Submarine pipelines installed on very uneven sea bottom usually results in many long free spans. If the free spans are above a certain length the pipeline will be subjected to vortex induced vibration (VIV) caused by sea currents or wave motions transverse to the pipeline direction. The onset and level of VIV will depend on several factors such as pipe dimensions and submerged weight, current velocity, seabed soil properties and span boundary condition. For uneven sea bottom, seabed intervention may be very costly if there are many high free spans that are longer than what is acceptable with respect to fatigue. There is obviously a possible economical benefit if longer free spans can be accepted due to improved fatigue performance, which may result in less intervention work by either excavation, trenching or rock/mechanical supports.
This paper will summarize results from a comprehensive full scale fatigue test program that has been performed in order to verify potential enhanced fatigue design parameters of high quality pipeline welds compared to standard fatigue design parameters according to DNV-RP-C203. The testing included pipe dimensions from 6 to 16 inch OD, and tests were performed on both as welded pipes and pipes subjected to simulated reeling strains. In addition to normal welds, also welds with deliberately introduced weld defects/weld indications, maximum acceptable axial misalignment and repaired welds were tested. The test results were compared with similar recommended design parameters for pipeline girth welds. Furthermore, the effect of reeling, weld residual stresses, weld defect and repair will be presented and discussed.