This research study focused on regular and random wave interaction with a pair of long, flexible cylinders in close proximity, representative of Tension Leg Platform tendons or risers in 1006 m (3300 ft) of water. The objective was to study the mean square and extreme response of the cylinders with an emphasis on modeling the collision behavior of the cylinders. Also of interest was the modification of the wave-induced response as a function of spacing and orientation of the cylinders with respect to the incident waves. Due to the complexity of the phenomenon and the inherent uncertainty associated with the wave kinematics, the study comprised of an extensive large scale experimental investigation of the phenomenon and analysis of the extreme response and collision behavior in a probabilistic framework.
The experiments were conducted in the deepwater wave basin at the Offshore Technology Research Center (OTRC) at Texas A&M University. A consistent methodology for the distorted scaling of flexible deepwater structures was developed and applied to the design of the models. Unique instrumentation and technique were also developed to estimate the inline and transverse displacement fields of the cylinders. Other measurements included the cylinder tension and reactions at the supports.
The single cylinder data were analyzed with an emphasis on understanding in the wave-structure interaction. Comparisons between the inline response predicted using a standard finite element model, and the measured experimentally indicated the deficiency of the finite element model to predict the response accurately, Interference ratios, comparing the paired cylinder root mean square response to that of a single cylinder, were determined as a function of orientation and spacing.
The relative motion process between a pair of cylinders in tandem was studied for long duration random wave simulations. The hydrodynamic coupling between the cylinders was identified as an important collision mechanism in addition to the cylinder pretension difference and spacing. The collision process was modeled by adapting first-passage time and barrier crossing formulations from probabilistic mechanics. Non-Gaussian extreme response estimates were formulated using the Hermite transformation technique. Comparisons between the models and the experimental data showed the Hermite models to predict the extreme response fairly accurately while the Gaussian estimates were unconservative.
