activities need to be not necessarily carried out in dry dock. Those
are 16 activities on the network. If an activity on CP can be performed during voyage,
time in dry dock can be shorter for certain. We tried to make shorter the time in dock by
transferring some activities to OBM. (Scenario 3 in TAB.1).
TAB.1
Scenarios for reducing the time in dry dock
Scenario
status
cost performance
0
field data of Past record
1
reduction by CPM technique
optiniize tile in dry dock
2
parallel activity introduced
optimize #workers within scheduled time
3
diversion to OBM
cut down dock expenses, reduce tile in dock
In Scenario 0 in TAB.1, actual time in dock was 188 hours with the
number of workers 234 persons. A simulation analysis was made so as to get the time in
dock shorter by increasing the number of OBM workers. CPM technique which minimizes the
time to complete the project was combined with the simulation study. CPM is the Critical
Path Method, which was originally the production control method. CPM aims at the optimum
expenses of work (activity) with restriction that predecessor element (activity) preceded
by the successor like PERT technique. In other words, if activity 1 finished, activity 2
starts. if activity 2 finished, activity 3 starts, and so on.
As for Scenario 1, the time in dry dock was reduced to 116 hours by 293
workers (see FIG.4). Cost performance was measured for every 10 hour reduction, with the
number of workers increased as shown in TAB.2. Dock workers are generally performed
sequentially. In some case, dock works are done in parallel. Activity 1 and activity 2 can
be performed simultaneously at a distance apart. Generally, works of dry docking is
conducted in parallel, say deck, engine and electric division. If deck activities in
sequence are switched to parallel activities as much as possible, time in dry dock is much
more reduced. As initial time in dry dock was 157 hours with 235 workers, some 'slow
moving' activitities were switched to 'fast moving' activities, thus activities on CP was
hastened to be 108 hours with 275 workers. This status is called Scenario 2. Cost
performance was measured by making time in dock every 10 hours shorter like Scenario 1 in
FIG. 3.
FIG.4 Cost change due to time in dry dock and the number of workers
Note: Bar charts shows the number of dock workers (use the scale, LHS),
graph shows cost(use the scale, RHS). The cost for scenario 3 does not include OBM costs.
