Neil Sheward outlines the surface modification techniques aimed at increasing the life of subsea components.
Subsea is one of the most hostile environments in which to operate critical equipment. With pressures increasing by 10bar for every 100m of depth, plans for ultra-deep oil sites only serve to increase the challenges. Materials used in these applications are often chosen for their surface properties and characteristics. This usually means expensive alloys with long lead times and difficult machining attributes. It is, however, possible to engineer the desired surface requirements using a variety of surface modification technologies.
Thermal spray coatings, for example, can be used to deposit a film of pure metal, alloy or ceramic oxide onto the surface. The coating can be selected to meet the exact demands of the operating environment; wear resistance, corrosion resistance, thermal insulation, electrical conductivity/insulation chemical resistance, erosion resistance, and so on.
Thermal spray coatings cover a range of processes including high-velocity oxy fuel, plasma, arc wire, flame spray and cold spray. The operating principle is the same for all and involves heating particles, usually to a molten or semi-molten condition, and propelling them at high speed onto the component surface, which is pre-roughened. When the particle hits the roughened surface it flattens and then shrinks as it cools, gripping the surface and creating a strong mechanical bond. The coating is then built up in layers to the desired thickness.
Extending fatigue life
Another critical area for subsea components that can be improved by surface modification is fatigue life. This can be extended considerably by inducing a layer of compressive residual stress into the component surface. Fatigue cracks propagate as a result of cyclic stresses that are often far lower than the static design stress considered for the component. The damage is cumulative and permanent, with failure occurring when cracks propagate to an extent where the remaining section is unable to withstand the application of a single load. Tensile residual stresses, often introduced during manufacturing, are effectively added to the operating stresses, accelerating fatigue damage. These stresses can be completely reversed with surface compression techniques.
The most cost-effective and well-proven method of inducing a protective compressive layer is to carry out a controlled shot peening operation. This involves firing spherical media at the component surface at a controlled velocity. The impacts produce spherical dimples in the material surface, causing elastic plastic deformation. When properly specified and applied, the compressive layer produced by shot peening can be driven deep enough below the surface to sit below pre-existing and post-manufacturing initiation sites such as pits, scratches and notches.
Shot peening can also be combined with thermal spray coatings to provide a protective layer beneath the coating interface, improving both surface characteristics and fatigue resistance.
These techniques can be applied to components such as mandrels, drill collars, rock bits, ball valves and valve bodies. In addition to improving the performance of new components, thermal spray and shot peening technologies can be used in the repair and overhaul of worn and damaged components.
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Neil Sheward is technical services manager (Derby) at Metal Improvement Company, Curtiss-Wright.