INSPECTION & ASSESSMENT OF DAMAGED SUBSEA PIPELINES: A CASE STUDY

The Central Area Transmission System (CATS) in the UK sector of the North Sea delivers natural gas through a 404 km pipeline from the CATS riser platform to the North East coast of England. During the summer of 2007 this 36 inch diameter natural gas pipeline was damaged by a vessel anchor. The anchor lifted the pipeline from under the seabed, dragged it across the seabed, bending the pipe and locally deforming it. This event resulted in a significant inspection, assessment and repair programme before the pipeline could safely return to operation. This paper describes the detailed structural assessment of the damaged pipeline and the inspection and repair operations. Following inspection of the pipeline by divers, the damage was assessed using the "Pipeline Defect Assessment Manual" (PDAM). The manual was prepared from research primarily for onshore pipelines: this paper discusses the strengths and weaknesses of PDAM and key differences in defect assessment for onshore and offshore pipelines. The paper highlights several very important lessons learnt from this incident, including: the complex stresses developed in a pipeline that is pulled and moved by an anchor; the need for damage assessment methods for pipe containing high compressive stresses and 'locked-in' stresses; the safety aspects and complexity of inspecting a pressurised and damaged subsea pipeline. These lessons learnt are then translated into recommendations for the industry, and advice to other subsea pipeline operators.