Page 267 - International safety guide for oil tankers and terminals

SPECIAL SHIP TYPES
14.1.3
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14.1.3.1
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
14.1.3.2
Slack Holds in Combination Carriers
General
Because of the broad beam and size of the holds, the very large
free surface in slack holds (i.e. holds not filled to within the
coaming) permits substantial movement of liquid, which can result
in both loss of stability and ‘sloshing’.
Loss of Stability
Particular care should be taken when loading or discharging liquid
cargo on combination carriers, and when handling ballast on such
ships, to ensure that the total free surface effect of cargo and
ballast tanks is kept within safe limits, otherwise a sudden, and
possibly violent, list could occur.
In compliance with government requirements, all combination
carriers are supplied with stability data and loading and unloading
instructions. These instructions should be carefully studied and
followed. Generally, these instructions will specify a maximum
number of cargo holds or tanks which may be slack at any one
time. Sometimes, it may be necessary to adjust the quantity of
cargo to be loaded in order to avoid slack holds. Where double
ึ
bottom ballast tanks extend across the whole width of the ship,
the free surface effect of water when these tanks are slack will be
as great as that of full cargo holds and account must be taken of
this fact.
Some combination carriers have a valve interlocking system that
limits the number of tanks which may be loaded or discharged
simultaneously. Such systems may fail or can be bypassed, and it
is a requirement that a conspicuous notice is displayed at the
cargo control station warning of the danger of free surface effect
and stating the maximum number of holds that can safely be
slack at any one time.
Before arriving in port, a plan should be prepared for the
anticipated loading or discharging sequence, bearing in mind the
free surface effect and distribution of all cargo, fuel and ballast at
all stages of the operation.
Terminal operators should appreciate that combination carriers
may be subject to loading rate limitations and to specific
discharge procedures. These arise from the danger of hatch seals
leaking if placed under excessive pressure, as well as from the
free surface effects.
If a loss of stability becomes evident during loading or discharge,
all cargo, ballast and bunker operations must cease and it is
prudent to disconnect the loading arms or hoses. A plan should
be prepared for restoring positive stability. If the ship is at a
terminal, this plan should be agreed by the Terminal
Representative.
The specific action required to restore stability will be determined
by the ship’s detailed stability information in relation to a particular
condition.
© ICS/OCIMF/IAPH 2006 233