CNG gains interest as alternative for offshore developments

June 20, 2005
Compressed natural gas (CNG), transported at ambient temperatures, is being looked at as a viable alternative for offshore stranded and associated gas.

Compressed natural gas (CNG), transported at ambient temperatures, is being looked at as a viable alternative for offshore stranded and associated gas. Although CNG is not a new concept, high costs of natural gas and cryogenics may give CNG an attractive niche market.

Several papers presented at the Offshore Technology Conference in Houston, May 2-5, 2005, give reasons that CNG may be an economical alternative to LNG.

One paper, “CNG Marine Transport-A Gas Transportation Company Perspective” by G. Cano and G. Stephen of TransCanada PipeLines Ltd., Calgary, describes the challenges and advantages of CNG marine transport from a gas transportation-system operator point of view. That included cost and reliability issues, risks, technical challenges, and safety.

CNG background

Cano and Stephen refer to test fleets of trucks and designs for CNG marine transport fleets dating back to the 1960s. Patents are in place for such things as coiled pipe storage, fuel tanks, and Knutsen’s Pressurized Natural Gas system (sidebar) which utilizes vertical steel pressure vessels.

Over the years, the only CNG transport service found to be economical, Cano and Stephen said, has been truck-based systems in specific niche markets or for maintenance outages and other intermittent or one-time uses. With declining North American production and increasing prices, however, CNG is getting more interest.

CNG projects, they said, are getting into the detailed design stage. And it is in this stage, they maintained, that “realism” must come into play as ideas move from concept to reality.

Project feasibility

Cano and Stephen said that when viewing the initial project feasibility for a CNG marine transport system, it is important to recognize CNG’s strengths and weaknesses. CNG marine transport is not as reliable as pipelines, they maintained, especially in areas with severe sea states, such as the North Sea. Therefore, where markets must be served 100% of the time, storage or backup fuel systems must be included in the project.

The main asset in a CNG system, said Cano and Stephen, is the ships or barges that generally comprise the majority of the capital and operating costs. But when gas is not moving at capacity, the asset is underutilized and capital is not being adequately deployed. In CNG ships and barges, the longer the ship or barge takes in loading or unloading, the less time in the cycle it is moving gas. Bigger ships or barges, they said, will not necessarily be better.

Such things as distance from supply to market, ship or barge speed, gas composition, and loading and unloading methods also affect the ship or barge size and must be optimized for a particular project. For a CNG project, there are no off-the-shelf answers.

There is, however, more flexibility to expand CNG transport capacity after initial construction by adding more ships and compression facilities. The authors feel that one of the strengths of a CNG system is the ability to start small and build up or redeploy capacity as market developments change.

Project design, construction

In detailed design of CNG marine transport systems, it is important to have an in-depth understanding of the process and system performance with the particular gas compositions and ambient conditions that the project presents. It is only with this understanding that the optimum CNG container design and ship or barge size can be determined.

For the ship hull or barge designs, experience and expertise in design and construction is available. Designs such as LNG carriers and shuttle tankers have similarities that should be incorporated into CNG marine systems. Because CNG marine transport is a concept in its infancy, utilizing as many tested and proven design concepts as possible will lead to easier acceptance of the systems and ultimately project success.

CNG ships or barges are generally composed of a basic or special hull design with a multitude of pressurized gas containers connected by a system of valves and piping. Because of the large number of containers that must be built and installed, it is important to consider such things as detailed time and motion studies for repetitive tasks, rigging studies for multiple lifts, assembly line procedures for fabrication, and prevailing material prices. In the past year, the authors said, steel prices have skyrocketed, affecting all components of the shipping systems.

Another major item that must be considered in the design and operations of CNG transport systems is the regulatory and classification society requirements. At the present, CNG transport-ship regulations are under development by at least two major class societies, Det Norske Veritas and American Bureau of Shipping. Also, Lloyds Register has reviewed systems and issued “Approvals in Principal” on a case-by-case basis.

During the detailed design phase, it will be important to work closely with one or more of the class societies as well as the appropriate regulatory bodies to ensure that designs and operating and maintenance procedures are acceptable.

Operations, maintenance

Cano and Stephen said that most facilities required for the loading and unloading of CNG marine transport ships or barges are already in operation in offshore and onshore installations. These include compression, dehydration, cooling, pumping, and associated utilities such as power generation and flare and vent systems.

One area that requires special attention is valves. The authors said CNG marine systems, because of the large number of containers on the ships or barges, will rely more heavily on valves than most other liquid energy-transport systems.

A recent study concerning valve failures in the offshore oil and gas industry, sponsored by the UK Health and Safety Executive, found that almost three quarters of all incidents were in gas service. For all the data sets studied, leakage through the valve seats was the predominant failure mode requiring repair or replacement.

In TransCanada’s fugitive monitoring program for all of its gas transmission facilities, said the authors, the largest single fugitive-emission source is leakage past valve seats, contributing about 36% of total emissions. Leakage past valve stems added about 5% to total fugitive emissions.

New technology and better monitoring and maintenance can go a long way towards minimizing this leakage; however, leaks will happen and must be planned for.

They believe the first project in CNG marine transport will be a small version or pilot project in a benign sea state area, and they are currently working to develop one or two of them.