The use of ultrasonic testing in condition monitoring is a relatively new method in the field of condition monitoring. It monitors the high frequencies sound that is generated by plant machines and systems. Ultrasonic sound is not audible by the human ear, and occurs at a frequency above 20 kHz. The different ultrasonic equipment records ultrasonic sound between 20 kHz and 100 kHz.
There exist two types of ultrasonic monitoring, airborne and structural. Both methods receive ultrasonic sound, airborne through the air and structural through solid surfaces. The airborne method can further be separated into two modes. The scanning mode primarily used in detection of gas leaks. It uses the fact that the instrument is only sensitive to ultrasound and that it is ultrasonic sound that is transmitted from a gas leak to register the leakage from valves, pipes etc. The contact mode is on the other hand used to locate turbulent or restricted flow in process piping. In this case study we are going to focus on the airborne scanning method of ultrasonic condition monitoring. 
A vital part of an offshore oilrig is the compressors. The compressors are used for many different tasks such as reservoir management, production enhancement and transmission of gas. If the compressor handles combustible gases, such as hydrocarbon gases, a leak could in worst-case lead to a fire or an explosion. A typical failure scenario is a compression unit offshore used for natural gas transmission to land. A leak in one of the bearings occurs, and because of the outdoor environment the gas does not make contact with the regular gas detectors before it has reached a high concentration. How will this be monitored and detected so that workers are warned in time before the gas concentration reaches a point were it could be ignited and cause a fire or explosion. 
There are several different types of equipment that can be used to detect ultrasonic sound from a gas leakage. The tool used to monitor this system is a non-portable tool named GDU-Incus Ultrasonic Gas Leak Detector for the detection of leakage, and then a portable ultrasonic detector to locate the exact location for the leakage. Airborne ultrasound occurs when gas moves from high-pressure (approximately above 2 Bar) to a low-pressure environment. The GDU-Incus will react instantaneous to the ultrasound from the leakage and show the real time decibel on the monitoring screen. An ultrasonic sound is a precisely line of site, in other words, it does not bend. The sound will be at its highest at the source – at the leakage. The portable ultrasonic detector will therefore be used to find where the maximum is located and a laser pointer can be used further to find the straight line to the gas leakage. The portable ultrasonic detector consists of a detector and a monitor. When an operator uses the portable detector they point the detector around the plant and listen to the sound through...