The Case of The Sandy Oilfield Thread Failure
An oil-well site required a temporary sand trap to allow gases and condensates to empty from down-hose portions of the well. In an effort to implement a speedy fix, a nearby sand trap was placed in service at the well. The sand trap had multiple functions in the operation of a new well. It was not uncommon for a sand trap to vibrate, move and shake at high well pressures with flow pulsations during well clean-out startup. There was inadequate structural supports and as a result the threaded pipe at the sand trap inlet and outlet failed while in service. A worker was severely injured. The worker did not have drawings, specifications or work instruction about the installation.
The sand trap inlet and outlet connection threads were dilapidated, yet the vessel was rated for 3,000 psi. There was visual evidence that the pipe threads were damaged and bent The vessel had not b
een in-service for several years. It had been 10 years since the worker had installed a similar sand trap at another well-site. The worker had never received training for sand trap installation. The sand trap was not on the company’s mechanical integrity list of vessels for routine safely assessments. Cyclic fatigue fractures occurred at nominal stresses below the yield strength of steel pipe after a sufficient number of loading cycles during the brief service-life of a screwed connection. .
The pressure in the sand trap, while attached to pipes from the well, exceeded the intended design capacity of the sand trap threaded pipe connections. The sand trap vessel moved in the cyclic regions of first order of harmonic vibrations while solely restrained by pipe threads of the inlet and outlet. The lack of support at these loads was the cause of the pipe thread failures.
The Lessons Learned:
The pipe threads failed as a result of being unsupported. The threads experienced low cycle fatigue and low-temperature embrittlement from frozen hydrates in the well flow stream. Low-cycle fatigue has a very distinct fracture appearance that was observed on the fracture surfaces of the failed threads.