A survey of incidents with pipelines in the North Sea has been carried out as a, part of the comprehensive Netherlands Marine Technological Research program (MATS) sponsored by the Ministery of Economic Affairs. Nine operators and four government agencies in the Netherlands, Great Britain, Norway and Denmark were interviewed with regard to their experience with submarine pipelines. The operators were asked to supply information about incidents which occurred with their pipelines during installation, testing and operation. The government representatives explained the safety philosophy and technical requirements adopted by their respective offices. The paper discusses the main findings of the authorities on pipeline trenching (or burial) requirements. Subjects discussed are anchor hookings, trenching, burial, testing, spanning etc. The most serious incidents with pipeline occurred during hydrostatic testing. On four occasions, a major pipeline ruptured during testing. In three of these four incidents cracks apparently started form scratches inside a dented area of the pipeline. These dents were made by the trenching equipment when it hit the line, these scratches acted as notches and the high stresses introduced at the root of this notch initiated a crack which subsequently led to failure of the pipeline. The incidents referred to above suggest that the trenching operations should be reviewed critically. There seems to exist little uniformity in the trenching requirements. Most government agencies involved in the safety of submarine pipelines are, however, prepared to discuss the trenching requirements. The results of this paper may be used for a safety assessment of submarine pipelines.
The safety assessment of offshore structures and submarine pipelines is becoming an important tool in the descision-making process of government agencies. Against this background, the Steering group-Pipelines of the Netherlands Marine Technological Research Organization (Mats) decided to initiate a research project which studies the safety of submarine pipelines. The Steering group-Pipelines consists of representatives from government, industry and research organizations.
Analytical methods and procedures for carrying out a safety assessment (reliability analysis) are already being developed by other Mats-project groups. The application of these methods, however, depends to a great extent on the availability of the required input data. The Steering group-Pipelines discussed this particular aspect and accepted a proposal to survey. the existing submarine pipelines in the North Sea, in an attempt to categorize all incidents which occurred with these pipelines during installation, testing and operation.
The budget available for this project dit not permit the execution of an exhaustive survey. A statistical evaluation of the survey results was not feasible either due to the limited scope of the survey, but also because most incidents occur infrequently and recurrence rates cannot be determined realistically. The survey should, however, highlight the present philosophy of operators and authorities with respect to trenching and burial of submarine pipelines.
It was decided to conduct the survey by means of interviews with selected pipeline operators and government representatives in the Netherlands, Norway, Denmark and England. Protech International B.V. was asked to carry out these interviews and to report to a Project Group (ProMats).