Mechanical Engineering Design 3

2013/14

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Mechanical Engineering Design 3

Alexander Baum – s1116447

Paul Dryburgh – s1123628

Seb Harrison -- s1129683

John Hutcheson -- s1113934

Evan Morgan – s1122600

Ross Thomson – s1103665

4-Apr-14

1 ABSTRACT

This report details the preliminary design of an FPSO for use in marginal oil fields in the North

Sea. The amplitude of the one in a hundred year design wave was calculated using wave height

data and probability. A cylindrical FPSO design was selected and basic mathematical models

were developed to analyse the main design parameters of the vessel. The design parameters for

a whole range of different cylinder dimensions were analysed and compared in order to select

the optimum design. Basic steelwork was also designed and basic stress analysis was carried

out, using the forces excerted on the cylinder by the design wave. Results from this analysis

were used to generate the section properties of the steelwork and drawings of the preliminary

structure were produced. Additionally different tether designs were researched, and the most

suitable configuration was selected. Finally corrosion prevention and manufacturing were

analysed in detail and preliminary procedures and treatments for both were selected.

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2 INTRODUCTION

The task is to design a floating production system for a marginal North Sea oil field. Seven of

the most common oil platform designs are displayed below in Figure 1.

Figure 1: The Seven Most Common Oil Platform Designs

For the purposes of this report the design to be analysed is a floating platform, hence, fixed

structures are not considered in the analysis. The Floating Production System (FPS) is typically

comprised of a monohull structure. They are usually anchored at a site for a relatively short

period of time before relocation if the exploitation field becomes redundant.

Floating Production Systems have progressively been utilised to produce oil in more remote

areas, and in deeper waters than what was previously thought economical. According to

industry forecasts, deep water oil production will constitute a third of the world’s total oil

supply by 2020 [1], hence, the short lead times compared to fixed platforms, and the ease with

which these systems can be reused or decommissioned gives them distinct advantages over the

fixed platform type.

3 DESIGN CONCEPT

The design brief required that an FPSO suitable for use in a marginal oil field in the North Sea

be designed. The FPSO was to be capable of storing 20,000T of crude oil and support a deck

payload of 2,000T.

Figure 2: Sevan 300

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A number of different oil platforms were researched when selecting a concept for the design. It

was found that most floating platforms were based on a cylindrical shape, with oil storage and

ballast inside the cylinder and a deck situated on top, holding the oil production equipment and

living quarters. The most relevant existing FPSO to the given design brief, was the Sevan 300.

The Sevan 300 is an FPSO based on a cylindrical design with a capacity of 36,500T, crude oil.

This FPSO contains a cylindrical oil tank in its centre, surrounded by a ballast tank containing

sea water. The Sevan 300 uses a number of tethers to secure it once it has been positioned

above the oil field. The Sevan 300 can be seen in Figure 2.

A number of design features of marine vessels were researched and their use in the FPSO was

considered.

3.1 Ballast Tanks

Ballast tanks are required on ships and other floating vessels to maintain stability when there

are large changes in the vessel’s overall mass. Water is commonly used as ballast as it can be

pumped into and out of the vessel, allowing the vessel’s mass to be altered and stability

maintained. Ballast tanks are very important in the design of this FPSO as the oil storage is

20,000 tonnes, meaning that the vessel’s weight can vary by this amount. Incorporating ballast

tanks into the FPSOs design means that the structure could remain stable no matter how much

is stored. A control system would have to be designed to ensure that the mass of oil and ballast

were such that the structure was stable.

3.2 Cofferdam

A cofferdam is a small space between two watertight bulkheads. They are required at the

boundaries of storage tanks to ensure that oil or other hazardous chemicals do not leak out into

machinery space or the sea. The minimum width of a cofferdam is 760 mm. It is important to

consider cofferdams when designing an FPSO to meet environmental protection standards.

3.3 Watertight Flats

Watertight flats are horizontal watertight partitions within the structure and serve a number of

purposes. Firstly, they strengthen the structure and provide structural rigidity. They also act as

watertight barriers that can prevent the whole vessel from flooding should the hull be breached.

Another important use for watertight flats is their ability to contain fire, a feature that is

especially important on a vessel carrying large amounts of oil. Finally, watertight flats can be

used to divide the hull into different areas such as machinery stores.

3.4 Double Hull

The purpose of a double hull is to increase the safety of the vessel by allowing it to stay afloat

should the outer hull get damaged. Since the Exxon Valdez oil spill of 1989 it has become

legislation that all oil tankers must be fitted with a double hull to prevent oil spills and

environmental damage should the hull become penetrated.

The FPSO being designed was based on the Sevan 300’s general layout, and was chosen to be

a hollow cylinder, containing oil and ballast tanks, with tethers holding it down to the seabed.

For simplicity, it was decided that the oil tank should be circular, and located in the centre of

the cylinder. The ballast tanks would go around the outside of the oil tank. The tanks would be

situated in the bottom of the cylinder to keep the centre of gravity low and maximise its

stability. The payload would be situated above deck. Figure 4 shows the basic layout of the

FPSO. Static and dynamic analysis was carried out to determine the optimum cylinder

dimensions for the given problem.

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Figure 3: Concept sketch of FPSO Figure 4: Cross-section of FPSO structure

The structure of the FPSO should be made out of welded steel plate, supported by a framework

of horizontal girders and vertical stiffeners. The framework should be made up of an outer

frame, supporting the plating for the hull, and an internal frame, making up part of the structure

for the oil and ballast tanks. Watertight flats were also included at various positions in the

structure, to reduce free surface effect. These were included to provide structural stiffness and

space to store equipment. Regulations state that there must be cofferdams at the boundaries of

storage compartments, therefore these cavities have been included in the design at the ballast

and oil tank boundaries. A mixture of concrete and water was used as ballast in order to

overcome the large buoyancy force present due to the large submerged volume. The concrete

was situated in the bottom of the ballast tanks, underneath the water ballast to keep the centre

of gravity as low as possible. Figure 3 shows the cross sectional layout of the FPSO’s internal

structure.

3.5 Human Considerations

For the FPSO to operate in a safe and effective manner it must be designed to support the

people who operate it day-to-day. It is therefore important that significant consideration and

influence is given to human factors within the design process. International standard ISO

13407 describes how to integrate human characteristics into design to optimise system

performance. Important factors identified as appropriate for the design of the FPSO are

maintenance access, ease of maintenance and working environments. The produced design

should be laid-out in an ergonomic fashion which is considerate of operators’ everyday duties.

Working environments should be designed so that there is no undue risk to human health e.g.

climate extremes, motion sickness, poor or blinding light, excessive noise, toxic materials,

electric shock and musculoskeletal injury.

4 DESIGN WAVE

To calculate the largest wave the structure is likely to experience in its design life (chosen here

as 100 years) a set of wave data must be analysed for the extreme values. The data comes in

the form a scatter chart, as shown in Appendix 2.

The range of wave periods shown in the chart below is 4-13 seconds. This information can be

used to define the range of resonant frequencies that should be avoided when designing the

structure.

. The period of the design wave was calculated

as .

Full details of these calculations are shown in Appendix 2

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Figure 5: Negative Log Plot Showing lines of best fit

The two most accurate fit lines are shown dashed. These yield a value of . The

following properties can also be found:

5 THEORY

A mathematical model of the structure floating in the sea was made to analyse each of the

design parameters for a specified set of cylinder dimensions. The model that was used to

analyse the problem was based on a simplification of the concept FPSO structure. The

simplification involved modelling the girders, stiffeners and plates as a steel tube of a uniform

wall thickness with two steel caps on the top and bottom, of the same thickness as the tube.

The ballast was modelled as solid masses contained within the cylinder. The layout of the

FPSO using in the mathematical model can be seen in Figure 6.

5.1 Design Methodology

To evaluate each of the design goals, various calculations were carried out. These calculations

largely revolved around the free body diagram of the can as seen in Figure 7. The calculations

undertaken to evaluate each design parameter are described in the following section.

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Figure 6: Mass element layout of FPSO Figure 7: FBD

5.2 Cable Tension

It was important to calculate the maximum tension in the tethers to ensure that they were

strong enough to resist the vessel’s buoyancy force. The tension in the cable was calculated

using an equation derived from the free body diagram of the floating structure, seen in Figure

7. The equation for the tension in the tether can be seen as Equation 1.

(1)

A combination of the vessel’s submerged depth and the weight of ballast dictated the tension in

the tether. The ballast used was made up of two components, water that could be added or

removed as required, allowing the tension to be adjusted, and concrete to provide permanent

ballast. The amount of permanent ballast required was calculated by specifying the minimum

amount of tension that the tethers could withstand, then calculating what weight of ballast

would be required to achieve this tension. The equation used to calculate this can be seen in

Equation 2.

(2)

The maximum volume of water ballast available was calculated using Equation 3, where is

the volume of concrete and is the internal diameter of the tank and is a scaling factor used

to set the tank height in relation to the cylinder height.

(

) (3)

The internal tank diameter, , was calculated by considering the volume that the mass of oil

would occupy. It was known that the tank was circular, and the height was specified by the

scaling factor therefore the diameter of the tank could be established based on this height and

the oil’s volume. Equation 4 shows this calculation.

√

(4)

With the weight of permanent ballast and the maximum ballast tank capacity known, the

maximum tether tension could be calculated. The worst case scenario tension that may occur is

when there is no oil being stored in the FPSO. In this case the ballast tanks would be full, as

this is the lightest the vessel could be. Equation 5 was used to calculate the maximum tension

in the cable.

(5)

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5.3 Mass of Steelwork

The mass of the steelwork was simply calculated by working out the volume of the tube and

caps and multiplying it by the density of steel, as seen in Equation 6.

(6)

5.4 Metacentric Height

To determine if the vessel was stable or not, its metacentric height, GM was calculated. A

positive metacentric height indicates that the structure is stable as the centre of buoyancy is

higher than the centre of gravity so there is a self-righting moment should the structure be

tipped slightly by a wave the buoyancy acts to right the structure.

(7)

was calculated by dividing the moment of inertia of the structure about its waterline by the

submerged volume, as seen in Equation 8.

(8)

was calculated using Equation 9. The centre of buoyancy was calculated by simply

dividing the submerged height by two, as seen in Equation 10.

(9)

(10)

5.5 Natural Frequencies in Heave and Pitch

It was important to calculate the natural frequencies of the cylinder in pitch and heave. This is

to ensure that they did not coincide with the range of ocean wave frequencies to prevent the

structure from resonating, which could be uncomfortable for occupants and could expose the

FPSO to abnormally large oscillations, causing the structure to fail.

The natural frequency in heave was calculated using Equation 11:

√

(11)

The natural frequency in heave was calculated using Equation 12:

√

(12)

The parallel axis theorem was used to calculate the contribution to the total moment of inertia

of each individual mass element in the vessel about the centroid. These individual

contributions were summed to obtain the overall moment of inertia.

5.6 MATLAB and Selection of Optimal Design Parameters

MATLAB was used to calculate and display the values for the design parameters for a range of

possible cylinder dimensions. The MATLAB code worked as follows:

A function was created that would take the basic cylinder dimensions as inputs and then

use the theory previously described to calculate values for each of the design

parameters; cable tension, metacentric height, mass, and the natural frequencies in

heave and pitch.

This function was nested inside a loop which varied the cylinder height and diameter

inputs to the function as it ran.

The output variables were saved to individual matrix, each matrix value corresponding

to a different H and D value.

These matrices were used to generate a surface plot for each of the design parameters.

The surface plots were used to compare the characteristics of each different

combination of cylinder dimensions, allowing the optimal set of dimensions to be

identified and selected.

The MATLAB code used to generate these surface plots can be seen in the appendix.

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5.7 Improving Model Accuracy

An iterative process was used to select the optimal dimensions. After the first iteration, the

optimal dimensions obtained from the MATLAB script were used to generate a CAD model.

This model was used to calculate the mass of the FPSO. The thickness of the steel in the

MATLAB model was changed until the mass was the same as the CAD model. With the new

mass in MATLAB, the script was re-run, and a new set of surface plots were obtained. These

plots were analysed again to select the new optimum cylinder dimensions. The mass of the new

optimal cylinder dimensions was compared to the Solid Edge structural model. The uniform

thickness used in the MATLAB script was adjusted until the total mass equalled that of the

CAD model. By using this process, the accuracy of the MATLAB script was improved,

yielding better, more accurate results. This process was repeated several times until the results

converged to a correct thickness and a specific cylinder height and diameter.

5.8 Analysis

Once the surface plots had been generated, each was analysed to identify the optimal cylinder

dimensions

5.8.1 Tension

The surface plot produced for the maximum tension in the tether showed some interesting

results and can be seen in Figure 8. It can be seen that the tension was very large for small

diameters, however as the diameter was increased, the tension dropped, eventually reaching

negative values, indicating that the structure would sink. This is because, whilst the buoyancy

force would be larger for larger cylinder diameters, the capacity of the ballast tanks would also

be a lot greater, allowing them to cause the structure to sink when full. Dimensions were

therefore selected, so that the tension of the cable would be as low as safely possible, without

causing the structure to sink. The plot showed that increasing the diameter had the greatest

impact on the tether.

Figure 8: Surface plot of cable tension Figure 9: Surface plot of structure mass

5.8.2 Mass

Figure 9 shows the surface plot showing the FPSOs mass. It can be seen that increasing the

height of the cylinder has a much smaller impact on the mass than increasing the diameter

does. Therefore, when choosing dimensions, in order to minimise the FPSO’s mass, and

ultimately its cost, the diameter should be increased as little as possible.

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5.8.3 Metacentric Height

Figure 10: Surface plot of metacentric height Figure 11: Colour gradient of metacentric

height

Figure above shows the surface plot displaying the variation in the metacentric height for each

combination of cylinder geometry. It can be seen that the wider, and shorter the cylinder was,

the larger the metacentric height was, and the more stable the structure would be. This was

taken into account when selecting the optimal cylinder dimensions. The dark blue flat spot on

the graph indicates cylinder geometries that are either zero, or negative, indicating an unstable

structure.

Figure 12: Natural frequency in pitch Figure 13: Natural frequency in heave

5.8.4 Natural Frequency in Heave

Equation 13 was used to calculate the range of submerged heights that were required to avoid

resonance in heave. It was found that the range of possible wave frequencies were

. This meant that the submerged height either had to be below or

above . It was not feasible to set the submerged height at 3.97 m so the submerged

height was fixed at .

5.8.5 Natural Frequency in Pitch

Figure 12 shows the surface plot for the natural frequency in pitch of the vessel. The blue /

turquoise surface represents the natural frequency of the vessel and the red flat surface

represents the maximum allowable limit for the frequency, based on the minimum wave

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frequency. It can be seen that the natural frequency for all geometry configurations fall under

the allowable limit. Some configurations however, gave complex natural frequencies, which

are shown on the graph as the flat blue surface. Inspection of Equation 12 concluded that the

only way the frequency could be complex, was if the metacentric height was negative.

Comparing Figures 10 and 12 it can be seen that the flat spots occur over the same range of

dimensions, indicating that the two variables are related.

5.8.6 Selection of Optimal Dimensions

Comparison of the surface plots showed that a height of and diameter of were

optimal.

A minimum height of was calculated by adding the submerged height to the amplitude of

the design wave. The submerged height was set as in the MATLAB model in order to set

the natural frequency in heave at a value that would not cause the vessel to resonate on the

ocean waves. The amplitude of the design wave was , therefore to avoid the design wave

from breaching over the top of the deck, the cylinder had to be at least high. It was

decided that there should also be a splash zone of included, making the minimum cylinder

height .

When choosing the diameter, a compromise had to be made between mass, tension, and the

vessel’s stability. As previously explained, a larger diameter increased the tension and mass of

the vessel, which is undesirable, however increasing the diameter, also increased the stability

which was desirable. It was decided that the diameter should therefore be as small as possible,

whilst still ensuring the vessel was stable. The metacentric height was found to be at its

minimum when the maximum design wave passed the FPSO, and caused most of the cylinder

to be submerged. Figure 11 shows the plot of GM for this situation. The point marked on the

plot indicates the selected dimensions for the cylinder. It can be seen from the graph that

increasing the height reduced the stability for a given diameter, therefore the height was chosen

to be the minimum possible value of 60 m. The diameter was selected as 54 m as it gave a

sufficient amount of stability, however was not excessively large, keeping the weight and cable

tension to a minimum. The design parameters for the optimal dimensions have been

summarised in Table 1.

Table 1: Design Parameters of Optimal Cylinder Dimensions

Property Value

Tension

Mass

GM

NF Heave

NF Pitch

5.8.7 Design of Steelwork

Equations for the minimum plate thickness and stiffener and girder section modulus were used

to calculate the basic steelwork requirements for the FPSO. [2] The results of these calculations

have been summarised in Table 2.

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Table 2: Section Properties of Structural Steelwork

Property Hull Tank

Plate Thickness (mm) 250 110

Stiffener Plastic

Section Modulus (mm³)

140909770 4839450

Stiffner Elastic

Section Modulus (mm³)

117120638.75 4200946.05

Girder Plastic Section

Modulus (mm³)

74914700 3328068.41

Girder Elastic Section

Modulus (mm³)

66161937.76 39369720

To check that the dimensions of the steelwork were realistic, basic stress analysis was

conducted for forced acting in surge and heave. It was found from the course notes [3] that the

surge and heave forces acting on a submerged cylindrical structure could be calculated from

Equations 13 and 14.

(13)

( ) [

] ( ) (14)

Equation 14 was simplified to give the maximum heave force, i.e. when the sine term was

equal to 1. The simplified equation can be viewed in Equation 15.

[

] ( ) (15)

The area over which each respective force was acting was obtained from the cross section of

the CAD model that each respective force was acting over.

A MATLAB script was used to calculate the surge and heave forces for the selected cylinder

geometry, this script can be seen in the appendix. Equation 15 was then used to calculate the

stress in the respective cross section of the structure. The force values, cross sectional areas

obtained from CAD and stress values can be seen summarised in Table 3.

Table 3: Results from stress calculations

Loading Condition Area (mm2) Force (N) Stress (MPa)

Surge 8.0593e+07 4.3073e+07 0.5345

Heave 5.6144e+07 9.3064e+07 0.0165

The results show stresses far smaller than the yield stress of the steel used to make the

structure. These results are believable as this analysis only considers the axial stress in the

structure. In reality bending would also occur, which would produce larger stresses, however

analysis of bending for this problem would be very complicated and would require complex

finite element models to do accurately. In addition, the equations used to produce the

dimensions of the steelwork include a safety factor, meaning the cross sectional area is larger

than is calculated necessary, and ultimately the stresses present under normal working

conditions will be low.

6 TETHER

6.1 Mooring Analysis

Station keeping refers to maintaining a floating structure in a specific location within the limits

set by safety, function and environment. This is achieved through a series of tethers and

anchors that can be divided into two main configurations: the catenary system and the taught

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leg system. A taught leg system, based on tension leg platforms, is designed for the FPSO in

question.

Analysing the mooring system of a Floating Production System involves the determination of

environmental loads, safety precautions, system lifespan and all other induced forces on the

tendons. Analysis can be approached in several different ways depending on the level of

sophistication required. For this system a time domain, quasi-static approach is used complying

with offshore standards (DNV-OS-E301 and API RP 2SK [4] [5]).

Pre-tension caused by the still water buoyancy of the main structure.

Effects on tension caused by set down and offset of main structure by 2.5% of water

depth.

Maximum and Minimum tensions possible according to the design wave (ensuring

tension is always positive).

Tendon weight for ease of installation and extra tension forces.

Limiting combinations of events.

Temporary loss in tendon tension is acceptable provided the minimum tension in at least one

tendon per grouping remains non-negative and a comprehensive analysis on the effect of the

removal of tension from all tendons is carried out.

The essential components should be designed so that they can be easily inspected for

maintenance purposes. Any mechanical components should be designed to be monitored in real

time, to detect signs of failure early. Live tension forces should also be monitored in case the

design loads are exceeded. All materials liable to corrode must be protected accordingly by

including sufficient extra thickness to meet the design life set.

6.2 Mooring Components

Mooring equipment for Floating Production Systems includes winches, chain, rope, fairleads,

shackles, and anchors. Anchors are an important component and require further attention so are

addressed later. Detailed analysis and calculations for each component should be carried out

separately.

The winch connects the chain to the structure and is responsible for tensioning in the tendon

and must be capable of adjusting to varying tensions. For mooring lines with a chain

component, a chain jack must be used as the winch. These jacks are used to safely tension

mooring lines in spread moored FPSOs. Chain jacks have high traction due to the shape of the

chains. This allows for extra tensioning capacity when installing mooring tendons. It also

allows for a pull in speed of around 2m/minute, significantly higher than that of a windlass.

There are two types of chain jacks, linear chain jacks and rotary chain jacks. Linear chain jacks

are a relatively new concept which produce less bending in the mooring line along with having

less moving parts (reducing the risk of failures). They also have a smaller footprint so extra

space on the structure can be utilised. Rotary chain jacks do however still have a higher

tensioning and load capacity as they are driven by a motor that is not limited by space.

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Figure 14: Studless to stud

link chain. [6]

Figure 15: Usage of synthetic fibre mooring lines. [7]

The most common component in mooring lines is the chain. The chain diameter and material

must be designed appropriately for the loads they are required to take. Two frequently used

chains are the studlink and the studless chain. The studless is often used for permanently

moored systems, whereas, the more commonly used studlink chain is used for moorings that

have to be reset in their lifetimes. The studlink chain offers a higher minimum breaking load at

the expense of weight. Where weight is an issue, the studless chain is a better option. The

studless and studlink chains are displayed in.

For deep water, a mooring chain is not a viable solution due to the enormous weight of chains

in excess of 100m long. Large sections of mooring tendons are often replaced with a wire rope

or synthetic fibre rope. Wire rope has a similar breaking load to chains without the excessive

weight. In taught leg systems, the higher elasticity of rope is also beneficial. Common wire

ropes consist of six or more intertwined steel strands (often in a spiral pattern) that are

terminated with a socket. Wire ropes generally tend to be more prone to corrosion and damage

than chains so must be protected. Synthetic fibre ropes are an emerging trend in deep water

mooring, seen in Figure 15. These ropes are often made of polyester of high modulus

polyethylene. The major benefits of synthetic fibre ropes are their lightweight and high

elasticity.

The shackle is a common connecter consisting of a bow closed by a pin. Shackles are used to

connect separate components of the mooring system (for example the mooring chain to the

anchor) and many different types are available suitable for the required application. Chain

shackles are used as a connecter for chain-chain sections or chain-rope sections. The anchor

shackle is a necessity in connecting the mooring line to the anchor.

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Figure 16: Chain and Anchor shackle [6]

6.3 Mooring Design

The tension is the dominant force in the tendons and is dictated by the buoyancy and weight of

the floating structure as stated.

(16)

Where, is the displaced water and is the weight of the structure. For the ultimate

limit state design this force must be calculated when the design wave is at maximum amplitude

and, in turn, the buoyancy force at its greatest. The maximum value of tension can then be

divided by the number of tendons to be installed on the structure.

The maximum force in each tendon must be able to withstand an offset margin of 2.5% of the

water depth when one tendon has completely failed.

(17)

Safety factors must also be included in the calculations but due to the relatively conservative

100 year return period, quasi-static accidental limit state safety factors may be used.

Table 4: Safety factor

Class Safety factor on mean tension

1 1.10

2 1.35

Consequence class 1 refers to mooring systems where failure is unlikely to lead to

unacceptable consequences such as loss of life, uncontrolled outflow of oil, capsize, or sinking.

Class 2 are systems in which the former consequences are likely due to system failure.

The elongation of the mooring rope section must be accounted for in design.

[ ( )]

[

( ) ]

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Table 5: Permissible Load per Tendon

Maximum Tension

Tension in tonne-force

Ultimate Limit Tension

Accidental Limit Tension

6.4 Component Selection

Tethering a cylindrical FPSO to the ocean floor takes similar techniques to the ones used when

mooring a spar platform. Combining that with the arrangement used by the majority of tension

leg platforms produces an array of 3 groups of 4 tendons each position apart, as shown in

Figure 17.

Figure 17: Mooring Pattern

Using the permissible loads dictated by the maximum tension of in each tendon

(1442.4 tonne-force) the mooring components are chosen accordingly from Vryhof Anchors

Anchor Manual (see Appendix 6).

Table 6: Suitable Chain Shackle

Table 7: Suitable Anchor Shackle

SWL 1500tonnes

Weight 2500kg

A 260mm

B 325mm

C 460mm

D 840mm

E 650mm

SWL 1500tonnes

Weight 2800kg

A 260mm

B 325mm

C 460mm

D 870mm

E 650mm

O 600mm

Table 8: Suitable Studlink Chain Table 9: Suitable HMPE Rope

Proof Load 19845kN

Break Load 25174kN Minimum

Breaking Load

21893kN

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Weight 671kg/m

D 175mm

Stiffness (EA) 1.22x106kN

Weight 15.3kg/m

Diameters 177mm

Table 10: Linear Chain Jack

Load Capacity 30000 tonnes

Weight Approx. 12 tonnes

Chain Stopper

Fixed Latch

(cannot be released

under load)

6.5 Suitable Anchors

A pile anchor is a hollow steel pipe that is driven into the seabed with the use of a piling

hammer. These piles are capable of withstanding both vertical and horizontal loads so are often

also used in catenary line systems. The holding capacity of the anchor is dictated by the friction

of the soil along the walls of the pile and the lateral soil resistance. These piles must be driven

to great depths in order to generate the required holding capacity.

Figure 18: Suction Caisson

The suction caisson is similar to the driven pile in that it is a hollow pipe but with a diameter

that is much larger. The holding capacity is generated by the friction and soil resistance making

the anchor capable of withstanding horizontal and vertical loads. A pump connected to the top

of the anchor is used to create a pressure difference and force the anchor into the ground. When

pressure inside the pipe is lower than outside, the pipe is sucked into the ground. Suction

anchors have become far more popular due to the rapid installation times and the lack of noise

produced when installing, an example can be viewed in Figure 18.

The vertical load anchor is a new development in anchoring technology. It is a modification of

the drag embedment anchor that is capable of withstanding vertical and horizontal loads. The

anchor is dropped into the water then reeled in once it has penetrated the seabed, which forces

it into the ground. Although the vertical load anchor can withstand vertical forces it does not

penetrate the seabed as far as the pile design anchors making it a more popular anchor with

smaller floating installations. VLAs are also popular in locations which require minimum

damage to the seabed.

6.6 Anchor Design

According to to DNV-OS-E301, suction piles require a safety factor of 1.5 to 2. This produces

a new force in which the piles must be able to endure.

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(18)

For the limit state design of the pile, the resistance produced by friction should be equal to or

greater than the tension force.

(19)

The resistance of the seabed can be calculated with the following formula.

Values for static undrained shear stress and cyclic direct simple shear stress should be obtained

through experiment but averages for a clay seabed are presented by TUDelft in the offshore

engineering handbook. Taking lowest values can allow the design of a worst case scenario

anchor.

Set up factor depends on the plasticity index and the soil sensitivity of the clay.

Typical values are displayed in Table 11 for simple DSS between and

.

Table 11: Setup Factor:

<25% 25-50% >50%

0.58 0.65 0.65

0.58 0.65 1.95/ , <1.0

The surface area of the pile that is in contact with the soil can be calculated for the worst case

, .

(20)

This area can be used to design the dimensions of the pile. Elasticity and buckling ranges of the

caisson are dictated by:

(21)

(22)

Existing suction anchors in The Tubular Bells field have a diameter 4.87m and length 29.57m

weighing 190 tonnes. These dimensions give a starting point for design of a suitable suction

pile. Using a diameter of 6 m and skirt thickness 50mm.

(23)

(24)

(25)

These dimensions give a suction anchor that produces a holding capacity of when

installed to a depth of by underpressure. The underpressure force required for

installation can be calculated from the total resistance of the soil and the submerged

weight of the anchor . For worst case scenarios is equal to

.

(

)

18

Where is the plan view inside area of the pile where underpressure is applied.

6.7 Finalised Tether

{1} Bardex’s Linear Chain Jack is used as the winch positioned on the side of the hull.

The maximum tension caused by the design wave is only 4.8% of the maximum

capacity of the hydraulic chain jack. [8]

{2} The studlink chain supplied by Vryhof Anchors has a 29% force margin at

maximum tension before it begins to elastically deform.

19

{3} Bardex’s BarLatch Fairlead and stopper is used to prevent wear on the side of the

main hull as well as keeping the tendons in line with the chain jack.

{4} The safe working load of both Vryhow Anchors shackles is 565kN higher than the

maximum tension.

{5} High Modulus Polyethylene is chosen as the synthetic fibre for the rope section to

reduce weight in the tendons (specific gravity of 0.975). HMPE elongates by only

11.6mm at maximum tension.

{6} Using a suction pile with 6m diameter and 20m length provides an easily installed

and capable anchor. The worst case scenario resistance due to the seabed is twice the

value of the maximum tension.

7 CORROSION CONTROL

7.1 Introduction

The harsh marine environment in which the FPSO system is to operate leads to the requirement

for significant consideration of the effects of corrosion. Exposure to seawater and salt air,

combined with the erosive cyclical impacts of wave action, can lead to serious corrosive

damage to the steel structure, severely limiting its effective life span and reliability.

Numerous maritime accidents in the 1990s saw a series of legislative changes occur which

greatly enforced the corrosion inspection criteria of all marine vessels. These changes

coincided with the widespread adoption of double-hulled structures, which vastly increased the

surface areas which required corrosion protection, maintenance and inspection by an estimated

300%. [5]

The costs associated with this monitoring and maintenance, combined with the remote

geographic operating location of offshore structures, means that any design rework or

production downtime caused by problems creates significant cost. It is estimated that the

application and maintenance of a robust protection system is between 4 to 14 times cheaper

than replacement of corroded steel. [9] It is, therefore, of critical importance that significant

thought and planning is given towards corrosion control within the design process.

7.2 Corrosion Protection Techniques

In simplest terms corrosion protection techniques aim to inhibit the corrosion reactions from

taking place. Protection systems used in the offshore industry can generally be classified under

three categories:

• Cathodic Protection

• Coatings

• Wastage Allowance

7.2.1 Corrosion Protection Theory

Corrosion is caused by a driving force of a refined metallic element desiring to return to a more

stable compound. It is an electrochemical process consisting of two voltaic half-cell reactions;

an oxidation reaction at the anode and a reduction reaction at cathode. The full corrosion

reactions for steel are introduced further in Appendix 7.

7.2.2 Cathodic Protection

Cathodic protection (CP) works by turning the protected metal surface into the cathode of an

electrochemical cell rather than the anode of a corrosion process. An opposing current is

supplied to the metal surface to provide a flow of electrons. This polarises the sites where the

anodic oxidation reaction is likely to occur to the same potential as the cathodes and thus

passivates the metal to corrosion.

There are two methods of supplying the polarising current in a CP system; sacrificial anodes or

impressed current.

20

The first system utilises a sacrificial metal alloy with more negative electrochemical voltage

potential than the protected metal to create a galvanic cell. When an electrical connection is

made the potential difference causes a current to flow and thus supplies electrons from the

anode to the protected surface. Galvanic corrosion therefore occurs to consume the sacrificial

metal instead of the protected metal.

Figure 19: Sacrificial Cathodic Protection

In an impressed current (ICCP) system, the current is provided by an external DC power

supply. This often takes the form of an AC-rectifier. An electrical circuit is formed by

connecting the protected metal to the negative terminal and an auxiliary metal anode to the

positive. The auxiliary anodes supply current to the protected surface in the same galvanic

fashion as with a sacrificial system; however they do not provide the electrons and are

therefore non-consumed. ICCP systems are most common for applications where a high

polarising current is required or where galvanic systems are simply too large and heavy to

implement. Table 12 provides a direct comparison of the advantages and disadvantages of both

CP systems [10].

Table 12: Advantages and Disadvantages of Sacrificial Protection and ICCP

7.2.3 Coating Systems

Coating systems simply create a barrier between the protected metal and the environment thus

inhibiting the corrosion reactions. Seventy years of offshore oil-and-gas production has led to

the development of many highly advanced coating technologies which are formulated for

specific properties and functions.

Research has identified a common four-coat system for usage on this FPSO. Firstly there is an

undercoat of a primer which contains small pigments of galvanic metal. Should an area of steel

become bare due to damage to the coating the pigments become anodic, providing a small

amount of galvanic cathodic protection. The protection does not provide significant current but

Galvanic Anodes Impressed Current

No external power system required External DC power required

Driving voltage fixed by anode material Voltage easily adjustable

Polarising current fixed by anode mass

(typically limited to 10-50 mA)

Adjustable unlimited polarising current (

unlimited but typically 10-100A)

Usually used in lower resistivity

electrolytes

Can be used in almost any resistivity environment

Usually used on small or very well coated

structures

Can be used on any size structure

Anodes add significant weight Low weight system

Low cost per unit Higher cost per unit

Low associated maintenance Higher maintenance

Higher cost per area of metal protected Lower cost per area of metal protected

21

does effectively stop rust creep and undercutting of the surrounding coating, confining

corrosion to the small area until the coating can be repaired.

The two most common types of anticorrosive pigmented primers are zinc silicate or zinc rich

epoxy. Zinc silicate primers have more effective anticorrosive properties however they require

a very high level of shot-blasted surface cleanliness for application. It is assumed that this level

of cleanliness will only be available at fabrication stage. Coatings which require less clean

surfaces (power tool cleaning) are usually selected for repairs and touch-up of coatings once

FPSO is in operation. [11]

The undercoat primers have low abrasion resistance thus requiring a protective coating. This is

usually achieved with a thick layer of high-build epoxy. A minimum film thickness is usually

specified depending on the area of application and typically, at least two coats are required.

These provide a high level of abrasion and chemical resistance however they are highly

sensitive to ultraviolet radiation from sunlight and quickly fade without protection from a

topcoat. An aliphatic-polyurethane based topcoat provides this as well as adding a smooth,

easily cleaned, chemically resistance surface finish. [8]

Finally, areas such as the platform decks, stairways, escape routes, helipads and tanks require

human access either for everyday FPSO operation or for means of regular inspection. It is

therefore important to provide a non-skid surface for safe means of travel. Coatings are

available which are pigmented with abrasive material to provide a high coefficient of friction

however a tread tile system such as the EPOK (shown in Figure 24) provides the best

simplicity of installation and maintenance.

7.2.4 Wastage Allowance

Wastage allowance involves addition of extra material to parts in order to over-compensate for

any corrosive effects. It often involves adding thickness to walls and hulls to account for an

estimated rate of corrosion.

Typical corrosion rates for steel in salt water can vary from 0.1-0.3mm per year depending on

numerous factors such as temperature, salinity, humidity, impurities etc. Guidelines exist to aid

in the selection of corrosion allowance however, as suitable data is not provided it is assumed

unsafe to rely solely upon this protection technique.

7.3 Design of Corrosion Protection System

7.3.1 System Selection

For efficiency, four separate corrosion zones are identified on the FPSO structure. Each zone

experiences different environmental and operational conditions thus it is necessary to examine

and identify the techniques most suited for each. The zones are:

• Submerged – Areas permanently below seawater.

• Splash – Areas intermittently wetted by seawater through action of waves. Extends 5m above

the height of the water line.

• Atmospheric – Areas unaffected by seawater but subject to salt air.

Figure 20: EPOK Tread Tile System

22

• Internal Ballast – Internal spaces permanently or intermittently in contact with seawater at

any point (ballast tanks).

• Underground Zone – Areas permanently situated below ground level (i.e. anchors and piling).

The optimal system is selected for each zone based on performance, lifetime, safety, ease of

inspection and maintenance. Initial purchasing costs are deemed insignificant compared with

cost associated with future problems.

Each zone is to be coated in a full four-coat system of zinc silicate primer, two coats of high-

build epoxy and aliphatic-polyurethane topcoat. Zinc rich epoxy is too used for repairs when

necessary.

Submerged, splash and internal zones are to also have cathodic protection systems as repairs to

damaged coatings are challenging in these areas. The size of FPSO is not particularly large in

comparison to existing designs and therefore the assumption is made that impressed current

system is not required. Due to its simplicity and mechanical robustness a sacrificial anode

system was deemed to be best suited.

The corrosion protection systems chosen, combined with the safety factor included in the hull

thickness calculations (Section 5), are assumed sufficient. Hence, specifying an overall

structural corrosion allowance is not deemed necessary.

The underground piling remains in place for the whole design life of the FPSO and therefore

requires a specialised system. The optimal solution for steel in soil is to account for loss of

thickness through allowance. Hot zinc-coating can also greatly increase the protection life.

Corrosion penetration of zinc is assumed to be 10µm for North Sea soil types and therefore a

minimum zinc thickness of 1mm is deemed sufficient. A protective epoxy coating layer is

applied to further protect the zinc coating.

7.3.2 Design of Galvanic System

Design of the galvanic cathodic protection system is based upon recommended practice DNV-

RP-B401, all corresponding tables are displayed in Appendix 7.2. [12]

Anodes are to be distributed so that a uniform current is supplied throughout the structure.

Each anode should also be sufficiently spaced from other anodes to avoid current interference

patterns occurring. For simplification of fabrication the anodes will be arranged in a 10m

rectangular array (as displayed on Drawing Sheet 4). This means that each anode must be

designed to protect an area ( ) of 50m2.

The approximate total surface area of metal to be protected for each zone ( ) is calculated.

This is divided by to calculate , the total number of anodes required to protect the surface

area.

The geometry and material of the anodes is selected so that they supply the required current

demand to maintain cathodic protection ( ). To select the final anode geometry a MATLAB

script was produced to carry out the design calculations. Full derivation of design equations are

displayed in Appendix 5.3. Data from recommended practice tables was input along with

parameters of existing commercially available aluminium anodes. Anode size was increased

until the design conditions were met.

The final anode selected had geometry based on the 280HAL anode from the company BAC

Corrosion Control Ltd. Drawing Sheet 4 displays a full technical drawing of the designed

anode. The anode mass is estimated at 27.2kg plus steel insert of mass 2.15kg. For

simplification of manufacturing the same size of anode was chosen for each zone. Table 13

displays the required number of anodes for each zone.

Table 13: Required number of anodes

Zone Protection Area ( )

Number of Anodes

( )

23

8 FABRICATION

Based on the period of the design wave there will be 4.21x108 cycles of loading in 100 years of

operation.

Figure 21: S-N Curve for Steel

Choosing a material with a yield stress in the range of the high would, give a fatigue

yield stress of , for loading cycles based upon Figure 21. This is still an

acceptable level as the maximum stress on the system is . The stress cycles are so low

and that is based upon the worst case scenario wave, it is unlikely the steel would have its

strength reduced to such an extent.

8.1 Fabrication Process

The design is split into 72 subsections for the outer wall and 48 for the inner wall. This allows

for each of them to be lifted by a standard overhead crane making the construction and lay

down of the sub-assemblies much easier. Generally, each subsection will be 2m high and 10m

wide. This gives general sub-assembly weight of 83.33 tonnes, which is lift able by standard

cranes, making manoeuvring the subsections easier.

The general plan for fabrication of an offshore structure as adapted from [13]

Preparation of support and staging

Rough setting of assembly main structure & position tacking

Dimension Control

Pre-weld Inspection

Weld out

Blasting

Install apertures

Painting

Final inspection and sign off

Deployment

This methodology is designed for a dry dock assembly.

For fabrication the structure will be broken into geometrically similar sub-sections. The metal

sheet will be brought in and laser cut on-sight. As the sheet metal will be difficult to

manoeuvre with typical lifting equipment, a magnetic spreader beam will be used to orientate

Submerged 9925 199

Splash 2550 51

Internal 11900 238

Total 24375 488

24

the sheet for profiling. The smaller sub-assembly panels on the inner and outer wall of the

cylinder will be assembled face down in sixths, this means no lifting equipment will be

required to position the material for welding and allows the initial assembly to occur inside a

fabrication bay. Generally the quality of weld and thus the speed of work in greatly improved

by working under shelter for as long as possible in the manufacturing process.

Once the first phase of assembly has been completed, the subsections should be transported

outside to the dock for the main assembly to take place. The final assembly will take place in a

dry dock with each sub section lowered in one by one for welding to its’ adjoining section.

Figure 22: Image showing similar subsections

stored dockside

Figure 23: A jig used to secure bent metal for

welding

The subsections of the outer shell are shown above. Each one will then be stacked on top of

one and other. Each sub-section is similar to a high degree to the others, this means that there is

a high degree of repeatability in the fabrication and a good opportunity for time saving

fabrication aids to be developed.

Fabrications aids will be crucial in ensuring the fabrication process proceeds smoothly as

planned. One aid will likely be a jig to support the steel plates during welding. This form of jig

must have a curved surface to run with the contours of the bent sheet. Shown above is a

possible example of this.

When the ballast tanks have reached 15m in height the concrete will be poured in and set to act

as a permanent ballast. At this point, the FPSO will be at the bottom of the dry dock thus the

concrete can be poured from normal methods from the dockside, as the height of the FPSO will

not exceed the level of the dockside. Care must be taken to ensure the pour rate is not too fast

or the high to great as this will cause the concrete to splash and cause the smaller particles to

move away from mixing with the larger particles forming a poor bond [14]. Each layer of

concrete will take between 45 minutes to an hour to set before the next layer can be poured.

Following this, the assembly of the structure will continue until all sub-assemblies have been

joined to the main structure. This is followed by the structure being shot blasted to improve the

surface condition. From this point on cherry pickers and scaffolding will be required to allow

workers access to the outside of the structure. Then cathodic protection must be attached the

units will be attached to the outside of the FPSO using a combination of welding as specified in

the following Corrosion System Manufacture section.

25

Figure 24: Image showing naval structure being shot

blasted for surface preparation

Figure 25: Image showing cheery picker

being used to allow painting

The final stage of production is to paint the outer shell of the FPSO. Again, the cherry pickers

and scaffolding must be used to give access. The paint coating should confirm to the should

conform to the standards set out in section 7. Guidance on health and safety is provided in

Appendix 8.

8.2 Site and Contractor Selection

The fabrication of the FPSO will be outsourced to a contractor for the construction work. Lists

the criteria used for selection of fabrication site. Details the minimum standard required by the

company and the preferred standard the company would like met. The large scale of the FPSO

means that there are no suitable sites available in the United Kingdom for the construction [15].

The likelihood is that a specialist yard such as Yantai Raffles Shipyard in China would be

utilised.

Factor Minimum Standard Preferred Standard

Machining Capabilities CNC plasma cutter and sheet

bender

CNC plasma cutter and sheet

bender

Lifting capacity 500 tonnes 1,000 tonnes

Assembly area Dry dock basin Dry dock basin

Fabrication area foot print 50,000m2 80,000m

2

Dimension control and

Inspection

Association with local

inspection firm, who have

worked on similar projects

Both integrated as part of

fabrication team, and working

on site.

Sheltered fabrication area 10,000m2 20,000m

2

8.3 Material Selection and Control Process

Material selection is based upon:

• Consequences of failure

• Degree of redundancy

26

• Presence of stress concentration

• Accuracy of analytical stress predictability

• Susceptibility to fatigue

• Crack arrest (e.g., leak before failure)

This process should be followed for each structure type within the FPSO. For this report only

the process for the primary steel structure will be detailed.

Failure of the outer structure would be catastrophic for the design and almost certainly lead to

total failure, with potential danger to many lives.

• The outside structure is integral to the functionality of the FPSO

• The outer walls of the FPSO are subject to loading from hydrostatic, hydrodynamic

pressures, and wave forces. Leading to high stress in the material.

• High degree of accuracy, little safety buffer required

• High susceptibility to fatigue, 100 year life

• Crucial that cracks, especially those occurring in the Heat affected zone of welds, can

be arrested before crack propagation occurs and leads to crack propagation

When considering the factors above it is clear that a strong high performing material, will be

required for the task from the design factors. Further fabrication factors also influence the

decision such as good weldability being desirable and the availability of material thickness,

based upon the required thickness calculated in section 5. It is industry standard to use steel for

use in the construction of offshore structures. Based upon the pressure calculations on the

outer-structure from the hydrostatic and hydrodynamic forces and in line with BS EN ISO

10225:2009, S355G10+N steel is selected. S355G10+N offers 460 to 620 MPa as the ultimate

tensile strength and a yield stress of 320 MPa.

The company should work with the fabricators in order to source a suitable material provider.

The provider must be able to work from CAD models to produce the specific parts of tubular

or box section. The tolerances of the sub-section are set at ±6 mm [13], thus each material part

must have strict tolerance of ±2 mm set out. Plasma and laser cutting has now become common

in industry [16].

It is crucial that the material providers are willing and able to fall in line with the company's

procedures on material traceability and identification. All individual parts of material should be

individually itemised and catalogued. This number should then remain visible throughout the

fabrication stage up until painting. The material history log should be maintained fully by the

fabricators up to the structure passing its final sign off inspection, at which point the material

log will form part of the technical documentation pack handed over to the company. Material

traceability is crucial in being able correctly assess the situation and decide which part of the

supply chain was at fault should there be a failure in the structure, and allow easy identification

of other components that are susceptible to failure.

8.4 Welding

All welders working on the contract shall be qualified in accordance with AWD D1.1/D1.1M.

The company should be notified of all welder qualification testing and should have the option

to bear witness. Traditionally for marine steel construction, shielded metal arc welding (SAW)

is used. SAW can be used for automated welding. As this job mainly involves the welding of

steel plates, SAW is ideally suited [13]. Automated welding generally requires a manual

labourer to set-up the consumables for welding. The use of automation will increase the speed

of production and guarantee similarity in welds [17]. It is important that the electrodes used in

welding should produce no more than 10ml/100g of diffusible hydrogen, in line with ISO

3690. The choice of electrode for the weld is selected in line with BSI BS EN ISO

26304:2011, based upon this a welding wire with specification ISO 26304-A S 55 0 H5, is

required. This welding wire is overmatched to the parent material for yielding but this does not

cause issue and provides a factor safety in the values of yield given for the wire. If the level of

diffusible hydrogen is too high then it can lead to hydrogen embrittlement of the weld,

27

compromising the structural integrity [18]. If welds are cracking during the welding process

then sufficient pre-heat will be placed upon the material to prevent weld cracking in future

production. These pre-heat temperatures should not fall below the range given in AWS

D1.1/D1.1M Table 3.2. No welds should be forced cooled or quenched. The only method of

cooling allowable is by exposure to the natural environment [19].

Full penetration welds will be made to all structural member joints. Partial penetration welds

are acceptable for joints where stress is high but the members are not key structural

components. Any ancillary joints will be welded using a fillet joint, as long at the stress is

suitably low and not dynamically loaded. Welds in water-tight elements and foundation

members must be double continuous welds [4].

The welding procedure should also include hardness testing. This testing will be performed on

the heat affected zone, the weld metal and the base metal. A Vickers test will be used as the

hardness test of choice. This hardness testing should be performed as specified in ISO 9015-

1:2001.

Sample full penetration butt weld procedure specifications are included in the appendix for

reference.

The welding required to fit the components of the cathodic protection will be performed using

SMAW welding, due to the smaller machine size required giving the welder greater freedom of

manoeuvrability when working at height. Furthermore, SMAW does not require a gas shield

thus it is more suitable for use in a potentially windy outdoor environment.

8.5 Inspection and Dimension Control

All welds will be subject to a full visual inspection. Other forms of testing that will take place

include radiography, magnetic particle inspection, ultrasonic and penetrant. NDT/NDE workers

should be qualified in line with ASNT SNT-TC-1A. The welds should be inspected for cold

fusion, excess slag, cracking, poor bonding and porosity. A dimension control team should be

used to ensure that the structure falls within the tolerance limits set out by the company. A

continual checking process should be implemented and each subsection should be subjected to

assessment both pre and post weld. For this process tools such as a calibrated ruler are not of

adequate precision. The dimension control team should use optical levels in collaboration with

appropriate software to produce 3D technical reports of the sub-structures conformity to the

allowable tolerances set out [20]. Further information on inspection is given in Appendix 8.

8.6 Transportation and floating

There are two possible methods for transportation of the FPSO to its site of operation. The use

of tugboats which attach to FPSO via chains or tethers, this constitutes a wet transport as the

FPSO is in the water for all points. From the port, the FPSO is then pulled into position by the

tug boats. Method 2 utilises a semi-submersible lift ship to perform a dry transport by carrying

the FPSO upon its loading deck. There are two methods possible for positioning the FPSO on

the lift ship’s deck. Either the boat is constructed in a dry dock, which is then flooded for the

FPSO to be towed out to deep water. The semi-submersible will then take on extra ballast to

sink its deck below the water line and under the FPSO. From here, ballast is lost and the deck

returns to above the surface, lifting the FPSO out of the water for transport. This option is not

suitable as the largest of lift boats are unable to sink to a depth below 35m, and the height of

the FPSO below exceeds this. Therefore, to get the FPSO onto the deck, assembly of the sub-

sections would have to take place out of the water beside the dock, such that the FPSO could

be slipped on to the deck directly. This method would still present difficulties with the height

of the FPSO as the boat would not be able to sink low enough to properly release the FPSO, as

such the FPSO would need to be tipped off from the ship. Furthermore with this method the

FPSO will not be built in the dock which means it will be high in the air when fully constructed

and may not fit under cranes making fabrication difficult. Due to the difficulties in

implementing the dry transport method, the wet transport method, using tugboats, was selected.

28

For safety, the tugboats used must be 11 times greater than the largest significant wave

expected. The towlines used must be 4 times the static bollard pull of the ship it is attached to

[13]. Furthermore, the towlines should also be 1.25 times the breaking strength of the most

powerful tugboat used. Based upon the use of Ocean Wave Class tugboats, the tethers used will

need a strength of 600 tonnes. Whilst other cylindrical FPSOs have been transported on semi-

submersibles, they can be accommodated due to their much lower draft. For example the Sevan

Voyageur Spirit has a draft of 18m, compared with 40m of this design.

Whilst the continuous bollard pull of 150 tonne is small compared to the mass of the FPSO, the

larger static bollard pull is sufficient to overcome inertia of the FPSO and get it moving.

Figure 26: Image showing the variation in

bollard pull of a tugboat against time

Figure 27: Image showing cylindrical FPSO

being moved by two tugboats

Two tugboats will be used to allow more control of the movement of the FPSO, generally only

1 tug provides a pulling force in the direction of motion. The other ship is there to provide

security and prevent the FPSO being pulled off course. This may be crucial when the tugboats

travel out to the rough North Sea waters. Furthermore, there is already precedent for using two

tugboats as can be seen above from the transport of the Sevan Voyageur.

8.7 Corrosion System Manufacture and Fabrication

Initial surface preparation of the steel is of vital importance to the overall success of the

corrosion protection system. The performance of the coating system chosen is highly

dependent upon its ability to strongly adhere to the surface of the steel. A standard cleanliness

grade of Sa 2.5 to Sa 3 according to BS EN 150 8501-1 is deemed accepted. This involves very

thorough blast cleaning until the metal is in near-white condition and surface is free from all

grease, oil, dust, mill-scale and rust.

Method and equipment used for application of coatings should follow manufacturer

recommendations. Temperature and relative humidity should also be closely controlled for

example application should only occur when the temperature of the surface is 3°C above the

dew point to prevent moisture problems. The specified number of coatings should be applied

within a time limit and to a set range of minimum and maximum dry film thickness. Drying

should also occur under strictly regulated conditions.

Manufacture of anodes should comply with the requirements stipulated by standards NACE

RP0387. All materials should be electrochemically tested prior to anode manufacture for

quality control purposes. Steel inserts should be chemically cleaned prior to casting within the

sacrificial aluminium. All anodes should be checked for surface cracks or irregularities and a

minimum of 10% of all anodes should be tested for verification of weight and dimension. Two

anodes should be subjected to destructive testing for internal inspection of defects and steel

inserts.

Anode attachment should conform to welding specifications as previously stated.

8.8 Corrosion System Inspection and Maintenance

Inspection of anode attachments should be carried out immediately prior to the FPSO

installation at site as they may be dislodged during fabrication, transportation and floating. A

29

regular inspection procedure should also be arranged whereby the anode condition is evaluated

through non-destructive means such as visual inspection, radiography, eddy currents etc.

Attachment welds should be inspected in accordance with specifications previously stated.

Critically damaged anodes must be replaced where necessary. Underwater contractors will be

required to inspect and weld in the submerged area.

Discontinuities in the coating system such as pinholes, voids and cracks are commonly known

as holidays, and are often invisible to the naked eye. Unrepaired holidays can quickly lead to

rust creep and undercutting of the coating and, therefore, it is important that they are detected

efficiently. Holiday detectors should be used which apply a voltage and detect if current flows

indicating a gap in the insulating coating. Regular adhesion testing must also be carried out and

recorded.

Where holidays are detected repair should involve surface preparation, before application of

the repair coating. Hand-held power tools provide sufficient cleanliness and roughening for the

application of the chosen repair coating, zinc rich epoxy.

9 COST OF CONSTRUCTION

There are many contributing factors that make up the total cost of building an FPSO, including

the cost of raw materials, labour, topside equipment, corrosion protection, insurance and

installation. The cost of raw materials can be calculated relatively easily by using the material

cost per unit weight and the mass of the steelwork of the FPSO. This analysis however requires

accurate material costing's which can vary dramatically depending on where and when the

material is bought, and the quantity in which it is purchased. It is therefore hard to accurately

estimate the raw material cost. The cost of the topside equipment is also hard to estimate as it is

highly dependent on the exact requirements of the FPSO. Labour and installation costs also

vary considerably depending on where the FPSO is manufactured and installed.

A better way to estimate the cost of production is to consider the construction costs of FPSOs

that have already been built. It was found that a high production purpose built FPSO typically

exceeds $700 million dollars, with the hull costing over $120 million dollars and the topsides

costing over $600 million dollars [21]. A specific example is the Girassol which operates off

Angola. This cost $756 million to manufacture, with the hull costing $150 million, the topsides

$520 million and project management and delivery $84 million [22].

The most relevant example to the designed FPSO is the Sevan Goliat, a Norwegian cylindrical

FPSO built for operation in the North Sea. The contract was awarded in 2010 for $1.1 billion

[23]. When adjusted for 2014 prices this is nearer $1.2billion. The Goliat has a hull, topside

and appurtenance weight of 52,000 tonnes which leads to an approximate cost of

$23000/tonne. Total weight of the designed FPSO (hull + payload) is approximately 40,000

and therefore, assuming similar costs to the Goliat, would cost approximately $920 million to

manufacture.

10 CAD DESIGN

10.1 Drawings

For this report, basic technical drawings have been produced. These however are for purely

illustrative purposes and to be used as a tool to gauge how the large work required is. No

fabrication or design work should be carried out based upon these drawings. Should the FPSO

design continue, as is commonplace in the industry, an outside drawing office with

draughtsman who have experience in offshore structures would be brought in to create full

technical drawings and CAD models. All future fabrication and technical work will then be

based upon these models.

30

10.2 Finite Element Analysis of Offshore Structures

After an experimental preliminary analysis on a thin walled cylinder of appropriate

dimensions, it was decided that the FE capabilities available were not sufficient for a model of

this magnitude. As the mesh size decreased, the stress never converged to a final value. This is

due to the size of the mesh compared with the thin wall section. A specialised tool for large

structures would need to be utilised rather than the facility built into Solid Edge [24]. The

difficulties in accurately modelling offshore structures and generating both useful and correct

results can be exemplified by the failure of the Sleipner A platform. The FEA on the structure

generated stress values 47% lower than the realistic values. This failure was a combination of

the incorrect boundary conditions in addition to a rough mesh size. This was carried out by a

professional design office using the dedicated FEA package, NASTRAN [25].

A brief FE analysis was performed on a chain link, this analysis should not be used to specify

the chain as the manufacturer rates each grade individually.

Figure 28: FEA of Chain, analysed using symmetry

The analysis could be used to show the points of highest stress (although the values should not

be taken as read) and these points could be regularly inspected for preliminary signs of failure.

11 CONCLUSION

This report has detailed the design of an FPSO for use in a marginal North Sea oil field. The

one in a hundred design wave was calculated and its characteristics were used to model the

worst case load scenario for the FPSO. The design wave was calculated as having an amplitude

of 13m. The FPSO being designed was modelled as a hollow cylinder with oil and ballast

inside. Mathematical models were developed to analyse several design parameters for a whole

range of cylinder geometries. The cylinder dimensions were optimised for stability, mass and

31

buoyancy and were found to be 60 m in height, with a diameter of 54 m. These dimensions

produced an overall steel mass of 39.53 thousand tonnes, a metacentric height of 4.115 m and a

tether tension of 169.8 MN. The mooring system was developed based on already existing

technologies. Finally corrosion protection and manufacturing methods were researched and

procedures were developed. A cathodic protection system was designed to ensure the integrity

of the system for the design life. An initial build methodology was developed and an

investigation into possible sites for its construction was researched. Finally, a cost analysis of

the structure was carried out based on the manufacturing costs of similar vessels. It was

estimated that the FPSO would cost about $920 million to manufacture.

32

13 REFERENCES

[1] J. Vidic-Perunovic, “Novel hull concepts address ultradeep Gulf of Mexico

production,” 3 April 2013. [Online]. Available:

http://www.ogj.com/articles/print/volume-111/issue-11/drilling-production/novel-hull-

concepts-address-ultradeep.html.

[2] A. B. S. ABS, “Spar Installations,” 2014.

[3] Det Norske Veritas, Design of Offshore Steel Structures, General (LRFD Methods),

DNV, 2011.

[4] O. S. Det Norske Veritas, “Position Mooring DNV-OS-E301,” 2010. [Online].

Available: https://exchange.dnv.com/publishing/codes/download.asp?url.../os-e30.

[5] V. Anchors, “Anchor Manual 2010,” 2010. [Online]. Available:

Available:http://www.vryhof.com/anchor_manual.pdf.

[6] D. a. E. S. i. R. T. a. M. S. Consultancy, “ Fiber Ropes for Ocean Engineering in the

21st Century,” 2011. [Online]. Available:

http://www.tensiontech.com/papers/papers/deep_mor/deep_mor.html.

[7] M. S. Bardex Corporation, “The Bardex Chain Jack Systems,” 2010. [Online].

Available: http://www.bardex.com/downloads/Mooring_brochure-2010.pdf.

[8] J. Johnson, “Ships - Summary and Alaysis of Results - Appendix O,” CC Technologies

Laboratories, Inc, Dublin, Ohio.

[9] C. Anderson, “ Protection of Ships - Lecture 1: Protection against Corrosion,”

Newcastle University, [Online]. Available:

http://www.ncl.ac.uk/marine/assets/docs/NclUni_Lect1_1103.pdf.

[10] T. B. Jr., “Designing Protective Coatings Systems for Offshore Oil and Gas Platforms,”

2004. [Online]. Available:

http://ppgamercoatus.ppgpmc.com/marketsserved/docs/PC_designOffshore_Oil%20Ri

gs.pdf.

[11] D. N. Veritas, “CATHODIC PROTECTION DESIGN,” Recommended Practice DNV-

RP-B401 , 2010.

[12] P. Mohamed A.El-Reedy, Offshore Structures: Design, Construction & Maintainance,

Gulf Professional Publishing, 2012.

[13] Indiana Govt., “Concrete bridge chapter,” Indiana Govt..

[14] Department of Energy & Climate Change, “The Capability & Capacity of the UK

Offshore Oil & Gas Fabrication sector,” DECC, Aberdeen, 2011.

[15] Y. H. Çelik, “Investigating the Effects of Cutting Parameters on Materials Cut in CNC

Plasma,” Materials & Manufacturing Processes, 2013.

[16] J. Percio, “Automated welding systems taking on added importance,” Plant

Engineering , 2013.

[17] A.Alvaro, “Hydrogen Embrittlement of a weld simulated x70 heat affected zone under

H2 pressure,” 2014.

[18] M. W. &. W. Lukens, Effect of forced gas cooling on GTA weld pools, AWS, 1986.

[19] J. Vose, Dumpy level work: a guide to its use in building and simple surveying, 1962.

[20] N. J. W. N, “A Story About FPSO Project Performance,” [Online]. Available:

www.fpso.net. [Accessed 04 April 2014].

[21] “FPSO World Fleet,” [Online]. Available: http://www.fpso.net/page7.html. [Accessed

33

4 April 2014].

[22] J. Alberch, “FPSO and FPSO Topsides Costs,” [Online]. Available:

http://www.projectcontrolsinternational.com/fpso-and-fpso-topsides-costs.html.

[Accessed 04 April 2014].

[23] P. Kurowski, “Avoiding Pitfalls in FEA,” Machine Design, 1994.

[24] D. N. Arnold, “The sinking of the Sleipner A offshore platform,” 2009. [Online].

Available: http://www.ima.umn.edu/~arnold/disasters/sleipner.html.

[25] A. B. o. Shipping, “General Requirements for Spar Installations,” Rules for Building

and Classing Floating Production Installations, pp. 492-522, 2014.

[26] A. &. J.V.Sharp, Saftey Factor Requierments for the Offshore Industry, Engineering

Failure Analysis, Elsevier, 2007.

[27] J. L. Meriam and L. G. Kraige, Engineering Mechanics Statics, 6th ed., Wiley, 2008.

[28] J. M. Gere and B. J. Goodno, Mechanics of Materials, 7th ed., Toronto: Cengage

Learning, 2009.

[29] Y. A. Cengel and M. A. Boles, “Chapter 10 Refrigeration Cycles,” in

Thermodynamics: An Engineering Approach, 4th ed., New York, McGraw-Hill, 2002,

pp. 567-572.

[30] Center for Environment, Commerce & Energy , “Seven Types of Offshore Oil

Production Platforms,” 2010. [Online]. Available:

http://cenvironment.blogspot.co.uk/2010/05/seven-types-of-offshore-oil-

production.html. [Accessed 14 March 2014].

[31] BAE Systems Defence Consultancy, “Human factors integration: Implementation in

the onshore and offshore industries,” Health and Safety Executive Books, Norwich,

2002.

[32] D. Cueva and F. Faria, “Roll Motions of FPSOs,” Oceanica Offshore, Sao Paolo, 2011.

[33] A. Le Cotty and M. Selhorst, “New Build Generic Large FPSO,” in Offshore

Technology Conference, Houston, 2003.

[34] C. Bollinger, 2012

[35] A. P. Institute, “Design and Analysis of Stationkeeping Systems for Floating

Structures,” 2005. [Online]. Available:

http://www.offshoremoorings.org/moorings/PDF/API%20RP%202SK.pdf.

[36] ABS, “Guidance notes on the Inspection, Maintenance and Application of Marine

Coating Systems,” American Bureau of Shipping , 2007.

[37] J. Ault, “The Use of Coatings for Corrosion Control on Offshore Oil Structures,”

[Online]. Available:

http://www.elzly.com/docs/The_Use_of_Coatings_for_Corrosion_Control_on_Offshor

e_Oil_Structures.pdf.

[38] T. Semerad, “Corrosion in the Oil Industry”.

34

14 APPENDIX 1 – DESIGN WAVE

To calculate the largest wave the structure is likely to suffer in its design life (chosen here as

100 years) a set of wave data must be analysed for the extreme values. The data comes in the

form a scatter chart, as shown in Figure 29.

Figure 29: Scatter Graph for the North Sea

The range of wave periods shown in the chart above is for 4-13 seconds. This information can

be used to define the range of resonant frequencies that should be avoided when designing the

structure. The equation below was used to determine this value.

35

Figure 30: Scatter Graph from Microsoft Excel

Shown above, in Figure 30, is the format in which it was analysed in Microsoft Excel. Firstly,

the design wave period was determined from below.

∑ ∑ [ ]

∑ ∑ [

]

To obtain the long-term distribution, initially, the following formula is used:

[ ] ∑ ∑ [ ]

( )

Where ∑ ∑ [ ]

Then ( ( [ ])) is plotted against .

1 1 2 0 0 0 0 0 0

2 18 19 14 10 5 1 1 0

5 30 42 25 18 14 5 2 1

2 21 46 39 24 14 6 1 1

0 11 47 50 25 8 4 1 0

0 2 33 38 25 7 2 1 0

0 1 17 35 23 10 5 1 0

0 0 5 27 21 9 3 1 1

0 0 2 21 20 12 3 2 1

0 0 0 9 20 11 3 0 1

0 0 0 3 11 10 3 1 1

0 0 0 2 6 10 4 1 0

0 0 0 0 4 10 5 1 0

0 0 0 0 2 5 3 1 0

0 0 0 0 0 3 2 0 0

0 0 0 0 0 1 3 1 0

0 0 0 0 0 0 2 1 0

0 0 0 0 0 0 1 0 0

36

Figure 31: Negative Log Plot

To find the design wave height , the probability of the significant wave height and the

coefficients for the line of best fit must be found and input below.

( ( ( [ ])) )

Where the come from the line of best fit

( ( [ ]))

In addition, is the significant wave height.

The significant wave height is defined as the height that will be exceeded by only one third of

the total waves recorded.

-3.000

-2.000

-1.000

0.000

1.000

2.000

3.000

4.000

5.000

6.000

7.000

8.000

0 2 4 6 8 10

-ln

(-ln

(P[H

SI))

HSI

37

Figure 32: Scatter Graph Showing the Cumulative Sum

The total number of waves recorded in this case is ,

the figure above

shows that the significant wave height for this case is . Rearranging the formula

displayed in class notes for ( ) gives:

(

) (

( ))

( )

Taking and , ( ) .

If not all the points are to be used for a line of best fit, this means that a family of best-fit lines

will be created.

4.5 5.5 6.5 7.5 8.5 9.5 10.5 11.5 12.5 Cumulative Sum

0.25 1 1 2 0 0 0 0 0 0 4

0.75 2 18 19 14 10 5 1 1 0 74

1.25 5 30 42 25 18 14 5 2 1 216

1.75 2 21 46 39 24 14 6 1 1 370

2.25 0 11 47 50 25 8 4 1 0 516

2.75 0 2 33 38 25 7 2 1 0 624

3.25 0 1 17 35 23 10 5 1 0 716

3.75 0 0 5 27 21 9 3 1 1 783

4.25 0 0 2 21 20 12 3 2 1 844

4.75 0 0 0 9 20 11 3 0 1 888

5.25 0 0 0 3 11 10 3 1 1 917

5.75 0 0 0 2 6 10 4 1 0 940

6.25 0 0 0 0 4 10 5 1 0 960

6.75 0 0 0 0 2 5 3 1 0 971

7.25 0 0 0 0 0 3 2 0 0 976

7.75 0 0 0 0 0 1 3 1 0 981

8.25 0 0 0 0 0 0 2 1 0 984

8.75 0 0 0 0 0 0 1 0 0 985

38

Figure 33: Family of Best Fit Lines

To illustrate the variety of values that can be obtained for the design wave, a set of best fit lines

are shown. These are best fit lines of two points each, extrapolated to the range of the data.

Shown below is the range of Wave Heights that come from these lines.

Table 14: Range of Design Wave Heights

a b H [m]

1.509 -2.083 14.5

1.068 -1.752 20.2

0.875 -1.512 24.4

0.829 -1.431 25.7

0.695 -1.129 30.2

0.715 -1.185 29.4

0.656 -0.992 31.8

-10

-5

0

5

10

15

0 1 2 3 4 5 6 7 8 9

-ln

(-ln

(P[H

SI))

HSI

39

a b H [m]

0.787 -1.485 27.1

0.791 -1.503 27.0

0.733 -1.227 28.7

0.835 -1.763 25.9

1.162 -3.640 20.2

1.111 -3.324 20.9

0.816 -1.330 26.0

1.391 -5.502 18.2

1.836 -8.944 15.7

1.387 -5.246 18.1

The more data points that are used to find the lines of best fit, the less extreme the values for

become.

Figure 34: Wave Height against Number of Points in Line of Best Fit

As expected, the values converge to an intermediate value. However, since this is an extreme

value analysis and the required value is a maximum, it is more pragmatic to take a value

0

5

10

15

20

25

30

35

0 5 10 15 20

Wav

e H

eig

ht

[m]

No. of Points [-]

2 Points

4 Points

6 Points

8 Points

10 Points

12 Points

14 Points

16 Points

18 Points

40

between that of the ‘14 Points’ and the maximum ‘2 Points’ Value. Re-evaluating the family of

best fit lines, Figure 35 shows the outlying lines removed.

Figure 35: Realistic Best Fit Lines

This reduces the number of possible wave heights as shown below.

Table 15: Range of Design Wave Heights

a b H [m]

1.509 -2.083 14.5

1.068 -1.752 20.2

0.875 -1.512 24.4

0.829 -1.431 25.7

0.695 -1.129 30.2

0.715 -1.185 29.4

0.656 -0.992 31.8

-10

-5

0

5

10

15

0 1 2 3 4 5 6 7 8 9

-ln

(-ln

(P[H

SI))

HSI

41

a b H [m]

0.787 -1.485 27.1

0.791 -1.503 27.0

0.733 -1.227 28.7

0.835 -1.763 25.9

1.162 -3.640 20.2

1.111 -3.324 20.9

0.816 -1.330 26.0

1.391 -5.502 18.2

1.836 -8.944 15.7

1.387 -5.246 18.1

The line that gives the maximum wave height, 31.8m, is the line with the smallest gradient

(highest on the left, lowest on the right). The most accurate fit lines are shown in Figure 34:

Wave Height against Number of Points in Line of Best Fit above as dashed lines. These both

represent a wave height of ~26m. In Figure 34: Wave Height against Number of Points in Line

of Best Fit the maximum values of wave height seem to converge to around 26m before

dropping off. This will be the value taken forward as the design value.

From the information yielded by the scatter diagram useful information about the wave can be

obtained. Firstly, the wave amplitude:

The wave angular frequency is found:

rad/s

The wave number is then acquired:

The wavelength is given by:

42

Figure 36: Images of Wave Pressure Animation

Animation performed in MATLAB to show pressure distribution across submerged cyclinder

over time.

43

15 APPENDIX 2 – THEORY

15.1 Structural Calculations

The structural calculations are based the assumption of a cylindrical plate steel shell, with

vertical stiffeners and horizontal girders. Both the stiffeners and the girders are I section. The

calculations to determine the appropriate thicknesses of these elements are based on the rules

set out by the ABS [26].

The calculations define the plate thicknesses, and the section moduli (elastic or plastic) of the

framework.

The MATLAB code is inserted below.

Additional Formula for calculation the FPSO’s mass

The mass of the tube was calculated using Equation 1. The mass of the cap was calculated

using Equation 2.

(

( )

)

(1)

( )

(2)

Where is the diameter of the cylinder, is the thickness of the steel and is the density

of steel.

Additional Formula for GM

BM was calculated using equation 3. Equation 4 was used to calculate the moment of inertia of

the cylinder about the waterline.

(3)

(4)

(5)

Where D is the diameter of the cylinder and hs is the submerged depth.

BG was calculated using equation 6. It can be seen for this calculation the centre of gravity and

the centre of buoyancy were required. The centre of buoyancy was calculated by simply

44

dividing the submerged height by two, as seen in Equation 7. The centre of gravity was

calculated using Equation 8.

(6)

(7)

(8)

To calculate the centre of gravity (COG) of the structure it was required that the level of the oil

and water in the tanks and the depth of the concrete were known. The following expressions

were derived to find these levels, as shown in Equations 9, 10 and 11.

(9)

(10)

(11)

45

16 APPENDIX 3 – TETHER

46

Safe Working Load 2-1500tonnes

Diameter 51-870mm

Weight 0.44-2800kg

Proof Loads 276-21586kN

Break Loads 324-27383kN

Weight 8-750kg/m

Diameters 19-185mm

Minimum Breaking Load 3648-24812kN

Stiffness (EA) 2.03x105-1.38x10

6kN

Weight 3.3-17.2kg/m

Diameters 81-187mm

Bardex Linear Chain Jack Approx. 12000kg

Bardex BarLatch Fairlead Approx. 4000kg

Studlink Chain (75m Long) 50325kg

HMPE Rope (140m Log) 2142kg

Shackles 7800kg

Suction Anchor & Pump (from mass of steel

used)

Approx. 151950kg

Single Tendon Total Mass 228217kg

47

17 APPENDIX 4 – CORROSION PROTECTION

The driving force of a refined metallic element desiring to return to a more stable compound

causes corrosion. It is an electrochemical process consisting of two voltaic half-cell reactions;

an oxidation reaction at the anode and a reduction reaction at cathode.

The anodic reaction in the corrosion of steel involves the oxidation of the metal to its ions:

At the cathode, several different reduction reactions are possible depending upon conditions.

The most common reaction is the reduction of oxygen, which produces an alkali at the surface

of the metal:

The products of the reactions at the anode and cathode combine to form iron oxides and iron

oxide-hydroxide which are commonly recognisable as rust:

( )

( ) ( )

( )

48

17.1.1 Recommended Practice Tables

17.1.2 Design of Anodes

49

Anodes are to be distributed so that a uniform current is supplied throughout the structure.

Each anode should also be sufficiently spaced from other anodes to avoid current interference

patterns occurring. For simplification of fabrication the anodes will be arranged in a 10m

rectangular array (as displayed on Drawing Sheet 4). This means that each anode must be

designed to protect an area ( ) of 50m2.

The approximate total surface area of metal to be protected for each zone ( ) is divided by

to calculate , the total number of anodes required to protect the surface area.

The geometry and material of the anodes is designed so that they supply the required current

demand to maintain cathodic protection ( ). is calculated by multiplying the total protected

surface area ( ) with the design current density ( ) and the coating breakdown factor ( ):

Values for current density are obtained from recommended practice tables with an arctic

climatic region assumed. Variables change with time and therefore calculations are made for

initial and final current demands (denoted by subscripts and respectively). A ‘mean’ current

demand (subscript ) is also calculated to represent the current demand when the CP system is

at a steady-state potential.

Coating breakdown factor is the predicted reduction in total current demand ( ) due to the

usage of the coating system. Initially the coating is 100% intact and therefore current demand

for protection is negligible. As the coating degrades current demand increases and therefore in

the mean and final cases is a function of operational environment, properties and time.

illustrates the typical coating breakdown experienced after five years usage on an FPSO.

Figure 37: Typical Coating Breakdown after 5 Years

For calculation of recommended practice suggests the usage of:

( )

50

Parameters a and b are experimentally calculated constants read from recommended practice

tables and is time in years. For the final case, time used is the design life of the CP system

( ) whereas in the mean case it is half the design life.

( )

( )

( )

It should be noted that if the design life is large and calculated value exceeds 1 then

should be used as this refers to uncoated metal.

Ideally the design life of the galvanic system should equal the design life of the FPSO,

however, this is economically and operationally unfeasible as it would require very large

anodes. A more viable approach is usually adopted whereby a design life of 5-15 years is

chosen combined with strict maintenance routines and the possibility of future system

retrofitting. A safe middle value of 10 years was taken for this design.

The choice of anodic material requires a high electrochemical capacity ( ) and a closed circuit

potential ( ) which is more negative than the design protective potential (

). For protection

of steel of -0.80V relative to seawater is deemed acceptable. Zinc and aluminium are the

most common materials used for galvanic anodes in the offshore industry. Recommended properties show that aluminium has a higher electrochemical capacity and more

negative closed circuit potential than zinc and was therefore selected for this design. It can

produce more current before depletion than zinc whilst also providing a higher potential

difference.

The material properties are used to calculate the total mass of anodes required to supply the

demand current over the protective design life:

In this equation 8760 converts design life from years to hours and is the utilisation factor.

This defines the percentage of the total material which can be safely assumed to provide

protection before the anode becomes unpredictable. The utilisation factor is dependent on the

anode design geometry; as it reaches its utilisation factor it may supply less current due to a

decrease in structural support causing a breakage in electrical connection or an increase in

resistance.

51

Figure: Utilised Anode

For this system, the design geometry of the anodes is based on currently available commercial

marine hull anodes. There are a huge range of available sizes with weights ranging from 1kg to

over 100kg and three main categories of shapes are in usage; stand-off, flush-mounted or

bracelet. Recommended practice tables show that stand-off have the highest utilisation factor,

however, after research into typical attachment methods it was decided that flat flush-mounted

anodes, shown below, are more appropriate for welding onto hulls or tank walls than stand-off

or mechanically bolted methods.

Figure: Flush Mounted Anode

Recommended resistance formulae for each anode type is also dependent on shape and

dimensions. For rectangular flush-mounted anodes resistance ( ) is:

Whereby is the seawater resistivity (0.2 for North Sea temperatures) and is the

arithmetical mean of the anode length and width. For the final case, it was assumed that

dimensions will have decreased in accordance with the utilisation factor:

The potential difference ( ) is termed the design driving voltage. It can be used to calculate

the current output per individual anode from Ohm’s law:

(

)

Aluminium

Anode

Steel Insert

52

To select the final anode geometry a MATLAB script was produced to carry out the design

calculations. Data from recommended practice was input along with parameters of existing

commercially available aluminium anodes. Anode size was increased until the design

conditions outlined below are satisfied:

53

18 APPENDIX 5 – FABRICATION

18.1.1 Methodology

The initial step in the method is to decide upon a design guideline in order to make material

and fabrication method decisions. For this design, a Load and Resistance Factor Design

approach was taken.

Generally, for all structures there is requirement to satisfy the state displayed below in equation

64 [4].

Where R is the resistance of the structure and L is the load placed upon it.

The four states that should be considered for a full offshore structure design are:

Ultimate Limit State

o Ultimate load carrying ability of the structure.

Fatigue Limit State

o Cyclical loading on structure, typically from environmental loads

Accidental Damage

o Ensuring that damage from a local accident does not lead to complete failure of

the structure

Serviceability

o Deformation, vibrations etc. that govern normal usage of the platform.

The Reserve Strength Ratio (RSR) should fall in a range of 1.8 to 2.5, as designed following

API RP2A [27] shown in equation 54.

The Load and Resistance Factor Design (LRFD) method defines factors for the material

resistance and the load. Essentially these factors act as insurance factors to account for possible

inaccuracies in the calculations and variances in material strength.

The load factor can be found from equation ?.

The resistance factor is defined by equation ?.

A decision was taken to focus the design based upon the ULS and the serviceability loading.

The accidental and fatigue limit stages are both difficult to quantify and require judgement on

how the FPSOs functionality would be effected by a certain failure.

For the ultimate limit state, two simulations must be performed with load factors as denoted by

54

Table 16: Load Factors for the Ultimate Limit State

Permanent

Load

Variable

Functional Load

Environme

ntal Load

Deformati

on Load

Loading

Case 1

1.3 1.3 0.7 1.3

Loading

Case 2

1.0 1.0 1.3 1.0

The material load factor is 0.85 for both the UTS loading cases [27].

18.2 Process

Plan of fabrication for sub-assemblies

Sheet steel brought in and cut to size

Rolled to correct circumference

Plates fitted together in jig and sized up

Tack welding to secure plates in correct position

Automated welding to make full penetration welds on outer structure steel sheet

Girders fitted into place and sized up and tacked on

Automated welding to fit girders

Dimension control on sub-assembly

18.3 Inspection

Decisions on welding repairs will be taken by company's on-site quality team. Should a weld

not meet the required standard the contractor will be bound to repair the weld to a suitable

standard. The contractor will prepare a weld repair specification for approval by the company.

No repairs should be commenced without approval of the company. If a method of repair for

the weld cannot be agreed, the contractor should dig the weld out and start again. Repairs of

welds will be subject to CVN impact testing. The CVN impact testing shall locate the V at the

original/repair weld highest dilution zone and immediately below the fusion line within the

weld. Hull plate that has been CVN impact tested shall be welded with a WPS that has been

CVN impact tested to the same (or lower) temperature and meet or exceed the plate impact

energy requirements. The impact properties of a weld joining steels with different impact

requirements shall conform to the more severe steel impact test requirement. CVN impact

requirements shall conform to the Classification Society Rules.

18.4 Weld Procedure Specification (Sample)

55

http://www.aws.org/certification/docs/Book_of_Specs_Eng.pdf

18.4.1 Health and Safety

It is important that a high standard of safety is maintained at all times and any potential risks

minimized or avoided where possible. The financial and publicity damage that would be

caused to the company should a serious accident happen on their contract could have

potentially seriously damaging effects on the company’s ability to continue to operate

normally.

On site there should be strict health and safety policy employed at all times. When on the shop

floor full personal protective equipment should be worn. This includes but is not exclusive to

Full Overalls

Protective footwear

Gloves

Safety glasses

Hard Hat

Further equipment such as ear defenders and masks may be required dependent on the situation

and should be made readily available for all members of staff by the fabricators.

A policy of no ignorance should be employed fully and workers on both the company and the

contractors’ side reminded regularly of their responsibility to report and correct and unsafe

actions or working conditions that they spot. A culture of ignorance should not be tolerated.

The company acknowledges the social stigma that can be attached to enforcing health and

56

safety policy to colleagues and often those of a higher rank. Thus there will be an anonymous

drop box where staff can report no urgent issues.

The company should have a health and safety officer at based at the site at least part time in

order to ensure site standards fall in line with those of the company.

Applicable standards for safety equipment are as follows:

1731:2006

14052:2005

20345:2004

458:2004

8468-4:2008

57

19 MATLAB CODE

58

59

60

61

62

63

64

65

66

67

68

69