A New User Defined Element for Nonlinear Riser-Soil Interaction Analysis of Steel Catenary Riser Systems

Abstract

Steel Catenary Risers (SCRs) provide a technically feasible and commercially efficient solution for the offshore field developments in deep waters. Fatigue design of SCRs in the touchdown zone (TDZ) is among the most complicated engineering challenges in riser design. The cyclic interaction of the riser with seabed leads to a number of complex nonlinear behaviors including soil suction, separation of the riser from the soil, trench formation and degradation of soil resistance during cyclic loading. Accurate simulation of the riser-soil interaction has significant effects on the fatigue performance in the TDZ. Few hysteretic nonlinear riser-soil interaction models have recently been introduced and some of them have been implemented in commercial software packages for analysis and design of riser systems. Due to complexity of the models and also limited access to special software packages with in-built nonlinear soil models, traditional simple linear soil models are still being used widely for riser analysis, in particular for fatigue design. In this paper, one of the existing nonlinear hysteretic seabed model, already been used in a commercial analysis program OrcaFlex [1], has been implemented into general finite element software Abaqus [2], through the coding of a user defined element (UEL) subroutine. The paper documents the implementation of UEL into Abaqus and the establishment of global riser model for both static and dynamic analysis on which the pipe is modelled efficiently as series of unidirectional beam elements from floater to seabed, resting on a bed of nonlinear springs. Longitudinal friction between pipe and seabed has also been considered. A series of simulations are performed to illustrate the capabilities of the model. All these results have good agreement with those from OrcaFlex. Results indicate that the proposed UEL is capable of modelling nonlinear riser-soil interaction phenomena and has been verified to be a cost-effective alternative to OrcaFlex in terms of global analysis of SCRs. In addition, as an open source code, UEL provides the required tool for future development on nonlinear soil models. A new type of nonlinear soil with bilinear soil shear strength is modeled and its effect on structural performances of SCRs is investigated.