SUBSEA PRODUCTION Control fluid considerations mount as field development heads deeper

Simon McManus
Wm Canning

• Comparison of water and oil-based fluids viscosity under pressure.

• Variation of hydrostatic head according to water depth.

• Pressure effect on thermal stability using Oceanic BTC 491 production control fluid.

• !-- FROM PAGE 46, NOT 48 --Relative toxicity of commonly used and recent Oceanic fluids on three marine organisms: the figure quoted is the concentration required to inhibit the growth of (EC 50) or to be fatal to (LC 50) 50% of the test species.

Hydraulic fluids designed for use in subsea systems are relatively low cost, and may lack the high-profile appeal of the engineering expertise that goes into subsea templates and umbilicals. But if the hydraulic fluid is not fit for the purpose, there is no point in the umbilical carrying it out to a subsea template that cannot operate.

For those of us who have to design equipment around hydraulic fluid, or hydraulic fluid around the equipment, it is comforting to realize that fluids have been developed to cope with the challenges of production from water depths of 3,000 ft or beyond. There are a number of factors to be considered when applying experience and knowledge to hydraulic fluids in extremely deep waters:

• Most systems under discussion will have to be operable over long distances.

• The extreme pressure exerted by the weight of the sea water can affect the operation of the systems.

• Many of the deepwater reservoirs are themselves under high pressure and temperature. Fifteen years ago, operating an hydraulic fluid under these conditions was unthinkable. Even five years ago, there were no fluids available specifically designed for these type of operations.

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.

As the pressure of a system increases, the thermal stability decreases. Further development of hydraulic fluids currently is focusing on higher temperatures with the up and coming environmental legislation influencing, but not controlling, the chemicals used in formulations.

From assessing the use of hydraulic fluids in very deep waters, it would appear that a balance has to be found between the low viscosity, high density and incompressibility of water-based fluids and the comfort of using a synthetic hydrocarbon at high temperatures.

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