Free Surface and a Partially Loaded Barge in a Beam Sea
A 12,000-barrel barge is loaded to about 60% in three center tanks, each roughly 30 feet wide and 60 feet long, with the remainder of the cargo in wing tanks. The barge is in a wide river with a beam sea and a 20-knot wind on the beam. The supervisor notices the barge has a noticeable list that changes slowly side to side, and the mate reports the barge 'feels tender' and rolls more than usual. Draft readings show the barge is within its load line marks and the trim is nearly level. The cargo tanks have no internal baffles.
1. Explain what is happening to the barge's stability and why the load line and trim readings do not reassure you.
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The barge is experiencing free surface effect. The three center tanks are only about 60% full, so the liquid in each tank is free to shift as the barge rolls. Each tank is roughly 30 ft wide, and with no internal baffles the free surface is large. The slow side-to-side change in list and the 'tender' roll are the signature: the center of gravity rises as liquid shifts to the low side, reducing the metacentric height (GM) and increasing the roll period and amplitude.
Load line and trim readings do not reassure you because they measure displacement and fore-aft attitude, not transverse stability. A barge can be within its marks and level and still have a dangerously reduced GM from free surface. The relevant quantity is the free surface correction to GM, which grows with the cube of the tank width - a wide, unbaffled, partially filled tank is the worst case. The beam sea and beam wind add a heeling moment that the reduced righting energy may not counter.
2. What immediate actions reduce the risk, and what is the trade-off?
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Immediate actions, in order of effect:
- Reduce free surface: either fill the center tanks further (to reduce the free surface area) or press them up/empty them - a full or empty tank has no free surface. The trade-off is that you may need to shift cargo or take on/transfer cargo, which takes time and must be done within the transfer plan and stability limits.
- Use wing tanks to lower the effective center of gravity: loading lower/outboard tanks and keeping high tanks full or empty reduces the free surface correction.
- Change the heading or reduce exposure: alter course to put the sea and wind off the beam, or slow/stop, to reduce the heeling moment while you correct the loading.
- Confirm the stability: run the actual free surface correction for the current tank configuration and confirm GM is positive with margin for the wind and sea.
The trade-off: filling tanks to kill free surface may raise the KG if the fill is high in the barge, and shifting cargo takes time and creates its own transfer hazards. The right answer is to compute the corrected GM for the proposed configuration before acting, not to guess.
3. The mate suggests just adding ballast to one side to 'level it out.' Evaluate that idea.
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Adding ballast to one side to counter a transient list treats a symptom, not the cause. The list is changing side to side because liquid is sloshing in unbaffled, partially filled tanks; a fixed ballast correction cannot track a moving free surface, and it may make the roll worse by adding weight high or by reducing the righting arm in the other direction.
Also, ballast added to counter a list raises the center of gravity if it is high, and it changes the loading condition without a stability check. The correct approach is to eliminate or reduce the free surface (fill or empty the tanks), verify the corrected GM, and control exposure to the beam sea and wind. If ballast is used at all, it should be part of a computed loading condition, not a seat-of-the-pants correction.
The judgment point: a changing list in a beam sea points to free surface, and the fix is to change the tank configuration, not to trim with ballast.