Friday, January 29, 2010

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.


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.



- 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


Unknown said...

Sweet Jeebus

Rick Spilman said...

A fascinating and a bit scary topic. It appears that with enough number crunching guidelines could be developed to help a ship's master avoid the critical combinations of speed, roll period and GM. I see that ABS developed a new "parametric roll class notation" about a year and half ago.

New Parametric Roll Class Notation

Paul, Dammit! said...

Blech- I'm having flashbacks to Hurricane Ike- double hull tankers in ballast condition can do the same damn thing in a head sea.

Velu said...

Nice post. Never been in one, and I hope it stays that way. Sailing on car carriers must be tough in bad weather. What with the bridge up front, everytime that thing went towards the water, the heart must skip a beat. I think I like it better on my big tankers. :)

Smooth sailing,

QMC(ret) said...

I've taken some crazy rolls on an LST in the Western Pacific (FYI 53 degrees rings the bell on the bridge) but I don't think they matched those of that cruise ship.

Sammy said...

You did the right thing after try #2...too many people think they have the textbook, logical answer and don't take into consideration the physics of the boat. Good job.