Abstract

The touchdown zone (TDZ) is the most important part of the steel catenary riser (SCR) under the complicated loadings. It is the key issue to evaluate the service life for the SCR in the deepwater. In accordance with the geological data in the South China Sea region, a computational model, which is with the feature of the solid-spring interaction under the real loadings of TDZ pipelines, is proposed herein based on Winkler elastic foundation model. The dynamic characteristics and response of the pipeline in TDZ is calculated by using the model upon the software ABAQUS. The nonlinearity of material and geometry, as well as the contact situations, are accounted. Stress, displacement response of pipeline in TDZ are given in this paper. The factors of environmental loadings are discussed. Simulation results show that when dynamic load with different frequency is applied on the pipeline, its dynamic response changes a lot. The natural frequencies of TDZ is changed with the external pressure, internal pressure and vertical displacement loading. The research results is valued to estimate the damage mechanism and the life time of the pipelines under the action of the multiple loadings in TDZ.

Introduction

Steel catenary riser (SCR) is a critical system connecting floating platform system (FPS) and subsea pipeline in deep sea engineering. The structure is relatively flexible compared with the fixed offshore platform. This greatly reduces the service life of the riser in touchdown zone (TDZ), as shown in Fig. 1. The critical point when the catenary riser firstly contact the seabed soil is the touchdown point (TDP). When SCR on service moves under marine environment loadings, a new TDP will be generated, forming a region for pipe-soil dynamic interaction, called touchdown zone (TDZ). Pipe-soil interaction in TDZ is really complex and it's one of the key issues in SCR structure design. The main techniques for investigating TDZ of SCR include experimental method and numerical analysis method. Bridge et al. (2002, 2004, 2005) and Hodder et al. (2010) respectively conducted full-scale test and laboratory scale model test of pipe and soil interaction in TDZ. Their previous work on pipe-soil interaction laid a solid foundation for the numerical model assessment and validation. In numerical simulation, a variety of assumptions were put forward by scholars. Palmer (2008) numerically investigated the pipeline subsidence process in TDZ based on rigid plastic seabed assumption. Guo Haiyan et al. (2009) simulated the interaction process of steel catenary riser and seabed soil by using the contact element based on the platform of ANSYS finite element software. Du Jinxin et al. (2008), Huang Weiping et al. (2013) and Bai et al. (2015) all established the finite element contact model of the interaction between a SCR and seabed based on an extensible curve beam with a large curvature and a beam on an elastic foundation. However, studies of the dynamic response in TDZ is very limited. This paper proposes a solid-spring model to explore the dynamic response of pipeline in TDZ.

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