Reciprocating Compressor Tie Bolt Root Cause Failure Analysis

A three-stage reciprocating compressor on a Floating Production Storage and Offloading (FPSO) experienced multiple incidents of tie bolt failures resulting in costly and disruptive shut downs. The failed tie bolts connect the first stage cylinders to the compressor frame via a distance piece. Metallurgical analysis was performed on tie bolts from various failures, and in all cases the tie bolts failed in fatigue. In most of the cases fatigue cracks initiated on corrosion pits, but in one case the initiation site was an inclusion. Analysis of unfractured tie bolts that had seen the same service as one of the failed bolts found evidence of localized yielding.

The repeated fatigue failures and yielding suggested that stress on the tie bolts was excessive. However, finite element analysis (FEA) predicted acceptable stresses in the tie bolts under typical operating conditions. By design the tie bolts were required to be pre-stressed by tightening to a specified torque in order for the required stiffness to be achieved. The FEA did indicate that a loss of the tie bolt pre-load would result in stresses with a high likelihood of fatigue crack initiation.

Except for a liquid shock event or nut loosening, KHE did not identify any loading scenarios that would yield the tie bolts and/or cause a loss of preload. The root cause failure analysis (RCFA) investigation noted several indications that process conditions and/or process controls could result in liquid carryover to the compressor. Therefore, KHE concluded the most likely root cause of the tie bolt failures was liquid carryover, although the possibility of nut loosening due to vibration could not be entirely ruled out. KHE provided several recommendations to address the loss of bolt pre-load, and to investigate the suspected liquid carryover issues.