Non-Linear Hysteretic Seabed Model for Catenary Pipeline Contact

Abstract

This paper presents a new mathematical model of the reaction force normal to the seabed, experienced by a pipeline or catenary riser in contact with the seabed. Such contact is currently often modeled using simple seabed contact models, and it is hoped that improved modeling of the seabed interaction will give more accurate predictions of system behavior, in particular for fatigue analysis. The model uses as its primary data the pipe diameter, the seabed soil shear strength profile with depth and the soil density. Additional parameters, in particular the maximum normalized stiffness of the pipe-soil response following reversal of motion, are used to derive non-linear hyperbolic functions that model the seabed resistance force as a function of the penetration. Different functions are used for the initial penetration, for uplift and for repenetration, and the function parameters are updated each time a penetration reversal occurs. This enables the model to capture the hysteretic behavior of the seabed response and the increasing penetration of the pipe under cycles of load in the vertical plane, although no attempt is made to model softening of the soil due to remolding. The paper documents the model equations and discusses their background and characteristics. The various non-dimensional parameters of the model that are used to control the resistance response are described and their effects are illustrated. The model is intended for use in practical engineering analysis and has been implemented in a commercial riser analysis program (Orcina 2008). The paper compares results obtained using the model against measured results for pipe-seabed interaction from laboratory and harbor experiments. It also presents results of using the model for engineering analysis of a riser under cyclic motions, and compares the resulting fatigue life with that obtained using a simple linear seabed model.