| dc.description.abstract | The service life of offshore stainless steel cylinders used for riser tensioner systems is lower than expected. This is due to the complex degradation mechanisms, like corrosion, wear and fatigue, that take place under the harsh environmental conditions encountered in the North Sea. After extensive research focused on increasing the service life of the riser tensioner systems, a viable solution using uncoated, super-duplex stainless steel has showed promising results. The disadvantage of using this system is that the low hardness, super-duplex stainless steel piston, is found to wear of at the mid-stroke area. This indicates that there are some tribological conditions that are promoting wear. Several lubricants and seals have been tested to improve the wear resistance of the system, however this thesis is focused on improving the water-glycol, fire-resistant lubricants used for offshore hydraulic cylinders.
By testing different commercially available polymer thickeners under both hydrodynamic and boundary lubrication conditions, the lubricating behaviour of the fluids that are used in piston cylinder applications have been evaluated. The studies show clear indications that high-molecular weight, non-Newtonian polymers are performing poor in hydraulic applications. A low molecular weight thickener with Newtonian behaviour is preferable.
The most promising thickener for improving the water-glycol lubricant was found to be Thickener C. This is a low molecular weight polymer that provided near-Newtonian shear stability, low degree of temperature-viscosity dependency, low COF at hydrodynamic lubrication and lower wear in boundary lubrication than the reference lubricant in use today. Thickener C may improve the service life of the direct riser tensioner system by reducing the wear at boundary lubrication and COF at hydrodynamic lubrication. | |
