Your bulk carrier is on a ballast voyage from Rotterdam to Baltimore in January. The vessel is in the North Atlantic in heavy weather — significant wave height 8 metres. The chief mate is asked by the master what structural risks to monitor and how to respond if a cargo hold begins taking water.

A. NORTH ATLANTIC WINTER BALLAST — STRUCTURAL RISKS AND RESPONSE: SPECIFIC RISKS IN HEAVY WEATHER ON BALLAST: (1) HOGGING: ballast vessel with empty cargo holds may have weight concentrated in ballast tanks at the ends — this creates hogging bending stress that is accentuated by wave-induced hogging at sea crests; (2) SLAMMING: with the bow riding high on ballast, the bottom of the bow may slap the water surface at high speed — impact loads (slamming) can be severe enough to crack bow structure; (3) GREEN WATER ON DECK: large waves on a partially filled deck can create significant hydrostatic pressure on hatch covers; (4) HOLD VENTILATION: ensure cargo hold vents are closed to prevent water entry; (5) BALLAST DISTRIBUTION: ensure ballast tanks are filled in the configuration stated in the loading manual for ballast conditions — avoid partial filling that creates adverse BM. RESPONSE TO HOLD FLOODING: (1) Pump the hold bilge immediately — assess whether pump controls the ingress; (2) If pump cannot control: notify master immediately and proceed to nearest port of refuge; (3) ASSESS STABILITY: any flooding changes trim and stability — recalculate; (4) In severe weather, consider heaving to to reduce slamming and wave impact; (5) PAN PAN or MAYDAY as appropriate — if flooding is progressive and uncontrolled, this is a distress situation.

B. A ballast voyage in heavy weather has no specific structural risks compared to a loaded voyage. The vessel is lighter and therefore under less stress on ballast.

C. If a cargo hold floods on ballast, drain it into the ballast system and continue the passage. Flooding of an empty cargo hold is not serious because the hold is not carrying cargo.

D. Structural monitoring is only required in port and during loading. At sea, the dynamic loads are too variable to assess and monitoring is impractical.

A. BENDING MOMENT AND SHEAR FORCE — CONCEPTS AND ACTION: BENDING MOMENT (BM): the internal moment within the ship's hull that resists the tendency of the vessel to "hog" (bend upwards at the centre when supported at ends) or "sag" (bend downwards at the centre when buoyancy exceeds weight amidships). BM is measured in tonne-metres and varies along the ship's length. SHEAR FORCE (SF): the internal vertical force within the hull at any cross-section — the tendency of the ship to "shear" (vertical displacement between adjacent sections). Caused by unequal distribution of weight and buoyancy. STILL WATER vs WAVE BM: still water BM is calculated at the loading port and is the controllable component. Wave BM is added at sea and is the classification society's additional allowance. MAXIMUM PERMISSIBLE STILL WATER BM: the value stated in the ship's loading manual is the maximum still water BM that the vessel's structure can withstand in still water, with a reserve for wave-induced BM at sea. EXCEEDING THE LIMIT: the proposed sequence results in 9,100 vs maximum 8,500 tonne-metres — this is 7% over the permissible limit. THE VESSEL MUST NOT SAIL in this condition. ACTION REQUIRED: (1) Modify the loading sequence — redistribute cargo between holds; (2) Add ballast to alter buoyancy distribution; (3) Reduce total cargo weight; (4) Recalculate until within limits; (5) Document the revised plan and recheck BM and SF on the loading computer.

B. The maximum permissible value is a conservative limit with a 10% tolerance built in. A bending moment 7% over the stated limit is within the safety margin and the vessel may sail.

C. Shear force and bending moment limits only apply in port loading operations. At sea, wave action overrides the still water values and the loading computer limits are not relevant.

D. Contact the classification society for a dispensation to sail over the limit. Classification societies routinely grant dispensations for temporary bending moment exceedances at loading ports.

A. HOGGING AND SAGGING — DEFINITIONS AND CAUSES: SAGGING: the vessel bends so that the midship section is lower than the bow and stern — the hull curves downward like a banana shape with the centre lowest. CAUSES: (a) Heavy cargo concentrated amidships with relatively empty ends — weight exceeds buoyancy amidships; (b) At sea: when the vessel is in the trough of a wave — buoyancy is reduced amidships (both bow and stern supported by wave crests). STRESS IN SAGGING: the keel and bottom structure are in TENSION; the deck structure is in COMPRESSION. HOGGING: the vessel bends so that the midship section is higher than the bow and stern. CAUSES: (a) Heavy cargo at the ends with a light or empty midships section; (b) At sea: when the vessel is at the crest of a wave — midships is supported by the wave while bow and stern overhang the troughs. STRESS IN HOGGING: the deck is in TENSION; the keel is in COMPRESSION. PRACTICAL SIGNIFICANCE: most bulk carriers are more vulnerable to hogging in heavy weather (empty holds or light cargo at midships with heavy ore at ends). Structural failures have occurred from excessive hogging loads in severe weather. LOADING TO BALANCE: a well-loaded bulk carrier distributes heavy cargo to minimise both sagging and hogging BM — check the loading computer output for the BM envelope diagram.

B. Sagging occurs when the bow is heavily loaded and the stern is light. Hogging occurs when the stern is heavily loaded and the bow is light.

C. Hogging and sagging refer to the list of the vessel, not bending. A hogged vessel lists to port, a sagged vessel lists to starboard.

D. Sagging is when the vessel is heavy overall and riding low in the water. Hogging is when the vessel is light and riding high. Both are stability terms, not structural terms.