Vibration Suppression Devices for Long, Slender Tubulars (1996)

ABSTRACT A new concept of vortex shedding suppression devices was developed with the aim of meeting the requirements of easy irrstallation and low costs. A laboratory test progromme was undertaken to determine the device performance in terms of their capacity to reduce the level of vibration in both the cross-flow and in-line direction. During the tests, the device efficacy was verified for different incident angles between the flow direction and the pipe axis, and for increased velocities exciting the higher modes. The experimental results showed that the effectiveness of the new devices is comparable to traditional helical strakes. The performance of the new devices decreases for oblique flow and for velocities, where the higher modes lock-on, observations that may also apply to traditional suppression devices. Introduction The offshore industry has often tackled the problem of static and dynamic response of slender tubular members of considerable length subjected to hydrodynamic loads of quasi-steady or oscillatory nature. That is the case of drilling and production risers, tethers of tension leg platforms, collecting pipes for deep ocean mining, slender members of jackets, free spans of submarinepipelines, etc. as documented in the subject literature. Different approaches have been developed to simulate the structural response either in 2D or in 3D, taking into account different force or response models, considering the material behaviour and the large displacement-rotation theory in different ways, etc. Sometimes, the static and dynamic analyses are combined, but the most commonly used approach consists in the determination of the quasi-static equilibrium configuration achieved under the long period excitation, and superposition of the dynamic response due to the short period cyclic loads acting on the structure. The offshore industry is currently embarking upon new challenging deepwater projects involving floating production systems with risers and pipelines for transportation of the hydrocarbons. Vortex induced vibrations (VIV) can he detrimental to the slender tubulars applied in these concepts because of the severe fatigue impact associated with the varying stresses resulting from the large amplitude vibrations. To counteract the fatigue impact, the VW need to be suppressed. Vortex shedding suppression devices can be installed on anchoring tethers of floating systems, on steel risers or on submarine pipelines free spanning over irregular seabeds, to assist in controlling the level of fatigue damage due to WV during the design lifetime. Major research efforts have focused on VIV and suppression devices for slender tubulars subjected to uniform flow along their length, often disregarding that in these special applications, the incident flow is variable along the component axis. In this case, to describe the interaction between the fluid and the tubular component is extremely difficult. High velocity gradients generate a multi-modal excitation where the tubular shows both excited zones and damped zones9'?z'w, Suppression devices which are satisfactory for certain excitation regimes may fail under different conditions.