LNG

Floating Compressed-Natural-Gas System Provides Simpler Path to Monetization

Monetizing offshore gas by compressing and shipping it in floating compressed-natural-gas (FCNG) carriers is simpler and, in most cases, less expensive than liquefying and shipping it in floating liquefied-natural-gas (FLNG) carriers.

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Source: Getty Images.

Monetizing offshore gas by compressing and shipping it in floating compressed-natural-gas (FCNG) carriers is simpler and, in most cases, less expensive than liquefying and shipping it in floating liquefied-natural-gas (FLNG) carriers. Seaborne compressed-natural-gas (CNG) transport requires essentially the same infrastructure as pipeline export, except that the gas is compressed into a ship rather than a pipeline. FCNG provides a new way to add to the world’s natural-gas supply by allowing smaller offshore gas fields to be exploited economically.

Introduction

Many offshore gas fields are too small or too remote to produce by pipeline to shore. The liquefied-natural-gas (LNG) industry has advanced to fill the need with respect to larger gas fields by creating an FLNG production concept where an LNG processing and refrigeration plant, complete with LNG storage, is integrated into a ship or barge and moored at a gas field. LNG ships are then used to offtake the LNG and deliver it to markets, where it is stored and regasified as needed. A simpler and less expensive way to produce gas from many fields is to avoid liquefaction altogether and instead compress the gas into CNG ships, which then deliver it to regional markets.

Virtually all floating production, storage, and offloading (FPSO) ships in operation today treat and compress natural gas, either to reinject associated gas or to export it by pipeline. This is the same technology required to produce CNG offshore.

CNG vs. LNG

To transport natural gas efficiently, its density must be increased. The principal way to achieve this is by decreasing temperature or increasing pressure. Decreasing the temperature to −162°C causes it to condense to a liquid at ambient pressure and results in a volume ratio of 600:1. Compressing the gas to 275 bar at ambient temperature squeezes the gas to a dense gaseous phase with a volume ratio of 300:1.

For many projects, CNG is a lower cost solution. In the past, no CNG ships were developed or approved, so this more-economical CNG solution was ignored. A decade of work on the regulations, engineering, and full-scale fatigue testing has paved the way for the first commercial CNG ships.

FCNG: A Shuttle Tanker Operation

An FCNG production vessel is a traditional gas floating production and offloading (FPO) unit with a high-pressure gas transfer system to load Coselle CNG shuttle ships instead of a subsea pipeline. CNG is transferred at near-ambient temperatures, so the hoses and transfer systems are the same as those used to transfer high-pressure gas and well fluids onto an FPO unit.

It is important to recognize that a CNG delivery system is a continuous-flow process without interruption or discontinuity (like a pipeline) and not a batch process like LNG or fuel oils; gas flows continuously with high reliability by use of the CNG shuttle ships.

FCNG Advantages

The advantages of CNG ship transport from an FCNG production vessel over a gas pipeline to shore are

  • CNG ships can reach more distant markets.
  • CNG ships have the flexibility to reach multiple markets. Once laid, pipelines are fixed to a particular market, typically the nearest landfall.
  • Pipelines must be fully written off over the life of the project to which they are attached, whereas CNG carriers can be moved to new fields, extending their operational life to 30 years or more.
  • Deepwater or submarine hazards (e.g., seismic) can make pipelines extremely expensive or technically impossible, whereas CNG ships are unaffected.

The principal advantages of FCNG over FLNG are

  • It is much simpler and less expensive to prepare raw gas for compression than for liquefaction.
  • Because the gas is not liquefied, the only issues with inert gases relate to the customers’ requirements for heating value.
  • Hydrocarbon liquids removal may be required by both CNG and LNG systems in order to maintain acceptable hydrocarbon dewpoints in the supplied gas. LNG systems also have to remove heavier hydrocarbons in order to avoid wax formation. CNG is more tolerant to keeping the heavier hydrocarbons in the process stream because the process temperatures are higher.
  • From a mechanical and operations perspective, FCNG has the following advantages over FLNG:
    • Risk is reduced. FCNG does not need to carry large inventories of natural gas onboard because the shuttle ships provide both storage and transportation.
    • Sloshing in cargo tanks is not a design issue because CNG is a gas.
    • Transfer systems are simpler. Offloading can be achieved through traditional tandem or buoy loading systems using high-pressure hoses, which are commonly used for risers and for reinjection of gas.
    • Offloading can be carried out in open-sea conditions.
    • An FCNG process is much easier to shut down and start up, thereby increasing safety and operational reliability.
  • From the perspective of energy efficiency, a CNG process consumes less than 5% of the gas compared to an LNG process, which consumes approximately 10% of the gas.

Because of the 2:1 density difference between LNG and CNG, respectively, the economic distance to market for CNG may be limited to approximately 2500 km before the cost of shipping begins to override the economic advantages of offshore compression vs. offshore liquefaction.

The Coselle CNG Container

A Coselle is a coiled pipeline made from relatively inexpensive small-diameter pipe. By coiling pipe, it is possible to create a very large pressure vessel that can fit compactly into a ship. Fig. 1 compares an equivalent volume of traditional gas-storage bottles (169) and Coselles (6). Each Coselle is a 21-km pipeline. Where 169 bottles require more than 300 connections, the six Coselles have just six.

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Fig. 1: Pressure cylinders vs. Coselles.

 

High-pressure gas must be contained safely. One significant safety advantage of the Coselle is that the diameter of the gas storage pipe is small. In the highly unlikely event of a full-bore rupture, the energy released is insufficient to damage the ship’s hull.

Coselle CNG Ships

A CNG carrier is a shuttle ship and needs to be efficiently designed, taking into account the Coselle and associated cargo systems. The Coselles are integrated into the ship structure, thereby making maximum use of the Coselle’s structural steel in the ship structure. This arrangement saves 30% of the structural steel that would be required for a similar ship without integrated Coselles.

To match the requirements of a broad range of projects CNG ships are designed in different sizes. A schematic of a Coselle ship is shown in Fig. 2.

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Fig. 2: Coselle CNG ship with 25 Coselles (C25).

The CNG Process

To assist in understanding the process by which raw gas is processed and delivered with CNG ships, the following example considers an offshore field with a natural gas production capability of 350 MMscf/D. The gas is lean gas, with a molecular weight of 18.2, a methane content of 88.5%, and a lower heating value of 955 Btu/scf.

In the following scenario, three C84 CNG ships (each containing 84 Coselles in 12 stacks of seven) are used to deliver gas continuously from an FCNG vessel that is 350 km away from a receiving terminal. It is assumed in this example that both the loading and discharging of gas is continuous. The CNG system essentially provides a floating pipeline.

Gas is received on board the FCNG vessel at 800 psig and 50°F. The gas is dehydrated on the FCNG vessel to a water dewpoint of −15°F. The dewpoint requirement results from the fact that, upon depressurization, the gas can reach temperatures of −8°F at the beginning of the unloading sequence. Fuel gas is taken from the dehydrated gas in order to power the compression equipment on board the FCNG at a rate of 6.3 MMscf/D. This leaves a flow rate of 343.7 MMscf/D, which is used on a continuous basis either to fill the CNG ship directly or to top up storage Coselles on the vessel.

Dehydrated gas is compressed from inlet pressure to 2,050 psig in the first stage of compression and is then cooled to 90°F using seawater coolers. This gas is then mixed with gas from storage and fills the incoming ship’s Coselles to a pressure of 2,000 psig, after which the second-stage compression is started; this raises the gas pressure to 4,040 psig. The connected shuttle ship’s Coselles are filled to 4,000 psig before disconnecting and travelling to the receiving facility.

The complete cycle for a ship is as follows:

  • The ship arrives at the FCNG facility with an average cargo pressure of 231 psig and an average temperature of 12°F. The ship moors and is connected to the FCNG facility.
  • Gas from the FCNG storage (initially at 4,000 psig) flows at 550 MMscf/D and is combined with gas from the first-stage compressor for a combined loading rate of 893.7 MMscf/D. This gas fills the Coselle CNG ship up to a pressure of 2,000 psig. This requires 121.5 MMscf of gas to be taken out of storage.
  • Gas from the FCNG storage is then stopped, and the second-stage compressor is started. The remainder of the Coselle CNG ship is filled at 343.7 MMscf/D. The total filling time for the ship is 15 hours, and the average temperature of the gas at the end of loading is 96.2°F at 4,000 psig.
  • As soon as the ship is no longer taking gas, the high-pressure discharge gas from the second compressor stage is directed back into the FCNG storage to make up for the gas that was used during dual loading.
  • The ship disconnects from the FCNG vessel lines and prepares to depart. The time for mooring, connecting, and disconnecting is 6 hr/cycle.
  • The ship sails to the receiving facility at a speed of 16 knots, which takes 12 hours.
  • The ship either moors at a buoy or docks at a jetty at the receiving facility alongside the preceding ship, which is nearly empty. The receiving terminal is configured so that two vessels can be connected by separate loading arms at a jetty or at separate buoys so that there is no interruption of gas discharge. The initial pressure of the cargo arriving at the receiving facility is 100 psig lower than when it left the supply facility because of consumption of fuel by the ship during the journey and temperature drop in the Coselles.
  • The ship begins discharging gas through the high-pressure discharge line at 273.3 MMscf/D while the preceding ship empties the last stack of Coselles through the low-pressure header down to the ship heel pressure of 250 psig at 66.7 MMscf/D.
  • Once the ship’s first stack is depressurized to 820 psig through the high-pressure header, the first stack is switched over to the low-pressure header, where it finishes discharging to 250 psig. The second stack of Coselles is connected to the high-pressure header and begins to discharge at the same time. The use of high- and low-pressure headers results in significantly lower compression requirements.
  • The ship is subsequently unloaded in a cascade fashion, stack by stack, until the last stack is switched to the low-pressure header. Gas for the high-pressure header is then supplied from the next ship, which has arrived and is connected.
  • The ship then sails back to the supply facility, using some of the remaining gas as fuel in the journey, so it arrives back at an average pressure of 231.4 psig, to begin the cycle anew.
  • Fuel for the scavenger compressor is taken from the ship discharge at a rate of 1.0 MMscf/D, leaving a continuous supply to the receiving facility of 339 MMscf/D.

Conclusion

FCNG serves markets that fall between subsea-pipeline markets and very-long-distance LNG markets. For market opportunities that cannot be reached by a subsea pipeline but are within 2500 km, FCNG provides a simpler and less expensive process than FLNG does, with more flexibility to deliver to multiple sites than pipelines offer.

This article, written by Editorial Manager Adam Wilson, contains highlights of paper OTC 23615, “Floating CNG: A Simpler Way To Monetize Offshore Gas,” by David Stenning, SPE, John Fitzpatrick, and Mark Trebble, Sea NG, prepared for the 2012 Offshore Technology Conference, Houston, 30 April–3 May. The paper has not been peer reviewed. Copyright 2012 Offshore Technology Conference. Reproduced by permission.