Partial Reconstruction of Wavelet Transforms: A Technique for Quantifying Amplitude Envelopes of Riser Response Modes

Knowledge of the frequencies and amplitudes and single-mode versus multi-mode behavior of deepwater riser responses is important for accurate fatigue analysis. The extraction of this information from measured riser responses is a significant challenge given the complexity of the phenomena involved with riser vibrations. An existing technique for obtaining modal amplitudes, wavelet ridge extraction (WRE), has been shown to return a smoothed version of the actual amplitude envelope, resulting in attenuated peak values. In this paper, we introduce a new wavelet-based technique, partial reconstruction of wavelet transforms, for accurately extracting amplitude envelopes of the modes from multi-mode signals. The partial reconstruction technique is proposed to circumvent problems presented by WRE by making use of information available from the inverse wavelet transform. It is demonstrated with synthesized test signals that the new technique provides much more accurate modal envelopes than does WRE. Finally, to illustrate the technique for practical riser response signals, the partial reconstruction technique is applied to representative acceleration fluctuations measured from a drilling riser in the Schiehallion field.