Crew Transfer Vessel (CTV) Performance Plot (P-Plot ... · Crew Transfer Vessel (CTV) Performance...

Research Project Summary

June 2017

Crew Transfer Vessel (CTV) Performance Plot (P-Plot) Development

Notice to the Offshore Wind Energy Sector

SUMMARY

This R&D Summary describes the results of research commissioned by the Carbon

Trust in order to better understand the performance of fast crew transfer vessels

(CTV). The work focussed on establishing the typical operational availability of CTV

with respect to increasing sea-state, primarily during their transit mode (voyage from

port to wind-farm and back) and transfer mode (push-on, step-across, transfer of

personnel to/from the CTV and wind-turbines). The objectives were: a. to provide wind-

farm developers with a better understanding of CTV performance (for contracting and

O&M modelling purposes) and b. to provide the industry in general with a more

detailed understanding of the factors that limit CTV operations and c. to establish a

benchmark performance of typical CTVs in the sector (to encourage improvements in

CTV performance). The results of the research are being widely disseminated.

1. Background

1.1 Development of offshore wind-farms has led to the need for specialist vessels to

transfer workforce and equipment to and from the wind-turbines during both the

construction and the operation & maintenance phases.

1.2 Conventional workboats were initially used for this purpose but more specialist

vessels quickly developed – increasing in size and with a trend towards catamaran hull

forms. However, their performance characteristics varied considerably and were, in

general, limited to operations in sea conditions with significant wave heights up to about

1.0 metre to 1.5 metres. Even then, it was not possible to be sure what the performance

of the vessels would be until they were put into operation.

1.3 This situation led wind farm developers to consider the need for a better

understanding of vessel performance, not only to improve the relationship between the

charter rates of CTVs and their operational capabilities, but also to increase vessel

availability in the more onerous environmental conditions.

1.4 The ensuing research was managed by the Offshore Wind Accelerator (OWA)

programme within the Carbon Trust, represented by Dong Energy, EnBW, EON,

Mainstream, RWE, Scottish Power, SSE, Statkraft, Statoil and Vattenfall. The research

project was undertaken DNV GL (Kema) and Seaspeed Marine Consulting Ltd, an

independent research organisation.

1.5 It is important to note that performance is used here to describe primarily vessel

motion characteristics, speed and ability to transfer personnel, in different sea

conditions and at different headings. It does not cover issues such as vessel

construction, system engineering, fuel economy or manoeuvrability.

2. Research Programme

2.1 The vessel performance research was undertaken in six main stages as follows:

a. Development of a standardised sea trial programme (Reference 1) and the

subsequent undertaking of sea trials on a range of CTVs.

b. An assessment of the CTV industry (2015/16) to establish the nature of the craft

being used and what the designs of their successors were likely to be like, along

with an assessment of the factors that limited vessel operations, such as vessel

motion and fender slip, and their associated acceptability threshold values.

c. Development of a range of baseline hull form designs representative of the

industry (2015/16), covering different hull forms (catamarans, monohulls and

Swath craft), vessel sizes (18, 22 and 26 metre lengths) and propulsion systems

(waterjets and propellers). The principal particulars of these baseline hull forms

are presented in Figure 1.

d. Computer simulations of these baseline designs in a range of environmental

conditions, to establish their likely performance and limitations in the transit and

loiter modes of operation.

e. Free-running scale model tests of these baseline designs to establish and

understand their performance characteristics in their transit, loiter and transfer

modes of operation across a range of environmental conditions. The transfer

mode was assumed to be a conventional push-on, step-across, rubber bow-

fender arrangement.

f. Consolidation of the results from the sea trials, computer simulations and free-

running model tests, along with acceptability criteria, into CTV benchmark

performance plots.

2.2 Whilst undertaking this research, the influence of a wide range of vessel and

environmental parameters were investigated, and in particular the mechanism of fender

slip in the Transfer mode was studied in detail. The more important findings from this

research are discussed in Annex A to this report.

3. P-Plot Development

3.1 The wind farm developers, through the Carbon Trust OWA research programme,

required a simple but realistic presentation of vessel performance, benchmarking the

relevant performance representative of the industry at the time. This requirement led to

the selection of what is often referred to as an operability diagram (referred to here as a

performance plot or P-Plot) to present this information. These diagrams provide the

approximate maximum speed and/or seastate below which the various acceptability

criteria concerned with transit or transfer are not compromised. The acceptability criteria

used in this project are taken from Reference 2 and presented in Figure 2.

3.2 Seastate is defined by significant wave height and wave period, these being the

primary defining parameters. In terms of the associated wave spectrum, it has been

assumed to be represented by the JONSWAP (Joint North Sea Wave Project)

spectrum. The sea-state data used for this research is presented in Figure 3.

3.3 With over 90% of CTVs being catamaran craft with a load line length of less than 24

metres, the majority of work was focussed on catamaran craft across three different

sizes (18, 22 and 26 metre craft).

3.4 The Transit and Transfer P-Plots are presented in Figures 4 to 9 and represent the

performance benchmark (based on the catamaran hull form). For clarity, the Transit P-

Plots are presented for individual significant wave heights (Hsig). The Transfer P-Plots,

having fewer variables, can accommodate all the main variables on one diagram.

3.5 Comparison of the performance of new or existing craft with the benchmark P-Plots

is possible either from performance predictions or from the results of sea trials.

Guidance on making such a comparison is presented References 1 and 2.

4. Commercial Implementation

4.1 The results of this research programme should assist wind-farm developers, and

this industry sector in general, in assessing the performance and operational availability

that can be expected from CTVs. A benchmark performance level has been established

along with an improved understanding of the influence of various design and operational

parameters on the performance and limitations of these craft.

4.2 For wind-farm developers, this provides for improved O&M modelling and, for

planning and contracting purposes, a better understanding of the operational availability

of these craft.

4.3 It is expected that for CTV designers, owners and operators, the benchmark

performance and associated technical information will assist in the development of more

capable and cost effective vessels and operational procedures.

4.4 In terms of contracting for CTVs it is becoming more common for developers to

require some form of prediction of vessel performance to compare with this benchmark

prior to contract (particularly for new or novel craft), and for the vessel performance to

be monitored in order to establish the achieved performance. With respect to vessel

monitoring it is intended that performance will be assessed over the longer term rather

than from a single sea trial.

4.5 It has been established that during the transfer mode, the monitoring of fender

forces is likely to become a high priority with respect to assessing the confidence of safe

transfer of personnel. Such measurements allow assessment of fender friction

(accounting for fender material properties and surface conditions) and reductions in

bollard thrust (due to wave forces, propulsor ventilation etc). Fender force

measurements may also be used to assess docking impact loads, including the benefits

of resilient fender arrangements.

4.6 It should be noted that the benchmark performance P-Plots do not directly account

for specific issues of tidal current, very shallow water or the effects of local topography

and these may need to be taken into account in any final assessment process. It should

also be understood that some vessels will perform above or below the benchmark and

that the P-Plots will be used as guidance rather than as a definitive standard.

5. Conclusion

5.1 This research programme resulted in the development of a benchmark of CTV

performance and a significantly improved insight into the variation in performance of

these craft with respect to vessel type, size, freeboard, propulsion system and bollard

thrust. It also provided a detailed understanding of the mechanism of fender slip during

the transfer mode, a parameter clearly at the heart of transfer safety.

5.2 It is intended that the results of this research will be used by wind farm developers

to improve their economic modelling processes and commercial contracting

arrangements.

5.3 It is hoped that by disseminating these findings, the industry as a whole will benefit

in terms of improved CTV design, operation and safety, leading ultimately to a reduction

in the overall cost of offshore wind energy.

6. Figures

Figure 1 - Baseline CTV Principal Particulars

18 metre Hull Forms

Monohull Catamaran (jet) Catamaran (prop) Swath

Waterline length 16.0 m 16.0 m 16.0 m 16.0 m

Overall beam 4.76 m 6.8 m 6.8 m 6.8 m

Hull beam 4.49 m 1.86 m 1.87 m 1.68 m

Hull CL separation n/a 4.4 m 4.4 m 5.12 m

Draft 1.19 m 1.07 m 1.1 m 1.68 m

Hull block coefficient 0.456 0.611 0.583 n/a

Strut width n/a n/a n/a 0.52 m

Displacement 40.0 t 40.0 t 40.0 t 40.0 t

LCG 6.69 m 6.89 m 7.19 m 8.4 m

VCG 1.92 m 1.92 m 1.92 m 2.05 m

Bow freeboard 2.33 m 2.29 m 2.26 m 2.4 m

Stern/wet-deck freeboard 1.53 m 1.49 m 1.46 m 1.76 m

Pitch gyradius 4 m 4 m 4 m 4 m

Roll gyradius 1.428 m 2.04 m 2.04 m 2.04 m

Bollard thrust, 85% MCR 7 t 6 t 7 t 7 t

Operational speed 23 kts 23 kts 23 kts 23 kts

22 metre Hull Forms



Overall beam 6 m 8.5 m 8.5 m 8.5 m

Hull beam 5.59 m 2.3 m 2.3 m 2.1 m


Draft 1.4 m 1.18 m 1.23 m 2.1 m


Strut width n/a n/a n/a 0.65


LCG 8.86 m 8.6 m 9.04 m 10.5 m

VCG 2.4 m 2.4 m 2.4 m 3.33 m

Bow freeboard 3 m 3 m 3 m 3 m

Stern/wet-deck freeboard 2 m 2 m 2 m 2.2 m





26 metre Hull Forms



Overall beam 7.14 m 10.2 m 10.2 m 10.2 m

Hull beam 6.67 m 2.71 m 2.73 m 2.52 m


Draft 1.45 m 1.21 m 1.28 m 2.52 m


Strut width n/a n/a n/a 0.78 m


LCG 10.15 m 10.32 m 10.97 m 12.67m

VCG 2.88 m 3.84 m 3.84 m 5.26 m

Bow freeboard 3.83 m 3.83 m 3.76 m 3.60 m

Stern/wet-deck freeboard 2.63 m 2.63 m 2.56 m 2.64 m





Figure 2 - CTV Performance Acceptability Criteria

Transit Acceleration and Motion Limits

Vertical acceleration, rms 1.5 m/s2 (approx. 0.15 g)

Lateral acceleration, rms 1.0 m/s2 (approx. 0.1 g)

Pitch, rms 5 deg

Roll, rms 6 deg

Transfer Motion Limits

Friction limit 95% waves pass with no slip above 300mm (or one ladder rung)

Roll limit, rms 3 deg

Freeboard limit 95% of waves below the average* freeboard

Note * average freeboard is the average of the wet-deck freeboard and the bow freeboard. This parameter is used for computer assessments of performance and is not expected to be used on sea trials assessments.

Figure 3 - Typical UK Sea Statistics

Sea Area Category

Limited Fetch (short period on P-Plots)

Exposed (standard period on P-Plots)

Ocean (long period on P-Plots)

Hsig, m

% Exceed’nc

Average Modal Period To, sec

Spread of

Modal Period To, sec

% Exceed’nc


Spread of Modal Period To, sec

% Exceed’nc


Spread of Modal Period To, sec

0.5 86 4.5 3.5 – 5.5 97 5.0 3.5 – 6.5 98 6.5 4.5 – 8.5

1.0 53 5.0 4.0 – 6.0 80 5.5 4.0 – 7.0 86 7.0 5.0 – 9.0

1.5 31 5.5 4.5 – 6.5 59 6.0 4.5 – 7.5 67 7.5 5.5 – 9.5

2.0 17 6.0 5.0 – 7.0 42 6.5 5.0 – 8.0 51 8.0 6.0 – 10.0

2.5 9 6.5 5.5 – 7.5 30 7.0 5.5 – 8.5 39 8.5 6.5 – 10.5

3.0 5 7.0 6.0 – 8.0 21 7.5 6.0 – 9.0 29 9.0 7.0 – 11.0

Figure 4 - Transit P-Plot for 18 metre CTV



Figure 7 - Transfer P-Plot for 18 metre CTV

7. References

Ref 1. Conduct of offshore access performance evaluation trials, OWA-S2-A-Y2-1 October 2015

Ref 2. Derivation and Presentation of Offshore Access Performance Plots, OWA-S2-A-

Y2-2, October 2015 More Information

Offshore Wind Accelerator

Carbon Trust

4th Floor, Dorset House

27-45 Stamford Street

London

SE1 9NT

Tel : +44 (0) 20 7170 7000

Email : [email protected]

Web : www.carbontrust.com/offshorewind

© Copyright : The Carbon Trust

ANNEX A - SUMMARY OF FINDINGS FROM CTV PERFORMANCE RESEARCH

A.1 Introduction

Whilst undertaking this research, a wide range of influencing factors were investigated

for both the transit and transfer modes of CTV operations. In particular, the issue of

fender slip during the transfer mode was studied in detail. The more important findings

from this research are discussed in outline below.

Performance is presented in the form of performance plots (P-Plots). These diagrams

provide the approximate maximum speed and/or seastate below which the various

acceptability criteria concerned with transit or transfer are not compromised.

A.2 Effect of Vessel Type

Three vessel types were investigated within this research project – a catamaran,

monohull and Swath craft: all were propeller powered. An additional model of a waterjet

powered catamaran was also tested. All vessel designs had the same length and

displacement.

It was found that the catamaran and monohull vessels exhibited very similar

performance characteristics with respect to transit, loiter and transfer, with the monohull

having slightly greater roll in beam seas. The difference between the propeller and

waterjet powered catamaran was negligible apart from some instances in the transfer

mode (primarily head and stern sea conditions) when the waterjet ventilated more

frequently thus increasing the slip frequency. Whilst manoeuvring onto the docking

poles was not investigated in detail, it is generally accepted that waterjet powered craft

are better suited in this respect. However it was noted that waterjet powered craft

generally had lower bollard thrust for a given engine power and thus the transfer

performance would be affected as described in A.4 below.

As can be seen from Figure A.1 and A.2, the Swath craft was seen to perform

particularly well in most conditions of transit and transfer.

In the transfer condition the Swath design exhibited a rather different limiting condition

to conventional catamarans, in which the bow did not so much slip (as predicted by the

Friction Line) but came away from the docking poles due to a loss in net horizontal

thrust – in stern seas due to propeller ventilation and in bow seas due to a strong effect

of wave orbital motion. This is represented in Figure A.2 by the free-fender test result

line (also calculated at a 95% confidence limit).

It is acknowledged that different hull forms and different vessel types – such as semi-

swath hull forms, surface effect craft, trimarans etc – will have a performance that is

different to that presented in the P-Plots. However, the catamaran P-Plot provides a

performance benchmark which is representative of the vast majority of the CTV fleet

and can be used to demonstrate benefits, or otherwise, of different designs.

Figure A.1 - Transit P-Plots for 22 metre Monohull, Catamaran and Swath

Figure A.2 - Transfer P-Plots for 22 metre Monohull, Catamaran and Swath

Monohull Catamaran

hull SWATH

A.3 Effect of Vessel Size

As expected for the transit and loiter modes, it was found that the larger the vessel the

better the performance in terms of motion characteristics across the range of sea

conditions. Clearly there are aspects of the design that may have a secondary

influence, such as freeboard, but in general, the larger the vessel the better the

performance.

However, for the transfer mode it was found that the ability to remain pushed onto the

docking poles was not primarily determined by vessel size. In general, the larger the

vessel the greater the forces acting to separate the vessel from the docking poles,

although this was very dependent on freeboard since once the forward wet-deck is in

contact with the wave crest then the vessel generally experiences bow fender slippage.

It was found that the main determining factors with respect to transfer performance were

bollard thrust, freeboard and propulsor ventilation.

In general, the larger the vessel the larger the freeboard, bollard thrust and propulsor

submergence and so as long as the increase in these factors was related beneficially to

the increased wave forces imposed on the vessel, then the transfer performance was

improved. At the time of the study, it was found that the relationship between vessel

size, freeboard and bollard thrust was most appropriately aligned for existing craft of

about 22 metres in length.

For small vessels, their freeboard was generally the limiting factor and their roll

characteristics often limited transfer performance in beam sea conditions. For the large

vessels, their bollard thrust was generally their limiting factor since they had sufficient

freeboard as a consequence of their size. Relevant performance plots can be seen in

Figures 3 to 8 of the main report.

A.4 Effect of Bollard Thrust

As noted in Section A.3 above, bollard thrust was found to be a primary driver in terms

of parameters affecting the ability of the craft to remain pushed onto the docking poles

in rough weather. This is because fender slip occurs when the fender friction force is

exceeded by wave induced forces on the vessel - and fender friction force is primarily a

function of bollard thrust and the fender coefficient of friction.

Interestingly, the Swath design was in general found not to slip, but in the extreme, to

experience longitudinal wave induced forces in excess of the bollard thrust and thus to

momentarily come away from the docking pole, rather than slip.

An example of the effect of bollard thrust on the operational envelope for transfer is

shown in a modified P-Plot in Figure 3.

Figure A.3 - Transfer P-Plot for 22m catamaran showing effect of bollard thrust

A.5 Effect of Freeboard

As noted in Section A.3 above, freeboard was found to be a primary driver in terms of

parameters affecting the ability of the craft to remain pushed onto the docking poles in

rough weather. This is because the wave induced forces on the vessel increase

dramatically once the forward wet-deck is in contact with the wave surface.

Associated with this was the finding that the ‘effective’ freeboard could be increased

somewhat by pushing onto the docking poles with a bow-up attitude, providing a slightly

increased operational envelope.

An example of the effect of freeboard on the operational envelope for transfer is shown

in a modified P-Plot in Figure 4.

Figure A.4 - Transfer P-Plot for 22m catamaran showing effect of freeboard

A.6 Effect of Vessel Speed

Clearly this is relevant to the transit and loiter modes only since during transfer, the

vessel is stopped.

For the catamaran and monohull craft the comfort level on board the vessel were very

similar and generally deteriorated with increasing speed, certainly in the head and bow

sea headings, as can be seen in Figure A.5 below.

Increasing speed not only affects vertical accelerations in head and bow seas but also

leads to other limiting factors such a slamming (generally in head and bow seas) and

broaching and/or deck diving (generally in stern quartering and following seas). These

limits are less straightforward to predict but are well known to vessel operators and

generally lead to voluntary speed reductions to maintain safe operation.

Figure A.5 – Vertical acceleration rms of 22m monohull showing effect of speed

A.7 Effect of Sea-State

For all modes of operation and all headings, the motion characteristics increased with

increasing sea-state for any given vessel speed. Whilst the description of sea-state is

often limited to a value of significant wave height (which is related to the total energy

within the sea spectrum) the associated wave period (related to the distribution of this

energy across a frequency range) can, in some circumstances, be almost as influential

with respect to vessel motions, particularly in transit conditions.

For the vessel size range of 18 to 26 metres and the sea-states of interest, the head

and bow quartering transit motion characteristics of the catamaran and monohull

increase almost linearly with significant wave height and to a slightly lesser extent with

an increase in wave frequency.

For transfer operations, the maximum wave height (as opposed to significant wave

height) is often stated as being the parameter of interest, since it is the maximum waves

which can be seen to cause fender slip. Whilst the relationship between maximum wave

height and significant wave height is relatively well defined for fully developed sea

spectra, it is acknowledged that for partially developed conditions (typical of coastal

conditions) it is less straightforward to predict. However, for a 95% confidence limit it is

considered that significant wave height provides a sufficiently reliable measure of

expected wave heights for most situations.

Figure A.6 – Vert. acceleration rms of 22m monohull showing effect of Hsig

A.8 Effect of Heading

In terms of operability in the transit mode, the performance of these craft is in general

far more sensitive to sea conditions and speed in head and bow quartering seas, as

opposed to other headings, possibly with the exception of the Swath craft which

appeared to be more sensitive in stern quartering and following seas.

In the transfer mode, the transfer performance of all craft was most sensitive to sea

conditions in the head and stern sea headings. This appeared to be due to the fact that

at these headings the approaching wave crests reached both hulls at the same time,

creating a greater buoyancy force peak than at the other headings where the crest

reached one hull first and then the other, inducing a certain level of roll, thus reducing

the overall buoyancy force peak.

In addition, at these other headings the vessel yawed and rolled in response to the

wave orbital motion, allowing the bow fender to ‘walk’ up and down the docking poles,

with each pole individually and alternately in contact with the fender as the waves

passed. It was also evident that in the head and stern sea conditions the propulsors

ventilated more readily than at the other headings, and when they did, both hulls of the

catamaran or both propulsors of the monohull ventilated together rather than

individually. As soon as ventilation occurred, the fender slipped and/or came away from

the docking poles.

A.9 Effect of Shallow Water

In terms of the effect of shallow water on vessel motion performance, ‘shallow water’ is

a relative term since it effects waves differently depending on their wave length. In this

context, shallow water is generally understood to mean a depth of water less than about

half the wave length. Since the sea state is made up of a large number different wave

lengths, this is not a particularly helpful definition – however, for typical CTV operations,

shallow water effects on sea states start to become noticeable if the depth is less than

about 30 metres and are clearly evident at depths less than about 15 metres. In shallow

water the waves become shorter and steeper and so motions that are sensitive to wave

slope (generally pitch and roll) are generally increased. This can exacerbate

performance in both transit and transfer, generally inducing greater pitch and roll

motions.

A.10 Effect of Current

Tidal current can affect the performance of CTVs in a number of ways. During transfer,

a current will generally reduce the performance of the craft by requiring the use of

steering to offset the yawing effect of the current (and thus reducing the net longitudinal

bollard thrust, leading to reduced fender friction). The current can also induce a

transverse force on the fender, leading in the extreme to the fender slipping off

transversely, unless restrained by a fender nib.

During transit, the current can have a beneficial or detrimental effect depending on its

direction relative to the vessel and/or the wind. Wind against current is well known to

induce steeper waves and increase vessel motions, and of course the current can slow

or speed up the vessel speed over the ground.

However, in general current is considered primarily to affect the approach mode

(manoeuvring to get into the transfer position) and the net effect on this mode will

depend on the relative direction of the current, waves and docking pole orientation.

A.11 Effect of Docking Pole Inclination

Some docking poles are not fitted in a simple vertical orientation and are sometimes

inclined at an angle to the vertical (angled away from the approaching vessel), generally

to ease installations on non-vertical structural components. The performance of CTVs

pushing up against inclined docking poles (up to an angle of 7 degrees from the vertical)

has been studied. It was concluded that no appreciable difference in operability could

be noted due to such an inclination. In general, the craft rode up the pole slightly more

than on vertical poles, and slips were generally instigated in an upward rather than

downward direction. However the net operational performance appeared to be largely

unaffected.

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