Investigations into Observed High Frequency Mooring Line Dynamic Behaviours

摘要

As part of improvements to the integrity management practices for two in-service FPSO mooring systems, detailed reviews of existing video inspection data were undertaken. These reviews identified separate high frequency mooring system dynamic responses in frequency ranges higher than typical wave frequency responses. In relatively benign conditions associated with ROV-based inspection activities, torsional vibrations in deep water wire rope sections were observed. In a moderate water depth system, vortex induced vibration (VIV) excitation of a wire rope segment, leading to high frequency oscillations of chain links within the chain hawse tubes on a turret moored system, were also observed. These oscillation frequencies were typically an order of magnitude higher than those arising from wave frequency motions. The implication of these observed high frequency motions was that there were potential fatigue and wear degradation mechanisms that had not been accounted for in the mooring system design. A review of the observed high frequency response mechanisms - torsional vibrations and VIV excitation of mooring line segments - was performed followed by an assessment of the implications on mooring system design and ongoing integrity. The theoretical foundations for the two mechanisms were identified, and the frequency and amplitude of the resultant motions and component stress ranges evaluated. The degradation effects of the two mechanisms were then assessed in terms of both the core mechanisms and the child degradation processes they may initiate (e.g. wire rope segment VIV leading to increased chain wear). A review of different mooring system configurations and water depths was performed to identify the propensity for these mechanisms to occur and their potential for adverse impacts on mooring line integrity were evaluated. For each mechanism, the key influence parameters on the observed high frequency response were identified. The potential impacts on overall mooring system integrity were evaluated, along with the implications for the inspection strategies to verify the ongoing condition of the mooring components. These mechanisms show that mooring system designers must be aware of the dynamic responses that can be excited with typical mooring systems. Without paying due attention to these potential effects at the design stage and during operation, the degradation mechanisms that may be initiated by these effects may be detrimental to the overall long-term integrity of the mooring system.