SHIFTING OF CARGO ON BOARD SHIPS, A SERIOUS THREAT TO LOSS OF INTACT STABILITY

Abstract

Ship's intact stability is one of the most important aspects that have to be taken into consideration when loading the ship as well as during the voyage. Despite the fact that many vessels left the ports with a proper intact stability, the experience proved that during the voyage, the stability has been lost. One of the factors that contributed to loss of intact stability was shifting of cargo during the voyage. In the present paper is examined the impact of cargo shifting on intact ship stability. Real case accidents involved losses of intact ship stability due to shifting of cargo are presented. Analytical equations of forces acting on a cargo piece during motion are explained.

Headnote

ABSTRACT

Ship's intact stability is one of the most important aspects that have to be taken into consideration when loading the ship as well as during the voyage. Despite the fact that many vessels left the ports with a proper intact stability, the experience proved that during the voyage, the stability has been lost. One of the factors that contributed to loss of intact stability was shifting of cargo during the voyage. In the present paper is examined the impact of cargo shifting on intact ship stability. Real case accidents involved losses of intact ship stability due to shifting of cargo are presented. Analytical equations of forces acting on a cargo piece during motion are explained.

Keywords: cargo, shifting, stability.

(ProQuest: ... denotes formulae omitted.)

1. INTRODUCTION

Transport is the most important link in economic relations. It is involved in creating of products and delivering it to consumers, provides the link between production and consumption, between different industries, between countries and regions. It affects the development of the economy as a consumer of petroleum products, grains, metal, timber, and many other products.

Every cargo transported has particular proprieties for loading, stowage and transport. These cargo proprieties can influence the nautical qualities and the seakeeping behaviour of ship. One of the nautical qualities, which is influenced, and in many cases affected by cargo proprieties, is the ship intact stability. Inability of acknowledge the proprieties of the cargo to be loaded, in many cases, is the result of dangerous situations, even to capsize.

Safe transport of cargo is not just a matter of safe stowage and securing of cargo, solely, but is strongly related with the design and construction of the ship, her outfit, as well as with the way the ship is being operated at sea in different environmental conditions.

According to the statistics, 10% of serious stability accidents of ships are caused by the shift of cargo. These kinds of accidents are quite common for general cargo vessels, container vessels, Ro-Ro vessels and bulk carriers.

Cargo shifting is a complex phenomenon. Usually it is caused by the motions of the ship and is largely influenced by the proprieties of cargo, lashings as well as operational aspects. During the voyage, a various number of different conditions is encountered by the ship influenced by sea state, ship's speed and heading towards the waves, and loading condition. Forces acting on the cargo and ships motions will be different in each such condition. The waves will have specific proprieties and in order to response to these waves the ship will move in a certain way. These ship motions will induce forces on the cargo, and if these forces are larger than what the cargo and its lashings can withstand the cargo

will shift. Thus, the probability of cargo shifting will differ in the various conditions.

Hua [3] has presented a concept of an equivalent roll angle and probabilistic approach to the problem of cargo shifting. He discussed the effect of linearization of the model on the predicted results.

2. CASUALTIES OF SHIPS RELATED TO SHIFTING OF CARGO

Due to various types of cargo transported, shifting of cargo is present on board of various types of ships. Here below, are presented some casualties where shifting of cargo resulted in loss on ship's intact stability

TRADEN - 8,188 GRT, Ro-Ro cargo ship

The vessel en route from Valencia to Norkoping, loaded with containers, pallets and roll trailers, occurred in emergency on 19th October 2001, due to a severe cargo shift. The cargo shifted when the ship got into storm for more than two days and into exceptionally troublesome confused seas [8].

KARIN KAT - 1,501 GRT, general cargo vessel

Vessel loaded with various stainless steel LNG cargo handling equipment was sailing from Antwerp bound for Ras Laffan. On 17th February 2003, the vessel encountered bad weather with winds up to gale force 8. The vessel was rolling 15-20 degrees and also pitching and heaving resulted in shifting of cargo inside vessel's holds. The investigations concluded that the rough weather and heavy swells, which caused the rolling, pitching and heaving of the vessel, has been a contributory cause of the foundering. Moreover, the cargo shifted because it was insufficiently secured to withstand the movement of the vessel during the prevailing rough sea [2].

ANSAC ORIENT- 16,715 GRT, bulk carrier

Vessel encountered, on 2nd February 2004, a swell on its starboard bow and rolled violently up to 20 degrees to each side. Logs fell from the after end on no.3 hatch allowing the lashing to slacken. Cargo then shifted athwartships and about 45 logs were lost overboard. Investigation revealed that the vessel's GM was of 3.38 m, the ship was very stiff and had a short period of roll [10].

SUNSHINE- 1,214 GRT, general cargo ship

The vessel loaded with marble in blocks sailed, on 22nd April 2007, in bad weather conditions, with heavy seas, north of Libyan coast. During Master's effort to maintain a reasonable course under the given circumstances, the vessel took, suddenly, excessive list and capsized within 10 minutes. The shifting of a single heavy block of marble has identified primary cause of failure [11].

RIVERDANCE - 6,000 GRT Ro-Ro trailer ferry

This vessel encountered, on 31st January 2008, stability problems in heavy weather during the course of a voyage across the Irish Sea towards the UK, and ultimately foundered on a beach on the UK coast. Among other factors that contributed to loss of ship stability, the investigations pointed out that shifting of cargo was one of the causes [9].

3. THE DYNAMIC FORCES ACTING ON A PIECE OF CARGO

The diagram of the forces acting on the cargo piece is presented in figure 1.

Fy and Fz are the components of the total force F in the global inertial coordinate system. These components can be derived from the Lagrange's equation as follows:

...(1)

...(2)

As can be seen from equations (1) and (2) force F has inertia, Coriolis and gravitational force components. The total force F is given by the components in the moving frame as

... (3)

... (4)

The equations (1) and (2) can be used for determining the required strength of the cargo lashing system. In this case, cargo shift is set to zero, i.e. u = du/dt = d2u/dt2 = 0 in equations (3) and (4), and the ship motions govern the forces acting on the cargo only.

4. THE EFFECT OF CARGO SHIFTING ON SHIP INTACT STABILITY

The effect of transversally cargo shifting is the moving of ship's centre of gravity from G to G1, as illustrated in figure 2.

In this way, the ship's centre of gravity has two components of movement: yvertical component and xhorizontal component. Both of components can be calculated by the formulas

...(5)

where:

x - horizontal movement of G;

y - vertical movement of G;

A - displacement of ship;

w - weight of cargo shifted;

a - horizontal movement of centre of gravity of w;

b - vertical movement of centre of gravity w.

Thereafter, the new righting lever GZ will be (figure 3):

...(6)

In the situation when only the horizontal movement is taken into consideration the loss of righting lever will decrease as the angle of list increases. In other words, the greatest loss of righting lever will be when the ship is upright.

The ship will come to rest at the angle of list in still water condition and the horizontal movement of ship's centre of gravity will represent a negative value of righting lever when the ship is upright (figure 4).

The effect of horizontal movement of G (when the vertical movement of G is ignored) is as follows [z60]:

* The initial GM remain unchanged (since only the vertical movement of G will cause this to change).

* All values of righting lever across the range of stability are reduces, particularly at the smallest angles of heel. When the ship is upright, the horizontal movement of G acts as a capsizing lever causing the ship to heel over to the angle of list.

* A reduction of dynamical stability (area under the curve). Since the ship is already listed, less work is required by the external forces to heel the ship over to dangerous angles of heel on the listed side.

* Range of stability is reduces (at both ends of the range for a ship having an initial range of stability less than 90°).

* The angle of heel at which the deck edge immersion occurs remains unchanged but there is less work required by the external forces to reach it on the listed side.

In the cases where the vertical movement of G is not ignored, the effects of cargo shifting is illustrated in figure 5.

This situation involves a decreasing of GM and this effect is reflected as follows [z60]:

* All values of righting lever across the range of stability are reduced, particularly at the larger angles of heel.

* Dynamical stability (area under the curve) is reduced making the ship less able to resist heeling by external forces.

* Range of stability is reduced.

* The angle at which deck edge immersion occurs remains unchanged as freeboard has not changed.

In both situations presented above, the effect of cargo shifting revealed that most aspects of stability are worsened.

Shifting of cargo on board vessels can lead to dangerous listing and ultimately capsizing. The majority of the casualties involved the consequences of the transverse movement of cargo in heavy weather.

Cargo movement can be attributed to contributory factors like:

* Movement caused by waves hitting the stow (for deck cargo), and exacerbated by movement of the vessel in rough sea.

* Failures in the methods of stowing and lashing the cargo.

Cargo is not always stowed in compact stows and large void spaces remain. These gaps allowed the stow to move especially when vessel was rolling in heavy seas. Lashing materials (wire rods, tumbuckles, rings, etc.) are in many cases old and poor maintained. Moreover, many are not certified and not adequate to be used like lashing materials in particular cases.

Provisions stated in regulations issued by ship's administration (such as Cargo Securing Manual) or international regulations (such as IMO Code of Safe Practice for Cargo Stowage and Securing) are often ignored with intention or by rashness.

In many cases, ship's Cargo Securing Manual did not contain any instructions on stowage and securing of particular types of cargoes, or any information or guidance for the crew related to stowage and securing of cargo on deck/hatch covers.

* Inadequate friction between the cargo and ship steel structures (decks, hatch covers, etc.).

The decks and hatch covers are made from steel and usually are covered by ordinary paint coating which does not provide a non-slip surface, especially when wet. In this way, would be of great benefit in reducing the risk of cargo shift if a proprietary high friction coating is applied on top of these surfaces where the cargo is to be loaded. An alternative method can be the mixing of paint with sand which can increase considerably the friction. A particular and very often case found in practice is that steel bands strap the bundles of sawn timber. These steel bands are the contact points at the lower part of the bundle with steel tops of the hatch covers. Steel-on-steel provides a very low level of friction.

5. CONCLUSIONS

The most important aspects regarding shifting of cargo on board ships were presented in this paper.

The analysis presented revealed that the phenomenon of cargo shifting is one of the main causes of ship stability loss. It were presented the forces acting on a piece of cargo during moving as well as the aspects behind the connection of cargo shifting with loss of ship

It can be concluded the fact that shifting of cargo, developed especially during the voyage when the vessel develops large oscillations due to encountering large waves in heavy seas, is a serious threat to intact ship stability and can have a negative impact on that.

In this respect, it can state that one of the most important objective for the people who work in maritime community and have to be taken into consideration to prevent dangerous situations involving shifting of cargo, is to take precautionary measures to avoid such situations. This objective can be achieved by a continuous education and training of the ship's officers, which are directly responsible for loading, stowage and lashing of cargo on board vessel, to understand the mechanisms of cargo shifting and moreover the dangerous situations involved. Use of simulators in this respect can be a useful tool for training of ship's officers to face the real problems that affecting the safety of ships and safety of navigation [1].

References

6. REFERENCES

[1] BELEV BL., GECHEVSKI PL., DUNDOV N.( Using Simulators for Education and Training Condition, Necessity, Development, Proceedings of BulMet 2005, Varna, 2005, p. 39-44.

[2] Danish Maritime Authority, The foundering of KARIN KAT 18 February 2003, Casualty Report, Case 199932267/12, July 29, 2003.

[3] HUA J., A probabilistic Study of The Effective Roll Angle, 1995.

[4] IMO Res. MSC.267(85), Adoption of The International Code on Intact Stability, 2008 (2008 IS Code), London, 4th December 2008.

[5] IMO/MSC.l/Circ.1281, Explanatory Notes to the International Code on Intact Stability,2008, London, 9th December 2008.

[6] IMO, CSS Code - Code of Cargo Stowage and Securing, Edition 2011, London, 2011.

[7] RHODES M. A., Ship Stability for Masters / Mates, Witherbys Seamanship International Ltd., 2008.

[8] Maritime Accident Investigation Board (MAIB), Report no.20/2006, August 2006.

[9] Maritime Accident Investigation Board (MAIB), MV RIVERDANCE Stability Investigation Report, August 2009.

[10] Maritime New Zeeland, Cargo loss en route to Tauranga on 5 February 2004, Accident Report no. 04 3395 Ansae Orient, 2004.

[11] Panama Maritime Authority, MV Sunshine sunk off Libyan coast on April 22, 2007, Final Investigation Report No.AD09038303, 2007.