The wave basin at the Offshore Technology Research Center provides the offshore industry and academia a state-of-the-are deep water wave testing laboratory. The basin is equipped with a multi-directional wave maker capable of generating directional regular and random waves. The recent addition of current and wind generating equipments allows modeling of combined load environments encountered in the oceans. The OTRC has previous experience with modeling risers using stain gages and load cells. Duggal and Niedzwecki (1992) reported on the behavior response of single and paired tendons/risers. In particular, the study of the collision process in the paired case required careful integration to obtain accurate displacements (Duggal and Niedzwecki, 1994).
Following this past experience, an experimental study to investigate the response behavior of a single deepwater riser subject to wave and current loading was carried out at the OTRC. One of the major objectives of this study is to investigate the use of underwater cameras to directly measure the riser deflections of the model, thus minimizing the complexity of both displacement data processing and model instrumentation. Underwater cameras have been used prior to this investigation (Tsahalis and Jones, 1981; Hartnum and Airey, 1997). However the details of how the image was processed is not reported in these cases and the investigators indicated some difficulties in their optical setup. The addition of current to the environmental loading is also expected to provide unique data.
The research study is divided into two major parts. The first part involves the design and instrumentation of a marine riser model that represents a prototype riser to be used in 500 meters of water. Inline and transverse displacements are monitored using a new acquired underwater camera and the optical tracking system already in use at the OTRC. The current generations is achieved with appropriate disposition of pump nozzles and valves apertures so that desired profiles are obtained at the target location. Current meters were used to determine those profiles. A test matrix is developed to obtained meaningful wave-current loadings, taking into account the capabilities of the wave basin and the properties of the riser structure itself.
The second part of the study focuses on the analysis of the data. The assessment of accuracy and validity of the tracking technique is established. The wave-current interaction for random waves and variable current profiles are studied. Comparisons to Huang et al (1972) wave-current interaction model are made. Examination of the transfer function relating the current modified spectrum to the initial unmodified wave spectrum is performed. A new finite element software package has been written to simulate the riser response in the time domain. It is based upon the formulation of Gardner and Kotch (1976), however it includes enhancements suitable for representing the presence of current as well as drifting due to mass transport, since a mean offset is expected in the wave/current direction (Dean and Dalrymple, 1991). Comparison of the numerical prediction performed by the finite element program and the experimental data is pursued for displacements and natural frequencies of the riser.
Related Publications: Niedzwecki, J.M. and Duggal A.S., “Wave Run-up and Forces on Cylinders in Regular and Random Waves, ” Journal of Waterway, Port, Coastal and Ocean Engineering, ASCE, Vol. 118, No. 6, 615-634, November 1992.
Duggal, A.S. and Niedzwecki J.M., “An Experimental Study of Tendon/Riser Pairs In Waves,” Offshore Technology Conference, Houston, TX, May 1993, Vol. 3, 323-333.
Duggal, A.S. and Niedzwecki, J.M., “A Probabilistic Model for the Collision Between a Pair of Flexible Cylinders,” Applied Ocean Research, Vol.16, No.3, pp.165-176, 1994.
Niedzwecki J.M. and Duggal A.S., “Collision Mechanisms and Behavior of a Pair of Long, Flexible Cylinders in Close Proximity,” Offshore Mechanics and Arctic Engineering Conference, Glasgow Scotland, June 20-24, 1993, Vol. I, 291-298. (Presentation)
Duggal, A.S. and Niedzwecki J.M. “Regular and Random Wave Interaction with a Long, Flexible Cylinder,”Offshore Mechanics and Arctic Engineering Conference, Glasgow Scotland, June 20-24, 1993, Vol. I, 283-290.
Guerandel, V., Niedzwecki, J.M. and Duggal, A.S., “Marine Riser Model Tests in Waves and Currents,” Proceedings of the 14th International Conference on Offshore Mechanics and Arctic Engineering, Copenhagen, June 1995.