This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper OTC 22407, ’Post-Macondo BOP Safety Upgrades,’ by Benton F. Baugh, John Vozniak, SPE, and Nathan Schmidt, Radoil, prepared for the 2011 Offshore Technology Conference Brazil, Rio de Janeiro, 4-6 October. The paper has not been peer reviewed.
Before the Macondo blowout, the offshore industry had an impressive safety record, especially considering the complexity and magnitude of the work performed in relatively harsh conditions. The failures and spillages of Macondo indicated that several safety areas have not experienced appropriate progress, in large part because there had never been a major failure of a blowout-preventer (BOP) stack at that water depth. This study addressed several areas of technology that can be implemented to increase safety on new deepwater drilling-BOP systems or retrofitted on current BOP systems in the field.
Depth-compensated accumulators represent a major safety improvement, and they were seeing market acceptance before Macondo. The problem with accumulators on subsea stacks is that more than 100 accumulator bottles would be needed instead of approximately seven for the depth-compensated version. Additionally, the depth-compensated accumulators can be retrofitted on the ocean bottom by use of multiple frictionless controls, another major safety upgrade.
The basic concept of a depth-compensated accumulator, as shown in Fig. 1, is that a dumbbell-shaped piston has pressurized nitrogen above it in the top chamber and working fluid below it in a second chamber. The second chamber is divided from the third chamber by a central stationary bulkhead. The third chamber is maintained at environmental pressure, typically through an intermediate fluid for corrosion reasons. The fourth chamber is empty or at a vacuum. Therefore, the pressure in the third chamber is not supported from below, but rather is mechanically added to the pressure in the second chamber. At any depth, the pressure in the second working chamber exceeds environmental pressure by the amount of the nitrogen charge.
When a shear ram moves forward, the accumulator pressure declines. There is never as much accumulator pressure available at the point of shear as was available in the accumulator when the shear command was given. In some cases, the accumulator pressure can be regulated down such that the shear-ram pistons always see the same pressure, but this simply dumps the excess pressure that may otherwise have been useful. Deepwater operations happen in a high-pressure environment. Therefore, if one is operating in 10,000 ft of seawater, the ambient pressure is 4,650 psi, and when it appears that 5,000 psi of pressure should be on one side of the piston and zero on the other, there actually is 9,650 psi on one side and 4,650 psi on the other.