Drilling out or working within small sizes of casing and liners requires the use of a drill string with small outside diameter tool joints to fit inside the casing/liner bore and, at the same time, a large enough connection internal diameter to pump actuating balls inside the drill string when needed. These requirements significantly limit the available options that can be used.

Historically, a drill pipe double shoulder connection with a 3⅛-in. outside diameter (OD) has been used for such operations, as it allows for multiple makeups and breakouts before it needs to be repaired. This is a great improvement compared to using small tubing premium connections that are somewhat limited on the number of makeups. However, the geometry constraints are such that the thin material envelope leads to torsional weakness in the connection, resulting in a higher than expected recut rate as connections can be overtorqued downhole in operation.

A research and development (R&D) project was commissioned to improve the connection performance significantly to mitigate the downhole overtorque. Exploring the acceptable connection envelope limits allowed for a slightly reduced internal diameter (ID) when compared to the previously used connection. The team considered different thread designs and decided to use the one that would provide the highest torque. The design process was then followed to develop and qualify a well-balanced connection.

The design validation was performed at an engineering technology center in Houston, Texas, where samples were destructively tested to compare the actual capacity of the new connection against the calculated values. It was confirmed that the torsional strength of the new design meets and exceeds the theoretical value, an improvement of at least 85% over the previously used connection, and a first string was built. It was subsequently deployed in the field and the recut rate was monitored to establish that the objective of delivering a connection capable of higher torque was indeed met to resist the downhole overtorque.

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