BHP unveils new compact LNG technology

• Optimized Nitrogen Cycle [46,967 bytes]
• Bayu-Undan LNG process [91,191 bytes]

BHP detailed its Compact LNG technology, which it says enables LNG production at volumes as low a 1 million metric tons/year, at last month's LNG-12 conference in Perth.

The process was developed by BHP and Linde AG of Hollriegelskreuth, Germany. It will be marketed by BHP and may be used to develop some of BHP's own gas finds, notably Bayu-Undan in the Timor Sea.

The Compact LNG process is based on the conventional nitrogen cycle that has been successfully deployed for decades by Linde and other engineering firms in small, peak-shaving LNG plants.

BHP says that the conventional cycle's low efficiency has prevented it from being used for large baseload LNG plants until now. BHP claims to have optimized the cycle and dramatically improved its efficiency.

The authors of the LNG-12 paper were: Chris Dubar, senior engineering specialist, BHP; Timothy Forcey, principal process engineer, BHP; Vaughan Humphreys, senior engineering specialist, BHP; and Hans Schmidt, senior process engineer, Linde.

Compact LNG

"The simple design, using a single refrigeration cycle operating in the single-phase gas regions, means that there are relatively few equipment items," said the authors.

"The liquefaction section of the plant consists of a single compressor, a turboexpander, a cold box, and three heat exchangers for the compressor coolers. This reduces the complexity and space requirements for the plant.

"No compressor suction or refrigerant surge drums are required, nor (are) their associated piping, valves, and instrumentation."

However, the nitrogen cycle's inefficiency at this scale is a barrier. It requires three times more power than a comparable baseload LNG plant using a mixed refrigerant cycle.

Also, the small rotating equipment of a peak-shaving plant operates at lower pressures than do the larger machines in base-load plants, and the lower feed-gas pressure of a peak-shaving system reduces the thermodynamic efficiency of the process.

BHP found that, by adding a second cold expander to the conventional nitrogen cycle and pre-cooling feed gas with a small auxiliary refrigeration cycle, the process efficiency could be improved significantly.

A simple nitrogen-cycle plant scaled up for baseload operation would require 69 MW of power to produce LNG at the rate of 6,279 kilomoles/hr, while a compact LNG plant would require only 48 MW of power.

"By adding the cold expander and optimizing the temperature levels and load split between the warm and cold expander duties," said the authors, "a much closer matching of the heating and cooling curves between the feed gas and nitrogen refrigerant can be obtained. This results in higher thermodynamic efficiency.

"The cumulative effects of these improvements is to reduce the specific power requirements to a competitive level for small-to-medium LNG plant capacities and yet to retain all the inherent safety and simplicity advantages of the simple nitrogen cycle."

Bayu-Undan

The company has designed a topsides with the Compact LNG process and storage tanks on a concrete gravity base structure installed on the seabed specifically for the Bayu-Undan discovery.

Bayu-Undan has estimated gas reserves of 4-5 tcf and lies in 90 ft of water in the Zone of Cooperation between Australia and Indonesia in the Timor Sea (OGJ, Feb. 10, 1997, p. 64).

BHP says the Compact LNG process is ready for use in commercialization of offshore gas fields, and that the design for Bayu-Undan could have many applications elsewhere around the world.

"Detailed model testing and safety and reliability studies confirm the future safe and reliable operation of all aspects of the LNG facility," said the authors.

"Detailed cost estimates confirm that offshore LNG production in the range of 2-3 million metric tons/year is competitive with larger land-based LNG facilities on a cost-per-ton-of-LNG basis."

The authors said that, while sufficient design, safety review, cost estimating, and independent auditing had been done to conclude that the Compact LNG process is ready for LNG baseload applications to develop remote, marginal gas fields, floating LNG production is still at the concept stage.

"Compact LNG technology may one day be used on floating LNG facilities," said the authors. "However, floating facilities require the future development of a safe and reliable cryogenic offloading system for the transfer of LNG from one floating vessel to another.

"Fortunately for the near term, the availability of shallow-water sites in the Timor Sea near undeveloped gas resources allows the cost-effective use of Compact LNG plants on gravity base structures.

"Such a man-made island would sit firmly on the seabed. This enables the use of conventional loading arms for LNG transfer. No new LNG offloading technology is required."

In Bayu-Undan, the current development concept calls initially for condensate to be produced and natural gas liquids to be stripped out of produced gas (OGJ, Nov. 10, 1997, p. 47). Lean gas would be reinjected for later recovery when gas utilization facilities, of which Compact LNG is one option, come on line.

The compact LNG design for Bayu-Undan incorporates a single pretreatment train to remove acid gases, water, and mercury. Two independent liquefaction and refrigeration trains each have capacity to produce 1.5 million tons/year of LNG.

"Studies show that process facilities for the production of 2-3 (tons/year) of LNG can be fabricated in a single process module that weighs approximately 10,000 tons," said the authors. "Such a module is similar in size to other offshore modules that have been constructed in a number of fabrication yards around the world.

The authors reckon the Compact LNG plant envisioned for Bayu-Undan field development would have an average availability of more than 96%: "This is on a par with world-class LNG plants in operation today."

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