This paper describes a Sour Gas Leak Detection System implemented by Novacorp International Consulting Inc.'s Pipeline Software Group. The emphasis of the paper is on the design considerations made in supplying a leak detection system for a gas pipeline system. The paper also presents the estimates of performance obtained from preliminary studies. The project's main goal was to provide reliable and accurate leak detection at low cost. That meant providing better accuracy than a simple volume balance at a cost considerably less than a transient model based leak detection system. A Compensated Volume Balance approach was selected since it is cost effective and sufficiently accurate when the pipeline is well instrumented. The pipeline's SCADA system design was not yet finalized so that additional pressure and temperature instruments could still be added to accommodate the leak detection system.
The pipeline is located in a sparsely populated area of Central Alberta, Canada. It is close to a small town and a provincial campsite and crosses a river at one point. The effect of a prolonged leak of sour gas in this environmentally sensitive area would be disastrous. Therefore, the steps taken by the client to protect the public and the environment are commendable.
The pipeline is 44 miles long and 12 inches in diameter with a 0.35 inch wall thickness. Elevation of the pipeline ranges from 3250 to 4500 feet. Depth of burial is approximately three feet. The pipeline is extremely well instrumented with pressure measurements spaced from one to six miles apart. Flow, pressure and temperature are measured at the inlet and outlet ends of the pipeline with pressure and temperature measured at four intermediate block valves and pressure only measured at another eight intermediate block valves. The following instrumentations accuracy is assumed: Measurement Accuracy (%) Range pressure 0.5 0-2200 Psig NOTE: Accuracy is given as a percentage of full range. The SCADA system receives data fiom each of the RTU's along the pipeline every two minutes.
On the low end of the performance scale is the Simple Volume Balance leak detection method. In its simplest form leaks are detected by comparing the volume into and out of the pipeline over time. This method might be considered for an incompressible fluid such as crude oil but is highly insensitive for gas pipeline application especially in short time fiames.
On the other end of the scale of leak detection algorithms is the Transient Leak Detection method. This method can provide a highly accurate means of detecting leaks in a liquid or gas pipeline. This method of leak detection comes in various forms but the basic principles are the same. The modelled state of the pipeline is used to determine the change in line pack between flow measurements.