SECTOR REPORT Ӏ RENEWABLES
Two new methods for North Sea lift by mammoet, SHL and dockwise
Cyprus-based Seaway Heavy Lifting (SHL), an independent offshore installation contractor with experience in the transport, installation and removal of offshore oil and gas platforms, subsea structures and the installation of wind turbine foundations, worked with Mammoet and maritime service provider Dockwise to perform an unusual North Sea lifting operation for Siemens which involved two revolutionary techniques. The task was to install the SylWin alpha converter platform, which will act as a giant ‘power socket’ for the DanTysk, Sandbank and Butendiek wind farms off the German and Danish coasts of the North Sea. Together the farms consist of 240 wind turbines and represent a generating capacity of 864 megawatts. The platform transforms the alternating current generated by the wind turbines into direct current that is transported by a 160km long seabed cable and 45km of underground cable to the onshore Büttel high-voltage station, where it connects to the German national grid. The converter platform is 83m long, 56m wide and 26m high and
weighs 14,000 tonnes; built by Siemens it is the largest ever installed in the North Sea. As it exceeds the lifting capacity of fl oating cranes, the 'fl oat-over method' was used to move it from a pontoon onto the already installed jacket or base of the platform. This approach has been used successfully in the Gulf of Mexico and in the Far East, but this was the fi rst time the method has been used in the North Sea. The platform was fl oated out to the jacket on its pontoon and was positioned over and between the jacket's legs. It was installed onto the legs of the jacket by ballasting the pontoon, thus lowering the platform. Dockwise was responsible for this phase. The second new procedure was devised by Mammoet. It used
strand jacks to lift the platform 15 metres up onto its fi nal position on the jacket. The use of strand jacks is not unusual but Mammoet devised a time-saving method, customised for this project, that allowed much faster removal of the strand jacks after the lift. In a revolutionary move, Mammoet reversed the normal positions of the lifting equipment to raise the platform. As a result, installation time at sea was reduced by 30 days. Converter platforms are designed to be lifted at sea after installation on the jacket. Each of the six platform legs has a jacking house built around it at deck level. The conventional method for the installation of converter platforms is to mount strand jacks at the jacking house and anchor blocks on top of the platform leg. The jacking house is a small space, which makes installing a
4500kg strand jack complicated work. A similar problem occurs during disassembly. Removal of the strand jacks from the jacking houses
Mammoet’s new strandjack method on the SylWin installation
involves rigging the anchor block, removing the wires and the anchor block, and fi nally removing the strand jack itself. Given that no fewer than 44 stand jacks were used on the lift, the procedure would have taken around 50 days. But Mammoet invented a way to install the system, which basically meant turning it on its head. Mammoet installed the strand jacks on top of the platform legs instead of on the jacking house, using special constructions called strand reel frames that were designed especially for this project. The reel frames were constructed so that the wires would wind back up on the reel while pulling up the platform, eliminating the need for time-consuming wire removal. In turn, the anchor blocks were installed at the jacking house. To remove the jacks Mammoet simply lifted the anchor blocks onto a pallet truck and rolled them in and out of the jacking houses. Since the anchor blocks weighed just 600-800kg apiece this presented few problems. The Mammoet method made it possible to remove the strand jacks straight after the lifting operation, without having to remove the wires from the unit fi rst. With the new method, it took just ten days to complete the operation at sea. Lifting only took one day, and removal of the equipment was just eight, making it possible for the platform to be commissioned a month early.
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