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

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 The paper has not been peer reviewed. 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.