On the behavior of suction buckets in sand under tensile loads

Abstract

Suction buckets can be used instead of driven piles for the support of jacket or tripod foundations for offshore wind energy converters (OWECs). However, due to the relatively small self weight of offshore wind structures, considerable tensile loads can occur for instance during storms; these loads usually govern the dimensioning of the buckets. Under rapid tensile loading, suction pressures are induced inside the bucket, which can considerably increase the tensile capacity. This paper presents results of numerical simulations based on a coupled pore fluid diffusion and stress analysis which allow for the description of the partly drained load-bearing behavior as well as the quantification of the tensile resistance. It is shown that a high pull-out rate leads to a large increase of the tensile capacity. The maximum capacity is reached when the soil behaves fully undrained or when cavitation of the pore water occurs. In this regard, the main influence parameters are the bucket geometry, the soil permeability, the pull-out rate (loading rate) and, regarding cavitation, also the water depth. It is shown that the mobilization of suction pressures requires a large heave of the bucket, which might be inadmissible with respect to serviceability requirements. Simulations in which variable tension loads are applied with a specific loading rate and then kept constant are also presented. It is found that an accumulation of heave over time occurs when the load exceeds the drained capacity of a bucket. This indicates that cyclic loading of buckets with tensile loads often exceeding the drained capacity might lead to excessive heave.