A combined leak-detection, gas-composition-tracking system, PIMS, has been developed, installed and operated for two wet-gas pipelines in the North Sea. One system has been operating for almost 3 years with no false leak indications to date. The system is based on a simplified, transient, multi phase, on-line model. The code is small and robust and can run on the same computer as the existing control system. PIMS also tracks gas composition so that the onshore terminal can properly blend low quality gas (high nitrogen content) with richer gases so that the minimum-Wobbe contractual obligations are met. In a field test of this system, the model was able to predict arrival times of packets of low-Wobbe gas to within 1 percent. PIMS also estimates the liquid content of the pipeline and informs the operator when the main trunkline requires pigging to reduce liquid hold-up. The model also provides an estimate of the slug size in front of the pig.
Another complementary, on-line model, VO, uses the liquid content estimates. VO controls the flowrate via the wellhead chokes to prevent or minimize ramp-up slugs from forming in infield flowlines. It also controls the flowrate from each well in order to minimize sand production and to keep wells from loading up. Finally, it selects wells and their flow rates in a manner that will control the gas blend to insure that the minimum Wobbe targets are met.
Figure 1 shows the layout of the gas development where the PIMS and VO systems are installed. In this development, gas is produced from three platforms. Two reservoirs, B and C, are produced from Platform 1. A 16-inch, 10 km flowline transports the well stream fluids to platform 3. Reservoir S is produced from platform 2 and is transported to platform 3 via an 18-inch, 30 km flowline. Gas from reservoir M is produced on platform 3. In order to reduce both OPEX and CAPEX, platforms 1, 2 and 3 are not-normally manned. The full well stream production is transported in flowlines 1, 2 and 3. At platform 3 the liquid and gas is separated so that the gas can be compressed, but the liquid is re-injected into the trunkline for transport to shore.
The produced gases range in liquid content from 1 to 20 BBL/MMSCFD. Because of both the unmanned operation and the high-liquid-content fluids, the control and operation of the fields pose a number of challenges which are addressed by the PIMS and VO systems:
Leak detection for multiphase systems is difficult. On an earlier development, the commercial leak detection system that was originally installed produced many false alarms. These false alarms resulted in the pipeline being shut down and, in some cases, helicopters being sent out to survey the pipeline route for leaks. For each alarm there was a high cost to identify that it was a false alarm. In the end, the leak detection system was removed. Despite the high false alarm rate, leak detection systems for multiphase flow lines are expensive. The project team wanted a low cost, leak detection system which would minimize false leak alarms while still providing a timely warning when a real leak occurred.
Gas composition can vary from reservoir to reservoir. For this system, reservoir B has a high nitrogen concentration which results in a low Wobbe Index. The Wobbe Index measures flame stability and is related to the BTU content of the gas. If the Wobbe Index falls below the minimum contractual value at the delivery point, then a stiff penalty is imposed.
When multiphase flowlines operate at low flowrates, liquid collects in the upward-sloping sections. Gas contracts frequently dictate large changes in flowrate (at least daily). When the flowrate is ramped-up, the accumulated liquid is released as a slug.