Timber Cargo-3 (Stability related issues)

Loading of timber on deck can cause problems like top heaviness due wrongly declared eights, extra ordinary absorption, use of bunkers from DBs, etc. Collapse of deck load; angle of loll; movement of logs in heavy seas; etc are the situations of impaired stability.

In many sheltered passages often the barges are loaded in stability-wise unsafe manner. There is a very high probability of capsizing of such barges, unless there is glassy sea with no swell. The Master, must always adhere to the standard methods of loading from the loading and stability manual. The securing should be done as advised in cargo securing manual. Local and conventional methods of loading may not be safe as was seen in many incidents in past. In case of doubt, a clarification from class must always be made. Thus, in one particular incident, the amount of cargo loaded was increased on a trial-and-error basis, as opposed to a more systematic approach based on specific stability data. The high GM values, obtained in calculations, were interpreted as an indication of good stability. Actuslly, they were merely an indication of stability in an upright condition, in contrast to stability at large angles of heel. The aspects of intact stability criteria involving the entire GZ curve must be checked and not the GM alone. Wind thrust can be very large on the deck load. This thrust in comparison of the size and stability of ship plays the vital role. Thus, in the case under consideration, the wind-driven waves flooded the deck, increasing water absorption by the logs. Movement of logs in bad weather is a positive warning of a bad scenario ahead, if the situation continues. In the case above, the movement of the topside cargo caused a heavy list in the rough seas. End result being stanchions on the port side were bent inward and starboard stanchions were bent over the side of the vessel.

The complication starts with initial shift itself. Incorrect actions subsequent to the initial disturbance of stow has been the root cause of accidents. A few tips learnt from several past accidents are s follows:

  1. Adequate amount of friction between a timber deck cargo helps. Dunnage should be positioned under the packages to create a higher friction. Large common area of contact of wooden face would help.
  2. Disturbed stow should be unloaded with extreme caution when in port. There may be voids created between logs. The crew should avoid standing on the stow and the use of cherry pickers should be considered for releasing the lashings and removing the tarpaulins. Sending crew on top of already disturbed poles is very risky.
  3. It is often extremely dangerous to venture on to the open deck of a vessel during heavy weather. Even the well lashed containers have shifted. A loosened lot after moving can cause damage with momentum.
  4. ‘Top over lashings’ do not adequately prevent sideways movement. They provide downward force on the deck cargo and this, in association with the friction between the timber packages and the top of the hatch cover, is expected to prevent transverse shift. In open sea an angle bar, or flat bar welded to the edge of the hatch cover are very good. Fabric webbing is considered beneficial. Steel uprights slotted into sockets attached to the hatch coaming, are found very effective.
  5. Any slackness may prove expensive. At times, the logs were not fitted into appropriate sockets and the rope tying them together did not provide a secure arrangement. If the uprights are connected with hog wires running in between the tiers of the cargo, a consolidated system is ensured. Hog wires positioned in this way are held in place also by the weight of the cargo.
  6. Many modern vessels have no uprights fitted. However, it should be noted that uprights with hog wires are best to secure a stow.
  7. The deck cargo not being integral with the ship, nor secured in the cell guides like containers is likely to always shift in bad weather. The shift in combination with broken lashings makes the consequences even worse. Thus, prevention of sideways shift is the most important consideration.

8. Large quantities of water can get under tarpaulin covers and around the cargo. Deck gets loaded with free surface effect intermittently.
9. Vibrations cause timber deck cargo to settle. This results in the loosening of lashings. Timber’s own weight practically is the only force to keep it in place. This problem is usually overcome by checking and if necessary tightening the lashings, regularly.
10. Jettisoning normally would be done in an emergency that is already set in. Releasing a timber deck cargo can be extremely dangerous, even when slip hooks are fitted on each wire lashing unit. Usually, someone is required to stand on the cargo which is to be jettisoned; this is like sawing off a branch while sitting on it! A remote device is the only safe way of jettisoning a timber deck cargo.
11. Timber rails should have adequate strength. The strength calculations must confirm the strength and verified by the class.

A high friction coating, applied to the tops of the hatch covers is of great benefit in reducing the risk of cargo shift. Static frictional force being proportional to the frictional coefficient as well as the load on top. Mixing sand with the paint, and applying this to these surfaces, is tried sometimes. A similar arrangement must be thought for a regular timber deck ship.

Timber must be appropriately loaded on main deck to avail the application of timber loadlines and alternate intact stability criteria in respect of timber.

For ships loaded with timber deck cargoes the Administration may allow the following criteria which substitute the relevant criteria stated for cargo ships. This is ofcourse, provided that the cargo extends longitudinally between superstructures (where there is no limiting superstructure at the after end, the timber deck cargo shall extend at least to the after end of the aftermost hatchway). Stow will extend transversely for the full beam of ship, after due allowance for a rounded gunwale, not exceeding 4% of the breadth of the ship. Also, the cargo is secured with supporting uprights, (which should remain securely fixed at large angles of heel), re fitted.

The area under the righting lever curve (GZ curve) shall not be less than 0.08 metre-radians up to θ = 40° or the angle of flooding if this angle is less than 40°.

The maximum value of the righting lever (GZ) shall be at least 0.25 m.

At all times during a voyage, the metacentric height GM0 shall not be less than 0.1 m, taking into account the absorption of water by the deck cargo and/or ice accretion on the exposed surfaces. Additional curves of stability may be required if the Administration considers it necessary to investigate the influence of different permeabilities and/or assumed effective height of the deck cargo. 25% permeability corresponds to sawn wood cargo and 40%-60% permeability corresponds to round wood cargo with increasing permeability with increasing log diameters. Since excessive GM values induce large accelerations, GM should preferably not exceed 3% of the breadth of the vessel, as indicated in 2008 IS Code. When determining the ability of the ship to withstand the combined effects of beam wind and rolling the 16° limiting angle of heel   under action of steady wind shall be complied with, but the additional criterion of 80% of the angle of deck edge immersion may be ignored. Ballast water exchange operations should be carried out in accordance with instructions in the Ballast Water Management Plan.

Stability booklet:
The ship should be supplied with comprehensive information, which takes in to account timber deck cargo. Such information should enable the Mater, rapidly and in simple way, to obtain accurate guidance as to the stability of the ship under varying conditions of service. Comprehensive rolling period tables or diagrams have proved to be very useful aids in verifying the actual stability condition.

Rolling period vs GM:
Nomogram is made in order to satisfy the relationship and provided for use.
GM0 = \left[ \dfrac{fB}{T_{r}}\right] ^{2}   where f is the rolling coefficient, B the moulded breadth and Tr is true rolling period.

Precautions when using the relationship:

  1. For GM of 20cm and below, results are less reliable.
  2. The formula is more suitable for dry cargo ships. In case of liquid carrier or with excessive unaccounted free surface the results may not be correct.

GM0 from Nomogram:
Nomogram that is provided consists of 2 scales with a line placed in between the two scales in order to provide the desired result without having to use the calculator. The values of breadth and rolling coefficient are joined by a straight line. This line joins the intermediate line at a point say M. Now draw a line joining rolling period and M. This line extended till the left side scale will give the GM0 of the vessel.

For ships carrying timber deck cargoes, the Administration may deem it necessary that the Master be given information setting out the changes in deck cargo from that shown in the loading conditions when the permeability of the deck cargo is significantly different from 25%.
For ships carrying timber deck cargoes, conditions should be shown indicating the maximum permissible amount of deck cargo, having regards to the lightest stowage rate likely to be met in service.

Operational Measures:
The stability of the ship at all times, including during the process of loading and unloading timber deck cargo, should be positive and to a standard acceptable to the Administration. It should be calculated having regards to:

  1. the increased weight of the timber deck cargo due to absorption of water in dried or seasoned timber and due to ice accretion, if applicable;
  2. variations in consumables;
  3. the free surface effect of liquid in tanks; and
  4. weight of water trapped in broken spaces within the timber deck cargo and especially logs.

The Master should cease all loading operations if a list develops, for which there is no satisfactory explanation and it would be imprudent to continue loading.

Before proceeding to sea, ensure that:the ship is upright;

  • the ship has an adequate metacentric height; and
  • the ship meets the required stability criteria.

The Master of ships having a length less than 100m should also:

  1. Make a good judgment to ensure that a ship, which carries stowed logs on deck, has sufficient additional buoyancy so that there is no subsequent overloading at sea.
  2. Be aware that the calculated GM0 in the departure condition may decrease continuously owing to water adsorption by the deck cargo of logs and consumption of fuel, water and stores and ensure that the ship has adequate GM0 throughout the voyage.
  3. Be cautioned that ballasting after departure may cause the ship’s operating draught to exceed the timber load line. Ballasting and deballasting should be carried out in accordance with the guidance provided in Code of safe practice for ships carrying timber deck cargoes, 1991 [resolution a 715(17)].

Masters and Mates must always be concerned about the sea condition; change of wind direction; shipping seas and sprays; etc. Any change of heading, speed, etc in time can save a bigger trouble later. The forces imposed on the cargo, structure and lashings must always be under check as much as practicable. The lashings are designed to provide a means of securing against imprudent ship handling in heavy weather. There can be no substitute for good seamanship though. Ship’s, in addition to the intact stability criteria (or alternate criteria) must comply with severe wind criteria with a comfortable margin.

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