The requirement for a dredger hadpresented a challenge for the Harbour Commissioners and vessel builders Goodchild Marine launched.
The requirement for a dredger hadpresented a challenge for the Harbour Commissioners and vessel builders Goodchild Marine, who were jointly involved in the design of a 24m spud leg barge launched in August.
Owned by the Harbour Commissioners and with its operation contracted to Goodchild Marine, the 21m by 10m pontoon dredger features a draught of only 1m and carries a JCB JS360 long reach excavator with 21m boom on board.
It incorporates the latest Ground Profiling System from Prolec, which provides a repeater monitor in the pontoon wheelhouse receiving information transmitted from the excavator cab, where it is also displayed. The system also performs the dual function of assisting with the positioning of the vessel itself.
With work in a fairly confined area encountering quite heavy traffic, "positioning and manoeuvrability were of optimum importance" said Harbour Master Robert Smith.
The dredger is fitted with a Cummins B Series 200hp turbocharged diesel engine supplied by Sykes Marine of Thurrock in Essex UK, which delivers up to 6 knots. The vessel is constructed with the capability of turning on its own axis, thanks to the remote controlled heavy duty Hydromaster self contained marine propulsion unit and Hydromaster heavy duty transmission, installed significantly with a reverse gearbox which removes the need to rotate the 580 rpm manganese bronze high thrust propeller 180 degrees to obtain reverse. All electrical power is provided by a 30kva generator set.
The vessel is steered through the Hyromaster"s RCC2 joystick control, which allows for manual positioning of the dredger using the Ground Profiling System monitor. This not only ensures accurate positioning but also enables work to commence precisely where it finished the previous day.
The heavy duty Hydromaster equipment was chosen to protect the vessel from impact shock through collision with underwater obstacles. The unit possesses a unique hydraulic transmission "kick-up" specifically applicable during beaching operations.
Division of the hull into 10 compartments by watertight structural bulkheads means that the barge will maintain stability even if three compartments were flooded.
Three 11.5m spud legs can be independently or simultaneously operated and will provide 6.3m of anchorage beneath the vessel. They incorporate a pioneering design of guiding tube casing which is split into two sections, (the upper removable) to allow the lower section of the spud leg to be retrieved if bent.
The initial six months of dredging will remove approximately 0.5m of coarse sand and shingle from the outer channel to the harbour entrance and involve the creation of 1.5m high pronounced sediment berms placed along its eastern side. The eventual tidal dispersal along the eastern shore will encourage crucial self-maintenance of the channel.
Dredged material from the jetty area will be placed on adjacent sand flats increasing the bed level by 1m and also to the south of the pontoons, to prevent the emergence of a second channel to threaten the main channel towards the quay.
When the initial phase of capital dredging is completed next March, the vessel will remain in service to counter the effects of channel shift by maintaining a minimum operational depth of 1m at low water neaps in the main channel and access channel to the jetty. This will facilitate continuous 24 hour access by wind farm support vessels and other commercial workboats involved in the upcoming Sheringham Shoal Offshore Wind Farm.