Despite the many LNG terminals planned in major market areas of Western Europe and North America, especially the US, the industry is moving to solve technical issues for offshore LNG terminals.
According to David Landry, vice-president of Freeport-McMoRan Sulphur LLC, two of the most important technological issues facing offshore LNG terminals are loading-arm design and design of an LNG pipeline for subsea movement of LNG to regasification facilities ashore or on a platform.
Landry spoke at the Offshore Technology Conference May 5 in Houston.
The loading arm on an offshore LNG terminal must be able to withstand the effects of heavy seas during transfer from tanker to offshore berth. A subsea LNG pipeline must be able to maintain the frozen state of the gas over several miles without damage from the cryogenic medium.
In response to the pipeline issue, Neal Prescott of Fluor Corp., Houston, described the design and testing of a new pipe-in-pipe pipeline configuration that uses a thermal nano-porous insulation in the annular space between an inner and outer pipe.
Insulation
Sealing metal or nonmetal bulkheads maintain the material under ambient pressure, he said. These bulkheads transfer the “contraction-induced axial compression load on the inner cryogenic carrier pipe to the external jacket pipe.”
The resulting pipeline, he said, addresses thermal contraction and expansion concerns without resorting to “expansion bellows or ultralow thermal contraction alloys.”
The inner, LNG carrier pipe that would be able to transfer thermal loads from the bulkheads would typically be 9% nickel steel; the jacket pipe, carbon steel.
The key ingredient, the thermal insulation, would be a “high-performance nano-porous aerogel product,” a blanket about 2 in. thick installed in the annular space without vacuum and under only ambient pressure, he said.
Testing
Prescott described what he called “Phase 1” testing, an effort sponsored by the US Department of Energy’s National Energy Technology Laboratory and conducted by Conversion Gas Imports LP in April 2004 at Atlanta Gas Light Resources’s Cherokee plant near Canton, Ga.
The goal of the testing, said Prescott, was to “design, construct, field test, and evaluate the performance of key components of a salt cavern-based LNG receiving facility....” At the center of the testing was the Bishop Process Exchanger, which involves direct vaporization of LNG and storage of the residue gas in salt caverns.
The test section of pipe consisted of an inner pipe being 6-in. OD, 0.280-in. WT, Grade 316L stainless pipe; the outer pipe was 14-in. OD, 0.250-in. WT, Grade B carbon steel. Total length of the test section was 42 ft; it was insulated with 2 in. of the aerogel insulation provided by Aspen Aerogels Inc., Northborough, Mass.
The 5-day test was conducted with flowing LNG, said Prescott, and employed 22 sensors: specialized fiber-optic heat-flux sensors developed by Aspen Technology Inc. to measure the thermal efficiency of the insulation and Fiber Bragg Grating fiber-optic sensors to measure temperature, pressure, strain, and contraction on the test piece.
Prescott said the test data, during partial and full flow conditions, were useful in “understanding the dynamics of LNG flow in a cryogenic pipeline and in defining the requirements and details of the pipe-in-pipe configuration.”
He said the test revealed the aerogel insulation to have high thermal insulating characteristics and to be sufficiently efficient to permit smaller thickness than conventional insulation. The “hydro-phobic” insulation was also shown to be ideal for marine environments by shedding or repelling water. The test, however, did not submerge the pipe section, but the section lay on steel supports that would allow it to move during the test if necessary.
Prescott said that posttest inspection of the insulation showed “no physical changes” resulting from the cryogenic exposure. ✦