Recent offshore natural gas opportunities in US waters often share the same obstacle: a lack of pipeline infrastructure to support offshore drilling and transport the gas to shore. One solution is the use of com- pressed natural gas vessels to transport gas from produc- tion sites to mar- ket (Fig. 1). While still in its infancy and working through commer- cial and regula- tory hurdles, the marine CNG industry may ultimately be the key to unlocking large oil and gas reserves in the US and abroad.

This article details the advantages of marine CNG, touching on current developments in the industry.

New production

The expiration of the 26 -year-old moratorium on drilling on the US offshore continental shelf could open vast new areas to offshore drilling, including sites along the East and West coasts previously off limits to new exploration and production. Several potentially high-yield natural gas discoveries are also under development in the deepwater Gulf of Mexico, with dozens of other new prospects identified and under evaluation. Many of these opportunities - in both deepwater and in new geographic regions - lack pipelines to transport natural gas to shore.

CNG vessels, such as the one pictured here, offer alternatives both for field development and for opening new markets (photo from EnerSea Transport LLC; Fig. 1).

Pipelines in the Gulf of Mexico often do not extend far enough to reach new deepwater fields. On the East and West coasts the longstanding ban on offshore drilling has even more sharply limited development of pipeline infrastructure. The lack of pipelines to serve new fields forces developers either to build new pipelines or seek other transportation solutions.

Stranded gas

More than one-third of the world's gas reserves cannot currently be developed and are thus considered stranded. Gas can be "stranded" for a number of reasons, including the presence of physical or technical barriers to extraction, such as lying in areas too deep to drill, and economic reasons, such as lying beyond the reach of current pipeline networks. Political considerations, such as the moratorium on OCS drilling and state or local opposition to drilling, can also cause gas to be stranded.

Stranded gas reserves can include deepwater plays, marginal fields historically considered too small to develop, and held s in newly opened regions, such as along the East Coast. Stranded gas can also affect offshore oil production, as natural gas is frequently a by-product of oil drilling operations (associated gas). While associated gas can be reinjected into a well to increase output, the US Minerals Management Service restricts reinjection to limited shortterm circumstances.

Excelerate Energy's Gulf Gateway Energy Bridge offshore port began LNG operations off Louisiana in 2005. Similar systems could be used both to load and unload CNG vessels (photo from Excelerate Energy; Fig. 2).

MMS similarly restricts flaring or venting unwanted gas at the wellhead during normal operations. Environmental and resource conservation concerns have also led to increased international restrictions on flaring and venting.

With reinjection, flaring, and venting generally prohibited, operators in the US must find solutions to transport both associated and nonassociated gas to market. The traditional solution is to build out existing pipeline networks, letting gas move to shore through interconnecting pipelines.