Skirted mudmats have been the preferred choice to support pipeline or flowline end termination assemblies on soft clay seafloors encountered at deep water sites. Usually, a sliding mechanism accommodates the line expansions and contractions, during the many shutdown and restart cycles over the field design life. In deep waters, where hydrocarbon lines operate under high pressures and temperatures, designing and fabricating sliding mechanisms accommodating line expansions of up to 4-m and horizontal forces in excess of 2,000 kN (under lock-up), can become challenging and lead to impractically large mudmats. As an alternative, mudmats that slide directly or are “tolerably mobile” are gaining popularity. While this is a conceptually attractive alternative, a proper design ensuring that the mudmat will not unduly sink, rotate or dig into the seafloor during the many shutdown and restart cycles in its design life is complex. Cathie et al. (2008) are believed to be the first to discuss the various challenges that needed to be overcome to ensure a steady state sliding without unwanted digging of the foundation and, most importantly, without a tendency for the foundation to pitch or roll excessively. During the last five years, numerous papers have been published on the behavior and design of sliding mudmats for subsea structures, each focusing on or offering methods to address some specific aspects of mudmat geotechnical design. This paper presents a review of the literature that forms the present state-of-the-art/practice about mudmats sliding directly on the seafloor and proposes a numerically based comprehensive modeling approach for their design. The authors believe a crucial modeling aspect currently missing is that the behavior of a sliding mudmat is critically affected by other parts of the interacting multipart system (mudmat, seabed, flowline and jumpers). This can lead to unconservative design or risking overstressing of the connections.
Jean M. Audibert, M. Kabir Hossain, SUT, ASCE and John T. Bryant ASCE, AIPG, PTI, ACEC
Keywords: None provided
Publisher: DFI