Hydraulic accumulators have been in existence for some 40 years but the exacting requirements of the offshore industry have driven the technology to new levels. Quality, reliability in service and minimum weight coupled with higher working pressures have required new ways of thinking and produced specialised product.
This paper looks at the changes that have occurred including the current state of the art and examines the technical considerations that face the system designer who has to incorporate the product.
Prior to the development of the offshore industry in the North Sea, there had been very little change in the basic design of hydraulic accumulators.
They had been used in industrial applications, typically machinery such as die casting equipment which had intermittent high flow rate requirements. As a result of this, some development work had been carried out such as fitting large fluid ports but the market was generally satisfied by the range of products then available.
In the early 19701s, we became involved in the offshore industry supplying accumulators for use on the B.P. Forties Project for operating ballast valves on the upending system. Supplying the industry in those days was relatively simple with technical specifications limited to one page and any Quality Assurance documentation to a similar size!.
As time progressed, accumulators became used on an increasing range of applications and the market became a significant part of our business.
Towards the end of the 1970's and the development of subsea systems, we supplied what we believe were the first accumulators to be used subset on methanol injection on satellite ‘PI’ on the North Cormorant field.
These units were adapted for long term installation and were fitted with low permeability bladders to reduce loss of nitrogen gas to a minimum, stainless steel fluid port assembly, metal to metal sealed gas valve assembly with grease filled protective cap and the unit finished with a special paint system.
(You may be aware that PI was retrieved mid 1993 and returned to A.B.B. Vetco Gray, Aberdeen for inspection. With the exception of minor paint problems and the fact that some of the anodes had been painted, all the equipment was in good condition and the bladders exhibited very little marking or deterioration. A paper was presented in December 1993 by Steve Cromer of ABB Vetco Gray at the Subsea '93 International Conference in London).
Further offshore development work was around an accumulator with a one piece shell incorporating a split flange connection and double seals on the gas end to prevent a possible loss of hydraulic fluid (mineral oil) into the environment. A low permeability bladder was now fitted as a standard requirement. This unit was supplied for several years but discontinued due to cost of interfacing with the system and long manufacturing lead times for the shell, which was available only in one size.