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.
In the late 60's offshore floating drilling operations were becoming common. The water depth during this time period was typically less than 1,000 feet and control of the subsea BOP stack was accomplished through the use of a redundant, piloted hydraulic control system. Two hydraulic hose bundles supplied hydraulic power from the surface to directional control valves located on BOP mounted control pods. Each hose bundle contained a quantity of small pilot lines, which actuated directional control valves located on control pods. These valves directed fluid to open or close the various functions on the BOP. In the center of each bundle was a single, one-inch ID hydraulic supply line, which provided hydraulic power to the directional control valves. BOP stack response times were, in part, a function of the velocity of the hydraulic fluid flowing through this one-inch hydraulic supply line.
A relatively straight, 1,000-foot long one-inch hose subjected to a maximum hydraulic control system pressure of 3,000 psi is capable of producing a mean flow rate of approximately 37 gallons per minute. A typical 18–3/4" 5,000 psi annular preventer requires 48 gallons to close. So, the quickest possible closing time for this preventer can be calculated by dividing the closing volume by the flow rate. In this example, the closing time would be in excess of 1.3 minutes. Note that this estimate is extremely un-conservative as it does not include the directional control valve actuation time or pressure drops due to hose and pipe bends. Also, surface system pressure decay was not considered. Taking these factors into account will further increase the closing time estimate.
The offshore drilling industry deemed response times in the range of 1.3 minutes too slow for safe well control operations. A larger hydraulic supply hose would produce quicker response times but was considered impractical. Instead, subsea accumulators were added to the BOP stack. Subsea accumulators are precharged on surface to an initial pressure with gas, typically nitrogen. Fluid is then pumped into the accumulators to a final pressure equal to the BOP control system pressure. Because nitrogen is compressible, it is capable of storing pressure energy. When the pressure of the hydraulic fluid drops, the compressed nitrogen expands, forcing the hydraulic fluid out of the accumulator and into the BOP control circuit.