Page 380 - Practical Ship Design

Stcihility and Trim - General

12.3 DREDGER STABILITY

12.3.1 Different types @dredger
Before discussing dredger stability, it is worth noting that dredgers are built to
undertake two distinctly different roles.
( 1 ) The first of these is the removal of spoil from estuaries and rivers to provide
navigational channels. Vessels for this purpose are generally fitted with
bottom hopper doors and having loaded up with spoil they sail out to sea to
specified dumping areas where the spoil is deposited by the opening of the
bottom doors.
(2) The other role is the extraction of sand or gravel from estuaries or from
shallow waters further out to sea and the transport of these materials to a
quayside where it can be discharged ashore for sale to the building industry.
These vessels do not require to have hopper doors and can be built with a
conventional double bottom. If intended for use at more than one port these
ships are generally fitted with self-discharge facilities, although some ships
of the type may be designed for a dedicated service and rely on port facilities.
Although this second type of dredger does not require hopper doors to fulfil its
role, these ships are quite often fitted with hopper doors. This may be done so that
the other role can be undertaken if it should become financially advantageous to do
so, but is more generally done to gain the reduced freeboard and greater cargo
deadwight for given dimensions which the hopper type of vessel is permitted to
have.
The paragraph on dredger freeboard in 3 1 1.2.1 indicated that these ships may be
assigned reduced freeboards and outlined the design features giving increased
survivability which justifies the reduced freeboards of these ships.
The bottom doors of hopper dredgers must be capable of being operated from
the bridge even if the main power fails. They must be capable of being completely
opened in not more than four minutes.
Both types of dredger must be designed with spillways so positioned as to limit
the hold capacity so that when the ship is full of saturated spoil of the heaviest
anticipated density the appropriate load line mark will not be immersed.
Many dredgers are built with a number of different spillways to facilitate
conversion to different spoil densities applicable in different operational areas.
Non hopper type vessels are required to undertake loading trials which must
demonstrate that the spillway in use is appropriate to the specific gravity of the
spoil in the operational area. If they move to another operational area with a
different spoil a different spillway is brought into use and further trials are
required.