Hydrogen Sulphide is a killer, but modern technology means that the region’s plants can reap rewards from even the sourest of gas, writes Greg Whitaker
As anyone involved in sweetening sour gas will know, it only takes tiny quantities of H2S to knock a man out within a few breaths. The dangers of sour gas were well known before an incident in Texas that saw an entire family killed by sour gas in 1975.
Indeed, there had been a further eight fatalities of oilfield workers in the state during the preceding months, but it took the deaths of members of the public to prompt a public inquiry resulting in new legislation and for the subject to become a ‘live’ issue in the minds of the public.
Safety implications from the accident in Texas went far beyond just the southern states of the US too: oil majors were under renewed pressure to design safety into natural gas installations, including the then untapped fields of the Middle East.
One of the problems in dealing with sour gas is that it doesn’t behave in the same way as sweetened gas, a fact that Total engineers discovered long ago. In 1951, during an exploration campaign in South Western France, an oil well known as Lacq 3 revealed a significant reservoir.
The major quantities of hydrocarbons were found at a depth of 3,450 metres, but in gaseous form. The pressure was around 650 bar, meaning that gas initially had to be flared. It took two months to withdraw the drilling assembly and plug the well.
When the drill string was pulled out, the engineers discovered an unusual phenomenon; for some reason the gas had altered the molecular composition of the steel and caused cracking in all the welded parts. Experts soon realised that the disintegration of the welded seams was actually due to the hydrogen sulphide reacting with the welding flux.
The Shah Processing Plant in Abu Dhabi’s Empty Quarter, operated by Al Hosn, has high pressure, exceptionally sour gas and the plant employs a number of modern techniques to minimise the risks to people. For a start, the inlet area on the western side of the plant is out of bounds to all but the most essential employees.
Maintenance is carried out when the plant has been depressurised and purged. The design of the plant has minimised the number of flanges and has entirely eliminated control valves with manual by-passes in order to reduce the opportunity for a leak to occur.
All of the remaining valves have been specially engineered by Swiss engineering firm ABB to ensure a high level of safety. For the Shah gas field, the firm has supplied low voltage switchgear, motor control units and variable frequency drives. The switchgear uses remote control systems and multifunction protection relays to make it as safe as possible.
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One key development used on sites like this is a Pipeline Monitoring System known as a PMS. Using a network of remote computer terminal units (RTUs) and programmable logic controllers (PLCs) the system can remotely monitor for signs of trouble, including any leaks. During normal operation, mode valves are in the ‘open’ position.
When a block valve needs to be closed, pipeline operation must be stopped. Valve line closures are required to perform a pressure test of the pipeline to find small leaks, to isolate a leakage between two block line valves and for safety reasons during pipeline maintenance.
“ABB technology for remote control systems is crucial in order to ensure the functionality and the integration of low-voltage switchgear in modern production plants,” explains Tarak Mehta, head of ABB’s Low Voltage Products division.
The Shah facility is not the only sizeable sour gas field in the UAE. Another vast natural gas works is the Bab oil field about 150km from Abu Dhabi city. The plant has a design capacity of over a billion standard cubic feet per day; the feed gas contains a high percentage of H2S and CO2, along with some heavy hydrocarbons such as benzene, toluene and xylene.
This field is owned by Adco, the onshore arm of the Emirate’s national oil company Adnoc. While the field produces both oil and gas, Anglo-Dutch major Shell has signed a joint venture agreement, where ADCO gets 60% and Shell has the remaining 40%.
Similar to Shah, the gas from Bab is typically around 15% H2S and up to 50% CO2. However, Bab is part of a heavily populated industrial city so the utmost care has to be taken to avoid an accident.
As well as featuring technology similar to Shah to monitor the pipelines during the extraction stage, the sweetening process employs Jacobs’s EuroClaus technology, which, to over-simplify, uses a series of thermal steps and catalytic reactions to separate the sulphur from the product gas, as well as the firm’s patented Sulfinol-X technology. The plan is to then shift the sulphur product by rail using the first line of the United Arab Emirate’s new rail network.
“The company has a strong track record in delivering complex sour gas projects safely, on time and on budget based on the integration of our vast operational experience and with technologies covering the whole of the sour gas value chain,” says Shell’s UAE country chairman Andrew Vaughn.
Nobody has ever suggested that dealing with sour gas safely will be straightforward or cheap. Advances in early warning systems and general plant design and management mean that people living near a plant can sleep soundly.
