The ship during the cargo operations in port: the stability plays its role Juan Olivella Puig & Ricard Mari Sagarra of AWfzW Sconce 6zn fa/aw 78,

Transcription

1 Transactions on the Built Environment vol 36, 1998 WIT Press, ISSN The ship during the cargo operations in port: the stability plays its role Juan Olivella Puig & Ricard Mari Sagarra of AWfzW Sconce 6zn fa/aw 78, Abstract This paper focuses on the possibility of ships' accidents due to stability loss while they are doing cargo operations in port. In the best of cases, stability calculations for the departure condition of the port, for an intermediate condition corresponding to the half of the voayage, and for the arrival condition to the port, are made. However, the evolution of the ship's stability while carrying out cargo operations is taken into account only for very specific cases and circumstances. In the present work, it is considered and recommended the necessity of computing the stability for the different phases of the cargo work. It is also made a qualification of the risk operations through a methodology based on the limit planes, that allows to forecast easily, but without quantification, the effect of each individual or group of operations under the actual situation of the ship, getting, finally, rules to help the master to make his ship more stable keeping the minimum values when she is operating in port. 1 Introduction Ships' accidents due to stability loss while they are performing cargo operations in port are not infrequent. In fact, this kind of accident, occurs with an unnecessarily and unjustified frequency. Moreover, in port ships can take advantage of floating in good sea conditions, but normally, with

2 Transactions on the Built Environment vol 36, 1998 WIT Press, ISSN Maritime Engineering and Ports the disadvantage that the bow vizor, stern door, side doors, watertight doors,..., are open due to reasons of service. Stability calculations to determine ships' conditions for departure, the half of the voyage and the arrival are made, or must be made. However, the evolution of the ship's stability while carrying out cargo operations is taken into account only for very specific cases and circumstances. Usually, control of the buoyancy and longitudinal stability is made, for practical and obvious reasons, but transverse stability is not considered, not even the initial transverse stability, due to the fact that operations for correcting the heel are made in a visual and experimental manner. Therefore, it is recommended to compute the stability for different phases of the cargo work, (generic term that here means any operation: charge, discharge and/or shift of weights). For this purpose, the cargo of the ship is classified into commercial cargo, or only cargo, and tanks, i.e., ballast, oil and other service tanks. 2 An example of risk due to enough stability during port operations On december of 1995 a general cargo ship of metric tons of dead weight with two holds, sank in front of the Tarragona coast just a short time after leaving the port of Vilanova i la Geltrii, being the meteorological conditions excellent, data that has been corroborated by both the Meteorological Service Information as well as by the crew. The study of the flotability and the longitudinal and transverse stability of the ship was made in order to determine these conditions during the cargo operations in port. The analysis of the studies carried out allowed to deduce that the deficiency of transverse stability could have been the main cause of the ship's loss. The ship arrived to Vilanova port with ballast and 289,5 metric tons of cargo stowed on 350 pallets and distributed into the two holds. In the Vilanova port she shipped metric tons also in pallets, distributed into the holds and on deck, with a high of two pallets situated on each hatch. To know better the ship, tables 1 and 2 contain the more important

3 Maritime Engineering and Ports 187 Transactions on the Built Environment vol 36, 1998 WIT Press, ISSN characteristics and tank capacities. Figure 1 also shows the diametral and horizontal planes with the location of the holds and the tanks of the ship. Table 1. Ship characteristics Length between perpendiculars Moulded breadth Depth Draught Displacement Dead weight Gross tonnage 63,00 m 10,80 m 6,10 m 5,18 m 2.785,00 Tm 2.085,00 Tm 1.171,00 Tm Table 2. Tank capacities Description Ballast FW tanks DO tanks Lub oil Volume nf 412,670 26,029 91,453 4,910 Weight Metric Tons 422,987 26,029 76,820 4,419 Several calculations were made for different cargo conditions, but we only present the results of two of them, due to their significance in the conclusions of our investigation. The cargo conditios for these two computations selected were: 1. Stability calculations of the ship loaded according to the cargo plan plus the double bottom ballast tanks full. 2. Ship's stability in the assumption that the ballast is discharged while the ship is finishing the cargo operations.

4 Transactions on the Built Environment vol 36, 1998 WIT Press, ISSN cr. 3 CD OQ O- « ' Figure 1. General ship distribution

5 Transactions on the Built Environment vol 36, 1998 WIT Press, ISSN Maritime Engineering and Ports Figure 2. GZ curves Figure 2 shows the corresponding GZ curves for these two conditions, and the curve of Rahola has been included as a relative reference. The curves indicate that the first cargo condition does not observe the IMO criteria, but in the second case the transverse stability is really critical. 3 Changes in the metacentric height: practical evaluation Now, our interest is focused on the effects that the cargo of a weight produces on the initial transverse stability due to the parallel inmersion.

6 Transactions on the Built Environment vol 36, 1998 WIT Press, ISSN Maritime Engineering and Ports The changes in the transverse metacentric height will be the algebraic sum of the variations produced in the second member terms of the following equation, (Figure 3 and see Nomenclature in page 10), h = (1) dh = (2) D + p (3) 6r = r, - * ~ / 6r = - (4) (5) Ft. F G C % y A A A ^i *, i A A Y ; A : ; h y ; Ai!5/. r \ y y y Y Figure 3. Parameters of the initial transverse stability

7 Maritime Engineering and Ports 191 Transactions on the Built Environment vol 36, 1998 WIT Press, ISSN Substituting in the eqn (2), the variation on the metacentric height will be, D + p (6) and the value of the new metacentric height, D + p - h - z 2 * (7) From eqn (6), that computes the changes in the metacentric height, the limit plane is deducted, whose aim will be to collaborate in the analyses of the effects that cargo of a weight relatively small with respect to the ship displacement produces on the transverse stability. The addition of the terms, % L z + - n 2 (8) determines the vertical position of the limit plane above the keel. In other words, the limit plane will be to a depth equal to the metacentric height below the final flotation, when h > 0, (Figure 4). This limit plane helps us to deduce the sign of the variation of the metacentric height according to the table 3. Table 3. Evaluation of the variation in the metacentric height 6h>0 <5h = 0 P -> 0 P < c 0 P : > 0 P < c 0 Load of the weight below the limit plane Unload of the weight above the limit plane Load of the weight in the limit plane Unload of the weight in the limit plane

8 i yl ivianiime nngineeniig anu runs Transactions on the Built Environment vol 36, 1998 WIT Press, ISSN h < 0 P -> 0 P «c 0 Load of the weight above the limit plane Unload of the weight below the limit plane Figure 4 shows the effect of the load or unload of a weight placed above the limit plane, either coinciding with it or below of the same. <L a Sh >0 <0 p< 0 p> 0 Piano limite V h Sh = 0 p> 0 p< 0 Sh >0 h <0 p> 0 p< 0 I Figure 4. Limit plane. Practical evaluation in the change of the metacentric height 4 Method for controlling the initial stability during the cargo operations Following with the previous exposition, a method is proposed for taking decisions in due form, fast and reliable during the cargo operations in port. In any case, it is mandatory to make some stability calculations for different "strategic moments" in the cargo plans. This strategic moments obey to the stability characteristics of every ship in particular. A previous and generic study based in the major number of cargo plans as possible, means an increment in the probabilities that the actual cargo plan will be compatible with one of them. The selection of each one of the strategic moments corresponding to a cargo plan is made according to the value of the metacentric height and its permissible variation for the weight unit,

9 Maritime Engineering and Ports 193 Transactions on the Built Environment vol 36, 1998 WIT Press, ISSN the last depending of the notability characteristics of the ship. The method considers two different levels of assumption of decisions. In the first level, it is analysed the sign of the Sh according to the relative positions of the limit plane above the keel, (Eqn 8), and the centroid of the weight, (Figure 4). In the case of a positive sign, the metacentric height will be higher, and the decision is immediate. If the sign is negative, it is necessary to go ahead with the second level of the method, in which the indicators of the variations for weight unit must be considered, (Table 4). Table 4. Cargo plan number XX. Strategic moments and permissible variations Mean draught Strategic moment h value Permissible variation 6h per weight unit By computing the variation of the metacentric height <5h, (Eqn 6), per weight unit and checking its value with the corresponding value of the column 4, (Table 4), the cargo officer will be in better position of taking the right decision. Operations capable of using the proposed method are not explicitly established in a determined cargo plan, like a charge or discharge of tanks of ballast, fuel or diesel oil, lubricant, fresh water, or extra cargo, as it is usual in containerships, for instance. 5 Conclusions - The benefit of getting standards of cargo plans with the information of the strategic moments. - Capability of making the adequate computations in a case of an actual cargo plan with no compatibility with the historic ones. - This explained method gives some friendly tools to the cargo officer for the treatment of the initial stability, in a similar way of difficulty as he/she calculates the heel or the trim of the ship.

10 194 Maritime Engineering and Ports Transactions on the Built Environment vol 36, 1998 WIT Press, ISSN Weaknesses of the method - A certain degree of complexity for getting the tables of cargo plans with the strategic moments, depending on the type of ship and traffic service. - In some cases the need of external support to introduce the methodology and in the implementation of the system. - Some refinements to be added due to the consideration of only the parallel inmmersion. 5.2 Highlights of the method - Ease of use, with low requirements of information and computations. - A quick and secure reply in front of the initial transverse stability questions due to a load or unload of a weight. - Professionalism and technocracy in the cargo operations from the point of view of the flotability and stability of the ship. Nomenclature D displacement GZ righting arm of tranverse stability h, 6h metacentric height and its variation 1% moment of inertia of the waterplane p weight r, Sr metacentric radius and its variation z draught 5z paralell immersion z^, 6zc height of the centre of buoyancy above the keel and its variation Zg height of the centre of gravity of the weight ZQ, 6zo height of the c. of g. of the ship and its variation ZM height of the netacentre above the keel Bibliography OLIVELLA PUIG, Juan. Teoria del Buque. Estabilidad, varada e inundation. Barcelona; Edicions UPC, RODRIGUEZ RIPOLL, Javier. &W/o ak /a wmmww 6k %/? mixto portacontenedores. Barcelona: Proyecto fin de carrera, 1997.