Friction and wear of coatings used in subsea valves operating in high temperature and pressure.

Subsea oil and gas production relies on technology for controlling well fluid under high pressures and temperatures. In this technology, valves are a key component for both controlling well fluid and injected chemicals, but also to act as a safety barrier. These valves must create a sealing interface for the well fluid, open on demand, and automatically close if necessary. To achieve this, great attention to the sealing surfaces which see relative motion must be met. Surfaces must have low roughness, provide low opening and closing force (low friction), and have high wear resistance. Sealing surfaces on valves are typically coated with a hard coating of carbides in a metal matrix which provides necessary hardness and wear resistance in addition to relative low friction forces. As oilfields with higher temperature and pressures are pursued, coating technology must be further developed. In this project, friction and wear of coating used in subsea gate valves are investigated. Results from identification of the friction and wear mechanisms together with correlation between small scale laboratory experiments and full scale valve testing will be used for optimizing coatings for subsea gate vales in the future. Coatings are produced by a process known as thermal spraying where combustion of fuel and air creates a high velocity jet flame. The coating material is injected in to the high velocity flame where it is accelerated and heated before impacting the surface to be coated. Successive passes of the flame builds up a coating layer by layer. The coatings of interest in this study consist of hard wear resistant tungsten carbides in a soft and ductile metal matrix of cobalt and chromium. During the deposition of these materials, the high temperatures experienced by the material may lead to decomposition of the carbides and changes in crystal structure. This may in term affect the friction and wear properties of these coatings. Part of this project is to identify the effect of varying the deposition process and the effect of sliding environment in small scale and large scale gate valve sliding interface. A wide variety of thermal spray processes currently exist, with one of the called Warm Spray. This process was developed by National Institute of Material Science (NIMS) in Japan, Tsukuba. This process combines a traditional HVOF thermal spray process with a technique for reducing particle temperature and particle degradation during deposition. As a novel process exhibiting exiting properties of the coating, these were investigated for friction and wear in simulated gate valve testing conditions with interesting results. In this project, tribological testing was done at small scale in various atmospheres and temperatures. The results point out the importance of correctly designing tribological test to better correlate friction and wear between small scale test and full scale valves.

Produksjon av olje under vann (subsea) er avhengig av ventiler i en rekke konfigurasjoner. En stor del av ventilene er såkalte "gate valves" der to flate overflater danner en forsegling under trykk. Ventilene fungerer som en sikkerhetsbarriere samtidig so m de brukes til å kontrollere strømmen av olje eller gass opp fra brønnen. Ventiler i samme konfigurasjon brukes også ved injisering av nødvendige kjemikalier ned i brønnen. Ventilene utsettes for enorme påkjenninger ved operasjon fra høyt trykk, store t emperaturvariasjoner og aggressiv brønnstrøm. Dette stiller store krav til overflatene som skal fungere som forsegling mot brønnstrømmen. Trenden i markedet er operasjon under enda høyere trykk og temperaturer som igjen øker kravene til ventilene. De bev egelige delene i ventilene som skal gi forsegling er tradisjonelt overflatebehandlet for å gi en sterk og slitasjebestandig robust overflate. En slik overflatebehandling består av å belegge delene med et veldig hardt materiale slik som wolframkarbider. Tr adisjonelt har beleggene tålt påkjenningene, men ved høyere trykk og temperaturer har de sine begrensinger. Prosjektet er startet for å forstå hva som skaper begrensningene og hvor disse ligger. En fundamental forståelse i de grunnleggende mekanismene so m degraderer og hvordan disse kan reduseres elle kontrolleres er nødvendig for å utvikle og produsere ventiler som fungerer under de tøffeste forhold.