Water intake riser – hydrodynamic damping modelling for fatigue assessment

The Prelude experience has underlined the potential strong impact of both the hydrodynamic damping and any potential superimposed current to properly assess the Water Intake Riser life time, refer to Minguez & al. (2020). This effect, resulting from the considered hydrodynamic modelling, remains qualitatively suggested within the different riser design standards (e.g. either the DNV-GL-RP-C205 2017 or the ISO-13628-7 and ISO 19902 2007) but no quantitative recommendations agree on the way to properly address such a topic. The proposed numerical developments underlined the gap between the different Best Practices and the impact in terms of fatigue assessment. The different recommended damping models are considered and experienced to assess the quantitative impact on fatigue assessment. Based on this critical review, a large-scale model test focusing on a simple riser response is proposed to position the different existing hydrodynamic models. This has been done by imposing forced motions to a rigid cylinder mounted under a spring system. The system, free to move, is plunged within a basin in which different current speed and direction are imposed. Both loads and dynamic responses of the model have been monitored. The numerical developments proposed in the frame of this research suggest both any damping and drag empirical formulation to properly address such fatigue concern from engineering point of view. This work finally recommends that caution be exercised when the DNV guidelines and the International Standard are followed due to the contradiction of the recommendations with the experimental evidences reported in this study.