THERMAL SPRAY FOR OFFSHORE APPLICATIONS

Thermal spraying of aluminum has been used to protect offshore platforms from corrosion for over two decades.  Combined with a paint coating system, the thermal sprayed aluminum (TSA) can provide corrosion protection of more than 50 years.  Aside from this better known thermal spray application, other applications are being used to provide wear protection to components used for offshore oil drilling.  Some these examples are provided below.

Plasma spray alumina-titania ceramic coatings, in combination with a nickel chrome bond coat, are applied onto offshore hydraulic components to protect against wear and corrosion.[i]^, [ii]  This combination of coating material is the same as those successfully used by the US Navy in nanostructure form to protect the bearing regions of propulsion shafts against wear and corrosion.[iii]

Gate valves are extensively used in the offshore oil industry to contain and control high-pressure and high flow rate fluids.  Certain surface regions of these valves, the seal faces, require protection from both erosion and abrasion from the sand present with the hydrocarbon fluid.  In 1988, a study estimated the cost to replace or repair damaged valves at around £30 million per year. [iv]  Although both WC-base and Stellite[v] coatings are used to protect the sealed surfaces, it is the former that dominates in numbers.  It is the favorable combination of high strength/hardness and fracture toughness that provides the excellent wear resistance in HVOF-applied WC-CoCr coatings.[vi]

HVOF coatings have gained considerable momentum as a viable alternative to hard chrome plating for numerous applications, including for drilling and pumping equipment.  Parts such as chokes, blowout preventers, mud pumps, sucker rods, heat exchangers and separators, packers, safety valves, and various types of downhole tools benefit from carbide-base and hard metal coatings.[vii]  Although line-of-sight requirements, higher application costs, inability to coat internal diameters do limit its use, thermal spray can provide cost savings and superior protection on large surfaces. 

Spray and fuse coatings are extensively used to protect drill bit stabilizers from severe abrasion, impact and erosion.[viii]  WC particles are added to further enhance the wear properties of the self-fluxing metal matrix.  These spray and fuse composite coatings are also used to reduce wear:

• between the internal radial bearing and its external bearing of a drive assembly for a mud motor used in directional drilling; and 
• of the screw conveyer surface in drilling fluid separators.

The information provided above is a cross-section of the various thermal spray applications used in the offshore oil industry.  Many more applications are proprietary to applicators and OEMs and are not publicly disclosed.  It is safe to say that as thermal spray, especially the HVOF process, proves itself to be a cost- and technically-effective alternative to hard chrome, the list of thermal spray applications for offshore applications will surely increase.

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[i] http://www.offshore-technology.com/contractors/corrosion/plasma/

[ii] United States Patent 5,077,139 – Coating Applied To Piston Rods of Hydraulic Cylinders – Dec. 31, 1991.

[iii] Lawrence T. Kabacoff, “Nanoceramic Coatings Exhibit Much Higher Toughness and Wear Resistance than Conventional Coatings”, The AMPTIAC Newsletter, Spring 2002, Volume 6, Number 1.

[iv] B. Vivien, Selecting process valves for in-service reliability, Proc. Eng. (1988) pp. 33-36.

[v]http://www.stellite.com/ProductsServices/Equipment/HVOFApplications/tabid/350/Default.aspx

[vi] D.W. Wheeler, R.J.K. Wood, Erosion of hard surface coatings for use in offshore gate valves, Wear 258 (2005) pp. 526-536.

[vii] S. Moore, Hard Chrome Plating…Its Past, Present and Future, http://www.pfonline.com/articles/web120301.html.

[viii] K.W. David Hart, D.H. Harper, M.J. Gill, Case Studies in Wear Resistance Using HVOF, PTAW and Spray Fusion Surfacing, Thermal Spray Surface Engineering via applied research, May 8-11, 2000, Ed. C.C. Berndt, ASM International, Materials Park, OH, pp. 1117-1125.

George E. Kim, Ph.D.

F.W. Gartner

Perpetual Technologies, Inc.

email: gkim@perpetualtech.ca