Page 220 - International safety guide for oil tankers and terminals

INTERNATIONAL SAFETY GUIDE FOR OIL TANKERS AND TERMINALS

propelling gas or liquid volumes, pressures, time required for the
pig to travel along the line, volume of residual cargo in the line,
---ใช้เพื่อการศึกษาเท่านั้น---
and the amount of ullage space available should be discussed
and agreed.

งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
During the pigging operation, the terminal should monitor the
pressure upstream of the pig to ensure that it is not stuck in the
line. Failure of the pig to arrive within the expected time period will
also indicate that free movement of the pig has been restricted.

On completion of the pigging operation, the terminal should
positively verify that the pig has arrived. Any residual pressure in
the shore line must then be bled-off before opening the pig trap or
disconnecting cargo arms or hoses.

Personnel at the receiving end should be aware that there may be
sediment in the pig receiver unit and there should be means in
place to deal with this, for example rags, absorbent material and
drums.

11.2 Stability, Stress, Trim and Sloshing Considerations

ึ
11.2.1 General
Single hull oil tankers usually have such a high metacentric height in all
conditions that they remain inherently stable. While tanker personnel have
always had to take account of longitudinal bending moments and vertical
shear forces during cargo and ballast operations, the actual stability of
the ship has therefore seldom been a prime concern. However, the
introduction of double hulls into tanker design has changed that situation.

11.2.2 Free Surface Effects
The main problem likely to be encountered is the effect on the transverse
metacentric height of liquid free surface in the cargo and double hull
ballast tanks.

Depending upon the design, type and number of these tanks, the free
surface effect could result in the transverse metacentric height being
significantly reduced. The situation will be most severe in the case of a
combination of wide cargo tanks with no centreline bulkhead, and ballast
tanks also having no centreline bulkhead (‘U’ tanks).

The most critical stages of any operation will be while filling the double
bottom ballast tanks during discharge of cargo, and emptying the tanks
during loading of cargo. If sufficient cargo tanks and ballast tanks are
slack simultaneously, the overall free surface effect could well be
sufficient to reduce the transverse metacentric height to a point at which
the transverse stability of the ship may be threatened. This could result in
the ship suddenly developing a severe list or angle of loll. A large free
surface area is especially likely to threaten stability at greater soundings
(innages), with associated high vertical centre of gravity.

It is imperative that tanker and terminal personnel involved in cargo and
ballast operations are aware of this potential problem, and that all cargo
and ballast operations are conducted strictly in accordance with the ship’s
loading manual.

186 © ICS/OCIMF/IAPH 2006