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.

The Ichthys Riser Support Structure (RSS) is a large subsea structure in 250 m water depth, which comprises a 110 m tall tower with a 130 m wide arch for supporting up to 25 flexible risers and umbilical's as they transition from the seabed to the semi-submersible Central Processing Facility (CPF). The foundation for the RSS comprises a skirted raft approximately 69 m x 38 m in plan with a central cut-out and 6.8 m deep skirts installed in uncemented carbonate sediments through a combination of self penetration and suction assistance. A 40 year design life combined with the requirement for no loss of containment during a 1 in 10,000 year metocean or seismic event made the geotechnical and foundation engineering design a challenge. This paper will describe the innovations adopted in assessing the RSS foundation capacity against the design in-place loads and the key results and observations with respect to the installation of the foundation.

Calcareous soil type is found at many locations, requiring careful selection of foundation type. Calcareous soil is mostly or partly composed of calcium carbonate in the form of lime or chalk derived from the underlying chalk or limestone rock. North-West Shelf of Australia is an example of site which consists of carbonate soil types wherein the majority of existing offshore facilities and platforms being installed using Drilled and Grouted (D&G) piled foundations and in some instances using Gravity based foundations. This paper discusses alternate foundation concepts on such soils, namely; (i) Micro-piles, and (ii) Inclined pile cluster, along with the common concepts of (iii) D&G piles and (iv) Gravity based foundations. The foundation concepts are discussed with focus on key aspects of the foundation structural configuration, vertical foundation capacity feasibility, and some serviceability related aspects. In addition, offshore operation and installation duration perspective are also discussed to provide some insight on how each foundation concept could suit the project preference which often influence the final selection of foundation concept. Risk/challenges and advantages of each concept are then summarized for overall comparison.