Offshore gas production is entering a challenging phase as many mature, near-shore fields approach the end of productive life. For example, annual proved reserves of natural gas have fallen consistently for a decade for water depths less than and beyond 200 metres in the Gulf of Mexico (GoM). Offshore gas production in the GoM fell from 10 billion cubic feet per day (bcf/d) in spring 2005 to 3.5bcf/d ten years on, according to US forecaster BTU Analytics.
While companies want to develop new gas reserves in increasingly complex and remote areas, many such regions lack pre-existing processing and pipeline infrastructure. Floating liquefied natural gas (FLNG) facilities therefore appeal as a development solution.
However, FLNG involves high initial capital expenditure (capex), and low oil prices mean many operators want to reduce the annual double-digit growth in capex and operational expenditure (opex) experienced since 2005. Offshore gas also faces price competition from US shale gas.
DNV GL set up an extraordinary innovation project (EIP) to tackle these cost pressures directly by exploring future options for producing offshore gas in remote areas. The EIP reviewed how installations can adapt to, and benefit from, subsea technology developments to meet criteria for future success. “In thinking through scenarios, maintainability, safety and the environment were to the forefront,” said Kjersti Aalbu, project manager for the EIP at DNV GL – Strategic Research & Innovation.
The business and safety caseThe EIP generated a high-level concept called Solitude (see graphic), based on an unmanned FLNG installation. The concept aims to reduce lifecycle costs as a lack of permanent personnel reduces life-support requirements and avoids the need for related superstructure such as living quarters. In addition, simplified safety systems could focus on better protection of topside equipment.
“Our new concept could cut 18% from annual opex, and increase overall safety while adding only 5% to capex compared with conventional FLNG,” Aalbu said. “Most savings would come from automated maintenance and simpler logistics with no personnel on-board.”
Solitude’s central feature is an unmanned, autonomous unit, monitored and controlled from shore, and powered by fuel cells. A rear dock berths special-purpose support and accommodation vessels.
Equipment is modular, and much routine maintenance and fault correction is by self-programming, autonomous inspection and maintenance robots.
The topside has rails along each process train, giving robots access to equipment. Wireless sensor networks feed information to a system overseeing fault detection, and providing proactive maintenance and repair planning.
“Changing the focus from maximum efficiency to maximum reliability, and selecting robust processing options with built-in redundancy, has let us develop a solution that ensures production levels and boosts economic viability of FLNG projects,” Aalbu explained.
Elements reducing risk to people include: lack of permanent personnel; autonomous units handling normal inspection and maintenance; and process plant shutting down when operators enter. The rear dock for special-purpose vessels boosts the safety of human interventions.
Safety factors that also reduce asset risk include a faster blow-down in emergencies, improved ventilation reducing risk of explosion and fire, and a nitrogen expander liquefaction process to reduce hydrocarbons on the topside.
The simpler processing system, and better monitoring and control, contributes to fewer leaks, flanged connectors and valves.
The concept exploits advanced, but mainly available, technology – in this case fuel cells – to provide power instead of relying on high-maintenance gas turbines. This improves power generation reliability and reduces the unit’s environmental footprint.
A smarter approachAalbu anticipates some scepticism: “There could be a perception of increased risk through sensors and autonomous units replacing eyes and ears, and a longer response time if unforeseen events required human intervention. Dropped modules might damage equipment and tanks topside, for example.”
The challenge, she said, will be to design robust, autonomous equipment and control systems to replace humans for many operation and maintenance tasks. It will need a modular FLNG topside, as well as shelf state and Class approval for remotely-operated and autonomous systems.
“That said, Solitude is a smart, reliable, robust and resilient system,” Aalbu stressed. “The simplified process plant needs less maintenance. Easily replaceable modules reduce downtime. Sophisticated monitoring systems predict component failure, thus enabling improved repair and maintenance planning.
“Many elements of Solitude can be implemented independently and some are already available; for example, operators control subsea installations and simple, fixed offshore installations from shore. With continuing advances, unmanned offshore installations are a natural development.”
Elisabeth Tørstad, CEO, DNV GL – Oil & Gas, added: “EIPs are our way of thinking out loud. Our aim is to present high-level concepts that can form a basis for discussion and be further developed in collaboration with the industry. We see Solitude as a new opportunity for the future.”
Read more about DNV GL’s unmanned FLNG concept at: dnvgl.com/solitude
 World Energy Outlook 2014, International Energy Agency, Paris, France
 ‘Federal offshore Gulf of Mexico proved reserves’, US Energy Information Administration data series
 ‘US offshore gas gets no respect, but could surprise’, Andrew Bradford (CEO, BTU Analytics), oilpro.com, April 2015
 'World LNG market forecast', Douglas Westwood, October 2015
 ‘Going remote: rethinking offshore operations’, DNV GL, May 2015