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

Article

Abstract

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.

Keywords

Currents Wind-generated inertial oscillations Measurements Simulations Northern North Sea 

Abbreviations

c

Empirical damping coefficient

CD

Drag coefficient

Cs

current speed

Csback

Background current speed

Csmax

Maximum current speed during an episode of wind-generated inertial oscillations

CsDir

Current direction, degrees clockwise from north towards which the current is flowing

D0

Mixed layer depth [m]

F

Wind stress force, x component

f

Coriolis parameter, 2Ωsinφ

G

Wind stress force, y component

Hs

Significant wave height

NCS

Norwegian Continental Shelf

φ

Latitude, °N

Ω

Rotation of the Earth, 7.29 × 10−5 s−1

ρa

Air density

ρw

Water density

q

Annual probability of exceedance

τ

Wind stress, vector

τx

Wind stress, x component

τy

Wind stress, y component

θ

Wind direction, degrees clockwise from north towards which the wind is blowing

u

Wind-stress induced current, x component

v

Wind-stress induced current, y component

W

Wind velocity

Ws

Wind speed

Wsmax

Maximum wind speed during an episode of wind-generated inertial oscillations

WsDir

Wind direction, degrees clockwise from north from which the wind is blowing

Notes

Acknowledgements

This work was made possible by funding from the Norwegian Research Council’s Industrial PhD-program (231832) and from Statoil. Sincere gratitude is expressed to chief engineer Simen Moxnes who secured Statoil’s funding.

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Statoil ASAStavangerNorway
  2. 2.Department of Marine TechnologyNorwegian University of Science and Technology (NTNU)TrondheimNorway
  3. 3.Department of Mechanical and Structural Engineering and Materials ScienceUniversity of StavangerStavangerNorway

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