Running-in of metal-to-metal seals and its influence on sealing ability with application to the design of biodegradable thread compounds

Abstract

Metal-to-metal sealing of casing connections is affected by running-in because it determines the topography of the gap between the contacting surfaces by wear and plastic deformation during assembly under the influence of the thread compound, coatings and the initial surface topography. The work in this thesis concerns the mechanisms related to these elements of the tribosystem and how they affect running-in of the metal-to-metal seal tribosystem and ultimately influence the sealing ability. The research was driven by a need to reduce costs in particular of the qualification of premium connections. Furthermore, increased understanding of the barriers in the well is important for well engineering. Understanding was needed on galling during assembly which is an often occurring failure mechanism as well as the protective mechanisms behind the applied coatings. In addition, the Oslo-Paris Convention for the protection of the Marine Environment of the North-East Atlantic (OSPAR) demands future substitution of mineral oils and the (heavy) metal additives used in the American Petroleum Institute (API) modified thread compound by biodegradable alternatives. To this end a combined experimental and modelling approach was applied. The existing thread compounds were shown, with pin-on-disc, anvil-on-strip and Shell Sealing Mock-Up Rig (SSMUR) tests, to provide relatively minor protection to initiation of galling in uncoated contacts. This was shown to be because of squeeze out of the formed tribofilms, limited adsorption of the additives and the lack of replenishment by the plan parallel contact configuration coming from the turned surface topography. These mechanisms only added 60mm of additional sliding length, before failure, with API modified thread compound compared to a plain mineral oil. In relation to the substitution of API modified for biodegradable alternatives, the elevated temperature degradation mechanisms of (environmentally acceptable) thread compounds were studied using Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), high temperature rheometry and pin-on-disc. Thread compounds were shown to fail because of evaporation and oxidation leading to starved lubrication conditions. The system subsequently enters a severe adhesive wear regime sometimes exacerbated by abrasive action of the hard metal oxide particles present in environmentally acceptable compounds. The found failure mechanisms and the developed test protocol were validated and successfully mitigated by the development of a prototype thread compound. The presence of phosphate conversion coatings proved to be a dominating factor in the running-in of the metal-to-metal seal. This was shown to be caused by two main mechanisms using various tribological experiments and analytical techniques. On the uncoated counter surface a durable tribofilm was formed by physical adsorption of phosphate debris particles through a shear stress activation mechanism. From the same debris particles a smooth glaze layer was generated on the phosphated surface which possesses a substantial hardness after dry sliding. However, this hardness was much lower after lubricated sliding and was shown to be related to the particle size which generated the glaze layer. The combination of these mechanisms resulted in a wear process that could satisfactorily be described by the energy dissipated in the sliding contact. Finally, it was shown with experimental data and a simple running-in model that the combination of plastic deformation of the waviness of the turned surface topography and wear of the phosphate coating determine the running-in behaviour. It was found that the surface runs-in within 40mm sliding length after which the wear regime transitioned to mild wear. The combination of severe initial wear by plastic deformation of the waviness and the generation of a smooth glaze layer created a conformal sealing configuration with multiple concentrated line contacts along the circumference. This created the most robust sealing configuration compared to configurations that did not have a distinct lay.