Pressure Cycling of Suction Buckets in Clay

Abstract

Suction buckets are being increasingly considered as foundations for offshore wind turbines, as they can be installed by pumping water from the interior of the bucket, minimizing acoustic emissions. A key design consideration is ensuring that the bucket can be installed under suction pressures that do not cause cavitation or structural buckling. Pressure cycling may be employed during installation in an attempt to reduce the required suction pressure. This involves reversing the pump flow intermittently to cause vertical displacement cycles with the intent that this will degrade the frictional resistance at the skirt/soil interface. This paper considers the effectiveness of this approach in clay seabeds through experiments at model scale in a geotechnical centrifuge and using data obtained at full scale at a site in the North Sea. The data indicate that although one-way pressure cycles are ineffective, two-way pressure cycles can reduce the required suction pressure significantly. A simple model is outlined that extends calculation approaches for monotonic installation by including an interface strength reduction factor established from cyclic cone penetrometer test data. This model is shown to provide good agreement with field installation records and provides support to the use of cyclic cone penetrometer tests in offshore site investigation for developments where pressure-cycled suction bucket installations may be required.