2.4 BALLASTING FACTORS

Selection of a ballasting arrangement is normally not an option for the tanker's crew. The locations of the SBT or CBT tanks dictate where ballast is to be carried. When additional ballast is carried because of anticipated heavy weather, or because of ballast tank cleaning or repairs, the following factors should be considered:

2.4.1 Stress on the hull

Hogging stresses induced in an empty, unballasted tanker; sagging stresses arc induced in a tanker loaded to its mark.

The location of the ballast contributes to three kinds of stress on the hull. An empty tanker is subject to bending stresses caused by the relatively heavy structure at the bow and the stern, (where there is less buoyancy), combined with the light structure and excess of buoyancy amidship. An empty tanker is said to be 'hogged'.
The main deck is in tension, and the keel is compressed by this bending force. Adding ballast to the midship area tanks reduces this bending force. The midship ballast tank should be filled first to minimise stress during ballasting. Likewise, the sequence of subsequent tank filling should be selected to minimise hull stresses during the filling operation.
Shear stress is caused by uneven distribution of forces at adjacent parts of the hull, such as at a transverse bulkhead separating a ballasted tank from an empty one. The ballast pattern must not produce shear forces which would exceed the yield point of the steel hull.
The calculation of bending and shear forces is complex for the designer. This calculation has been simplified for the master/chief officer by providing a set of stress moment tables or approved computer program. The chief officer can quickly determine the stress numerals for a proposed loading plan by using a customised computer program.

Example of computer plot of tanker stresses. A Maximum allowed seagoing shear force. B Maximum allowed bending moment (hogg). C Actual bending moment curve. D actual shear force curve.

Each tanker must also be provided with a stability booklet, with which an officer can calculate the stability and stresses of his ship using only pencil and paper. The stability book is essential because computers fail, and because they are most likely to be unavailable when a ship is damaged, and a stability calculation is most needed.
Localised structural stresses are also produced by the ballast pattern. In general, the longitudinal bulkheads of a tanker are more resistant to outward force (due to the heavier wing tank framing), than to inward pressure. It is better to ballast the centre tanks than the wing tanks. Also, a 'checkerboard' ballast pattern tends to stress the brackets in the ballasted tanks as it sets up a wave deflection pattern in the longitudinal and transverse bulkheads.

1 Undesirable ballast arrangement - four additional bulkheads are placed under bending stress and four corner intersections are heavily stressed (A - heavily stressed bulkhead intersection). 2 Preferred ballast arrangement - provided hull bending and shear stresses are within acceptable limits.

Where ballast will be changed during the voyage, the stability and stress must be evaluated for each step of the procedure to ensure that the maximum allowable at sea stress conditions will not be exceeded while changing from one set of ballast tanks to another.

2.4.2 Draft and trim
The minimum draft for manoeuvring a tanker in SBT or CBT ballast is mandated by IMO convention. Beyond that requirement there are other considerations the master and chief officers must be aware of. Sufficient propeller immersion is important to prevent cavitation erosion of the propeller blade tips. Fair weather requires about one meter of immersion, two meters for moderate weather, and three meters for heavy weather.
The forward draft should be sufficient to prevent the bow from slamming or pounding in heavy seas, but not so deep that seas are taken over the bow.
Trimming for speed is possible if the behaviour of the ship in ballast has been carefully studied. Gains of 0.4 knots for the same fuel consumption are possible through careful adjustment of the ballast trim. The optimum trim for a 140,000 DWT ship is about four meters.
Trim and the ballast pattern together influence hull vibration. Vibration increases hull fatigue and the incidence of cracking. A ballast plan which reduces vibrations is preferred. To ensure that maximum time is spent at best draft and trim, the tank washing and ballast changing operations should be completed in the shortest time possible.

2.4.3 Motions
Placement of ballast within the hull affects the motions of the ship at sea. The rolling period of the ship can be increased by placing ballast outboard, in the wing tanks, structural strength permitting (see Section 2.4.1).
Pitching can be reduced by ballasting tanks near amidships, instead of at the ends of the hull. This also helps to reduce 'pounding'.

2.4.4 Fuel consumption
Although heavy weather may be anticipated on a voyage, taking full heavy weather ballast on departure will significantly increase fuel consumption. It is preferable to take ballast for moderate weather and to divert the vessel's course as recommended by a weather routing service. A weather routed vessel normally arrives earlier, or not later than, one which pursues the direct route. Weather routing produces less wear and tear, and lower fuel consumption than the ship which slogs through the storm.

2.4.5 Corrosion
Intelligent ship owners ensure that the ballast tanks are protected against corrosion by tank coatings and/or cathodic protection. Where cathodic protection is carried the tanks must be filled completely for the under-deck structure to receive the protective benefit of the anodes. The best way to insure this is to open the tank lids after the vessel is at sea, and fill the ballast tanks to overflowing. Do not do this in port! It is better to fill the ballast tanks to a safe ullage in port, and press them up to the vents or tanktops after departure.

2.4.6 Repairs/cleaning
If tank repairs, tank piping repairs, or tank cleaning are to be completed during the ballast voyage, the ballast plan must be prepared to permit washing and ventilation of the tank(s). When the cleaning is for entering drydock, it may be necessary to clean both the CBT/SBT ballast tanks and the cargo tanks.

2.4.7 Clean ballast
If cargo tanks will be washed for clean ballast during the voyage, the vessel must depart with the selected tanks empty to permit early cleaning after departure.

2.4.8 Ballast records
A comprehensive file of ballast plans should be maintained. The record should indicate all weights in the ship (ballast, fuel, and water), along with the drafts produced, and an indication of the speed and fuel consumption achieved. The record is best maintained by computer files, or in a bound journal with the facing pages being a ballast diagram and a page of text and notes of the ballast voyage particulars. All ballast handling details must also be recorded in the oil record book. Exact times and positions of discharging ballast or decanting slops should be recorded and may be invaluable if the ship is investigated because of oil pollution caused by another vessel.

2.4.9 Efficiency
A good ballast plan achieves maximum efficiency by providing the best trim to achieve the maximum speed during the voyage, and by structuring the tank washing and ballast change operations to minimise washing and ballast pumping time.