Design & Operation of Wind Farm Support Vessels, 29-30 January 2014, London, UK

XSS – A Next Generation Windfarm Support Vessel M Jupp, R Sime and E Dudson, BMT Nigel Gee, UK SUMMARY The global offshore wind industry has undergone a huge expansion programme in recent years and the market for supporting and maintaining offshore wind turbines is highly competitive among turbine crew transfer operators. However, the current fleet of windfarm support vessels has been developed to serve near shore windfarms and the limited ability of these vessels to operate in rough weather often results in windfarms being unreachable. With windfarms being built further offshore in more exposed locations, increased seakeeping ability of support vessels is of paramount importance. This paper describes the work undertaken by BMT Nigel Gee in the development and optimisation of an innovative ‘extreme semi-SWATH’ (XSS) hullform designed to meet the exacting needs of the offshore wind industry. The paper discusses the challenges encountered, as well as the potential benefit of the vessel to the offshore wind industry. 1.

INTRODUCTION

Catamarans of up to 20 metres in length currently operated by crew transfer operators are designed to service wind farms located relatively close to the mainland. For wind farms located further offshore, where significant wave heights will be greater and transit times will be longer, conventional catamarans of this size are unable to meet the requirements for high speed transit and zero speed push-up operations in high sea states and non-optimum wave headings. This paper starts by outlining how the offshore wind industry is changing and goes on to describe the design and build of the XSS, a vessel which BMT believe meets these increasing industry requirements. 2.

INDUSTRY REQUIREMENTS

With the global expansion of the offshore wind industry into locations further from shore, the requirements for vessels which are used in the support and maintenance of turbines has changed. From an earlier analysis of locations and environments conducted by BMT [1] in which these windfarms are sited along with industry collaboration, the following vessel requirements were established:  Increased Operability & Utilisation – vessels must offer a higher level of operability in existing windfarms, as well as being capable of serving more demanding far shore locations.  Improved Safety and Comfort – vessels must deliver higher levels of comfort in transit and at zero speed, along with improved safety during push-up and transfer operations.  Performance and Capability – power requirements and fuel consumption levels must be comparable to conventional catamarans, whilst providing flexible deadweight and cargo capacity.

© 2014: The Royal Institution of Naval Architects

2.1

VESSEL REQUIREMENTS AND CONSTRAINTS

A vessel with a load line length of under 24m, with a limited passenger capacity of 12 does not require STCW certification of the crew. In addition, a vessel of this length and passenger capacity can be classified to the requirements of the various windfarm service vessel rules, with a significantly less onerous service notation than vessels of 24m+. The principal particulars for the vessel are listed in Figure 1. Hull Length Overall Load Line Length Maximum Beam Moulded Displacement (full load) Draught (full load) Passengers Crew Cargo Capacity Fuel Capacity Class Notation

25.40 m 23.96 m 8.50 m 98 t 1.82 m 12 3 10 t 9800 ltr DNV 1A1 HSLC R2 WINDFARM SERVICE 1 Figure 1 – Principal Particulars – Design Requirements 3.

HULLFORM TECHNOLOGY

3.1

OPTIMUM HULLFORMS FOR SEAKEEPING

Typical methods of improving the seakeeping performance of a catamaran in high seas are to increase length and displacement, adopt suitable ride control systems and reduce the waterplane area of the demihulls. The latter option can be achieved by incorporating narrow hull sections at the waterline along the length of the vessel and moving the hull volume (centre of buoyancy) far below the waterline. The effects of these design features have been proven in commercial and military applications by Small Waterplane Area Twin Hull (SWATH) vessels which demonstrate by far the

Design & Operation of Wind Farm Support Vessels, 29-30 January 2014, London, UK

best seakeeping performance in large waves, but at the expense of very high powering requirements, high capital costs and much higher running costs in comparison to less complex, fuel efficient catamarans. To achieve improved seakeeping whilst maintaining reasonable powering and fuel consumption levels semiSWATH hullforms have been developed such as BMT’s ModCat, as discussed in Section 3.2. 3.2

MODCAT TECHNOLOGY

In 2001, BMT Nigel Gee undertook a research and development project sponsored by the US Navy Office of Naval Research (ONR) to develop a catamaran hullform offering significantly better seakeeping performance than a conventional catamaran, but with minimal resistance penalty. The advanced semi-SWATH hullform adopted narrower sections at the waterline, with a lower centre of buoyancy and a slender bulb at the bow. In comparison to a conventional catamaran hullform with identical principal particulars, physical model tests demonstrated that the vertical accelerations of the ModCat were up to 50% lower than those of the conventional catamaran, with only a 5% increase in power required to achieve the same speed. The ModCat hullform has subsequently been adopted for military applications in the Atlantic Ocean (e.g. 79m 'Sea Fighter' for US Navy) and for rough water ferry operations in the Pacific Ocean (e.g. 57m 'Betico II' for Sudiles).

developed preliminary designs for the following hullform technologies which were considered to be the best potential solutions for windfarm support vessels:     

Monohull Conventional Catamaran Semi-SWATH SWATH Trimaran

BMT developed a scoring model to compare the five selected hullforms against a range of technical and commercial performance attributes. These attributes were: Technical:   

Seakeeping / MSI – in a range of sea states / zero speed Seakeeping / Speed loss – in a range of sea states / zero speed Deck area

Commercial:    

Vessel complexity Maintenance costs Fuel consumption Capital Cost

The results for the vessel types based on Technical criteria were: Platform SWATH Semi-SWATH Trimaran Conventional Catamaran Monohull Figure 3 – Technical Results

Rank 1 2 3 4 5

Figure 2 – ‘Sea Fighter’ Using the fully proven ModCat hullform as a basis, the XSS has been developed to go beyond semi-SWATH technology, specifically to meet the exacting requirements of the offshore wind industry. 4.

CONCEPT DESIGN DEVELOPMENT

4.1

HULLFORM COMPARISON

Prior to any development of the XSS hullform, BMT undertook a review of existing hullform technologies to identify which offered the best attributes when considering the design requirements outlined in Section 2.

The results for the vessel types based on Commercial criteria were: Platform Monohull Conventional Catamaran Semi-SWATH Trimaran SWATH Figure 4 – Commercial Results

Rank 1 2 3 4 5

Each vessel type has advantages and disadvantages. The SWATH is by far the best technical solution, yet is the most expensive and complex. Conversely the monohull and conventional catamaran are technically poorest but likely to be the most cost effective.

Based upon the same nominal principal particulars, capabilities and regulatory requirements, BMT

© 2014: The Royal Institution of Naval Architects

Design & Operation of Wind Farm Support Vessels, 29-30 January 2014, London, UK

4.2

EXTREME SEMI-SWATH CONCEPT

In order to develop the next generation vessel it was considered that the vessel should achieve more of the technical benefits of a SWATH without the associated cost and complexity. This vessel should ideally balance the requirement for an affordable vessel with the requirement for significantly improved seakeeping over what is currently available with conventional hullforms. In the commercial world the semi-SWATH has been developed to offer improved seakeeping compared to a conventional catamaran, but this improvement is limited. The semi-SWATH has been developed for the commercial fast ferry market and therefore the design has always been constrained by the increased fuel consumption tolerated by the industry. This limited increase in fuel consumption has in BMT’s opinion restricted the level of improvement in seakeeping exhibited by semi-SWATH designs. BMT have therefore developed the extreme semiSWATH (XSS) concept which aims to offer an improved level of seakeeping over existing designs without the performance and cost penalty exhibited by a full SWATH vessel. For this hullform, with the required level of utilisation, a higher powering requirement will be considered acceptable in return for significant improvement in seakeeping capability and operability.

4.3

The XSS hull lines are based largely on BMT’s ModCat hullform. The main differences are that the ModCat’s semi-SWATH sections have been modified to shift the vertical centre of buoyancy further below the waterline, and to further reduce waterplane area. Indicative XSS hull sections are presented in Figure 6, alongside typical sections for equivalent ModCat and conventional catamaran hullforms.

Figure 6 – Comparative Sections – Left to right; Conventional Catamaran, ModCat, XSS 5.

MODEL TESTING

Calm water resistance and seakeeping tests were carried out at the Haslar ship tank in Gosport, UK. The same model was used for both tests. 5.1

The variation in vessel characteristics between a conventional catamaran and a SWATH is illustrated in Figure 5.

XSS HULLFORM

CALM WATER RESISTANCE TESTS

5.1 (a) Resistance Tests Calm water resistance tests were carried out at the estimated full load displacement of the vessel. Trim and interceptor optimisation tests were undertaken at 21 and 27 knots to identify the optimum combination of static trim and interceptor settings. Subsequent calm water resistance tests were then carried out at speeds between 10 and 32 knots using these settings. Waterjet thrust data supplied by the waterjet manufacturer was used to develop the powering and speed predictions. 5.1 (b) Powering Predictions

Figure 5 – Relative Merits of Alternative Hullforms The ideal balance between technical and commercial requirements is a matter of opinion for individual operators. However, considering the vessel requirements outlined at the beginning of this paper, it can be seen that the XSS hullform bridges the gap in technical ability between a SWATH and a semi-SWATH, with only a limited reduction in commercial performance.

© 2014: The Royal Institution of Naval Architects

Figure 7 shows the 24m XSS powering requirements, plotted against the powering requirements of a 24m conventional vessel and a 19m conventional vessel. In each case the vessel displacement is equivalent to 12 tonnes of deadweight being carried. The 24m XSS has the highest powering requirement of the three vessels due to the SWATH-type sections and some additional T-Foil drag. However, for a given deadweight the achievable speed is comparable to existing 19m conventional catamarans. The 24m XSS is superior to the 19m vessel as it has a higher deadweight capacity and can comfortably service far shore wind farms.

Design & Operation of Wind Farm Support Vessels, 29-30 January 2014, London, UK

The 24m conventional catamaran has a higher top speed than the 24m XSS and a high deadweight capacity, but would not be able to comfortably service far shore wind farms due to its inferior seakeeping performance.

During each run measurements of heave, pitch and vertical acceleration were recorded. Heave and pitch were measured using potentiometers, one mounted at the forward perpendicular (FP) and the other mounted at the LCG. 5.2 (b) Seakeeping Test Results

Figure 7 – Ship Brake Power and Speed Capability Comparisons (with 12 tonne dwt) 5.1 (c) Range The 24m XSS is designed with a large fuel capacity of 9800 litres, enabling the vessel to reach far shore wind farms and remain on station for as long as possible. The range achievable in calm water (sea state 2) conditions at 25 knots is over 480 nautical miles. 5.2

The seakeeping tests demonstrated that the XSS hullform, in combination with an active ride control system (RCS) would offer motions and accelerations experienced by passengers within long term tolerable levels for heavy work. The XSS hullform is ideally suited to the use of T-foils, as the reduced waterplane area and low pitch ‘stiffness’ result in lower values of moment to change trim than for conventional catamaran hullforms. This lower level of stiffness allows a correctly sized ride control system to work very effectively at reducing vessel motions and accelerations. Initial predictions by the ride control system supplier, NAIAD Dynamics, indicated that reductions in motions of up to 80% could be achieved. Following the conclusion of the model testing and a further round of preliminary design development, Turbine Transfers decided to proceed with the build of the first 24m XSS. The general arrangement for this vessel is presented in Figure 9.

SEAKEEPING TESTS

5.2 (a) Model Setup The model was towed using two conventional heave posts in a side-by-side arrangement (one post in each hull) as shown in Figure 8. The posts were connected to the model at the intersection of the longitudinal centre of gravity (LCG) and the thrust line. The model was free to trim and heave.

Figure 8 – Seakeeping Model Test Setup The sea keeping tests were carried out in the estimated full load condition with the optimum static trim setting from the calm water optimisation tests. The model was ballasted prior to testing to achieve the correct vertical centre of gravity (VCG) and the correct pitch radius of gyration.

Figure 9 – General Arrangement

© 2014: The Royal Institution of Naval Architects

Design & Operation of Wind Farm Support Vessels, 29-30 January 2014, London, UK

6.

MACHINERY AND STABILITY

6.1

PROPULSION MACHINERY

6.1 (a) Engines The XSS is installed with the following propulsion machinery: Main Engines Propulsors

2 x MTU 12V2000M72 (1080kW) 2 x MJP 550DRB Waterjets

The extreme semi-SWATH section shapes presented several challenges in the design of the machinery arrangement, in particular the position and alignment of the main engines. As Figure 10 shows, the extreme semi-SWATH section shape prevented the engine from being positioned within the lower part of the hull. As is more common with SWATH vessels, the engine is positioned in the ‘haunch’ area, and connected to the waterjet by a vertical offset gearbox with no down angle (Figure 11). The engine itself is set at an angle of 6.5.

The engine foundations are located on the ring frames of the haunch section of the hull. There is no requirement for the usual longitudinal girders (as would be found on the hull bottom of a standard windfarm support vessel) as the lower section of the hull effectively acts as the longitudinal stiffening. As the engines are located higher above the waterline than in a standard vessel, the required extent of structural fire protection is greater. Figure 10 shows that the insulation extends down to roughly the bulbous section of the hull. The engine beds are all insulated, with an added vapour barrier in way of the sump, effectively forming a drip tray. 6.1 (b) Waterjets To accommodate the relatively high shaft angle and the rise of keel aft, while retaining high propulsive efficiencies, a bespoke intake duct and output nozzle were designed and supplied by MJP. As Figure 11 shows, the main waterjet body and impeller are set at the same angle as the shaft. The nozzle is then shaped with a 6.5 discharge angle, so that the output returns to the horizontal. This simple solution allows more flexibility in siting the engine and eliminates the need for multiple shaft sections and inefficient joints. Importantly, this solution also avoids the large trimming moments that would result from a high thrust angle combined with low waterplane area. 6.2

AUXILIARY MACHINERY

6.2 (a) Motion Damping System The motion damping system for the XSS has been designed and supplied by NAIAD Dynamics and comprises of the following:  

Figure 10 – Engine Section View

Pair of interceptors, each of span 1.3 metres Pair of T-foils, each of area 0.75m2

At the time of design, the 0.75m2 T-foils were significantly smaller than any that NAIAD Dynamics had previously produced. Additionally, it is understood that these are the first aluminium T-Foils developed by NAIAD. 6.2 (b) Cargo Handling Cranes The XSS has been built to carry cargo handling cranes on both the fore and aft decks. The structural foundations for these cranes have been designed to accept a variety of commonly used marine cranes.

Figure 11 – Engine and Jet Arrangement

© 2014: The Royal Institution of Naval Architects

The fore deck crane is located on the port side, near to the bulwark, where it does not hinder the use of the cargo loading area. The aft deck crane is located on the port side, where the foundation is built into the transom. If not

Design & Operation of Wind Farm Support Vessels, 29-30 January 2014, London, UK

required, the cranes can be removed, leaving the decks flush and clear for other uses. 6.3

7.

CONSTRUCTION AND SEA TRIALS

7.1

CONSTRUCTION

STABILITY

6.3 (a) Classification As discussed in Section 2.1 of this paper, the Load Line Length of the XSS has been limited to below 24 metres, to allow classification under any of the current Windfarm Service Vessel rules offered by the major classification societies. By doing so, the more stringent criteria of the High Speed Craft Code are avoided.

The XSS was built between August 2011 and March 2013 at the Sepers Group Shipyard, The Netherlands. The entire structure was 3D CAD modelled and then CNC cut. All plating is Aluminium alloy 5083-H321 and extrusions 6082-T6.

The XSS is currently classed to the DNV Tentative Rules for Domestic Service Craft, January 2011. It has the following class notation: DNV 1A1 HSLC R2 WINDFARM SERVICE 1 6.3 (b) Loading & Stability Results The XSS has a 10 tonne cargo carrying capacity, which allows 5 tonnes to be carried on both fore and aft decks, either containerised or loose. The vessel passes all stability criteria (both Intact and Damage Stability) with the maximum 5 tonnes of cargo on either the fore or aft deck alone, with no counter loading required (i.e. cargo does not have to be evenly distributed between decks). This offers greater flexibility and cargo carrying options to vessel operators.

Figure 13 – Hull During Construction

Figure 12 shows a 3D model rendering of the XSS in the worst damage case and loading condition. In all cases of damage no counter flooding is required to keep the vessel within the maximum acceptable angle of inclination. As the image shows, there is no deck edge immersion anywhere on the vessel and there are dry-shod evacuation routes to either side of the vessel for embarkation into the liferafts.

The XSS hullform is more sophisticated than a standard windfarm support vessel and contains areas of complex 3D curvature.

As the superstructure is a separate and resiliently mounted unit, this allowed for construction in parallel to the hull units. Pre-outfitting prior to attachment to the hull also enabled the construction programme to be compressed. 7.1 (a) Challenges

BMT realised the importance of reducing man-hours associated with shaping plates and therefore developed rolling templates to allow plate suppliers to cut and roll plates at their own facility. The additional man-hours spent at this stage of the design saved many more hours that would have been used later on in production, if the yard had been required to form the plates themselves. Once delivered to the yard, the components were assembled with the aid of detailed isometric drawings developed by BMT. No additional cutting or component fabrication was required. 7.2

SEA TRIALS

7.2 (a) Calm Water Trials

Figure 12 – Rendered Model of Worst Damage Stability Equilibrium Condition

Before entering service a limited set of sea trials were undertaken in calm water to confirm the speed capability of XSS. Speeds of up to 28 knots have been measured, and the vessel is capable of operating comfortably at a service speed of around 25-26 knots.

© 2014: The Royal Institution of Naval Architects

Design & Operation of Wind Farm Support Vessels, 29-30 January 2014, London, UK

7.2 (b) Seakeeping Trials Trials conducted by NAIAD on the XSS have demonstrated the effectiveness of the ride control system in rougher seas (SS3). The vessel was trialled off the North East coast of the UK, during which the active ride control system was intermittently switched on and off to quantify the effects of the active RCS on vessel motions. Figure 14 and Figure 15 show the time histories for pitch and roll rate when the RCS is ON (active T-foil deflection and active interceptor deployment) and OFF (fixed zero T-foil deflection and fixed fully retracted interceptors).

% Reduction in % Reduction in RMS Pitch Rate RMS Roll Rate Heading Data Data Data Data Set 1 Set 2 Set 1 Set 2 Head 60.7 / 44.2 / 45.0 / 46.2 / 63.0 47.2 55.9 52.1 Ave. = Ave. = Ave. = Ave. = 61.9% 45.7% 50.5% 49.2% Stern Quartering 41.7 / 50.4 / 60.2 / 67.1 / 46.5 49.1 58.0 68.0 Ave. = Ave. = Ave. = Ave. = 44.1% 49.8% 59.1% 67.6% Figure 16 – Measured RMS Pitch & Roll Rates In addition to the low vessel motions, only a very small speed loss in waves has been observed.

Figure 14 – Time History in Head Seas

Figure 17 – XSS During Trials 8.

DESIGN FEATURES

Figure 15 – Time History in Stern Quartering Seas Figure 14 and Figure 15 show that when the RCS is OFF, the pitch and roll motions are considerably higher than when the RCS system is ON. The corresponding RMS figures are presented in Figure 16, where it can be seen that pitch and roll rate reductions of between 42-68% were achieved in bow and stern quartering seas. It is evident that the NAIAD active RCS is extremely effective at reducing vessel motions when combined with the XSS hullform.

© 2014: The Royal Institution of Naval Architects

Figure 18 – XSS Design Features The XSS advanced hullform is complemented by a number of additional design features to improve vessel performance and ensure the safety and comfort of maintenance technicians and crew, including: 8.1

ACTIVE INTERCEPTORS & T-FOILS

Active T-foils and interceptors are fitted to give a very high level of motion control, as demonstrated during the full scale sea trials.

Design & Operation of Wind Farm Support Vessels, 29-30 January 2014, London, UK

8.2

ACTIVE FENDER SYSTEM

9.

The XSS is fitted with a modular Active Fender System, which has been developed to minimise the impact loads experienced by the turbine foundations by up to a factor of 3 compared to a conventional fender system. 8.3

There is no requirement for a dynamic positioning system, which enables the system to be installed on standard windfarm support vessels. A further major advantage of TAS is that at no point does the system attach to the turbine. Comprehensive HAZID and HAZOP assessments have been undertaken with vessel operators, windfarm developers and turbine suppliers. Fitted to a 24m windfarm support vessel the TAS has undergone extensive sea trials and successfully demonstrated increased operability. 8.4

SUSPENSION SEATS

Suspension seats are provided for all passengers as well as crew, to deliver increased comfort levels during transit and reduce whole body vibrations. 8.6

FLEXIBLE CARGO STOWAGE

Containerised cargo can be stowed on both fore and aft decks, using standard flush mounted container locking mounts. Loose cargo can also be secured in place via the numerous flush mounted deck tie down points. 8.7

  10.

CARGO HANDLING

The XSS is fitted with foundations and all of the necessary powering to carry cargo handling cranes on both the fore and aft decks. These cranes can be positioned depending on the specific operator requirements.

All year round support for far shore installations High speed transit in high sea states Zero speed motions comparable to SWATH hullforms Increased safety and comfort for personnel No ballast systems CONCLUSIONS

This paper has described the design development, testing, construction and in-service performance of the XSS windfarm support vessel. The result is a vessel which offers exceptional capability in high speed transit as well as zero speed push-up operations in high sea states. The paper has demonstrated that through the adaptation of an existing proven hullform, combined with several highly effective technological features, the ability to service the increasing number of far shore windfarms has been achieved. 11.

REFERENCES

1.

COCKBURN, C. L., STEVENS, S., DUDSON, E., ‘Accessing the Far Shore Wind Farm’, RINA Marine Renewable and Offshore Wind Energy – April 2010, 2010

12.

AUTHOR BIOGRAPHIES

RESILIENTLY MOUNTED SUPERSTRUCTURE

The resiliently mounted superstructure is designed to eliminate the structure borne vibration and reduce the ambient noise levels in the cabin, leading to a more comfortable experience for both crew and technicians. Noise levels achieved are below 65 dB. 8.5

  

POTENTIAL TURBINE ACCESS SYSTEM (TAS)

Developed in partnership with Houlder, the Turbine Access System (TAS)TM is a lightweight, heave compensated gangway system which has the potential to significantly improve the safety of personnel transfer and will also allow transfer in higher wave conditions.

BENEFITS OF XSS TO THE OFFSHORE WIND INDUSTRY

Matthew Jupp holds the current position of Naval Architect at BMT Nigel Gee Ltd. He is responsible for all aspects of design and engineering for projects ranging from commercial vessels to yachts. His previous experience includes key involvement with numerous windfarm support vessel and crew boat projects. Matthew is an Associate Member of the Royal Institution of Naval Architects. Rob Sime holds the current position of Naval Architect at BMT Nigel Gee Ltd. He is responsible for a wide range of naval architectural duties from concept design through to detail design, including hull lines development, stability calculations, performance predictions, model testing and sea trials supervision. Rob was heavily involved in the early design development of XSS. Ed Dudson holds the current position of Technical Director at BMT Nigel Gee Ltd. He graduated from the University of Southampton in 1990 and joined BMT Nigel Gee the same year where he has worked continuously with the exception of a year’s sabbatical in MARINTEK. He is a Chartered Engineer and Fellow of the Royal Institute of Naval Architects.

© 2014: The Royal Institution of Naval Architects