“The sky darkened and calm waters turned violent when suddenly a colossal, solitary wave emerged from nowhere, like a wall of water ready to swallow the ship whole without a warning”.
This could have been part of the many dangerous journeys described by mariners in the early days of ocean exploration. These tales were often dismissed by skeptics as the wild imaginations of superstitious sailors. However, they actually carried a kernel of truth. Today, we know these waves as ‘freak waves’ or ‘rogue waves’. In this article, we will explore this phenomenon, bridging the gap between ancient maritime myths and modern scientific understanding.
A freak wave is an unusually large and unexpected wave that is significantly higher than its surrounding waves. Stories about these waves have been told for hundreds of years. In the early ages, these stories were considered to be made up or just exaggerations of what actually happened. During the Middle Ages, tales of giant waves were widespread, but the scientific understanding was still very limited. Also, sea monsters were part in explaining the disappearance of ships during this time. Several historical incidents in the 20th century involving 20-30 meter high waves, provided more and more evidence for the existence of such phenomena.
The Draupner wave
The existence of these freak waves was finally confirmed when, on 1 January 1995 at 15 UTC, a freak wave of 25.6 meter high was measured at the Draupner gas platform. This platform is located in the Norwegian North Sea block 16/11, 160 kilometer offshore from Norway (Figure 1).
On that first day of 1995, the North Sea was facing adverse weather conditions due to low pressure near Scandinavia. A hindcast reanalysis was done by Cavaleri et al. (2016) to get a clear overview of what happened on that day. From that reanalysis, it appeared that a so-called ‘polar low’ (a small-scale, short-lived but active depression occurring in cold air masses) caused the high waves over the North Sea. The reanalysis showed a modeled significant wave height close to 11 meter near the Draupner platform at the time of the freak wave. A significant wave height of over 12 meter was modeled just northeast of the platform (Figure 1). This corresponds well to the significant wave height of almost 12 meter that was reported on the platform.
The measured freak wave of 25.6 meter is significantly higher than its surrounding waves. As a general rule, the largest individual wave one may encounter is approximately twice as high as the significant wave height (the average of the highest third of the total waves), which means that waves could not have reached a height of over 24 meters in the Draupner case. The actual measured freak wave could thus not be explained with a significant wave height of almost 12 meters.
Figure 1: the wind direction (arrows) and the significant wave height (color; shaded) over the North Sea on 1 January 1995 at 15 UTC from the Cavaleri et al reanalysis. The black triangle indicates the Draupner platform location.
Definition of a freak wave
In oceanography, a freak wave or rogue wave is officially defined as a wave with a height exceeding two times the significant wave height, like the Draupner wave. Freak waves are primarily generated by wind and wave interactions in the open sea. Waves of different sizes, periods and speeds travel across oceans in various directions. Most waves pass through each other without noticeable effects, but the perfect conditions can cause waves to amplify each other, forming a freak wave.
Freak waves can occur in many sizes and are not necessarily the size of a 4-story high building. Of course, these monster waves are the ones that spark our interest and may cause the most damage. Due to recent advances in satellite observations, freak waves are actually more common than previously thought. It is however, due to the very local character, not too often experienced.
Impact on marine operations
Freak waves are often characterized by their steepness and unpredictability, making them particularly dangerous for maritime activities. In the past they have been known to capsize ships or bringing substantial structural damage to offshore structures as oil rigs and wind turbines. As a result, engineers now account for the potential impact of freak waves when creating safety protocols and structural designs.
Atmospheric models, wave models, and the availability of observations have significantly improved in recent years. However, forecasting freak waves remains almost impossible due to their localized nature and the perfect combination of conditions required for their formation.
Conclusion
Freak waves are no longer just sailor’s tales. Transitioning from ancient myths to modern scientific understanding, we now know that freak waves are both common and a serious maritime hazard. Despite advancements in technology and wave modeling that have improved our understanding, these waves still pose a significant threat to ships and offshore structures due to their unpredictability and destructive power. The Draupner wave continues to drive ongoing research and advancements in marine safety. While we cannot yet forecast freak waves , the near future may hold promising developments in this area.
Sources:
Cavaleri, L., F. Barbariol, A. Benetazzo, L. Bertotti, J.-R. Bidlot, P.A.E.M. Janssen & N. Wedi, 2016: The Draupner wave: a fresh look and the emerging view. Accepted for publication in Journal of Geophysical Research – Oceans.
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