Wärtsilä Oil & Gas Systems AS is building upon the solid start it made in the provision of reliquefaction plants for the LNG shipping industry. Over the past year the engineering company has secured contracts to supply such equipment for eight LNG carrier newbuildings. The new orders augment the reliquefaction plants already supplied for 35 LNGCs currently in service.
Of the existing vessels, 31 are Q-flex size ships of 210,000-216,000m3 built for charter to Qatargas and RasGas and the carriage of large volumes of LNG produced at Qatar’s Ras Laffan industrial complex. Each Q-flex LNG carrier is powered by a pair of traditional low-speed diesel engines that run on oil fuel.
As such the reliquefaction plant on each vessel is called upon to process all the cargo boil-off gas (BOG) generated during the course of normal operations and return it to the tanks as LNG. The arrangement enables cargo losses to be minimised and outturns to be optimised. Construction of the Q-flex fleet was shared between Daewoo Shipbuilding & Marine Engineering (DSME), Hyundai Heavy Industries (HHI) and Samsung Heavy Industries (SHI), and the ships were delivered between October 2007 and January 2010.
The other four LNGCs in service fitted with Wärtsilä reliquefaction plants are a series of 170,000m3 ships built for BG Group by SHI and completed in 2010. These vessels have tri-fuel diesel-electic (TFDE) propulsion systems, an option that enables the use of either natural gas boil-off, middle distillates or heavy fuel oil to propel the ship.
The combination of the TFDE propulsion arrangement and the reliquefaction system allows the shipowner to run the vessel almost exclusively on gas and to process any excess BOG not utilised by the engines for returning to the cargo tanks as LNG. It is up to the owner to decide which fuel to burn and his range of options is enhanced by the fact that the partial reliquefaction plants of the type fitted on the BG ships are still sized to process all the cargo BOG generated on the vessels if required.
Over the 2012-2014 period all 31 Q-flex vessels were docked at the Nakilat-Keppel Offshore & Marine (N-KOM) repair yard in Ras Laffan for their five-year surveys and maintenance drydockings. Servicing of the reliquefaction plants on the ships was a key part of these scheduled visits to the repair yard.
Wärtsilä Oil & Gas Systems has developed a rigorous service package for these units which places emphasis on preventing downtime through the remote support and monitoring of the process plant. This helps the vessel operator to limit planned maintenance, beyond the routine servicing which can be carried out by the crew while the vessel is in employment, to the ship drydockings that take place every fifth year.
The reliquefaction plant on a Q-flex ship comprises two BOG compressors, each able to handle 100 per cent of the maximum anticipated load; two 100 per cent companders; and one 100 per cent cold box. There is also an auxiliary equipment complement which includes nitrogen booster compressors, LNG pumps and a fully automated control system that forms part of the ship’s integrated automation system (IAS). The Wärtsilä reliquefaction plant’s compander, which is supplied by Atlas Copco, is an integrated, three-stage turbo compressor with expander stage, all mounted on one gear and powered by either a 4.5MW or 5.5MW motor.
“The reliquefaction plant on the Q-flex ships is the primary pressure control system for the LNG cargo tanks, and any downtime of the overall system will have major consequences,” states Torgeir Paulsen, business unit director, aftermarket for Wärtsilä Oil & Gas Systems. “Our plant design provides full redundancy with respect to the rotating machinery which, in turn, contributes significantly to the high overall uptime of the plant. The large compressors normally only need maintenance during drydocking, while the rest of the plant consists of standardised equipment that requires little maintenance beyond what the crew is able to carry out.”
Leslie Fernandes, senior service engineer with the company’s Dubai support team, further explains: “Wärtsilä has focused on providing operational support to the operators of ships with reliquefaction plants, as system issues are more critical than any mechanical faults that may occur in the redundant subcomponents.
“Our LNG support team is based in Dubai, close to the customers, and everyone in the reliquefaction plant supply chain has 24/7 access to Wärtsilä’s expert engineers in Norway. This arrangement is offered to all customers as part of a support contract. On a number of ships our support team can connect remotely, giving them access to the same screens as those in front of the cargo control officers on board.
“Typical issues that can be resolved remotely,” continues Mr Fernandes, “include signal errors in instrumentation and control cabinets. After such a fault is tracked down by our support team, it can be repaired by the crew on board. More diffuse issues due to off-spec cargo tank conditions have also been resolved by the support team.
“In addition to fault-finding and support, the data link is also used to collect operational data as part of a programme called Operational Performance Improvements (Operim). The collected process data is not only used to detect operational issues in individual plants but also to look for fleetwide trends and patterns.
“After having analysed data from nearly 200 sessions, a relatively small modification was implemented to the reliquefaction plants on a number of the ships in order to improve the power efficiency and capacity and thus increase overall system efficiency. The effect of the modification is currently monitored by analysing new Operim data.”
The modification work was carried out during the recent five-year surveys and drydockings of the Q-flex vessels. It also encompassed modifications that ensure improved flexibility, including faster ramp-down of the capacity, allowing the reliquefaction plant to run at full capacity for a longer time when approaching the unloading terminal. In addition the ability to seamlessly switch from running the ship’s gas combustion unit (GCU) to running reliquefaction was provided, as were improvements related to warm start-up.
Further modifications were made on some ships which yielded improvements in free-flow operation during the ballast voyage, ie, without running the BOG compressor. This measure allows the carriers to keep the cargo tanks cold during the ballast voyage in a more efficient manner.
During the drydocking maintenance work on all 31 Q-flex vessels at the N-KOM yard in Qatar the large compressors were overhauled by their manufacturer, Atlas Copco, under Wärtsilä supervision and project management. LNG
This article was written by Mike Corkhill
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