This paper describes and validates a method for estimating quadratic transfer functions (QTF) for floating structures based on model experiments with irregular wave loading. With the QTF for a specific floating moored structure the slow drift forces acting on the structure can be estimated. This serves as input in the design phase of e.g. the mooring system. One method for experimental determination of the QTF involves a large number of bi-chromatic wave conditions in order to cover the two-dimensional frequency space that defines the QTF. This is a time consuming and tedious approach. In the present study, a method based on a time-domain analysis of measurements of forces or motion responses of a floating structure was implemented. The aim is to develop a practical engineering tool that, based on a limited number of irregular sea states, can estimate the QTF for a floating structure. The method was validated by three test cases:
a synthetic response signal generated based on a known QTF,
a measured force signal on a fixed structure, and
a measured surge displacement signal on a floating moored structure.
The synthetic test signal showed that the method is capable of reproducing the test QTF. For the real case example where wave force measurements from the fixed structure was applied a good agreement was found between measured forces and those reconstructed based on the estimated QTF. For the floating structure some discrepancies were seen between measured and reconstructed motions. The method was found in general to be applicable for estimating the QTF. It is recommended to apply the method based on a measured wave force signal on a fixed structure. This requires that a single irregular time series with force measurements on a fixed structure should be incorporated in the experimental test program when the estimation of the QTF is of interest.
