Abstract
The Turret Mooring System (TMS) for the Shell Prelude FLNG (Floating Liquefied Natural Gas) facility consists of a Top Mounted Internal Turret (TMIT) integrated at the bow of the FLNG hull. Permanently connected to the seabed through a dedicated mooring system, this Turret allows the FLNG facility to passively weathervane in response to prevailing weather conditions and enables transfer of fluids and utilities (in both directions) between the subsea wells and FLNG topsides. This paper describes the challenges faced during the engineering and construction of the most challenging TMS ever executed to date.
Prior to Prelude, SBM Offshore (SBM) had designed for Shell a TMS suitable for a large range of gas compositions from a portfolio of remote offshore gas fields, including North-West Australia, in water depths ranging from 165m to 1,350m, with the generic FEED vision "engineer one, build many". The Prelude TMS was finally developed with a detailed design to meet the specifics of the 250m field water depth and to remain safely on station for 25 years in cyclonic weather conditions. To achieve the robust design target, field proven solutions were revalidated for application at a greater scale, for a longer design life, harsher environment and the high reliability demands of the project. The execution strategy followed the requirements of TMS contractor direct and end Clients.
The TMS, located very close to the bow, moors the 488m long, 74m wide Prelude FLNG hull. The system will ultimately accommodate 8 production risers, although only 4 are presently installed. The mooring system design is governed by the 10,000-year storm, resulting in a weathervaning system comprising 40 bogies, 40 radial wheels and 32 lower pads transferring the load to the hull. The mooring arrangement is composed of 4 bundles of 4 lines each, with 175mm R4 chains at top and bottom, and a mid-water section of 162mm sheathed diameter steel wire rope.
Several challenges have resulted from the requirement to comply with a complex set of codes and standards, the severity of the cyclonic conditions, the record size of the FLNG hull, and from the required capacity of mooring components pushing beyond the existing industry experience base.
Lessons learned from a selection of key challenges encountered during the engineering and construction of this one of a kind TMS will be presented with a view to facilitating future developments offshore North West Australia.
