Increasing Productivity and Safety of Pipe Lowering Through Finite Element Analyses

It is common practice in the construction of buried pipelines that pipe sections are welded together at surface level and then lowered into a trench. The stresses induced to the pipe during lowering operation are seldom evaluated or considered in the pipeline design. Scarce industry guidelines and procedures are empirical. Simplistic (while not always conservative) calculations typically used to prevent yielding lead to restrictive spacing between lift points. This paper shows that the use of finite element analyses to realistically simulate the pipe lowering operations can significantly increase the distance between side booms (and hence the productivity) while increasing safety. Case study analyses were performed using data from a real pipeline under construction, showing that limiting the pipe span length is not sufficient to prevent yielding, while by adequately studying the lowering procedure, significantly larger spacing between lift points was achieved. The analyses were performed using a commercial software package widely used in offshore pipeline analyses. The effect in terms of induced stresses of additional issues such as route curves, which are difficult to evaluate in simple calculations, was also discussed. Further advantage can be taken from being aware of the actual stresses imposed to the pipe during its construction, for example by using weld defect acceptance criteria based on fracture mechanics rather than workmanship.