Estimating Quadratic Transfer Functions for Floating Structures using Model Test Data from Irregular Sea States

摘要

This paper describes and validates a method for estimating quadratictransfer functions (QTF) for floating structures based on modelexperiments with irregular wave loading. With the QTF for a specificfloating moored structure the slow drift forces acting on the structurecan be estimated. This serves as input in the design phase of e.g. themooring system. One method for experimental determination of theQTF involves a large number of bi-chromatic wave conditions in orderto cover the two-dimensional frequency space that defines the QTF.This is a time consuming and tedious approach. In the present study, amethod based on a time-domain analysis of measurements of forces ormotion responses of a floating structure was implemented. The aim isto develop a practical engineering tool that, based on a limited numberof irregular sea states, can estimate the QTF for a floating structure.The method was validated by three test cases: 1) a synthetic responsesignal generated based on a known QTF, 2) a measured force signal ona fixed structure, and 3) a measured surge displacement signal on afloating moored structure. The synthetic test signal showed that themethod is capable of reproducing the test QTF. For the real caseexample where wave force measurements from the fixed structure wasapplied a good agreement was found between measured forces andthose reconstructed based on the estimated QTF. For the floatingstructure some discrepancies were seen between measured andreconstructed motions. The method was found in general to beapplicable for estimating the QTF. It is recommended to apply themethod based on a measured wave force signal on a fixed structure.This requires that a single irregular time series with forcemeasurements on a fixed structure should be incorporated in theexperimental test program when the estimation of the QTF is ofinterest.