In the pre-Macondo blowout era the offshore industry had an impressive record of safety, especially considering the complexity and magnitude of the work being done in relatively harsh conditions. The failures and spillages of Macondo have indicated that there are a number of safety areas which have not received appropriate progress in large part because history had not experienced a major failure of a BOP stack at that depth of water. This paper addresses several areas of technology which can be implemented to increase safety on new deepwater drilling BOP systems or retrofitted on current BOP systems in the field. These safety areas include better use of accumulator capacity, greater control over BOP stack functions, more redundancy of control, and improved ability to shear wellbore components. The ready availability of these safety upgrades and methods for full BOP Subsea operation and control, high volume ROV operations, and shearing drilling collars represent significant upgrades to Subsea drilling equipment safety systems post Macondo.
Depth compensated accumulators represent a major safety improvement post-Macondo, although they were already 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 version1. Additionally, the depth compensated accumulators can be retrofitted on the ocean bottom by using multiple frictionless controls, also a major safety upgrade presented later in this paper.
The basic concept as seen in figure 1 is that a dumbbell piston has pressurized nitrogen in the top chamber and working fluid immediately below in a second chamber. The second chamber is divided from the third chamber by a central, stationary bulkhead. The third chamber is pressurized with environmental pressure, typically through an intermediate fluid for corrosion reasons. The fourth lower chamber is basically empty or a vacuum. This means that the pressure in the third chamber is not supported from below, but rather is mechanically added to the pressure in the second chamber. This means that at any depth, the pressure in the second working chamber exceeds environmental pressure by the amount of the nitrogen charge.
Increased safety impacts of depth compensating accumulators are:
High nitrogen (or helium) pressures associated with conventional accumulators in deepwater situations are not required. As the piston area in chamber 2 is slightly less than the area in chamber 1, the nitrogen pressure never exceeds the working fluid pressure.
Complex calculations are eliminated, the accumulators are simply run and operate subsea similarly as they operate on the surface.
When the accumulator comes to the surface, dumping the nitrogen in the accumulators is not required.
When the accumulator is re-run, the nitrogen does not have to be recharged,
There is no danger that only some of the nitrogen banks have been recharged, but not all of them.