It resembles a tripod, but is asymmetric and with one leg missing. That is because the installation had to be made easier, Project Engineer Thijs Visser explains.
By Anne Korsgaard and Karsten Prinds
Designed for water depths from 20 to 60 meters and turbines up to 6 MWs, the main advantages of the Self Installing Wind Turbine (SIWT) is easy installation and simple construction. The support structure is sailed to site in one piece – with the turbine mounted.
- We selected the asymmetric construction to enable transport and installation from a standard flat top barge, DP vessel. It is therefore very versatile from a transport and installation viewpoint, whereas symmetrical jackets or tripods are not. Besides this, the asymmetric layout allows easy horizontal fabrication before upending, which is not possible with symmetrical tripods. This reduces fabrication costs. At SPT Offshore they prefer the term ‘Braced Monopod’ about this type of construction. The structure has a long history:
Tripods in the shape of braced monopods have been used all over the Gulf of Mexico, but are assembled in a totally different way than the symmetrical tripods. The braced monopods basically use a driven caisson and several driven diagonal piles that are clamped at the top.
SIWT will fit on any standard barge with a length of about 120 meters and a width over 36 meters. Damping of the suction piles in the water and the relatively high centre of gravity of the SIWT results in a benign behaviour of the barge. Installation can be performed in sea states of up to 1,5 meters.
At the seabed, the SIWT uses suction piles for foundation. The greatest challenges we see are the high sea fastening loads and the problem of how to incorporate these within the structure at minimal cost. At this stage, everything has been calculated and numerically modelled:
- We plan to perform model testing in a wave tank and commence detailed design for a demonstration project, provided we can find a client who wishes to participate in it.
How to install the support structure and turbine using a barge and a pump
1. Suction piles are placed temporarily at harbour seabed.
2. Assembly of support structure, nacelle and rotors on top of the foundation.
3. Sailing it to location.
4. After lowering to the seabed, the structure is installed with the suction piles.
Illustrations: SPT Offshore