This paper reviews various methods available for surveying and monitoring the condition of laterally buckled flowlines in deepwater, with sufficient accuracy to meet integrity management requirements.
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 compressive loads by buckling. In deep water developments, various methods have been adopted to encourage and control lateral buckling as a cost-effective and practical solution. However, to demonstrate that the selected method is effective in practice and, hence, that a flowline is fit for purpose, it is necessary to obtain positional data of high accuracy to feed into the lateral buckle response and fatigue models, post-lay, after start-up, and throughout operational life. Experience from the integrityassessment of a number of pipelines has shown that estimating loads within lateral buckles by matching FE-generated profiles to survey-generated profiles is not easy, even in relatively shallow water. This is primarily due to survey accuracy limitations, which tend to increase with water depth. These challenges are demonstrated with examples from actual projects.
The operating condition of HPHT (high-pressure hightemperature) flowlines means that they are susceptible to lateral buckling. A lateral buckle results from instability due to axial compressive loading, otherwise known as Euler buckling. Extreme conditions can develop within a lateral buckle; on first load, the stresses can exceed yield and may involve significant plasticity, whilst in normal operation, regular shutdowns can lead to high stress cycles.
The key limit states in a lateral buckle are:
In the case of lateral buckling, local curvature in the buckle is the critical parameter, and there are methods for measuring this. However, most monitoring methods involve determining the position of the flowline, and deriving the curvature from this. Clearly, the accuracy of the determination of load and fatigue accumulation therefore depends upon the accuracy with which position, or shape, can be measured;