Advances in Pipe-soil Interaction Methodology and Application for SCR Fatigue Design
Authors: Sharma Partha Pratim , Aubeny Charles Paul 
Published: May 2011

Pipe-soil interaction is a complex phenomenon which is dependent on several factors including shear strength of the soil, pipe diameter, rate and magnitude of loading. This paper presents recent advances in the methodology for modeling pipe-soil interaction effects and its application to SCR fatigue design. The pipe-soil interaction model used in this study is based on laboratory measurements and finite element analysis. The model has also been calibrated to a limited number of field measurements. The proposed methodology is applied to fatigue analysis of a 20-inch oil export SCR in 1500 m water depth.

The global motions of the riser suspended from a semi-submersible are determined for different sea-states. The response time series is then post-processed to generate the range of vertical and horizontal motions of a point near the seabed. The motions of the point are analyzed using the calibrated pipe-soil interaction model to obtain the stabilized trench depth and seabed stiffness. The trench profile and the seabed stiffness play a significant role in computed fatigue damage estimates. The evolution of the trench profile and seabed stiffness under repeated cyclic loading is presented herein. Guidance is provided for selection of the stabilized trench geometry and seabed stiffness for use with global riser analysis software.

Introduction

The steel catenary riser (SCR) is the preferred riser solution for the cost effective development of deepwater fields. Conceptually, the SCR is relatively simple, easy to fabricate and install, and has been successfully adapted to variety of floaters including Spars, tension leg platforms (TLPs), semi submersibles and floating production storage and offloading vessels (FPSOs). SCR design is often driven by the fatigue damage considerations at the floater hang-off location and the touchdown zone (TDZ). The fatigue damage mechanism at the floater hang-off point is relatively well understood and is mitigated by using specialty joints such as flex joints and tapered stress joints. The fatigue damage assessment at the TDZ is more challenging, primarily due to the non-linear interaction of the riser pipe and the seabed. Further, this area of the riser is not easily inspected, and due to the inherent uncertainty in modeling the riser seabed response, design standards such as API RP 2RD require a higher factor of safety for the fatigue design of the riser pipe in the TDZ.

In the last decade, several joint industry projects such as CARISIMA and STRIDE have investigated the non-linear pipe-soil interaction. More recently in 2006, the erstwhile Minerals Management Service supported a research project at Texas A&M University to study the seafloor interaction with SCRs. The study resulted in an analytical framework [1] to describe the complex SCR seafloor interaction. The analytical framework is based on experimental data [2] [3] [4] as well as 2D finite element simulations of the local pipe soil response. This analytical model has been further refined and calibrated by more extensive model tests at the Norwegian Geotechnical Institute (NGI) [5] and [6]. This paper provides a brief description of the soil model presented in [1] and [7] and discusses the application of this model to fatigue assessment of SCRs. Results from fatigue analyses are presented for flat and trench seabed profiles.