Decommissioning is a rapidly growing business in the offshore oil and gas sector, albeit one that requires careful, cost-effective management.
To help companies undertake work in this field, Longitude Engineering, part of the London Offshore Consultants (LOC) Group, has devised the concept for a self-styled “decommissioning barge”, intended to assist with the safe removal of small oil and gas platforms – thereby providing an alternative to expensive heavy-lift vessel hire.
The barge has been specifically developed to serve PTT Exploration and Production (PTTEP), a subsidiary of Thai state-owned oil and gas company PTT, though Longitude tells Offshore Marine Technology: “The platform types in the Gulf of Thailand present similarities with other South East Asia region areas where this barge could be used for decom.”
The concept has been fuelled by the aim to develop a “viable, cost-effective alternative to conventional ‘reverse installation’ through the use of heavy-lift crane barges, for the removal of the topsides and jackets,” Longitude states.
The engineering specialist elaborates: “The [barge] solution was shown to be cheaper when looking at the overall platform removal scope of work – including prep work, mobility of ROVs, lifting, skidding, disposal, and so on.” As well as producing the naval design of the barge, Longitude’s input has included structural design, definition of operational weather windows and definition of electrical systems.
The barge’s primary task will be the removal of between 90-100 of PTTEP’s minimum facilities platform assets in the Gulf of Thailand, which feature topside weights of up to 800tonnes and jacket dry weights of up to 1,000tonnes. PTTEP is reportedly keen to commence this decommissioning programme, and is currently in discussion with Thailand’s Department of Mineral Fuels (DMF – the governmental agency responsible for overseeing offshore oil and gas-related developments) with a view to obtaining approval for these removals in the coming years.
Longitude comments: “With the implementation of new decommissioning regulations, all concessionaires must submit the decommissioning plan with cost estimation to DMF for financial security placement at the first stage.” It is hoped that the ‘cheaper’ cost of the barge, in comparison to heavy-lift vessel deployment, may help PTTEP to secure DMF approval. PTTEP has subsequently logged a petty patent for the barge solution.
The proposed barge would use reverse float-over and onboard lifting to remove a wide range of different topside and substructure types, using the same vessel and without requiring any modification work to the barge. To achieve this aim, Longitude called on German hydraulics tech developer Bosch Rexroth to develop a viable heave-compensated lifting and skidding system.
Longitude continues: “The current candidate used for the preliminary study is 120m in length, 33.5m-wide and has a depth of 7.6m and a summer draught of 5.4m.” The barge would accommodate up to 57 persons, spread across 30 cabins (27 double, three single), granting them autonomy for a period of up to 40 days.
An eight-point mooring system would enable the barge to maintain its position during operations, once it has been towed to its destination via tug. “Towing gears are sized for up to 100 metric tonnes bollard pull,” Longitude adds.
The next move will be to select a shipyard to effect the barge conversion. Longitude explains: “A pre-listing was put together through a preliminary request for quotation [RFQ] process, but PTTEP might expand the invited yard list once the design is ready for conversion.” The delivery date will be based on PTTEP’s first platform removal date – which, Longitude estimates, will not be earlier than 2022.
Jean-Baptiste Meier, Longitude’s lead engineer, adds: “The South East Asian fields present many similar small platforms where innovative thinking can…make an economy of scale in the removal operations. It is crucial to clearly define the standards and procedures for the decommissioning work in the Gulf of Thailand, to ensure that the solution is not only the most innovative and cost-efficient, but also environmentally friendly.”