ABSTRACT

After only months in service, a tubing-to-annulus leakage was detected in a subsea gas injection well located on the Norwegian continental shelf. This well had been completed with UNS(1) S41426 martensitic stainless steel tubing. Upon retrieval of the tubing string, the source of the leakage proved to be a full tubing wall thickness penetrating crack which had initiated in and was confined to, tubing make-up tong die marks on the outside surface. Further, the initial investigation showed that the temperature/pressure gauge instrument line encapsulation material had been severely degraded, revealing the galvanized steel bumper wire. This instrument line was clamped onto the outside of the tubing.

This paper describes the failure analysis and laboratory corrosion tests performed trying to replicate the field failure. The investigations proved that the tong die marks had severely cold-worked the external surface of the tubing and, thus, locally altered the metallurgy and hardness. Further, as the galvanized steel wire was exposed, an effective galvanic cell between the zinc coating and the martensitic stainless steel tubing was established, allowing hydrogen-assisted cracking of the tubing.

INTRODUCTION

A gas injection well completed mid-2019, started leaking after only approximately four months in service. The well was initially back flowed for a period of approximately two months before it was put on gas injection service. After another two months on gas injection, a tubing-to-annulus leakage was registered and the well had to be shut in to re-establish all barriers. As a result of the tubing-to-annulus leakage, this well was worked over and the tubing replaced. The retrieved tubing joint with the identified leak was subject to a failure investigation.

The initial investigations identified the location of the leakage, a full tubing wall thickness crack, to be confined to tubing make-up tong die marks on the tubing's outside surface. Further, when the completion was pulled out of the well, it was observed that the temperature/pressure gauge instrument line encapsulation material had been severely degraded, revealing both the instrument line (UNS N08825) itself and the galvanized (zinc coated) steel bumper wire/steel rope (mechanical protection of the instrument line).

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