Offshore wind turbines have several advantages over their dry footed counterparts, not least their better power output. But construction is much more difficult. Marine contractor Seacore has been working from its Deep Diver jack-up pontoon, battling against appalling weather to erect five wind turbines onto rock socketed monopile foundations it had drilled into the seabed, 4km off the south western coast of Gotland, Sweden.
The 2.5MW windfarm in the Baltic Sea is for client and power generating company Vindkompaniet. The five 500kW turbines, each supported on a single tubular steel tower and standing in 7m of water, are arranged in a 60 V pattern and spaced 300m apart to minimise interference. Each windmill, with its 37m diameter three bladed rotor, is bolted to a 2.1m diameter steel monopile grouted into a mating socket drilled about 10m in the seabed rock.
'We originally planned to use 400t-500t concrete caissons for each turbine foundation simply because two similar and earlier projects in Denmark opted for concrete,' says Vindkompaniet partner and project leader Gte Niclasson. 'However we changed our minds at the last minute after we found out about Seacore's monopile technique.' Seacore convinced the client that it was
a viable alternative. 'The monopile solution saved us about 10% to 15% on the overall cost of the windfarm project.'
Vindkompaniet started investigating using offshore windpower early in 1995 following planning problems with a land-based windfarm. 'Wind power needs a much larger area than is available on land,' says Niclasson. 'Wind turbines are also far more efficient offshore in the open sea as they are not affected by land contours.' Wind at the tops of 200m-300m high hills is similar to that over the open sea. 'But in Sweden, most of the land is forested which acts as a wind break. So there would be a big difference in the yield from sea and land-based windfarms.'
Niclasson indicates that on a clear open onshore site one of its 40m hub height turbines would generate 1.3M.kWh/year, but this would drop dramatically to around 800,000kWh/year if the same turbine was in the middle of Gotland and 15km from the sea. 'However if we go offshore with an identical 40m high turbine we get 1.6M.kWh/year, that is about double the output,' says Niclasson.
Seacore moved on to site at the beginning of September.
The firm did a very good job, says Niclasson. 'We spent a lot of time planning the logistics and had to work round some extreme wind and weather conditions to complete the drilling and erection before winter,' says Seacore project manager Jason Clark.
The extreme weather resulted in Seacore staying on site for about three months although the actual work only took four weeks. 'We are convinced that had the client chosen the original massive concrete caisson foundation system, which would have required much heavier lifts, the delays due to bad weather would have been far greater,' adds Seacore director Peter Clutterbuck.
Seacore drilled all five rock sockets and installed the first 21m long monopile sections of the turbine towers prior to adapting the Deep Diver jack-up pontoon to complete the windfarm erection. The drilling team used a long conductor or guide tube to ensure the drill bit bored a straight and vertical socket hole. The 2.25m internal diameter conductor was temporarily connected at its toe, to a casing shoe section. The assembly was lowered on to the seabed.
Seacore's 30t reverse circulation hydraulic rotary drill was lifted on to the top of the flooded conductor. The bit was lowered through the rig and down the guide tube to the seabed and bored through the mudstone and limestone rock. Compressed air forced the seawater and drilled chippings mixture back up the centre of the hollow drill pipe. The drill bored a 2.25m diameter hole and as the bit progressed the 2.29m outer diameter casing shoe with its attached conductor followed closely behind. The casing and conductor assembly usually stopped after the casing had penetrated about 500mm into the rockhead. Drilling then continued to full 10m penetration.
With the hole complete, the bit and bottom hole assembly was withdrawn and conductor tube detached. The shoe was left proud of the seabed to prevent the open hole from collapsing and ready to accept the first monopile section. Seacore floated the buoyant piles and towed them 8km from its onshore base at Burgvik harbour.
The pontoon was jacked down to sea level to act as a wind break. Its onboard crane was used to lift the buoyant tube which was slowly lowered as water was pumped in. In its final position, about 3m protruded above sea level. Grout was tremied down to fill the base void and the annulus between the tube and cased hole.
After all five monopile base sections were installed, Deep Diver returned to Burgvik for the drill rig to be removed and the special turbine rotor assembly jig to be mounted.
The pontoon's jacking legs were also extended by 11m to 45m to allow the erection of the five turbines' remaining tower sections, nacelles and rotors.
The water inside the rock socketed pile was first pumped out and a 1.8m tall steel ice shield lifted into place. An electrical transformer and control box were fixed.
The next 15m tall 15t tubular mast section was positioned on top of the monopile base and both sections were bolted together. Seacore jacked up Deep Diver about 12m and similarly positioned the 18.5m high, 13.5t tapered top tower section.
Raising the pontoon a further 9m gave a 21m air gap between the underside of Deep Diver and the sea. This allowed the main nacelle, weighing 21.5t, to be lifted. The three rotor blades had earlier been placed in a cradle on the pontoon ready for lifting. Work could then shift to the next turbine.