Riser Interaction Model: A Combined Time/Frequency Domain Approach

Summary

Objective

A time domain model for riser interaction was reported by Huse in 1993 and within the same timeframe a probabilistic model was developed and reported by Niedzwecki and Duggal (1993). Each of the models has limited predictive capabilities that restrict the applicability of the models to practical design. This was recognized and discussed at length at a workshop sponsored by the offshore industry and held in Norway in 1998. As a result, a new round of experiments at Marintek and refinements to the Huse model were pursued. In the experimental study multiple instrumented vertical cylinders were towed at various speeds and cylinder spacing with instrumentation quite similar to the earlier tests performed at the OTRC. However, the focus of the new experiments was on excitation due to uniform current flows. The probabilistic model developed by Duggal and Niedzwecki (1995) considered only deepwater wave excitation of risers with identical properties at various spacings and cylinder positions with respect to the lead cylinder. Issues of variable cylinder pretension and material properties cannot be addressed adequately in part due to the lack of experimental data. Collectively these factors lead to a rethinking of the riser interaction modeling approach by Niedzwecki and Diao in 1998.

A combined preliminary frequency/time domain modeling procedure has been conceived and is presently being explored. The initial results look promising but a serious research effort needs to be initiated to complete the model. Basically, the new model would recognize the nonlinear frequency dependent nature of the wave interactions and utilize classical nonlinear models for interpretive and predictive purposes. The objective of this proposed study is to develop a spacing dependent model that is general enough for potential use in design practice.

Approach

1. Model refinement based upon the analysis of existing wave excited response data.
2. General characterization of nonlinear frequency dependence (complex spectral model) as a function of riser spacing and other key parameters.
3. A limited comparison of new model predictions with high order spectral analysis interpretations in collaboration with Dr. Powers.
4. Pursuit of a closer collaboration with Statoil Norway (Dr. Teigen) and NTNU (Dr. Naess) to gain access to the new Marintek data
   • analyze select data sets from Marintek experiments
   • extension of probabilistic considerations

Anticipated Results

1. New predictive model for riser interaction
2. Verification of the order and types of nonlinear and suggested models

Related Publications

Niedzwecki, J.M. and Liagre, P-Y. (2002). “System Identification of Distributed-Parameter Marine Riser Models,” Journal of Ocean Engineering, Vol. 30, 1387-1415.
Teigen, P. and Niedzwecki, J.M. (1998). “Experimental and Numerical Assessment of Mini-TLP for Benign Environments,” 8th International Offshore and Polar Engineering Conference, May, Montreal, Canada.

Teigen, P. and Niedzwecki, J.M. (2003). “Wave Diffraction Effects and Wave Runup around Multicolumn Structure,” ISOPE 2003, May 25-30, 2003, Honolulu, Hawaii.

Liagre, P-Y. and Niedzwecki, J.M. (2003). “Estimating Nonlinear Coupled Frequency Dependent Parameters in Offshore Engineering,” Applied Ocean Research, doi:10.1016/j.physletb.2003.10.071.

Sibetheros, I. A. and Niedzwecki, J.M. (2003). “System Analysis of the Interactive Behavior of Flexible Cylinders under Unidirectional Wave Loading,” ISOPE 2003, Vol. 3, May 25-30, 2003, Honolulu, Hawaii.

Niedzwecki, J.M. and Liagre P-Y. (2003). “Interpreting Data on Marine Riser Response Behavior Using System Identification, OTRC Deepwater Mooring Symposium, October 2-3, 2003, Houston, TX.

I.A. Sibetheros and J.M. Niedzwecki, “Analysis of single and tandem cylinder data using an orthogonal Volterra model approach” Ocean Engineering, Volume 32, Issue 2, February 2005, Pages 135-156