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# Optimizing Berthing of Crew Transfer Vessels against Floating Wind Turbines: A Comparative Study of Various Floater Geometries

Written By

Laurent Barthélemy

Submitted: November 25th, 2021 Reviewed: December 14th, 2021 Published: March 25th, 2022

DOI: 10.5772/intechopen.102012

From the Edited Volume

## Wind Turbines - Advances and Challenges in Design, Manufacture and Operation [Working Title]

Prof. Karam Youssef Maalawi

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## Abstract

Securing the return on investment for commercial floating wind farms by a proper estimate of the operation and maintenance (O and M) downtime is a key issue to triggering final investment decisions. That is why crew transfer vessel (CTV) weather stand-by issues should be assessed together with new floating wind floater concepts, to boost their cost attractivity. However, such issues as the numerical investigation of the landing manoeuvre of a service ship against a floater reveal complex to calculate. Based on similarities with seakeeping, we investigate various floater geometries. To estimate the weather limitations associated with each configuration. Most recent works find that calculation compares with 5% accuracy to an experiment from a test tank at a model scale. Method description: (A) Vessel seakeeping: (1) assess vessel responses (amplitude and phase angles) and (2) compare them with vessel responses of available publications, as a benchmark. (B) Vessel berthing: (1) model both vessel and floater, (2) account for the wave masking effect of existing floater designs, and (3) compare the ratio of wave vertical force over wave horizontal force and the grip coefficient at the interface between the vessel fender and the floater boat landing. Findings: The wave masking effect calculation for a square floater is cross-checked favorably with an existing demonstrator.

### Keywords

• operation and maintenance
• crew transfer vessel
• floating wind farm
• significant wave height
• wave period

## 1. Introduction

The development of floating wind farms implies the issue of offshore O and M workers safety. It is therefore of upmost importance to know the constraints and acceptable conditions for berthing a CTV.

For berthing with the “bump and jump” method, a CTV comes and pushes its fender against the boat landing ladder. The fender studied here is the stiff fender [1].

The present work relies on the results of a study performed by HSVA [2] and endeavors to meet the results obtained in its CTV model tank test. However, our approach here is different from HSVA numerical berthing calculations, which are more sophisticated.

## 2. Method description

### 2.1 General

The calculation is based on a simplified linear diffraction-radiation model applied in the frequential domain [1].

The studied ship is a CATamaran CTV (CAT CTV) [2, 3]: 27 m long, 8.2 m wide, twin hulls 3.2 m wide. It is modelled with Wigley hulls (Figure 1) [1].

The software used are GMSH for meshing [4] and NEMOH for hydrodynamics [5].

### 2.2 Loads of a unidirectional wave on CTV (seakeeping)

The Wigley hull, due to the wave excitation, moves in a vertical plane as follows (Figure 2) [1]:

 1 rotation against her floatation centre 1 degree of freedom θ (pitch) 1 translation against her original position O 2 degrees of freedom τx (surge)τz (heave)

The equations of dynamics are:

I+IaX¨+BẊ+KX=FexcitIX¨=FexcitIaX¨BẊKXIX¨=FextE1
 I=I110I150I330I150I55=m0mZG0m0mZG0IG+mZG2 Fexcit: vector of wave loadsIa: matrix of added inertia(both calculated by NEMOH [5]) B=BR+BV,BV=ΛH0−ΛH220b30−ΛH220ΛH33 b3≝2ρBCgH×bcλ≝Λωwith:Λ≝4ρCCdxmax+Hθmax/3πBR=radiation damping matrix(calculated with NEMOH [5])BV=linearised viscous damping matrix K=0000PBCG+P0−P−mgCG+mgB212H P = propeller thrust. If:P=m+maag/hωthenlimk→0xm/a=0

### 2.3 Loads of a unidirectional wave on CTV (berthing)

The friction coefficient without sliding is, about the principle of action and reaction (Figure 3) [1]:

f=Tangential forceatvertical wallNormal forceatvertical wall=TNf=Fextz/FextxE2

Assumptions:

1. A thrust P is added to N, in order never to reach N < 0:

P=Px=2GxavecP<0etG<0E3

2. For a low friction berthing:

ΣFextz=I33Z¨E4

The equations at the berthing point A = A- become therefore:

Fextz=I33Z¨andFextx+P=I11X¨+I15θ¨E5
f=I33Z¨I11X¨I15θ¨+Pf=Z¨X¨ZGθ¨+Gω2whereG=P/m1+CM1ω2E6

We define tT and tN respectively as the time phase corrections required to get the calculated loads T(t) and N(t) in phase with HSVA test results THSVA (t) and NHSVA (t) [2].

Z=ZmCOSωttT+φz
X=XmCOSωttN+φx
θ=θmCOSωttN+φθE7

We also define the following notations:

Ttanωt/2AZmcosωtT+φz,BZmsinωtT+φz,CXmcosωtN+φx+ZGθmcosωtN+φθ,DXmsinωtN+φx+ZGθmsinωtN+φθf=AT22BT+A/CGT22DT+C+GE8

In order for the function f(t) to get relative extremes, the numerator of the quotient in Eq. (8) must have a positive discriminant δ′:

Moreover, the denominator in Eq. (8) must never be null. Physically that means that the CTV propeller thrust P should be great enough in order never to get N < 0. Mathematically that implies both that its discriminant Δ′ must be negative and that GC<0:

<0andGC<0P>mω2D2+C2E10

If the conditions (3) and (4) are met, then, over a wave period, the friction coefficient will reach its extremes at the instants t+ and t, which correspond to the following values T+ and T:

Then the maximum friction coefficient over a wave period is:

fmaxT=maxfT+fTE12

We must therefore choose. Physically, that is the CTV surge over which the CTV captain has the time to adjust the propeller thrust P, in order for the fender never to lose contact with the boat landing. Nevertheless, the CTV is limited by her maximum thrust Pmax. We infer that:

P=minmGω2PmaxE13

Or, in other words:

G=minLPmax/mω2E14

The selected criterion for CTV boarding at the berthing point is that the friction coefficient must never exceed the grip factor:

fmaxT<fgripwithfgrip=0,8rubberverywetsoil6E15

## 3. CTV against monopile

For benchmarking purpose, the first calculation models the “bump and jump” against a monopile (Figures 1 and 4). The water depth is 29 m.

The studied monopile has a 5 m diameter [2].

It may be noted that the CAT CTV is wider than the monopile: therefore the former is not masked from the waves by the latter.

Figure 5 compares for 2 m significant wave height (Hs) the calculated ratio of wave vertical force over wave horizontal force (T/N) with the grip coefficient of rubber against very wet soil (fgrip) [6].

For comparison reference [7] estimates the berthing limit to be for a ratio wavelength over boat length of 1.85 (λ/B): both results meet with 5% accuracy.

## 4. CTV against 13 m diameter cylindrical floater

The second calculation models the “bump and jump” against a planned [8] cylindrical wind turbine floater (Figures 6 and 7). The water depth is 70 m [8].

The studied cylinder has [8]: a 13 m diameter, 14 m draft, 2000 T displacement.

This time, the floater masks the CAT CTV from the incidental waves, therefore the horizontal incident wave loads are masked, while the vertical incident wave loads are only the ones passing below the floater keel.

Figure 8 compares for 2 m Hs the calculated ratio T/N with fgrip versus λ/B.

This time berthing may take place for 2 m Hs whatever the wavelength.

## 5. CTV against 41 m diameter cylindrical floater

The third calculation models the “bump and jump” against a cylindrical wind turbine floater (Figures 9 and 10). The water depth is 23 m.

The studied cylinder has: a 41 m diameter, 7 m draft, 9300 T displacement.

One more time, the floater masks the CAT CTV from the incidental waves, therefore the horizontal incident wave loads are masked, while the vertical incident wave loads are only the ones passing below the floater keel.

Figure 11 compares for 2 m Hs the calculated ratio T/N with fgrip versus λ/B.

This time berthing may take place for 2 m Hs whatever the wavelength.

## 6. CTV against 36 m side parallelepipedal floater

The fourth calculation models the “bump and jump” against a parallelepipedal wind turbine floater (Figures 12 and 13). The water depth is 23 m.

The studied cylinder has: a 36 m sides, 7 m draft, 9300 T displacement (same draft and displacement as in Section 5).

One more time, the floater masks the CAT CTV from the incidental waves, therefore the horizontal incident wave loads are masked, while the vertical incident wave loads are only the ones passing below the floater keel.

Figure 14 compares for 2 m Hs the calculated ratio T/N with fgrip versus λ/B.

This time berthing may take place for 2 m Hs whatever the wavelength.

## 7. CTV against FLOATGEN floater

The fifth calculation models the “bump and jump” against an existing [9] square hollow floater (Figures 15 and 16). The water depth is 23 m [10].

The studied square has [9, 10]: 36 m side, 7 m draft, 6000 T displacement (same draft as in Sections 5 and 6).

Once again, the floater masks the CAT CTV from the incidental waves, therefore the horizontal incident wave loads are masked, while the vertical incident wave loads are only the ones passing below the floater keel.

Figure 17 compares for 2 m Hs the calculated ratio T/N with fgrip versus λ/B.

One more time berthing may take place for 2 m Hs whatever the wavelength.

## 8. Results

Table 1 sums up the Hs found for berthing to occur whatever the wavelength.

CaseFloater geometryDepthMaximum Hs for berthing
15 m diameter monopile23 m1.5 m (no masking)
213 m diameter cylindrical floater70 m2.1 m
341 m diameter cylindrical floater123 m2.5 m
436 m side square floater123 m2.6 m
536 m side square hollow floater (FLOATGEN)23 m2.3 m

### Table 1.

CAT CTV berthing limits for monopile various floater designs.

Those floaters have been chosen to have same displacement and draft.

## 9. Interpretations

### 9.1 General

FLOATGEN 2.3 m Hs berthing limit calculations compare precisely with feedback from offshore site test with full scale prototype [11]:

“Transfer up to 2.3 m significant wave height with no motion compensation”.

From Table 1, it may be noted that berthing limits are influenced by:

• Hardly by floater geometry if they have the same displacement.

• Significantly by their displacement: the greater the displacement, the greater the berthing limit.

It is possible to propose an approximative analytical of the berthing limit due to the floater masking effect. Indeed, the calculated ratio T/N is (see Eq. (8)):

fT±=AT±2+2BT±ACGT±2+2DT±C+GE16

where:

T±tanωt±/2,AZmcosωtT+φz,BZmsinωtT+φz,CXm,cosωtN+φx+ZGθmcosωtN+φθ,DXmsinωtN+φx+ZGθmsinωtN+φθ.G=minLPmax/mω2Pmax=maxm+maag/hωCTVmaximum thruster forceE17

At the large wave periods we have the following behavior:

limω0fmaxT=maxlimω0fTlimω0fT+E18

Where (see Eq. (11)):

In the case of box barge of same displacement, length and draft, we have:

### 9.2 Surge amplitude at large wave periods

xma=ηω3Imϑω2+Reκω2+ζω4+Reϑω2+Imκω+Reμ2αω6+γω4+εω22+βω5+δω3+ϵω2ReκK55b3+ΛωH3/3k3Fxm/a+ΛωH2/2k3Mym/aImκ+ΛωH2/2PFzm/aReμ+P2+K55k3Fxm/aεm+maP2b3K55ΛHωk3m+maK55+ΛωH22/12ϵΛHωP2+k3K55K55gGC+gB2/12H+KZA+2k3/Hglimk0xma=limω0Reκω2+Imκω+Reμ2εω22+ϵω2limω0Fxm/a=Oω3limω0Mym/a=limk0m+maH2B212Hghω+m+maghωZa++Oω3limω0ImFzm/a=g/Hsinhkz0+h/sinhkh+OωE20

Note: since the vertical incident wave loads are only the ones passing below the floater keel then, if its draft is |z0|, zm/a = zm(z = z0) = sinh[k(z0 + h)]/sinh(kh) [12] (refer to equations written in Figures 6,9,12, and 15).

Therefore:

limω0Reκ=limω0Oω3+ΛH22k3m+maZa++H2B212Hghω2limω0Imκ=limω0Oω2
limω0Reμ=limω0Oω2+K55k3Oω3limω0ε=limω0k3m+maK55+Oωlimω0ϵ=limω0ΛHωP2+k3K55limk0xma=limω0Oω32+Oω3+Oω2+K55k3Oω32k3m+maK55ω22+ΛHOω2+k3K55ω22limk0xm/a=OωE21

### 9.3 Surge phase angle at large wave periods

cosφx=+ηω3Imϑω2+Reκωαω6+γω4+εω2+ζω4+Reϑω2+Imκω+Reμβω5+δω3+ϵω/αω6+γω4+εω2ι2+βω5+δω3+ϵω2/ηω3Imϑω2+Reκω2+ζω4+Reϑω2+Imκω+Reμ2sinφx=+ηω3Imϑω2+Reκωβω5+δω3+ϵωζω4+Reϑω2+Imκω+Reμαω6+γω4+εω2+ι/αω6+γω4+εω2ι2+βω5+δω3+ϵω2/ηω3Imϑω2+Reκω2+ζω4+Reϑω2+Imκω+Reμ2cΛΛH/m+malimT+cosφx=cΛ/1+cΛ2limT+sinφx=1/1+cΛ2E22

### 9.4 Heave amplitude at large wave periods

zma=ImΓω4ImΠω2+ReΣω2+ImΞω3+ReΠω2+ImΣω2αω6+γω4+εω22+βω5+δω3+ϵω2ReΠIGA15PFxm/am+maPMym/aImΠm+maK55+B11B55B152ImFzm/aReΣB15PFxm/a+B11PMym/aImΣKB55+K55B11ImFzm/a
IGA15mzGma1H/2B11λHB15λH2/2B55ΛωH3/3limk0zm/a=sinhkz0+h/sinhkhE23

### 9.5 Heave phase angle at large wave periods

cosφz=ImΓω4ImΠω2+ReΣωαω6+γω4+εω2ImΞω3+ReΠω2+ImΣωβω5+δω3+ϵω/ImΓω4ImΠω2+ReΣω2+ImΞω3+ReΠω2+ImΣω2/αω6+γω4+εω22+βω5+δω3+ϵω2sinφz=+ImΓω4ImΠω2+ReΣωβω5+δω3+ϵω+ImΞω3+ReΠω2+ImΣωαω6+γω4+εω2/ImΓω4ImΠω2+ReΣω2+ImΞω3+ReΠω2+ImΣω2/αω6+γω4+εω22+βω5+δω3+ϵω2limT+φz=0°E24

### 9.6 Pitch amplitude at large wave periods

θma=ReDω3+ReEω2+ReFω2+ImCω4+ImEω2+ImFω2αω6+γω4+εω22+βω5+δω3+ϵω2ReEm+maPImFzm/a=m+maPg/HImEIGA15m+maZa+k3P/aReFk3B15Fxm/ak3B11Mym/aImFB11PImFzm/alimk0θma=limω0m+mag/hωK55H2+Za+/m+maB212H2+a2limk0θm/a=0E25

### 9.7 Pitch phase angle at large wave periods

cosφθ=+ReDω3+ReEω2+ReFωαω6+γω4+εω2+ImCω4+ImDω3+ImEω2+ImFωβω5+δω3+ϵω/ReDω3+ReEω2+ReFω2+ImCω4+ImDω3+ImEω2+ImFω2/αω6+γω4+εω22+βω5+δω3+ϵω2sinφθ=+ReDω3+ReEω2+ReFωβω5+δω3+ϵωImCω4+ImDω3+ImEω2+ImFωαω6+γω4+εω2/ReDω3+ReEω2+ReFω2+ImCω4+ImDω3+ImEω2+ImFω2/αω6+γω4+εω22+βω5+δω3+ϵω2
limT+cosφθ=11+Za+/a1/2B2/12H2_1+Cm1H/a2limT+sinφθ=Za+/a1/2B2/12H2_1+Cm11+Za+/a1/2B2/12H2_1+Cm1H/a2φx1Za+/a+1/2B2/12H21+Cm1H/alimω0cosφθ=1/1+φx12limω0sinφθ=φx1/1+φx12E26

### 9.8 Friction coefficient at large wave periods

Therefore:

limω0A=limω0asinhkz0+h/sinhkhcos360°=az0+h/h,limω0B=limω0asinhkz0+h/sinhkhωtT=Oωlimω0C=OωcΛ/1+cΛ2+ZGOω1/1+φx12=Oω,limω0D=Oω1/1+cΛ2+ZGOωφx1/1+φx12=Oωlimω0G=Llimω0T±=limω0ALA2+Oω2L2AOωOωOω2AOωOωOω+OωL+OωLlimω0T±=limω0ALAL+Oω2/Oωlimω0T=LIMω0az0+h/hLaz0+h/hL+Oω/Oω=E27
limk0fT=A20+A020L20+0+L02=az0+h/hLlimω0T+=LIMω0AL+AL+Oω2/Oω=Oω=0limk0fT+=A02200+A0L022D0+0+L=az0+h/hLE28

Eventually, we get the following formula for berthing to possible [1]:

limk0fT±=az0+h/h/L<fgripE29
iffgrip=0,8rubberverywetsoilrefer toEq.9

Table 2 sums up the analytical Hs found for berthing to occur whatever the wavelength, by using Eq. (29).

CaseFloater geometryDepthMaximum Hs for berthing (Hs = 2a)1
15 m diameter monopile23 m1.5 m (no masking)
213 m diameter cylindrical floater70 m2.0 m
341 m diameter cylindrical floater23 m2.3 m
436 m side square floater23 m2.3 m
536 m side square hollow floater (FLOATGEN)23 m2.3 m

### Table 2.

CAT CTV analytical berthing limits for monopile various floater designs.

The CAT CTV fender length is L = 1 m [1].

As can be seen results from Tables 1 and 2 meet with less than 12% discrepancy. The reason why is that the analytical calculation does not account for the diffraction forces.

Eventually, some practical considerations must be accounted for (see Figure 18):

The more the wave direction varies, the more a large floater width becomes necessary to allow berthing by masking the waves.

## 10. Conclusions and recommendations

The present study results find that berthing a CTV against an offshore wind turbine is not yet optimized, at least from a marine maintenance point of view: most designs do not provide sheltered waters.

Some proposals focus on improving the CTV:

• ESNA promote the use of Surface Effect Ships (SES), to minimize their heave. However, the lack of commercial success of the solution seems related to the high fuel consumption of such boats [7].

Other proposals focus rather on the floater side:

• IDEOL have designed a pontoon-like floater which provides sheltered waters. At best that solution will prove successful for floating wind farms. Obviously, that is not a solution for fixed wind farms [11].

• FLOATING POWER POINT propose a floating wind turbine combined with a wave energy convertor (WEC), to provide an artificial harbor downstream, thanks to the wave energy extracted by the WEC [13].

Another possible axis of development would be to design an additional wall to existing boat landings, providing a sheltered water.

Eventually, since the present study only applies to a unidirectional wave, the next studies will focus on multidirectional waves, in order represent a more realistic sea state.

## Acknowledgments

The author gratefully acknowledges the support from ENSM and its director of research and industrial relations, Mr. Dominique FOLLUT, for throughout the present research work.

## Acronyms and abbreviations

 Cat Catamaran CTV crew transfer vessel DP dynamic positioning Hs significant wave height HSVA Hamburgische Schiffbau-Versuchsanstalt GmbH (Hamburg Ship Model Tank Test Facilities) O&M operation & maintenance RAO’s response amplitude operators 3D three dimensional 2D two dimensional WT wind turbine O, x, y, z absolute reference frame M operating point fixed to the CTV ρ water specific gravity h water depth B ship length H ship draft m (or Δ) ship mass (or displacement) G ship centre of gravity IG ship inertia at G C ship centre of buoyancy I matrix of ship own inertia Ia matrix of ship added inertia K matrix of stiffnesses B matrix of dampings Fexcit vector of wave loads λ wavelength A- berthing point O, X, Y, Z reference frame attached to the ship F centre of floatation g gravitational acceleration xG, zG coordinates of ship gravity centre xC, zC coordinates of ship buoyancy centre b3 ship heave damping factor k3 ship vertical hydrostatic stiffness Cd shipdrag coefficient Cm1 added mass coefficient in x direction ma added mass in x direction X vector of ship motions τx ship surge τz ship heave θ ship pitch angle T regular wave period k wave number a wave amplitude (half crest to through) x (or bc) flat rate heave damping coefficient (%-age of critical damping) xmax or τxm max. surge estimate (for quadratic damping force) ω wave pulsation θmax adimensional max. pitch estimate (for quadratic damping force) Λ 4ρCCdxmax+Hθmax/3π cΛ ΛH/m+ma G CTV surge over which the CTV captain has the time to adjust the propeller thrust P, in order for the fender never to lose contact with the boat landing xm max. CTV surge zm max. CTV heave θm max. CTV pitch φx max. CTV surge φz max. CTV heave φθ max. CTV pitch L fender length Za+ CTV propeller elevation Z0 floater keel elevation

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Written By

Laurent Barthélemy

Submitted: November 25th, 2021 Reviewed: December 14th, 2021 Published: March 25th, 2022