Large-scale experiments of a marine riser

Abstract

The focus of this research is on the development and testing of a large-scale model riser (130 m in length) undergoing high mode vortex induced vibrations (VIV) in the ocean environment. This large scale model will provide an intermediate step between the common riser models (8-10 m in length) that have mainly been used to research VIV to date and the actual 3,000+ m deepwater risers being used in industry today. During offshore drilling operations, marine risers carry mud and debris from below the sea floor, and during production operations they are responsible for transporting oil or gas from the hydrocarbon reservoir to the surface platform. The integrity of a marine riser is therefore critical to the success of offshore drilling and production. The flow of seawater around marine risers is subject to vortex shedding which excites oscillations known as vortex induced vibrations (VIV). When the VIV frequency approaches one of the natural frequencies of the structure, resonance, or lock-in occurs. This results in enhancement of the vibration amplitude of the structure and may have potentially destructive consequences due to high bending stresses and fatigue damage of the riser. At present, the prediction of this phenomenon is one of the most challenging areas in the offshore industry. In experimental investigations of VIV, large aspect ratio risers, namely long cylinders with relatively small diameters, pose a modeling challenge. This paper focuses on the development, field testing and data analysis of a large-scale model riser capable of transmitting data in real-time under realistic operating conditions. The large scale riser was mounted to the port side of a small vessel and towed off the east coast of Newfoundland through a series of vessel velocities corresponding to realistic operating conditions for deep-water risers. This paper includes an analysis of the modal vibrations experienced by the riser as well as the in-line and cross stream motions, over a series of vessel speeds.