Visiting an offshore turbine, whatever the weather

OFFSHORE: When planning an offshore wind farm, developers often calculate their investment costs on the basis of the turbines being serviceable for 95% of the time.

Safe transfer to the offshore turbine is a priority
Safe transfer to the offshore turbine is a priority

So, in order to maintain an economically viable wind farm, and to meet the requirement of warranties, the transfer of crew and equipment to the turbine foundation needs to be possible for that 95% availability period.

The aim of a wind farm, onshore or offshore, is to keep the turbines running, especially when the wind is at its strongest. But an unplanned production stop offshore in such conditions is hard to handle, because the greater the wind strength, the more difficult a turbine is to reach — access is most awkward when it is most needed.

Best availability

Two objects are equally crucial when designing an operations and maintenance (O&M) access system that can reach this desired 95% availability: finding the best vessel to transport personnel and heavy spare parts to the specific site, and finding a suitable means of transferring them onto the turbine.

Speed is also important, particularly when the turbine is located far from shore, such as in the German sector of the North Sea. Travelling time counts as working time, so the longer it takes to reach the turbine, the less time is available for the employee to work on the repair. Speed, however, must be balanced against the comfort of the worker — there is little point in transferring somebody who is then too ill to be able to effect repairs — and the fuel consumption, which increases with speed.

Most crew members are not sailors and cannot always be expected to deal with discomfort as do professional seamen. At Horns Rev offshore farm, in the North Sea off Denmark, owner Vattenfall allows transfers at a maximum significant wave height (SWH) — a standardised measure of the height of waves — of 1.3 metres to ensure crew comfort, which reduces access to around 64% of the time. For this reason, some wind farms have been equipped with helicopter landing platforms. While this is an expensive option, and considerably reduces the ability to deliver heavy parts or tools, it can significantly raise access potential.

Because offshore wind turbines are placed where the wind speed averages around eight to 11 metres/second, the resultant waves are inevitably a problem. In the worst possible weather, the O&M solution must cater for a possible SWH of around 2 metres. In effect, this can mean waves in excess of 3.5 metres from trough to top. The distance covered over the time it takes the full length of the wave to pass — the wave period — can be more than 100 metres.

Countering waves and swell

A vessel has six directions of free movement in open water — heave, pitch, roll, sway, surge and yaw — and the system of transferring personnel either to or from the turbine must be able to counteract these movements. It is the large swell far offshore that has a greater impact on the movement of the vessel in relation to the static wind turbine foundation. As a wave passes, the energy it releases will be enough to move the entire vessel a large distance within one or two seconds. So the transfer method, whether a ramp, gangway or other, must be capable of a large stroke length, as well as very fast adjustment, in order to maintain the desired position relative to the foundation.

The vessel must also be able to counteract the currents around the turbine; possibly up to five knots during tidal variation, particularly around the UK coastline.

Safe return

When weather conditions deteriorate beyond an acceptable level, there is a delay of approximately one hour before the sea conditions similarly deteriorate. Therefore, the crew on board the vessel must be able to forecast the wave height to plan and execute the safe recovery of personnel and equipment from one or more vessels. Here, sticking to a maximum number of people deployed per vessel becomes important.

For ships and other vessels to work in such a harsh environment, size becomes extremely important. Linked closely to size is, of course, the cost of the vessel, making the transport and transfer system the single most expensive aspect of an O&M plan.

When some of the first offshore commercial turbines were installed in 2001 off Middelgrunden, Denmark, a small rigid inflatable boat (RIB) capable of carrying up to four personnel, or a small 12-person vessel, were used for access to the landing arrangement of the turbine foundation. The crew then climbed up the ladder onto the turbine platform. This was generally successful with no known incidents.

In sea areas with low waves, such as the Nysted offshore farm off Rødsand, Denmark, the access ladders can rotate fully around the turbine foundation, providing shelter from the waves and minimising the movement of the vessel. But in areas with higher waves, such as the North and Irish seas, the access platform has to be placed much higher above sea level, requiring fixed long-ladder arrangements and intermediate platforms to reach the top, and preventing adjustment to protect the access point against the wave.

Additionally, as turbines are sited further away, smaller vessels become less cost-effective, travel time increases and the need to position the crew closer to the wind farm becomes greater. In the German sector of the North Sea, some developers are looking at using a mother vessel offshore where O&M crews can live for longer periods and then be deployed to wind farms around the area, an expensive strategy. Others are looking at the island of Helgoland as a base. It is close to several wind farms ,and has a port and an airport to accommodate the harbour facilities needed.

For the increasing number of wind farms that will be sited further out to sea, and where intermediate offshore bases are not available, there are four types of vessel to be considered for access for routine O&M work: the traditional monohull vessel, the standard catamaran, the SWATH vessel and the lift boat (see box on previous page).

Calculations for safe transfer of personnel to the turbine have shown that a monohull vessel will transfer safely up to 1.5 metre SWH, allowing access to the wind farm 88% of the time. However, a SWATH vessel can transfer people safely up to 2.5 metres SWH, providing access 99% of the time. This much higher availability makes it the preferred type of vessel. While it is often rejected due to cost, it is worth noting that the cheapest option, the monohull, was recently found to deliver only 89% availability at a Baltic Sea wind farm. As this is lower than the supplier's warranted availability, and the onus is in the owner to provide the transport, the supplier may be released from part of the warranty cover. The SWATH, on the other hand, exceeds the manufacturer's warranted availability, which could even lead to an improvement in the turnover of the wind farm.

Safe transfer to the turbine

Once delivered to the location of the wind turbine, the wind farm owner must ensure the safe transfer of personnel from ship onto the wind turbine itself.

The boat landing system uses bumpers and shock absorbers fitted to the transport vessel to allow it to nose up to the turbine base. These have been successfully used to deploy personnel from monohull vessels without any incidents known to this author. However, it has only been approved for operation in up to 1.5-metre SWH, so misses the required 95% accessibility. If used with a SWATH vessel, it would increase accessibility.

Another system of transferring personnel is to use a gangway, which extends and retracts to compensate for the wave-induced movement of the vessel. These fit to the transport vessel and extend out to the wind turbine, in some cases establishing a fixed connection. They are expensive and require a large vessel in order to be stable.

Other systems are available or being trialed. Selstair, marketed as a life-saving system, is a safety staircase fitted onto the transfer vessel and a line is run up to the turbine platform. However, as it appears to be difficult to attach it to the turbine, it is likely that one would need to be fitted to each turbine and operated remotely. This would increase costs considerably.

A smaller gangway system that is currently available for use between two moving vessels would require careful design and alterations to enable operation between one moving vessel and a fixed foundation, such as a wind turbine. If achieved, however, it would provide a smaller, lighter and faster system than the other solutions. Various manufacturers are also looking at remotely operated cranes to lift the crew to the turbine.

Best option

To design the transport and transfer system to achieve 95% accessibility means combining the transport and transfer systems. Until now the best transfer vessel has been the monohull, with its benign movement in all weather types. But, as offshore wind turbines go further out to stormier conditions, they should be between 30 and 60 metres in size; few such boats are available.

Recent changes in the design of SWATH vessels has greatly reduced the effect of waves hitting the sides of the boat and, with an appropriate transfer system, the crew will be able to pass safely even in poor weather.

Newer transfer systems should not be considered by wind farm owners until they have been fully tested. This leaves two possible solutions for the near future. One is the current boat-landing system, where the vessel keeps a forward thrust larger than the wave and current effect. The other is the gangway system, which reacts to wave motion and is not rigidly attached to the foundation, and which is designed to cope if contact with the foundation is lost. Both of these systems are in operation today and, so far, neither has led to loss of human lives.

Until the reliability of offshore wind turbines means that maintenance becomes virtually unnecessary, there remains much work to be done to provide safe, proven access vessels.

Kurt E Thomsen is CEO of Advanced Offshore Solutions Aps

Parting the waves four vessel types to ease access for offshore owners

Monohull vessels

Simple, stable during calm, good for cargo, large vessels needed for more distant sites

Sea-keeping rating (0-5): 3

This is the most common type of ocean-going vessel. It is more stable than others in calm seas and, while not particularly comfortable, it can operate effectively in wind and waves from all directions. It has a large waterproof hold for cargo.

Some models are designed to compensate for high wave forces. Larger vessels are in short supply.

Catamarans

Fast, steady when facing waves, unsteady side-on, limited storage

Sea-keeping rating (0-5): 2

A catamaran has two parallel hulls with a deck suspended between them, along with a bridge and accommodation module. They have good sea-keeping abilities until the weather becomes severe. They travel at high speed and are more steady in head-on waves than the monohull but can become uncomfortable when hit by waves from the side. They cannot carry any significant payload unless the vessel is large and, as they are made from glass-reinforced plastic or aluminium to keep the weight down, they cannot always stand up to the rugged type of work they are required to perform, particularly when docking onto the turbine foundation.

Redistributing weight issues restrict ability to fit bow thrusters to manoeuvre and stabilise around the wind turbine, but they can use propellers to rotate around a centre axis. Less expensive to operate than a SWATH but more than a monohull. SWATH vessels

Good availability in high waves, comfortable at speed, steady when facing waves, poor storage

Sea-keeping rating (0-5): 2

The Small Water Plane Area Twin Hull (SWATH) vessels have twin torpedo-type buoyancy hulls approximately 2 to 2.5 metres below the surface. The biggest difference between this and a catamaran is that the SWATH's twin hulls are completely submerged. With a smaller surface in the splash zone than a monohull, these vessels are less influenced by wave forces, making them more comfortable to work from and able to lie stable in the water when the waves come from the front or rear. But they do suffer even more from low cargo capacity than catamarans and are more costly to build. It is also difficult to install bow thrusters but, like catamarans, can use propellers to make them rotate around a centre axis.

Lift boats

Slow, stable, fixed position so unable to move around turbines

Sea-keeping rating (0-5): 3

Lift boats are rugged, self-propelled three-legged jack-up vessels aimed at long-term work in one position. Can carry up to 32 people, but most are fitted as eight-person cabins.

A lift boat simply puts down legs in front of the turbine, removing many of the risks of transferring people from a moving boat deck to the turbine in bad weather. But it is limited to deploying people to the turbine in front of which it is jacked and can only rescue those workers if the weather deteriorates. Its use, for routine maintenance, therefore, is likely to be limited.

Global challenge Design an Access system

How do you transfer personnel and equipment safely from boats to offshore wind turbines in three-metre wave heights?

That is the challenge set by UK non-profit company The Carbon Trust. Recognising the need to improve access to larger and more distant offshore wind turbines, a competition was launched to find new design solutions. "Successful innovators will see their solutions power the next phase in the UK's offshore wind expansion," says Charles Hendry, UK energy minister. The global market for access solutions is estimated to be worth over €2.3 billion by 2020, with the UK market accounting for up to 50%, according to The Carbon Trust.

Competition entries were due in November. For more information, go to www.carbontrust.co.uk/offshorewind

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