Reciprocating compressor installations on offshore platforms and FPSOs present piping dynamic challenges that don’t exist in onshore applications. Engineers who apply standard onshore analysis methods to offshore installations consistently underestimate the vibration risk.
The Platform Structure as a Boundary Condition
In onshore installations, compressor foundations are connected to grade — effectively to an infinite rigid boundary. In offshore installations, the compressor is mounted on a platform deck or FPSO hull structure that has its own dynamic characteristics.
The platform structure is not rigid. It has natural frequencies of its own, and if any of those frequencies coincide with compressor excitation frequencies, the interaction can amplify vibration levels significantly. An acoustic and piping model that ignores platform flexibility and treats the foundation as rigid will produce non-conservative results.
The Correct Modeling Approach
For offshore applications, Tech Transfer includes the platform deck or FPSO hull structure in the dynamic model. This requires structural data from the platform designer — deck plate thickness, stiffener geometry, and connection details — in addition to the compressor and piping data.
The combined model calculates the natural frequencies and mode shapes of the entire system: compressor, piping, and supporting structure together. This is the only way to accurately predict vibration behavior and ensure that the analysis recommendations will be effective in the installed condition.
Motion Effects on FPSOs
FPSOs introduce an additional complication: vessel motion. Ship roll, pitch, and heave create inertial forces on the compressor and piping that vary with sea state. These quasi-static loads must be combined with the dynamic compressor loads to determine the governing stress condition.
Our FPSO analyses include vessel motion loads as a standard analysis case, combined with the pulsation-induced dynamic loads to determine the worst-case stress condition across all design scenarios.
