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  GNL - Novidades Tecnológicas
  Autor/Fonte: Offshore, May 2004
  Data: 2004-08-26

    New LNG conversion technology expands terminal options

DOE research describes offshore salt cavern potential

Michael M. McCall and Lance J. Van Anglen, Conversion Gas Imports, LP
James F. Davis, Paragon Engineering Services
Max Krekel, Bluewater Offshore Production Systems (U.S.A.) Inc.

Combining offshore LNG receiving terminals with gas storage in man-made salt caverns offers an opportunity to improve the security, economics, and scale of the LNG value chain in the US. With particular focus on the Gulf of Mexico, LNG receiving terminals linked to salt caverns could combine the advantages of easy vessel access, improved security, large-scale storage, and high send-out volumes.

The mooring and transfer system is remotely operated and normally unmanned.

To study the concept of LNG regassification into salt caverns, the US Department of Energy awarded a cooperative research agreement to Conversion Gas Imports LP for a comprehensive analysis of the processes, including full-scale field-testing of critical components.

Technical validations are proceeding, with full-scale field tests to be completed soon. The data obtained will be incorporated into process models, equipment lists, and cost estimates for both onshore and offshore terminal designs. The government-industry effort is intended to accelerate the move from concept to full commercialization.

Salt caverns make up 5% of the US natural gas storage capacity, but provide 15% of US gas deliverability. In fact, the entire US Strategic Petroleum Reserve of more than 630 MMbbl is stored in salt caverns along the Gulf Coast.

Numerous design schemes are being developed for salt-dome-based LNG terminals. One design under consideration is based on Bluewater Offshore''s Big Sweep mooring and LNG offloading system, combined with a platform designed by Paragon Engineering.

The Bluewater design, developed for operation in the Gulf of Mexico, is a shallow-water version of their Big Sweep system. Key components include:
•A monopod structure with a swivel deck piled to the seabed
•A rigid-truss arm suspended from the monopod
•A mooring outrigger fitted at the forward end with its aft end terminating in a buoyant column
•An LNG transfer system starting at the LNG carrier''s manifold and ending at seafloor at the monopod structure.

This design calls for the LNG carrier to hook-up to the mooring outrigger fitted on the forward end of the truss arm by means of a bow hawser. The length of the rigid arm positions the buoyant column near the midship cargo manifold. By adjusting the length of the mooring hawser, the carrier''s cargo manifold can be lined up for vessels ranging from 125,000 cu m to 200,000 cu m storage.

A quick connect and disconnect manifold will fit forward of the LNG carrier''s existing midship manifold. This will pipe-up with removable spool pieces and allow use of the existing manifold at conventional terminals. Flexible jumpers, suspended from a manipulator, connect to the skid onboard the LNG carrier. The manipulator is remotely operated and allows connect/disconnect of the cryogenic flow path without manual intervention.

The loading arm trails the monopod but can be "parked" away from the LNG carrier''s line of approach using its own propulsion. In this position, the loading arm assembly cannot be damaged by a failed mooring approach of the carrier. Emergency disconnection can take place in two ways – by a "quick disconnect", allowing time for closure of valves/pumps, or by activating full power on the thrusters to clear the rigid arm from the carrier.

Platform process equipment
The capability of salt caverns to handle high-volume gas flow rates accommodates the need for LNG tankers to unload quickly at rates from 8,000-14,000 cu-m/hr of LNG (an equivalent flow rate of 4+ Bcf/d). Warming the LNG from a nominal -260° F to a cavern gas injection temperature of 40° F requires a high volume flow rate vaporizer and cryogenic LNG pumps that can operate at send-out pressures that vary from 1,000 to 2,000 psi.

The Bishop Process Exchanger (BPE) is a modular pipe-in-pipe design comprising an inner 6-in. 316L SS pipe for the LNG and an outer 12-in. UV-coated polymer pipe. Within the annulus, seawater, brine, or fresh water is pumped to warm the LNG. The 2,000-ft length of the inner pipe is folded into a 250-ft "folded 4-pass" configuration. The "pass" refers to the number of separate water injection points along the length of the inner pipe required to achieve the proper heat transfer. The folded 4-pass unit will vaporize 300 cu m of LNG per hr into about 6.3 MMcf/hr or 180 MMcfd equivalent flow rate.

The outer polymer pipe ends in a slip-sleeve seal enabling the inner pipe to move longitudinally with temperature variations. A jumper connection – much like a radiator hose – links one straight section of pipe to the next. The outer polymer pipe is rated for less than 3-bar, as its role is to direct the flow of water and is not structural. Centralizers placed every 10 ft provide support points for both the inner and outer pipes.

Potential salt dome-based LNG terminal locations are scattered across the GoM shelf.

For direct vaporization into the gas send-out pipeline at pressures below 1,300 psi, a low-pressure BPE can be designed with ASNI 600 specifications supplied by cryogenic pumps. For injection into uncompensated salt caverns at pressures from 1,000 to 2,000 psi (depending upon how much gas is stored), a high-pressure BPE is designed to ANSI 900.

Paragon Engineering has modeled LNG pump configurations required to work against this variable pressure at the required flow-rates. By linking multiple high-pressure BPEs into a common high-pressure header, currently available LNG pumps without variable frequency drives can satisfy both the pressure variation and the flow rates required to offload a LNG tanker in the allotted time frame. Multiple low- and high-pressure BPE units can be combined into a vaporizer system for a particular location.

The design of the salt caverns and wells are conventional, but sized for the high rates of injection and discharge and adapted for offshore locations. Multiple caverns (four to seven for each terminal) and multiple wells (two per cavern) of large size work best. Each terminal will be able to deliver 3 bcf/d with 100% turndown. Four initial caverns would be placed into service with 8-10 bcf working storage capacity and would continue to be solution mined to an aggregate capacity of about 20 bcf. Three additional caverns would be added to increase the ultimate terminal storage capacity to exceed 30+ bcf.

Multiple offshore sites in the Gulf of Mexico at varying water depths and with an array of offshore and onshore pipeline connections would be capable of large volume imports into the US. A salt dome that extends over much of Vermilion blocks 164-179 forms the basis for the design work in the DOE cooperative study. Details of the mooring and LNG transfer systems, process requirements, cavern designs, projected costs, and operating characteristics are largely applicable at other locations. All offshore sites studied are under the Department of Interior''s Mineral Management Service and the applicable leasing and right of way arrangements with the government are being developed.

The National Energy Technology Laboratory (NETL) is funding 70% of this research, while CGI is managing the project. The other 30% of funding comes from industry participants, including AGL Resources, Bluewater Offshore, BP, Carter Cryogenics, Dominion Resources, Ebara, EnCana, ExxonMobil, Fluor Daniel, FMC, HNG Storage, Hoegh LNG, Marathon, Nikkiso Cryo, Northstar Industries, Paragon Engineering, PB

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