PANIC! Well, at least severe concern. That should be your reaction if your ship developed a permanent list, and you eliminated the possibility of any extra weights. The only other possibility is an angle of loll. Angle of loll shines like a bright red warning sign, indicating serious stability problems. You need to track down this cause, because the next warning may be when your ship capsizes. Today we discuss an angle of loll (AOL): what it is, how to find it, and what to do about it.
An angle of loll (AOL) is a symptom of serious stability problems. Basic stability theory imagines a point called the metacenter, which represents the stability of the ship. (Point M on Figure 2‑1) For a stable ship, we want the ship’s center of gravity (point G) to be vertically below the metacenter.
When a ship has poor stability, the points M and point G are very close together. With an AOL, when the ship is upright, the point G is actually above M. End result: the ship is unstable within a small range of heel angles. That is very bad.
We can also see this instability in the righting arm curve. (Figure 2‑2) Initially, the curve drops negative, and moves back to positive after some angle of heel. That crossing point is the angle of loll.
Sure, an AOL is annoying. But the real concern is that an AOL is a symptom of a borderline unstable ship. And unstable ships capsize.
Don’t jump to find zebras when you only hear hoofbeats. A permanent heel may only be a list, which is far less concerning than an angle of loll (AOL). These are some classic symptoms of an AOL.
AOL’s typically result from some change in your vessel’s tanks. This can drastically affect the free surface moment and create an AOL. Check for any of these changes to your tanks.
Angle of loll (AOL) is hard to detect. The best solution is to bring in an expert: call a naval architect. But naval architects are expensive. Maybe you want to check things yourself before opening the wallet. The following procedure helps indicate if your ship has an angle of loll. Just read all the warnings and if you have any safety concerns, a naval architect is the best route to go.
To test this, you will need the following:
The goal here is to fill the heeling tanks on deck until the ship changes from the list angle back to zero heel, and slightly past. If the ship has a true list, it will remain at the new angle. If the ship has an AOL, once you pass zero heel, it should very quickly continue to heel further. For an example, assume our problem ship has an initial heel of 2.5 deg to port.
This test gives you a clear and very noticeable distinction between AOL and genuine list.
This test does have some risks for the ship. For safety planning, assume you have an angle of loll (AOL). This means, when you create the counter moment, you created a list angle. The list angle will compound with the AOL and heel farther on the opposite side than you started with. This potentially leads to very large heel angles and possible deck edge immersion. This following video shows an excellent example of how that can happen.
Check your load cases before hand to ensure the vessel can tolerate the larger heel angle. If you are unsure about the safety aspects, consult with a naval architect.
Fix two problems for the price of one! Address the underlying stability issues first, and the angle of loll will resolve itself. If this was a recent change to your vessel stability it probably stemmed from free surface moment problems. The most likely culprit is cross connects between port / starboard tank pairs. Cross connects should normally be closed. Check all your cross connects and repair any bad valves.
If this problem slowly developed over time, then it is time to call in the experts. Hire a naval architect to do a stability test, stability assessment, recommend corrective actions, and update the trim and stability booklet. These are not quick and easy solutions, but they are necessary. The angle of loll is a major warning about stability. Ask yourself which is easier: preventative maintenance, or dealing with a capsized ship.
An open cross connect valve often results in an angle of loll. Cross connects are normally associated with the port/starboard tank pairs and often sealed with a valve. These valves lie deep in the double bottom and can be devils to find.
How to find the hidden cross connect? Start by filling only the starboard tanks of your port/starboard tank pairs. Leave the port side empty. Then take continuous soundings of the full tanks. Any tanks with open cross connects, and you will quickly notice a change in the sounding. It should take no more than ten minutes to detect a definite change in tank level.
Repeat this process with the port tanks full and starboard tanks empty. You want to check it both sides to ensure you only have one unknown opening.
Angle of loll is actually a blessing. It provides a clear warning about underlying stability problems with your vessel. Not all ships show this warning before bad things happen. If you have an angle of loll, take advantage of the warning. Speak with a naval architect to address the stability issues and get your vessel floating straight and tall.
|||K. J. Rawson and E. C. Tupper, Basic Ship Theory, Oxford: Butterworth-Heinemann, Fifth edition, 2001.|