An investigation of VIV response of a steel catenary riser model

Slender catenary members are common structural elements used in the design of many offshore scientific and engineering applications. These catenary components generally have a small diameter to length ratio and are susceptible to flow induced vibrations. A more complete understanding of this flow induced phenomenon and the resultant response behavior is explored through a series of laboratory measurements. The intent is to provide data in a relatively controlled environment in order to limit the response behavior with the goal of eventually providing additional insight needed as the basis for better prediction methods to be used in the design of prototype systems. In this study the flow induced response behavior of a steel catenary riser model that was towed to simulate excitation by a uniform current flow field is analyzed and discussed. The flow-induced, i.e. vortex induced, vibrations (VIV) are treated as random processes which is a significant departure from the standard approach. The vibrations measurements were obtained using a series of bi-axial accelerometer equally spaced along the model catenary. Different perspectives from which to view the measurements of VIV are presented using coordinate transforms, spectral analyses and probability distributions. The vortex induced vibrations observed in laboratory model tests or in actual offshore systems are developed as a consequence of flow fields where the fluid flow is unsteady and nearly steady. This can be related to laboratory tests where the towing begins when the flow field is essentially quiescent, i.e. the flow velocity is essentially zero, then as the towing carriage increases its velocity to the desired speed for a short time before slowing and eventually stopping as the tow carriage reaches the end of the tow tank. This study investigates the consequence of including or neglecting the effects of these two transient flow speed regions on the interpretation of the response measurements and provides an interesting perspective of this complex response phenomenon.