A Qualitative Comparison of Measured and Theoretical Metocean Data and its Impact on Flexible Riser Design

Abstract Dynamic analysis of marine structures is usually based on standard metocean report data, where the environmental data are combined in defined limit states based on return periods. For currents in particular, the available data are often, to which an assumed profile defines the velocity distribution throughout the water column. For design analysis, current is usually defined relative to wave propagation, and a combination of the most onerous scenarios governs the design process, which leads to potentially unrealistically high conservatism in the dynamic analysis. This paper presents the impact of the difference between theoretical current profiles and waves, and in-situ measurements made by a seabed-mounted current profiler at an oil field in the North Sea. A global response analysis is performed using a detailed model of the flexible riser system in OrcaFlex with two different sets of environmental input: one based on actual measurements and the other based on theoretical values from the metocean report. Characteristic loads and motions are extracted from the two analyses, and it is found that the current velocity profile directionalities and magnitudes impact on the riser responses, and that there is no clear trend regarding which hindcast model is most conservative. From the dynamic simulations it is evident that achieving measured data provides an important pointer towards the validity of forecast and hindcast data as illustrated regarding current directionality and velocity as function of water depth.