This paper covers aspects of subsea production control fluid development, use and experience related to high pressures and high temperatures. It covers material qualification, metal and polymer compatibility test methods and equipment selection at temperatures from 130°C up to 250°C. The paper also discusses the limitations of using fluids at high temperature, such as degradation rate and boiling risk in smart wells and downhole safety valves (DHSVs).
The testing method and experience of hardware from the old British Hydrodynamic Research (BHR) DHSV test rig in the 1990s, to current qualifications on BP Rhum field, are discussed. The paper also covers field experience and what can be learned. There are currently over 50 projects operating over 100°C, and some have been around for over 15 years, yet there are only a handful of projects operating over 150°C and these have very limited experience.
For future subsea control projects, this paper will cover recent research and successful qualification of water based fluids up to 220°C. The conclusions show that current testing protocols produces meaningful results for an appropriate cost, but to ensure future reliability, testing is required to mimic the hot environment where the subsea fluids are required to perform.
The subsea hydraulic system controls the produced hydrocarbon flow and provides the pressure safety system. Historically, these systems have been hydraulically driven, which reduces component size and increases reliability in a hostile environment.
Many subsea engineers do not understand the challenges associated with the development of hydraulic fluids. Control fluids need to meet stringent technical performance and environmental requirements. It could also be speculated that the industry as a whole does not fully appreciate the impact that subsea control fluids have on subsea design and operation.
From the outset, traditional hydraulic fluids were not suitable for this type of equipment. They were relatively viscous with exceptionally low seawater tolerance and poor environmental profiles. Special fluids, oil and water based, were either developed for this purpose or adopted from the aerospace industry. In the early 1980s, the first "modern" water-based subsea control fluid, Oceanic HW540, was used in the North Sea on the Sun Balmoral control system. Castrol had similar success with its hydrocarbon based Brayco fluids with Shell.
For experienced subsea production engineers, the subsea control fluid has been regarded as the lifeblood of the subsea system. The fluid keeps the system moving and touches all components from the topside hydraulic power unit (HPU) controls down to the DHSV within the wellbore, and even on to the smart well controls within the reservoir itself.
The subsea fluid has a major influence on the physics, materials, cost, reliability and environmental properties of the subsea production control system. The importance of this liquid and the environment in which it operates has greatly affected development and qualification of fluids. Subsea systems have become larger and more complex, but the two major influences over the last 15 to 20 years have been the environmental regulations, which have evolved significantly
