An Introduction to Crew Transfer Vessels
A range of vessels is used during the construction and maintenance and operation of offshore wind farms. While the majority of vessels will work in different markets, Crew Transfer Vessels (CTVs), have emerged as a specialist area. These vessels are used to transport wind farm technicians and other personnel out to sites on a daily basis.
Dalby Ribble - 26m
The majority of CTVs have been designed to be efficient and effective and built to work in the sector. They are usually aluminium catamarans accommodating 12 passengers. Transit speeds range between 15-25kn although some vessels have a top speed of up to 30kn.
Approximately 25% of CTVs are driven by fixed pitched propellers but around 14% have controlled pitched propellers. Water jets are also used in the market. Approximately 25% of the fleet fitted with jets as they make the vessels highly manoeuvrable plus have a shallow draft. This is critical for some sites.
Passenger comfort on board is a top priority as technicians need to arrive in the field feeling well before transferring. Most vessels have individual suspension seats which are designed to minimise travel fatigue and stress caused by vessel motion. The vessels also have facilities including a kitchen, television and entertainment system. One or two berths will be available for use by the crew when on extended operations.
CTVs can be used to take small amounts of cargo out to sites, such as components and equipment for the installation and servicing of the turbines. Accessible foredeck space is required with a load capacity ranging from 1t to as much as 30t. Deck cranes are used for lifting cargo from the quayside onto the deck but on site, lifting is done by crane on the turbine itself. Some vessels also provide refuelling services where they carry fuel for the turbine generators offshore.
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 77% of CTVs in Europe are fully classed enabling them to work further offshore than MCA Cat 2 vessels which are limited to working with 60nm of a safe haven.
Iceni Vengeance - 23m
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 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 pushes 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 to 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 are limited to sea conditions of 1.5m significant wave height maximum although accessibility is also affected by wave frequency and length, and current. Operational decisions are made on a daily basis by the site marine coordinator.
Walk to Work
To expand the transfer window, a number of ‘walk to work’ systems have been developed. These consist of a heave compensating bridging mechanism which attaches to the j-tubes or ladder. from the vessel. This allows personnel to ‘walk to work’ as the bridge remains stationary relative to the turbine and can be used in higher sea states. There are number of systems on the market such as Ampelmann, Maxcess and Houlder TAS . However, these systems are only suitable for much larger vessels than CTVs. Systems for the smaller crew boats have been developed but not proven viable in the market.
Other New developments
New vessels designs are continually being developed to meet the changing demand of the offshore wind sector. Vessels have increased in size as larger vessels provide better sea keeping qualities as well a 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 regulation applies to the build which adds to the overall cost.
Costs are very important to charterers. Fuel and lubricants are paid for by the charterer outside of the day rate, so fuel efficient vessels are becoming increasingly important but this has to be balanced with performance. GRP (glass reinforced plastic) is a much lighter material and has been used for the construction of a number of vessels which have proven to be more economic than aluminium vessels but the lighter vessels have other limitations.
CTV Market Summary
The last 10 years has seen the crew transfer market expand significantly with a steady evolution in design as the operators work to become more competitive. While new and better vessels keep entering the market, some are leaving as they are no longer fit for purpose.
The development of larger offshore wind farm sites, further offshore with larger capacity turbines has changed the operational model for vessel support. Service Operational Vessels (SOVs) and helicopter support is becoming more common place reducing the need for CTVs. As a result, demand is stabilising despite the increasing offshore wind capacity coming online.
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CWind Typhoon Tow - 25m