Epoxy Oilserv is a supplier of Subsea control fluids in Nigeria

Subsea Control Fluids

Subsea Control Systems are used to remotely control the flow of oil or gas from producing wells using subsea hydraulic control fluids (SCFs). These fluids are the ‘blood’ of the system so it is essential that the hydraulic fluid doesn’t damage the subsea equipment in any way and careful consideration is given to which fluid is chosen. It is extremely expensive to repair subsea equipment, so extensive testing should be carried out with all materials (metals/elastomers & coatings) that could come in contact with the fluid, to check that they are compatible.

Fluid types

Different Types Subsea control fluids can be divided into two main categories: Water Based (most common) and Oil based. Both have advantages and disadvantages; the main advantage of Oil Based fluids is the superior lubrication properties, oil is naturally a good lubricant, whereas water is not. Traditionally oil based fluids were not environmentally acceptable, however new fluids coming onto the market, for example ‘CLEO’ is both oil based and environmentally acceptable.
Water based fluids are generally cheaper and are much more widely used, so this article will concentrate on them.

Chemical Make‐up

There are two main components that make up the ‘base’ for all water based SCFs, these are Di‐mineralised water and mono ethylene glycol (MEG). The ratio of water/MEG will change depending on the climate, with more MEG used in colder climates and in gas wells for hydrate control. Then, an additive package is added which includes boundary lubricants, extreme pressure additives, corrosion inhibitors, biocides, surfactants, dyes, stability additives etc. This package will be very variable between the different brands of SCFs.

Long umbilicals

At  present, long umbilicals are operating as far as 35 km in thermoplastic (as in the Arco UK Welland Field, supplied by Multiflex), and 45 km in steel pipe (the Gulf of Mexico Popeye Field supplied by Alcatel). In the near future, distances envisaged are as long as 60 km in thermoplastic (as in Duco’s consignment to the Poseidon development) and 100 km in steel – the Mensa Field, also supplied by Duco.
To keep the cost of an umbilical down, it is advisable to have the minimum number of hydraulic lines, and for these lines to have the smallest functional bore. Power loss over such long distances is the major operational consideration.
The two main properties required in a subsea control fluid that can help in this area are:
(1) Low viscosity: A low viscosity hydraulic fluid will reduce power loss over long hose or pipe lengths and, combined with a relative incompressibility, will give a faster and more easily controlled response at the subsea template.
(2) Low toxicity: Discharging the fluid into the ocean once its pressure has been spent negates the need for a hydraulic return line. That means there is one less length of hose. Discharging hydraulic fluids decreases the cost of an umbilical and increases the power transfer efficiency.
The need for low viscosity in a subsea control fluid has been known for a long time and there are fluids on the market that have similar properties to water in this respect. Recent legislation, however, has limited the volume of fluid that can be discharged into the ocean. High profile environmental policies mean that operators will only discharge chemicals if all the relevant ecotoxicological information is available to prove there will be no harm to the environment.
Recent developments have produced control fluids that are in the region of 90-100 times less toxic than fluids available five years ago. Oceanic HW443, for instance, can be discharged into UK waters at a rate of 1,000 tons per installation per year (DTI and the Ministry of Agriculture, Fisheries and Food Classification Group E). Oceanic HW540, the market leader, can be discharged at a rate of 100 tons per installation per year.
The issue of fluid toxicity grows in importance as legislation tightens and each project is assessed individually for the associated chemicals that are proposed to be discharged. In Norway, fluids that were allowed to be discharged only a few years ago are now being refused permits because they are not the most environmentally friendly product on the market.
Pressure pitfalls
The problems encountered with extreme depth are apparent: inaccessibility and high pressure due to tonnes of sea water between the subsea template and the surface, possibly 6,000 ft above.
Pressure is the most influential factor for umbilical and mechanical design on systems used in extremely deep water. Designing a hydraulic fluid for deep waters should take into account certain requirements: as workover is virtually impossible, the stability and reliability of the fluid should be paramount. Also, its ability to cope with small amounts of contamination and to remain stable over the typical lifetime of the project (up to 30 years) are essential. This will reduce the need for workover due to corrosion or blocked control lines.
In shallow waters, the hydrostatic head created by the seawater and the fluid in the control line are very similar and any differences are negligible. As the water depth increases, the pressure differential due to the weight of the control fluid can become significant.
Water-based fluids have a specific gravity approaching that of seawater, therefore the pressure differential rises only slightly as the seawater depth increases. It should also be noted that pressure differential increases with water-based fluids and decreases with oil-based fluids. So, if a subsea line were left unpressurized and a leak occurred, a water-based fluid would slowly leak out of the system due to the positive pressure differential.
However, an oil-based fluid, which has a negative pressure differential, would tend to draw the seawater in. This increases contamination in an oil-based system and escalates the potential need for a workover. It should also be noted that hydraulic oils tend to be a lot less compatible with seawater than do water-based control fluids. Conceivably, the pressure differential at 6,000 ft could be as high as 40 bar for the oil-based system, easily enough to operate a valve unwittingly.
HTHP production
The major frontier in production includes not only deep water, but also the higher pressures and temperatures of the hydrocarbons being produced. As explained previously, water-based control fluids are less affected by high pressures but tend to be the poor relation in terms of the high temperature stability of synthetic hydrocarbon control fluids.
Wm Canning is currently developing water-based fluids to operate at higher temperatures. The first water-based HT fluid, Oceanic HW443, is operated by Total and is capable of withstanding 145 degrees C. The oil-based equivalent currently used by Shell, Oceanic BTC491, can operate at up to 200 degrees C, depending on the pressure required. From tests it would appear that the pressure has more of an effect on the thermal stability of hydraulic oils than on water-based fluids.

Manufacturers
The main brands of SCFs on the market today are:
Pelagic 100 Subsea Control Fluid
Supplied by Niche Products Ltd
Castrol Transaqua (including HT/HT2/EE1/EE1/EE3)
Supplied by Castrol Offshore
Oceanic HW (including 540/525/443/540E/740/740E)
Supplied by MacDermid Plc

Due to the Deep offshore activities in the Nigeria Oil and gas industry, Epoxy Oilserv is now a supplier of these subsea control fluid to make products readily available to customers as at when needed.