Why the roll?

So with yet another video of a big container ship taking big rolls I’ve got a question for everyone. Why?

Nothing wrong with a heavy roll if it’s unavoidable, it’s the ocean, shit happens, move along. But for a container ship in the midst of the North Altantic with nothing but deep water all around it seems like it would be avoidable.

But my experience is all small research vessels, we were top-heavy as is, and if we weren’t able to then we’d just be breaking gear and people for no reason in any sort of roll so we’d get out of it if we could or do our best to minimize it. What’s the difference here? Is making the schedule important enough that the ship can afford to have every piece of gear that isn’t tied down flying around the bridge? Does a container loss not bother the bottom line? Does loss of life not occur. I watch these videos and think to myself “all that is stopping that ship from capsizing and killing everyone are lashing bars and the hope that the deck crew knew their stuff”.

So is it just part of the job, an order, or poor seamanship, that leads to these situations?

Looks like synchronous rolling from a long, heavy swell from the quarter.

It’s tricky to deal with this situation because you can sail for hours with little rolling and then take a series of heavy rolls. Because of the ship’s speed and relative swell direction, small changes in the wave period result in large changes in the ship’s behavior. If a less then major course change is made it might not help because the rolling will then occur with a different set of swell

The rolling can usually be stopped by a slight course change till that particular set passes then return to orginal course. Sometimes just putting the rudder over is enough to break the roll momentum. Problem is that often the bridge watch doesn’t realize that the ship is rolling synchronously until about the third roll. Even then mariners who have never encounted this type of rolling are left not knowing what to do. Even if they know it still takes fairly good sea-sense to accomplish. I"ve known some very experienced mariners to get in trouble this way.

That makes a lot more sense, thanks. Think I experienced that effect on Georges Bank then. Only spent two weeks on the ship I was on so was chalking it up to the ship or the mixed seas but it seemed like we could get out of the rolls for a few minutes then it suddenly would pick back up. Now I’ll know what to look for assuming I ever get back out.

The underlying problem is that we haven’t incorporated necessary safeguards against (1) stability loss in waves, (2) parametric rolling and (3) vulnerability to surf-riding and broaching in Intact Stability rules yet. Present IMO Intact stability rules analyze and provide safeguards as per the behavior of the vessel only in calm water (uniform waterplane). Second Generation Intact Stability criteria under development by IMO would incorporate safeguards against all the above-mentioned three dangers. I haven’t seen the video you mention in your post (no link!) but Kennebec Captain’s post seems to suggest that the container vessel is experiencing parametric roll.

Parametric roll basically happens because of change in waterplane area as a wave passes along the ship - in waves, the vessel’s waterplane area keeps continuously changing. The changing waterplane area in turn changes the GM of the vessel, and leads to changes in restoring forces acting on the vessel due to the changes in GM (and hence GZ). The other important requirement is that the encounter frequency and natural roll frequency of the vessel attain a certain ratio.

The criteria that lead to parametric roll to kick in are highly complex relationships between:

(i) The length and depth of the vessel
(ii) Difference in GM between the highest and lowest drafts of the vessel
(iii) Wave encounter frequency of the vessel at the time of operation (which are also affected by both its velocity and heading)
(iv) Operation speed of the vessel
(v) Appendages that increase damping (eg. bilge keel)

A ratio that may be signify a probability of parametric roll is GMa/GMo > 0.12, GMa is the mean of the difference between the lowest (Gmin) and highest (Gmax) GM for one passage of a wave crest along the ship hull, and the GMo is the mean of Gmin and Gmax. Each vessel has its characteristic speed envelope where it is vulnerable to probabilistic rolling. Merely satisfying these criteria does not make parametric rolling a certainty, but there are further tests that may confirm parametric rolling conditions, and it typically involves calculating the maximum roll angle, which is a fifth degree equation. In case the damping is sufficiently large to overcome the energy gain from waves, the roll will decay, and no parametric resonance roll will be experienced. In case the roll equation’s solution does not decay (ie, insufficient damping), it develops into parametric roll.

It has typically been observed that containerships, (unfortunately) RoPax and cruise vessels are most susceptible to parametric rolls. Tankers, bulk and gas carriers and naval vessels are considered “safe”. General cargo vessels, fishing vessels and workboats come in the “grey” zone.

From my personal experience, there was a case where a sister vessel of a decently operating vessel was found to have parametric roll issues after elongating it by a mere 20 ft. The problem was solved by installing a huge anti roll tank just below the pilot house - this also goes on to show that making a vessel more top heavy does not necessarily make it more susceptible to parametric rolling - it is a combination of factors mentioned above that land a vessel in the danger zone. I concede however that cruise vessels, RoPax vessels and general cargo are typically more top-heavy than a naval vessel or a tanker.

IMO has guidelines for operation in dangerous situations. MSC.1/Circ.1228 “REVISED GUIDANCE TO THE MASTER FOR AVOIDING DANGEROUS SITUATIONS IN ADVERSE WEATHER AND SEA CONDITIONS” is a good guide for Master Mariners. It gives guidance on speeds and angle of encounters to avoid depending on the length of the ship, time period of the wave, etc.

Incidents of parametric roll should decrease once the second generation criteria are incorporated into the IMO Intact Stability Code in the near future. Problems like parametric roll can and should be tackled at the design stage than leaving these problems to Mariners to slug it out with the elements in high seas.

For further information on Second Generation Intact Stability criteria, I suggest reading “The Second Generation of Intact Stability Criteria: An Overview of Development” by W.Peters, V.Belenky, C.Bassler, K.Spyrou, N.Umeda, G.Bulian, B.Altmeyer - SNAME (2011).

In case someone has never seen parametric roll, here’s an experiment we did at the university with a model of a RoPax vessel in head seas: