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 windfarm 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. Page 1
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 bowfender arrangement. Page 2
f. Consolidation of the results from the sea trials, computer simulations and freerunning 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. Page 3
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. Page 4
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 Pitch gyradius 6 m 6 m 6 m 6 m Roll gyradius 2.14 m 3.06 m 3.06 m 3.06 m Bollard thrust, 85% MCR 16 t 13 t 16 t 16 t Operational speed 25 kts 25 kts 25 kts 25 kts Page 5
Figure 2 - CTV Performance Acceptability Criteria Transit Acceleration and Motion Limits Vertical acceleration, rms 1.5 m/s 2 (approx. 0.15 g) Lateral acceleration, rms 1.0 m/s 2 (approx. 0.1 g) Pitch, rms Roll, rms 5 deg 6 deg Transfer Motion Limits Friction limit Roll limit, rms 95% waves pass with no slip above 300mm (or one ladder rung) 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. Hsig, m Limited Fetch (short period on P-Plots) % Exceed nc Average Modal Period To, sec Figure 3 - Typical UK Sea Statistics Spread of Modal Period To, sec Sea Area Category Exposed (standard period on P-Plots) % Exceed nc Average Modal Period To, sec Spread of Modal Period To, sec % Exceed nc Ocean (long period on P-Plots) Average Modal Period To, sec 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 Page 6
Figure 4 - Transit P-Plot for 18 metre CTV Figure 5 - Transit P-Plot for 22 metre CTV Page 7
Figure 6 - Transit P-Plot for 26 metre CTV Figure 7 - Transfer P-Plot for 18 metre CTV Page 8
Figure 8 - Transfer P-Plot for 22 metre CTV Figure 9 - Transfer P-Plot for 26 metre CTV Page 9
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 : dan.kylespearman@carbontrust.com Web : www.carbontrust.com/offshorewind Copyright : The Carbon Trust Page 10
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 semiswath 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. Page 11
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 Page 12
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. Page 13
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. Page 14
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. Page 15
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. Page 16
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. Page 17
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. Page 18