Five years after it offloaded its first shipment of LNG, the Prelude Floating Liquid Natural Gas (FLNG) facility is still going strong off the coast of Australia. As the biggest offshore facility ever built, it remains a remarkable piece of engineering.
By Joanne McIntyre, Stainless Steel World
When Shell first announced the project in the early 2010s, the dimensions of the Prelude were astonishing. It was five times larger than the largest LNG carrier at the time, measuring 488m long and 74m wide, and would displace more water than six aircraft carriers. Stood on end, it would be taller than the Petronas Towers in Kuala Lumpur. Easily taking the records for the world’s largest FLNG vessel and the biggest offshore facility, Prelude is operated by Shell, in a joint venture with INPEX (17.5%), KOGAS (10%) and OPIC (5%).
Original feature story
Read our feature story based on interviews with key engineers from SBM and Shell Global Solutions when Prelude was under construction. ‘FLNG Prelude: the world’s biggest floating structure’ provides insights into the duplex and stainless steel equipment on board and the challenges of testing the massive forgings. CLICK HERE
Bigger is better
At just under half a kilometre in length Prelude contains 260,000 metric tons of steel – more than was used in the original World Trade Centre in New York. A huge amount of stainless steels, including duplex grades, are used in the project. The FLNG was designed to produce 3.6 MMtpa of LNG, 1.3 MMtpa of condensate and 0.4 MMtpa of LPG per annum – that’s more than Hong Kong uses each year. Although Shell did not disclose the cost of the vessel, Reuters reported that industry analysts estimate the price tag to be between USD 10.8 – 12.6 billion. The first shipment of LNG was offloaded from Prelude in June 2019.
What is FLNG?
Floating LNG (FLNG) consolidates the traditional offshore to onshore LNG infrastructure into a single facility that is based over the fields. The FLNG facility gathers, processes, stores and offloads natural gas and condensate products at sea. FLNG removes the need for pipelines to shore, dredging and onshore works and therefore significantly limits the disturbance to the surrounding environment and in the right conditions, reduces development costs. It is also a competitive solution for fields like Prelude, that are very remote and hard to access.
The Prelude FLNG facility is moored in 250 metres of water, by four groups of mooring chains. Each mooring chain is held to the sea floor by piles. The facility has been designed to withstand severe weather, including a ‘10,000 year’ storm, and will remain onsite during all conditions.
Seven production wells will feed gas and condensate from the reservoirs via four flexible risers into the facility. All subsea connections join the facility via the turret. The turret’s swivel design enables the facility to pivot according to wind and sea conditions while it remains fixed to the sea floor.
The Prelude FLNG facility has thrusters to ensure it remains steady during production and offloading, but it is a fixed facility, with no means of propulsion. The management of subsea wells and manifolds is carried out via umbilicals connected through the turret to the control room on the facility. The processing of gas and condensate occurs in modules onboard that occupy an area approximately one quarter the size of a typical onshore LNG plant. Safety of the FLNG facility has been paramount during its design, and its safety profile is predicted to be inline with modern offshore oil and gas facilities. The FLNG design has gone through extensive testing programs and simulations to ensure it has the ability to remain connected and moored to the sea floor throughout all weather conditions.
17200 kg duplex castings
Given the impressive scale of the Prelude FLNG it’s no surprise that some of the individual forged parts are truly enormous. For example, the Gas Production Swivel Inner Part, forged in solid duplex, weighs 14500 kg and measures 2450mm outside diameter. The Gas Production Swivel Outer Part, also forged in solid duplex, weighs 17200 kg and measures 2990mm outside diameter. During fabrications there were concerns that during the solution heat treatment of these very thick items, the interior would cool down much more slowly than the exterior of the part, increasing the potential for the precipitation of detrimental phases. The SBM team developed special methods to deal with this challenge: see the text box on the previous page ‘Original feature story’ to find out more.
Go bigger to be smaller?
It’s hard not to picture Prelude as a giant of the seas, but paradoxically it’s the miniaturisation that the design represents that makes the vessel economically feasible. The huge hull is still far smaller than an onshore refinery, and Shell Oil describes it as “more environmentally friendly” than an onshore plant. Given that it is located far out at sea, Prelude does not have the same environmental impact as onshore refineries with respect to land use, pipelines, and risks to local populations.
Six-year maintenance contract
This July, Wood secured a six-year contract to provide brownfield engineering, procurement, and construction management (EPCm) solutions for Prelude. Ken Gilmartin, CEO at Wood, said, “LNG is a key transition fuel as (the) industry balances the need for global energy security with the importance of urgent reduction in carbon emissions. We are delighted to build on our
70-year global relationship with Shell to deliver integrated brownfield engineering solutions for Prelude, the world’s largest floating offshore gas facility.”
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