The pipelines and associated structures are major components of the subsea deep water oil and gas production. Deep water production requires the transportation of multiphase hot products under high pressure and temperature. In deep water the soil is usually very soft clay of high plasticity with high water content. Both of these elements are major aggravating factors for pipeline instabilities: lateral buckling and pipeline walking. The paper presents and illustrates practical measures which are developed by the contractors for mitigating the risks of failures associated to these instability phenomenons. The analyses which are carried out for the lateral buckling and pipeline walking are generally very conservative. Such attitude is fully understandable with regards to the risks of failure in deep water. However the mitigation measures should not be inducing unwanted effects.
The instability phenomenon associated to the variations of pressure and temperature become more acute in deep water fields. When dealing with lateral buckling and pipe walking it is considered safer to provide mitigation measures to prevent these conditions than attempting to design the pipeline system to accommodate the loads and displacements. With time contractors have developed different techniques to mitigate the risks. The purpose of this paper is to discuss the different techniques used in the industry. It will present also the negative effects which may be resulting from some measures and will describe some techniques which are new or nor yet in usage.
The pipeline displacements induced by pressure and temperature will result in stresses in the pipeline itself and the connection spools and subsea structures. The continuous production cycles of start up and shut down result in cyclic loading and progressive movements of the flowlines. This will result in cumulative fatigue damage to the flowlines, which, if not considered in the overall design of the flowline system, can lead to failure and rupture of the flowline. Failures may occur on different types of pipe: single pipe or pipe-in-pipe as illustrated in the Figure 1. The fracture surfaces are smooth indicating a fatigue failure with a number of cycles in the range of one hundred thousand to one million.