Weather conditions need to be assessed diligently, even when a mishap might occur. There are some pertinent questions that one needs to answer in advance - in how much time a helicopter can access the site, is landing possible and if not, how to...

By Ritesh Gupta

It is fascinating to follow how offshore wind construction specialists respond to each weather forecast and hazardous operating conditions, and take a call on how to proceed with the task at hand.

There are times when one is left with no option, but just avoid any construction activity. For instance, in the case of the DanTysk offshore wind farm project, on one occasion a forecast indicated waves of up to 12.5m. Accordingly, the guard vessel was called back into port. It only returned after the storm had cleared. The project has now touched the half-way mark with 40 turbines in place. The 288 MW project is located 70km west of the island of Sylt in the German North Sea.

Weather conditions need to be assessed diligently, even when a mishap might occur. There are some pertinent questions that one needs to answer in advance - in how much time a helicopter can access the site, is landing possible and if not, how to hoist the medical specialist from the helicopter etc.

Understanding the impact of weather

As larger and more complex wind farms are constructed, the effects of weather down time can be spread over several separate installation cycles, giving an overall reduction of the weather down time percentage than for a one off installation.

“Local weather effects have to be understood as fully as possible in order to accurately budget using statistically correct forecasting of schedules,” says Samuel Park, Construction Manager, RES Offshore.

But Park quickly points out that weather does not understand statistics and has a habit of foiling or assisting as it pleases, which drives the necessity to plan and budget accordingly.

For its part, RES has dealt with this issue by developing its in-house ‘CORE’ model, which is capable of processing metocean, local weather, tidal streams, operating and associated cost criteria, and then producing various options (quickest, shortest, least cost etc.) that can be tailored to suit the risk profile of any developer.

The model can accommodate multiple port and wind farm locations and show the effects of the different combinations. Park shared that it can be used to identify bottlenecks in the installation process, compare installation options, identify preferred installation commencement date, and estimate installation costs.

CORE has been successfully deployed at Rhiannon Wind Farm (UK), Lincs Wind Farm (UK) and St Brieuc Wind Farm.The model can assist by being used at various stages of the project, from development through to offshore construction. The CORE model was validated against real downtime data collected by RES during the construction of an existing wind farm.

“Improved weather forecasting helps on an operation by operation basis, with almost real time satellite imagery allowing confidence in the forecasts received. Even so, the windows required per operation have been increased by projects and their insurers to mitigate the risk profile,” explains Park.

Overall, as Park points out, it would be fair to say that the main construction improvements are due to an increase in operability of vessels, the increased wind limits due to better handling equipment and better forecasting or in-field monitoring.

Risky operating conditions

Risky operating conditions at sea also call for apt handling of the staff, especially the task of transportation of service staff.

Park acknowledges the criticality of this aspect, and mentioned that the transportation and transfer of offshore personnel has long been an issue the industry has had to tackle; over a decade ago rigid-inflatable boats were often in use but their limitations were quickly shown and from then the industry’s fleet of dedicated transfer vessels has grown significantly both in terms of numbers and in the size and capability of the vessels themselves.

“Multi-hull and wave-piercing SWATH vessels have much improved comfort for passengers, ensuring personnel are as fresh as possible to commence their site work,” says Park.

“Tentatively, the industry has also been getting used to the idea of helicopter transfers, particularly for the more remote sites being built now.”

For near shore sites, the use of these specialised Crew Transport Vessels (CTV) has matured dramatically and is now a much safer operation, although CTV incidents are still occurring. Vessels typically are shaped to push onto the foundation boat bumper to provide a stable platform for personnel access and egress, and the industry has counted up many transfers using this method.

According to Park, transfer does depend to a degree on the skills, experiences and confidence of personnel themselves, including the vessels crew. To counter this personnel training is now better defined so that everyone travelling offshore is much more aware of what is required of them, what the risks are and how to minimise them.

“One way of mitigating the personnel competency element is the use of “walk to work” systems which are becoming more common-place on larger high-endurance vessels,” says Park. These systems eliminate the need for climbing up a vertical ladder from the boat landing, and help to increase the safety of personnel transfer system as a whole. They are not without downsides though – their reliability linked to their inherent complexity plus any failures by providers to integrate them fully as a system onto the vessel, have caused issues for the industry.

As things move further offshore new philosophies will have to be employed, be it mother-ships (‘flotels’), accommodation platforms or other new access concepts that will come to the fore in the coming years.