With increasing field development costs and stable high gas prices, the oil and gas industry is searching for innovative ways to produce hydrocarbons from frontier development areas while minimizing life-of-asset costs. Frontier areas include deep and ultra-deep water as well as shallowwater areas where there is limited infrastructure.

EnerSea Transport LLC developed a floating gas production and compressed natural gas

When the GPSS is used for production, well stream fluids flow from subsea wells to a subsea manifold, then onward through one or more flexible risers connected to a loading buoy. (Images courtesy of EnerSea)
(CNG) carrier called VOTRANS to addresses the challenges operators are facing. The company has improved upon VOTRANS, creating an all-in-one gas production and transport system called GPSS (gas production storage shuttle).

Many operators are now investigating small-scale floating liquefied natural gas (LNG) facilities. EnerSea’s vessel, which allows production, storage, and transportation, provides significant advantages over the floating LNG solutions. The GPSS shuttle vessel concept offers operators the ability to eliminate expensive and unproven floating liquefaction technology and expensive cleaning and pre-processing of raw gas wellstream required for LNG. The GPSS is analogous to a floating production, storage and offloading vessel (FPSO) used in oil service with the added capability of transporting its gas product to market, EnerSea said. Additional advantages of an FPSO, such as risk management and re-deployability, also apply to the GPSS.

Floating LNG vs. GPSS
The GPSS combines all of the features and capabilities of EnerSea’s VOTRANS CNG carrier, including proprietary gas containment and gas handling technologies. The GPSS offers direct operational control and support for the subsea gas field, as well as processing systems for the produced fluid onboard. The shuttle vessel 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. A tandem buoy configuration can be used with multiple GPSS vessels operating in a shuttling fashion for uninterrupted production from gas reservoirs.

The GPSS concept can also be considered for extended well testing or as an early production system. Having the ability to produce substantial volumes of gas from large, complex reservoirs prior to making the ultimate field development decision provides a highly attractive risk management option for E&P companies. Cost-effectively using the GPSS for extended well tests to gain confidence in the reservoir’s long-term productivity characteristics allows field operators to lower the risk associated with high capital cost field developments and very expensive wells.

EnerSea completed a study with Kerr-McGee Corp. to develop the conceptual design and assess the technical and commercial viability of a gas production system for an ultra-deepwater gas reservoir in 8,202.5 ft (2,500 m) of water in the Gulf of Mexico. Additional assessments are currently underway with two E&P companies, applying the GPSS concept in one case as an early production system and in another case using the GPSS as a transport system for a full-field development, EnerSea said. Both assessments are for harsh environments.

According to the American Bureau of Shipping (ABS), EnerSea’s VOTRANS CNG systems are
Artist’s rendering of a shore-based CNG handling facility.
ready for project development. “EnerSea has successfully completed all the requirements in ABS’s AlP (Approval in Principle) granted on April 16, 2003, and ABS considers that the VOTRANS design is ready to move into construction phase for specific projects,” said Phil Rynn of ABS Americas.

Construction time for the GPSS is estimated at 28 to 30 months from the time of contract award. Qualified cost estimates and project schedules have been created in the development process for both the VOTRANS and GPSS systems.

Gas field subsea production features
When the GPSS is used for production, well stream fluids flow from subsea wells to a subsea manifold, then onward through one or more flexible risers connected to a loading buoy. Dual buoy terminal design can support continuous production operations from the field with a staged handover of control when switching from one GPSS unit to the other, EnerSea said.
The gas reservoirs currently being evaluated by EnerSea are over-pressured. This condition allows the GPSS to be loaded using direct reservoir energy. Modifications can be made as pressure declines. The GPSS design allows for a compression module to be installed so reservoir evacuation can continue and the economic field life can be extended. Because EnerSea’s patented designs use relatively modest storage pressures, a compression module would not be required until late in the field life.

To contend with the high flowing wellhead pressure, high-integrity pressure protection systems (HIPPS) will be installed at the subsea manifold and on top of each buoy to protect downstream flow path elements from high pressure. Flowing well stream pressure control
will be managed by subsea chokes at the wells and the subsea manifold, as well as at chokes on top of each buoy.

In general, flowing well stream pressures and temperatures are kept at levels as high as practical throughout the flow path to the GPSS units so heat can be dissipated subsea.
Umbilicals will be installed to control the manifold and each of the wells and to provide a facility for injection of a hydrate control fluid. The umbilicals also will provide communication pathways for data acquisition from the wells and strategic points of the subsea production system. Flow splitting/direction control of a valve arrangement at the manifold or pipeline end manifold (PLEM) can also be provided for dual buoy systems to allow for seamless production transfer from one GPSS to the other, EnerSea said.

Production/loading terminals
Numerous types of loading buoys available in the marketplace can meet the required temperatures and pressures expected during loading. The main criteria for buoy selection are met-ocean conditions and required uptime. EnerSea has selected submerged turret production (STP) buoy systems, supplied by Norwegian company, Advanced Production and Loading (APL) for harsh weather applications. The STP buoy connects the GPSS to the mooring system when pulled into a mating cone in the GPSS hull. The buoy submerges to a depth of about 98 ft (30 m) when the turret is disconnected.

The buoy incorporates a turret connected to the mooring and riser/umbilical. At the top of the turret, a flange and a mate-able quick connect provide a robust connect/disconnect mechanism. STP equipment in the GPSS is housed in a compartment with the buoy locking mechanism. The mooring system can be traditional chain-wire-chain or fiber-wire-chain, depending on water depth. Suction or drag type anchors can be used depending on seabed soil conditions.

GPSS reliability and operability
While the sea states in harsh environment regions are very rough, there are databases that reflect a large number of ships operating in these regions, including loading ships equipped with disconnectable STP and submerged turret loading (STL) systems. Based on these data, it is expected that the GPSS units, which will be equipped with appropriate dynamic positioning capability, will be able to load and offload in these weather conditions with a high degree of uptime.

EnerSea is designing its vessels and systems according to the specific sea state and weather environment conditions expected to be encountered along a project’s transit route to provide the necessary reliability.

By using a two-ship fleet, it is possible to support continuous production with rapid, intermittent cargo offloading operations. This can be achieved by offloading the cargo into a market with sufficient capacity to accept high rate intermittent deliveries or through the use of a CNG storage unit at the port. CNG storage can be provided if necessary to deliver gas rateably to the market.

GPSS vessel specifications
The general arrangement for the GPSS reflects a double-hull design that can be built and outfitted cost-effectively at many shipyards around the world. The gas containment system on each GPSS consists of arrays of tall, vertical cylinders comprising 42-in. API5L X80 pipe segregated into tanks consisting of multiple cylinders. The individual cylinders are designed to be manufactured according to recognized pressure vessel codes. The GPSS also will be designed with a proprietary volume-optimized gas handling and containment system aboard.
Gas will be chilled and stored at between 110 and 135 barg (depending on gas composition) at -23°F (-30ºC). The ship holds outfitted with the CNG containment cylinders will be insulated, made inert with nitrogen, and refrigerated to offset environmental heat loads and maintain the temperature of the storage enclosure and gas cargo. The deck and hull spaces adjacent to the turret compartment support the production and gas handling equipment. The helm, accommodations, and main control stations for the gas handling facilities are located aft.

Gas containment capacity and duty requirements are largely influenced by the volume and composition of gas, the transport distance and project-specific considerations such as contractual obligations between the gas seller and purchaser and between the field operator and gas transporter. The fleet transport capacity is easily expandable to accommodate higher rates by increasing transport speeds, decreasing storage temperature (to effectively increase storage capacity on an existing ship), or adding one or more additional shuttles to the fleet, EnerSea said.

Cargo gas will be used to provide fuel for propulsion, ship services, and process requirements onboard the GPSS. This is far more cost effective than using expensive bunker fuel for these operations. A dual-fuel diesel electric system will be used to generate power to support these functions.

Power can be shared efficiently, since the peak power requirements for process work (gas loading and offloading) occur when the ship is moored and propulsion needs are at a minimum. Conversely, when peak propulsion power is needed, gas processing needs are at their minimum. Propulsion systems in these vessels will be designed to support operational speeds of between 15 and 18 knots.

The estimated fully loaded operating draft of the GPSS will be less than 24.5 ft (7.5 m) to allow for docking at the offloading port and for dry-docking, which will be required every five years.

Typical gas field production and processing facilities designed for subsea well applications are located on each GPSS. These include condensate and water separation and handling facilities for methanol. Methanol or ethanol is injected at the wellheads to prevent hydrate formation. As the produced fluids arrive on the GPSS, simple processing equipment separates the gas and liquid flow streams. The gas stream is then cooled to a storage temperature of -23°F, while the pressure is throttled and controlled to the initial loading pressure as it is injected into the gas containment system at the design production rate.

Offloading terminal
CNG cargo is transferred out of the cargo containment system to a shore-based gas handling facility through loading arms at a jetty or dedicated port. The discharging cargo flowstream is directed into a flowline designed for full pressure conditions and low-temperature service.
Gas is displaced from the GPSS by pumping displacement fluid (ethylene glycol/water) from a cold storage tank on shore. The gas product stream is transferred out of storage at constant pressure. The cargo flow stream is scrubbed, metered, and offloaded as a single gas phase stream.

After approximately 95% of the cargo gas has been displaced from a storage tier, the remaining gas can be expanded to displace the glycol from the storage tier back into the glycol storage tank. Heel gas should be limited to less than 2%, as 3 to 4% will be reserved for fuel gas for the return trip to the field.

Hydrocarbon liquids (e.g., gas condensates) separated at the field and transported in dedicated containment to the offloading delivery point are discharged by a separate flow line to appropriate shore-side processing and storage facilities.