Rolling Behavior of a Ship with Fin-Stabilizers in Heavy Seas Simulation Analysis and Training Course Development
US Merchant Marine Academy, USA
F. Eugene Guest
MarineSafety International, USA
Abstract: A cruise ship equipped with fin-stabilizers encountered heavy seas. The stabilizers were functioning in a very effective manner, and the ship was proceeding smoothly at cruising speed in heavy quarter-astern-seas. However, after several hours of heavy loading, the fin-stabilizers ceased to function. Immediately, the ship began large rolling, causing injury to some of the passengers. Rolling dynamic behavior under the observed heavy sea conditions was examined in a series of simulation runs including with fin stabilizer effects. Results obtained in the study indicated:
1. The fin-stabilizer was very effective under heavy quarter-astern-sea conditions.
2. However, heavy loads on the fin-stabilizers were clearly evident under the given sea conditions.
3. Slowdown was effective to avoid large rolling motions and heavy load on the fin-stabilizer.
Accordingly, in order to prevent similar incidents in the future, the following two guidelines are described for shiphandling in the quarter-astern-seas:
a. Resonance-avoidance maneuver
b. V/T diagram guidance.
In the late afternoon one day in mid-January, a cruise ship started the voyage from Long Beach, California towards Puerto Vallarta, Mexico. After the departure, the ship encountered rapidly deteriorating weather conditions. The ship was proceeding on a southeasterly course at a speed of 18 knots in the quarter-astern sea (see Figure 1) with fin stabilizers out and working properly. Approximately at 22:35, the ship encountered heavy rolling motion, causing injury to some of the passengers on board. The first engineer found that the portside fin stabilizer was damaged.
A simulation analysis was made to examine rolling behavior of this cruise ship under the sea conditions during that evening. The ship is a 590 ft (177 meter) long cruise ship, equipped with two 13,300 HP main engines. At the time of departure, forward and aft drafts were 23.6 ft and 23 ft, respectively. The ship is equipped with Denny-Brown fin stabilizers. Figure 2 shows the perspective view of the unit. Outreach (span) and chord length of the fins are 4.382 meters and 1.8 meters, respectively, as shown in the figure.
At 16:00, the cruise ship started the voyage from Long Beach towards Puerto Vallarta. The following are the meteorological and oceanographical conditions encountered by the ship during the late evening on that day:
From 16:00 through 19:00, winds were noted to have increased to WSW Force 8 by 20:00, per entries in the ship's deck log. Some three hours later, i.e., about 22:00, the deck log noted a wind shift to the WNW at Beaufort 9. (Figure 3 shows NOAA's sea state photograph as an example for Beaufort 8 and 9.)
According to the deck log, at 22:00, the ship position was proceeding on the southeasterly course at speed of 18 knots. Stabilizers were out and working properly. Wave data from NDBC were (see Figure 4a):
1. HW = 7.3 meters, TW average = 9 sec
2. Wave and wind direction = 310 deg
Figure 1 A Ship in Quarter Astern Sea
Figure 2 Fin Stabilizer
Sea State Examples for Beaufort 8 and 9