Mobil develops center section hull with enhanced stability
G. Z. Gu
G. C. Hoff
Mobil Technology Company
Mobil has developed a unique floating liquefied natural gas (LNG) plant. The facility is a production, storage, and offloading platform, designed to produce 6 million tons/yr of LNG and 55,000 b/d of condensate.
All production and offloading equipment, weighing close to 100,000 tons, is supported by a square, donut-shaped concrete hull. The hull contains storage tanks for 250,000 cu meters of LNG, 650,000 bbl of condensate, and adequate ballast water. LNG and condensate can be directly offloaded to shuttle tankers. During its life, the movable plant is expected to produce from several fields.
Floating LNG plants have a number of advantages over onshore facilities, including greater mobility, comparable or lower costs, and faster schedule. With a well-designed hull to satisfy stringent motion requirements, a floating LNG plant can be a very attractive and economic option for many fields.
Hull configurationMobil formed a multi-discipline team to determine the feasibility of a floating LNG processing plant. In the project's early stage, many hull shapes were examined for housing the proposed plant. After several iterations, an innovative square-donut hull shape was developed. Numerical analysis indicated this hull shape to be superior to others in many aspects and is uniquely suited for the application of a floating LNG plant.
The unique square-donut hull provides:
- Sufficient deck area for an LNG plant with a throughput of 6 million tons/yr and storage space for LNG, condensate, ballast water, amine, and other materials
- Excellent hydrostatic stability for the process
- Excellent motion characteristics under ocean waves from any direction
- Protected inlet for risers and water intakes
- Sufficient safe area for locating living quarters, maintenance shops, escaping conduit, and other personnel activities.
The hull is 540 ft square, with a moonpool. Viscous dampers and unique gratings improve vessel motion and reduce wave runup. The large moonpool has the effect of reducing the barge motions, improving hydrostatic stability and providing a protected passage for risers and cooling water intakes. Most of the process is located 20 ft above the main deck, eliminating salt water contact with the main processing facilities.
Inside the hull are LNG tanks, condensate tanks, and ballast water tanks. Four square corner chambers provide buoyancy and storage space. LNG is stored in special tanks fitted in the hull chambers and condensate is stored directly in the concrete compartments. The ballast water tanks are in the hull perimeter, thereby providing additional impact protection for the LNG storage tanks. This layout also provides maximum trim ballast efficiency.
The unique shape allows the concrete barge to be built in four sections in different dry docks, and mated together in a protected offshore location. As a result, many drydocks world wide are capable of building a single section, reducing costs and shortening the overall construction schedule.
Hydrostatic hull stabilityDue to the large water-plane area and wide beam width, the hull is extremely stable during storms. The meta-centric height (GM) can reach up to 88 meters in normal operating conditions and can sustain severe hull damage and flooding in the event of a ship impact. For example, in the event all three ballast tanks and both corner chambers on a side are flooded, the hull will tilt only 5 degrees without active ballast on the other side.
The hydrostatic stability makes the hull relatively insensitive to topside weight and the height
of heavy equipment. This is particu larly important for an LNG plant since it has a considerable amount of heavy and tall equip ment. Also, an LNG plant is very sensitive to long-term tilt which can significantly reduce its efficiency. The high meta-centric height and wide beam of the hull make it less likely to have large angle, long-term tilt caused by wind, offloading, or hull damage.
Hull motionsLNG plants have signifi cantly more stringent motion require ments than typical oil processing topsides, due to both gas processing and cryogenic off loading system demands. An in-house numerical analysis using a hydro dynamic program devel oped by the Massachusetts Institute of Technology in a joint industry program (WAMIT), and its Windows user interface developed by Win+Tool (WAFRONT), showed the unique hull shape has excellent motion characteristics.
The roll/pitch viscous damping was estimated using well known empirical formulations and the maximum hull motions were calculated for a site offshore northwest Australia. To verify the numerical results, a model test was conducted in the wave basin of the Offshore Technology Research Center at Texas A&M University. The test results confirmed the numerical predictions. A slight over-prediction by the numerical model can is attributed to scale effects of viscous damping.
The benign hull motions are achieved through a combination of the square donut shape, low natural frequencies, low center of gravity of the concrete hull, and the viscous dampers. Also, the symmetric geometry makes the hull insensitive to wave, wind and current directions.
Hull materialIn the process of selecting hull material, both concrete and steel have been examined. While both materials are believed to be feasible for this application, concrete has many advantages over steel including:
- Durability due to strong fatigue and corrosion resistance
- Minimal maintenance requirements
- Strong resistance to LNG spills, fire, ship collision, and dropped objects
- Improved motion characteristics due to heavy hull weight, high mass moment of inertia, and low center of gravity
- A stiff hull and strong deck for heavy and tall equipment support
- Good thermal insulation
- A short, cost-effective schedule
- Low construction labor skill requirement.
MooringFor the base case site located at a water depth of about 200 meters, in-house mooring analysis show ed that the concrete hull can be spread-moored with 24 6-in. diameter chain lines, six at each corner. The maximum offset in a 100-year typhoon condition is about 37 meters.
Although much larger than those used on most of the existing FPSOs, the size of the mooring chains is certainly within the capability of the industry. It is believed that the mooring system design can be further optimized through additional studies.
LNG offloadingWith the benign motions of the floating barge, LNG offloading to a shuttle tanker can be achieved using the FMC-designed bow-to-bow loading arm. This specially designed double-diamond pantograph system can accommodate six degree-of-freedom motions between the FPSO and the shuttle tanker's bow. Two loading arms, located at two opposite corners, will be used to assure a high offloading availability during all seasons.
The uniquely designed concrete hull has many excellent hydrostatic and hydro dynamic characteristics as compared to a turret-moored ship-shaped hull. This type of design can provide a stable and highly reliable base for offshore LNG manufacturing and offloading.
The authors would like to thank Mobil Technology Company for granting permission to publish this paper and acknowledge other members of Mobil's Floating LNG Plant team: Marie Naklie, Subir Bhattacharjee, Dave Garrett, Ed Grave, Kailash Gulati, Joe Jeffery, Roy Johnson, Carl Kinney, Raul Lopez, Jin Mok, Roy Rolph, Harvey Schulz, Jon Sonka, Walt Spring, Bert Sweetman, John Weeks, and Kris Yost of Mobil Technology Company, Tony Urbanelli of Mobil Oil Shipping and Transportation, and Cary Iverson of Mobil New Exploration & Producing Ventures.
Naklie, M., "Novel Design for a Floating LNG Plant," Mobil Technology Company, Oil & Gas Journal, June 16, 1997.
Copyright 1997 Oil & Gas Journal. All Rights Reserved.