The development of an offshore wind farm generally encompasses four different stages, specifically pre-installation surveys, installation, operations and maintenance and decommissioning. This means that the sector requires a wide variety of support vessels in order to carry out the multitude of different tasks effectively. In recent years, bottlenecks in the supply of such vessels have affected the sector, mainly because of the lack of availability of specialized vessels. Consequently, wind farm developers have tended to turn to the oil and gas sector for assistance.
Vessel shortages causes delays in the construction phase and this has been a regular problem in the past. For example, the Alpha Ventus wind farm was delayed for a year because of such shortages, and when a vessel was provided, it was 20 times larger than had originally been planned. North Hoyle offshore wind farm required a dedicated turbine installation vessel, the MPI Resolution, the construction of which was delayed. The project also had to use construction vessels in order to keep to deadline but was consequently delayed by about six months. The installation vessel for the London Array was delayed by three months, requiring the hire of an additional vessel in order to keep to deadline.
Given the rate of growth in the offshore wind sector, there is likely to be more than 5,000 operational offshore wind turbines in UK waters by 2025. The average distance from shore to wind farm is likely to increase in accordance with Crown Estate leasing rounds, for example the Dogger Bank Offshore Wind Farm will be constructed in water depths of up to 63 metres with a likely transit distance of 290 kilometers, requiring a nautical journey of over 5 hours.
This in turn will have an impact on Operations & Maintenance (O&M) strategies and an increasing impact on vessel design. Vessels will therefore have to become more durable and efficient in the future and so the market for these vessels is constantly evolving. There may also come a time when, as with the oil and gas sector, wind farm technicians will have to operate from and live in bases close to the wind farm site. In the oil and gas sector the period spent at such a base is typically two weeks. Although the most economical solution for near-shore operations is likely to be one based on workboats, sites in deeper waters further from shore will have to be supported by accommodation modules or a ‘mother ship’, larger support vessels or jack-ups, depending on the O&M strategy employed.
The distance of wind farms from shore will therefore inevitably have an impact on costs and for this reason wind farm operators have to deploy equipment that is suited to severe weather conditions. Entire conferences are now being held specifically covering the subject of offshore wind farm support vessel design.
Currently, major turbine and foundation installation vessel suppliers include A2Sea, MPI Offshore, Scaldis Salvage, Seajacks International, Jack-Up Barge, Ballast Nedam, BARD, GeoSea, International Marine Construction and Seaway Heavy Lifting. Most of these companies are based either in the UK or the Netherlands. Subsea cable installation companies include Global Marine Systems, MPI Offshore, Nexans, NKT, Prysmian, Mika, Offshore Marine Management, Peter Madsen Rederi, P&O Maritime Services, Technip (Subocean), Visser & Smit Marine Contracting (VSMC) and Stemat.
Ocean Surveyor [Image source: Geological Survey of Sweden, Flickr]
Survey vessels are used to conduct geographical and climatic surveys prior to the construction of an offshore wind farm. The range of vessels used for this purpose varies widely, depending on the depth of the sea bed and distance from shore.
Cable laying vessels
Cable laying is one of the first tasks to complete during the installation of an offshore wind farm. Cable laying vessels are specially designed for laying subsea cables that carry the generated electricity from the wind farm to shore. 4C Offshore maintains an entire database of such vessels and they can take many forms. For example, Van Oords Nexus vessel is a new design with an extensive deck area which can be customized to carry dedicated cable-laying equipment. It has a main crane with a lifting capacity of 100 tons at 15 metres outreach and an auxiliary hoist with a lifting capacity of 10 tons at 34 metres outreach. The vessel can also provide accommodation for 90 people.
The standard feature of these vessels is the turntable or carousel which stores the cable without the risk of excessive bending. It also has cable guiding sheaves and installation devices, including Remotely Operated Vehicles (ROVs) which are used for trenching. This is one of the most important jobs which involves cutting 3 metre deep trenches in the seabed for interconnecting power cables. It is one of the more challenging roles in the sector with up to 80 percent of insurance claims for cables damaged during installation.
These vessels also have Dynamic Positioning (DP) systems to hold them in place even in the roughest weather conditions.
These vessels can take a variety of forms and often include a range of multi-purpose ships that are not ideally suited to the task, due to their use in other sectors. This means they are only hired in for a short period of time. These multi-purpose vessels (MPVs) include jack-up barges which are non-self-propelled platforms that can lift themselves above the surface of the sea on legs resting on the seabed. Such vessels provide stability in rough weather conditions, but they are slow and require support ships to tow them to the site. Jack-up vessels operate on a similar basis except that they have self-propulsion systems, although they are still limited by water depth and their multi-purpose role.
Crane ships are based around large pedestal mounted cranes which deny the use of the vessel for any other function, such as carrying equipment, as well as limiting their speed.
A crane ship in the Port of Rotterdam [Image source: Frans Berkelaar, Flickr]
More recently, the construction of offshore wind farms has involved the use of a specialized wind turbine installation vessel (WTIV), a completely new class of ship that has only started to appear in the sector in the last few years, replacing crane ships borrowed from the oil and gas sector. These vessels generally have a flat rectangular hull in order to accommodate an extensive load area and a large crane. This tends to increase fuel and electricity consumption given a maximum speed of 10 to 12 knots from shore base to construction site. Electric rudder-propellers (thrusters) or Voith-Schneider propellers are used to control propulsion and positioning and the ship must remain stable in position while 4-6 jack-up legs are deployed so that the ship stands firmly on the seabed. Installation engineers are accommodated in a multi-storey structure on the bow with a helicopter landing pad on the top.
The first two WTIVs deployed in the sector were Sea Power and MPI Resolution. Both are still in service after ten years. Sea Power is a converted freighter but MPI Resolution was specially built for the job in 2003 and thus is the first true WTIV. From 2003 to 2009, the industry tended to use ships chartered from the oil and gas sector, with specialist WTIV construction starting thereafter. A total of 25 WTIVs entered service from 2009 to 2014 with 7 ships available in 2012. The WTIV sector is now engaged in a process of modification as it is becoming clear that the demands upon them are increasing. Fred Olsen will be launching the sister ships Bold Tern and Brave Tern shortly. These two vessels have jack-up legs that have been extended by 14 metres while the crane has been extended by 10 metres. They have been constructed specially for the UK’s Round 3 of offshore wind farm construction.
When WTIVs aren’t available, developers often bring heavy-lift crane vessels (HLCVs) into play. These were originally built for other purposes but are well-suited to wind farm construction. For example, the Svanen was originally built to install a large bridge. It can lift a load of 8,700 tons and its charter fee is fairly inexpensive as it rarely does anything else these days. Thialf has a 12,000 ton crane capacity and is one of the largest crane ships in the world. However, it is rarely deployed in the offshore wind sector because it is very expensive to charter.
A leg-stabilized crane vessel is a lighter version of a jack-up and is only really suitable for the installation of smaller 2 MW turbines in shallow waters. They are becoming more of a rarity in the sector as wind farms move further away from shore.
New vessels are entering the sector that are specifically designed for operating in rough seas, such as the Wind Server developed by Danish offshore service provider DBB Jack-up Services. This will be able to work at a wave height of 2.0-2.5 metres, whereas the norm for most other vessels is 1.5-metres. This will in turn allow the vessel to operate for around 320 days a year as opposed to 200, in turn generating significant cost reductions.
Crew transfer vessels
Crew Transfer Vessels (CTVs) can travel at a speed of 25 to 30 knots and are mainly used for operations and maintenance operations. In the early days of the sector, vessels such as fishing boats and survey ships were used to transport technicians to offshore wind sites but with the growth of the sector, the crew transfer market has evolved steadily.
Germany’s Alpha Ventus site is supported by boats in spring and summer and helicopters in winter. Regulations governing the site prevent voyages being made when the wave height is greater than 1.5 metres, but helicopters can be used even in relatively high winds, although their use can be restricted by poor visibility. Helicopters can usually transport three technicians plus their equipment at a top speed of 245 km/h, compared to around 45 km/h for CTVs. As well as crew transfer, helicopters are used for rescue operations and medical support. In German waters they also play an active role in the construction phase. When Meerwind was being constructed, Siemens Wind Power managed to reduce transport time by 80 percent for the transport of technicians as well as avoiding seasickness. Helicopters were also used during the construction phase of Borkum Riffgrund and Nordsee Ost.
Workboats are a tried and tested method of gaining access to offshore wind sites, helicopters less so. Vessels are relatively inexpensive and they can carry a large number of technicians, but their response times and ability to access sites is limited by weather conditions. Helicopters are expensive and can carry only a small number of technicians, but they can respond quickly and their performance is not affected by the sea state. They have been used for years in the oil and gas sector, but are new to offshore wind. For this reason, there is a degree of uncertainty regarding their potential usefulness in the sector. The two modes can be complementary, with workboats being used for scheduled preventative maintenance when there are no risks to the turbines and helicopters for more urgent situations and correct maintenance in which response time is critical to reducing turbine downtime.
Many companies operate catamarans which have speed and weight advantages over other types of vessel. Most catamarans are made from aluminium as this is less expensive than composite materials. However, composites are lighter and that allows the vessel to carry a greater payload, which in turn reduces operating costs. One of the latest designs on the market is EVOC22 (efficient versatile offshore catamaran, 22 metres in length) developed by CTruk, based in Essex in south east England. The vessel has a 7.6-metre beam, a 1.25-metre draft and incorporates a moveable wheelhouse and flexible deck pod system which enables the vessel to switch from carrying up to 12 technicians to providing 72 square metres of deck space for equipment. The vessel is powered by two 800hp turbocharged diesel engines and carries 24,000-liters of fuel.
Small Waterplant Area Twin Hull (SWATH) vessels are catamarans with a reduced hull cross-section at the sea surface where the energy from the waves is most powerful. This gives the vessel extra stability, especially at high speeds. However, SWATH vessels are expensive and have greater maintenance requirements.
These vessels are likely to be deployed increasingly as wind farms become more complex and move further out to sea. Converted cruise ferries are used alongside roll on/roll off ferries. These vessels can usually accommodate up to 100 people and can include conference and meeting rooms with the some of the larger ships also having swimming pools on board and lounge areas on board.
DONG Energy uses an offshore accommodation platform for its Horns Rev 2 project in Denmark. The platform accommodates 24 people and enables access to the wind farm’s transformer via a gangway. The transformer platform at Global Tech 1 has accommodation for 34 operations staff as well as a crane, a helicopter landing pad and containerized spare parts.
Several UK projects have used converted ferries as floating accommodation. These are anchored close to the wind farm, thereby reducing the transfer time for CTVs and also their fuel consumption while increasing productivity.
Written by Robin Whitlock