The Ichthys Riser Support Structure (RSS) is a very large subsea structure for supporting up to 25 flexible risers and umbilicals as they transition from the seabed to a semi-submersible Central Processing Facility. The Ichthys field is in a region affected by both local shallow crustal earthquakes and major subduction earthquakes emanating from the Australian/Eurasian plate boundary. Large beds of uncemented carbonate silts and sands are present at Ichthys, which are highly prone to excess pore pressure accumulation and liquefaction under cyclic loads. Any structure placed on the seabed will suffer from both co-seismic and post seismic settlement after an energetic earthquake event.
This paper presents dynamic non-linear numerical analyses (2D and 3D FEA) of the coupled soil and structure that were performed in order to estimate the seismically induced settlement and the potential extent of soil liquefaction beneath the RSS, which is founded on a large skirted raft foundation. Extensive site specific laboratory test data, defining the material behaviour during and after cycling, was collected to provide the necessary input to this analysis.
Significant areas of liquefaction (in the form of cyclic mobility, not flow liquefaction) were found under DLE conditions and hence the combined co-seismic and post-seismic foundation settlements were predicted to be substantial although tilts were modest due to low eccentricity of the centre of gravity. The RSS was demonstrated to be tolerant to such large settlements and hence the results were shown to be acceptable in this case.