Deep and ultradeep water of many of the world's most promising hydrocarbon provinces has technically and economically challenged developers and operators in their field development approaches, especially for natural gas.
As industry continues to push boundaries of ultradeep water and remote location finds, we have addressed the challenge by taking a compressed natural gas (CNG) carrier concept and expanding it into an all-in-one deepwater gas production and transport vessel called Gas Production Storage Shuttle (GPSS).
This shuttle vessel concept offers exploration and production (E&P) operators the ability to eliminate much of the infrastructure traditionally required for remote gas field development, such as expensive ultradeepwater pipelines and dedicated production facilities. Cost analyses have shown that savings of approximately 20% to 25% in project capital costs for field production operations support, storage and transportation for a deepwater gas field development may be achieved across a broad range of applications by employing the GPSS concept.
Recently a feasibility study was completed with Kerr-McGee Corp. and the Research Partnership to Secure Energy for America (RPSEA) to develop the conceptual design and assess the technical and commercial viability of a gas production system for ultra-deepwater gas reservoirs in 8,200 ft (2,500 m) of water in the Gulf of Mexico.
The GPSS is analogous to a floating production, storage and offloading (FPSO) vessel used in oil service with the added capability of transporting its gas product to market (Figure 1). It combines all of the features and advantages of EnerSea's Votrans (Volume Optimized Transport and Storage) CNG carrier, including proprietary gas containment and gas handling technologies, with direct operational control and support for the subsea gas field and processing systems for the produced fluid onboard. The shuttle vessel concept also serves as a storage facility for gas and liquids and, when filled to capacity, disconnects from its production buoy/mooring to deliver the gas to market. Utilization of a tandem buoy configuration and multiple vessels operating in a shuttling fashion allows for uninterrupted production from remote fields.
Application
This development solution may be considered worldwide wherever relatively lean offshore gas fields exist beyond economic pipeline access and up to 2,500 miles (4,000 km) from market.
While the system provides a robust solution for gas resources located in deep and ultradeep seas - up to 9,840 ft (3,000 m) - the gas production and shuttle storage concept also offers a cost-effective solution in much shallower waters (e.g. less than 328 ft or 100 m). The GPSS delivers all of the advantages of a FPSO, including risk management and re-deployability.
Table 1 illustrates some of the basic differences between a GPSS-based field development and one utilizing a long reach deepwater pipeline.
Offloading of gas cargo to the market can be made through offshore terminals sited conveniently but safely away from population centers. These offshore locations lessen the risks to sensitive areas from acts of sabotage or other events, thus reducing exposure to, and safety issues associated with, large onshore gas terminals.
Current industry practice for developing ultradeepwater reservoirs requires aggregation of fields - hub development - to accumulate enough production volume to justify the high cost and risk of deepwater pipelines that are not re-deployable. The GPSS will allow multiple reservoirs to be tied back through the initial installation. Alternatively, with GPSS, operators can also develop smaller reservoirs sequentially with a system that can be re-deployed once the initial reservoir is depleted.
Earlier this year Votrans technology was granted "Class Approval in Principle" by the American Bureau of Shipping. Project initiatives for application of this technology have recently been announced and prototype-testing programs are underway. We anticipate a commercial project using a Votrans CNG vessel to be adopted within the next 12 months, with gas production and transport operations commencing in 2008. Vessel construction typically takes 28 to 30 months from the time of the contract award.
Feasibility
The feasibility study advanced the concept design for producing raw reservoir fluids directly onto the GPSS, which processes and stores separated gas and condensate streams, and then transports and unloads the gas into an existing gas trunk-line system located approximately 120 miles (350 km) from the field. Two GPSS vessels, each with a capacity to transport 220 MMscf of gas, would be used to support continuous production from the reservoir in this field development scenario.
The GPSS will be initially loaded through use of direct reservoir energy. As pressure declines, the GPSS has been designed to allow installation of a compression module to continue reservoir evacuation and to extend the economic field life.
General GPSS description
The subsea field production system will be connected to a short subsea offtake flowline. This flowline connects to the bottom of a simplified hybrid riser tower (HRT), which rises from the seabed to within a few hundred feet or meters of the water surface (Figure 2). At that point, a pair of flexible jumpers extends laterally to a pair of submerged production buoys. The HRT technology is currently deployed at Total's Girassol field in Angola in approximately 5,900 ft (1,800 m) water-depth and can be readily extended to the water depth of the project.
Control, monitoring and chemical injection of the subsea equipment is performed through a multi-conductor umbilical connecting the subsea system through the turret connected to the GPSS. Wellhead valving and a high-integrity pressure protection system provide isolation of reservoir pressure upstream of the connection between ship and buoy.
Operations
The GPSS ships are dynamically-positioned, twin-screw vessels with diesel-electric propulsion which also allow convenient load-sharing with the onboard gas handling facilities. Deck and hull spaces adjacent to the turret compartment support the production and gas handling equipment and bulks. The cargo block amidships is comprised of insulated holds that are tightly enclosed to maintain a cold, nitrogen-inerted storage environment. The helm, accommodation and gas handling facilities' main control stations are located aft.
Typical gas field production and processing facilities designed for subsea well applications are located on each GPSS, including condensate and water separation and handling facilities for methanol. Methanol injection at the wellheads is used to prevent hydrate formation. As the produced fluids arrive on the GPSS, separation equipment recovers the methanol and also separates the gas, water and condensate flow stream. The gas stream is then cooled to a storage temperature of -20?F (-29?C), while the pressure is throttled and controlled to the initial loading pressure, typically 1,700 or 1,800 psig, as it is injected into the gas containment system at the design production rate. Gas containment on each GPSS consists of arrays of vertical 42-in. API5L X80 pipe cylinders, each 80-ft (24.4-m) long, and segregated into CNG modules consisting of multiple cylinders. Individual pressure vessels are designed to be manufactured according to ASME Section VIII Division 3 Code Case criteria.
Production from the subsea wells flows through the swivel to the inlet of the topside processing facilities. As one GPSS is filled to near capacity another GPSS, returning empty after unloading, connects to the unoccupied STP buoy to begin loading. Control and production flow are switched over to the second vessel in a phased manner to ensure continuous control and production.
The completely loaded GPSS then begins its transit to market. Upon arrival at the offloading station the GPSS connects to a single unloading submerged turret loading (STL) buoy to discharge its cargo into a subsea pipeline within 24 hours.
Condensate separated at the field and transported to the offloading delivery point can be commingled upstream of the swivel, yielding a two-phase export flow into the pipeline. In other applications, these streams could be delivered through separate export pipelines if desired. Product is offloaded at the required pipeline pressure and minimum design temperature.
Optimization of this concept in certain applications may include installation of a permanently moored barge or buoy at the field to provide subsea production support and storage operations. In this "GPSO" mode, a smaller capacity fleet of more typical Votrans CNG carriers can be quickly and intermittently loaded for delivery of gas to market. Tandem or bow-on loading would be used in this scenario with only one CNG transport ship connected to the GPSO and loading at one time.
Commercialization
Qualified cost estimates and project schedules have been calculated in the development of both Votrans and GPSS systems. This work indicates first gas can be produced within 3 years of project sanction following completion of front-end engineering design. The US Coast Guard has confirmed its leadership role in approval of the gas transport concept and offloading terminal. The terminal should pose very few permitting problems as the US Department of Transportation (DOT) recently approved a similar STL buoy system for an offshore gas port, which is projected to be installed and in operation in 2005 and will pave the way for the permitting process of the GPSS system.
The GPSS is a viable concept using a combination of industry-proven technology and Votrans CNG technology. After an extensive feasibility and conceptual engineering program, no technical roadblocks have been identified that would prohibit the successful permitting, design, construction and operation of the GPSS as described herein.
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