Page 43 - Practical Design Ships and Floating Structures

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The smooth tank surface provides a better quality surface preparation and coating application. The
stiffer double side structure also eliminates the flexing or fatiguing of the side frames. Although the
initial costs for a double hull bulk carrier will be higher than for a single side skin bulk carrier, the life
cycle costs may be less because of lower maintenance and repair costs.

10 BALLAST WATER MANAGEMENT

Another area of loading and discharging aboard ships that can affect structural safety of both tankers
and bulk carriers is the handling of ballast water. This concern has developed as a result of new
regulations for ballast water management. The purpose of ballast water management is to prevent the
spread of harmful aquatic species in ports. The regulations require ships to exchange ballast in open
waters (away from ports) before entering a port. Some examples of harmful aquatic species are zebra
mussels, which rapidly multiply and clog pipes and attach themselves to ships. In the U.S. it is
estimated that the zebra mussel alone has caused more than US%5 billion in damage to water pipes,
boat hulls and other surfaces in the Great Lakes region.
How did ballast water management come about? Beginning in 1989 governments started to institute
regulations to protect local jurisdictions from introduction of unwanted marine organisms. IMO
adopted guidelines for management of ship ballast water in 1997. There is now a growing list of
countries imposing mandatory national and local requirements, including Canada (‘89), Australia (‘92),
Great Lakes (‘93), Israel (‘94), Chile (‘93, Alaska and Great Lakes (amended) (‘96), New Zealand
(‘98). IMO is now progressing toward ever widening international regulations.
There are five approaches to minimizing unwanted organisms from ballast water:

(1) Retention of ballast on board - this is practically not feasible, as there is not sufficient ballast
reception facilities in ports.
(2) Reducing organize taken on board - this can be done whenever practical by delaying the loading of
ballast until the ship is in open ocean waters. Ballasting should be avoided in very shallow waters,
in stagnant areas, in the vicinity of sewage outflows and dredging operations, in areas where
organisms are present, etc
(3) Exchange of ballast at sea - these methods are about 95% effective in eliminating aquatic
organisms.
(4) Shipboard ballast treatment - although ballast water treatments are currently being investigated,
none has yet been shown to be practical or cost effective for general use by most ships (biocides,
separators, ultraviolet light)
(5) On-shore ballast water treatment has advantages to shipboard treatment. However, many ships do
not currently have the capability in their piping system to discharge water ashore.

Exchange of ballast at sea is presently the most practical approach. There are two methods used. One
is the Sequential Empty and Refill Method wherein ballast tanks are completely emptied and refilled
with open-ocean water. The other is the Flow Through (or Overflow) Method wherein open-ocean
water is pumped into a full ballast tank, overflowing the tank to three times the tank capacity.

In the sequential method, emptying of certain tanks may lead to significantly reduced stability, higher
stresses, high sloshing pressures, andor reduced forward drafts. A secondary effect of reduced
forward draft would be an increased probability of bow slamming.

Theflow through method achieves 95% replacement. Applying the method does not alter the stability,
stress, and ship attitude. Venting and overflow arrangements for each tank must be reviewed to insure
that flow through is a practical alternative. It is also important to assess piping and overflow