The Prelude floating LNG design challenge

Designing Shell's Prelude floating LNG project involved shrinking a complex LNG plant and making it safe in cyclonic seas on top of the world's largest offshore facility.

The Prelude floating LNG design challenge

This story was originally published in The West Australian on 1 July 2017 with the headline "Prelude crew in a safe space." © Peter Milne.

Containing as much steel as 36 Eiffel Towers and spanning three times the length of the oval at Perth Stadium, Shell’s Prelude floating LNG facility will cut an imposing figure even in the vastness of the open seas between Asia and Australia.

The world’s biggest vessel was last night continuing on it journey from South Korea to its new home, 475km north-east of Broome.

It’s jaw-dropping scale presented more than a few headaches for engineers but the main difficulty was not accommodating the vessel’s girth, it was dealing with how close its crew would be to the millions of tonnes of gas that Prelude would process.

The greatest danger in the oil and gas industry is the low but very real probability of a release of gas or an explosion. To reduce the effect of any incident, areas of the process plant are separated from each other and from where the most workers are.

Prelude’s engineers had little room to play with to achieve this separation.

Woodside’s Pluto plant, that produces 4.9 million tonnes of LNG a year, spreads over an 80ha site near Karratha.

Prelude will produce 5.3 million tonnes of LNG, condensate and LPG a year on less than 4ha.

The confined spaces below deck, where gas clouds and blast pressure from an explosion cannot disperse, contain the least hazardous equipment, the product tanks.

On deck, the most safety- sensitive gas-processing plant, the turret and the flare tower were placed as far from the living quarters as possible. Lower-risk equipment like the power generation was placed nearer the living quarters that are protected by a blast wall.

The four big process plant modules are separated by 20m-wide safety gaps to help disperse any gas released and impede the spread of any fire.

The layout also allowed for possible LNG spills that, at -162℃, can make steel brittle.

Dealing with a moving sea

Safety was the primary concern in the Prelude layout, but there were other layout problems caused by vessel motion.

Some equipment that contains a mix of liquid and gas is sensitive to motion and was placed near the centre of gravity of the ship, where the movement is least.

The motion of the Prelude also creates repetitive loads on the equipment and piping that would never occur on land.

Unless addressed in the design, these could cause a fatigue failure, where small micro-cracks in the metal gradually grow with each movement until, after many years and millions of movements, the components fracture.

LNG carriers are usually near full when they deliver cargoes and near empty when they return. In either case, there is little force on the tank’s insulated walls from sloshing of the LNG inside the tanks when the ship moves.

On the Prelude, though, the interiors of its LNG tanks were designed to withstand the forces of LNG sloshing about in a half-empty tank during a cyclone.

At onshore LNG plants, an LNG carrier is tied up to a stationary jetty before LNG loading arms connect and fill the tanks. Prelude required loading arms that can swivel, rotate and follow the motion of the LNG carrier for the 15-hour loading process that occurs every five to six days. These loading arms were built and tested in France using liquid nitrogen and a rig that simulated movement in extreme sea conditions. The loading arm had to be able to pull itself into place to connect and release quickly in an emergency.

During loading, an LNG carrier will not face the worst case of being pushed sideways by the weather as Prelude rotates around its turret to face the prevailing sea state. Three giant thrusters under the vessel will keep the Prelude steady as the carrier is loaded.

Keeping Prelude attached to the seabed

While some of Prelude’s engineers were challenged by how small Prelude was for an LNG plant, the engineers designing the turret had to deal with Prelude’s massive size.

The turret is a vertical rotating cylinder built into and protruding below the hull.

It has the dual role of attaching the Prelude to mooring lines while allowing production from the subsea wells to flow into the vessel as it swivels about the turret.

The turret must withstand the load of a fully laden 600,000-tonne Prelude subject to the worst storm expected in 10,000 years while fixed to the seabed 250m below by 16 huge mooring chains.

Unsurprisingly, the turret is the biggest ever built: 94m high, 26m wide and weighing over 11,000 tonnes.

The Prelude is not only connected securely to the mooring chains by the turret; it is connected permanently. Prelude is designed for a field life of 20-25 years and cannot disconnect for cyclones or go into port for maintenance.

Under the Prelude, away from the mooring lines, are eight water intake risers.

To cool the machinery the riser suck in 50,000 tonnes of water an hour from 150m below the surface where the water is 6-10℃ cooler.

When the Prelude starts producing LNG off Australia’s coast, it will be due to the extraordinary number of problems identified and solved by its design engineers.

Main image: Prelude floating LNG facility leaving shipyard in Korea. Source: Shell