Extracting modal parameters of offshore structures under ambient excitation is crucial for the structural health monitoring. In this paper, a procedure combining Natural Excitation Technique (NExT) and Polyreference Complex Exponential (PRCE) method to identify modal parameters of an offshore jacket platform under ice excitation is presented. First, a numerical study is conducted to verify the method using displacement response under simulated ice load. The selection of reference channel is found to play an important role in modal parameter identified result. Then the measured response acceleration signals obtained from a sea testing were analyzed applying the proposed method. Modal frequencies and damping ratios as well as corresponding mode shapes identified from different segments of the whole response time history are in good agreement, which validates the proposed methodology.
As the most common kind of offshore structures, steel jacket-type platforms have been widely used in offshore oil and gas exploitation. These platforms are subjected to various kinds of environmental loadings. During an offshore structure's service life, structural damage caused by environmental loads would be continuously accumulated. The damage may result in a significant change in the modal properties of the platform, such as natural frequencies, damping ratios and mode shapes. Thus, the modal parameter estimation, which is the process of determining modal parameters such as natural frequency and damping ratio from test data (Maia et al., 1997), of an offshore platform structure, is the essential step for modal-based damage detection. The dynamic testing of off shore platform is performed in the field, and the most practical means of exciting the structure is ambient test which is defined as the excitation experienced by a structure under its normal operating conditions (Farrar et al, 1999). The offshore platforms are subject to ambient excitation from sources such as wind, waves, ice, etc. The input ambient excitation is often unmeasurable. Therefore, the modal parameters of these structures must be identified by output-only modal identification methods.
The study of extracting modal parameters of structures from response testing data has a long history (James III et al., 1995). There have been plenty of different approaches, including both frequency domain such as peak-picking from Power Spectral Density (PSD) functions and time domain methods such as Ibrahim Time Domain (ITD) method, the complex exponential method (Prony's method) and Eigensystem Realization Algorithm (ERA) and so on. Most of the current modal identification techniques have been based upon the measured data being the frequency response function (FRF) or the equivalent impulse response function (IRF) (Maia et al., 1997; Ewins, 2000). However, ambient excitation does not lend itself to FRF calculations because the input excitation cannot be quantified (James III et al., 1995).
