Page 1 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.
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Page 1
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.
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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.
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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.
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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.
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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
Monohull Catamaran (jet) Catamaran (prop) Swath
Waterline length 20.0 m 20.0 m 20.0 m 20.0 m
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
Hull CL separation n/a 5.5 m 5.5 m 6.4 m
Draft 1.4 m 1.18 m 1.23 m 2.1 m
Hull block coefficient 0.405 0.590 0.56 n/a
Strut width n/a n/a n/a 0.65
Displacement 65.0 t 65.0 t 65.0 t 65.0 t
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
Pitch gyradius 5 m 5 m 5 m 5 m
Roll gyradius 1.8 m 2.55 m 2.55 m 2.55 m
Bollard thrust, 85% MCR 11 t 9 t 11 t 11 t
Operational speed 24 kts 24 kts 24 kts 24 kts
26 metre Hull Forms
Monohull Catamaran (jet) Catamaran (prop) Swath
Waterline length 24.0 m 24.0 m 24.0 m 24.0 m
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
Hull CL separation n/a 6.6 m 6.6 m 7.68 m
Draft 1.45 m 1.21 m 1.28 m 2.52 m
Hull block coefficient 0.385 0.56 0.53 n/a
Strut width n/a n/a n/a 0.78 m
Displacement 90.0 t 90.0 t 90.0 t 90.0 t
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
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.