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Different Types of Ships and
Barges
There
are various types of ships and barges used to carry oil. Each
presents its own challenges to the performance of accurate
measurement. This section discusses most of the basic types
and the potential problems each may cause in obtaining proper cargo
measurements. When going on board a ship for the first time,
the ship's general arrangement plans should be reviewed to see where
all the tanks are and whether the ship is single- or
double-hulled. Barges usually do not carry such plans and the
bargeman should provide the required information.
SINGLE HULL VESSELS
The
most common type of ship and barge currently is the single hull or
single skin vessel. These generally cause the least problems
for measurement because their single skin design usually allows easy
access to all compartments. Even on these, however,
obstructions may prevent gauge tapes from getting all the way to the
bottom of the tank. In addition, it is not uncommon to have
some leakage of cargo between tanks due to small cracks in internal
bulk- heads. Therefore, it is important that all tanks be
gauged and the reference heights of each tank carefully noted and
recorded
DOUBLE HULL SHIPS
Double
hull ships are becoming more and more common due to increased
environmental and industry regulations. These ships differ
from
the single hull vessels in that the cargo tanks are surrounded by
another hull or series of tanks. Because of the "clean" tank
design of the ships cargo tanks, there usually are no obstructions in
the cargo tanks to hinder the taking of good innages.
However,
because the main cargo tanks are located above a series of other tanks,
it may be difficult to measure any material, cargo, water, bunkers,
etc. in those tanks. Double hull ships also usually have
fewer
tanks to gauge than does a conventional, single hull ship.
Some
of the problems with double hulls are similar to those of OBOs/OROs
MID-DECK TANKERS
One
of the detriments to the double hull design is that ships can lose as
much as 33% of their potential cargo-carrying capacity to the empty
spaces comprising the double hull. In order to help remedy this
problem, alternate hull designs have been proposed by ship
owners. The mid-deck tanker is one of several designs that
provide protection of the cargo in case of a hull rupture without
significantly reducing the vessel's carrying capacity. The
existence of a two-deck cargo tank system, however, can cause various
potential measurement problems, many of which may not be fully realized
until ships of this type come into full service. Some of the
problems would be similar to those noted in OBOs or OROs
COMBINATION CARRIERS - OBOs and
OROs
Combination
carriers, such as ore/bulk-oil carriers (known as OBOs) and ore-or-oil
carriers (known as OROs), are specialty vessels designed to carry
either dry or liquid bulk car- goes. The basic procedures
described in this publication apply to combination carriers.
In addition, because of the unique design of combination carriers,
extra care should be taken when liquid cargoes are measured on board
these vessels. The following points should be noted:
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Combination
vessels usually have double bottoms and side and wing tanks in addition
to normal cargo tanks.
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Bottom
lines on combination vessels may not be calibrated and often run
beneath the cargo tanks.
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Because
of the large width of each ore/bulk-oil tank, trim and list corrections
are critical, and error scan be magnified if the corrections are not
correctly applied.
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Some
combination vessels have cargo ducts instead of piping systems.
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Because
the bottoms of combination vessels' tanks may sustain random
deformities resulting from dry bulk handling procedures, OBQ/ROB
determinations may be affected, especially in regard to the
establishment of the liquid plane.
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Such
deformities may affect the validity, development, and application of
the vessel's VEF.
Training
Many
aspects of measurement on board vessels require thorough knowledge and
experience so that an accurate survey can be produced.
Without adequate and recurrent training of personnel, many errors may
be introduced during the measurement and sampling process.
Inaccuracies
in the survey obviously have an economic impact on the seller and
buyer, and periodic training and review is required to maintain
measurement skills. Training and review are essential in
maintaining awareness of improved techniques and equipment that allow
better measurement and sampling of cargoes on marine tank vessels.
Although
this describes parties for proper measurement and sampling of liquid
cargoes on board vessels, it is not intended to be a training
manual. Additional training should be provided to those
involved
in measurement activities and should be based on current API
material. Appropriate training in shipboard operations and
safety
practices should be provided to all personnel working on board any
vessel.
Sections of text taken from
API - MPMS Chapter 17 Marine Measurement
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