Suction Caisson Foundation Design for Vortex-Induced Vibration Loading
Authors: Al Khafaji Zaid , Audibert Jean M.E. , Hossain Mohammad Kabir , Templeton J.S. , Clukey Edward C. , de Jong P.R. 
Published: October 2008

Vortex induced vibration (VIV) can be an important source of loading on deepwater production structures, and one that is drawing increased attention in recent times. The fact that cyclic loading due to VIV can be an important foundation design issue is discussed for the first time in this paper. This paper presents the methods that have been applied for the BP Holstein project to design the suction caisson foundations, which are subjected to cyclic loads from VIV. Both traditional cyclic pile analysis methods and effective-stress finite element analysis methods are discussed.

The methods presented in this paper are in principle applicable to any offshore pile with cyclic loads, but the principal applications will be to suction caisson foundations for deepwater production systems with significant vortex induced cyclic mooring line loads.

There are many possible sources of cyclic loading effects including:Effects built up during sub-yield cycling due to (small) inelastic behavior, notably the accumulation of plastic strain in small amounts per cycle over large numbers of cycles;Effects of cycling at (or near) yield due to accumulation of plastic strains in larger amounts per cycle; andSoftening effects of repeated cycling at yield on the dry side of the critical point.

Conventional cyclic analyses can incorporate the effectsof the first and second sources of cyclic loading effects mentioned above on skin friction capacity through a phenomenological approach fitted to cyclic test data. In the work presented in this paper, incorporation of these effects did not lead to significant reduction (or degradation) in skin friction capacity because the cyclic load component was not large enough to accumulate in the relatively small (compared to storm wave loading cases) number of cycles. The effective stress finite element analyses can incorporate the second and third sources of cyclic loading effects through a fundamental approach. For the skin friction, hardening effects may cause some cyclic changes in skin friction capacity contribution. For the end bearing, softening effects dominate, eventually causing some cyclic loss of effective stress end bearing capacity due to repeated incremental work softening on the dry side of the yield surface. For the end-bearing zone, the effective stress-captured effects were found to be significant.

The paper gives insight and guidance into the design of suction caisson foun dations for effects of long period VIV cyclic loading conditions. Discussed are adaptation of traditional methods of accounting for VIV cyclic load effects on caisson skin friction capacity plus new methods forinvestigating cyclic load effects on reverse end bearing. The authors believe that this is the first application of such combined methods to account for VIV loads on suction caisson foundations.

Introduction

BP plans to develop its Holstein prospect in Blocks 644 and 645 of the Green Canyon Area in the Gulf of Mexico, which includes installation of a Truss SPAR structure anchored to the seafloor by four suction anchor clusters of four suction caissons each.

The objective of the study was to investigate the holding capacity of the suction caissons for long period VIV cyclic loading conditions.