Oxford University, Civil Engineering: Research: Offshore Structures and Foundations: Caisson Foundations for Wind Energy Applications

Caisson Foundations for Wind Energy Applications

Suction Caissons are a novel foundation that might be used in a variety of applications. For example the developing offshore wind industry has expressed considerable interest in using this design for offshore wind turbines and anemometer masts. This case presents an interesting problem, as there are very high moment and horizontal loads, but low vertical loads, a very different pattern to that experienced by oil and gas structures. The wind and wave directions may not be co-incident, so the base shear and moment are not in the same direction. Considerable uncertainty therefore surrounds how the foundations may perform. An external group (Offshore Data Ltd) has been considering the viability of using these foundation types. Several field scale tests have been undertaken to prove various aspects of performance. Some of the work was sponsored by EPSRC (£16 500) and the Rhodes Trust (B.W. Byrne) and is in collaboration with Offshore Data Ltd (A joint venture which comprises of EDRO Limited, Mustang Marine (Wales) Limted and Solar Wheel).

First Trial at Tenby Harbour, South Wales - August 1998

An initial trial was staged from the Quay at Tenby Harbour. The trial was used to assess the ease of installation of a suction caisson. Once installed the caisson was subjected to fast and slow tensile loading tests to assess theoretical calculations of capacity. The photo on the left shows the caisson being prepared for installation, with Prof. Guy Houlsby standing in front for scale. Workers on top of the caisson are attaching hoses through which the suction was applied during the installation. The photo on the right shows the caisson being lowered steadily into the water by a crane. Once a firm seal with the sea-floor was made suction was applied and the caisson embedded itself. The crane used for lowering the caisson in to the water was also used to apply the tensile loading. Once the loading tests were completed the caisson was removed using overpressure and installed in a different location to further the understanding of the installation process.

Second Trial at Burry Port, South Wales - September 1998

A second trial was held at Burry Port to assess the moment capacity of the caisson. The photo on the left shows the set-up of the caisson and loading frame at low tide before the caisson was installed. The photo on the right shows the layout during the loading test. A horizontal load was applied at some distance above the top of the caisson - applying a moment to the caisson. Rotation, pore pressure and horizontal load were recorded during the test, which enabled an assessment of the rotational capacity of the caisson. Once the test was completed the caisson was removed using overpressure.