Pipe/Soil Interaction Behavior During Lateral Buckling

Abstract

This paper addresses lateral pipe/soil interaction behavior at the large displacements that occur with lateral buckling of a pipeline. Force-displacement-response models were developed by the Safe-buck joint-industry project (JIP) to replace the use of simple friction-coefficient approximations. Such simplistic models are unrealistic for modeling large lateral displacements or the building of soil berms that occurs with cyclic lateral loading. The models are based on large- and small-scale tests carried out by the Safebuck JIP on deepwater soils from the Gulf of Mexico and west Africa, as well as on kaolin clay. To this database was added project-specific test data donated by JIP participants. Four stages of pipe/soil interaction are considered: Embedment of the pipe at installation. Breakout during buckle formation on the basis of different levels of initial pipe embedment. Large-amplitude lateral displacement as the buckle forms. Cyclic lateral displacement influenced by the building of soil berms. While breakout loads have been the subject of much research and published papers on pipeline stability, there is little guidance on modeling lateral resistance at the large displacements experienced in lateral buckling. There is also little guidance on modeling subsequent large-amplitude cyclic behavior, which occurs with each shutdown and restart of the pipeline. New equations were proposed where appropriate, and recommended models for each part of the characteristic response were developed. These models provide a valuable basis for lateral-buckling design guidance. They currently are being applied by JIP participants on a number of projects in which pipelines are being designed for lateral buckling. Background of the Safebuck JIP The aim of the Safebuck JIP (Bruton et al. 2005) is to raise confidence in the lateral-buckling-design approach and to improve understanding of the related phenomenon of pipeline walking. Experimental work is being undertaken at the Welding Inst. on low-cycle fatigue-materials performance and at Cambridge U. on axial- and lateral-pipe/soil interaction. The JIP has been very well supported by the offshore industry. BP, ConocoPhillips, ExxonMobil, Petrobras, and Shell, as well as the U.S. Government through the Minerals Management Services, participated in Phase I, with installation contractors and suppliers represented by Allseas, JFE-Metal One, Technip, and Tenaris. Additional participants including Acergy, Chevron, Statoil, and Saipem have joined Phase II, which will run through 2005 and 2006.