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
