Subsea control fluids

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Subsea control fluids are broadly divided into two categories, those used for Drilling, for example blowout preventer (BOP) Fluids and those used for Production, Subsea Production Control Fluids. These fluids provides medium to hydraulically control and operate subsea hardwares from drilling rigs or offshore platforms. This article gives an overview of the properties of these fluids and also some information on chemical make‐up, manufacturers and future trends.

Key Properties

Subsea fluids (both BOP and Production Control fluids) must offer some key physical properties to allow them to do their job effectively and without causing any damage to the subsea system.

These properties include:

  • Lubrication
  • Corrosion protection
  • Bacterial protection
  • Stability

In addition there other considerations when choosing a fluid including:

  • Compatibility
  • Environmental profile
  • Cleanliness
  • Equipment qualification
  • Field experience

Lubrication and anti‐wear

Lubrication and anti‐wear additives must offer boundary lubrication and torque reduction, both at low and high pressure and must prevent excess wear on the system. Falex testing and 4 Ball testing are often used to test the lubrication and anti‐wear parameters, so ask for these results to allow you to compare how different fluids perform.

Corrosion Protection

When adding water based fluids to a system containing metal components corrosion is an obvious problem. Typically the pH of the fluid is raised to help protect steel components, but a high pH can cause attacks on yellow metals (for example Al Bronze, Copper) so the pH needs to be carefully balanced and some yellow metals must be avoided.

Protection in the vapour phase is also highly desirable, so it is best to check if VPI (vapor phase corrosion inhibitor)is present in your chosen fluid.

Bacterial Protection

The formation of bacteria can be a major issue for subsea and BOP systems; bacterial infections initial manifest as a bad odour, followed by a rapid drop in pH as acid forming bacteria multiply. As discussed earlier a low pH can damage metals present in the system and at a very low pH the fluid can destabilise and separate. Therefore it is essential to prevent bacteria formation and this is done by adding a biocide or biostat. A biocide will kill the bacteria, but can be consumed and depleted, whereas the biostat will prevent the bacteria from growing. Additionally the fluid must buffer against acid formation. As a bacterial attack can shut down the system it is important to know how your chosen fluid will protect against bacteria.

Stability

The subsea fluid can remain in the system for the whole lifetime of a field, without being fully changed for fresh fluid. Therefore it is essential that the fluid remains stable for a project lifetime, often in excess of 25 years and at high temperatures. Stability of control fluids at temperature is critical in the surface-controlled subsurface safety valve (SCSSV) section as it is a key safety area with long, narrow lines where the fluid is not replaceable.

In the BOP system it is equally important that the fluid remains stable and does not separate, form precipitate solids or insoluble ‘slime’. These problems can occur despite the fluid being regularly displaced during drilling operations. Therefore fluids need to be extensively tested to ensure that they remain stable for the lifetime of the project.

Compatibility

All metallic components/families and internal coatings which come into contact with the fluid must be individually tested to confirm compatibility with the fluid. Additionally multiple metals must be tested in the fluid together to investigate the possibility of galvanic corrosion.

Seawater is very corrosive so all the testing must be repeated with seawater present to observe the interactions of the fluid and metals. Any elastomers which come into contact with fluid must also be thoroughly tested as certain fluids will cause elastomers to swell, harden or soften, which can cause major problems in the system. Due to environmental legislation changes or technical problems it is sometimes necessary to change the control fluid or BOP fluid on an existing asset. In this case it is essential that the old and the new fluid are completely compatible. Extensive testing is performed at various mix ratios, temperatures and with/without the addition of seawater.

Environmental Profile

The environmental profile of subsea fluids is becoming increasingly important as countries around the world tighten up environmental legislation to protect the environment. Each country has it’s own legislation, with Norway considered by many to be leading the way to protect the oceans from the discharge of harmful chemicals.

Norway uses a colour system to classify fluids according to their risk, with black being the most harmful, through Red, Yellow (Y3, Y2, Y1, Yellow Zone) and then Green/PLONOR which are considered to Pose Little Or NO Risk. Black and Red fluids are now banned for discharge and others down the list will continue to be banned in the coming years.

The UK runs a similar system, using the letters A‐E for subsea fluids with A being the most harmful (and banned) to E which is the least harmful and preferred. Therefore it is important when choosing a fluid that the environmental profile is excellent; this will help prevent expensive changeovers of the asset in the future if a harmful fluid is banned.

Cleanliness

All subsea fluids must be microscopically clean to prevent blockages of small orifices and the gathering of sediment in valves. The standard minimum specification is ISO 4406 17/15/12 (equivalent to NAS 1638 Class 6). Fluids should be checked by the supplier both in the mix vessel and in the packaging to ensure the product cleanliness standards are met. It is also important to adhere to clean procedures on the vessel to ensure no contamination is introduced in service.

Equipment Qualification

Before any fluid is used in a subsea system it is essential to check if the subsea equipment used is compatible with the fluid. Fluid suppliers work closely with Original Equipment Manufacturers (OEMs) to qualify fluids with their equipment. Therefore checking what equipment a fluid is qualified with is an important step in choosing the correct fluid for the project.

Field Experience

Extensive laboratory testing is very important in the development and qualification of a fluid, but field experience offers direct feedback on how a fluid will actually work in the field. When choosing a fluid look into what assets it is used on, for what length of time and also the subsea equipment used on asset to compare the performance with similar systems.

Subsea Production 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.

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

The Future

As technological advances are made in drilling, hotter and higher pressure wells (HP/HT) are becoming much more common, which the maximum pressures and temperatures of the subsea equipment increasing every year. This has a big impact on which fluid can be used and extensive testing must be performed on the fluids at high temperatures, to ensure they remain stable for the lifetime of a project.

The highest rated water based fluid with significant field experience has an upper operational temperature limit of 180°C/ 356°F but the highest rated oil based fluid is operational at 250°C/ 482°C. So on new assets it could become the case that oil based fluids increase in popularity as they are able to reach higher temperatures.

Blow Out Preventer (BOP) Fluids

Blow Out Preventers (BOPs) are an important safety device which installed once a well has been started. Drilling is conducted through the centre of the BOP and is controlled hydraulically to seal the well if oil and/or gas pockets are discovered. The BOP has several safety features and can close around the drill‐string or cut it in half to seal the hole quickly. In shallow water jack‐ups and barges use topside BOPs and in deeper water semi‐subs and drillships use subsea BOPs. Subsea BOP fluid is used in these systems, it is used in large volumes so it is sold in concentrate and diluted on board the vessel. Subsea BOP fluids have to be water soluble, environmentally friendly for discharge into the ocean and very reliable.

Chemical Make‐up

BOP fluids are sold in concentrate form and mixed on the asset with fresh or potable water, typically a dilution ratio of 1‐5% BOP fluid to water is used. Additionally BOP fluids are often mixed with Glycol anti‐freeze in cold climates.

It is extremely important to keep the concentration correct to the manufacturer’s recommendation. Letting the concentration of BOP fluid fall too low will reduce the fluids effectiveness to lubricate, prevent corrosion & prevent bacterial growth. Some fluid manufacturers offer a free fluid maintenance service, whereby samples from the system are sent to the laboratories and the fluid is tested for pH changes, bacterial contamination.

Manufacturers

The main brands of BOP fluids on the market today are:

The Future

On the drilling side the current focus for new projects is often in harsh, yet environmentally sensitive environments, for example the Arctic regions in Norway, Alaska and Russia. In these areas the environmental profile of the fluid is extremely important as stricter legislation is often in place. Therefore new products emerging onto the BOP fluid market will need to offer excellent technical properties for these harsh environments, coupled with an excellent environmental profile. Such an example is the Pelagic Green/PLONOR range of BOP Fluids which are specified for use in such areas and meet all the relevant environmental guidelines.


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