J.L. Lye – D.T. Brown, BPP Technical Services
This presentation details how FPSO technology advancements have created the opportunity to process stranded gases from marginal offshore fields to produce – at sea – high-value methanol and other synthetic hydrocarbon “gas-to-liquids” products.
The authors say it is now technically and economically feasible to safely convert gas to liquids (GTL) at offshore sites, allowing the liquids to be transported to market in a cost-effective manner. (In this regard, GTL refers to any process that converts a gas to a liquid, rather than specifically to the Fischer-Tropsch technology).
Feasibility studies have been conducted on design of a number of offshore platform types to be used to process and store GTL products. Designated as Worley Gas Terminal (WGT) vessels, they would be floating platforms having low motion and optimum multiple-product cargo capacities, yet maintaining acceptable stability and draft throughout complete loading/offloading cycles. These design features and dimensions were optimized through the use of dedicated design software to minimize motions resulting from wind and waves while minimizing overall construction costs.
As water depths increase and combined reserves decrease with new development projects, existing proven resources must be fully exploited. With natural gas, a solution to the problem of offshore “stranded” gas must be solved.
When an offshore field is too small or too remote to be economically handled via conventional pipeline to shore transport, land-based technologies such as LNG and GTL have been proposed for use on site-specific floating platform structures similar to FPSOs.
A highly desirable feature of such a floating gas facility would be its ability to move from one stranded gas source to another, taking advantage of a series of small reserves and converting them to a high value liquid at each site. Having the liquefaction site offshore allows the liquid products (LNG or liquid/oxygenated hydrocarbons) to be more easily loaded and transported into appropriate tankers for delivery to worldwide markets.
Well-proven shore-based gas conversion technologies can be used offshore to convert gas into gasoline, high-value methanol, or synthetic hydrocarbon products such as syncrude, diesel, and naphtha. These products can be transported in a manner similar to “traditional” petroleum products, rather than like LNG, which requires cryogenic tankers, or CNG, which calls for multiple vessels with high-pressure containers.
In the feasibility studies, the proposed location for the subject platform would have the following characteristics:
- Mild sea conditions, with a long predominant wave period
- A likelihood of tropical squalls
- Moderately deep water
Each platform design was optimized for the following design drivers:
- Deck area – Must be sufficient for all required equipment
- Cargo storage capacity – Must be sufficient for a given cycle time and optimum transport volume
- Stability – The platform must provide for optimum stability throughout the entire
- Draft/freeboard – Should not vary excessively during loading/unloading cycle
- Motions in waves and wind – Should be kept to a minimum for optimum operations and efficiency.
In general, feasibility studies indicated that any viable platform for GTL conversion and oil and gas production should have storage tanks for two or more products, low motion response to waves and wind, as well as a large deck area. Traditional FPSO hull technology does not meet the necessary criteria of the WGT hull form.
The proprietary hull form features two hulls, with the lower hull suspended below the upper hull by vertical, structurally integrated columns. This hull design was chosen for a number of reasons, including increased roll dampening, greater control of the vertical weight distribution, and greater separation between dual cargoes.
Along the beams of the upper hull is a series of motion control tanks whose bottoms are open to the sea and which contain valves to control airflow into and out of the tanks. This gives the platform the ability to alter the roll period, which is particularly useful in environments where the predominant wave period is close to the platform’s harmonic roll period.
The authors address the novel WGT platform’s topsides and tank layouts, principal dimensions and design optimization using an in-house generated design tool (software) to produce the large number of variables stemming from an equally large number of proposed hull designs.
The authors conclude that the increased number of variables and interactions involved with the novel hull form necessitate a systematic approach to design optimization. Using automated procedures, they note, it is possible to perform the sophisticated analyses necessary to verify the feasibility of a given hull form.
