This paper addresses lateral pipe-soil interaction behaviour at the large displacements that occur with lateral buckling of a pipeline. Force-displacement response models were developed by the SAFBUCK JIP to replace the use of Coulomb friction approximations, which are unrealistic for the large lateral displacements that occur upon buckle formation and inappropriate for modelling the development of soil-berms that occur with lateral cyclic displacements in operation.
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 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
Break-out, during buckle formation based on different levels of initial pipe embedment;
Large amplitude displacement as the buckle forms;
Repeated cyclic behaviour, influenced by the building of soil berms.
The models provide an important basis for lateral buckling design guidance; they are being applied by JIP Participants on a number of current projects where pipelines are being designed for lateral buckling.
The aim of the SAFEBUCK JIP is to raise confidence in the lateral buckling design approach and improve understanding of the related phenomenon of pipeline walking; with experimental work at TWI on low cycle fatigue materials performance, and at Cambridge University on axial and lateral pipe-soil interaction.
The scope of work for the SAFEBUCK JIP is summarised in Figure 1, which highlights the key parts of the lateral pipe-soil interaction scope of work covered in this paper.
Figure 1 - Scope of work for the SAFEBUCK JIP (available in fullpaper)
Subsea pipelines are increasingly being required to operate at higher temperatures and pressures. The natural tendency of a pipeline is to relieve the resulting high axial stress in the pipewall by buckling. Such uncontrolled buckling can have serious consequences for the integrity of a pipeline. Consequently, the industry has generally sought to restrain pipelines by trenching, burying and rock-dumping; or relieving the stress with in-line expansion spools. A far more elegant and cost-effective solution is to work with rather than against the pipeline by controlling the formation of lateral buckles along the pipeline. Controlled lateral buckling is an efficient solution to the relief of axial compression.