Mid-water arches are often used for supporting flexible risers and umbilical's from floating structures onto seabed. If the structure needs to support a number of flexible risers and umbilicals located at a site which is prone to soil liquefaction during seismic conditions, then the selection of an appropriate foundation system and designing the super structure to suit the ground condition becomes a challenge and it is the behaviour of the foundation system that dictates the design of the structure.

The Ichthys Field located about 220 kilometres offshore from the NW coast of Western Australia in average water depths of 250 m is such a site. The Ichthys LNG Project Field development includes a large subsea Riser Support Structure (RSS) supporting up to 25 flexible risers and umbilicals in a "Fixed-S" configuration as they transition from the seabed to the semi-submersible Central Processing Facility. The RSS consists of two key components, namely a 130 m long arch structure and a steel tower with a suction foundation base. The total height above seabed of the RSS is 110 m with a total weight of 7300MT in air. The RSS has been conceptualized to suit the design, fabrication, installation and inplace operations. The RSS was installed using the conventional method of launching at site, self-upending and crane assisted lowering onto the seabed with a controlled flooding of tower legs and Auxiliary Buoyancy Tanks (ABT). The tower is anchored permanently to the seafloor by suction cans.

The objective of this paper is to present the key aspects of the concept and detailed design development of the RSS, including the innovative approach to the environmental and seismic conditions to avoid problems with due consideration of fabrication, transport and installation. The scope of this paper is limited to the RSS structure and will not cover the detail geotechnical foundations design aspects which are the subject of another paper.

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