By larsas | Tue, 10/25/2022 - 08:31

EXWW and VXWW

EXWW (Ekofisk eXtreme Wave Warning) and VXWW (Valhall eXtreme Wave Warning) are two long lasting special forecast services; at Ekofisk for Philips Norway since 1991 (now ConocoPhillips) and at Valhall for BP Norway since 2004 (now AkerBP). Focus is to warn in due time for waves above given thresholds so that necessary safety measures can be taken offshore. Criteria for these actions to be in effect (started) are directly comparable to forecasted values. A group of 7 scientists from the division of Oceanography and Marine Meteorology with expert knowledge on wave observations and forecasting models are trained for this purpose. Procedures for communication and forecasting are developed in close contact with the end user and revisited every year if necessary.

The detailed forecasting service early showed the necessity of having good wave measurements; Instruments fail from time to time, backups are needed. Also, sensors  mounted on platforms may measure altered wave fields in some wind or wave directions due to disturbances from platform constructions. The high focus on waves has initiated deployment of several wave sensors, making Ekofisk one of the best equipped in that sense. Two famous freak or rogue waves are measured: the Andrea Wave (on 7. November 2007) and a triple rogue wave known as the Justine Sisters (on 30. November 2018). Frontier research is being done with stereo video camera systems to analyze the aerial distribution of rogue waves, their dynamics and tendency to break (or not), among others. 

Bilder:

Waves, example
Figure 1: Part of a large wave near a platform in the North Sea.
Waves, example
Figure 2: A high crest breaks through the supporting construction of a bridge
Ekofisk
Figure 3: Ekofisk oil platform
Valhall
Figure 4: Valhall oil platform

References: 

Magnusson A.K. and M.A. Donelan, 2013: The Andrea wave.     Characteristics of a measured North Sea Rogue Wave. Journal of Offshore Mechanics and Arctic Engineering, Aug 2013, 135(3): 031108. OMAE-12-1062, https://doi.org/10.1115/1.4023800.      

Magnusson, A.K., K. Trulsen, E.M.Bitner-Gregersen, O.J.Aarnes, M.Malila, 2019: “Three sisters” measured as a triple rogue wave group. Proceedings paper of OMAE2019, June 9-14, Glasgow, Scotland. https://doi.org/10.1115/OMAE2019-96837            

Malila, M.P., Barbariol, F., Benetazzon A., Breivik, Ø., Magnusson, A.K., Thompson, J., and B. Ward, 2022: Statistical and dynamical characteristics of extreme wave crests assessed with field measurements from the North Sea. Submitted 2022 to Journal of Physical Oceanography.

Benetazzo, A., Serafino, F., Bergamasco, F., Ludeno G., Ardhuin F., Sutherland P.,  Sclavo, M., Barbariol, F, 2018 : Stereo imaging and X-band radar wave data fusion: An assessment. Ocean Engineering, Volume 152, 2018, Pages 346-352, ISSN 0029-8018,https://doi.org/10.1016/j.oceaneng.2018.01.077