A Global Migration Risk to Partially Constrained Long Pipelines Subject to Localized Thermal Gradient Cycles

In recent years the progressive migration of pipelines along their length when subject to routine cyclic operation has become a serious design problem for a number of deepwater developments. Due to parameter uncertainties and need for simplified modelling assumptions a high level of uncertainty can exist in regard to the final design outcome. This often necessitates a ‘wait and see’ approach in regard to installing costly pipe anchors or evaluating the risks imposed by displacement loads at or in the vicinity of vulnerable structures located along the flowlines. This paper addresses a recently discovered phenomenon identified during studies of the global movement of long pipelines. These flowlines may be seated on variable slopes with significant routine thermal heat-up/cool-down gradients from both general production and shutdown dieseling operations. Traditional global mitigation involves short or non-cyclically-constrained pipelines which move towards their cold end or down seabed slopes. The topic described within this paper is for long partial or cyclically constrained pipelines which are capable of moving towards either end of the pipeline if subject to exceptional loading conditions. In this paper the new type of pipeline global migration phenomenon or ‘caterpillar-type’ locomotion is discussed. An analytical method has been developed to capture the mechanism and highlight its sensitivity in regard to changes in design parameter. Worked examples from design projects are given and the overall findings discussed. The phenomenon and the design solutions presented should be topical to current deepwater pipeline designs within the oil and gas industry.