BOP CONTROL SYSTEM: A System Analysis
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Further development of oil and gas resources includes moving petroleum operations into areas of deeper water depths. As the operation moves into these challenging regions, the pressure loss in return lines increase. Earlier, this was solved by venting hydraulic fluid into the sea. Today, this alternative is normally not accepted as governed organisations desire a zero exhaust policy to the Norwegian Sea. From this environmental point of view, many operators require a return conduit to surface. (1) Thus, alternative solutions must be considered to stay within industry requirement for the operation of BOP functions. Subsea BOP is one of the two required pressure barriers in subsea drilling and intervention operations. The BOP control system aims to operate large BOP functions as fast as possible, which includes rapid changes in the control fluid flow rate, i.e. by activation of subsea valves. These changes in flow rate result in a potential risk of water hammer generation in selected parts of the control system. This thesis presents basic theory and description of different BOP control systems performances, and explains the principles of the water hammer phenomenon. Special attention has been given to problems related to the requirement of zero exhaust to sea and the effect of water hammer in pipeline systems. Further, the report includes the use of a software program, FLOWMASTER V7, in order to perform a complete sensitivity analysis of water hammer in pipeline systems. Water hammer is high pressure shock waves that are developed due to sudden change of flow. The magnitude of water hammer is affected by parameters such as the inner diameter of the pipeline, the elastic properties of the pipe, length of the pipe, etc. The effects of these parameters have been investigated to provide a general understanding of potential mitigating measures from a water hammer point of view. FLOWMASTER V7 is also used for a study on pressure losses generated in return conduits and the effects this pressure loss has on the exhaust rate of control fluid. The three scenarios that where investigated include direct exhaust to sea, direct exhaust to surface via a return conduit, and exhaust of control fluid via a return line with connection to a subsea accumulator bank. The simulation results showed that the demand of zero exhaust to sea increases the response time for BOP functions. Moreover, that the implementation of an accumulator on the return line reduces the exhaust rate compared to direct exhaust to surface. Another important observation made from the simulations is that the water hammer phenomenon can be expected in the return line when no accumulator is connected to the return conduit. Water hammer pressures in pipelines under great water depths can result in potential dangerous failures, as a negative initial pressure spike in combination with the ambient pressure can result is collapsing hoses and ruptured fixtures.