Regular readers of Cranes Today will know that Horns Reef, off the coast of Denmark, is the world’s largest offshore wind farm. The 160MW wind farm will be operated by Elsam. Danish firm MT Højgaard was main contractor for the civil works and it subcontracted the installation of 80 monopiles, each weighing 165t and measuring 33m long, to Mammoet Van Oord Windmills BV. The 80 turbines were placed on the foundations by A2Sea, as subcontractor to turbine manufacturer Vestas.
Both A2Sea and Mammoet Van Oord Windmills were established to undertake offshore lifting work in general and turbine installation specifically. The latter commissioned a E25m jack-up barge specially for the project. The Jumping Jack is fitted with a 1,200t capacity Manitowoc M1200 Ringer.
Foundation work began on 18 March this year. However, technical problems delayed delivery of the barge to such an extent that the Horns Reef (Horns Rev in Danish) foundations were all in place before it ever had chance to prove itself on the project. Mammoet Van Oord Windmills instead hired two other barges, the Buzzard from Ballast Nedam and the Wind from De Brandt, and was still able to finish its part of the project by 3 August, a full four weeks ahead of schedule.
A2Sea began in mid April once the first monopiles were in place and also finished well ahead of schedule, on 21 August. As previously reported, its vessels – Ocean Hanne and Ocean Ady – are standard cargo ships converted to carry two wind turbines at a time, complete with blade sets, and each fitted with stabiliser legs, and a Demag CC 2500 on a pedestal.
Kurt Thomsen, technical director of A2Sea, says that at peak it was installing turbines in less than six hours and a total round trip with two turbines – i.e. loading, sailing and installation – was being completed in less than 26 hours.
Work was not always conducted at peak speeds, of course, and he admits that A2Sea also had its own technical problems. This is not surprising, he says. ‘It is foolish to to think that two ships could be fitted with a system never used before and then work without repairs and modifications on the go. Therefore we allowed for intensive testing prior to installing the first turbine to detect and correct all the small items that we knew would arise.’ Spooling devices were added to winches and tests were conducted to measure any excess accelerations on the turbines or crane during transit and installation.
Thomsen admits: ‘One problem kept bugging us and the reason was not obvious, as the calculations of forces, loads and safety factors were not exceeded at any time. It was not an item that influenced the integrity and safety of the vessel, but the ropes that lift the stabiliser legs wore extremely quickly and had to be changed quite often. Unfortunately the problem did not arise until mid May, when work was well under way, and it took some time to solve, as the reason was not obvious.
‘Finally the reason for the frequent breaking of the lifting ropes was determined. The bearing material in the underwater sheaves could not endure running dry for the amount of time it took for the legs to be lowered under water. Thus the bearings were shot out of the sheave and they ran without any bearing at all. The problem was solved by changing the type of bearings that we were using.
‘This meant, however, that the ships had to be taken out one by one and have the bearing material in the underwater sheaves replaced, an operation that lasted five days per vessel. Fortunately this coincided to some extent with the bad weather periods that occurred, preventing work anyway, but some delay in the installation process was endured. After the repair, the system worked superbly.
‘We had just about caught up when we experienced damage to three of the baseplates [from the stabiliser legs] that unhinged themselves and dropped to the seabed around the wind farm. Three new baseplates were produced within five days in a Rotterdam shipyard and Ocean Ady was back in business within a week. During that time Ocean Hanne managed to install 12 turbines within 10 days, and with Ocean Ady back we managed to install a total of 15 turbines within 14 days.
‘The overall performance of the vessels has been formidable, and the crew working both on the pier and on the turbines has continuously been achieving better loading and installation times, starting from 18 hours down to 8 hours 30 minutes for a total turbine. In the end, the project was carried out faster than expected, finishing 13 days early despite a two week delay at the start.’
Thomsen says that, even with the hitches, A2Sea could have finished its part in the project a month earlier but for a decision made by the client at the outset to take things slowly at first. ‘When installation first started, the decision was made by the client to sail out just the towers to allow the crew to get used to working in the new environment. We had installed 17 complete turbines before we were allowed to transport blades and towers together. We could have done it in half the time. Had this decision not been made we would have installed the total wind farm one month early.’
