Page 69 - Practical Ship Design

40 Chapter 2

2.5 REQUIREMENTS OF SERVICE SHIPS AND OFFSHORE WORKING
VESSELS

Each of the ship types shown in the service ship and offshore working sectors of
Fig 1.2 has its own quite different requirements.

2.5. I Tugs
The requirement that overrides all others for a tug is the bollard pull. This is
determined by the size and type of ship the tug is designed to assist, the number of
tugs that will share the work, and the currents, tides and winds in which towage
may take place.
Tugs for long-distance offshore towage must be very seaworthy vessels and
have ample fuel and stores capacity.
The turning capability should be tightly specified. All tugs, but particularly
harbour tugs, must be highly manoeuvrable, not least to avoid the danger of
capsize that can arise if the tow rope pull goes round to the beam and by the same
token tugs must have very good stability.
One way of providing a high manoeuvring capability is by fitting one or more
Voith-Schneider propellers or steerable thrusters in lieu of conventional propellers.
If these are positioned near the bow rather than at the stern they operate to pull the
tug rather than push it and, as a result, the danger of a sideways pull is almost
completely eliminated.
A required free-running speed may be specified, but this is usually of lesser
importance and in practice is often determined by the power installed to give the
bollard pull. However, if a relatively high free running speed would be advantage-
ous, this should be stated, as a variable pitch propeller can significantly improve
free running performance.

2.5.2 Dredgers
The leading requirements for a dredger are the types of spoil that it must handle and
the range of water depths in which it must be able to operate. Next in importance is
the quantity of spoil that has to be removed per day and whether the removal of the
spoil to the dumping ground is a function of the dredger itself or is carried out by a
separate hopper vessel or by pipeline. If the ship is to have its own hopper, then the
spoil deadweight together with its lowest likely specific gravity which is required
to determine the hopper size must be stated together with the distance to the
dump ground. If the spoil reaching the hopper is likely to be thixotropic, this
should be stated as this has a major “free surface” effect on stability.
Hopper dredgers with bottom dump doors permitting the jettisoning of the spoil
in an emergency and open hoppers from which the spoil will spill before the ship