The invention relates to a hydraulic fluid for offshore use. The invention further relates to a riser tensioning system comprising the hydraulic fluid.

Hydraulic fluids are used in e.g. hydraulic systems for the transfer of power. Ideally, the fluid in a hydraulic system should be able to last for the lifetime of the system. Therefore, hydraulic fluids should preferably be compatible with all the seals, guiding bands, and metals and alloys inside the hydraulic system, and they also need to have the necessary viscosity for carrying the loads of the system. Preferably, they should contribute to in crease preservation of the system, e.g. by providing low friction in the seals and comprise corrosion inhibitors to avoid corrosion of the metallic parts of the system.

Hydraulic fluids used in offshore applications additionally need to comply with very strict regulations such as fire resistant and non-explosive properties. This forces the use of large quantities of water in the formulation of the hydraulic fluid which limits the perfor mance of the fluids. The choice of additives in such fluids is therefore of great importance for obtaining an optimal performance. For example, for water-based hydraulic fluids, the additives need to be soluble in water. One of the main drawbacks in using hydraulic fluids in environments where seawater is present is that the additives added to them need to be compatible with the salts present in the seawater, as there may be a slow exchange of seawater from the surroundings with the hydraulic fluid. Since some hydraulic systems are to be used for many years, even an extremely slow exchange may result in large prob lems over time. The issue may be particularly important for hydraulic systems in downhole applications where the pressure can become very large. Another drawback of using water in a hydraulic fluid for offshore use is that the conditions offshore may cause the hydraulic fluid to freeze if the water content is too high or too low. In order to control the freezing point of the fluid, it is necessary to formulate it with a suitable ratio of water and pour point depressant, typically a glycol. Water-based hydraulic fluids typically comprise a thickening agent to improve the viscosity of the hydraulic fluid. Depending on the operating conditions of the system, some degree of viscosity of the hydraulic fluid is needed to keep a fluid film for lubrication. Suitable thickening agents may be organic polymers which at low temperature occupy small vol- umes and therefore have a low association with the bulk lubricant. In situations where the temperature is raised, an increase in surface area of the polymer is expected, and there fore the association of the polymer with the bulk lubricant increases. As a result, the vis cosity of the hydraulic fluid increases. Unlike mineral oils, polymer-thickened hydraulic fluids exhibit non-Newtonian behavior. This means that when the fluid is subjected to shear stress, its viscosity will drop until it approaches the level of polymer-free fluid (shear thinning). Since the thickening efficiency of a polymer is typically proportional to its molec ular weight, high molecular weight polymers are commonly selected to increase the vis cosity. However, for higher molecular weight thickening agents a larger drop of viscosity is seen after the shearing (see Q. J. Wang and Y.-W. Chung, Encyclopedia of tribology, vol. 150, no. 1 -2, 2013).

A thickened hydraulic fluid should maintain a critical viscosity level under high shear rates typically encountered in the hydraulic system. It is therefore necessary to minimize the shear thinning behaviour of the hydraulic fluid, which in consequence might cause exces sive wear of the sliding components. One known thickening agent is polyalkylene glycol copolymer prepared from 75 mole% of ethylene oxide and 25 mole% propylene oxide. This copolymer thickening agent has been widely used in many water-based hydraulic fluids under the trade name UCON 75-H. The recommended concentration necessary to thicken water-based hydraulic fluid may reach 18%. Another known group of thickening agents is polyethers with molecular weights between 20000 and 40000 Dalton. Examples of such polyethers are pentaerythritol, trimethylopro- pane, and glycerol-initiated ethylene oxide/propylene oxide polyether. A final concentra tion between 15% to 20% is recommended to reach ISO VG 46 (46 mPas at 40 °C) vis cosity. The thickening action of these polymers is based on the solvation of macromolecules in water. The decrease in viscosity due to shear reaches around 2% (ac cording to DIN 51382 test) (C. R. Rasp, A. G. Leverkusen, and W. Germany,“Water- based Hydraulic Fluids Containing Synthetic Components,” J. Synth. Lubr., vol. 6, pp. 233-251 , 1989.) Hydrophilic and hydrophobic polyether urethane may be used together with emulsifiers. The thickening effect is due to the interaction of hydrophobic ends of the emulsifier mole cule with hydrophobic regions of the base thickener that form bulky micelles which has a hydrophobic nucleus and hydrophobic outer area. This thickener is characterised with a self-healing behaviour which occur when shearing and tearing of lubricant occurs 8 (C. R. Rasp, A. G. Leverkusen, and W. Germany, “Water-based Hydraulic Fluids Containing Synthetic Components,” J. Synth. Lubr., vol. 6, pp. 233-251 , 1989).

One of the main disadvantages with the hydraulic fluids used today is that they tend to form sludge, which may cause the system to function poorly and parts of the system to be worn down faster. The sludge can be removed or filtered, but this causes system down time while removing the sludge or changing the filter. This problem may be particularly difficult to overcome in water-based hydraulic fluids where the possible choices of addi tives may be more limited.

The invention has for its object to remedy or to reduce at least one of the drawbacks of the prior art, or at least provide a useful alternative to prior art. The object is achieved through features, which are specified in the description below and in the claims that follow.

The invention is defined by the independent patent claim, while the dependent claims de fine advantageous embodiments of the invention.

More specifically, the invention relates to a hydraulic fluid for offshore use, the hydraulic fluid comprising water, a pour point depressant, a chelant, a friction modifier, and polyvi nylpyrrolidone as a thickening agent. This combination provides a hydraulic fluid which forms significantly less sludge during long-term use offshore than prior art hydraulic fluids do. Additionally, polyvinylpyrrolidone is hydrophilic, non-toxic, biodegradable, and stable to high levels of shear stress, which makes it an advantageous choice as thickening agent.

The concentration of polyvinylpyrrolidone may be 0.5-30 wt%. More preferably, the con centration of polyvinylpyrrolidone may be 0.5-25 wt%, and even more preferably the con centration of polyvinylpyrrolidone may be 10-25 wt%.

There is also disclosed a hydraulic fluid for offshore use, wherein the hydraulic fluid com- prises water, a pour point depressant, a chelant, a friction modifier, and a thickening agent, wherein the thickening agent may be carboxymethylcellulose, polyacrylic acid, pol- yalkylene glycol, polyvinyl alcohol, or a combination of these. In such a hydraulic fluid, the concentration of the thickening agent may for example be 0.5-30 wt%, 0.5-25 wt% or I Q- 25 wt% for carboxymethylcellulose, and 0.05-30 wt%, 0.05-25 wt%, or 0.05-5 wt% for pol yacrylic acid. Polyacrylic acid, preferably with a high molecular weight such as e.g. 1250000 Dalton, can absorb and retain water molecules which causes a swelling effect of the polymer. Polyacrylic acid is known to be relatively chemically stable, so it does typical ly not undergo any chemical reactions under moderate to slight severe thermal conditions. Carboxymethylcellulose is a modified cellulose that easily dissolves in water. It is chemi cally derived from natural cellulose and it is an abundant, cheap resource. Furthermore, it has the advantages of being non-toxic, biodegradable, and biocompatible. The applicant has discovered that the undesired formation of sludge in prior art may be caused at least partly by the degradation (e.g. polymerisation) of the thickening agents used. For example, thickening agents based on polyethylene glycols have shown the ten dency to form sludge under high temperature conditions around 80 °C, and even at tem peratures above 60 °C. In addition, these types of thickening agents are not suitable for applications operating under elevated temperatures due to phase separation. The sludge formation may for example be catalysed by the presence of metal ions such as copper, aluminium and iron.

On the contrary, polyvinylpyrrolidone in combination with a suitable chelant does not con tribute significantly to the formation of sludge in the hydraulic fluid, whereby the hydraulic system can function properly for a longer time, greatly reducing the need for costly maintenance.

The chelant can be any molecule which forms chelation with metal ions, for example ni- trilotris(methylene)triphosphonic acid, sodium diethyldithiocarbamate, and/or ethylenedi- aminetetraacetic acid. The concentration of chelant may for example be 0.05-5 wt%. Therefore, when a chelant is added to the hydraulic fluid, it can form chelation with metal ions in the fluid, whereby the metal ions cannot interact with the thickening agent. The metal ions can thereby not catalyse the formation of sludge from the thickening agent.

The friction modifier may for example be octanoic acid, decanoic acid, dodecanoic acid, tetradecanoic acid, hexadecanoic acid, octadecanoic acid, nonanedioic aicd, decanedioic acid, dodecanedioic acid, decanol, dodecanol, propylamine, or a combination of those. Friction modifiers are typical additives for a hydraulic fluid, however, the applicant has discovered that they may also contribute to the formation of sludge. Sludge may for ex ample arise if the friction modifiers are deactivated by the presence of bivalent metal ions, e.g. calcium and magnesium coming from seawater. If metal ions bond with the friction modifiers, these can be deactivated, i.e. lose their beneficial properties. This may result in increased friction and wear on many parts of the hydraulic system, therefore leading to an increase of the temperature of the hydraulic fluid, which may cause the thickening agent to form sludge. In addition, presence of bivalent metal ions may cause the friction modifi ers to be sequestered in the fluid, thus creating a thick precipitate product similar to sludge. The chelant thereby protect the friction modifier against deactivation and contribu tion to sludge formation by forming a complex molecule with any metal ions present in the fluid. Therefore, in addition to preventing the thickening agent causing sludge formation, the chelant also prevents that the friction modifier causes sludge formation.

The pour point depressant, typically a glycol, may also be referred to as an antifreeze agent, and it will ensure that the hydraulic system functions properly also at low tempera tures. The pour point depressant may for example be monoethylene glycol, diethylene glycol, triethylene glycol, or a combination of any of these. A typical concentration of the pour point depressant may be 35-50 wt.% to provide a hydraulic fluid that can work at -20 °C with a mixture of the three mentioned glycols irrespective of the ratio. To operate at lower temperatures, the glycol concentration should be higher, but generally not higher than 60 wt.% as water is also required. If the glycol concentration is lower than 35 wt%, the operating temperature will be higher than -20 °C, which will make the hydraulic fluid less suitable for offshore use.

Water causes the hydraulic fluid to be fire retardant, and a water-based hydraulic fluid is also more environmentally friendly than an oil-based hydraulic fluid. The concentration of water may typically be 35-50 wt.% for the hydraulic fluid to be suitable for offshore use.

In one embodiment, the chelant may be ethylenediaminetetraacetic acid (EDTA). EDTA is highly soluble in water, and addition of EDTA does not require addition of other molecules or ions to the hydraulic fluid, which may be the case for other chelants, for example when added as salts. EDTA is therefore particularly advantageous as a chelant for hydraulic fluids for offshore use, where the hydraulic fluid is desired to function for many years. Any additives which are not required may potentially cause a problem at some stage during the desired life time of the hydraulic system.

In one embodiment, the hydraulic fluid may additionally comprise a corrosion inhibitor, whereby corrosion of the hydraulic fluid can be minimized or avoided. The corrosion in hibitor can for example be morpholine, dimethylethanolamine, diethylaminoethanol, or dibutylamine. The preferred concentration of the corrosion inhibitor depends on the de sired pH of the system, but keeping a pH above 8 is typically desired. In order to achieve this value, a concentration corrosion inhibitor higher than 0.5 wt.% is typically required.

In one embodiment, the hydraulic fluid may additionally comprise an antioxidant, for ex- ample y-tocopherol or L-ascorbic acid. The presence of antioxidants may ensure that oth er components of the hydraulic fluid does not oxidize, thereby possibly destroying the ef fect of said other components. The preferred concentration of an antioxidant is typically higher than 0.1 wt.%.

In one embodiment, the hydraulic fluid may additionally comprise a dye. This may have the advantage than any leaks in the system, also subsurface, will clearly and easily be identified, whereby maintenance time will be decreased.

In a second aspect, the invention relates to a riser tensioning system comprising the hy draulic fluid according to the first aspect of the invention. The riser tensioning system may be particularly useful in offshore and subsea or subsurface applications, were saltwater may slowly bleed into the system.

In the following is described examples of preferred embodiments.

General composition of the present water based hydraulic fluid

The following general composition has excellent corrosion protection and tribological per formance: · 35-45 wt.% distilled water

• 20-45 wt.% pour point depressant

• 0.05-30 wt.% polyvinylpyrrolidone as thickening agent

• 0.05-5 wt.% chelant

• 0.1 -2 wt.% amine

· 0.05-1 wt.% carboxylic acid

The pour point depressant may typically be a glycol, for example monoethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, or a combination of any of these.

Polyvinylpyrrolidone may typically have an average molecular weight between 40000 - 1000000 Dalton. Other thickening agents such as carboxymethylcellulose, polyalkylene glycol, polyvinyl alcohol, and/or polyacrylic acid may also be added in the hydraulic fluid. The chelant may be a nitrilotris(methylene)triphosphonic acid, a sodium diethyldithiocar- bamate and an ethylenediaminetetraacetic acid.

The amine may be a morpholine, a dimethylaminoethanol, a diethylaminoethanol or a dibutylamine. The carboxylic acid may be an octanoic acid, a decanoic acid, a dodecanoic acid, a tetra- decanoic acid, a hexadecanoic acid, an octadecanoic acid, a nonanedioic aicd, a decane- dioic acid, a dodecanedioic acid or a combination of those.

Properties of hydraulic fluid according to the general composition:

Dynamic viscosity: 46 mPas at 40°C

· Newtonian characteristic up to 500 s ^_1 shear rates

The pour point approx. -25°C

Low friction (COF=0.12) under harsh boundary conditions (2 GPa contact pressure, 0.26 cm/s sliding velocity, room temperature, ball on disc con figuration)

Good corrosion protection

No sludge formation at elevated temperatures

Preparation of the hydraulic fluid

Preparation of a typical hydraulic fluid may be conducted as follows. As a first step, pour point depressors such as glycols may be added to the water and stirred until a homoge nous solution is achieved. Subsequently, between 0.1 and 2 wt.% of amines may be add ed to the solution while stirring. After this step, saturated or unsaturated C10-C18 carboxylic acids may eb added to the solution with a concentration not lower than 0.05 wt.%. There after, a chelating agent may be added. Finally, the thickening agent is added. The concen- tration of the thickening agent depends on the type and molecular weight of the agent, and on the level of viscosity desired.

Example 1

Polyvinylpyrrolidone base fluid:

• 52 wt% distilled water

· 20 wt% Monoethylene glycol

• 20 wt% Diethylene glycol • 6 wt% polyvinylpyrrolidone (Mw = 100000 Dalton)

• 1.2 wt% dimethylaminoethanol

• 0.5 wt% ethylendiaminetetraacetic acid

• 0.3 wt% dodecanoic acid When a base fluid with this formula was tested in a ball-on-disc tribometer under harsh boundary conditions (2 GPa initial maximum contact pressure) with a pair consisting of super duplex stainless steel plate and alumina ball, the coefficient of friction obtained was 0.12. The test was performed at room temperature and at a sliding rotating speed of 0.26 cm/s. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodi ments without departing from the scope of the appended claims. In the claims, any refer ence signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.