Page 111 - Introduction to Naval Architecture

98 THE ENVIRONMENT

maintain the temperature and humidity at such levels as people
find comfortable. The problems of atmosphere management are
most severe in submarines where systems are fitted to remove
carbon dioxide, add oxygen and remove a wide range of
impurities.
(2) Levels of illumination. These will depend upon the activity within
a compartment but typically, in terms of lux, will be about 75 in
cabins, 100 to 150 in public rooms, 50 in passageways, and 150 to
200 in machinery spaces.

MARINE POLLUTION

As well as the effect of the environment on the ship, it is important to
consider the effect of the ship on the environment. In 1990, after the
serious pollution that followed the grounding of the Exxon Valdez, the
USA required all tankers using their waters to be of double hull
construction. The IMO, 1993, accepted that the double hull would
reduce oil outflow in many cases but also recognized that alternative
design configurations were possible, and could be even more effective in
certain types of incident. One alternative is the mid-deck design. By
venting the lower tanks their tops are kept at atmospheric pressure.
Penetration of the bottom leads to the entry of sea water, displacing the
oil into 'overflow' tanks provided. The differential pressure is aug-
mented by the fact that sea water is denser than the oil. The discussion on
the best ways of reducing pollution following an incident, continues.
Spillages are only one aspect of marine pollution.
The governing regulations are those developed by the IMO, known as
MARPOL 73/78 and which became internationally accepted in 1983.
They deal with the discharge of oily water, sewage, and other waste
products arising from the day-to-day operation of a ship. They also
control the deliberate dumping of chemicals and so on. The detailed
provisions of the regulations should be consulted, but broadly the
limitations imposed relating to sewage are that raw sewage may not be
discharged at less than 12 nautical miles (NM) from land; macerated and
disinfected sewage at not less than 4 NM; only discharge from approved
sewage treatment plants is permitted at less than 4 NM. No dunnage may
be dumped at less than 25 NM from land and no plastics at all. Levels of
pollution from all effluents must be very low.
The rules can have a significant affect upon the layout of, and
equipment fitted in, ships. Sources of waste are grouped in vertical
blocks to facilitate collection and treatment. Crude oil washing of the
heavy oil deposits in bulk carrier oil tanks and segregated water ballast
tanks are becoming common. Steam cleaning of tanks is being