In a gas processing plant, releases of flammable gas may occur and early detection is important. There is an automatic gas detection system, but in addition, the gas release may be detected by two operators working in the area. The following information is provided:

Each operator spends 30% of her time in the area and can only detect gas when she is in the area. The operators work independently of each other, and the time an operator is present is independent of whether other operators are present. The nominal probability of detecting gas (NHEP) for an operator is 0.3. Both operators are trained in the same way to look for gas.This training acts as a PSF and implies an adjustment of 0.4. The operators are normally busy, because of other tasks. This is a second PSF, with an adjustment of 2.0. Use the THERP-method to calculate the probability that at least one of the operators will detect a gas release.