It was examined whether some of repair and maintenance works could be
switched to OBM. The training ship, which was built by Ishikawajima-Harima Heavy
Industries Co., Ltd. in 1967, has 701.14 gross tons, with overall length 45m, width 9.2m,
and depth 4.9m. Days at sea in recent years is about 200 days. Actual dry docking period
was 188 hours in 1987. The number of works during dry dock was 147 in deck division and 63
in engine division. We call this original status Scenario 0 for convenience. See TAB 1.
Japanese steamship companies get cargo freight paid by dollar. Then, in
order to compensate sudden change in yen exchange rate, management want to pay their
repair cost in dollars. The number of days at sea was 200 days, the number of days in port
100 days, the number of days in dock 65 days, which is the annual operation framework of
an average ship. The operating hours of main machinery was 3,200 hours (＝200days ×
24hours X 60%). It is half of the operating hours of a general merchant ship, whose
average operating hour of the main machinery is 6,000 hours. Therefore, we can give the
framework to the training ship which is double of the time between maintenances of general
merchant ship. The dock works which is spent on deceleration machine, turbo-charger,
pumps, heat exchanger, piston extraction of the main engine and so on seem to be the
excessive maintenance.
Critical path (CP) of the docking operation was calculated by the PERT
network technique. A series of dry dock works were drawn in FIG. 3 as a PERT network, and
can be rescheduled. Since the operation that is not on CP has in general the slack in the
scheduled days of start and finish, we can move the days within the slack so that the
total completion period can be made shorter. Then the schedule can be optimized when
project budget and management resources such as OBM workers and tools are restricted.
For Scenario 0, by increasing the number of OBM workers we can make the
activities on CP shorter (see TAB. 1), so that the whole completion time can be minimized.
In other words, by adopting either 'slow moving' or 'fast moving' for each operation or
activity, total schedule is optimized by applying CPM technique (Scenario 1 in TAB. 1).
Secondly, by switching a serial operation to a parallel operation in consideration of the
relation between predecessor and successor, time in dock was further reduced (Scenario 2
in TAB. 1). Some
FIG. 3 Schedule Control of Docking Activities according to PERT
Technique
