Comparison of Various Offshore Industrial Gas Technologies
Authors: Wilson Adam 
Published: April 2018

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper OTC 27939, “Application of Gas Industrial Technologies Offshore,” by Carlos G. Saavedra, Saavco International, prepared for the 2017 Offshore Technology Conference, Houston, 1–4 May. The paper has not been peer reviewed. Copyright 2017 Offshore Technology Conference. Reproduced by permission.

Offshore oil-exploration drilling and testing are key for the production of oil; however, a number of associated challenges, particularly the handling of associated gas, must be overcome. This paper discusses the alternatives for processing the associated gas and transporting it to markets. The technologies described in this paper are applicable to nonassociated-gas projects as well. Fig. 1 provides a description of gas-handling value chains for some of the alternatives.

Floating Liquefied Natural Gas (FLNG)

Liquefied natural gas (LNG) is natural gas that has been converted to liquid form for ease of storage or transport. It is a gas cooled to −162°C and has a volume that is 1/600 that of the gas at room temperature. The gas has to be processed at both ends of the shipping chain in order for LNG to be produced and used. It can be transported by specially designed cryogenic seagoing vessels (LNG carriers). At the destination, the LNG is offloaded to a receiving terminal that stores it and revaporizes it into a pipeline that takes the natural-gas product to the end users.

Floating Compressed Natural Gas (FCNG)

Compressed natural gas (CNG) is a concept for gas transport over intermediate distances. The FCNG technology involves storing natural gas at high pressure in a carrier ship to transport it from an offshore location to a typically onshore location. The high pressure and possibly reduced temperature increase the density of the gas, making it more economical to transport.

CNG is stored and distributed in cylindrical or spherical vessels at pressures up to 275 bar and at ambient or sub ambient temperatures. Higher pressures and lower temperatures allow more gas to be contained per unit volume. The typical volume reduction is 1/300.

An FCNG production vessel is a traditional gas floating production and operation unit (FPO) with a high-pressure gas-transfer system, instead of a subsea pipeline, to load CNG shuttle ships. The CNG is transferred at near-ambient temperatures, so the hoses and transfer systems are the same high-pressure hoses and systems used to transfer high-pressure gas and well fluids onto an FPO. The CNG shuttle carriers provide both storage and transport, thus avoiding the necessity for storage on board the FCNG vessel.

Any CNG-delivery-system technology should offer a continuous-flow process without interruption or discontinuity (like a pipeline). Gas should flow continuously with high reliability through the CNG shuttle ships. Major buoy-system suppliers have reviewed CNG transfer and found that their systems can be adapted to CNG transfer without developing new technology.

CNG appears to be a potentially suitable and promising technology for deployment despite not having been deployed commercially to date. It has no technical barriers. The principal commercial barrier stems from CNG being a solution that lies between a pipeline for shorter distances and LNG for longer distances.

FCNG technology has existed in a design state for several years, with numerous studies having been conducted; however, no facilities have been built yet.