Gas-to-liquid FPSOs: The way forward

In recent years, a growing interest has emer-ged regarding economic and environmentally acceptable solutions for developing remote stranded and associated gas offshore. While LNG technologies may be a solution in some instances, these also involve establishing a costly and time-consuming supply chain.

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J. López-Cortijo, Abel Méndez
Izar Fene

In recent years, a growing interest has emer-ged regarding economic and environmentally acceptable solutions for developing remote stranded and associated gas offshore. While LNG technologies may be a solution in some instances, these also involve establishing a costly and time-consuming supply chain.

An alternate is to convert the gas into premium grade liquid products through GTL technologies. In certain instances, this also presents the most cost-efficient solution.

As there are only two large-scale land-based GTL plants in operation worldwide, the concept of a floating GTL plant may appear premature and optimistic. The environmental and sustainability arguments against flaring, the differing market situation for GTL products compared to LNG, the large scale of offshore stranded gas reserves, and the strategic value of the GTL products all encourage further study to develop the concept.

Presently, efforts are being mounted by specialized process companies to tackle the challenges associated with marinization of existing onshore GTL technologies for use on floating facilities. The biggest concerns identified by Izar and its Fene shipyard in northern Spain (formerly Astano) relate to the sea keeping behavior of the floating structure. This must satisfy the motion and acceleration requirements of the FPSO process equipment that, to date, has performed successfully in benign weather scenarios.

The latest generation of FPSOs operating off West Africa, such as Girassol, and others undergoing construction or engineering development (e.g., Kizomba, Erha, Agbami), have production rates of about 200,000 b/d and oil storage capacities of around 2 MMbbl, with approximate dimensions of 300 m by 60 m by 32 m. Based on these facilities, with a typical gas-oil ratio of 700:1, the net amount of associated gas, after deduction of 19% for power generation and 13.5% for LPG and condensate shrinkage, is about 98 MMcf/d. This corresponds to a produc-tion capacity of GTL products of about 10,500 b/d (7,900 b/d of diesel and 2,600 b/d of naphtha). Additional capacity for produced water and consumables is also required.

One new concept is the ultra large floating production system (Ulfps). Based on a single hull, it has the following features:

  • Dimensions: 350 m by 100 m by 32 m
  • Storage capacity: 4.5 MMbbl
  • Topsides weight: Approximately 60,000 tons
  • Available deck area: 30,000 sq m.

The Ulfps concept is a compact solution for production and storage of large volumes of oil and GTL products. It is a single unit FPSO with adequate dimensions to provide the required deck area for process equipment and tank storage capacity. The resulting dimensions for the anticipated design parameters are well above conventional FPSO typical sizes for modern large-storage capacity units (e.g., 2.4 MMbbl for West African developments).

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One possible realization of the proposed Ulfps concept.
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The unit's unusual size necessitates specific features, some of which need to be further evaluated. A bow external turret is provided in order to minimize environmental loads and especially the roll motion. The available deck area, roughly 300 by 100 m, allows for a high flexibility to accommodate the process equipment needed for combined production of oil and GTL products.

The utility systems can be conveniently combined without compromising the necessary redundancies. Offloading to a shuttle tanker for both stabilized oil and GTL products can be performed by means of a floating hose, an offloading buoy, or a combination of the two.

Another possibility is the combined FPSO (C-FPSO) being investigated by Izar Fene as a solution to some of the drawbacks associated with increasing the size of single units, such as the Ulfps. The C-FPSO concept consists of two units, with the following combined characteristics:

  • Main dimensions: two units of about 300 m by 58 m by 32 m
  • Combined storage capacity: 4.5 MMbbl
  • Topsides weight: Approximately 60,000 tons
  • Available deck area: 30,000 sq m.

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The two units of a C-FPSO are connected by an ASL.
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The C-FPSO comprises two standard size FPSOs operating in tandem, with similar or different functions (e.g., crude oil and GTL), by means of an articulated arm spring link (ASL). It provides the benefit of using proven designed concepts instead of challenging new ones. Of course, several disadvantages are identified compared to the single unit concept.

The station keeping system of the C-FPSO relies on an external turret installed in one of the FPSOs. Thus, only one mooring system has to be deployed. The design of the mooring system is based on the 100-year storm with both units connected. To provide adequate heading stability for the C-FPSO (to avoid "fishtailing" effects), thrusters will probably need to be installed in both vessels. The number and capacity of such thrusters is still to be confirmed by computer simulations/model tests, although a reduced required capacity is anticipated.

The ASL is designed so that undesired degrees of freedom are released by means of hinged joints, and only tension-compression capabilities are retained. This element resembles the mooring hawser of a typical tandem offloading system of an FPSO with shuttle tanker connected at the vessel stern. However, compression capability is added to the system to limit the clearance between the units. The dynamic characteristics of the ASL are adjusted by a spring-damper element. Its dimensions are a function of the two FPSOs' maximum expected draft difference and their relative end motions. The ASL system includes an enclosed gangway for personnel access and small piping and cabling routing from one unit to the other. Transfer of main fluids for treatment (gas for GTL processing, for instance), can be arranged either by hanging the pipes (catenary option) from gooseneck elements, or preferably by means of floating hoses.

It may prove convenient for certain systems to be shared by both FPSOs to avoid unnecessary duplication of functions (i.e., accommodation, utilities, stores). However, this first requires careful examination of all associated risks and safety issues. Multi-body computer simulations are currently being performed in co-operation with the Marin Research Institute in The Netherlands to further refine the concept, and model tests will be needed for final validation.

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