Cyclic Tests of Model Pipe in Kaolin

Abstract

Summary Soil-pipe interaction is an important factor in the design of steel catenary risers. Pertinent aspects of this issue include the backbone curve defining soil resistance under conditions of virgin penetration, equivalent soil spring stiffness during unload-reload cycles, and permanent displacements under cyclic loading. This paper presents the findings of single-gravity model tests in soft kaolin that were conducted to investigate these issues. Cyclic tests were conducted for small amplitude loading involving no reversal in the direction of the soil-resisting force, and large amplitude loading involving such reversals. Presented in this paper are measured relationships of soil secant stiffness as a function of displacement and load cycle for conditions of unloading and reloading. The rate of accumulation of permanent displacements under cyclic loading is strongly affected by the magnitude of loading. Backbone curve measurements during virgin penetration are in general agreement with analytical estimates. The laboratory-model test data were used to derive seabed stiffness parameters for a seabed-riser interaction model. An example analysis is presented for a typical steel catenary riser touching down on a soft seabed.