Tuesday, March 23, 2010

Lessons Learned—A ship capsizes while loading cargo—Part III

Excerpt from U.S. Coast Guard “Proceedings of the Marine Safety & Security Council” magazine. To read the first two parts, see Part I at our March 16 post and Part II at our March 18 post.

The First Lift
The first load went smoothly. The cranes were slewed out and positioned over the wharf and swing ballast was loaded into tanks on the inboard side. The combined effect of the weight of the booms and the ballast listed the ship slightly toward the wharf, as desired.

At this point, someone would have checked the stability pontoon, since it would be effective only if the list did not exceed 2.5°. While someone checked on the pontoon, people on the wharf hooked the generator to the cargo falls, which took a few minutes due to the size and weight of the lifting hardware.

When everyone was in position, the winch operator took a strain. The captain also gave the order to begin filling ballast tanks on the outboard side before the list could get to a point that the pontoon could come out of position and lose its effectiveness. If it had come out of position, the ship might have capsized onto the wharf.

Reaching the point of full tension on the falls, the crew started to slew the booms inboard, exercising caution to ensure the load didn’t get ahead of the swing ballast. From time to time, the movement of the cargo was stopped to allow the swing ballast to catch up to the cargo.

Eventually, the two large port-side ballast tanks were filled and the generator was positioned over the hatch, three meters to port of the ship’s centerline. From here it took a few more minutes to lower it down into the hold. Four crewmen down in the hold unhooked it and began securing it for sea. Unfortunately, they were still in the hold during the second lift.

The Second Lift
The second lift began the same way as the first, except for how the ship was ballasted. The swing ballast from the first lift remained in the large tanks on the port side. Using that setup, the captain could plan on discharging the starboard side swing ballast while the load was slewed aboard.

The derricks were slewed out so the heads of the booms were over the wharf with the 308-ton generator. The boom heads were now 13 meters to starboard from the centerline, which caused a list to starboard. Additionally, the captain had one large starboard-side ballast tank filled to cause a maximum safe list before taking a strain on the cargo runners.

The generator was hooked up with hardware weighing over 20 tons. Meanwhile, the captain and cargo superintendent checked the stability pontoon to ensure the crewmen operating the winches and pumps were in position and ready. Additionally, there were four men down in the hold securing the smaller generator for sea, and a few others were in their staterooms or in the galley.

As the runners took a strain, ballast was discharged from the large starboard side tanks and the empty ballast tanks on the port side were filled. All of this happened faster than it should have. By the time there was full tension on the cargo runners, all port tanks were full and both of the large starboard tanks were empty.

The First Mistake
This meant that there was no place to put ballast on the port side in case there was a need for it. The port side had no “reserve ballast capacity,” a phrase coined by the Coast Guard’s technical advisor to the investigating officer.

With full tension on the cargo runners, the captain had the idea that he could discharge port-side ballast while slewing in the derricks to bring the generator aboard and prevent the ship from listing toward its port side.

The Second Mistake
This would not have been a problem if the stability pontoon had remained in position. The derricks were slewed in faster than they should have been; but the real mistake was that no crewmember was stationed to observe the pontoon for this second lift.

At some point before the port-side ballast discharge began, the pontoon became submerged. This meant that the ship, with its suspended load, was unstable but the captain didn’t realize it. The suspended weight was hanging slightly to starboard and all the port-side ballast tanks were full. At that point, the captain realized the ship was listing to port.

The best course of corrective action would have been to ballast. Stability teachings dictate ballasting an unstable ship first on the low side, so the ship can’t flop from a port list to a starboard list and capsize. However, since all the port-side ballast tanks were still nearly full, the alternative should have been to ballast on the starboard side; that is, ballast anywhere possible to make the ship less unstable.

The Third Mistake
Unfortunately the captain didn’t realize the condition of instability caused by the submerged pontoon and continued discharging port-side ballast in hope of alleviating the port list. The center of gravity got higher with every ton of ballast discharged, and the situation became more dangerous.

Then, the inevitable happened—the ship capsized (to its port side, away from the wharf) with unnecessary personnel aboard. In addition to the loading crew, there were four down in the hold, a cook in the galley, and other crewmen were in their staterooms.

In part IV we will explore the aftermath.

For more information:
Full article is available at www.uscg.mil/proceedings. Click on “archives” and "2008 Volume 65, Number 2" (Summer 2008).

Subscribe online at http://www.uscg.mil/proceedings/subscribe.asp.

Direct requests for print copies of this edition to: HQS-DG-NMCProceedings@uscg.mil.