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February 24, 2019

Vessel master, watch out for the dangerous swell!

Image of sea and crystal ball
The conditions at sea can be surprisingly different with same wave height.

Even though the significant wave height is the same on two different days. This article describes how and why this is the case.

Every day is different

Fighting the weather offshore is sometimes rougher than you can tell beforehand. Standing by the coastline and getting ready to head out in very weak winds, thinking that this will be an easy day can be so wrong. You have a look at the weather forecast and the significant wave height is predicted to be around 1.5m, perfect, just as yesterday. 

“Ship ahoy!” time to sail out, but didn’t you forget something? Yes, you did - out in the big blue you’re finding yourself in rougher conditions than yesterday despite same significant wave height and calmer winds. If you work professionally at sea or being a skilled sailor, you for sure already know what went wrong this day - but maybe not why. Regardless of your previous knowledge, keep on reading to learn more.

Definition of significant wave height

First a brief theory on significant wave height. The definition originates from observation of waves and is defined as: “The mean wave height of the highest third of the observed waves” and is abbreviated SWH or Hs.

Different types of waves interfere with each other in the wave pattern. Here the two main waves are the wind wave and the swell and will therefore be part of the total height of the significant wave. These two wave types are physically different from each other. Therefore, the mix of wind waves (also called seas) and swells is important to know, as the wave conditions will differ depending on this.

Here we have the very reason for why the conditions for our sailor in the text wasn’t as yesterday - the mix of wind waves and swells was different. If he would have known anything about wind waves, he would have become suspicious about the significant wave height being the same as yesterday - despite the much lower wind speed. Perhaps, a look at the forecast for swells would have been a good idea in order to avoid the unpleasant surprise of rougher conditions.

Diving deeper
A diver
Diving deeper...

The wind wave is created locally and are based on the wind and fetch in the area – you can learn more about wind waves in this article: Do you know how wind waves form?
Meanwhile, the swells originates from winds from several days before and from an area far away. This means that the swells have travelled much longer before reaching the area and that the swells are totally independent of the wind in the local area.

Furthermore, the features of the wave change during the travel. The wave length and the wave period of the swell becomes longer over time while the height only falls slowly.
Comparing a swell and a wind wave with the same height, means that the swell has a longer wave length than the wind wave. This does also relate to the period, that in general is larger for a swell than for a wind wave.

Jack-up vessels and swell

The rolling of a vessel is partly depending on the wave period – the waves will therefore feel different on the vessel. The swell, that have the larger period, does affect the rolling of a vessel more than a wind wave does, assuming the wave height is the same. This is something you have to take seriously if you are going to be jacking up a drilling rig, an offshore installation vessel or another type of jack-up vessel. 

When the hull of a vessel is going to be elevated from the water surface, it's crucial that the vessel motions caused by the waves are below the design criteria. It's therefore important to know the dominating wave periods as the energy in the waves, that passes a vessel, is higher for the long-crested swell than in the short-crested wind waves at the same height. If the wave period of the swell is near the eigenfrequency (the roll period) of the vessel or platform, it may cause damage to the legs or the entire vessel structure.

Long swell waves will be able to lift or lower the entire vessel. This means that the entire weight of the vessel can be placed on the leg that hits the seabed first. As the vessel also can be moved horizontally by the waves, the directional movement and the mass of the vessel can load an extreme bending moment on the leg. This can obviously cause severe damage and must always be avoid.

A sailing offshore installation vessel
A sailing offshore installation vessel in moderate conditions
Breaking waves

It is not only the rolling and accelerations of the vessel that may cause problems - breaking waves comes with a lot of power. Waves break when they grow too high. This depends on both the width and the height of the waves. A wider wave has a firmer base to stand on while a narrow wave has a more fragile base. The swell with the larger wave length, has a lower likeliness to break than the wind waves when being in open water.

When being closer to shore the story is different, as it is not only the wave length and the horizontal distribution of the wave that matters. The vertical distribution of the wave downward in the water column is approximately half of the wave length - this is the depth of the wave. When the waves reach shallower water, the bottom of the wave begin to interfere with the seabed. This will be pushing the wave together and the former low swell will rise and cause higher tops of the waves that eventually may break.

Still this is not the only trouble waves can do. A dreadful combination in rough and stormy conditions is a combination of the two wave types - from different directions! This can seriously damage or even be sinking a vessel if not being careful. Today’s scope was about a day in a calmer kind of weather, so let us save this topic for another time.

The above text can be summed up like this: Be careful to understand what weather conditions you actually can expect at sea and thus how to interpret the weather forecasts correctly. If you only look at the forecast for significant wave height, you will sometimes get an unpleasant surprise – remember to have a look on the swell forecast as well.
What is the weather going to be like? Swell!