Hydraulic Fluids for Offshore Applications – the Lubrication Mechanisms of Water-Based Fluids
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Components used in offshore drilling applications such as hydraulic cylinders wear out earlier than expected due to the combined effect of multidegradation processes like friction, wear, mechanical stresses and corrosion. The origins of these degradation mechanisms lie at the atomic/nano-scale of the whole system and can cause enormous economical loses, and potential environmental and safety consequences. This PhD thesis was, therefore, initiated with an overall objective to establish a fundamental understanding of the offshore hydraulic cylinders failure mechanisms together with an approach of improving the tribological performance of the system. In order to increase the lifetime of hydraulic cylinders, three different approaches were selected: - Improving the frictional performance of the hydraulic cylinder system by matching seal materials with the lubricants used in the system. - Reducing frictional and wear losses by the application of soft-polymer brush coatings on the piston rod surface. - Development and formulation of new water-based hydraulic fluids with the improved performance as compared to the currently used ones. In the first part of the PhD thesis, two different materials were tested in lubricated reciprocating pin-on-plate contact geometry using a water-based lubricant. During this study, it was revealed that the proper selection of the seal material plays a crucial role in reducing the friction of the system. Materials used for seal applications like Ultra High Molecular Weight Polyethylene (UHMWPE) showed very good compatibility with water based lubricants and improved the tribo-performance, while Polyketone (PK) was discarded as potential candidate due to the high friction and wear rates when tested under the same lubricating conditions. The very good frictional performance of UHMWPE was explained by the formation of thick transfer film layer on the countersurface, which activated an easy shear mechanism and therefore caused both friction and wear reduction. In addition, it was found that the incubation of both UHMWPE and PK in the water-based lubricants showed a beneficial effect on friction and wear, which was explained by the visco-elastic behaviour change of the polymer structures. The effect of the water content on the water-based lubricant on the tribological performance of the seal materials was also studied. In UHMWPE, increasing the water content in the lubricant resulted in a steady growth of the transfer film which could be related to the decrease of lubricant viscosity and consequently to the shift of the system towards the boundary lubrication regime. The second part of the PhD thesis focused on the friction reduction strategy of the hydraulic cylinder based on the potential application of a soft polymer brush coating on the piston rod. Within this work, two different types of polymer brush coatings (poly(lauryl methacrylate) – PLMA and poly(vinyl pyrrolidone) – PVP) were grafted-to a silicon surface and subsequently investigated with a microtribometer, atomic force microscopy and interferometric profilometer. PLMA brush coating could significantly reduce friction and wear under boundary lubricating conditions for both pure synthetic non-polar based fluid (poly-α-olefin, PAO) and polar lubricant (polyethylene glycol, PEG). However, when water was added to the polar lubricant (PEG), the frictional reduction of PLMA brush coating became negligible. For water-based lubricants, a new type of brush (PVP) surface modification strategy was suggested and studied in detail. It was found that under pure aqueous lubrication conditions, the PVP brush coating could deliver endless ultralow friction for a tailored combination of coating thickness and lubricant composition. Unfortunately, the addition of PEG (which is a crucial ingredient in the hydraulic fluid for the industrial application) in the water increased the adhesion between the two sliding surfaces leading to wear of the coating. The last approach of this PhD thesis involved the formulation of a new water-based fluid consisting of water, pour point depressant, thickener, corrosion inhibitor and friction modifiers. Since, friction and wear reduction was prioritized, a great attention was paid to find a water-compatible friction modifier. For this, a thorough study dedicated for understanding the friction reduction mechanisms of saturated carboxylic acids with a chain length varying from 10 to 18 carbon atoms in polar media (i.e. water) was performed. The work was first started from understanding and comparing the adsorption mechanisms of carboxylic acids in both polar and less-polar solvents. The work was subsequently continued with a tribological study of the lubricants additivated with carboxylic acids in a laboratory scale ball-on-disc tribometer. During this study, the effect of carboxylic acid concentration and the chain length was evaluated in both water-based and synthetic non-polar (poly-α-olefin, PAO) lubricants. It was observed that in both cases (polar and less-polar lubricants), the surface coverage of carboxylic acids increased with increasing the length of carboxylic acid chain. In polar lubricants, very dense multi-layered formation was promoted, whereas in the less-polar lubricants carboxylic acids adsorbed to the surface by evenly spreading on it rapidly. Friction reduction seen for carboxylic acids dissolved in the non-polar lubricant was not as efficient as in the case of the polar lubricant. This was explained by the more pronounced multilayer formation of carboxylic acids in the polar lubricants, which enabled higher friction reduction as compared to the adsorption of carboxylic acids in a dense monolayer form seen in the less-polar lubricant. Under water-based lubrication, a decrease in friction was found for all tested carboxylic acids with a hydrocarbon chain longer than 12 carbon atoms. On the other hand, wear reduction was only seen for the carboxylic acids with a chain length longer than 16 carbon atoms when the concentration of carboxylic acids was low. However, this could be reversed by increasing the concentration of the shorter carboxylic acids above the Critical Micelle Concentration (CMC). In addition, a set of experiments to study the effect of artificial seawater contamination in lubricants formulated with carboxylic acids was designed and performed in order to match the offshore working conditions. It was found that when the lubricant is contaminated with seawater, the frictional performance becomes worse while the wear is improved. Indeed, by increasing the concentration of seawater in the lubricating fluid, the ability of carboxylic acids to adsorb to the steel surface decreased leading to higher friction. This was explained by the formation of polyelectrolyte complexes of metal ions (seawater) and carboxylic acid molecules. These carboxylic salts improve wear resistance of the system due to the formation of micelles, which additionally support the load and separate the surfaces in contact. To improve the harmful effect of seawater on frictional performance, a chelating strategy was successfully implemented by using Ethylenediaminetetraacetic acid (EDTA). Moreover, an approach of determining the most suitable pour point depressant for hydraulic cylinder applications was made. Among the effective glycol candidates were: monoethylene glycol, diethylene glycol and triethylene glycol. However, it was observed that the thermal degradation stability of glycols decreases when increasing chain length of the glycol and can be also accelerated in the presence of copper ions. The typical degradation products of glycols were oxalic acid, glycolic acid and formic acid. Frictional performance of glycol-based lubricant was also dependent on the additives dissolved in the system. However, a general trend of decreasing friction for increasing glycol chain length was observed.