Abstract

High-pressure and high-temperature (HPHT) flow conditions in a pipeline that are typical of deep water applications can induce large axial compressive forces and buckling, which can lead to pipeline failure. Introducing buckling initiators to control lateral buckling is considered the most economical and effective approach to mitigate the hazards associated with uncontrolled buckling of the pipeline. In current pipeline design, snake-lay, sleeper and distributed buoyancy modules are the three most commonly used types of initiators. However few, if any, systematic studies have been published on the relative effectiveness of these three initiator types to control pipeline lateral buckling.

This paper describes a parametric study on the effectiveness of these three initiators, individually and in combination. The effectiveness of buckling control is considered in terms of critical buckling force, which is defined as the required axial compressive force at the buckle's ‘crown zone’ to initiate lateral buckling. A wide range of geometric design parameters for snake-lay, sleeper and distributed buoyancy module—type lateral buckling initiators and a wide range of soil strengths are considered.

1. Background

In a deep water depth field, a production pipeline is more likely to experience high pressure and high temperature (HPHT) due to environmental circumstances and flow assurance requirements [2]. As a result, the high thermal loading along with large internal pipe pressure can cause extremely severe axial expansion. Meanwhile, opposing longitudinal resistance from the soil-pipeline interaction can cause high compressive axial forces to be accumulated in the pipeline, which can lead to buckling. If an uncontrolled buckle develops, failure can happen with attendant negative consequences.

The mechanism of pipeline buckling depends on three key factors [4–8]: the pipeline driving force, critical buckling force, and the imperfection. The pipeline driving force is the axial resultant force derived from consideration of HPHT and soil resistance; the critical buckling force is the predicted force when a buckle will occur; and the imperfection is the amount of the pipeline's out-of-straightness (OOS) which also affects the critical buckling force directly. As the HPHT increases, the driving force in the pipeline will increase until the critical buckling force is reached, then it will generate a buckle in a direction influenced by the imperfection.

A buckle can be detrimental to a pipeline and it has been usually dealt with by restraint practices to avoid buckles from occurring; such as by rock dumping, pipe burying, or trenching. Another, less typical solution involves the use of devices; such as an in-line spool to dissipate the compressive axial force. The restraints increase the characteristic critical buckling force by imposing stronger confining force. It is recognized that the restraint approach is an effective and proven way to avoid buckling, particularly for a pipeline in shallow water. However for deeper water depths the foregoing practices have been reconsidered and revised due to increased HPHT conditions and the increased cost and installation difficulties associated with the traditional practices.

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