Hybrid Solution to the Grand Challenge of Developing Deepwater Stranded Gas
Authors: Carpenter Chris 
Published: May 2018

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 27686, “A Revolutionary ‘Hybrid Solution’ to the Grand Challenge of Developing Deepwater Stranded Gas,” by Richard Moore, SubCool Technologies, 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 The paper has not been peer reviewed.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.

While the floating liquefied-natural-gas (FLNG) option initially looked promising, high capital expenditure and very high operational expense (OPEX) have limited its potential application, with four potential FLNG projects being canceled in Australasia in the last 2 years. A new solution involving a proprietary hybrid concept has emerged to meet this challenge of developing deepwater gas fields. It is neither surface nor subsea, but the combination of both, offering fundamentally more-efficient and hence lower-cost processing.

Background

Subsea and Deep Water. When the oil and gas industry moved into deeper waters in the 1990s, subsea processing operations were not available. Techniques were developed to prevent hydrates and corrosion, including addition of chemicals such as glycol and monothylene glycol, insulation, and specialized techniques. More recently, this has included heating in the form of direct electric heating or trace heating in specially constructed pipe-in-pipe-type pipelines. These techniques have proved very successful in shallow waters and for moderate tieback distances. However, with the industry moving to deeper waters and greater distance, these same techniques are proving to be too expensive. The lack of first-stage bulk produced-water separation greatly adds to the costs, resulting in large volumes of glycol and water, as well as salt in the glycol, which requires expensive regeneration methods. Long distances are also a key issue for subsea compression. Power transmission and variable-speed control are viable for short distances but are challenged over long distances (greater than 100 km).

Costs of Surface Processing Facilities. Implementation of both conventional gas surface processing and FLNG facilities has often resulted in large mega-facilities that are cost-ineffective. Additionally, they have high OPEX and operational and safety challenges be-cause of the number of people required offshore. The cost impact is greater for surface facilities in deeper water beyond fixed-platform depth. In these cases, deepwater risers and flow-assurance facilities add to a more weight-sensitive floating substructure.

Key Differences Between Liquid and Gas Processing. To date, the significant cost of the surface facility has been seen as the driver for long-distance subsea tiebacks. However, there is a key distinction between processing liquids and processing gas. Nevertheless, this distinction has not been considered, debated, or published, to the author’s knowledge. Liquids can be processed on the sur-face relatively efficiently. It is the high-pressure surface gas systems that have a disproportionate impact on size, weight, and cost of surface facilities. Importantly, in regions such as Brazil and West Africa, using a floating production, storage, and offloading (FPSO) vessel to produce liquids is not questioned. Processing liquids offshore on an FPSO is clearly seen as more cost-efficient than bringing the liquids to shore.