Kjersti Bruserud

Comparison of wave and current measurements to NORA10 and NoNoCur hindcast data in the northern North Sea

The objective of this study is to compare metocean design criteria for waves and currents based on measured and hindcast data and by that provide some insight in the expected differences. At the Norwegian Continental Shelf (NCS), the Norwegian Reanalysis Archive (NORA10) hindcast for wind and waves and the Northern North Sea Current Hindcast Study (NoNoCur) for currents are available. A comparison of NORA10 wave and NoNoCur current data to recent wave and current measurements during May 2011 to October 2015 at four locations in the northern North Sea has been done. For waves, significant wave height (Hs), spectral peak period (Tp), and wave direction are compared, and for currents, current speed (Cs) and direction at two water depths. Scatter and qq-plots of Hs, Tp, and Cs and directional roses, summary statistics, and time histories of Hs and Cs are provided. In addition, the extreme values of Hs are estimated, the conditional log-normal distribution for Tp given Hs is discussed, and the Hs-Tp contour lines are established. Good agreement between NORA10 and measured wave data in the northern North Sea is demonstrated. The NORA10 Hs is found to be slightly more conservative than the measured Hs. The NoNoCur data corresponds well to current measurements in the northern North Sea. However, the NoNoCur data does not correspond as good as the NORA10 data corresponds to measured data. Consequently, NORA10 can be recommended to be used for wave design criteria at NCS, while NoNoCur must be further developed and used with caution.

Uncertainties in current measurements in the northern North Sea

AbstractA met–ocean measurement program of waves and current profiles at five locations in the northern North Sea was performed over a period of approximately 5 years. Despite quality control, the measured current speed data contained more noise than expected and large discrepancies were observed between overlapping current speed data measured by different current profilers at the same locations and water depths. Some of the noise and discrepancies can be explained by the influence from surface waves. The current measurements from instruments attached to a surface buoy indicated that these suffered from the influence of surface waves. Further investigations of the uncertainties in current speed data were carried out through three phases of a current verification study, where both additional current measurements and data analysis were done. Comparisons of overlapping measured current speed showed large deviations, suggesting that the accuracy of current measurements is not as good as the user expects. Thes...

Simulated wind-generated inertial oscillations compared to current measurements in the northern North Sea

Measured current speed data show that episodes of wind-generated inertial oscillations dominate the current conditions in parts of the northern North Sea. In order to acquire current data of sufficient duration for robust estimation of joint metocean design conditions, such as wind, waves, and currents, a simple model for episodes of wind-generated inertial oscillations is adapted for the northern North Sea. The model is validated with and compared against measured current data at one location in the northern North Sea and found to reproduce the measured maximum current speed in each episode with considerable accuracy. The comparison is further improved when a small general background current is added to the simulated maximum current speeds. Extreme values of measured and simulated current speed are estimated and found to compare well. To assess the robustness of the model and the sensitivity of current conditions from location to location, the validated model is applied at three other locations in the northern North Sea. In general, the simulated maximum current speeds are smaller than the measured, suggesting that wind-generated inertial oscillations are not as prominent at these locations and that other current conditions may be governing. Further analysis of the simulated current speed and joint distribution of wind, waves, and currents for design of offshore structures will be presented in a separate paper.

Extreme Loads on a Jacket Based on Joint Metocean Data

In lack of simultaneous metocean data for wind, waves and currents, Norwegian design regulations recommend a combination of metocean parameters for estimation of extreme metocean loads on offshore structures assumed to be conservative. The possible conservatism in the design regulations and also the effect of currents in the estimation of extreme loads are considered. A simplified parametric load model for a jacket, based on waves and currents, is assumed. Both measured and hindcast wave data are combined with different measured current data into load time series and the extreme loads estimated. The extreme load according to the recommended approach is also estimated. This is done at four locations in the northern North Sea. When compared to the recommended approach, the other approaches yield a reduced estimated extreme metocean load. Current is found to have an effect on the total extreme load. The results are intended be illustrative and not suitable for use in design.Copyright