

which is under the engine room, usually has a wider channel without a transverse floor plating

but stiffened longitudinally on the shell and deck plate. Large cut-outs are provided on the

longitudinal girders in order to have the required flow without much resistance. The size of the

cut-outs required are determined by the flow requirement that needs to be supplied to the

machinery equipment. All the structural surfaces inside the channel (including profiles,

brackets & etc.) will be treated with anti-fouling coating as they will be in contact with the sea

water throughout their lifetime.

The seawater is sucked in through the sea chests and fed into the cross channel after passing

through filters. Water is taken out for various purposes through the suction pipes, which are

inserted into the channel through the tank deck plate. The important factor here is the flow rate

of the water through the cross flow channel rather than the volume capacity. The required flow

rate is determined by accumulating the required amount of water for each of the equipment.



2.3 Present Production process

At Lürssen shipyard, the production of the cross flow channel is done at the block assembly as

a part of the whole block. The construction is carried out from separate longitudinal structural

groups starting from the centre and moving outwards transversely. The different structural

groups can be seen below indicated in different colours in fig. (2).

Figure 2. Single structural groups for construction

2.3.1 Sub-Assembly

Sub-Assembly or Pre-Fabrication is where most of the parts and components, which are

comparatively smaller and consist a few number of structures are built. Mainly, each parts of

the longitudinal girders and transverse frames are welded with the respective stiffeners or

profiles it is meant to have. In this case, as the deck plates at the crossover region needs

additional stiffening, the welding of the stiffening profiles on the deck plates is prefabricated

as well.

The parts and profiles are cut to the required sizes, prepared with the weld preparation and

given with unique names before it reaches the facility. After each sub assembly is built, they

are given with another unique name of the sub-assembly, which will be used in further process.

In addition, some of the modules are completely constructed in the pre fabrication. Especially,

the low sea chest, which is part of this block and an important component associated with the

cross flow channel, is built as a complete module here.



The walls of the low sea chest will be weld with the stiffening profiles and brackets first and

the module of the sea chest box will be completed. Different other modules and the reason to

be pre-fabricated as a module will be explained in the following chapters.

2.3.2 Block-Assembly

The block assembly is where the complete block is built with the sub-assemblies and modules

that are prepared at the pre-fabrication. Placed on a customisable jig setup, the block is built

upside down. From the production point of view, building upside down has the main advantage

of welding downwards. Since the cross flow channel is built as a part of the block, let us have

a brief about the construction of this particular block in this chapter.

As the first step, the tank deck plates are placed on the jig and weld with themselves together.

Next, one longitudinal section at the centre line is built with the respective sub-assembly parts.

It begins with the Central girder being placed first and each part of the transverse frames

associated along with it to the length of the girder in one side is welded together. One section

is then completed after welding with the required brackets, collar plates or other profiles.

Similarly, the next girder is placed and the transverse profiles follows and the construction of

the longitudinal section on the other side is done. After the completion of the central sections,

the keel plate is placed and welded together. The same procedure of section based construction

is followed until the block is constructed with the longitudinal and transverse members and the

shell plate is welded finally. Then the construction workers and welding technicians have to

get inside the double bottom in order to weld the frames, girders and the with the shell plate.

Finally, the modules of the sea chest, engine mounts, block joints and other required brackets,

collar plates and profiles will be weld together with the block.

After the completion of the block, it will be transported to the on board assembly for block

erection. This is the typical process of building the cross flow channel, which is an integral part

of the block in almost all of the projects at Lürssen shipyard.



2.4 Problems encountered in present method

After studying the current method and discussing with a few experts in the production and

design facilities, certain issues or problems were addressed.

Accessibility and reachability

The most important and major concern is the accessibility to the work. During the construction

of this double bottom block, the workers have to go through manholes carrying all equipment

they require to work with. Reachability is another concern, taking welding as example, the

welding torch should be able to reach the point of welding and the worker needs to have clear

vision on it. Deformation of structure due to uncontrolled welding is high and straightening the

buckled plates are a tough job as it requires large equipment to be carried inside the double

bottom.

In addition, small human errors also gets amplified due to this accessibility problems. As an

example, if a worker forgets any of his essentials such as glass or gloves, he has to go back

through the groves outside and come back again. This is a big issue though not quite addressed.

Missing parts

Another major issue encountered is the quite a large amount of time wasted in searching for

the parts or components at the block assembly and eventually having need to fabricate the parts

again. This is due to the large number of components involved and transportation, storage and

handling are always an issue.

Unsafe working conditions

A typical Yacht built at Lürssen shipyard would have a double bottom of 1.5 m height

maximum at the centreline in the engine room and the usual frame spacing is 650 mm. Hence

the compartments are very small and working inside such congested space creates an unsafe

working environment for the workers.

There’s very low ventilation, no natural light and the workers dealing with large electric

equipment might pose a risk of safety in those conditions. It will also be difficult to escape

during an emergency.



Other issues

Apart from these main issues, it is required to find and fix the favourable position of welding

very earlier in the design stage and place the structure according to that, as the block cannot be

turned. Which means that, everything has to be planned earlier and there’s no room for changes.

In addition, the idle time of workers are large as they need to wait for a machine to cycle or

waiting for the preceding operator to finish their work. When some changes needs to be done,

this might increase further.

2.5 Requirements of a Crossover

Similar to the problems encountered in the present production process, there are also several

requirements for a crossover, which are unable to be achieved due to the complex structure and

difficult construction process. Since the modular design and construction should be applied

through the optimization of the cross flow channel, let us understand the requirements of the

crossover and how modular design can help in such improvements to be made.

The important requirements of a crossover can be categorised under functional and operational

requirements, which are discussed below.

2.5.1 Functional Requirements

The first and foremost importance goes to the functional characteristics in the crossover. The

flow velocity in the crossover is usually maintained between 0.5~1.1 m/s to have a certain flow

rate that can meet the need of water from different machinery. The flow rate required will be

calculated by summing up the entire water intake rate of each equipment and on an average it

is found to be around 2000 m3/h. It is required to have low flow resistance for the cooling water

and good flushing.

At Lürssen shipyard, since it is the double bottom structure, which is used as the cross flow

channel, the flow is maintained at 0.5 m/s to avoid resistance due to the high number of

structure and profiles inside the channel. However, having a lower flow velocity lets room for

marine growth and this will again increase the resistance to flow and reduce the flow rate of

water which is necessary to feed the machines connected to it.



Hence, in terms of functional requirements, it is necessary to have a higher flow rate to reduce

the risk of marine growth while keeping the resistance down. It is also important to avoid air

bubbles or air cushion in the crossover to make sure that the suction pipes intake only water

from the crossover and not air which may damage the machinery.

2.5.2 Operational Requirements

The next most important aspect is operation and maintenance. As experienced from many

different vessels in the past, it is found that nearly after a year or two of usage, the cross channel

structure gets deposited with mud/sludge and marine growth which resists the flow and

eventually block the flow inside the channel. This problem has been a major concern, as it is

again a strenuous job to get inside and clean the double bottom structure.

The main cause for this is the sharp corners and edges of the structure, which can easily get

deposited by sludge. As discussed in the previous chapter, having a low flow velocity is another

reason behind this. Hence, it is a major requirement to avoid sharp corners and edges to reduce

the sludge formation and which also can reduce the resistance to flow.

The channel should have enough space to be able to get accessed inside for inspection and

maintenance purposes, preferably there should be space for a person to get in if necessary.

Additionally, the cross flow channel should be suitable for anti-fouling or chemical/electrical

fouling protection or for cathodic protection. With the idea of optimising the cross flow

channel, it is also required to have as little surface as possible to avoid fouling of larger areas.



3. OPTIMIZATION

3.1 Different possible solutions

In the early stages of the thesis, different possible solutions were discussed regarding the

optimization of the cross flow channel. As discussed in the previous chapter, the major concern

is to reduce structures or profiles from the cross flow channel. Hence as a simple solution, it is

discussed to build the stiffening profiles to the outside of the cross flow channel. In addition,

to reduce the sharp corners, a set of curved cover plates can be placed between two

perpendicular plates. The concept is represented as follows in fig. (3).

Figure 3. Idea of profiles built outside and additional cover plates

However, it is not possible to eliminate all the structures or profiles from the inside of the cross

flow channel. Also the concept of cover plates wouldn’t make much sense since not all the

corners can be covered and it will also increase the difficulty of building it. Moreover, not a lot

of benefits be achieved and the advantage of modular construction cannot be utilized.

Nevertheless, from the idea of curved cover plates, a new solution is discussed, which is using

a large pipe as the crossover instead of the cross flow channel. We know that this will definitely

increase the complexity of the structure and the process of construction. However, it is where

modular construction can better play its part.

Hence, the final decision is to integrate a large pipe inside the double bottom, which can carry

along with certain advantages over the conventional cross flow channel. The design process

and the benefits of the new structures will be discussed in the further chapters.



3.2 Integrated pipe in double-bottom

As mentioned above, the idea is to integrate a large pipe in the double bottom, which can serve

as a crossover instead of the conventional double bottom cross flow channel. The pipe should

be large enough to achieve the required flow as well as to give access inside for inspection and

maintenance. The basic idea is that a pipe of diameter 800-900 mm would be suitable for this

particular application.

3.2.1 Selection of Pipe

The most important factor in the selection of pipe is the flow requirements of the machinery

and equipment to which the crossover supplies seawater. After analysing the flow rate

requirements of the vessels built in the past, we can obtain an average value of 1856.6 m3/h of

flow rate usually required by the machinery & equipment. As the flow rate required was higher

only at a few occasions, we will choose a flow rate of 2000 m3/h as an average requirement in

a general perspective.

However, higher and lower rates of flow can be achieved depending on the cross section and

the flow velocity in the pipe. Considering the fact that the pipe does not poses too many

structures or profiles inside, a higher flow velocity can be achieved which is about 1 m/s.

Hence, the size of the pipe should be selected according to the flow requirement and achieved

velocity. The cross section area required can be calculated from formula shown in Eq. (1).

(1)

For an average flow rate of 2000 m3/h and a water flow velocity of 1 m/s, we get the cross

section area necessary is 0.55 m2.

To have a minimum cross section area of 0.55 m2, a standard size pipe with an inner diameter

of 843.6 mm is selected. With a cross section area of 0.558 m2, this pipe is capable of achieving

2012.17 m3/h, which is a little above what we expected to have. However, it is possible to

increase the flow velocity up to 1.1 m/s and hence it is possible to reach up to 2213.39 m3/h

with this standard size pipe.



Selection of pipe thickness is important as the buckling stiffness at a point where a stress around

70-100 MPa is experienced. Most of the Yachts built at Lürssen shipyard are classified under

DNVGL classification and there are certain rules, which are considered while selecting

thickness of plates.

According to DNVGL-RU-YACHT Pt.3 Ch.4 Sec.4., the minimum thickness for plating

of detached tanks is 3.0 mm and for stainless steel, the minimum thickness can be

reduced to 2.5 mm.

According to DNVGL-RU-SHIP Pt.3 Ch.6 Sec.3., the net thickness of plating shall not

be taken less than:

𝑡 = 𝑎 + 𝑏 ∙ 𝐿2 ∙ √𝑘 (2)

Where a=4.5 and b=0.02 for Inner bottom spaces. The factor k may be taken as unity.

Hence, for a ship of an average 100m, the minimum thickness would be about 6.5 mm.

According to the DNVGL rules of Sea going ships, I-1-1 Sec.12, the minimum plate

thickness of all structures in tanks shall be not less than:

𝑡 = 5.5 + 0.02 ∙ 𝐿 (3)

Hence, for a ship of an average 100m, the minimum thickness would be about 7.5 mm.

Usually the minimum thickness rules for tank boundaries would cover around 90% of the

buckling cases. However, extra stiffening is still considered for extreme cases. Hence, the

double bottom structure is usually stiffened with 15mm thickness on all transversely stiffened

plates and will be more than sufficient at a point so deep in the double bottom.

Considering the extra-stiffened girders and to comply with the minimum thickness

requirements, the pipe thickness should be selected between 7-15 mm. Hence, an available

standard size of 10 mm thickness is selected for the chosen pipe. However, it is only an entry

point and further investigation on the strength is necessary to validate it. With 10 mm thickness,

the selected pipe will have an outer diameter of 864 mm.



3.2.2 Crossover Pipe and fittings

A standard pipe of 864 mm outer diameter and with 10 mm thickness, which will serve the

need, is selected as the crossover pipe. On either end of the pipe, to the top, an opening with a

connection and a flange is provided in order to be fixed with the outlet of the pump through a

filter of seawater coming from the sea chests. This will be the inlet of seawater into the

crossover pipe. Additionally, the top of the crossover pipe will be provided with number of

cut-outs for the suction pipes to be sent through.

The crossover pipe is fitted with blind flanges on both sides in order to give access inside

whenever it is necessary to inspect or for the purpose of maintenance. The pipe also can be

given with only one opening at one end while the other is permanently closed. But there might

be some obstructions in getting through the pipe as it will be installed with many other suction

pipes to the middle. Hence, both ends are given with openings to make sure to have easy access

inside the pipe. The crossover pipe and its fittings are modelled and are as shown in fig. (4).

Figure 4. Crossover pipe

It should be noted that, with 210.5 kg/m weight of this standard pipe, together with the flanges

and after given with cut-outs it will bring in additional weight of about 1.6 tons to the structure.

However, the weight of the ship is maintained with no increase with several structural

modification, which will be explained in the following chapter.



3.3 Structural Modifications

Integrating the pipe inside the double bottom is a task that needs a lot of considerations from

the structural point of view, especially strength is a major concern.

3.3.1 Girder openings & strength

The crossover pipe should be placed in transverse direction that it would interrupt the

longitudinal girders in the double bottom. Usually, the cut-outs on the girders are made in such

a way that the opening is about 60 % of the web height of the girder, taken as a rule of thumb.

As this optimization work is based on an example of one particular vessel, this vessel has a

certain dead rise angle and the longitudinal girders have different heights at different positions,

the highest being 1.5 m of the centreline girder and the lowest being 1.02 m where the pipe

interrupts.

According to our case, having a cut-out as large as 0.86 m will make the cut-out being at about

85% of the web height on the smallest girder which is not acceptable even though it makes

only 57% at the centreline girder. Hence, to fit the pipe transversely through the girders, and to

comply with the strength standards, the girders are raised by 220 mm to achieve the cut-out

being at least 70% of the web height at the shortest girder. However, all the girders that are

interrupted should be raised to have an equal level.

It is important that the continuity of the girder should not be interrupted and hence the girder

is raised for a length of two transvers frame spacing and the raised height is tapered down to

the level of the tank deck for a length of one frame spacing. The knuckled edge is provided

with edge blends. The modified structure of a girder can be seen in the fig. (5).



Figure 5. Modified longitudinal girder dimensions

By modifying in such a way, we have obtained a decent percentage of size of opening on the

girder below which is permissible. The space between the shell and the bottom of cut-out of

the smallest girder is 105 mm while the minimum requirement is 100 mm. However, this is

only an entry point to the solution and it would certainly need further investigated in a detailed

FEA with potential optimization strategy.

Additionally, the raised part of the girder needs additional stiffening to prevent buckling, as it

is not supported by the deck plate anymore. A belt profile of 120×10 is used in this case bent

to have a knuckle to run along the raised part of the girder. Also, stiffeners of 50×8 are used on

either sides of the raised part of the girder at the bend point, connecting the belt and the tank

deck. The completely modified girder with all the added profiles can be seen in fig. (6).

Figure 6. A completely modified girder with additional stiffeners and belts



It should be noted that the raised girders do not interrupt the platform on top of the deck as it

is built 1 m above the tank deck. Since most of the pipes in the machinery room are placed in

longitudinal direction, there is very minimal disturbance, which can be adjusted easily.

3.3.2 Modification of the deck

The pipe used is quite large and is placed inside the double bottom in such a way that the top

of the pipe reaches almost the same level of the deck. Hence, it is not possible to have a flat

deck at this point as in the conventional cross flow channel. Since there will be many suction

pipes installed on the pipe, it is decided to have an open deck along the length of the pipe.

The deck plates are knuckled downwards at 40˚ angle just after the frames before the pipe and

are connected to the pipe having an opening of the deck for a certain area exposing the pipe.

The edges of the opening on the deck is provided with edge blends to reduce the effect of stress

concentration. The modified new deck arrangement is as shown in fig. (7).

Figure 7. Modified deck

The knuckled plates are placed below the flat deck plate 40 mm away from the frames in order

to leave space for the weld seam of the frame on deck plate and 20 mm inside to the edge of

the flat deck in order to weld them together. The arrangement is as shown below in fig. (8).



Figure 8. Knuckled deck plate arrangement

3.3.3 Addition of floor plate and support

In the conventional cross flow channel, usually a floor plate will be eliminated to give smooth

flow of seawater in the channel. The lost strength will be compensated with additional

stiffening of the shell, deck and longitudinal girders with long profiles spanning at least two-

frame spacing on either side of the missing floor plate.

However, the optimized structure is not a cross flow channel or a tank anymore and to give the

shell, girders and the pipe a good support, a floor plate is added below the pipe along its length

where it was missing before. Along with the pipe being welded on top of the floor plate, this

structure resembles a perfect girder and hence it gives an advantage of being able to eliminate

all the additional stiffening done in the conventional cross flow channels. This will be discussed

in detail in the following chapters.

Additionally, an extra plate spanning two-frame spacing is placed longitudinally at the end of

the floor plate where the pipe terminates. The structures added additionally to support the

bottom shell, girders and pipe can be seen below in the following fig. (9).



Figure 9. Added floor plate and additional support

Both of these additional structures would bring half a ton added weight to the ship, which

however is compensated by the structures eliminated.

3.3.4 Elimination of additional stiffening

Due to the addition of the floor plate & support and the open deck design as mentioned in the

previous chapters, a large number of stiffening profiles and brackets are eliminated in the new

structure which were present initially due to the lack of support strength on the deck and the

shell plates.

The elimination of such a large number of structure brings us certain benefits. One of the most

important in terms of structure is that, it helps maintaining the weight of the block. That means,

about 1.7 tons of weight is been removed due to the elimination of these profiles. Other added

advantages will be explained in later chapters.



3.3.5 Other modifications

There are certain other modifications carried out to have a much precisely detailed structure

concerning other issues that can arise. One of them is the modified and additional cut-out on

the transverse frames on either side of the crossover pipe. Initially, the part of the frames at

keel had one larger cut-out on each giving smooth an uninterrupted flow of seawater through

it. As the optimized structure is not a cross flow channel, such a large cut-out is not really

necessary. However, the additional longitudinal support added at the end of the pipe had

created a new compartment behind it which do not have an opening to give any access into it.

Hence the initial cut-out of 1050×800 is reduced to 850×700 and an additional cut-out is

provided on the same frame but on the other side which can give access to the new compartment

at the end of the crossover pipe.

Similarly, other larger cut-outs on the floor plates are also reduced in size of opening as it is

not necessary since the structure in not the cross flow channel anymore and there are no

chambers which requires access often.

Due to the elimination of certain profiles and brackets, the end cuts of certain profiles on the

transvers frames and longitudinal girders also had to be changed. Many of these profiles, which

initially had an ‘end connection with brackets’ are given with a ‘snipped’ end cut as the bracket

would have probably been eliminated.

3.3.6 Final modified structure

The final structure after all the above mentioned structural optimization is as shown in fig. (10)

Figure 10. Optimized cross flow channel – cut section



According to the modifications made and calculation of weight of all structures added and

removed, the final structure has an increased weight of about 1.5 tons in total. However, this is

later compensated by the weight of anti-fouling paint which is been reduced by about 85% due

to the reduction of the size of the cross flow channel, we will look in detail in later chapters.

The size and number of pipes depends on the equipped machinery and requirement. As an

example, the model below in figures 11-12 are shown with possibilities of installing a number

of suction pipes of different sizes.

Figure 11. Installation of suction pipes

Figure 12. Installation of suction pipes – section view

However, it should be noted that it is only an idea and it is very specific for this case. Different

vessels will have different equipment and capacities, sometimes more or sometimes lesser.

All of these modifications are based on one particular ship and it will differ for different ships.

For example, a vessel with much lower dead rise angle could be integrated with a larger and

longer pipe while possibly without the need to increase the girder height.



3.4 Impact on general design

According to an expert of basic design at Lürssen shipyard, the cross flow channel is treated as

a permanently flooded compartment but not permanently opened to the sea. And since the

crossover pipe is a part of the double bottom and as long as it is placed below the tank deck, it

is not concerned with SOLAS regulations of damaged stability.

However, a simple FE model is used for comparison of the modified structure to the

conventional structure to identify the effects on the longitudinal girders using Siemens NX

advanced simulation tool. It should be noted that it is only a comparison between two structures

and not a complete assessment of the local strength of the modified structure.

3.4.1 Idea behind the FE model

In a conservative approach, a longitudinal girder under the engine room between two bulkheads

is chosen, as it is necessary to analyse the impact on the girder and it is placed at an area of

very high stresses. The shell and deck plates attached up to 1.5 m on either side of the girder

are also modelled along with all the transverse frames between the bulkheads. The exact

spacing of the girder will not be a concern as we are not interested in the absolute stress results.

FE models for both the new and the conventional structure are modelled side by side as shown

in fig. (13) to have an easy comparison with same conditions. The two models have no

connection between them and given with same constraints and load conditions but separately.

Figure 13. Models of conventional structure & modified structure



The basic idea is to create a simple model, which is heavily idealized since the goal is to

compare the two different structures. The girder can be considered as a beam fixed at both ends

with a uniformly distributed load. The clamping condition produces a lot of reaction forces on

longitudinal girders while actual stress is carried by the them and not by the transvers frames.

Only one girder is modelled since it is a first approximation and only for comparison.

3.4.2 Mesh

2D shell elements are used with consideration of typical properties of steel (Modulus of

Elasticity = 210,000 M/mm2 and Poison’s ratio = 0.3) which are inbuilt in the program. Models

are given with a mesh of element size of 150 mm each. This is considered as a reasonable

number of elements for a first approximation as we are interested in static stress. Too much

detail or a very fine mesh might lead to the calculation of Notch stress. It is also a better choice

in terms of time required for simulation. The more finer the mesh, the more time consuming

the simulation is. Below in fig. (29) we can see the model meshed as described above.

Figure 14. Mesh with 150 mm of element size

It is also important to avoid triangles in the mesh. Triangles are more stiffer than squares and

might lead to underestimation of stress values during calculations. However, the FEM tool used

is not meant particularly for shipbuilding and not an accurate solver. Hence, the model had to

be meshed with ‘allowed triangles’ to be able to get the results without errors, which otherwise

wouldn’t run the simulation at all for the element size given.



3.4.3 Constraints and Load condition

As discussed above, the longitudinal girder can be considered as a beam fixed at both ends.

Hence, the horizontal edges at the ends are fixed in X and Z-axes while the vertical edge is

fixed in all directions, however all of those edges are left free to rotate along each axes. Another

constraint of fixed displacement only at Y direction is given to all of the edges of the transvers

frames, shell and bottom plates as well as for the edge of the pipe, keeping in mind that those

elements are continuing further in Y axis without a constraint until the next girder. This way,

it suppresses constraints in X and Z-axes, which is a perfect symmetry. It is a matter of

completeness and however, the rotations can be left free as it does not constitute much. Below

in fig. (15) we can see the constraints given at different edges of the models.

Figure 15. Constraints given on different edges

The load condition applied is a uniform static pressure of 100 kPa as shown in fig. (16), which

is considered to be a reasonable value of pressure to be acted on the double bottom. No special

sea condition is taken into account as it is only for the purpose of comparison.

Figure 16. Load condition



3.4.4 Simulation and results

The simulation is run using Siemens NX 9.0 Advanced simulation tool and the results are

obtained in terms of nodal displacement, rotation, elemental & nodal stress and Reaction force

& moment. However, we will have a brief on the displacement and the elemental stress as it is

the most important aspect to analyse. The obtained results are as follows.

Displacement

The nodal displacement obtained after simulation is show below in fig. (17).

Figure 17. Nodal displacement – Optimized structure vs Conventional structure

It can be noted that the displacement is minimal in both the cases. However, there seem to be

a slight improvement in the optimized structure considering the whole as a beam. This is due

to the addition of the floor plate which is missing in the conventional structure. The additional

stiffening in the conventional structure may serve the purpose but not as good as a floor plate.

Elemental Stress

This is the most important aspect that needs to be analysed, as the main concern is to identify

the impact on girders due to the structural modification. The following is the obtained von-

Mises stress distribution acting on the girders of both different structures shown in fig. (18).



Figure 18. Elemental stress – Optimized structure (a) vs Conventional structure (b)

As it can be seen clearly from the figure that the stress acting on the optimized girder seemed

to be significantly lower than that of the conventional structure. The main reason for this is the

large square shaped cut-out in the conventional structure, which has very high stress

concentration levels at the edges of the cut-out. In the modified structure, we can see that the

stress concentration around the cut-out is pretty much evenly distributed and are less than half

in values compared to the other. This is due to the circular cut-out and additional support with

a floor plate and a pipe attached with it.

Hence, according to this simple FEM analysis, it is clear that the optimised structure has a good

and actually, much better behaviour to the application of static pressure compared to the

conventional structure. For the purpose of this thesis, we can conclude that the optimised

structure satisfies the need.

However, as mentioned in the beginning of the chapter, it should be noted that these results do

not justify that the modified structure complies with required structural strength criteria. It is

only a comparison and we can only conclude that the optimized structure might possibly be a

good solution, but still needs further investigation in this regard. It is also important to note that

the simulation tool used is not meant particularly for shipbuilding and hence it cannot guarantee

the results to be perfect for the given case. Also as mentioned earlier, this tool had used triangles

for errorless working but it might underestimate the actual stress impact.

Hence, in order to analyse the exact impact on the structure, a strength assessment of the entire

bottom grillage should be made with all the possible sea conditions and with the right FEM

tool, which is much more time consuming and out of the scope of this thesis work.



3.4.5 The necessity to increase the height of girders

This is an important aspect from the structural strength point of view. According to the experts

of basic design at Lürssen shipyard, the girder should be increased if the cut-out is too large.

As a rule of thumb, about 60% of the girder web height is allowed for a cut-out.

Hence, to justify the answer, an FEM simulation of such a model is done and the results are as

follows. It is been compared to the results from the simulation of the optimised structure which

is discussed in the previous chapter.

Figure 19. Nodal Displacement –New structure without increased girder vs Conventional structure

As you can see from the above fig. (19) that there is a slight increase in deformation at the

central part of the girder and the shell plates compared to the FEM analysis done in the previous

chapter although it is not so significant. However, the elemental stress is the important factor

to be analysed and is obtained as follows.

Figure 20. Elemental Stress – Optimized structure, with raised girder (a) vs without raised girder (b)



In the above comparison shown in fig. (20), it can be clearly seen that the stress concentration

at the opening of the girder is significantly higher in the model without the raised girder.

Though it is only a comparison, it can still interpret that the girders are need to be increased in

height if the size of the cut-out is increased.

3.4.6 Open-deck above the pipe

To see this question from the structural strength point of view, another model is prepared

accordingly and an FEM simulation is carried out to see the outcome and are presented as

follows.

Figure 21. Optimized structure, with open deck (a) vs conventional flat deck (b)

Looking at the elemental stress as shown in fig. (21), we can see that the stress concentration

also has no significant difference. In this way, this model can also be chosen as the right

optimised structure. But the major consideration to use the ‘open deck’ concept is that the

requirement of “easy to build”.

Although the knuckled deck plate concept needs extra plate cutting, it can still be worth during

the construction process. This is because, the crossover pipe is large and is placed in such a

way that it is almost at the level of the deck. Welding the deck plate with the curved surface of

the pipe needs extra profiles and brackets to be placed in between them while there is very tiny

space available. And the welding job will be very congested at those points while reachability

is also another concern to be worried of. In addition, the open deck concept has another

advantage that, the suction pipes are installed directly to the crossover pipe, which can also be

pre-outfitted.



4. MODULAR DESIGN AND CONSTRUCTION

4.1 Literature Review: Modular Construction

Modular construction was obtained from the idea of the lean production, which is one of the

theories in ship industry. A Module is combination of sub-assemblies joined together that can

be transported from one facility and to be assembled in another. The meaning of modularization

differs from field of work, but the general idea is to divide large systems into smaller, self-

sufficient parts.

Looking at the origin of Modular construction, Henry Kaiser’s introduction of Group

technology for the Liberty ships (to attain benefits associated with production lines) lead to

development of modular assembly for shipbuilding business during WWII bringing about an

industrial revolution within the industry. This concept of modular construction had come up

due to the requirement of optimizing shipbuilding production process by reducing costs and

increasing competitiveness without investing in new facilities, machines and tools.

Figure 22. Concept of modularization

Modular construction is now a common method applied in shipbuilding industry. Modular or

prefabricated construction is a procedure that practices a prefabricated building process, which

are gathered on-site to create a stable or temporary prefabricated building. Modular space earns

lots of benefit of well-ordered fabrication surroundings together with the plan flexibility of

classic structure procedures to yield superior stable or impermanent prefabricated structures

for some request. The way these modules are combined makes a final unique design.



Modularization allows complex structures to be manageable in production, allows parallel

working and can accommodate future uncertainty. Discretizing the complexity down to self-

sustainable building blocks, where each module has defined system borders and demands, the

engineer is able to manage large and complex systems in a structured way. Modularization

techniques can be applied to platform construction and systems design. Larger modules can be

built with more fit-out and testing undertaken on land earlier in the build process.

Compared to conventional construction, modular construction needs a high degree of

interaction among construction activities, with planning of many of these activities to occur

early in the project as shown in fig. (22). Modular construction redefines relationships among

activities that are usually independent in conventional construction. Unlike standard

construction, where most of the design, engineering and construction activities are carried out

in sequential order, activities for modular construction involve additional interdependency

since those works can be performed in parallel in the same time at various fabrication shops or

at various construction sites.

4.2 General design aspects

We know that every sub assembly is a module; even a block is nothing but a module. There is

still a reason to select a certain size and particular number of components while designing a

module, each having different individual aspects. In this chapter, we will have a brief on the

important general aspects to be considered during implementation of the modular design

concept.

4.2.1 Tolerances

Tolerances in block mounting would be a big concern as there will be quite large imperfections

on fitting large structures. Considering this, modules can be chosen to have as less interfaces

as possible to avoid these issues.

4.2.2 Size standards

Modules can also be chosen with the standard sizes of sheets, plates, pipes and other type of

available material. This way, it is possible to avoid extra work such as cutting or welding.



4.2.3 Accessibility or Reachability

The most important work done in construction sites will be welding. When the structure

becomes large with increasing complexity, a module can be chosen that should give easy access

to all the places that needs to be welded.

4.2.4 Working position & conditions

It is always recommended to avoid upside welding as much as possible. A module can be

considered when there are large number of components to be welded to the top. It can give the

advantage of turning around to fit the most convenient welding position. It is important to

maintain a safe working condition during welding, cutting or other works. A module can be

selected considering the congested working spaces in the complex structures.

4.2.5 Number of parts

Another important aspect is the number of components involved. Transportation, handling and

storage of quite a large number of components are always an issue and it more often leads to

missing parts and wastage of time corresponding to it. Eventually, the missing parts might have

to be fabricated again. Modules can be selected with lesser number of parts, which does not

require larger storage and handling while reducing the same from the block assembly.

4.2.6 Better utilization of the facility

As much as it is important to have a facility, that is capable of dealing with modular

construction, it is also important to make use of it for the largest extent. For example, cranes

can be utilized more with a bit larger modules while reducing the work load from the block

assembly. At Lürssen shipyard, the cranes at the prefabrication facility can easily handle

modules up to 25 tons. This could be another important factor to consider modular construction.



4.2.7 Special parts/components

Some structures might integrated with special parts or components especially like larger pipes

and fittings. Modules can make it easier to construct with the surrounding structure with very

congested space around. Parts to be assembled closer to the shell can also be in this category.

Some components could belong to two or more different levels or blocks, which would make

sense to be built as a module.

4.2.8 Possibility of subcontracting

In addition, when modular design is standardized for a particular component, it can be sub-

contracted rather than building it by own. In addition, when special parts exist in structures, it

is beneficial to sub-contract it. While saving much time in our production facility, sub-

contracting might possibly be cheaper than building on our own.

Let us look at a few examples of modules, which are pre-fabricated at Lürssen shipyard and

the importance to do so.

Outside walls of the superstructure

The walls of the superstructure has a lot of small parts and components involved especially a

lot of stiffeners which are placed horizontally on a vertical wall. Hence, to have the most

convenient welding positions and to avoid handling of too many parts in the block assembly,

this part is built as a module. In addition, extra work can be avoided when the modules of the

walls are selected to the standard size plates available in the market.

Bulwark

The importance to do so is that the bulwark should be built along the curved shell on top and

it will be difficult to weld the profiles. There will also be problems of tolerances as the bulwark

is a long and large structure. With a module, the bulwark can be prefabricated with the most

convenient welding position with the possibility of turning around.



Sea chests

The low sea chests usually stand on top of the tank deck. It will be difficult to build it along

with the double bottom block as the block will be built upside down and it is much easier to

build the sea chest as a module separately with the necessary plates and stiffening included.

Meantime, a part of the high sea chest should be integrated in the double bottom, another side

being part of the bulkhead while it had to be welded with the shell also. This makes it a very

complex situation in terms of construction. Hence, the top part of the high sea chest is built as

a module with the required stiffeners, brackets etc. This can also help avoid issues of

alignments. After the double bottom block is completed and turned upright, both the high and

low sea chests can be laid down and mounted on top of the double bottom structure.

Engine/Machine foundations

In this particular case, it should be noted that the machinery to be mounted and hence its

foundation belongs to two blocks. To avoid the risk of imperfect alignment, these components

are built as modules.

Bow Thruster

A bow thruster is a very special component, which has to be placed on the bottom of the ship.

Since it has a very large opening that interrupts the girder, the shell of the thruster tunnel has

to be provided with extra stiffening as shown in fig. (23). Along with the motor, rotor blades

and all other machinery, the whole structure with additional stiffening is subcontracted as a

module. When the surrounding structure is built, the module of the thruster is placed and

mounted with the rest before the shell plates being weld.

Figure 23. Bow Thruster. Available from https://www.vethpropulsion.com/products/bow-

thrusters/tunnel-thruster/ [Accessed 26 December 2018]



4.3 Modular design of the cross-flow channel

As we already have an optimised structure of the cross flow channel, now we will study the

modular design and construction process of this particular cross flow channel structure in the

following chapters.

We know that the complexity of the cross flow channel structure is been increased due to the

modifications made, especially the addition of a pipe inside the double bottom structure. This

certainly rises the need for a modular design in terms of construction of the structure. However,

we should keep in mind that the structure is optimized to utilise the advantages of modular

construction. Following are the major and important requirements for a module.

Considering important aspects such as tolerances, accessibility and working condition issues,

number of parts involved and most importantly special part in this case as there is a large pipe

being integrated in the double bottom, a modular design is made for the cross flow channel.

The final module is as shown below in fig. (24).

Figure 24. Module of Cross flow channel

The module includes a total number of 137 single parts and constitutes 8.5 tons total in weight.

Pre-outfitting and painting of anti-fouling and anti-corrosion might increase a half a ton more.

However, the pre-fabrication facility at Lürssen shipyard is more than capable of handling such

a large module.



4.4 Selection of suitable substructures

In this chapter, we will have a brief about different major components or substructures, which

are a part of the module and the main reason for such selection.

4.4.1 Crossover pipe

As obvious, the crossover pipe is the core of the cross flow channel. Hence, the crossover pipe,

which is already prepared with the required openings, fitted with flanges, holes for suction

pipes and possibly with the pipe fittings for the suction already been welded will be built as a

module with the surrounding structure.

4.4.2 Transverse Frames and floor plates

Since building the structure around the crossover pipe is the main challenge, it is important to

have both the adjacent transverse frames in the module. The floor plate added newly supporting

the pipe below is also selected to be in the module. However, the parts of frames to be built as

module are limited to the length of the crossover pipe.

4.4.3 Longitudinal Girders

As we know that the crossover pipe is in transverse direction, the longitudinal girders have to

be limited in length in order to fit in the module. Considering the adjacent transverse frames

and to keep the continuity of the longitudinal girders, they were limited to the length of 200

mm away from the transverse frames to the outside on either sides. It is necessary to have such

an allowance to deal with tolerances and this way, it will be also convenient to join the girders

with the remaining structure during mounting of the module. The parts of the girders are cut

into half allowing the crossover pipe to be placed from above which otherwise wouldn’t be

possible at all.

4.4.4 Deck and Shell plates

The modified deck plates and the shell plates up to the length of the pipe are selected to be built

as module. These parts are also limited to a length of 200 mm away from the transverse frames.



4.4.5 Stiffeners, brackets and docking profiles

All the stiffeners, brackets and shear plates associate mostly with the transverse plating and as

well as the deck and shell plate are selected to be pre-built as sub assembly with the

corresponding structure before being mounted as a module. The docking profile are also cut

200 mm away from the transverse frames in order to be built with the module.

4.5 Sub-assemblies

Before the construction of the module being started, the sub structures are built in the pre-

fabrication, so to say, as smaller modules. This way, the number of parts handled will be

reduced further while moving into the construction of bigger a module.

4.5.1 Crossover pipe

Since the pipe we are using is of a standard size, it is bought and then prepared for the modular

construction as a smaller and much simpler module by providing with the required holes, cut-

outs, flanges and fittings as shown in fig. (25).

The holes and cut-outs required for the inlet of sea water and suction pipes are cut out of the

pipe and the necessary flanges are weld with it. It is also possible to party pre-outfit the pipe,

i.e., to weld the pipe fittings for the suction pipes at this stage as the pipe is freely accessible

without any obstructions.

Figure 25. Crossover Pipe sub-assembly

Once the pre-fabrication is over, the pipe can also be coated with the anti-fouling paint already

in this stage leaving only a little space where the girders will be weld, which can be painted

later.



4.5.2 Longitudinals

The upper half of the longitudinal are also pre-fabricated with the stiffening belt to be welded

on top of them. It can also be welded as one single parts after completion of the mounting of

the module to avoid a little extra cutting work. However, in this case we will consider the pre-

fabrication of the upper half of the longitudinal girders as shown in fig (26), which can also

help on providing a horizontal surface for convenient placing on the jigs during the construction

of the module.

Figure 26. Sub-assemblies of parts of longitudinal girders

4.5.3 Transvers frames and floor plates

The transverse frames consist of additional stiffening and they are pre-fabricated to become

single components before being built as a module. These are the most important groups of sub-

assemblies since they involve in many different single components with different size scales.

Plate stiffeners, corner brackets and shear plates are weld together with the corresponding floor

plate.



4.6 Construction of the module

The sub-assemblies discussed in the previous chapter along with the remaining selected parts

are constructed together to form a module. In this chapter we will have a look at the

construction of the module in steps for better understanding the procedure.

Each important step is presented with step drawings, which can help the workers picture the

procedure immediately to give the basic idea of how it can be done. In the step drawings, parts

indicated in a transparent manner are the ones which are placed first or welded/joint in the

previous step and the parts indicated orange in colour are the ones which are welded in the

current step.

Step-1

The construction of the module is done upside down, starting with a jig, on which initially, the

sub assembly of the upper half of the longitudinal girders will be placed. Then the horizontal

parts of the deck plate will be weld together with the upper half of the longitudinals as hown

in fig. (27). Also, the upper half of the additional support given to the crossover pipe and a

stiffener will also be weld on the deck plates.

Figure 27. Step 1 – Module building



Step-2

Now the crossover pipe is placed on the semi-circular cut-outs on the girders and the point of

contacts on either side of the girders can be weld easily. Later the inclined parts of the deck

plate are placed and can be welded with the girder as shown below in fig. (28).


Step-3

The next step involves in welding of the lower half of the longitudinal girders and the newly

added transverse floor plates below the crossover pipe as shown in fig. (29). This way there is

more than enough room around the connection points to be weld. However, this step can be

done in different ways to fit the workers convenient, which means changing the order of parts

being weld.




Step-4

In this step, the rest of the sub-assemblies of the transverse frames are placed and welded with

the module as shown in fig. (30). While the outer edges can be welded very easily, the inner

edges can also be welded conveniently, as there is free access for the workers around the

module and the module can also be tilted or rotated if necessary.


However, both of these steps 3 & 4 could be done in a combined manner with different

sequences to obtain different convenience of the workers. As an example, a group of structure

including a lower half of longitudinal girder and all three transverse frames attached to it can

be welded first which will follow with the next set of parts progressing from one end of the

crossover pipe to the other.



Step-5

Before placing the shell plate, the docking profiles are placed and welded together with the

transverse frames and the shear plates as shown in fig. (31).


Step-6

The final step is to place the shell plate and welding it with the module and is as shown below

in fig. (32).




It must be noted that at certain points, it can still be difficult to get access for welding of the

shell plate, especially with the longitudinal girder. However, there are alternate options like

plug welding or slot welding for those areas with limited space. Following fig. (33) can give

better understanding of the method.

Figure 33. Plug welding (a) and Slot welding (b)

In the case of our module, a weld backing which is a small plate strip of about 30-40 mm wide

can be welded along the bottom of the girders, as it might not have problems of accessibility.

On top of these backings, the shell plate with holes can be plug welded or a slot of 5-6 mm

between two shell plates can be filled to have a slot weld as shown in fig. (34).

Figure 34. Slot welding with weld backing on girder

Out of these two methods, slot welding is preferable over plug welding due to the effects of

stress flow through the material. In plug welding, the stress will tend to flow through the base

material regardless of the weld fill while in case of slot welding, there is a complete fill with

the weld material and the stress will flow straight regardless of the presence of backing.

Though it might be necessary to have special welding techniques even during a modular

construction, the required work is minimal compared to the advantages of accessibility

achieved though the module. In this case, the maximum length of above-mentioned special

welding jobs, which might require is only about 1.3 m, which is the distance between 2 frames.



5. Review of benefits and drawbacks

5.1 Optimized structure

The optimized structure itself poses certain benefits and drawbacks as well, which are

discussed in this chapter.

5.1.1 Functional benefits

As we discussed in the earlier chapter, there were certain requirements related to the function

of the crossover, which need to be achieved during its optimization. The optimized structure

did achieve those requirements giving certain advantages over the conventional structure of the

cross flow channel.

Flow velocity

The flow velocity up to 1.1 m/s can be achieved in the crossover pipe as it does not poses sharp

corners or edges. However, the suction pipes inserted inside needs to be considered. With high

flow velocity, it is possibly to achieve the required flow rate with minimal flow area, which

means that a further larger cross flow channel is unnecessary. High flow velocity also reduces

the risk of marine growth.

Reduced resistance

The conventional cross flow channel used to have a flow velocity of 0.5 m/s due to the

resistance caused by large number of structures and profiles inside of it. Since the new structure

has only a crossover pipe, resistance to flow is reduced even at high speeds. It is important to

make sure the water flow is not interrupted and is flowing continuously in the crossover and

being supplied to different machinery.

Reduction of air bubbles

As the crossover pipe is free from profiles and structures, the amount of air bubbles and air

cushion are reduced. It is important to make sure that the machinery which intakes water from

the crossover takes only water and not trapped-in air, which can damage or reduce the

efficiency of the machinery.



Flow rate

As we discussed earlier, the flow rate is the important consideration and not the volume of

water. In this case, with such a large crossover pipe, it is possible to supply about 2000 m3/h

while reducing the volume of water being carried by about 90%. This possess certain other

advantages, which are discussed in the following chapter.

5.1.2 Operational Benefits

Reduced mud/sludge formation and marine growth

One of the most important requirement of the optimization was to reduced the mud/sludge

formation and marine growth on the corners and edges of the structures which are exposed to

sea water. As the optimized structure has only a pipe in which the seawater flows through, this

problem is reduced due to the smooth and curved walls with no corners. Higher flow velocity

is another factor that can help reduce the marine growth in the pipe.

Although this issue is reduced, it cannot be completely eliminated as there will still be marine

deposits as a crossover is a closed confined space with flow of sea water, those of which are

perfect environment for marine growth. To further reduce this problem, antifouling is done on

the walls which are exposed to sea water.

Reduced surface to avoid anti-fouling

Anti-fouling can further prevent marine growth on the structure. Considering the conventional

cross flow channel, anti-fouling can be a difficult task to complete with a more expense, as the

structure is complex and confined for work.

The conventional cross flow channel has a very large surface area of around 285m2 with a large

number of profiles placed inside the cross flow channel. Each of these plates, profiles, brackets

and every single structure inside the channel has to be coated with anti-fouling paint, which is

expensive and needs to be coated with 8 layers usually. The painting job can also be hectic, as

it is required to go into the double bottom to do the work. Each layer needs to be given time

for curing and that will also waste so much of time.



In the optimized structure, only a pipe is been used as the crossover and it will be the only

component, which is exposed to seawater. Hence, it would be sufficient to coat the inner surface

of the pipe with anti-fouling paint, which is only about 18 m2. It will also be much easier in

painting as the surface is smooth and curved. However, the rest of the structure must be painted

with anti-corrosive paint, but which is not so expensive and is effective with only two layers

of coating. According to a simple cost estimation, which is explained in the following chapter,

the total painting time can be reduced by 30% while the total expenses related to anti-fouling

painting is reduced by 62%.

Easy inspection and maintenance

This is the most important benefit that is achieved through the optimization of the structure.

The crossover pipe has openings on both ends, which can give access from either side. The

pipe is large enough to allow an average person inside, enabling easy inspection of the

crossover and maintenance work if necessary.

In addition, the crossover only being the pipe, which does not poses sharp corners or edges

makes it easier for the maintenance work to be carried out.

5.1.3 Structural & Production related benefits

Elimination of many structures and profiles

Due to the addition of the floor plate below the pipe and the additional support, a large number

of stiffening profiles and brackets are eliminated in the new structure which were present

initially. This can be an advantage as the fabrication of all of these structures and the

construction of work in the block assembly is eliminated. The material, cutting jobs and very

difficult welding jobs associated to those parts can save a lot of time and money, which is

further investigated with the cost analysis discussed in the following chapters.

Reduced stress concentration

The stress concentration in the edges of the cut-out on the girder have been reduced by a big

margin since the squared-shaped cut-out is changed into a circular one and hence the stress is

more evenly distributed. In addition, the pipe and the floor plate below together act as a girder

and increase the strength at that point.



Space saved in double bottom

The initial cross flow channel was structured between about 4-5 frame spaces and had about

40 m3 in volume. The new structure however is limited between 2 frame spaces reducing the

volume to about 10 m3. We know that space in double bottom is highly valuable, but it depends

on the type of ships. On a merchant ship for example, the benefit is much more bigger because

there will be more space for the goods whereas on a mega yacht it depends extremely on the

situation. Hence depending on the requirement, this can also be an added advantage.

However, since the cross flow channel is now reduced only to a pipe, the volume of water

carried is reduced from 40 m3 to 4m3, which is 90% reduction. This reduces enormously the

issues of corrosion, sludge formation and marine growth and subsequently the antifouling and

maintenance work associated.

Reduced weight of antifouling paint

As we discussed earlier, the area to be painted with anti-fouling paint has been reduced due to

the reduction of the cross flow channel into a crossover pipe. The standard anti-fouling and

anti-corrosion paint used has surface densities of 0.75 kg/m2 and 0.2 kg/m2 respectively.

Considering the conventional cross flow channel, the weight of the paint itself will constitute

about 1.7 tons to the structure while the optimized cross flow channel will have about 220 kg

only. This is one of the biggest benefits as it compensates the added weight due to the addition

of pipe, floor plate and other supports.



5.1.4 Drawbacks or issues not resolved

There are a few drawbacks and some issues which still needs further study and investigation.

Extra cutting and welding

Though there is an advantage of elimination of a large number of parts and the corresponding

jobs, there is again addition of similar work due to the modified shape the structure, especially

the deck plate. Initially it was a flat deck plate but now it has to be knuckled and opened above

the pipe, which requires additional cutting and welding jobs. The crossover pipe can be bought

as it is a standard one but it still requires additional fittings and other preparations.

Air bubbles/cushions will still exist

Although the pipe does not has any profiles to interrupt the flow, air bubbles/cushions will still

exist due to suction pipes installed inside the crossover pipe. Standard reducers can be fixed at

the edge of the suction pipes to reduce turbulence as a remedy, but cannot eliminate the it

completely. However, this needs further investigation in terms of flow which is out of the scope

of this thesis work.

Additional suction pipe connections

Suction pipes should be installed into the cross flow channel to intake water for different

purposes. As we discussed in the earlier chapters, the conventional cross flow channel is

designed and built in such a way that, both forward and aft engine rooms will have a part of

the cross flow channel under the deck. This serves the need of seawater taken from suction

pipes from both of the engine rooms.

The area open to suction pipes is reduced by a large margin with the new crossover pipe, as it

is not a channel under the double bottom anymore. And in the present optimized design, the

crossover pipe is placed under the forward engine room and additional flaps and connections

need to be installed in order to reach it from the aft engine room.

A principle of alternate engine room arrangement is proposed as a remedy. That means, the

engine room bulkhead can be placed on the axis of the crossover pipe and hence suction pipes

from both of the engine rooms can be installed without needing to extra piping through

bulkheads. This again needs further strength investigations and additional piping would still be

feasible economically.



5.1.5 Cost estimation for overall comparison of production

A simple cost analysis is carried out to identify the potential benefit in terms of material and

construction due to the optimization. The comparison is done between the structures eliminated

and the structures added and the major aspects considered in the cost estimation are the material

and labour cost in production.

The cost model includes the material cost, labour cost associated to the working hours including

difficult and easiest welding positions according to the part being constructed, time and

expense for placing parts or components and overheads. In addition, the anti-fouling paint and

the labour associated in painting is studied during the cost analysis, which changes the

comparison dramatically.

We studied earlier that a large number of stiffening profiles were eliminated in the process of

optimization. This saves a big part of the expenses in terms of material as well as labour

involved. However, the addition of the large crossover pipe and the new modified deck

arrangement plays a big role in the increased expenses in the new structure. Moreover, we

reach to a point where the expenses are almost neutral considering only the modification made.

The majority of the expenses saved comes from the anti-fouling paint and the painting work

associated. As we discussed earlier, only the crossover pipe needs to be coated with anti-fouling

paint and the rest only requires anti-corrosion. But the anti-fouling paint is expensive and

usually needs to be coated with 8 layers while the anti-corrosion needs only 2 layers of coating.

This saves a large part of the expense as the area of the cross flow channel is now reduced from

285 m2 to 18 m2 area of the crossover pipe which is about 93% reduction. Hence, only 18 m2

needs to be painted with anti-fouling paint which is the expensive one. In addition, the painting

work would become much easier as the surface needs to be painted is a curved interior of a

pipe and the rest being reduced to 2 layers of coating. Finally, by doing the cost analysis

considering these aspects we get an estimated reduction of about 62% in the expenses related.

Finally, we can identify from the obtained estimation that about 100 working hours have been

saved and we have achieved about 30% reduction in the overall expenses of material and labour

with the optimized structure. This can be a good starting point as we reach to a conclusion that

the optimized structure possess profit in terms of material and production while the other

advantages we discussed in the previous chapters especially the maintenance work, which

continues for years to come cannot be estimated properly until it becomes practical.



5.2 Modular construction

In this chapter, we will review about the benefits achieved through the modular construction

method of the cross flow channel and as well as the issues unresolved or problems raised due

to method followed.

5.2.1 Benefits achieved

Accessibility, Reachability and working conditions

One of the biggest problems during block construction is the accessibility to the working place.

In case of this module, the problem of going through groves or manholes carrying all the

equipment is eliminated. Concerning the reachability issues, it is now easily possible to reach

the point of work due to working with a module though the new cross flow channel has much

more complex structure.

Additionally, the time wastages due to human errors can be reduced since the workers are not

working inside complex structures. Talking about the same example we spoke earlier,

forgetting equipment or essentials wouldn’t waste so much time as they can easily pick it up

again and come back to work.

Welding position

One of the most important advantage is that the welding can be done always downwards.

Usually when the preliminary welding is done, a specially trained welding team comes in to

do the complete welding of all the joints with plates and profiles. This work can be done with

the most convenient position since the module can be rotated at any required angle. The errors

during welding can be reduced and a good quality weld can be expected.

Number of components

The number of components involves in the module are limited now and the construction is

relocated to the pre fabrication. This means that the time & expenses of transporting each &

every part to the block assembly and their storage & maintenance will be eliminated.

Meanwhile in the pre fabrication, there’s no need for storage as the number of parts are limited

and in addition, there will be minimal problem of missing parts or wastage of time in finding

the right one.



Safe working conditions

The working condition is quite safe as there is no issue of ventilation or any other risks since

the workers are not working inside a double bottom in this case and in case of emergency, it is

easy to escape the facility.

Less overall process time

Since the module, which is a part of the block is now being relocated to the pre-fabrication, it

is possible to work in parallel with the block assembly. Which means that the module can be

built in the prefabrication while the block is being assembled. This can save time in the block

assembly, as the module is built in the prefabrication in a much easier way. The idle time of

the workers can also be reduced by distributing the work. This way, the overall process time

can be saved as well as the cost associated to it.

Possible pre-outfitting

To increase the advantage of modular construction, it is also possible to pre-outfit the module

of the cross flow channel. We know that the large crossover pipe should be installed with many

other smaller suction pipes. Hence those pipes can be pre-outfitted, welded and painted earlier

before the block assembly which can again save a large amount of time and money in the

outfitting work to be done later with difficult conditions.

Other benefits

There are few other benefits achieved such as;

Straightening the plates from welding distortion would become easier.

A better utilization of the cranes, which are capable to deal with up to 25t of weight as

well as the availability in prefabrication facility.

As we spoke about anti-fouling earlier, the painting job can be done much more easily

before the mounting of the module.



5.2.2 Drawbacks

Although a number of benefits are achieved through the modular construction, there are certain

disadvantages compared to the conventional construction method.

Extra cutting

Now that a part of the block is being built as a module, several parts and components has to be

cut to separate them which otherwise would have been one single part. The longitudinal girders,

deck and shell plates are such important parts, which need extra cutting due to being part of the

module. These are additional work compared to the conventional method.

As a remedy, the production process could be designed in such a way that, the plates and

profiles of the rest of the block can be standard sized and not cut due to the module. This may

reduce the extra cutting job a little but it cannot be completely eliminated.

Module mounting

After the module is built, it has to be mounted with the block either at the block assembly or at

the ship erection site. This is again a difficult job with concerns of tolerance issues especially

with girders. The module need to be given with extra material on one or both sides and mounted

very carefully to avoid misalignments. To avoid or reduce this problem, the module of the

crossflow channel can be built, placed first and then the construction of the rest of the block

can be started around it. However, the module-mounting job will result in additional expenses

compared to the conventional method.

Module handling and transportation

The module while being built and after needs very careful handling especially with the cranes.

Since the module is large and almost 10 tons heavy, a careless lifting may pose a risk of

permanent deformation in the structure.

Along with careful handling, transportation of the module is also a concern as it might require

additional lifting gear. Transportation of this large module will be an additional expense, which

wasn’t the case in the conventional method.



5.2.3 Cost estimation of modular construction

A simple cost analysis is done to find out the benefits and drawback of modular construction.

It includes the time and expenses saved due to working with the module as well as the

additional cutting and module mounting expenses. The estimation of total labour time in

construction is done with consideration of the easiest welding positions in the module and as

well as the difficulties in the module mounting job.

We can see from the obtained results that there is some time saved in mounting and welding of

profiles and plates in the module rather than doing the same in the block assembly. Precisely,

around 20 hours of work can be saved in welding and 13 hours in mounting due to working in

a module with respect to this cross flow channel structure. However, there is extra labour

necessary for the mounting of the module and it has been estimated with quite a large number

of labour hours compared to the time saved due to the module. This result in extra work of

about 120 hours with a significant expense corresponding to it.

We have discussed about advantages in overall process time but evaluation of total process

time can only be found precisely after the implementation of the method. In addition, unlike

the previous cost analysis, this one does not have a comparison since this modified structure

have not been constructed before.



6. CONCLUTION

In this thesis work, we have seen the modular design and construction of the cross flow channel

and its structural optimization by taking advantage of modular design. Modular construction

has always been the favourite option for the workers in production and during the study of this

thesis, it is very highly recommended by them to build this particular structure as a module.

The biggest advantage of modular design and construction, which we utilised in this thesis, is

that we will get the opportunity to modify and optimize the structure to obtain certain

requirements or benefits, which were initially excluded to avoid complications in construction.

Combined with the structural optimization, the modular design has achieved plenty of benefits

with possibilities for more. Especially, in terms of functional and operational benefits achieved

through structural optimization could lay a foundation to the continuous benefits in years to

come through the life of the vessel.

We learned that the modular design of this cross flow channel structure brings slightly extra

expenses according to the simple cost analysis, which can suggest that it might not be

economically feasible. But keeping in mind that it would be much more complicated the other

way, the potential problems which are already been identified could be studied further for

remedies. However, if we consider both the cost analysis of optimization and modular

construction, we arrive almost at a neutral position. But the important aspect to be noted is that

these cost analysis are only in terms of material and labour in construction and we already

learned that there’s much more benefits been identified regardless of material production.

As the world moves towards high tech ship building processes, modularization can

revolutionize the way ships are built. As we spoke earlier, we could go for more complex

geometries by taking advantage of modular design. A well-planned, zone-oriented modular

design process can shorten the duration time and bring down costs.

Finally we can conclude that the modular design and construction of the cross flow channel is

been studied with its possible structural optimization and we have identified the potential

benefits as well as drawbacks while the purpose of this study is justified through the results

obtained.



ACKNOWLEDGEMENTS

This thesis was developed in the frame of the European Master Course in “Integrated Advanced

Ship Design” named “EMSHIP” for “European Education in Advanced Ship Design”, Ref.:

159652-1-2009-1-BE-ERA MUNDUS-EMMC.

The thesis topic was proposed by the EMship – SAB industrial partner Fr. Lürssen Werft

GmbH & Co. KG and the complete work is carried out at the their facility in Bremen, Germany

during the period of 1st of July to 10th of November.

I gladly acknowledge with grateful thanks the help, information, comments and encouragement

afforded to me by the following personnel in the shipyard:

My supervisor Tim Stockhausen, Atanas Atanasov, Joschua Fleck, Philipp Becker, and the

department head Mr. Heiko Buchholz from structural design department.

Uwe Meggars from basic design department, Anastasia Stobert from nesting department, Jan

Becker from estimation department, Micheal Kropp from machinery department, Rafael

Herdzina from Piping department, Max Pitschke from design standards, Hagen Niekamp from

prefabrication, Pascal Czytrich from block erection and Mate Konya & Sebastian Koop from

IT support.

A special thanks to Nicole Schenk for arranging this internship and making it possible along

with Prof. Robert Bronsart of University of Rostock.



REFERENCES

[1] Ahmad Alliff Anuar Mokhtar, Mohamad Abu Ubaidah Amir Bin Abu Zarim, Kdr Halim

bin Abdul Aziz TLDM (Bersara) and Ainnur Hafizah Anuar Mokhtar, 2016. A Review of

Modular Construction Shipbuilding in Malaysian Shipyard. Proceeding of Ocean, Mechanical

and Aerospace Science and Engineering, Vol.3, 135-143.

[2] Rajko Rubeša, Nikša Fafandjel, Damir Kolić, 2011. Procedure for Estimating the

Effectiveness of Ship Modular Outfitting. Faculty of Engineering, University of Rijeka,

Croatia. Available from: https://hrcak.srce.hr/file/104820 [Accessed 11 June 2018]

[3] DNV GL Rules for classification, Edition 2018: Ships, Part 3-Hull, Chapter 6-Hull local

strength, Section 3-Minimum thickness, page 17-19. Available from:

https://www.dnvgl.com/rules-standards/index.html [Accessed 24 October 2018]

[4] DNV GL Rules for classification, Edition 2016: Yacht, Part 3-Hull, Chapter 4-Metallic

hull girder strength and local scantlings, Section 4-Structural arrangements, page 42. Available

from: https://www.dnvgl.com/rules-standards/index.html [Accessed 24 October 2018]

[5] DNV GL Rules for classification and construction, Edition 2016: I-Ship Technology, 1-

Seagoing ships, Section 12-Tank Structures, page 12-2. Available from:

https://www.dnvgl.com/rules-standards/index.html [Accessed 24 October 2018]

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Sagaya Punithasegaran Arrshan...built-in parts (e.g. double-hull or stainless steel tanks, shaft strut connection, etc.), together with the surrounding shipbuilding structure with