Norwegian Deepwater Program: Analysis of Vortex-Induced Vibrations of Marine Risers Based on Full-Scale Measurements

Abstract

Abstract As a part of the Norwegian Deepwater Program (NDP) three drilling risershave been instrumented with accelerometers and rotation-rate meters formeasurement of vortex-induced vibrations (VIV). In addition, current wasmeasured at number of depths. The paper describes how the riser displacementswere derived from the measurements and compared with the current. A major taskhas been to rid the acceleration measurements of the influence of gravity dueto the riser's rotations out of the vertical and include the measurements ofangular motion in a consistent way. This has been done using modaldecomposition and a least-squares method to estimate the modal weights. Themain purpose of the work was to provide data for calibration of computerprograms for prediction of VIV. Examples of results are given. Introduction To design marine risers for use in deep waters it is important to be able topredict the character and amount of vortex-induced vibration the riser mayhave. In general VIV will be important with respect to fatigue as well as dragforces on the riser. The VIV response of a marine riser is a complicated process involving boththe hydrodynamical and the structural properties of the riser. Model testinghas given valuable insight in VIV. Different types of experimenting have beendone, e.g. forced motion with rigid cylinders in a uniform flow (Ref. 1), spring-supported rigid cylinders in uniform flow (Ref 2) and scaled risermodels in uniform and sheared flows (Ref. 3). Ref. 4 gives a comprehensiveintroduction to the phenomenon of VIV in general, while Ref. 5 is an account ofthe state of art when it comes to VIV of marine risers. Still, full-scale data are needed to verify VIV models at realistic Reynoldsnumbers and in realistic currents that vary with depth. The task of obtainingdata by instrumentation and processing of the measurements is not trivial and, if done incorrectly, may lead to results of questionable value.