Dynamic Response of a Suction-Caisson Foundation-Structure System through Numerical Simulations and Model Testing

Suction caissons are widely recognized as a preferred foundation solution for both subsea structures and wind farms. Yet, the seismic performance of these systems is often not fully captured. Typically, analyses focus solely on foundation capacity by applying a quasi-static acceleration, which overlooks the crucial inertial interactions between the structure and the surrounding soil. A thorough design verification analysis should rigorously document the complete motion of the unit as well as the dynamic response of piping and other equipment on the unit and of any external pipes connected to it. The work described in this paper is the earthquake design verification performed for a four-caisson-supported manifold at the Shah Deniz field in the Caspian Sea. The work also included documentation of the earthquake performance of the jumper pipes and spool pipes connected to the manifold at one end and to the single wells and flowline termination assembly in the other end. Detailed finite-element time series analyses were performed accounting for interaction between the soil and the manifold structure. The soil model consisted of nonlinear springs accounting for hysteretic loading–unloading features. The development of this model was supported by large centrifuge tests. Earthquake time-history motions were applied at the far end of the springs. Examples of the resulting manifolds global motions are presented in this paper. The manifold structure as such is robust in resisting the earthquake motions. The main concern is related to piping and other dynamically sensitive equipment. In this paper, examples of the separate analyses performed for jumper and spool pipes are described, including modeling of soil resistance and earthquake input motions. The main features of the response, such as displacements, accelerations, and forces at different locations, are described and presented in the paper.