Rolls-Royce: Positioning a Giant
Rolls-Royce thrusters are in place to power Shell’s Prelude, the largest floating structure ever built, which will move liquefied natural gas production off land and out to sea.
Owned by oil giant Shell, the Prelude floating liquefied natural gas (FLNG) production facility is the world’s first FNLG development and construction is well underway.
It will be fitted with three Rolls-Royce electrically powered USL 455 azimuth thrusters at the Samsung Heavy Industries shipyard in Geoje, South Korea, one of the few yards in the world big enough to build the vessel. They will complete factory acceptance testing (FAT) during November, and will leave the factory in Rauma at the end of the year.
FLNG technology is a significant new innovation for the LNG industry. It combines two existing proven technologies – offshore gas production and LNG processing – in a single facility. This means that extensive gas pipelines to onshore processing facilities, compression facilities to flow gas to shore, dredging and jetty construction and onshore infrastructure including roads can be eliminated.
Prelude is 488m long, 74m wide and 44m deep from the keel to the main deck and will have a displacement of 600,000 tons, when all internal tanks are fully loaded. Offshore gas is received via insulated production flowlines and flexible risers, and impurities such as water, mercury and CO2 are removed. It is then chilled to -162°C reducing its volume 600 times and changing it from its gaseous state into a liquid, which is then stored in onboard insulated tanks. Ocean-going LNG tankers can then dock alongside to offload and export the LNG to world markets.
Once towed into position 200 km offshore the coast of Western Australia, in an area known as the Browse Basin, the production platform is not expected to see land again for over 20 years. It will remain operational around the clock and the thrusters will ensure it can keep its heading against the wind, current and waves to allow safe tanker berthing. It is moored at the bow via a turret, the largest ever built, with four groups of mooring chains anchored by 16 driven piles.
Novel thruster installation
This 24/7 operation means any maintenance of the underwater mountable thrusters, which are each rated at 5.2MW will have to be carried out on board over the coming years, which demanded a unique approach to how the thrusters were installed.
Team working within Rolls-Royce, the Shell technical team and the Samsung shipyard team was essential in arriving at the best solution, with safety our number one concern,” says Knut Eilert Rosvik, VP Propulsion - Commercial Marine. “Together we were able to develop a concept that will allow each thruster to be lifted from its operating position at the bottom of the stern of the vessel, 23m below sea level, into a dedicated workshop on a higher deck for maintenance. We had previously delivered a somewhat similar thruster arrangement for a much smaller floating production storage offshore vessel (FPSO), working with subcontractor Beacon Finland. So we were able to draw on this experience and working arrangement in developing the Prelude installation.”
Rolls-Royce teams in Norway, Finland, Korea and the UK were involved in tailoring the solution specifically for Prelude. To facilitate thruster overhaul or replacement while at sea, each electrically powered thruster is located at the base of a purpose designed trunk which links to the higher workshop area with its dedicated cranage. The maintenance space has all the functions and tooling found in the Rauma production facility.
Thruster condition monitoring system (CMS) is also part of the Rolls-Royce supply, type approved by ABS, Lloyds Register and DNV GL and in service on over 100 similar units around the world. CMS data can be used to determine the actual condition of the thrusters through life, without the need for internal and visual inspections. Maintenance and overhaul intervals can be extended to match the operating profile of the vessel. The CMS is wirelessly linked to an onshore Rolls-Royce operations center where the data is analyzed to produce the trending and operational information. The data analyzed represents a carefully considered balance of thruster information for maximum effectiveness in highlighting any potential operating anomalies, long before they can be seen or heard.
Prelude is one large floating facility with and enormous job to do. Every effort is being made by all concerned to maintain the highest quality standards, for the best in reliability to minimize the risk of any downtime. Once operational it is expected to produce 3.6 MMT/a of LNG, enough to meet Hong Kong’s annual natural gas demand – as well as 0.4 MMt/a of LPG and 1.3 MMt/a of gas condensate.
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