Analytical Formulation of Distributed Buoyancy Sections to Control Lateral Buckling of Subsea Pipelines

Subsea pipelines designed to operate under high pressure and high temperature (HP/HT) conditions tend to relieve their axial stress by forming buckles. Uncontrolled buckles can cause pipeline failure due to collapse, low cycle fatigue or fracture at girth welds. In order to control the lateral buckling phenomenon, a methodology was recently developed which consists of ensuring regular buckle formation along the pipeline. Distributed buoyancy is one of the most reliable initiation techniques utilized for this purpose which have been recently applied in some projects. The behavior of pipeline sections with distributed buoyancy is normally evaluated by Finite Element Analyses (FEA) even during preliminary design when analytical models could be adopted. FEA are also utilized in order to support reliability calculations applied within buckle formation problems. However, the referred analyses are usually time-consuming and require some experience to provide good results. This paper presents an analytical formulation for a pipeline section with distributed buoyancy, which can be utilized during preliminary design in order to evaluate the influence of buoyancy sections over buckle shape, feed-in length, tolerable Virtual Anchor Spacing (VAS), etc. Regarding buckle formation, this paper also presents a methodology to determine an expression for the critical buckling force to be used as part of the limit state function in reliability analyses, which combines the results obtained from the referred analytical formulation with Hobbs infinite mode.