Challenges of Lowering a Live Subsea Buried Gas Pipeline by 6m

Abstract

Abstract There are hundreds of kilometers of subsea pipelines around the world, forming a network for the extraction and transportation of oil and gas products. When near shore subsea pipelines cross shipping channels to ports, these pipelines can limit the size of the ships that enter the port. This is because the shallow burial depth of the existing pipeline can prevent any additional dredging required to deepen the shipping channel to accommodate larger vessels. An attractive solution under such circumstances is lowering the pipeline section throughout the width of the channel so that the channel can be deepened. The option of shutting the line down or installing a re-routed new line has cost implications. However, the alternative of lowering a pipeline while it is fully operational has engineering risk, operational challenges and the offshore industry is not very experienced in such projects. This paper presents a case study of such a key project where a 16" gas pipeline was successfully lowered from -3 m to -9m below the seabed whilst fully operational. The live gas pipeline was crossing a shipping channel and was buried at 3m below seabed. In order for the port to expand and allow bigger vessels to enter the port, the shipping channel needed to be deepened. Thus the pipeline was required to be lowered a further 6m for a stretch of 350m where the pipeline crosses the shipping channel. The lowering operations had to be carried out whilst the pipeline was fully operational as it was a 70km pipeline with key supply. This paper presents detailed overview into engineering challenges and operational issues faced on the project. The paper discusses all the stages of the project, risk assessments; integrity assessment for pipeline lowering; geotechnical assessment of trench stability; detailed pipeline lowering stress assessment; pre-operational planning; pipeline survey and pipeline lowering operation; post lowering integrity assessment. The pipeline lowering was successfully completed to meet the project requirement after 14 lowering passes. This successful 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 are presented in this paper.