Forward Predictions for Life-of-Field Assessments Based on the Observed Lateral Buckling Behavior of Operating Deepwater Pipelines

Making forward predictions of a pipeline's behaviour from an operational base state requires a working knowledge of the pre-existing stresses and strains. To gain this working knowledge for a pipeline that has undergone lateral buckling requires either an engineering assessment that can accurately predict the displacement of the pipeline on the seabed, and from which the operational stresses and strains can be estimated and extracted, or instrumentation on-board the pipeline that records stress and strain information which can be uploaded and sent back to the analyst. Whilst it is hoped that the pipeline analysis carried out in design stage be focused and accurate enough to capture and present the required data for life-of-field assessments the reality is otherwise. The purpose of the design strategy would have been to attain code compliance and the required safety level for the pipeline system, which are targets that may be at odds with predicting most realistically the pipeline's state in the operating condition. This is a result of the design needing to accommodate multitudinous conservative assumptions about the pipeline geometrical properties, the seabed profile, operating pressure and temperature profiles, as-laid out-of-straightness and pipe-soil axial and lateral resistances and also the need to attend to code requirements which will have their own partial safety factors embedded. It must be admitted, reluctantly, that the pipeline response realized in the field may never be an accurate reflection of the outcome of the design process. This paper presents data pertaining to the surveyed as-built configurations of deep water HP/HT pipelines and a small diameter piggyback pipeline and a method for making forward predictions from such pipelines about their observed base states. The information contained in the paper may be used as input to future design strategies or as key considerations for the successful completion of such re-analyses by other operators, contractors and consultancies.