Choosing the right FLNG topsides equipment can be a complex process. Audrey Leon reports on some of the solutions presented by Black & Veatch at this year’s Offshore Technology Conference.
The Carribean FLNG unit. Photos from Exmar.
Operators are looking to floating liquefied natural gas (FLNG) solutions to help bring uneconomic gas reserves to market. In a paper presented at this year’s Offshore Technology Conference (OTC) in Houston, Javid H. Talib, of Black & Veatch, discussed some of the challenges faced during FLNG topsides equipment selection before presenting some solutions.
There are many challenges associated with offshore production facilities. Deck space is often limited and then there are weight considerations, not to mention the severe weather and motions in open sea conditions floating systems are often subjected to.
FLNG facilities also need to have some level of complexity (which also comes with more expense) to process raw gas, unless they’re lucky enough for it to be very dry. Therefore, Talib told an OTC audience, FLNG topsides equipment needs to be more flexible and should have a smaller footprint. “Everything goes back to weight and cost,” he said.
The unit was previously built for Pacific Exploration & Production to operate offshore Colombia.
The size and scale of liquefaction facilities depends on the reservoir size and also the LNG export (volume) strategy, Talib and his co-author Bob Germinder, wrote in their OTC paper, “Game-changing floating LNG solutions” (OTC-27182-MS). They wrote that, in general, a 1 Tcf gas reservoir will supply gas for producing LNG at about 1 MTPA for 20 years. Reservoirs containing 1-4 Tcf reservoirs are considered small, requiring small-to-midscale range (0.5-4 MTPA) liquefaction production capacities.
To make FLNG viable, the liquefaction technology should be proven, reliable, space-efficient, and simple to operate, Talib says. He compared two technologies: single mixed refrigerant (SMR) and dual mixed refrigerant (DMR). He said that the SMR process is the best fit for small-to-midscale offshore liquefaction operations, while DMR can provide the option of higher LNG production in a single larger train for midscale applications, according to the OTC paper.
Black & Veatch has its own SMR and DMR options under the PRICO product line.
Left: PRICO-DMR process. Right: PRICO-SMR proces. Illustrations courtesy of Black & Veatch.
Advantages of PRICO-SMR
- Single refrigeration system; no series systems
- Reduced equipment count
- Compact layout
- Single module for entire liquefaction train
- Simplified operation
- Modular philosophy and process concerning available drivers
- Small refrigerant inventory
- No venting of refrigerant during shutdown
- Application across a broad range of plant sizes
- Efficiency competitive with baseload systems
- Use of a single body compressor
- High turndown ratio
- Economies of scale
- Quick startup after shutdown
- Lower capex and opex
PRICO-DMR, according to Talib, has the same functionality as SMR, with two mixed refrigeration circuits and four brazed aluminum heat exchanger zones contained in either one or two cold boxes. Equipment would include a warm mixed refrigerant compressor, a cold mixed refrigerant compressor, refrigerant expander and a LNG expander.
Talib said that DMR, paired with a compact layout, is an optimal solution for single, larger train FLNG applications because the brazed aluminum heat exchangers minimize motion concerns, while requiring fewer components to be stored for refrigerant makeup.
According to Talib and Germinder’s paper, the exchange works by pressure feeding the vapor refrigerant to multiple cores through a header, while the liquid refrigerant is pumped into the cores and independently controlled. The pressurized refrigerant flows through passages in the core unaffected by the position or motion of the core. This process, the two authors said, eliminates the possibility of any maldistribution of refrigerant fed to multiple cores.
While comparing the two options for midscale FLNG, Talib noted that larger trains can offer economies of scale (thereby reducing capex) and a reduced footprint, but less operational flexibility. Talib told the OTC audience that choices around equipment will be limited with longer delivery periods, which could potentially affect the economies of scale.
The smaller trains, while having fewer economies of scale, offer their own benefits. “Smaller trains offer operational flexibility, easier startup, and when one shuts for maintenance the others keep producing,” he said, which means annual maintenance can be staggered. He also noted that with multiple smaller trains, you have a wider selection of fabrication yards and suppliers.
Black & Veatch has a number of projects underway, including Exmar’s Caribbean FLNG, which consists of a simple barge with five-day LNG storage that is supplemented with a separate floating storage unit. The vessel’s main features include one, 0.5 MTPA PRICO liquefaction train, a GE LM2500+ gas turbine driver, refrigerant compressor with interstage cooling using seawater, a brazed aluminum heat exchanger for the main refrigerant exchanger, gas treatment and dehydration modules to remove CO2 and water, and a boil off gas handling module. The vessel’s hull is 140m-long with 16,500cu m storage.
However, the Caribbean FLNG project is currently in turmoil. Earlier this year, Exmar terminated an agreement with Pacific Exploration & Production after the company delayed project startup in early 2015. Back in 2012, Exmar was contracted to build, operate, and maintain the unit, which was meant to serve the La Creciente field in Colombia’s lower Magdalena Valley basin. Wison Offshore and Marine’s fabrication yard in Nantong, China, was chosen to construct the vessel. Exmar is looking for other employment opportunities.
Black & Veatch is also involved with the Golar GoFLNG conversion of the Hilli Moss LNG carrier, for Perenco’s Cameroon FLNG project. The vessel, delivery planned by Q1 2017, features four 0.5-0.6 MTPA PRICO trains, pre-treatment, liquefaction, boil-of-gas, and heat recovery steam generator, a GE LM2500+ G4 turbine/compressor, brazed aluminum heat exchanger, and seawater interstage cooling. Other projects in the pipeline include Golar’s Gimi and Gandria GoFLNG conversions planned for mid-2018 and Q1 2019, respectively.