Simultaneous Stochastic Model of Waves and Currents for Prediction of Structural Design Loads

Simultaneous data of metocean parameters, such as wind, waves and current, of sufficient quality and duration are necessary to establish reliable, joint models of metocean loads and load effects on marine structures. In lack of such joint models, the Norwegian design standard, NORSOK N-003, recommends combinations of metocean parameters for load estimations assumed to be conservative. However, the degree of conservatism is rather uncertain. The possible conservatism in NORSOK N-003 for combinations of wave and current conditions in the northern North Sea has been assessed. To perform such an assessment, precise knowledge about the wave and current conditions is required, as well as simultaneous wave and current data of high quality and long duration. Available measured wave and current data during nearly five years, at selected locations in the northern North Sea, are described. A thorough assessment of the current conditions at these locations is given, with the following important findings; (1) the quality of measured current data is poorer than anticipated, (2) the dominating current conditions at some locations is wind-generated inertial oscillations, (3) the seasonality of current conditions at these locations is very distinct due to the inertial oscillations and (4) significant inter-annual variations in current conditions is found. For waves in the northern North Sea, both measured and hindcast data are found to be of appropriate quality and duration for joint considerations, but neither current measurements nor hindcast have the required quality or duration. To generate adequate current data, a simple model for wind-generated inertial oscillations is applied and validated at one location in the northern North Sea. With that, simultaneous wave and current data of sufficient quality and duration for joint modelling are available and a joint conditional model for waves and currents is proposed. The anticipated conservatism in NORSOK N-003 for load estimations is assessed by a case study. A simplified model for a generic static load on a jacket, caused by waves and currents, is assumed. For the northern North Sea, metocean loads are estimated first according to the NORSOK N-003 recommendation, and then directly from a load times series. Comparison of the two different approaches gives a clear indication that the NORSOK recommendation is not necessarily conservative in the northern North Sea. Due to several simplifications in the steps leading up to the load estimations, this result is intended to be illustrative.