Developments in Accumulator Technology: A Review of Fluid Power Options in Subsea BOP Control Systems

摘要

The problem of delivering the required volume of hydraulic power fluid to subsea BOP control functions from gas-charged accumulator bottles has been well documented of late. Conventional nitrogen precharged subsea accumulator bottles currently require charging up to very high pressures to counteract the effect of seawater hydrostatic pressure in order to remain functional at the working depth. Research shows that when taking into account the effects of nitrogen compressibility, adiabatic expansion and temperature at these elevated pressures, the volumetric efficiency of the accumulator and available differential pressure are greatly reduced with increasing water depth. The reduction in fluid power availability has been typically underestimated, leaving conventional stack- mounted accumulator bottles unable to provide the required fluid volumes within specified response times to stack functions in the ultradeepwater environment. In addition, the subsequent installation of additional subsea accumulator banks to compensate for the loss of usable volume increases the weight and complexity of the subsea BOP stack. A new type of subsea gas-charged accumulator has recently been developed to tackle the deficiencies of conventional accumulators head-on. The constant differential accumulator (CDA) compensates for the increase in seawater hydrostatic pressure while being run to any depth, allowing it to function without requiring additional charge over and above the initial surface charge. In theory it should provide virtually the same usable volume of hydraulic power fluid at the same differential pressure, independent of depth. Subsea BOP design is becoming more complex as drilling contractors consider the options available for upgrading the operating depth of older generation rigs while equipping the latest generation rigs for ultra-deep waters. The following research aims to reduce the uncertainty involved in accumulator choice by defining the most effective working depth range of conventional and constant differential accumulators from the standpoint of fluid power availability.