Should Subsea Pipeline Stability in North West Shelf of Australia be designed Against 10,000-Years Return Period Storms? (Preliminary Insight)

Abstract

It has been noticed a pick up in the storm level across the world, which have been recorded to induce significant wave heights greater than 100years return period (RP). The events have been partly blamed on the climate change. This is one possible reason which has triggered pipeline designers, nowadays, to consider 10,000years RP storms for subsea pipeline on-bottom stability assessment in North West Shelf (NWS) region. Another possible reason is the remote locations and harsh environment, which would make it difficult or otherwise, costly, to carry out repairs, inspections, and measurements activities of the subsea pipelines in the area. Furthermore, it has been acknowledged that an offshore platform design has applied similar approach, although this is more likely due to the fact that the platform might be manned during its operation. Therefore, one might accept an overly robust design as the solution to the above issues. However, one might question the basis of 10,000years RP storm selection as a solution, since this can necessitate a costly design. This paper presents some alternatives to the existing 10,000years RP storms requirement. A program was developed to carry out structural reliability analysis (SRA) of pipeline on-bottom stability, while considering the occurrence (RP) of the loadings. The program was validated and calibrated to DNV RP F109 results, and then used for the higher RP storms assessment. Design cases were generated and data were adopted from an existing project within NWS region. Preliminary SRA results demonstrate that the selection of lower design RP is negotiable. The discussion also touched on cost versus risk analysis (other method of selecting higher RP storm), as well as soil stability during this level of storm. The amount of infrastructure savings could be significant if a lower design RP can be accepted, while not compromising the design reliability.