A study for estimating elongation and deflection in deepwater pipelines during hydrostatic testing using analytical methods and FEM

The contribution of pipelines, towards the transportation of crude oil and gas and refined petroleum products is immense. The issue of pipeline integrity holds a great importance for continuous and safe operation. Hydrotesting is the most common practice for testing both onshore and offshore pipelines for maintaining integrity before or after putting to service. The present study is related to a regulatory issue. The primary concern is about the change in length of the pipeline during flooding of the structure for the purpose of hydrostatic testing. It is expected that the change in length could be sizable. Excessive change in length can jeopardize the pipeline structure and the terminal facilities too. The solution methodology is based upon a static analysis approach. Two scenarios are considered i.e. when the pipeline is supported throughout the length and when it is free spanning. In the first scenario the elongation is calculated. A coupled development of elongation and deflection is investigated in the second scenario. Both analytical and numerical approaches are employed to obtain the solution for different loading configurations. The analytical solution is obtained by employing Thick Walled and Thin Walled pressurized cylinder theories while FEM is implemented for the numerical approximations. ANSYS software is used for implementing a numerical solution. SHELL63 and SOLID45 elements have been incorporated in the first and second scenario respectively.; It is found that the overall structural behavior of the pipeline, regarding elongation, would be that of an uncapped vessel. Thick Walled and Thin Walled Pressurized Cylinder theories can be used along with FEM successfully. Choosing between different methods should be a matter of required degree of accuracy only. The elongation ranges from .035 mm/m to .09 mm/m from the depths ranging from 800 m to 3500 m of water for a fully supported structure. A 50 m free spanning empty pipeline length section can experience a vertical deflection of about a meter which is quite significant. Deflection due to bending of free spans during the flooding of deepwater pipelines for hydrostatic testing, thus, requires attention too.