ABSTRACT

Drill string elements, used in the oil & gas industry commonly employ rotary shouldered connections. These connections are either standard API connections or connections that are developed proprietarily to provide increased torque capacity, fatigue performance and higher connection reliability. A wide variety of alloy families are used for drillstring components, and they exhibit very different galling behavior during make up. The non-magnetic austenitic stainless-steel drillstring components from the CrNiMo family or the nickel alloy family are prone to galling at the connection. This study focuses on thermally diffused zinc (TDZ) as an effective solution to galling issues seen on rotary shouldered connection. The effectiveness of the coating was studied by performing both multi-cycle, lab-scale galling tests as well as full-scale connection testing. Its corrosion performance was studied using electrochemical methods such as open circuit potential and cyclic potentiodynamic polarization, along with high temperature immersion testing. Its corrosion and fatigue impact on the different substrate materials were evaluated using rotating bending fatigue testing and stress corrosion cracking tests. Finally, metallurgical characterization tests were performed to understand the diffusion process and phase formation at the interface on the different alloy substrates. The results from this study established that TDZ is an effective gall resistant coating for CrNiMo austenitic stainless steel-based drill string components.

INTRODUCTION

Rotary shouldered connections are commonly used in the oil and gas industry as the preferred connection method between drill string elements. A wide variety of materials are used in the drillstring make up, each with different properties and galling behavior. Non-magnetic austenitic stainless steel (AuSS) alloys are frequently used in measurement/logging-while-drilling (M/LWD) tools. CrMnN AuSS were commonly used to produce M/LWD tools and have decent galling resistance but are prone to failure due to stress corrosion cracking (SCC) during operation. 1-3 As drilling environments become increasingly more aggressive in terms of temperature and environmental composition, more corrosion-resistant alloys such as CrNiMo AuSS or nickel-based alloys are becoming the material grade of choice in M/LWD tools. However, these alloys are prone to galling at connection sites leading to a need for reworking these connections. Historically, to combat the galling problem of these alloys, "saver subs", which are short subs made from a copper alloy are used. The copper alloys which meet the required strength for use as saver subs, also have low toughness and fatigue strength and fracture often in service. Thermally diffused zinc (TDZ) coating is a potential candidate for use as an anti-galling coating that can withstand multiple connection make-ups.

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