Parametric Rolling of a Car Carrier in a Head Sea

A 200 meter long  PCTC upbound Westerschelde River (photo by K.C.) 
Under certain conditions a large car ship in head seas may experience sudden, unexpected, heavy rolling, a phenomena know as parametric rolling.

I have experienced this type of rolling myself while hove to aboard a 200 meter ( 656 ft) long PCTC (Pure Car Truck Carrier)  in 10 meter seas

On the 10th of March, 2008. (satellite data here) enroute from the English Channel to the Straits of Gibraltar,   about 60 miles SW of Cape Finisterre, we experienced about four hours of high wind and seas starting about noon on the  10th. When the weather first began to deteriorated at about 0800 (8 a.m.)  I began  continuously  and gradually reducing  speed. The  seas continued to become   higher and  closer together and finally, around noon, in 65 + knots of wind and with seas 10 -14 meters (33 - 46  feet), to minimize ship motion  I  turned the ship   into the seas, and reduced engine speed to bare minimum revolutions  required to maintain heading.

At noon, in spite of the huge seas, the ship seemed under control, by adjusting the engine speed between  slow  and half ahead we were able to  maintain steerage and avoid pounding, the ship was pitching heavily but without too much drama.

After about 20 minutes of successfully encountering these huge seas the ship was climbing up the face of one of the bigger seas, perhaps 12 meters, when suddenly and without warning the ship took a sharp, deep, heart stopping  roll to starboard.   This was followed by the same amplitude  roll to port and again to starboard. On the third roll the main engine lost lube oil suction and the automation  shut down the engine due to low oil pressure.

With the engine stopped the high winds caused  the heading to fall off and drifting rapidly (about 6 knots sideways) downwind,  we experienced heavy rolling in beam seas  but nothing as bad as the three big rolls experienced with head seas. Once main engine power was restored I again  turned the ship into the sea and again experienced a series of quick, heavy rolls and for the second time the main engine cut out.

After the second experience of heavy rolling while encountering head seas  I gave up on the idea of maintaining a heading into the sea and instead experimented with different tactics, settling upon running dead slow with the seas more of less on the starboard  beam with the wheel hard over to starboard,  turning up more into the sea when encountering the biggest seas by increasing engine speed and reducing revolutions, allowing the heading to fall off and slowing the ship during the smaller sets.

Over the next four hour the seas gradually diminished and by about 1600 (4pm) we began to increase speed and a couple of hours later we were on our way at close to full speed.

The ship suffered no damage with the exception of a parted wire on the  starboard accommodation ladder, there was no cargo damage. However likely not all the cargo would  stayed lashed in place had the rolling lasted much longer. Even with a crew continually tightening lashings some of the lashing on the  cargo was starting to loosen.

Luckily we escaped with minimum damage, it could have been much worse.

Lessons:

Lesson learned  - avoid heavy weather.  Hidden  flaws,  the so-called latent condition, in this case a hull shape with a propensity to roll in certain head sea conditions,  are more  likely to reveal themselves  when the ship is being tossed about in bad weather, just when  you can least afford to cope with it.

K.C

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- Recordings of head sea parametric rolling on a PCTC (pdf)

In February 2003, the Wallenius PCTC M/V Aida experienced sudden violent rolling in rough head sea southwest of the Azores. Roll angles as large as 50 degrees were read off the bridge inclinometer. When this incident was post-analysed it was found that the conditions, in terms of the relation of wave encounter period and natural roll period, were such that parametric rolling was the most likely cause.

- With big container ship this phenomena become more widely known after the APL China suffered huge losses after being overaken by a Pacific storm in 1998.

- Video of a cruise ship experiencing parametric rolling.


- From: Parametric rolling--the why and wherefore:

The fact that inclining at reduced stability alternates with righting at increased stability can only lead to excitation of roll if this alternation is repeated regularly and sufficiently often. This is only possible at parametric resonance, when the period of encounter approximately equals or is approximately half the effective roll period

Is an Unexpected Wave a Rogue?

This YouTube clip is labeled "freak wave" but is this a freak (rogue) wave, or not? (The wave is at the very end of the clip)



The Ocean Prediction Center (OPC) defines a rogue:

They are generally considered to be unexpectedly high waves which in some instances come from a direction different from the predominant waves in the local are

The wave that came aboard was not much bigger then the ones previously encountered. An experienced mariner should not have been surprised to ship water in that sea. In fact the informed mariner should expect an occasional wave twice as high as stated in the forecast (for details, see below). So - no rogue in this case.

Here is another example of a boat being struck by an unexpected wave, in this case fishing vessel The Aleutian Ballard jogging in reported 40 kts seas and 60 kts winds in the Bering Sea - this clip is from Deadliest Catch:



In this case the wave that struck from the starboard does seem larger then the sea at the time and it apparently came from an unexpected direction.
Was it a rogue? A case could be made that it was but it is not uncommon during heavy weather to encounter two separate swell systems. The boat may have been struck by a breaking wave from a second, larger, swell system.

An unexpected wave is not necessarily a rogue (as seen in the case of the yacht). This raises the question, what size waves should a mariner expect to encounter at any given time? The answer is, as stated above, we should expect an occasional wave twice as high as stated in the forecast.

From the OPC again:

The wave height most commonly observed and forecast is the significant wave height. This is defined as the average of the one third highest waves. The random nature of waves implies that individual waves can be substantially higher than the significant wave height. In fact, observations and theory show that the highest individual waves in a typical storm with typical duration to be approximately two times the significant wave height.

For a specific example from the NWS (PDF file) :

a forecast of 10-foot seas in open waters means a mariner should expect to encounter a wave spectrum with many waves between 6 and 10 feet along with a small percentage of waves up to 16 feet and possibly even as large as 20 feet!

So in a typical storm we can expect a wave approximately twice as high as the height used in the forecast. So what is a rogue?

A rogue wave estimated at 18.3 meters (60 feet) in the Gulf Stream off of Charleston, S.C. At the time, surface winds were light at 15 knots. The wave was moving away from the ship after crashing into it moments before this photo was captured. Photo from NOS
A common definition of a rogue is a wave that is more then twice the significant wave height this is the definition found in Wikipedia.

Digging a little deeper and looking at some other definitions of a rogue wave, the photo above comes from this National Ocean Service site. They define a rogue as follows:

Rogues, called 'extreme storm waves' by scientists, are those waves which are greater than twice the size of surrounding waves, are very unpredictable, and often come unexpectedly from directions other than prevailing wind and waves.

This makes the case for calling the wave that struck the Aleutian Ballard a rogue stronger. We don't know if the forecast included a large swell from that direction but in any case it is not possible to predict when a wave in the open sea will break.

The weather services intended audience for forecasts is the mariner. Scientist and engineers are interested in the phenomena of rogue waves as well. They create and use their own definitions.
For example the book "Rogue Waves" By Michel Olagnon, Marc Prevosto differentiates between what they call classic extreme waves and rogue waves.

Classic extreme waves are rare members of a population of wave events defined by modeling the surface process as a piecewise stationary and homogeneous slightly non-Gaussian random field.

In other words classic extreme waves are rare waves that fit inside the forecast model.

Freak (rogue) waves are defined as:

A freak wave event is an event that represents an outlier (crest height, wave height, steepness or group of waves) when seen in view of the population of events generated by a piecewise stationary and homogeneous second order model of the surface model.

In other words waves that are not predicted by the model, an outlier.

This leaves us with the following:

1. The unexpected wave. - Waves that strike the uninformed and the unwary are often bigger then expected. These waves may be both common and easily explained.

2. Freak or rogue - These terms are defined in different ways. They are unexpected and sometime not understood.

3. Extreme wave - Sometimes used interchangeable with #2 but also may mean waves that are rare and while the may be unexpected, can be simply explained.

So, the unwary yacht was hit by an unexpected wave and the Aleutian Ballard - well, it got slammed by what mariners call "a big one".

K.C.

Here is a good site with some interesting photos and graphics:Freak waves, rogue waves, extreme waves and ocean wave climate

Waves and the concept of a wave spectrum

In Search of Severe Weather has a great post: Rogue Waves

Here is my post: Significant Wave Height - A quiz