This is the third article on Operational Aspects, with the first two printed in DNV Bulk Carrier Update No. 1 2009 and No. 2 2009.
Main provision for a safe and efficient loading or unloading operation is given through SOLAS regulation 7.3 of chapter VI by requiring a loading/unloading plan which is to be agreed between master and terminal representative. This plan has to be prepared and communicated well in advance to calling port and has to take into account both the safe operational limits of the ship and those of the terminal. Lack of adherence to the agreed loading/unloading plan may lead to overstressing ship’s structure and potentially loss of life and property.
It is generally recognised that there is a lack of alignment between the loading efficiency of some bulk carriers and the loading efficiency of dry bulk cargo terminals in terms of loading time. Some terminals have increased their loading performance over the years due to an increased demand on raw materials, but this increase in capacity has not been always been matched by an increase in loading efficiency on the ship. Bulk carriers of Panamax size and above, involved in the iron ore and coal trade, are particularly affected.
The loading efficiency of a ship is a complex function of loading rate, number of pours, vessel strength (both global and local) and de-ballasting capacity (both main system and stripping system).
This article will briefly explain the issues: Loading rate, number of pours, vessel strength (both global and local), but discuss the de-ballasting issue (both main system and stripping system) in more detail.
The loading rate is one of the performance indicator of the terminal with respect to its through-put and so for its profitability. Important for overall performance in loading a ship is the average loading rate, which is the proportion of total loaded cargo mass to elapsed time from start to end of loading. The nominal loading rate indicates the loading capacity of the individual ship loader.
Number of pours indicates how often a cargo hold is touched in the loading process. This number is influencing the average loading rate considering time to shift a ship loader from one hold to the other. Safe operational limits of a ship are covering structural strength, stability and manoeuvrability.
With respect to strength of ship’s structure, the safe operational limits are set by the envelope of permissible hull girder still water bending moments and shear forces and the local cargo hold mass diagrams reflecting permissible or required cargo hold masses as a function of draught. See Fig. 1 in the printed version.
When performing loading and unloading operation, it is to be verified after each step that the mass of cargo and possible double bottom content is in the allowable region of individual hold mass diagram in addition of verifying hull girder, stability and floating values. See Fig. 2 in the printed version.
During planning of loading and unloading process, sufficient stability is to be verified for all steps involved. Safe manoeuvrability is to be ensured at all times when approaching berth considering ship’s specific characteristics with regard to local navigational waters, traffic and weather condition.
Inefficient ballast water systems are one of the main reasons for lack of loading rate capacity. Although total installed ballast water pump and stripping eductor capacity might be sufficient to keep de-ballasting rate time wise synchronized with the loading rate the connected piping system may not.
Time wise synchronized in loading process is meaning that de-ballasting should be finished with the termination of cargo loading or just before carrying out final trimming pour. This does not necessarily imply that ballast tanks in way of holds being loaded are completely empty at end of each loading step provided strength limitations are not exceeded.
Especially on older bulk carriers the piping system might be already eroded and increased friction do not allow utilisation of pump capacity. But even in newly built bulk carriers it is observed that the size of piping diameter imposes certain restrictions in performing ballast water operation. Where main ballast lines are well adjusted to the installed pump capacity the individual branch lines and their valves are found to be of reduced size not allowing for full flow rate without exceeding recommended fluid velocity in pipes.
According to JIS standard fluid velocity in pipes should not be in excess of the following values:
Water cooling:
ND 20 to 40 – abt. 1,8 m/s
ND 50 to 100 – abt. 2,5 m/s
ND 100 and above – abt. 3,0 m/s
Ballast water: abt. 3,0 m/s
HFO: abt. 1,5 m/s
To overcome with excessive fluid velocities required Loading and Unloading Sequence manuals containing advises such that one pump has to serve two tanks simultaneously. Applying such criteria the water velocity in the branch line is even in line with the recognized standard.
An example may illustrate this. A given Capesize bulk carrier is equipped with two ballast water pumps having a capacity of 2500 m³/h each, which is a typical size and in general sufficient for all kinds of ballast water operations. Main ballast water lines consist of two pipes having a diameter of 550 mm allowing a water velocity of about 3m/s in accordance with standards. The branch lines serving individual water ballast tanks consist of one pipe having a diameter of 400 mm. Using full flow rate the fluid velocity in the branch pipes would be about 5.5m/s. Therefore, two tanks have to be served simultaneously allowing decrease of fluid velocity to about 3m/s. Fig. 3 illustrates piping arrangement. See Fig. 3 in the printed version.
In practical terms only one pump at a time is in operation and the other is on standby. Branch lines with same diameter as main lines would increase efficiency of ballast water system drastically.
Therefore it can be concluded that applying a system where one pump is serving to ballast water tanks is inefficient and may lead to a practice where both pumps are used irrespective of consequences on piping condition in lieu to meet high loading rates at some terminals.
A practical solution to increase de-ballasting efficiency for given concept is to use one pump each for pair of ballast tanks on each side of ship. In this case due attention is to be paid to control valves of forward tank (considering aft trim) avoiding suction of air and in-time starting of stripping. See Fig. 4 in the printed version.
Another example for a less efficient ballast water system shows the following instruction on the rated pump capacities for a bigger Panamax bulk carrier. See Fig. 5 in the printed version.
As can be seen from above statement availability of electrical power is a further issue having impact on efficient ballast water operation.
It is observed that some Loading and Unloading Manuals provide sequences which are of no practical use for masters as they have certain shortcomings. First of all, most lack on time indication particular for minimum time needed to de-ballast ship including stripping. Further on, if times are stated they take into account full pump capacity in general rather than consider pressure loss and stripping time. From that point of view, it might be advisable to include a de-ballast trial in building specification for newbuildings. Although, strength limitations are observed throughout the loading sequences efficient de-ballasting might be restricted due to unfavourable trim preventing pumps form taking suction as bell mouths are located at aft end of tanks in general.
Therefore, it can be stated that in general the provided loading and unloading sequences in the manual are only a demonstration that the ship is able to perform them and to be in line with relevant regulations.
The fundamental question concerning loading operations for a master is “What is the real capability of the installed ballast system?” So it is necessary to optimise loading sequences for standard loading/unloading conditions by structured processes.
To meet today’s and tomorrow’s demands on loading flexibility in terms of required loading time, it is essential to know ships’ capabilities and to derive cost-effective measures if necessary.
Further cost-effective measures to up-grade ballast water system have to look into:
Set-up and performance of loading computer
Maintenance of ballast water lines (e.g. cleaning, partial replacement) and pumps
Arrangement of stripping lines and educators
Electrical power supply and management
Against the background of ballast water treatment to be installed it is to be carefully considered what kind of system is most appropriate to guaranty efficient ballast operation. Particular systems which treat ballast water on the discharge side may have certain influence on de-ballasting rate.
In DNV we have the expertise and experience needed to assist owners and managers in operating their bulk carriers efficiently.