Low-profile tanks targeted for offshore use
  from: Offshore Engineer
  by: Jennifer Pallanich
  Wednesday, April 09, 2008

LNG technology continues to improve, Jennifer Pallanich looks at new storage and vaporization technologies.

Mustang Engineering has turned LNG storage on its side by offering a horizontal tank for small-scale LNG storage.

Each LNG Smart Horizontal Tank can be isolated with its own insulation to offer full-containment LNG storage, suitable for about 5000m³ in a Type C tank.

‘The real virtue of this is that it’s low profile [compared to vertical tanks],’ says Brad Hubbard, Mustang’s director of technical development, midstream project management.

He says this type of tank, the design for which Mustang has just received a patent, ‘has not really been used for LNG storage’.

The tanks can feature a concrete cold box onshore or steel cold box on a platform or FPSO barge. Mustang is opting for a 9% nickel steel or stainless steel that is cryogenic. Between the steel tank wall and the outer cold box wall. Mustang plans for 1m of perlite or foam insulation.

One benefit of the tank’s modular design, according to the company, is reduced construction time. A topmounted LNG pump can fill and empty the tanks, or the tanks can fill from the top and empty through the bottom with manifold piping and an external pump. The tank supports development of remote fields and despite its smaller footprint is easily expanded, according to Mustang.

The company is promoting the horizontal tanks for floating barges but the concept ‘hasn’t quite caught’, Hubbard says, citing the difficulty of getting any company to be the first to use a new technology or product.

For its purposes, Mustang is focusing on FPSOs and conversion of existing steel hull LNG carriers and tankers either for liquefaction or revaporization.

‘Concrete barges have gotten out of the mindset,’ Hubbard says. But ‘if it’s going to be moored, we think concrete’s a viable alternative’.

The math that makes this possible is the fact that LNG is half as dense as water. Hubbard says this means it would be possible to use most onboard space for LNG storage without being required to dedicate room for buoyancy for floating.

‘This can be built in Ingleside. It can be done wherever they can construct a graving dock,’ he says.

He says he’s seeing a growing interest in both large-scale and small-scale LNG FPSOs, especially if they have three key features: flexibility, ease of operation, and simplicity of equipment.

‘We’re seeing a lot of activity and interest in floating liquefaction,’ Hubbard says, especially for the Australian gas fields.

The horizontal tank is just one component of Mustang’s LNG efforts. The company has also been busy perfecting how it vaporizes LNG.

Warming up cryogenic LNG requires quite a lot of heat. That’s either a lot of gas burning or seawater, if using traditional methods to turn –260°F LNG into a gas. Mustang decided to use something more readily available in the temperate climes of the Gulf of Mexico: ambient air. In summer, it’s more than hot enough, and certainly humid enough, to vaporize LNG using Mustang’s recently patented LNG Smart Air Vaporization Process (SAV), which can be used offshore and onshore. They have worked on applications of SAV in waters as deep as 800m.

‘The heart of our technology is that we use air as a source of heat,’ Hubbard says. In short, air comes into a fin fan heat exchanger where it heats up an intermediate fluid of a potassium formate (KF) and water solution to approximately 50°F, before the fluid enters the surge tank, the KF pump, the KF/glycol exchanger and finally the LNG vaporizer before returning to the air heat exchanger at a noticeably colder 20°F. On chilly days, a supplemental heat system kicks in.

An estimated 75% of existing regas terminals internationally use seawater through open rack vaporizers (ORV) as a source of heat to vaporize LNG. The others use gas-burning water heaters such as submerged combustion vaporizers (SCV).

The fundamental difference between SAV, ORV and SCV is where the heat comes from, Hubbard says. SCV uses spiral tube heat transfer conduits and a submerged combustion heat source. In ORV, the system runs filtered warm seawater along tubes carrying the LNG and releases the seawater, which may be as much as 15°F cooler. Biocides may be used along the route to prevent marine growth in the equipment. The use of ORV, however, has effectively been disallowed in the GoM for new regas terminals.

‘Typically Louisiana fishermen are not treehuggers, but . . .’ Hubbard says about the push for a shift in the technology allowed in the GoM.

The SAV technology is sensitive to temperature and humidity levels, functioning at its prime in 70°F and 80% humidity and with efficiency dropping with humidity and temperature levels.

‘Our system works really well down to 35°-40°F. Any temperature above that and we’re still getting heat out of the air,’ Hubbard says. Conditions of 70°F and 80% humidity however, guarantee that 100% of the heat required to vaporize the LNG will come from the air, Hubbard says. ‘Forty to fifty percent of the heat source for us is the water vapor.’

If operating in a non-temperate climate, or when the weather gets chilly, it is necessary to supplement the heat. Ideally, Hubbard says, the SAV units will operate in a location where they can provide at least 90% of the heat for the year from the air, which would slash the annual fuelburning bill by a similar percentage.

Mustang and partners have developed four floating LNG regas concepts: a disconnectable floating regas storage unit, a column stabilized floating platform, a concrete floating regas storage unit and a steel hull floating storage regas unit.

Next year, Mustang will see its first SAV unit go online at Trunkline LNG’s facility in Lake Charles, Louisiana. BG has rights to the capacity at the Trunkline facility. Hubbard says the client could save up to $50 million a year on its fuel bill by using SAV for vaporizing its 2.1bcf send out capacity instead of the vaporizing system in use now. The SAV process should be in service in mid-2008 and will use the existing SCVs as a backup heat source for cold days.

The move will also drop NOx, CO and CO2 emissions substantially. OE

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