ABSTRACT

The subsea pipeline industry has recognised for over two decades that sediment transport, scour and liquefaction mechanisms have potentially significant impact on the validity and accuracy of existing pipe-soil interaction models under extreme metocean conditions. This is highlighted in the recently-published DNV-RP-F109 code in Sections 4.4–4.5. Appendix B of the code also discusses the need for specific models to be used in calcareous soils. The STABLEpipe JIP is presently in Phase 2, with the objectives of undertaking research activities which could achieve a step change in the science and engineering of subsea pipeline stability design. A summary of the proposed work and present status of the JIP will be presented.

INTRODUCTION

Australia's North West Shelf (NWS) and the Gulf of Mexico both feature significant energy resource projects in areas prone to severe tropical cyclones or hurricanes, leading to on-bottom stability issues for subsea pipelines in intermediate and shallow water. On-bottom stability is particularly challenging on the NWS due to the presence of calcareous soils that are either partially cemented or mobile. In addition, NWS developments typically involve gas rather than oil, leading to lighter pipelines that are less stable on the seabed. A number of up-coming large gas projects will encounter significant costs (A$500M to A$2,000M in total) to achieve stabilisation using current approaches, including costly secondary stabilisation measures. The difficulties associated with existing approaches to the analysis of on-bottom stability include: • Inadequate definition of the appropriate pipe-soil interaction model to be used on calcareous soils, as acknowledged by DNV in the commentary on this soil in Appendix B of DNV-RP-F109 (DNV, 2007). This appendix culminates in the statement that "An appropriate model [for pipe-soil interaction]… has not been presented within the public domain.

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