They also enjoy more consistent winds, with average annual wind speeds approaching 10 metres per second (m/s), and therefore produce considerably more energy. The further from shore a site is, the more wind is available and the greater the output will be.
However, this additional generation comes with new challenges. Construction and operation of offshore wind farms is more complex and far more dependent on the weather, particularly the roughness of the sea and the capabilities of vessels to operate in that sea.
Far-shore sites will usually have higher winds and be in deeper water, but with higher winds come proportionally higher waves. These make access to the turbine for servicing difficult and, depending on the turbine, there will always be a wind speed limit above which it is not permitted to work in the nacelle - usually this is around 17m/s. For far-shore sites in deep waters, the higher waves can build up a greater rise and fall and come thicker and faster. Wave height is expressed in terms of significant and maximum wave height (Hs and Hmax), and every vessel from a work boat to an ocean liner will have limits up to which it is safe to operate before it must return to port or run for shelter. Thus wind farm operators must check weather forecasts and information from wave buoys or other instrumentation in the wind farm before any vessel sets out with a maintenance crew onboard.
Too much of a good thing
If conditions are too poor, or are forecast to become too bad before the task can be completed, the vessel does not go and is "weathered off". As high waves correlate with high winds, having a stopped turbine awaiting service in these conditions is obviously bad news. Having to pay for the vessel and crew to remain in harbour - while the stopped turbine is unable to operate during a strong wind generation period - is the worst combination for any wind-farm operator.
What's more, because adverse weather is usually permitted downtime in the availability calculation of most operations and maintenance (O&M) contracts, there is no way to recover the lost generation revenue through liquidated damages.
For near-shore sites, it is usually simple enough to have a service base at a nearby port and use day vessels to go out to the wind farm every morning. This is a typical strategy of the first and second round of UK wind farms, and a host of small work-boat designs have evolved to suit. These boats are usually aluminium catamaran designs, with overall lengths ranging from 14 to 20 metres and either standard propeller or water-jet propulsion.
At the larger end of this range the vessels can be more comfortable, carry more equipment and be more versatile in their use, but Maritime and Coastguard Agency rules drawn up by the UK body and adopted by most European countries stipulate that they cannot carry more than 12 passengers and remain within the work-boat rules. This limits the benefit of using larger vessels, and a small fleet of 12-person work boats making day-trips is all that is required.
As longer hull forms bring greater stability and allow technician to transfer from vessel to turbine or vice versa in higher waves, this also limits the wave heights within which these types of boats can work in, typically up to 1.5 metres significant wave height and up to 2 metres maximum wave height. This is more or less the wave height that usually correlates with a good 10 to 12m/s mean wind speed.
Travel time to the far-shore site is also a major factor and the near-shore wind farms' strategy of day-trips from shore is no longer a feasible solution. This approach would result in more travel time to and from the site and a higher fuel consumption cost. But, more importantly, there would be less working time in the turbine. When you factor in that in European waters the technician crew must work within European Union working time directive rules - and face daylight hours constraints for winter work - then two hours' travel time each way is about the maximum before a different strategy is needed.
Take to the skies
Alternative strategies currently in place and being developed for deeper water far-shore sites include using helicopters, service/accommodation platforms and hotel/workshop ships. Helicopters bypass all the constraints on wave height but are expensive, have limited payload capacity and can only carry one technician team at a time. As the helicopter cannot land on the wind turbine, the team must "heli-hoist" down to the turbine nacelle one at a time hanging on to the winch-wire. This involves a level of training and risk that some developers, manufacturers and service providers are not comfortable with for regular turbine access operations.
However, helicopters often remain the best option for an outage at the offshore substation platform, which are usually fitted with a helipad for safe landing. Substation downtime events are rare but when they do occur restoring wind-farm output without delay is paramount.
Accommodation beyond fifty turbines
Eliminating the journey time to the site by staying out at sea is the other alternative. Hotel ships that can both accommodate the technician crews and provide an offshore workshop and spares store are one way to achieve this. A fixed accommodation platform is another. Both options have their relative merits and drawbacks (see box, previous page) and neither will be very economic for smaller offshore wind farms with fewer than 50 turbines that do not form part of a cluster.
In between the fixed and floating options is a third way. This is not always an option because it requires access to a suitably placed island nearby. If you are lucky enough, you may have the ideal deep-water far-shore site - high average wind speed, but with a relatively short transfer potential.
The key to making it work well is to have the right kind of vessels: bigger and more capable in rough weather than the standard work boat designs described earlier and able to carry more than 12 technicians in comfort. Small waterplane area twin-hull (SWATH) vessels fit this requirement nicely. Whether ballasted or always submerged, these designs provide good stability and space in 25-30 metres overall length, and have been reported as achieving safe crew transfer in up 3 metre significant wave height.
The bespoke solution
In summary then, deep-water far-shore sites need a different approach from their previous near-shore equivalents and are themselves different from one another. It will be the specifically tailored solution, based on comprehensive understanding of the site wind and wave regime - and that is itself sufficiently flexible to adapt to the changing conditions and access opportunities - that delivers the best availability and the highest generation revenues.
LIVING AT SEA FLOATING WORKSHOP VERSUS FIXED PLATFORM
There are two options for in-field accommodation and operations and maintenance (O&M) - fixed and floating. Fixed platforms are expensive, but they can provide all the necessary home-from-home comforts for the technician crews from a rigid platform, with as close to an "onshore feel" as possible. However, they cannot put into harbour to let everyone off for a few days' leave when a big storm comes, and they still need a vessel to deliver supplies and exchange crews.
A floating vessel that can be both a hotel and a workshop would seem to be a better alternative, but the ever-moving deck and fuel cost of keeping position and generating power are serious concerns, too.
Both options still need to offer a way of taking technicians to the wind turbines that need attention, and this is when things start to become complicated. One German project plans to uses helicopters to ferry crew from their accommodation on the offshore substation platform to the turbine, and another plans to use powerful little rigid inflatable boats (known as fast-rescue craft) that can be plucked out of the water by a heave-compensated hoist - a specially adapted davit crane with a winch. Other options include warship-like docking bays, where the stern of the mother ship can be opened up to the sea, allowing the smaller vessel to enter for docking; and taking the O&M vessel itself around to all the turbines to drop off the crews en route using a motion compensation system to maintain position.
The trouble with using daughter craft is that they need to be big enough to do the task required of them in these rough seas, yet small enough to fit within the mother ship or under the O&M platform. This could mean that either the mother ship will need to be oversized - and therefore expensive - for the task, or in the case of the platform, the lifting gear and space required to take several smaller vessels would add significantly to the complexity and weight of the platform.
Platforms with the additional weight of accommodation, services, vessel storage and gear would also increase the weight of the support structure required. As a result, only the largest, most expensive vessels on the market would likely be able to install these platforms. A typical offshore substation platform for an 80-turbine wind farm weighs around 2,500-3,000 tonnes and is several storeys high. Only very large vessels and specialist heavy-lift vessels and jack-ups can lift this much to a sufficient height above sea-level to fit on the jacket structure. A suitable vessel, such as the Stanislav Yudin from Seaway Heavy Lifting, the Rambiz from Scaldis Salvage and Marine Contractors or the Thialf from Heerema Marine Contractors. Day-rates for some of these vessels can be as high as EUR500,000 a day so a ten-day hire to install an offshore substation platform can be very expensive.
No wonder then that two of the biggest suppliers of offshore wind turbines are going for floating accommodation and workshop. They are offering service contracts of ten or more years, and need to remain flexible. Vessels can be upgraded, replaced, retasked, sold on or shared around - not so easy with a platform.
It all comes down to size
Ultimately deciding on fixed or floating accommodation depends on many factors, most crucial being the size of the wind farm in terms of area and the number of turbines within it. For smaller deep-water far-shore sites the floating option will likely be the best solution. But as a project size grows the case for fixed platforms improves substantially. For extremely large projects, such as the UK's 9GW Dogger Bank Round 3 site, hundreds of turbines and multiple platforms will be spread over a huge area. In this case a combination of fixed and floating options that evolves as the project is built will be the best approach.
VESSEL STRATEGY PLANNING YOUR O&M FLEET
- Integrated team Work closely with the project manager and ensure access to technical support on construction issues.
- Sizing and cost Vessels come with a range of capabilities, but significant wave height for operations and crew transfer, and the 12 passenger threshold set by the Marine and Coastguard Agency are the two main drivers.
- The wave regime at the wind site is driven by the wind resource and the length of "fetch" in the predominant wind direction. Fetch is the distance over which the wind can blow unhindered before it reaches the site. The longer the fetch the bigger and more persistent the waves will be.
- The significant wave height capability of the vessels chosen should comfortably exceed the mean wave height at site. This provides additional comfort that you can take someone off the wind turbine after you have put them on if the weather worsens, and that you can still have access in the rougher (and windier) winter months.
- Larger wind farms will need a small fleet of vessels. This allows for a range of craft with different capabilities. During easier summer weather smaller craft can do the job, and may be cheaper to run.
- Additional crew and access capacity will be required during the summer annual servicing season. Chartering in extra vessels may be an option, but this is a busy time and you will need to plan ahead in order to secure the capacity.
- A floating hotel and workshop vessel will require service and replenishment downtime itself, and that it too can become a weak-link in the chain if it fails. Have a back-up option.
- Fixed platforms are expensive and less flexible in terms of upgrade and redeployment but are the ultimate home-from-home base for service teams, especially for large or very distant sites. This provision may aid recruitment and there are significance lessons to be learned from the oil and gas industry regarding design, comfort and operation. For example, consider a design that avoids motion sickness issues and providing communication and recreational facilities for off-shift staff. Heed these, but tailor your options for offshore wind farm operations.
Stuart Brown is senior engineer and technical team leader at renewable energy consultancy Sgurr Energy