Developments in Accumulator Technology: A Review of Fluid Power Options in Subsea BOP Control Systems
Authors: McCurdy Paul J.A. 
Published: February 2009

Abstract
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.

Introduction

Background.
The inefficiencies of conventional nitrogen pre-charged accumulator bottles for use in the subsea environment has been the topic of some discussion in recent years, with most authors on the subject, notably Sattler (2002) and Curtiss & Buckley (2003), attempting to improve both the understanding of accumulator discharge and accumulator sizing calculations. The consensus of opinion concluded that current calculation methods and the thermodynamic processes used to model the accumulator discharge were not wholly representative of the actual processes taking place at very deep to ultra-deep water depths. Industry regulations pertaining to usable volume calculations were found to be lacking in the required complexity and detail with which to design accumulator systems accurately.
API Specification 16D, the most detailed regulatory document available at the time, came in for particular scrutiny. The API standards committees were aware of the shortcomings and Spec. 16D was updated (2nd Edition, 2004) along the lines highlighted by several authors to improve its accuracy and relevance to the deepwater drilling industry. This was achieved to some extent, and with built-in safety factors provided an acceptable improvement in accumulator sizing calculations from the standpoint of usable volumes of hydraulic power fluid.
Where Spec. 16D and recent recommendations by other authors fall short however, is in the consideration given to the non-ideal behaviour of compressed precharge gas. The effects of nitrogen compressibility have still not been fully addressed, leading to an overestimation of fluid power availability in conventional accumulator systems.