There are hundreds of kilometers of subsea pipelines around the world, forming a network for theextraction and transportation of oil and gas products. When near shore subsea pipelines cross shippingchannels to ports, these pipelines can limit the size of the ships that enter the port. This is because theshallow burial depth of the existing pipeline can prevent any additional dredging required to deepen theshipping channel to accommodate larger vessels. An attractive solution under such circumstances islowering the pipeline section throughout the width of the channel so that the channel can be deepened. Theoption of shutting the line down or installing a re-routed new line has cost implications. However, thealternative of lowering a pipeline while it is fully operational has engineering risk, operational challengesand the offshore industry is not very experienced in such projects. This paper presents a case study of sucha key project where a 16” gas pipeline was successfully lowered from -3 m to -9m below the seabed whilstfully operational. The live gas pipeline was crossing a shipping channel and was buried at 3m belowseabed. In order for the port to expand and allow bigger vessels to enter the port, the shipping channelneeded to be deepened. Thus the pipeline was required to be lowered a further 6m for a stretch of 350mwhere the pipeline crosses the shipping channel. The lowering operations had to be carried out whilst thepipeline was fully operational as it was a 70km pipeline with key supply. This paper presents detailedoverview into engineering challenges and operational issues faced on the project. The paper discusses allthe stages of the project, risk assessments; integrity assessment for pipeline lowering; geotechnicalassessment of trench stability; detailed pipeline lowering stress assessment; pre-operational planning;pipeline survey and pipeline lowering operation; post lowering integrity assessment. The pipelinelowering was successfully completed to meet the project requirement after 14 lowering passes. Thissuccessful lowering of a live gas pipeline by 6m is considered to be world’s first such lowering.Recommendations on how a pipeline lowering project should be approached, assessed and executed arepresented in this paper.
展开▼
