Engineers have proven that offshore storage of liquefied natural gas could soon be a commercial reality. Alan Sparks reports.
CONSTRAINTS PLACED on gas supply by the capacity limits and vulnerability of pipelines could soon be lifted, thanks to the future development of liquefied natural gas (LNG) offshore infrastructure, ICE members were told last week.
An Offshore Engineering Society meeting heard that oil companies were 'very keen' to take on the huge engineering challenge involved.
'Concern over single gas supply sources via pipeline makes the security of LNG very attractive, ' said vice president of specialist oil and gas sector contractor MW Kellogg, John Sheffield. LNG also compares favourably with alternative conventional energy sources, he said.
Nuclear power is frequently pushed as a low-emission, environmentally friendly energy source, but falls down on the hugely expensive and toxic fuel reprocessing and waste disposal required. Coal is considered a heavy pollutant and gas, although clean and cheap, is supplied from politically unstable countries.
'By having a floating production storage and offloading (FPSO) facility, we can supply from several sources, ' claimed Sheffield. 'And with the use of combined cycle power stations, gas is an increasingly efficient option.'
Sheffield was describing work undertaken by a consortium of European engineering companies in an EU funded programme known as Project Azure. This developed concepts for potential LNG production on an FPSO, looking at options for 3Mm 3/year plant and 1Mm 3/year facilities.
Consortium members for Project Azure were petrochemicals giants Bouygues Offshore, MW Kellogg, Chantiers de l'Atlantique, Fincantieri, FMC Europe, Gaz Transport & Technigaz, Bureau Veritas, Registro Italiano Navale and Institut de Recherches de la Construction Navale.
The main objective of the scheme was to demonstrate that LNG FPSOs and Floating Storage & Regasification Units (FSRU) are safe, technically feasible and economically viable.
'We have proven that such facilities can be engineered, ' said Sheffield. But whether such a scheme cuts the financial ice is still under review, with costs expected to be in the region of £600M. Sheffield believes that economic viability will depend on where a plant is sited.
By liquefying the gas at temperatures of around -120infinityC, conventional gas then becomes transportable. But handling LNG poses many unusual technical challenges.
Calm sea is vital. Sheffield shudders at the thought of engineering such a facility to operate in the North Sea. But where piping gas to shore is thought too expensive - say for remote fields or limited reserves - then the use of LNG becomes a viable alternative. An additional push in this direction comes in the form of pressure from environmentalists to stop burning off, or flaring, the gases extracted alongside oil. There is increasing pressure to capture and use them.
Shell is developing a facility known as the Sunrise project, which is a 5Mt a year facility that would cover an area of 350m by 80m - the size of six football pitches. This is slightly larger than the options considered under Project Azure.
In terms of design, the partners in the Azure consortium had their own particular challenges. For starters there are no proven hose systems to transfer LNG, and so rigid steel pipework is needed. But coping with large 3D movement required a clever piece of design. French engineer FMC, which built a one fifth scale model to demonstrate its success, is also developing a rigid rod connection from the FPSO to the FSRU.
Careful attention to the behaviour of containerised liquids at sea was needed to gauge how the FSRU would behave. Tankers get around the problem of destabilising 'swilling' when at sea by travelling with tanks either entirely full or entirely empty.
But the FSRU would normally be partially full, with potential for extensive movement of liquids in rough sea states.
Extensive fluid dynamic analysis was carried out on various levels of LNG by Italian naval engineers.
'Weight was not really an issue for the FPSO, ' said Sheffield. 'In fact if anything, we needed to add extra mass.
But distribution of weight was obviously a key concern.'
Designers concluded that a concrete hull best met the design criteria.