Dehydration on the seafloor could help build a business case for stranded gas deposits. Elaine Maslin reports.
Australia’s SubCool has developed a hybrid solution for stranded gas deposits. The concept would fully process the gas at the seafloor so that topsides facilities only need to deal with low pressure liquids, dramatically reducing topsides facilities weight, size and complexity.
For Richard Moore, SubCool’s CEO, the technology is about creating a business case for deepwater gas fields that are otherwise uncommercial and stranded, particularly those in 175m+ water depth and 100km+ from shore, with 0.5 Tcf+ of reserves.
“The problem with deepwater gas is that the process facilities need to be mega-facilities,” he says. This is because they have to be high-pressure gas systems, which has an impact on size and weight – and cost. High-pressure risers, high-pressure emergency shut-down valves, flare and utility systems for high-pressure, etc., are needed.
Floating liquefied natural gas (FLNG) facilities have been offered as an alternative, but, in Australia, four FLNG projects have been cancelled in 18 months, he says. “Capex and especially opex are high – it’s complex.” Subsea processing, meanwhile, has shown great potential, but is so far limited to shorter distances, he says. “Subsea projects are a great success for short distance projects. But for long distances, it’s a problem.”
When processing gas, you have to knock out the water and liquids and need low temperatures to do that, Moore says, and it makes sense to do that where it’s cold – subsea. This would also mean hydrate inhibitors wouldn’t need to be used to avoid flow assurance issues, which could be caused by leaving the water and liquids in the pipeline.
SubCool’s subsea dehydration system would comprise first stage active subsea cooling (making use of the nearby cold seawater with a heat exchanger) and separation (with traditional or inline separators) to separate out the condensed liquids and water, followed by a dehydration process, called “Sales Gas Subsea” by SubCool, to enable dry single phase gas export. This cools the separated cooled gas to below ambient seawater temperature to condense the remaining condensable liquids and produce a single-phase dew-pointed gas. “The bulk of the dehydration is done by cooling the gas,” he says. “What then comes to surface is lower pressure liquids, radically reducing the requirements of the topsides system.” The surface unit would also provide power for the subsea facilities, and potentially could be unmanned.
Moore says the result could be a compact, low pressure surface unit for liquids processing and surface support, at 4000-tonne, instead of 24,000-tonne, and 20 bar instead of 200 bar. “Distance wouldn’t be an issue and it would enable simpler, lower cost local subsea compression, as and when needed,” he says.
SubCool’s subsea dehydration system. Image from SubCool.