Buckle Propagation and Its Arrest: Buckle Arrestor Design Versus Numerical Analyses and Experiments

Abstract

Buckle propagation under external pressure is a potential hazard during offshore pipeline laying in deep waters. It is normal design practice to install thicker pipe sections which, in case of buckle initiation and consequent propagation, can stop it so avoiding the lost of long pipe sections as well as threats to the installation equipment and dedicated personnel. There is still a series of questions the designer needs to answer when a new trunkline for very deep water applications is conceived: • What are the implications of the actual production technology (U-ing, O-ing and Expansion or Compression e.g. UO, UOE and UOC) on the propagation and arrest capacity of the line pipe, • How formulations for buckle arrestors design can be linked to a safety objective as required in modern submarine pipeline applications. The answers influence any decision on thickness, length, material and spacing of buckle arrestors. This paper gives an overview of buckle propagation and arrest phenomena and proposes a new design equation, applicable for both short and long buckle arrestors, based on available literature information and independent numerical analyses. Partial safety factors are recommended, based on a calibration process performed using structural reliability methods. Calibration aimed at fulfilling the safety objectives defined in DNV Offshore Standards OS-F101 and OS-F201.

Stress concentration factors at welds in pipelines and tanks subjected to internal pressure and axial force

“…However, increased wall thickness or stiffeners may be required at supports and at crack arrestors in deep water pipelines (e.g. [5]). This may introduce stress concentrations that make the stress cycling in the axial direction of the pipes important.…”

“…For steel with n ¼ 0.3 the elastic length becomes l e ffi 0:78 ffiffiffi ffi rt p (5) and with reference to Fig. 2 the following definitions are made:…”

Parametric Study of Offshore Pipeline Wall Thickness by DNV-OS-F101, 2010

“…where    = characteristic resistance for external pressure (collapse) (Torseletti et al, 2003). The propagating buckle criterion is as below: (1) Internal over-pressure…”