(See my previous article Grounding [Initial action to take])
Effects of Grounding include failure of the hull, virtual loss of stability (proportional to the fall of tide), loss of strength (extent unknown), capsizing (due listing, bilging or virtual loss of stability), ship breaking (excessive bending due localized upthrust), etc. Constructive total loss, injury to crew, loss of life etc. are the other possible eventualities. Bilging of a compartment in grounding represents one of the most problematic situations because it causes bodily sinkage and a loss of buoyancy in addition to breach of hull. Moreover, there is no freedom to quickly re-float and go.
A compartment that is partially full of liquid and is also damaged, will cause the effect of free communications and free surface to certain extent. The variation of load on the ship’s hull can always worsen the situation. Each one of these effects occurs separately and should be calculated in an independent manner.
Heeling or listing may occur due to;
- Unequal weights due unsymmetrical flooding.
- Negative metacentric height (GM).
- A combination of the two items above.
- Technically FSE should not cause any harm as the vessel is resting at bed but the phase lag between the in and out communication of water may cause transverse shift of COG of the ship.
Whenever, there is heeling or listing, it is of vital importance to determine the cause before any corrective action is taken because improper corrective actions may aggravate the situation. All the six drafts are checked to ensure that the ship is neither hogging nor sagging. If the ship is aground at one end, sagging stresses are increased. If she is aground on a ledge or pinnacle amidships, hogging stresses are increased. Weight should be removed from the bow, stern or amidships and the tanks ballasted / deballasted to smoothen the stresses.
The situation of grounded vessels deteriorates quickly due to different factors such as weather and the nature of the bottom where the vessel is. Nowadays, the environmental concerns have become very important. Removal of bunkers from grounded vessels is fast becoming mandatory in order to mitigate environmental damages, thus avoiding heavy penalties and lawsuits.
In any condition of grounding, where the hull is intact, the ground reaction can be found as follows:
1. Reasonably accurate displacement prior to grounding would normally be, known to the ship’s staff.
2. The six side-drafts at any stage of grounding are read and new displacement is determined. The difference between the two displacements is the ground reaction. ‘Difference in the hydrostatic drafts x 100 TPC’ also can give ground reaction.
3. In the case of pinnacle or pivoted grounding the draft at point of contact is indicative of rise or fall of tide, as has also been discussed in last article. Change of hydrostatic draft is not equal to the rise of tide required to refloat vessel unless the vessel is aground at centre of floatation. The correct thing to remember is change of dragt at point of contact is rise of tide required to refloat the ship.
4. As has also been told in last article that setting up of a pendulum (log line), at point of contact must be done as soon as the vessel is aground. Tide level on this line must be hourly recorded to closely study the pattern of tidal streams at the very location.
5. In case of dry dock, the critical period and critical moment are crucial in ensuring that the stability is adequate during this phase. This is because after critical moment the ship is additionally supported and rests all over the bottom. The similar phases may occur grounding too but with no choice of sea bed. In case of grounding, the problem of stability does not end after sitting all over because the GM, draft, water plane area, etc. continue to reduce, with no choice about list, trim, contact area, etc.. Moreover, the vessel continues to be exposed to the severity of weather.
6. A grounding calculation sheet to calculate critical draft may be made. If the tide is receding, the probable results of stability and stresses must be considered. Lowering of the ship’s center of gravity may have to be done by adding more weight in low tanks, shifting weights down, etc. As a last resort jettisoning weight, located high may have to be done.
6. It is interesting to note and it is very important to understand that the tidal stream is neither uniform in rate nor direction over the cycle. It generally follows a simple harmonic pattern and is understood from the picture below. The rate of tidal stream is uite low just after the slack waters and very strong at the middle of slack waters.
The resources available to the ship include;
1. Tidal force;
2. Correct use of trimming & stability principles;
3. Engine power; etc. The first help, which may be available from shore is a tug or a barge. These will be helpful in traction pull ground tackle, etc. Under these limited resources, the refloating is quite a challenging task. Following is worth noting:
i. Reduction of the Static Grounding Reaction can happen automatically with rise of tide or can be caused by using stability principles such as change of trim by transfer of liquids or load / discharge of weights.
ii. Static Traction in the direction of deep waters can be achieved by dropping the anchor windwards at the initial stage or causing ground tackle subsequently.
iii. Increase in the depth of the water in the site of the grounding can be done by various manual means and can naturally happen with increase of tide. In some cases of partial grounding, success in refloating is obtained by maneuvers associated with changes in the positioning of weights onboard, adjustment of ballast and consequent changes in trim.
iv. Traction force > µ (F1+ F2+….+FN). where µ is coefficient of friction and depends on nature of bottom and the common contact area. F1+ F2+….+FN = (Δ – residual buoyancy from submerged hull. v. The application of static traction, requires magnitude of force, sufficient to overcome the forces of friction, force caused due to grounding. The force of friction in the mud is proportional to the area in contact with the bottom. Removing the mud from around the hull will help in the refloating operation. The force of suction can be minimized with fore and aft movements of the grounded vessel or by the scouring effect that may increase the flow of water past the hull. The use of dredging and scouring may increase the depth of the water in the site of the grounding.
In the cases of vessels aground, during the refloating, control of weights is important and their effect on the displacement, trim and stability should be reconfirmed before applying it. The same applies to the placement onboard of salvage equipment, the placement of such weights should be avoided in high positions.
In flooding and grounding the location and extent of maximum bending moment(BM) and shear force(SF) may not be found in stability documents provided. Thus, BM may occur at a point that is not close to the midship section (amidships). The same can happen with the location of sheer forces.
The tidal variations may increase or decrease the grounding reaction. During ebbing, the ground reaction will increase. It will decrease during flooding. This will also change SF & BM values. During the development of refloating operations, it is important to know the height of the sea during the tidal variations. In case such values are not tabulated and assuming that tide follows a cosine graph, we can obtain a good approximation of the values by using the method explained in the diagram below. The tabulated information maybe wrong sometimes, as was discovered in case the of Sea Empress, when she had grounded in Milfred Haven. It is also worth noting that the spring tides generally occur one or two days after the full / new moon.
The change of tidal height is minimum at slack waters (about half an hour after HW and LW). The change is maximum at half way between slack waters.
The grounding reaction at the point of contact works upwards. It move virtual centre of gravity in opposite direction. Centre of buoyancy will depend on the current draft. These forces must be visualized and given due regard when applying the traction force. These forces are likely to influence the pivot point too. The grounding reaction, for the large majority of cases, assumes that the distribution is uniform throughout the whole grounded length. An exception applies when groundings occur on sharp rocks or similar bed. In these cases a small area of the hull will be in contact with the ground and the distribution of the grounding reaction cannot be determined with accuracy.
(I will show the grounding calculations in my next article)
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