Going beyond traditional detection technology
Traditionally, gas leak detection offshore has been entrusted to infrared point or open path sensors. According to Innova GasSonic, these technologies have one crucial limitation: for the leak to be detected, the gas must be either in physical contact with the detectors or within the pre-defined path of the infrared beam. On exposed platform or rig decks, wind has a habit of changing direction, causing a dilution or shift in the leaking gas plume that may be beyond the detectors' reach.
Since 1999, Copenhagen-based Innova GasSonic has been offering an alternative to the offshore sector, the MM0100. This is a range of fixed instruments that can detect the airborne ultrasound emitted from gas escaping at high pressure. This noise (inaudible to the human ear) is not affected by wind direction. In tests, the company says, its system has been proven to sense a gas leak up to 12 m.
The MM0100 is designed to 'listen' to noise associated with escaping gas.
null
Unlike conventional gas detectors, the MM0100 does not provide a read-out showing the concentration level (LEL) of the escaping gas. Nor does it have to wait until the gas accumulates into a potentially dangerous cloud. As soon as it detects the noise from the gas, it will activate an alarm.
Innova says its ultrasonic system is also the only one providing the facility for the user to determine the system's detection coverage. This requires prior knowledge of the following:
- Gas pressure within the system at risk from a leak; acoustic ultrasound is generated when escaping gas travels from high to low pressure at the speed of sound
- The maximum rate of gas escape that might follow a leak on the installation, measurable by the user in kg/sec
- Background noise from process plant, such as turbo-compressors. Most will be in the audible low frequency spectrum (20-15,000 Hz), which can be filtered out by the MM0100. But there will also be a small amount of ultrasound mixed in. To prevent this from accidentally triggering an alarm, the prevalent ultrasound has to be defined, using an ultrasonic mapping meter, such as Innova GasSonic's hand-held 1700 device. This will allow the user to adjust the MM0100 to be above the ultrasonic background noise.
Spontaneous air releases from certain industrial sources could theoretically activate the warning signal. But according to Innova, ultrasound from leaking gas lasts longer than from spontaneous air emissions, and this should be recognized by the system's in-built delay function (thereby preventing a false alarm).
The system can be installed offshore on a standalone basis, or in combination with conventional gas detectors. It can be connected into the installation's distributed control system, or configured to have its own centralized alarm network. The system's six main components are a corrosion-resistant microphone, an ultrasonic high-pass filter circuit, an alarm detection circuit, alarm delay circuit, alarm relay output, and a rectifier.
Innova provides a portable test unit, the 1701, which can simply be placed over the gas leak detector's sensor head. This can be used to verify that the delay function is working, through generating a continuous leak noise. Or it can be applied during the plant maintenance campaign, even though, Innova says, the MM0100 does not need calibrating.
According to Innova GasSonic spokesman Mads Kornbech Rasmussen, 1,500 of the company's detectors have been installed worldwide since the technology was introduced in 1996. Offshore users include NAM in the Dutch North Sea and Statoil on the Sleipner complex, as well as BP, Shell, ConocoPhillips, and other operators in other parts of the world.
"We can detect leaks at rates as low as 0.1 kg/s," Rasmussen says. "We can also provide an onsite engineering service to determine the optimum location of sensors, as well as system commissioning. Last year, Britain's Health & Safety Executive (HSE) issued a report suggesting that only 62% of gas leaks in the UK sector were being picked up by traditional gas detectors. HSE is currently testing our system to see if it will also work with liquid releases."
