Norwegian Deepwater Program: Improved Predictions of Vortex-Induced Vibrations

Abstract

Abstract The paper describes the activity within the Riser and Mooring Project of the Norwegian Deepwater Program to improve the confidence in the prediction ofvortex-induced vibrations of deepwater risers. A drilling riser wasinstrumented to measure the riser vibrations during three deepwater drillingcampaigns offshore Norway. The full-scale data were analysed and used tocalibrate the input parameters of the SHEAR7 analysis program and to benchmarkother available prediction tools. The activity included an experimental testprogram to assess the effect of wave action and/or floater motion on thevortex-induced vibrations of risers. The paper gives an overview of the workconducted and presents the lessons learned. Introduction The Norwegian Deepwater Program (NDP) was formed in late 1996 by the oilcompanies which were awarded deepwater license shares under the Norwegian 15th Licensing Round [1]. The objectives of this 4 years R&D program was tojointly address technological aspects of deepwater developments common to theawarded blocks. The blocks are found in and around the Vøring Plateau in waterdepths ranging from 500m to 1400m. Five different projects were initiatedwithin the NDP:Metocean,Seabed,Riser & Mooring,Subsea, andEnvironmental Issues. The activities described in this paper are a partof the Riser & Mooring (R&M) Project. One of the technological challenges identified as being common to all thedeepwater blocks, was the ability to predict vortex-induced vibrations (VIV) ofrisers with reasonable accuracy. The VIV response of marine risers becomes moreimportant as the water depth increases. There are two main reasons for thisconcern. Firstly, the increased water depth leads to longer and more flexiblestructures, and secondly, the current-induced excitation becomes more importantrelative to the wave-induced excitation as the water depth increases. Theresulting VIV response adds contribution to the fatigue damage accumulation, but may also become a problem for deepwater drilling operations due to theincreased drag loads which follows as a secondary effect of the VIVresponse. The semi-empirical VIV prediction tools available to the industry are allbased on empirical data for sub-critical Reynolds numbers. The main objectivesof the work described in this paper have been to calibrate and validate VIVprediction tools against high Reynolds number data obtained under realisticenvironmental conditions. The work was carried out in three phases.