Rogue waves turn out to be not that rare

http://www.damninteresting.com/?p=701

Executive summary: rogue waves (>15m high) turn out to be not as rare as thought before, especially in North Atlantic and off the Namibia coast (both known ship graveyards). "During a three-week study in 2001, radar scanning detected ten monster waves in a 1.5 million square kilometer area."

Hmm... implications for design of offshore boats..?

Kaa

 

It amazes me how little thought goes into most "findings" like this. This is old news to anyone who has had to design for the open ocean in the last 40 years and has had access to real-time monitoring. My orginization was pointing out that data supported such occurances as early as 1989 when the NDBC came on line.

Some points that you must always consider.

1) A 15m wave height is actually a very common wave depending on where you are. See the JONSWAP studies. The wave is only of statistical significance compared to the significant wave height.

2) The term "rogue wave" has no place in considered description of seaways. Such large waves are a statistical certianty, not a unusual event.

3) The occurance of large waves is a function of energy available in the seaway, reguardless of significant wave height.

4) From a statistical viewpoint, the propibility of encountering such a wave is actually very good.

Lets take an example of a swell heavy NATO SS 3, Hs = 1.5m, Tp = 8 sec.

In 6 minutes there is a
5% chance of a 5.4m wave (3.6 x Hs)
1% chance of a 6m wave (4 x Hs)
0.001% chance of a 7.6m wave (5.1 x Hs)
a 1 in 1,000,000 chance of a 8.8m wave (5.9 x Hs)

In 100 hours, the heights increase to
5%= 7.8m
1%= 8.2m
1 in 1,000 = 9.4m
1 in 1,000,000 = 10.5m

All this from SS 3. If I was in a storm with a significant wave height of 5m ( say SS 6), I would expect to encounter a 15m wave and I would be designing for a 30m one.

EDIT to add.

As an aside, it is wave slope, not absolute wave height, that makes wave dangerous to vessels. A "steep" or breaking wave that buries the vessel is much more dangerous than a wave of extreme height of "normal" proportions. Express 32's have been sunk in SF Bay by 1m chop, while they commonly sail in 4m swell outside the Gate.

 

Wave Height related to Design

Interesting info, John.

When you say, "I would be designing for 30m" what, does that mean, specifically?

Chris

 

Hs is the standard deviation (sigma) of the wave spectrum. The probability density function of the single largest in “n” peaks is normalized on the standard deviation, i.e. Hmax = Yn * Hs. Yn is a function of number of peaks and the probability of those peaks exceeding Hmax. The more peaks you are exposed to, the higher the probability of exceeding some fixed Hmax. To choose a Yn of ~6 for a marine structure such as a ship or platform, implies a 99% probability of not exceeding the Hmax in the life (~20 years) of the structure. The choice of Yn is an economic and cultural one, but if you want to "within the will of God" NOT to loose the structure then you should select Yn=6. However, there will alway be some small, but real, chance that a larger peak will occur.

Once you have selected Hmax, then you can work through the physics of the wave...such as wave length cannot be less than 7*wave height and other physical effects that drive the design. As I pointed out before, wave steepness may drive a lesser height to be a greater threat, so that must be analyized.

 

Well, that was just an executive summary and so unclear.

Rogue waves are actually a more or less accepted term -- e.g. "Rogue waves, sometimes also known as freak waves, are a special kind of ocean waves that is distinguished by an instant, singular, and unexpected wave profile with an extraordinarily large and steep crest or trough." (http://www.glerl.noaa.gov/res/Task_rpts/2002/ppliu02-3.html) The same source later defines a rogue wave as having "maximum trough to crest wave heights greater than two times of significant wave height".

In fact, there are conferences on these waves -- e.g. Rogue Waves 2004 (http://www.ifremer.fr/web-com/stw2004/rw/index.html).

The problem with a rogue wave is not (only) that it's tall -- the problem is that it's steep and completely unexpected. It is my understanding that such waves are unexplained by the standard (gaussian) models of wave action. There are several models attempting to account for their existence, but no consensus yet, as far as I know.

As to probabilities, it depends on where you sail, of course, but we are not talking about encountering 15+ m waves. We are talking about encountering a single 15+ m wave in 5-7m seas.

Kaa

 

Rogue Discussion on another tread

Welcome to the forum Kaa.

Just a friendly reminder that you might utilize the "search" button above and look at what might have preceeded in discussions on the same subject.

For instance the Rogue Wave subject here:http://boatdesign.net/forums/showthread.php?t=4365

 

Point taken

Kaa

 

No problem. I didn't know how to use that function when I began posting.

I just think its nice to keep very similar subject matter 'together' unless a subject thread gets 'hi-jacked' by some unrelated discussions (happens on occassion)

 

Kaa, totally unpredictable freak waves aint as waves get higher in a storm
prolonged storm, tides, gulfstreams and stormfronts meeting is a know course of freak waves
apart from the hight, waves grow to 1/3 piece of a pie before falling back as said

 

My experience is that smaller vessels see far more damage from the boat falling off the wave and stoving in the impact side than from the breaking/collapsing wave, the damage is far less from a steep to breaking wave that sweeps and buries the boat. Holes and troughs can also occur and are also dangerous .


With ships it’s a bit different the breaking/sweeping wave does damage them they don’t fall off and they don’t give to a big breaking wave either, and suffer the consequences .

Some of the pyramidal waves around 3 times the average wave height seem to be particularly dangerous to smaller sailing vessels.

The sea generates more dangerous unpredictable waves in certain geographical areas for a variety of reasons, the number relative frequency and amplitude of different wave trains.


Interestingly the big southern Ocean rollers are often very manageable and predictable despite their fearsome size.


I’m sure any of us that have spent time at sea will have seen the monster wave rear its bulk between you and the horizon quite frequently.

 

I've been TDY so I couldn't get back to this sooner.

Kaa, there is really not enough bandwidth and I really don't feel like writing a dissertation (and I really can't) on my data or experience. Liu and Wu are just two that are approaching this from a lakes point of view. I would suggest reading Wiegel (Oceanographical Engineering) and Bhattacharyya (Dynamics of Marine Vehicles) for an understanding of the basis of wave spectral analysis. While the apperance of any single wave is not a gaussian distribution (it is a Rayleigh distribution), the formation of single event large waves can be modeled as a gaussian process due to the realities of spectral mechanics (i.e. spectral energy distribution and phaseing interactions). Now there is a lot of ivory tower bickering going on about how to do develop a "realistic maximum wave" in the time domain, however data I have supports classical probability thoery in this respect.

As for large waves being "unexpected"...well duh!...a seaway is a random process..all waves are unexpected and vessel motion phaseing differences account for 90 of structural loading and submergance/emergence. Just understand that regular periodic solution analysis inadaquate and get over it. Energy methods are much more successful.

 

Just one thing with all this "Nature don't understand maths" so you can extapulate all you want with theories and technical stuff but when it comes to the bottom line a big (or should I say B I G wave) normally (note that word) is no problem as long as you ride it, As Mike said above it's a different thing if you span a couple but I guess most of us are talking small boats? The Problem comes as the damn thing starts to break, if your mixed up in that you may have a problem - it would appear a couple of you understand this so why am I reinventing the wheel?

 

Big Wave

In, 2000 I was delivering a 52’ sailboat from Hawaii to San Francisco, we had enjoyed a pretty nice trip most of the way—Jib Top reaching for three days then lifted in medium breeze with a #1 and full main for the best part of 5 days. Then motor sailing at 2200RPM for 3 more days at 40N heading east. All Good! Except for the 1035mb High we were happily motoring through was smashed up against a deep thermal low over California. And there was a lane about 400 miles wide ware the wind was blowing 45+ for weeks. I had been watching this hoping that it would disappear before we arrived at the boundary at about 130W, but it didn’t. Long story short we pressed on, battened down the hatches and about mid way through were nearly capsized by a huge out of the ordinary wave. Ordinary was about 22ft. according to the WEFAX, but this one was a monster, hit us forward of the beam, and knocked us flat, the tip of the 75’ mast was under the horizon but not in the water, some how we made it. It was scary! Common or not, they are out there and they are dangerous.

 

Just a quick update...

For those of you interested, I have now found a fairly good explination of waves and wave spectral mechanics on the internet.

http://www.usace.army.mil/publications/eng-manuals/em1110-2-1100/PartII/Part-II-Chap_1-ppi-60.pdf

http://www.usace.army.mil/publications/eng-manuals/em1110-2-1100/PartII/Part-II-Chap_1-pp61.pdf

http://www.usace.army.mil/publications/eng-manuals/em1110-2-1100/PartII/Part-II-Chap_1-pp62-121.pdf

About 6.1Meg but a rather handy thing to have if you want to get deep into predicting extreme events.

 

Can anyone state the angle (or middle and upper angle if it is curved) of the wave face and back with regards to the horizon? Would like to draw some profiles to see what these beasts look like.

thanks