Wind Farm Service Vessels - An Overview

Article published on Monday, 22 April 2013

One of the most dynamic areas of the offshore wind sector is the Wind Farm Service Vessel (WFSV) or Crew Transfer Vessel (CTV) market. These vessels are used extensively to support the construction and the operation and maintenance of sites. Currently there are over 270 vessels of this type working on sites across Europe with a further 70 vessels coming into operation over the next 18 months – a rate of almost one a week.

The reason for the rapid growth in the supply of vessels is that the demand on each site can be very high, for example during the construction of London Array Phase 1 there were over 35 WFSV working at one point. So far the demand for WFSV on sites in other parts of Europe has been lower than the UK but with limited installed capacity in Germany and other countries it is perhaps too early to make a thorough assessment of demand sector wide.

Ellida Array built for London Array Phase 1

Ellida Array built for London Array Phase 1

Vessel design

The majority of WFSVs have been specifically designed and built to work in the sector. In the early days fishing boats were deployed before the catamarans became more common place. The design of the vessels has continued to be developed to be more efficient and effective in transiting personnel to site. The vessels need to be fast and highly manoeuvrable to access the foundations in a wide range of conditions.

A WFSV is usually an aluminium catamaran which can accommodate up to 12 passengers and up to around 10t of cargo. Transit speeds range between 15-25kn with some vessels having a top speed of up to 30kn.

Approximately 50% of WFSV are driven by fixed pitched propellers but around 10% have controlled pitched propellers. Water jets are also very popular in the market with approximately 36% of the fleet fitted with these as it makes the vessels highly manoeuvrable with the shallow draft which can be essential on some offshore wind farm sites.

Passenger comfort onboard is a priority. It is important that the technicians arrive in the field feeling well before transferring to the turbines. Many vessels have individual suspension seats which are designed to minimise travel fatigue and impact stress caused by the motion of the vessel. Other facilities on board normally include a small kitchen, television and entertainment systems. WFSVs often have one or two berths available for use of the crew when on 24 hour operations but these are not usually used by the passengers.


WFSVs are also used to take small amounts of cargo out to sites, such as components and equipment for the installation and servicing of the turbines. This means that accessible deck space is required with a load capacity ranging from 1t to as much as 50t. In addition, vessels can have a small crane for lifting cargo from the quayside onto the deck but most lifting is done by the lifting gear on the turbine itself. Some vessels also provide refuelling services where they carry fuel for the turbine generators offshore.

Vessel Classification

Vessel coding or classification ensures that vessels are built and equipped to a recognised standard enabling the charterer to be sure that the vessel will meet their requirements. Currently over 50% of WFSVs in Europe are coded to MCA Cat 2 meaning they can operate up to 60mn from a safe haven. To work further offshore would require an MCA Cat 1 vessel (up to 150nm) or a vessel coded by a classification society such as DNV (Det Norske Veritas) or GL (Germanischer Lloyd).

As the UK has led the market so far in offshore wind development, most sites can be serviced by MCA Cat 2 vessels. Fully classed vessels are more expensive to charter as the build costs are higher; therefore MCA Cat 2 vessels are more frequently used.

Despite the current use of MCA Cat 2 vessels, the development of wind farms beyond the 60nm limit will result in a shift toward more capable vessels. In addition, the project increase in Germany will result in a higher demand for classed vessels, which are required as minimum standard.

Transfer methods and developments

One of the problems of transferring from a vessel to a turbine foundation is that the structure is fixed to the seabed, resulting in considerable relative movement between the vessel and the foundation. The majority of transfers take place using what is known as the ‘bump and jump’ method (although there should be little bumping and no jumping), whereby the vessel is pushed on bow first to the ‘j-tubes’ which run vertically on the outside of the access ladder. The vessel uses sufficient thrust to enable it to remain stationary at the point of contact with the foundation and allow personnel to step over onto the ladder.

The turbine access ladder is set back from the j-tubes by 450mm which provides a safety zone to prevent anyone on the ladder from being crushed should the vessel move during the transfer procedure. Large waves, especially if there is a strong current across the side of the vessel can, on occasion, cause the vessel to lose position.

Although it is not a hard and fast rule, most transfers using the bump and jump are limited to sea conditions of 1.5m significant wave height or less. Other factors which will affect accessibility will be the wave frequency and length, and current conditions.

Walk to Work

To expand the transfer window, a number of ‘walk to work’ systems have been developed. These generally consist of a heave compensating bridging mechanism which attaches to the j-tubes or ladder so that personnel can ‘walk to work’ as the bridge remains stationary relative to the turbine. There are number of these systems on the market such as Amplemann, Maxcess and Houlder TAS and not only do they make it safer for personnel to transfer, the transfer can often take place in higher sea states.

Iceni Defiante fitted with the Maxcess transfer system
Iceni Defiante fitted with the Maxcess transfer system

Another type of system is the one used by Mobimar on the vessel the Wind Servant. In this case a mechanism on the vessel bow clamps onto the j-tubes and slides up and down before gripping securely once the vessel is a suitable position. The designers claim that transfers in 2.5m significant wave height can be safely achieved.

Wind Servant - trimaran fitted with Mobimar system
Wind Servant - trimaran fitted with Mobimar system

Despite the above advantages, there are downsides to these systems. The equipment takes up valuable deck space, payload capacity and has a power demand. Also there is either a capital cost or charter cost to be accounted for. More systems are in development - as the distance from shore increases so does the average significant wave height. This means that the use of transfer systems is expected to rise.

Other New developments

Vessel operators are designing new vessels to meet the changing demand of the offshore wind sector. Larger vessels will be built to provide better sea keeping qualities as well providing greater payload capacity. The number of passenger is also increasing but this is not a simple process of just adding more seats. Once passenger numbers are over 12, more stringent classification rules apply to the build which adds to the overall cost.

In the drive to build faster, more efficient and more stable vessels there are some trimarans now entering the market such as the new Austal built Cable Bay for Turbine Transfers. Designers claim that these vessels provide stability and efficient fuel consumption. Unfortunately, good sea keeping and fuel efficiency do not necessarily complement each other with other hull forms.

Fuel and lubricants are paid for by the charterer outside of the day rate, so fuel efficient vessels are becoming increasingly important. GRP (glass reinforced plastic) is much lighter and has been used for the construction of a number of vessels which have proven to be more economic to operate than aluminium vessels.

WFSV Market Summary

The last 10 years has seen the Wind Farm Service Vessel market expand significantly with a steady evolution in design as the operators aim to become more competitive and meet charterer’s requirements. Key areas of interest are fuel efficiency and the ability to access wind farms in higher sea states.

The large number of new vessels coming into the market would indicate that operators are able to find the financial investment needed to both develop designs and place firm orders at ship yards. However, the buoyancy in the market undoubtedly started before the significant delays in the development of the offshore wind market started taking effect.

The number of vessels in the market appears to be going to peak around the middle of 2014 at about 350-400 but there may be still more vessels being built speculatively in shipyards outside of Europe. The key message is that the market needs to be monitored closely.

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