Shear forces and bending moments, ship movement in a seaway, springing, hull stress monitoring, stability, free surface effect, angle of loll, flooding, sloshing, hogging and sagging, squat, effects of list and heel, change of trim due to change of density
THIS CHAPTER contains reminders of the strength, stability, draft and trim considerations which must be taken into account when operating a bulk carrier if damage is to be avoided and operations are to be efficient. It is not intended as a primer in ship construction and stability, subjects which can only be adequately studied from books devoted to the subject.
Such books are available and some are listed in the sources at the end of this chapter. Possible causes of bulker casualties are discussed in Chapter 26.
Shear forces and bending moments All bulk carriers which are Panamax sized or larger, and all which are strengthened for loading in alter- native holds, are provided by their classification societies with maximum allowable still water values for shear forces and bending moments. These values are stated in the ship's loading guidance and stability information booklet and are included in the program of the ship's loading instrument, if she has one. These values, which are provided to ensure that the ship is not damaged by incorrect loading, must be calculated for every stage of a loading or discharge and of a voyage, and must never be exceeded.
Normally two or three sets of maximum values will be stated. The in-port values for shear forces and bending moments are the maxima to which a vessel can be subjected whilst in the 'still' (i.e., sheltered) waters of a port, where she is not exposed to swell conditions. It is permissible to incur a higher level of stress, (up to the in-port limits), during stages in the loading or discharging provided that the stresses are reduced to lower at-sea levels before the vessel puts to sea. The in-port values are higher than the at-sea values because the latter take account of the additional stresses to which a ship is subjected when moving in a seaway.
A ship which is strengthened for heavy cargoes may be provided with two sets of maximum allowable values for bending moments in at-sea conditions, with one set being for 'Alternate Hold Loading Condition', and the second set for the 'Ballast or Uniform Hold Loading Condition'. The lowest bending moment values are allowed when alternate holds are loaded, since this is the condition in which the greatest stresses are created.
The shear forces and bending moments must be calculated before commencement of any of the follow- ing processes:
1. Planned loading and deballasting sequence.
2. Planned discharging and ballasting sequence.
3. Any change of ballast.
4. Any change in loading or discharging sequence.
5. Any instance when deballasting is delayed and becomes out of sequence with loading.
6. Any instance when ballasting is delayed and becomes out of sequence with discharging.
7. Taking of bunkers, step by step (i.e., tank by tank).
8. Consumption of bunkers, step by step (i.e., tank by tank).
9. Docking.
If the allowable values are exceeded there is danger that the ship's structure will be permanently dam- aged—it is even possible for the ship to break into two.
The importance of completing the calculations and ensuring that the stresses are not exceeded cannot be stated too strongly. The most likely reasons for failure to comply with this requirement are the underlisted;
they must be avoided.
• Failure to understand the calculations.
• Data provided in language which is not understood.
• Computer breakdown.
• Inability to make the manual calculations when the computer has broken down.
• Stability data unreadable.
• Change in loading/discharging programme.
• Failure to follow loading/discharging programme.
• Pressure of work.
• Negligent practices.
• Commercial pressure.
• Routine procedure undertaken without planning.
Small vessels up to and including handy size may be provided with no maximum allowable values or programs for calculating shear forces and bending moments. This is because the short length and comparatively greater scantlings of a small vessel make it impossible to expose her to excessive values of shear force and bending moment unless she is jump loaded (loaded in alternate holds).
Ship movement in a seaway
Ships are designed to withstand the weather conditions which are to be expected at sea, provided that they are handled carefully. When heading into adverse weather damage is likely to be suffered if the ship is allowed to pound with the forefoot crashing down upon the sea surface, or to slam with the bows plunging into the swell. Shipping green seas over the bows in adverse weather should be avoided. It is likely to cause damage to deck fittings and hatch covers as a consequence of the weight of the water shipped and the violence of its impact, particularly in ships without raised forecastles.
A ship beam-on to a steep swell may roll very heavily. Such an attitude may occur as a result of the course being steered or as a result of an engine failure.
In addition a ship which has had the weather astern
BULK CARRIER PRACTICE 103
102 THE NAUTICAL INSTITUTE
may be caught in the trough of the swell if an attempt is made to turn into the wind in order to heave-to.
Such heavy rolling may lead directly to damage to a vessel's hull and superstructure, and may also cause the shifting of cargo which can also damage the ship or reduce her positive stability.
One result of rolling is that the side shell plating is repeatedly plunged into the sea and then removed.
This results in repeated variations in the water pressure applied to the side shell. This panting effect may have a long-term weakening effect upon the side shell plating, and the structure to which it is attached.
(The conduct of ships in heavy weather is described in Chapters 15 [loaded vessels] and 17 [vessels in ballast].)
Springing
Seafarers report52 having occasionally experienced abnormal springing of the hull of their ships. This effect has also been named flexing, whipping and wave excited hull vibration. It may be visible as a succession of waves, flowing along the steelwork of the main deck of the vessel, associated with heavy vibration, or shud- dering, of the structure of the ship. This phenomenon is sometimes the result of propeller damage or the isolating of one main engine unit.
Alternatively, it may be a dynamic response induced by waves or swell when the ship is loaded in a particular way. In this case it is usually possible to stop the flexing by an alteration of course. When course cannot be altered, a change of speed or a ballast change may stop the motion. Such violent working of the ship's hull is likely to damage it and every effort should be made to avoid it.
Hull stress monitoring
A few large bulk carriers have in recent years (early 1990s) been equipped with hull stress monitoring systems which measure longitudinal and slamming stresses.
Longitudinal bending stresses are measured by long base strain gauges situated at several selected points along the deck. This process is continuous at sea and in port, permitting stresses due to cargo operations and ballasting as well as the ship's movement in the seaway to be monitored.
Slamming stresses are measured by accelerometers usually placed so as to measure vertical acceler- ations in the fore part of the ship. High-pressure trans- ducers, which detect the very high pressures experienced with slamming, are placed in positions near to the accelerometers.
The readings from each of these instruments is relayed to a desktop computer, usually placed on the bridge, where it provides the master and watch officer with a visual indication of the stresses induced in the hull. If required, the data can be retained on a hard copy print-out or disc.
Stability
A stable ship is one which will return to the initial position when inclined by an external force. An unstable ship is one which tends to heel still further when inclined to a small angle. One of the objectives of the ship's master and officers is to ensure that their
104 THE NAUTICAL INSTITUTE
ship remains stable throughout her life and cannot capsize.
An approximate indication of the ship's stability can be obtained from the metacentric height (the GM), which can be readily calculated provided that the positions of all weights in the ship are known with reasonable accuracy. The GM must be corrected for free surface effect, described below, to obtain the fluid GM.
When the fluid GM is large the ship will be very stable, or 'stiff. A stiff ship is uncomfortable in a seaway. She rolls violently and rapidly. Unfor- tunately, this condition is common aboard bulk carriers when they carry high density cargoes such as heavy ores and steel. A ship with a small fluid GM is less stable. She can be inclined more eaily, and will roll more slowly. This condition, known as 'tender', is common when low density cargoes such as coal and coke are carried.
The ideal stability condition for a ship lies some- where between stiff and tender. Aboard a bulk carrier the ship's stiffness will be governed primarily by the design of the ship and the nature of the cargo carried.
A ship is prohibited from undertaking a voyage in too tender a condition—she must satisfy the minimum stability criteria at all stages in the voyage—but there are no rules which forbid a ship from sailing in a very stiff condition, and bulkers are often required to do so.
Before a ship is permitted to go to sea she must comply with the requirements of the International Leadline Convention which call, amongst other things, for a more extensive assessment of her stability than is provided by the calculation of fluid GM alone.
The ship's loading guidance and stability manual will provide details of the calculations required, which are also fully discussed in Bulk Carrier Practice Chapter 10.
The stability manual also states the minimum permit- ted values for areas under the statical stability curve for the righting lever and for the fluid GM.
The rules which require the calculation of a vessel's stability before she puts to sea are intended to ensure that no ship will go to sea in an unstable condition and subsequently capsize. They are in the best interests of every seafarer and deserve to be followed with care.
If the calculations show that the ship has, or at some point in the intended voyage will have, insufficient stability, adjustments must be made. It may be possible to increase the positive stability by reposition- ing weight lower in the ship, by the addition of weight such as bunkers or ballast low in the ship, or by the removal of weight from high in the ship. Another option is the rearrangement of the contents of bunker and ballast tanks to reduce free surface effect. If all else fails it will be necessary to reduce the amount of cargo which can be accepted. (Planning of the loading is dis- cussed in greater detail in Chapter 9.)
Free surface effect
When a tank in a ship is part filled with a liquid—
normally ballast water, fresh water, fuel oil, diesel oil or lubricating oil—the liquid within the tank is able to 'slosh about' as the ship moves. This reduces the ship's positive stability by an amount which depends upon the dimensions of the tank, the density of the liquid and the displacement of the ship. Free surface
effect must never be neglected; it can transform a stable ship into to one which will capsize.
In many tanks the surface area of the liquid changes with the quantity of liquid in the tank. As a conse- quence the value of the free surface effect also changes.
Some stability tables quote only the maximum value of free surface moment for each tank. If this value is used, any error in the result will be a safe one. The ship will be as stable as the calculations show or she will be more stable than they show.
When making any stability calculations it must be assumed that any tank which is not full, or which is to be used later, has free surface effect. There may be times when free surface occurs, or is proposed, in a cargo hold as a result of hold washing or flooding.
Loading manuals usually provide no method of calculating the effect of such free surface.
When considering sea water in a hold the free surface moments (FSM) can be calculated from the formula:
FSM = 1.025 x LBV12 tonnes-metres, where L = length of compartment, measured fore and
aft, in metres and
B = breadth of compartment, measured athwart - ships, in metres.
The virtual rise of centre of gravity, in metres (G^o) = FSM/Displacement(tonnes).
A worked example of this calculation is provided in Appendix 8.1.
Angle of loll
Ships which become slightly unstable will list to an angle of loll. This condition can often be recognised by the fact that the ship will 'flop over', which is to say that she will list quite noticeably first to starboard and then to port (or vice versa) dependent upon such factors as the direction from which the wind is blowing, and the way the vessel heeled when the last alteration of course was made. If derricks or cranes are topped up when a ship is resting at an angle of loll that angle will increase.
A loll has in the past often been associated with ships carrying lumber or woodpulp on deck. If the stability calculations have been inaccurate it is possible for such ships to complete their voyage with no positive stab- ility and to commence to loll as more bunkers are con- sumed and negative stability develops. However, there are often other possible explanations for any list which such a ship develops towards the end of the voyage. The list may be due to the quantity of water which has been absorbed by the deck cargo on one side of the ship, perhaps because that was the weather side for most of the passage. Alternatively, the list may simply be due to an inbalance between the weights of bunkers and cargo to port and starboard of the centre line. In these cases, of course, the ship will remain listed to one side and will not flop from side to side.
The occurrence of loll must always be taken very seriously, since it shows that the ship has become unstable, and the reason for it must be sought with care. On one notorious occasion the master and crew of a dry-cargo vessel which developed a large list were so convinced that the list was caused by the large tonnage of cargo on deck that they completely over- looked 400 tonnes of water which had flooded one of their holds!
If a ship is found to be at an angle of loll the follow- ing steps should be taken to improve her stability.
• Slack tanks should be filled where possible to eliminate free surface effect.
• Weights should be lowered in the ship where possible, for example for transferring bunkers from deep tanks to double bottom tanks.
• If empty double-bottom (DB) tanks are available the stability can be improved by filling them with water ballast or with fuel oil, as appropriate and as available.
When filling such tanks there are two important rules to remember: pairs of small DB tanks should be filled before pairs of large DB tanks; and in each case the tank on the low side must be filled before the tank on the high side.
For example, if the vessel has taken up an angle of loll to starboard (Fig. 8.2) the starboard double- bottom tank must be filled before the port one. This will initially result in a further increase in the angle of loll to starboard, but the increase will be gradual, and well controlled. Thereafter, when the opposite double- bottom tank is filling, the angle of loll will diminish and eventually disappear, provided that the filling of the first pair of double-bottom tanks is sufficient to eliminate the negative stability.
If these rules are ignored, and the double-bottom tank on the high side is mistakenly filled first the ship will, at some time during the process, roll over vio- lently from her angle of loll to starboard to take up a similar angle of loll to port. The object of filling pairs of small tanks before pairs of large tanks is to ensure that the temporary increase in list is kept to a minimum.
Flooding
Accidental flooding of a compartment will almost invariably cause an unexpected list to develop. Flood- ing is also often associated with sudden increases or reversals of list, and any of these effects should ring warning bells in the mind of the duty officer and prompt him to make an urgent search for an explanation.
When an empty compartment such as a cargo hold is flooded the free surface effect will be the maximum for that compartment. If the compartment contains cargo the free surface effect will be reduced until such time as the water surface rises above the level of the cargo.
Sea water which enters a hold may do so by a number of alternative routes. Water may enter direct through holes in the ship's side or deck, it may pass through leaky hatch covers or through ventilators which are damaged or inadequately closed. Ballast water may leak from ballast tanks, and water may pass into the hold through eductor systems or bilge lines with faulty valves or as a result of faulty procedures.
The flooding of one hold of a bulk carrier which is loaded with a low density cargo such as coal is unlikely to cause such a vessel to capsize. Since the hold will be filled with the cargo the loss of buoyancy and the increase in free surface effect will not be excessive. If the bulker is loaded with a high density cargo, the level of flood water is more likely to rise above the surface of some or all of the cargo and the free surface effect will be larger. However, a ship carrying a high density cargo will have a much larger initial GM so this vessel,
BULK CARRIER PRACTICE 105
INSTRUCTIONS
t ENTER SHIP SPEED IN KNOTS ( point A ) 2 DRAW LINE A B TO INTERSECT APPROPRIATE
WATER-DEPTH CONTOUR AT B
) DRAW LINE B C PERPENDICULAR TO A B TO INTERSECT CONTOUR FOR BOW OR STERN APPROPRIATE TO THE AT-REST TRIM OF THE SHIP (POINT C)
4 DROP PERPENDICULAR. C D TO INTERSECT APPROPRIATE SHIP LENGTH CONTOUR AT D
5 DRAW LINE D E PERPENDICULAR TO C 0 TO GIVE BOW/STERN SINKAGE IN METRES (POINT E)
CONVERSIONS 1 METRE — 3 2808 FEET 1 FOOT — O 3O48 METRE
SQUAT ESTIMATION CHART
Fig 8.4 [Courtesy Dr Ian Dand]
106 THE NAUTICAL INSTITUTE