This paper reviews the experience, development and degradation issues of direct-acting riser tensioning (N-line) cylinder piston rod surfaces. A hydraulic cylinder used in a direct-acting riser tensioning system on board a deepwater drilling vessel operates in the splash zone, thus working in an extremely harsh environment. The piston rods are exposed to a corrosive environment in combination with continuous wear action and high static and cyclic mechanical loading. Operational experiences show that the coatings on these piston rods have dramatically shorter life than expected. The goal is to improve the service life of these piston rods. This will be achieved by establishing fundamental knowledge about the combined effects of corrosion, wear and fatigue degradation of selected tribological surfaces such as piston rod surfaces. The experience survey information shows that plasma sprayed ceramic coatings for the N-line piston rod application are being degraded to total surface failure in less than one year of operation, mainly due to substrate corrosion. The N-line piston rods with HVOF metallic coatings on Super Duplex stainless steel substrate typically fail within 2-3 years of operation. Observations suggest that it is the effects of cyclic mechanical loading, possibly enhanced by wear induced failure mechanisms which are the primary cause of failure and not so much corrosion on these piston rods. The experience input data indicate how operational factors and properties of material combinations affect the total degradation. Previous research and qualification on corrosion and wear resistant coatings have focused on standardized test methods, mainly verifying the ability to withstand single degradation mechanisms such as corrosion, wear and mechanical induced failure alone.
Direct acting riser tensioning systems are often used during ultra deepwater offshore drilling, when exploring for oil and gas. These direct acting riser tensioning systems, also called N-line or DAT systems, consists of 6 off long stroke hydraulic cylinders ensuring constant tensioning of the drilling riser as the drilling vessel is operating on ultra deep waters up to 3000 meters (m) and heaving due to sea waves and tide. In order to control the well during drilling for oil and gas from an offshore drilling vessel a riser pipe, with typical outer diameter (OD) of 30 inch is fixed to the seabed connecting the well to the drilling vessel. The riser pipe acts as an extension of the drilling well from the seabed to the drilling vessel drillfloor. For most of the new exploration wells the water depth is between 2000 and 3000 m, thus equally long riser pipe has to be used. A riser tensioning system is a vital system onboard a drilling vessel to prevent the riser pipe from collapsing and losing control of the well. The tensioning system functions just like a giant pre-loaded spring, compensating for the offshore drilling vessels heave movements due to the sea actions. The total tensioning requirement can reach up to 2000 metric tonnes (mT); this load is distributed equally on each of the 6 tensioning cylinders.
