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In general, the offshore drilling industry has long accepted the concept that subsea accumulators are required for offshore BOP control systems in order to satisfy certain industry standards and regulations. However, the actual energy derived from subsea accumulators may fall short of the perceived benefits. Moreover, in some instances subsea accumulators may not be necessary, provided an appropriately sized rigid conduit line on the marine riser is utilized. In the early days of offshore drilling, the industry relied solely on the energy stored in accumulators located subsea to, in part, speed the response times of the BOP. BOP control systems of this period typically utilized a one-inch inside diameter hose that transported hydraulic energy from the surface to the BOP stack functions subsea. The head loss due to friction from this relatively small hose over long distances was significant, resulting in BOP response times that were deemed too slow by the industry. As such, accumulators were mounted directly to the BOP stack to provide a shorter hydraulic conduit, thereby reducing head loss and response time. However, current research indicates that the benefit of these accumulators deteriorates rapidly with increasing water depths. Also, research suggests that utilizing a properly sized rigid conduit line to supply hydraulic energy to the BOP stack can provide acceptable control system response times without the use of subsea accumulators. This paper analyzes the effects of water depth and other variables on the available energy in subsea accumulators. Also, it analyzes the available energy through BOP control system supply lines of various sizes. The paper concludes with recommendations on how to best maximize the required energy to operate subsea BOP stacks. |
