Some rotating machinery units on an off-shore oil platform were experiencing high vibration exceeding accepted levels. Failure to a bearing, electrical connection, or supporting structure would cripple drilling operations and/or create a safety hazard. With little information to go on, 6D relied on its experience to determine a potential root cause and engineer a candidate solution to be subsequently verified.
Determining the Root Cause
6D was called after earlier attempts to correct the problem had proven unsuccessful. Field troubleshooting rarely follows any game plan; and this was no exception. Because of the remote location and urgency of the problem an initial diagnostic trip to collect data was not an option. 6D was provided with minimal information (pictures of fatigue crack damage and design model) yet needed to arrive on-site with a solution in-hand.
During the design phase, the unit’s system integrator had constructed a Finite Element model of the system and predicted the base twist mode well above the operating speed. Hence, no problems were expected. However, reviewing the model 6D saw problems with some of the component stiffness representations. Based on their experience 6D realized that a base twist mode was likely coinciding with the operating speed and was probably the source of the problem. A more accurate model was developed using more realistic stiffness. This more accurate model was used to develop candidate fixes: one aimed at stiffening the base twisting vibration mode and one aimed at stiffening the base bending mode.
Implementing the Solution
6D arrived on-site with solution candidates that could be adjusted if necessary to account for other variables. On-site testing indicated that the base twist mode was indeed aligned with the operating speed. A fix to stiffen the twist mode while not affecting the bending was quickly installed. 6D was also prepared to stiffen the bending mode if needed. Either way, the problem would be solved.
Results
After implementing the improvements, 6D ran tests to validate the solution during the same on-site visit. Testing confirmed that the twist mode fell exactly where predicted and the bending mode remained well below the operating speed. The units were quickly brought back on-line and up to full operating capacity.