Shell is planning to develop the Prelude and Concerto gas fields, located in the Browse Basin, 475 km north-northwest of Broome, Western Australia, using what could be the world’s first floating LNG (FLNG) facility.

The company chose FLNG technology as the development option for the field in late 2009. A consortium of Technip and Samsung Heavy Industries is currently working on front-end engineering and design for the project, targeting a final investment decision in early 2011 and first gas as soon as 2016.

FLNG for new frontiers

Most of the world’s large and easy-to-find gas reserves have been discovered. This means that to meet the world’s expected energy demand – and growth in the LNG market – smaller and more remote gas supplies need to be developed.

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FLNG is now technically possible and can be a more cost-effective method of monetising remote gas reserves than traditional development options as the infrastructure and energy requirements to pipe the gas to shore are not needed. In this way, FLNG also leaves a smaller environmental footprint.

Going forward, FLNG is likely to complement traditional onshore developments, not replace them. The best option for larger reserves – particularly those closer to shore – may well be an onshore development.

Shell’s FLNG

Shell’s proposed FLNG facility would be one the largest floating structures ever built. It will be 480 m long by 75 m wide, and will weigh 600,000 tonnes when fully ballasted. The facility is designed to produce approximately 3.5–4 million tonnes per annum (MMt/a) of LNG, as well as LPG and condensate, depending on field gas composition.

Shell’s FLNG facility will be able to receive all fluids flowing from the reservoir and to process and treat gas, condensate, aqueous and solid byproducts and wastes. The main liquefaction process uses steam-driven refrigerant compressors and utilises cold seawater from a depth of approximately 150 m for cooling.

The combination of the liquefaction efficiency, water cooling and elimination of gas compression to shore within Shell’s FLNG design yields a considerably reduced environmental footprint as compared to a typical offshore-to-onshore development.

This reduction can be measured in terms of greenhouse gas emissions, which are typically 15 per cent lower, the use of fewer construction materials by approximately 50 per cent, and a 90 per cent reduction in disturbance of land/seabed.

FLNG factory

In July 2009, Shell signed master agreements with Technip and Samsung Heavy Industries to design and build multiple FLNG facilities over a period of 15 years. As such, Shell’s FLNG facilities will be built in Samsung’s shipyards in Korea, one of the few places in the world large enough to accommodate construction of a facility of this size.

The FLNG facility to be used for Prelude is generic in design, following Shell’s “design one, build many” approach, meaning that the facility can be easily replicated for other projects.

The design builds on Shell’s experience in LNG technology, LNG shipping, floating production, storage and offtake vessels (FPSOs) and offshore operations. The FLNG facility concept is similar to that of an FPSO.

A key feature of Shell’s FLNG design is that it is very flexible; it can process a wide range of gas compositions, can accommodate a wide range of sub-sea configurations, has multiple solutions for hydrate management, can export LPG as well as LNG, and uses an efficient dual-mixed refrigerant liquefaction cycle.

The facility can also stay on station, its sheer size allowing it to withstand very severe weather conditions. With a hull life of 50 years, the facility is also able to be re-deployed after one field has been fully developed.

Safety first at Shell

Managing safety has been at the centre of technical studies since Shell first embarked on FLNG in earnest in the 1990s. Areas of focus have related to those aspects of FLNG which are different to other offshore structures. These include compactness, extreme weather survivability, product loading, motion responses and turret and mooring systems.

Importantly, Shell’s FLNG facility is designed to be capable of withstanding a 1 in 10,000 year weather event, which is more extreme than a category 5 cyclone.

Perfect for Prelude

The Prelude Field, in which Shell holds a 100 per cent interest, is located in WA-371-P in the Caswell sub-basin of the northern Browse Basin. Being remote and containing relatively small gas reserves, Shell believes that FLNG is the best development option for the field.

The FLNG facility will stay for 25 years over the Prelude Gas Field, and will be designed to produce up to 3.6 MMt/a of LNG, 0.4 MMt/a of LPG and 1.3 MMt/a of gas condensate during that time. Shell has said that the production rate may be backfilled in later years with gas sourced from the nearby Concerto, Crux and Libra fields.

The proposed project facilities will include wells, four flowlines approximately 4 km in length, umbilicals and flexible risers, and the FLNG facility. Onshore support facilities will potentially be located within established industrial areas either in Broome, Western Australia, or Darwin, Northern Territory, subject to state and territory government approvals.

Designed for gas

The plant will include liquefaction units, production storage and loading facilities, associated utility systems and a control room, maintenance facilities and accommodation. Along with the ability to process gas from the field, the FLNG facility needs to be able to store and offload the processed products.

In Shell’s design for the FLNG facility, as described in the Prelude environmental impact statement, the processed condensate, LNG and LPG is transferred directly to atmospheric pressure storage tanks in the inner hull of the facility. The double-hull design of the facility provides additional containment and protection for the tanks.

The inner hull houses six LNG storage tanks with a total capacity of 220,000 cubic metres, four LPG storage tanks with a total capacity of 90,000 cubic metres and six condensate storage tanks with a total capacity of 126,000 cubic metres.

Hard loading arms with swivel joints and quick connect/disconnect flanges will be used for transferring the LNG and LPG to export tankers. The condensate export tankers will be moored to the rear of the FLNG facility and condensate will be transferred by a floating hose as is done on FPSO vessels.

In terms of export shipping activity from the facility, Shell expects weekly LNG carriers, monthly LPG carriers and fortnightly condensate tankers.

If the FLNG fits...

The factors that make projects suitable for development via FLNG are not all the same. Every gas field and potential development is different in terms of the relative technical, economic, environmental and social parameters. As such, it is difficult to define which projects would or would not be suitable for an FLNG development.

FLNG is complementary to onshore developments, says Shell. Use of FLNG facilities could be considered for very remote or smaller gas reserves that otherwise wouldn’t be financially viable using a conventional onshore development. As all of the processing facilities must fit on the floating platform, the processing capacity of the plant is limited, as is the possibility of project expansion.

Shell is also a joint venture participant in the Greater Sunrise Gas Fields Development, located in the joint petroleum development area in the Timor Sea, which is also to be developed using Shell’s FLNG technology.

Greater Sunrise proponent Woodside Petroleum’s Chief Executive Don Voelte has said that the FLNG development option has a both lower capital cost than an onshore plant in Timor-Leste – by $5 billion – and a lower technical risk profile.

Prelude to a movement

FLNG provides an exciting promise for the development of remote offshore gas fields. “Prelude” is perhaps a rather apt name for the world’s first FLNG project.