Wind Farm Access Vessel

4th Year Group Design Project Offshore Wind Farm Access Vessel

Sam Naylor – Nicholas Brophy - Boyang Wang

Department of Naval Architecture, Ocean and

Marine Engineering

The University of Strathclyde

May 2014

1

Contents

INTRODUCTION .................................................................................................................................. 2

HULL FORM ......................................................................................................................................... 3

INITIAL DESIGN PROCESS ...................................................................................................................... 3

THE BENEFIT OF A SWATH OVER A CONVENTIONAL CATAMARAN ....................................................... 4

THE DISADVANTAGES OF A SWATH .................................................................................................... 4

CHOSEN DESIGN ................................................................................................................................... 5

SPEED OF INCEPTION ............................................................................................................................. 6

FIN DIMENSIONS .................................................................................................................................... 6

PERSPECTIVE VIEW OF THE HULL .......................................................................................................... 9

BASIC PRINCIPALS OF MOTHERSHIP ...................................................................................................... 9

RESISTANCE AND POWER PREDICTION ................................................................................... 10

SEAKEEPING ANALYSIS ................................................................................................................. 13

DATA INPUT ........................................................................................................................................ 13

DATA OUTPUT ..................................................................................................................................... 13

MOTION SICKNESS INCIDENCE ............................................................................................................ 14

RAO’S ................................................................................................................................................ 14

PROPULSION AND MACHINERY SELECTION .......................................................................... 14

HOW MUCH POWER WE WANT ............................................................................................................. 14

GREEN CREDENTIALS ......................................................................................................................... 15

EXISTING DIESEL-ELECTRIC SYSTEMS ................................................................................................ 15

MORE TRADITIONAL OPTION ............................................................................................................... 16

CREW TRANSFER SYSTEM ................................................................................................................... 17

GENERAL ARRANGEMENT ........................................................................................................... 18

APPENDICES ...................................................................................................................................... 20

APPENDIX A: DESIGN BRIEF ............................................................................................................... 20

APPENDIX B: CATALOGUE OF COMPARABLE SWATH DESIGNS ........................................................ 21

APPENDIX C: SEAKEEPING RESULTS ................................................................................................... 23

APPENDIX D: MSI ............................................................................................................................... 26

APPENDIX E: RAO’S ........................................................................................................................... 32

APPENDIX F: WEIGHT DETAILS OF SWATH ....................................................................................... 39

2

Introduction

In 2008, The Crown Estate announced ‘Round 3’, the largest offshore wind

programme in the world, proposing nine offshore zones to develop wind farms of

varying sizes. Due to the much larger scale of this round, there was the potential to

produce triple the collective energy of rounds 1 and 2, which in combination with

round 2 extensions and Scottish Territorial Waters projects would provide a total

output of nearly 50GW.

As a result, there has been a drive for designs which can operate safely and efficiently

within such global locations, which are outside of current operable vessels’ abilities.

This includes areas which are further from land and exhibit sea states greater than

those which are operated in at present. These zones provide suitable locations with a

sufficient area to host these vast wind farms with up to 1500 turbines, which is much

larger than current wind farms. Consequently, the design and construction of transfer

vessels with the ability to operate in a safe and efficient manner in areas portraying

the inevitable increase in wave characteristics is crucial.

A small water-plane area twin hull (SWATH) design is selected as a base for the

vessel, as this is fundamentally an efficient design in reducing the motions of the

vessel whilst stationary. With the inclusion of fins protruding from the torpedo hulls,

the Munk Moment (which can be detrimental to the SWATH hullform) can be

reduced to improve the selected design whilst travelling at a forward speed.

Performing a seakeeping analysis on a model of the vessel allows its seakeeping

abilities to be established for the specified sea state. The resistance prediction and

therefore powering requirements are calculated, allowing for the selection of a main

propulsion system. An appropriate method of crew and supply transfer is included in

the design, with the general arrangement of the vessel described.

3

Hull Form

Initial Design Process

The principle design imperatives that the new design had to address were:

The ability to carry out all operational aspects of crew transfer to turbines at

significant wave heights of greater than 2m.

A top speed in the range of 25-30kts.

A method of rendezvous with a mothership, with freedom to determine the

design basics of both the method and the mothership.

An appropriate range to be able to operate on deep-water windfarms that may

consist of 1000-1500 turbines up to 300km from shore.

The initial design brain-storming process was approached in two steps.

Firstly, several innovative ideas for the transfer (daughter) vessel with widely varying

concepts were sketched out. In parallel to this, the group came up with some concepts

for a mothership and rendezvous system for the daughter craft.

Second, once the general craft-type and approximate dimensions were decided on

along with an appropriate design for a mothership, a focus was made on the method of

transferring crew to the turbine safely.

With regards to the design of the daughter craft, emphasis was made on the ability of

the vessel to remain stable in waves. The brain-storming technique produced some

rather unconventional ideas including floating ‘stabilisers’, a hybrid between a

catamaran and a SWATH and a tri-SWATH with azimuthing torpedo hulls. Several

designs incorporated integral attachment systems for connecting to turbines although

some merely provided a reasonably stable platform on which to mount an existing

standalone motion-compensating gangway.

The conclusion was reached that the best hull type for providing a stable platform

would most likely be a SWATH.

4

The benefit of a SWATH over a conventional catamaran

SWATH stands for Small Waterplane Area Twin Hull. It is a class of catamaran

defined by very narrow hulls at the waterline, with the vast majority of the buoyancy

underneath the free surface. The hull itself may be considered as 3 different parts; the

torpedo-shaped lower hulls that provide the buoyancy; the thin struts that pierce the

waterline and connect the lower hulls to the wet deck, which sits above the waterline

between the struts and houses the superstructure.

There are two main advantages for a SWATH over a conventional catamaran, both of

which are related to the fact that the waterplane area (that is to say the horizontal

sectional area through the struts at the waterline) is exceptionally small. The first

advantage is improved sea-keeping qualities as a vessel’s response to a wave

excitation is dependent on waterplane area. Considering a wave profile raising the

level of the water above the at-rest waterline on the strut, the increase in hydrostatic

pressure against that part of the hull is increased leading to an increase in buoyancy,

hence upthrust. For a thin strut (in relation to the vessel’s total breadth) the volume

contributing to the change in buoyancy is very small and so the upthrust has little

effect on the vessel. The effect is to make the vessel slow to react to waves, hence it

has very large natural periods of roll, pitch and heave - much larger the periods of

wave normally encountered at sea – leading to small responses in normal conditions.

The second advantage is related to wave-making resistance, which is smaller for a

SWATH than a conventional catamaran (excluding any hull interactions) as the

wavemaking body at the waterplane is thin.

The disadvantages of a SWATH

Despite the gains to be made with the reduction in wave resistance, there is a penalty

to pay in terms of viscous resistance, composed of frictional and viscous pressure

resistance. Due to their largely cylindrical shape, the wetted surface area of the lower

hulls is higher than it would be for a catamaran of equal displacement. This leads to a

large frictional component of resistance. The form factor (1+k) correction made to

frictional resistance takes account of the 3-dimensional shape of the wetted surface.

In general it is much higher for a catamaran than for a monohull, and higher again for

a SWATH. Therefore it is not only the wetted surface area that is larger for

5

SWATH’s, but also the sectional shape leads to higher total viscous resistance which

can outweigh the reduction in wave resistance.

Although the small water-plane area gives favourable sea-keeping characteristics,

particular care needs to be given to the Munk Moment. The small water-plane area

means a SWATH vessel has a pitch restoring moment which does not change with

speed. The largest portion of its underwater volume is contained in the deeply

submerged torpedo hull. Therefore as a SWATH vessel moves forward the Bernoulli

pressure will generate a destabilizing pitching moment which is known as Munk

Moment, which is proportional to the square of the speed. The result is that at a

certain speed the Munk Moment will be as large as the pitch restoring force, leading

to pitching instability. In order to prevent pitch-instability fins are added to the lower

hulls. Their foil section means they create lift in proportion to the square of speed so

therefore they balance out the Munk Moment.

Chosen Design

After compiling a catalogue of data for existing SWATH designs (see Appendix B)

and drawing comparisons between them the principal dimensions for the current

design were selected (given below). It was decided to make the SWATH design

smaller than the majority of those researched in order to maximise the operational

feasibility of the mothership. The target maximum speed chosen was 25 knots.

Displacement 28.581 tonnes

Volume 27.883 m^3

Draft to Baseline 2.2 m

Immersed depth 2.2 m

Length on water line 12.407 m

Beam on water line 6.104 m

WSA 185.719 m^2

Max cross sect area 3.213 m^2

Water plane area 20.268 m^2

Cp 0.699

Cb 0.598

Cm 0.859

Cwp 0.956

LCB from zero pt. (+ve fwd) m -0.264 m

LCF from zero pt. (+ve fwd) m -0.24 m

LCB from zero pt. (+ve fwd) % Lwl -2.127 %

LCF from zero pt. (+ve fwd) % Lwl -1.932 %

6

Speed of Inception

The speed of inception is the critical speed where the munk moment equals to the

pitching moment. It can be found as follows:

Speed of inception:

√

√

As the vessel is required to travel at 25kts a method of remaining pitch stability

needed to be used.

Fin dimensions

Assumed Inputs:

Max speed: U=25.5 knots = 13.117 m/s

Added mass: A33=0.5*displacement=14.29 tonnes

Lower hull viscous lift coefficient: A0=0.7

Water density: ρ=1025 kg/m^3

Acceleration of gravity: g= 9.81m/s^2

Aspect ratio: Ar=1.2 (assumption)

Strut length: Ls=10.28 m

Fins centroid separation from the LCG: l=4.2m

7

Calculation of Static Moment:

The static moment of the lower hull about LCG is:

∫ ( ) ( )

The calculation of the static moment can be done using Microsoft Excel.

X(m) B(x)m S.M Area

product

Distance

from

LCG

Moment

product

-6.482 0.996 stern 1 0.996 -6.002 -5.977992

-5.184 1.384 4 5.536 -4.704 -26.041344

-3.888 1.472 2 2.944 -3.408 -10.033152

-2.592 1.472 4 5.888 -2.112 -12.435456

-1.296 1.472 2 2.944 -0.816 -2.402304

0 1.472 midship 4 5.888 0.48 2.82624

1.296 1.472 2 2.944 1.776 5.228544

2.5925 1.472 4 5.888 3.0725 18.09088

3.888 1.472 2 2.944 4.368 12.859392

5.184 1.382 4 5.528 5.664 31.310592

6.482 0.62 bow 1 0.62 6.962 4.31644

42.12 5.2805 17.74184

Moment 22.9934246

Therefore the static moment is about 23 m^3.

The lift coefficient is:

( )

√ (

)

The longitudinal metacentric height GMl can be estimated with equation below:

( )

The area of one fin is:

8

( ) ( )

The span & chord of the fin:

Span:

√ √

Chord:

The max thickness can be assumed as:

The area of both aft fins is:

The area of both forward canards:

Increment of the aft fin area due to canards:

Total fin area:

Fin dimension summary

Span 1.17 m

Chord 1 m

One fin

area

1.168 m^2

Two

forward

area

2.34 m^2

Two aft

area

4.68 m^2

Total

area

7.02 m^2

9

Perspective view of the hull

Basic Principals of Mothership

It was decided that the mothership for the vessel should be of a similar design to semi-

submersible naval amphibious assault support ships or superyacht transport ships.

These comprise a large dock at the stern of the ship. The ship is equipped with a

ballasting system which allows it to ‘sit down’ at the stern to flood the dock and

permit the daughter craft to enter or exit. Upon the arrival and docking of a daughter

vessel the ballast system is emptied, causing the mothership to rise. This lifts the

dock out of the water until it becomes dry. This allows maintenance to be carried out

on the daughter craft and prevents the underwater area becoming fouled when not in

use.

10

Resistance and Power Prediction

Importing the Maxsurf Modeller file which was created in to Maxsurf Resistance, the

resistance of the vessel for various forward speeds (also depicted by Froude number)

and consequent powering requirements are calculated. The software uses the slender

body method, and the form factor (1 + k) is calculated automatically for the hull using

the Molland formula and is found to be 1.594.

The calculated resistance and power graphs against speed and Froude number are

shown below.

Resistance vs Speed (& Froude Number)

11

Power vs Speed (& Froude Number)

Tabulating allows for a direct comparison of the required power and resistance of the

vessel for the range of speeds from 0 knots to 30 knots.

12

These values are for the towing resistance and power. Based on previous studies for

a SWATH a quasi-propulsion coefficient of 70% was chosen with a further seaway

Speed Resistance Power

Knots KN KW

0 -- --

0.75 0.09 0.03

1.5 0.4 0.31

2.25 0.85 0.99

3 1.58 2.43

3.75 2.18 4.2

4.5 3.25 7.52

5.25 4.97 13.41

6 6.25 19.3

6.75 10.83 37.61

7.5 12.17 46.95

8.25 11.46 48.66

9 15.81 73.22

9.75 24.59 123.34

10.5 34.16 184.53

11.25 42.23 244.43

12 48.23 297.71

12.75 52.09 341.64

13.5 54.67 379.7

14.25 56.55 414.56

15 57.69 445.16

15.75 58.77 476.17

16.5 59.94 508.76

17.25 60.83 539.8

18 62.1 575

18.75 63.29 610.49

19.5 64.78 649.88

20.25 66.5 692.79

21 68.14 736.17

21.75 70.12 784.55

22.5 72.09 834.4

23.25 74.5 891.04

24 76.74 947.46

24.75 79.12 1007.34

25.5 81.61 1070.59

26.25 84.26 1137.91

27 87.27 1212.14

27.75 90.07 1285.85

28.5 93.04 1364.14

29.25 96.04 1445.15

30 99.35 1533.25

13

margin of 10%. The seaway margin is lower than traditionally for a monohull as a

SWATH is less impeded by wave action. With the Margins applied the total required

shaft power at 25 knots was 1616kW.

Seakeeping Analysis

As a vital step in any vessel design process, a seakeeping analysis was performed on

the chosen hullform. This gave an indication of the vessels responses to differing

wave characteristics at various speeds.

Data input

This analysis was carried out for speeds from 0 to 30 knots, with an increment of 10

knots. The headings selected are head (180°), beam (90°) and stern/following (0°)

seas. The wave characteristics are defined using the ITTC (2 parameter) spectrum,

based on Sea States 4 (HS ≈ 1.25m-2.5m), 5 (HS ≈ 2.5m-4m) and 6 (HS ≈ 4m-6m) of

the World Meteorological Organization (WMO) sea state code. All of these values are

utilised as, whilst the given wave characteristic value is HS > 2m, the design brief

does not specify how large this value can become in these locations and so this

selection of data will provide results of the vessels seakeeping ability in all of these

sea states.

The vessel type is selected to be a catamaran and the spacing between the underwater

torpedo hulls is 5.24m. The pitch and roll radii of gyration are given in the Seakeeper

program as 25% of LOA and 50.91% of BOA, respectively for this type of vessel.

Also given is the VCG value, which is equal to 60% of the depth of the vessel.

Data output

The sea-keeping analysis is performed by Maxsurf Seakeeper and the results are

tabulated and represented pictorially by Microsoft Excel. The heave (m), roll(°) and

pitch (°) values for the three aforementioned headings are plotted for the three sea

states of increasing HS. These results can be found in Appendix C.

14

Motion Sickness Incidence

The MSI graphs (Appendix D) are provided for the differing speeds and sea states by

the software from the motions analysis, and is based on the ISO 2631 method. These

graphs, showing the severe discomfort limitations, are given by means of a graph of

RMS Acceleration versus Encounter Frequency.

RAO’s

The RAO’s for roll, pitch and heave against wave period are given in Appendix E.

This is again provided for the three selected headings and for the increasing speed of

the vessel from 0knots to 30knots with 10knot increments.

Also included in these graphs are the relative added resistances of the hull due to the

motions of the vessel and consequent change in wetted surface area.

Propulsion and Machinery selection

How much power we want

The total required installed propulsive power was 1616kW. The SWATH form lends

itself naturally to having twin engines; one in either lower hull. This provides several

benefits, including:

Redundancy in the case of one unit becoming inoperative. There is another

propulsor to ‘get you home’ which is vital when operating far offshore and

away from rescue.

Increased manoeuvrability. Having two propulsors gives far more

manoeuvring ability than a single engine. The large hull separation distance of

a SWATH magnifies this advantage which is useful for close-quarters

manoeuvring/holding station at a turbine.

However there are one or two drawbacks:

Reduced economy of scale. Using two smaller engines results in a smaller

overall fuel efficiency as loss are suffered in both engines.

15

Increased maintenance costs/downtime.

On the basis of using twin engines, the total installed power required in each was

1616/2 = 808kW.

Auxiliary power would be supplied by stand-alone generators. Based on data from

other SWATH designs a generation capacity of 15kW was sought. This would be

enough to run the on-board systems as well as provide power for the MaXccess crew

transfer vessel.

Green Credentials

Part of the design brief required that the vessel be “as green as possible and

economical to run”. To this end the suitability of a diesel-electric system was

investigated; the idea being that the propellers would be driven by electric motors in

turn powered by electrical power from either a diesel engine, a battery bank or the two

in tandem. The diesel engine can be used to provide power at moderate to high

speeds, although at lower speeds power could be supplied by the battery cells thus

avoiding the larger losses incurred when running an engine below its optimal loading.

This is especially useful during low-speed manoeuvring when the power drawn

undergoes large fluctuations. Extra power can be provided instantly from the batteries

and the losses due to constant revving of the engine are eliminated.

At sea the diesel engine is used to charge the battery; meaning the engine can be run

at a constant optimal loading. In this way the engine can always be run as efficiently

as possible. A further benefit would be the possibility of charging the batteries from

the mothership when the daughter craft is docked/stored there. Economies of scale

apply here as the electrical power generated by the much larger equipment aboard the

mothership would be more efficient in terms of fuel per kWh.

Existing Diesel-Electric Systems

There are mainly two companies can provide the system directly MAN B&W and

Rolls-Royce.

From Rolls-Royce there is the HSG system which has five running mode: Boost mode;

Diesel-electric mode; Parallel mode; Transit mode and shore connection mode.

16

The maximum output of this system is 8500 kW, which is well beyond the operational

needs of the SWATH in this study.

From MAN B&W we can find an example which can provide more than 10800 KW.

Again this is far in excess of the current requirements. However it may be possible to

commission a lower-powered system specifically for the current design. Therefore if

the design is pursued further hybrid power could be an option.

More traditional option

As mentioned above there is currently no example of a hybrid system fit for the

current design so it was decided on balance to seek a conventional medium speed

diesel engine for the prime movers.

The unit chosen was a MAN D2842 LE410 medium duty engine which has a

maximum output of 809kW, which almost exactly matches the required output.

Its details were as follows:

Make MAN

Model D2842

Variant LE410

Max Output(kW) 809

Length (m) 1.795

Breadth (m) 1.23

Height (m) 1.105

Dry Weight (kg) 1860

Max Consumption(l/hr) 202

Two generator were chosen for redundancy, with the intentional of running one at any

one time. The other would be in maintenance/standby mode. Therefore a generation

capacity would be retained in the event of one breaking down.

Make Kohler

Model 15EOZD

Max output (kW) 15

Length (m) 0.95

Breadth (m) 0.58

Height (m) 0.66

Wet Weight (kg) 308

Max Consumption (l/hr) 5

Voltage (V) 24

17

Crew transfer system

In order to transport the crew and the equipment safely we have to make sure that

there is no motion at all when people walking on the connection facility. Although the

ship is SWATH type which already decrease the motion a lot compare to other type of

boat but we still need a transfer system from the market.

The one is available called MaXccess-T12 sold by OSBITPOWER.

Specification:

18

General Arrangement

The traditional propulsion system is the first choice of us and as mentioned we need

two engines to supply the power. Consider the vessel has two parallel underwater

hulls we can use a parallel power configuration which means the each one of engines

will be mounted in one hull and of the consumption like fuel will equally separate.

The crew transfer system will be mounted on the deck and all of the systems will be

controlled by a power monitor in the control cabin.

Plane View:

Profile View:

19

20

Appendices

Appendix A: Design Brief

21

Appendix B: Catalogue of Comparable SWATH Designs

Name Type Length (m) Breadth (m) Draught (m) Disp (t) Power (kW) Speed (kts) Range (km)

USNS Able Military 72.0 29.0 7.6 3293 1200 9.6 N/A

CCGS Frederick G. Creed Hydrographic Survey 20.4 9.8 2.6 151.4 N/A 14 1610

USNS Impeccable Oceanic Survey 85.8 29.2 7.9 4870 3750 12 N/A

Kilo Moana Oceanic Survey 57.0 27.0 7.6 2507 N/A 15 15000

Planet Research 73.0 27.2 6.8 3500 N/A 15 N/A

Skrunda Class Patrol Boat 25.7 13.5 2.7 113 1627.5 20 1900 (12kts)

Sea Slice N/A 32.0 17.0 N/A 163 N/A 50 N/A

Tri Swath Turbine Transfer 27.2 10.5 2.4 N/A 1800 23 N/A

Sea Breeze Turbine Transfer 24.8 10.6 2.5 102.5 1800 18 N/A

Fob Swath Turbine Transfer 25.0 10.6 2.5 86 1800 23 (cat-mode) 1850 (cat-mode)

Cwhisper Turbine Transfer 19.5 7.8 1.5 30 915 23 N/A

Elbe Pilot Boat 49.9 22.6 5.9 1500 2720 14 N/A

Dose / Duhnen Pilot Boat 25.2 13.0 2.7 125 1580 20 N/A

25m SWATH A&R Pilot Tender 25.7 14.3 2.7 N/A 1420 18 N/A

Natalia Bekker Turbine Transfer 26.4 13.0 2.7 N/A 1800 18 N/A

Silver Cloud Pleasure Yacht 41.0 17.8 4.1 544 1640 12.5 N/A

Tabulated data for a variety of SWATH vessels with a large range of sizes.

y = 0.100x + 0.144R² = 0.924

0.01.02.03.04.05.06.0

7.08.09.0

10.0

0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0

Dra

ugh

t (m

)

Length (m)

Draught vs Length

y = 0.335x + 3.819R² = 0.924

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0

Be

am (

m)

Length (m)

Beam/Length

A very strong correlation can be seen between both length vs draught and length vs

beam. There is very little variation in the length to draught and length to beam ratios

which have respective mean values of 2.3 and 9.8.

22

0

500

1000

1500

2000

2500

3000

3500

4000

0 1000 2000 3000 4000 5000 6000

Po

we

r (k

W)

Displacement (tonne)

Power vs Displacement

0

1000

2000

3000

4000

5000

6000

0.0 20.0 40.0 60.0 80.0 100.0

Dis

pla

cem

en

t (t

on

ne

)

Length (m)

Displacement vs Length

23

Appendix C: Seakeeping Results

24

25

0

1

2

3

4

5

0 50 100 150 200

Met

re

Heading

Heave with SW5m

0 10 20 30

0

5

10

15

20

0 50 100 150 200

Deg

Heading

Roll with SW5m

0 10 20 30

0

5

10

15

20

25

0 50 100 150 200

Deg

Heading

Pitch with SW5m

0 10 20 30

26

Appendix D: MSI

Speed=0 Wave state 4:


27



28



29



30



31



32

Appendix E: RAO’s

Speed=0 heading=0

Speed=0 heading=90

33

Speed=0 heading=180

Speed=10 heading=0

34

Speed=10 heading=90

Speed=10 heading=180

35

Speed=20 heading=0

Speed=20 heading=90

36


Speed=30 heading=0

37

Speed=30 heading=90


38

39

Appendix F: Weight Details of SWATH

Item 1 item (l)/ (kg) No. Items Weight (kg)

Fuel 5075 2 8526

Waste 381 2 762

Gen Fuel 508 2 853.44

Lub Oil 200 2 180

F. Water 254 2 508

Tanks 10829.44

Engine 1860 2 3720

Gen 308 2 616

Transfer System 1500 1 1500

Structure (carbon fibre) 6000 1 6000

Fit-out 3000 1 3000

Provisons 500 1 500

Cargo 2500 1 2500

Total 1 28665.44

Ballast tanks Capacity (l) No. Items Capacity (tonne)

Ballast Fore 651 2 667.275

Ballast Aft 809 2 829.225

Ballast Strut 3750 2 3843.75

Total 10420 1 10680.5

Documents

Wind Farm Access Vessel