Challenges in Riser Design - The Ichthys Gas Field Development

Abstract

Abstract The Ichthys Gas Field is located 220 kilometers offshore from the North West coast of Western Australia. The field is located within a cyclone-prone region and has two permanently moored floating production units in a water depth of approximately 250m. The floating systems consist of a semi-submersible Central Processing Facility (CPF) and a Floating Production Storage and Offloading (FPSO) facility. This paper presents an overview of the riser solutions developed for each facility while documenting challenges faced in design, cognisant of installation limitations and complex interface management within a large EPCI project. Both facility riser systems contain aspects that are first of their kind within the region and were developed with the added complexity of a 40-year riser design life and the requirement to withstand a 10,000 year return period survival cyclonic event. Within a single mooring sector to the North of the CPF, 25 risers are supported on a single riser support structure (RSS) in a fixed-S configuration. The RSS, which is the subject of another paper [1], has an arch length of 130 m to support the risers at a height of 110 m above the seabed. This paper explains the design aspects of the system including optimizing riser spacing and riser gutter geometry in close proximity with varying minimum bending radii. A robust design was required that allowed for future phase riser installation and riser replacement. A lazy-S riser configuration was developed to support 15 risers from the FPSO. The solution consisted of three mid depth buoys (MDBs), one in each mooring sector. The MDB design is the subject of another paper [2]. Design challenges included significant dynamic response of the permanently moored FPSO in 10,000 year return period survival cyclonic conditions. MDB dynamic response was found to be significantly impacted by added mass, which provided challenges in maintaining positive tether tension and minimizing interface loads into the MDB structure and foundation. Diffraction analysis of the MDB structure was used to determine accurately the added mass coefficient in order to refine interface loads into the MDB and its foundation.