Evolution of riser-soil stiffness in a soil crust layer

Abstract

Site investigation data have shown that a crust layer may occur at the surface of many seabeds, with the crust exhibiting undrained strengths up to an order of magnitude higher than that of underlying normally consolidated sediments. This may affect the fatigue life assessment of steel catenary riser (SCR) as a result of riser-soil interaction. The paper presents data from centrifuge model tests, investigating the changes in riser-soil stiffness during cyclic motion of a rigid model pipe (simulating a riser element) within a model crust layer. The paper reports data from a cyclic test with 2000 cycles at a cyclic amplitude of 0.01D. The soil stiffness decreased initially for the first 300 cycles, but increased thereafter, indicating the importance of quantifying the long-term stiffness changes in this type of soil layer. The results are discussed in relation to current guidelines for estimating suitable values of seabed stiffness for SCR fatigue design.