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March 14, 2005

ABS approves new LNG containment tank concept

ABS has given its approval In principle (AIP) to energy major ConocoPhillips for a new proprietary Prism/Pyramid tank concept for large LNG carriers.

ISeparately, ABS also announced that it has given approval in principle to a novel concept from ABB Lummus Global (ABB) for a liquefied natural gas and liquefied petroleum gas, floating production storage and offloading unit (Niche LNG FPSO).

The key feature of the pyramid tank's unique shape reduces free surface area, thus reducing the high impact sloshing loads and resonance period in the tank.

The free surface reduction resulting from the design is important because, according to hydrodynamic experts at ABS, the impact pressure due to sloshing motion from the cryogenic liquid cargo inside tanks is one of the most critical load factors when designing containment systems for LNG carriers.

 "The magnitude, effective area and duration of the impact load are all important when considering structural response of the containment system.  It is also important to examine the spatial and temporal pattern of the impact load in concert with structural response," says Dr. Hoseong Lee, Staff Consultant, ABS Technology.

ConocoPhillips Marine contracted for model tests on the Prism/Pyramid tank design with the Marine Technology Research Institute (Marintek) in Norway.

The tests were attended by engineers from ConocoPhillips and ABS.

The critical ship motion responses and sloshing impact conditions were calculated with North Atlantic environment conditions. In conjunction with the model tests, ABS applied its proprietary numerical simulation tools to perform complex calculations that predict the dynamic and sloshing pressures acting on the membrane tanks in a seaway.  

Specifics of the tank testing and motion responses involved a four tank scenario for an LNG vessel in the range of 235,000 cu.m .

Vessel designs for this size historically have called for five or six tanks.

Irregular wave conditions were simulated with three different filling levels for the tanks and various ship headings. The test results of the comparison and pressure tests showed the design was acceptable and the loads on the ConocoPhillips Prism/Pyramid tank were equal to or less than those experienced on a traditionally designed 138,000m3 ship.

"We are pleased to have ABS technical input as a continuing partner in the development of new technology for LNG transportation," says Peter G. Noble, Manager, Marine Transportation, ConocoPhillips.  Noble adds there are a number of technology development initiatives underway at ConocoPhillips relating to gas carriers and terminals.

The ABS AIP process draws upon engineering, testing and risk assessments in order to determine if the concept provides acceptable levels of safety in line with current offshore and marine industry practice.  The methodology relies heavily on risk assessment techniques as a way to better understand and anticipate structural and operational issues related to a new or novel concept. ABS evaluation of the overall tank design included an assessment of the tank containment system to the requirements of ABS Rules, an International Gas Code (IGC) structural strength feasibility study and an analysis of the tank support system.


Niche FPSO

ABS has also recently given its approval in principle (AIP) of a novel concept from ABB Lummus Global (ABB) for a liquefied natural gas and liquefied petroleum gas, floating production storage and offloading unit (Niche LNG FPSO).

The concept is the offshore application and marinization of the NicheLNGSM process, a proprietary dual turbo-expander based LNG liquefaction scheme developed by ABB Lummus Global.

The equipment layout is similar to a typical FPSO, however, the concept for this new purpose-built FPSO allows for processing facilities onboard which incorporate both the gas feed pre-treatment (removal of CO2, Mercury etc., dehydration of the gas and LPG extraction) and LNG liquefaction.

LNG and LPG have different compositions thus requiring storage at different temperatures.  LNG must be stored at extremely low temperatures (-162 degrees C) while LPG can be stored at a much higher or warmer temperature (-40 degrees C).

Described by ABB as primarily a floating liquefaction facility for LNG production, storage and export, its process facilities are compact enough to fit onto the deck of a 312-metre long Niche LNG FPSO, yet is capable of delivering 1.5 million tonnes per year combined output from its three LNG production trains.  The design storage capacity is in the 200,000m3 range, with six tanks total---four for LNG storage and two for LPG storage.  The design features a flat deck making it easier to arrange and accommodate the topside process modules.

ABS was uniquely qualified to review this new vessel configuration because it is the only classification society to have experience with the type of liquid gas containment system specified for the Niche LNG FPSO.

The vessel will utilize a storage concept such as Ishikawajima Harima Heavy Industries, Co., Ltd's. (IHI) Self-supporting, Prismatic-Shape, IMO Type-B tank system or SPB. ABS has previously classed the only LNG carriers to use this containment system and also classed the first LPG FSO newbuild, the Escravos, and the first LPG FPSO newbuild, the Sanha, both operating offshore Angola.

According to ABS' Project Manager Phil Rynn, the uniqueness of the proprietary liquefaction process from ABB on the vessel required close review by ABS.  The 'dual turbo-expander cycle' to chill the gases to a liquid form for transport involves cooling and condensing the methane rich stream available from the gas pre-treatment step.  Gas is liquefied in two independent cycles, using methane as refrigerant for the first stage, and then moving to nitrogen, with both cycles adopting turbo-expanders.  These refrigerants are always in gas phase, which simplifies the equipment layout compared with traditional LNG refrigeration systems.

"Since there is a complete LNG process plant onboard which also incorporates LPG extraction facilities, there are more equipment concerns with regard to spacing and general arrangements," says Rynn.  During preliminary design review of all the drawings, Rynn says special attention was given to the process facilities, general arrangements, cryogenic transfer and facilities for loading and offloading the LNG and LPG products, as well as in-service inspection and security issues for the design.

"Industry has successfully dealt with oil and gas processing facilities on Floating Production Storage Units for a number of years.  However, the liquefaction and cryogenic storage of LNG and LPG offshore is a whole new arena," comments ABS Vice President of Energy Development William J. Sember.  

"This transition requires a comprehensive approach toward the integration of marine and typically shore-based gas processing facilities in the design of a floating concept," he adds. "A key issue for a floating concept like this is designing for the relative motions likely to occur during loading/offloading operations."

Transfer of LNG at subzero temperatures through a loading hose or arm presents industry with a technological challenge.  ABS is working with industry to evaluate appropriate technologies to optimize reliability and flexibility for the LNG transfer in an offshore environment.

The ABB Niche LNG FPSO design has incorporated a number of measures to minimize the effects of the motions experienced on a floating platform. The vessel will be held on station via an external turret mooring system at the bow and offloading will most likely be conducted tandem-style from the stern although side-by-side offloading is a possibility and is being explored.

The power generation demands for this concept call for an onboard power plant with the capability of providing 40 megawatts of generation.  Three main gas turbines will drive the main compressors. Together the three trains can liquefy 225 million cubic feet per day of gas.

AIP from ABS included design review of the following: general arrangements, topsides and system integration, process flow diagrams (or PFDs), heat and mass flow diagrams (or MFDs), safety shutdown and firefighting system philosophy, as well as review of the hull structure, cargo tanks, refrigeration and re-liquefaction plants in accordance with the International Gas Code (IGC).

Commenting on the 'Approval in Principle' Program just concluded, Robert R. Huebel, General Manager Randall Gas Technologies, a division of ABB Lummus Global, said "The rigorous program of design documentation reviews conducted by ABS, and its participation in HAZIDs has validated the Niche LNG FPSO as a safe and environmentally sound concept, fully in compliance with current marine practice, Society rules and International Standards, and suitable for its intended function, that is, for the monetization of gas reserves (both associated and non-associated) from offshore fields."



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