Abstract
As offshore drilling plans increase drill pipe sizes and push into deeper water, emergency accumulator systems for BOP control are also being pushed to maximum performance conditions. Recently some systems have failed subsea deadman/autoshear testing despite shore calculations that predicted they should work. Changing the accumulator precharge led to success in each case, but at the expense of costly stack pulls. This paper explores the causes of these failures and provides a new strategy to help prevent similar avoidable downtime events.
Optimization of subsea BOP lower stack deadman/autoshear accumulators should include consideration of temperature and pressure uncertainties that are inherent in the process of setting precharge pressure. For each well application, calculations typically compute an optimum precharge which provides the maximum usable fluid for the emergency function sequence. For single function sequences, this theoretically produces a subsea condition in which all of the fluid in the accumulators is usable. Precharge pressures below this optimum will provide more fluid, but the usable portion of this fluid gradually diminishes as precharge pressure is reduced. Precharge pressures above the optimum rapidly diminish the total amount of fluid (and thus usable fluid) in the accumulators. The usable volume penalty is therefore much higher for precharging above optimum than for precharging below optimum. As systems are pushed to maximum performance conditions, the margin between an optimum condition and a failing condition can be within normally accepted tolerances.
If temperature and pressure uncertainties involved in setting the precharge according to plan are considered, the most practical precharge plan will target slightly below the mathematical optimum. This will balance the asymmetric effects of uncertainties in the precharge settings. Correct calibration of the precharge pressure gauge and correct adjustments of precharge pressure for ambient temperature are both critical for reduction of precharge condition uncertainties. Adjustment of precharge to a specified plan is inherently difficult in practice for several reasons; therefore training of rig personnel and awareness of the effects of precharge condition uncertainty by planners on shore can prevent costly downtime.
In multi-function emergency sequences, accumulator precharge pressures are typically optimized to balance the needs of all functions. The same thought process described for single function sequences can be applied to multi-function sequences to keep the band of precharge uncertainty within the operating limits for all functions in the sequence.
