Exploration • Drilling • Field Services
How to make subsea components last longer
Neil Sheward outlines the surface modification techniques aimed at increasing the life of subsea components.
S
ubsea 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. Tis 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. Termal spray coatings, for example, can be used to deposit a film of pure metal, alloy or ceramic oxide onto the surface. Te 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. Termal spray coatings cover a range of processes including high-velocity oxy fuel, plasma, arc wire, flame spray and cold spray. Te 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. Te 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. Tis 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. Te 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.
Tese stresses can be completely reversed with surface compression techniques.
Fig 1. A thermal spray booth in action.
Te most cost-effective and well-proven method of inducing a protective compressive layer is to carry out a controlled shot peening operation. Tis involves firing spherical media at the component surface at a controlled velocity. Te 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. Tese 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. l
For more information ✔ at
www.engineerlive.com/iog
Neil Sheward is technical services manager (Derby) at Metal Improvement Company, Curtiss-Wright.
www.cwst.co.uk
www.engineerlive.com 39
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100