On dredgers, especially hopperdredgers, it is –at times- necessary to perform crane work in open sea conditions.
Onboard dredgers three types of cranes can be found:
a. Fixed cranes, this is: with a fixed foundation.
b. Travelling cranes with . 1. boom on top
c. Travelling cranes with .2. boom on transversal beam
Fixed cranes are generally cheaper in investment and maintenance, but travelling cranes are more suitable for offshore conditions, especially the type with a lower placed boom.
A ship at sea is subjected to motion induced by waves and / or swell. This ship’s motion (roll and pitch) hampers cranework and renders it dangerous.
A swell can be present, almost unnoticeable, but can affect crane work badly.
This makes crane work at sea dangerous for the unexperienced.
The suspended load in the crane has a tendency to swing, according the ship’s motion(s). In a worst case; the suspended load swings into resonance with the ship’s motion, which causes the amplitude of the swing to increase, and get out of control, fast.
The crane operator can –to a certain degree- avoid the load from swinging by
I. Counter acting this swing by short movements of he crane’s boom.
II. Keep the suspended load on a short hoisting wire, and
III. From (II.) follows: keep the boom as horizontal as possible. This can best be done with travelling cranes with a low placed boom. The shorter the hoisting wire, the less the chance of a heavily swinging load.
Small loads (up to appx. 1 ton) can quite easily be kept under control with manropes.
Special care has to be taken with crew lifting in a man-riding basket:
a. The crane has to be certified for such operation.
b. The crew basket itself has to be certified.
c. Crew has to carry a safety belt, directly connected to the crane hook.
Lifts heavier than 1 ton, at sea, are always a liability and must be considered high risk.
When lifting of heavy loads is “really” necessary, with swell present, one can proceed as follows:
a. Choose a heading for the ship, minimising roll / pitch
b. Ballast the ship down as much as possible
c. Stop the ship completely. If the load starts swinging out of control, one still has the option of dipping the load in the water, to dampen the swing.
d. Let the best crane operator onboard operate the crane.
e. Make radio contact between deck and crane cabin.
f. Keep in mind the length of the crane-job.
Once the crane is committed to the lift, and he weather deteriorates, risk increases.
Even with all this precautions, safety is not guaranteed.
Experience is a necessary ingredient and an error of judgement can cause heavy damage or bodily harm.
Captains, consider the sentence “When lifting of heavy loads is really necessary”…. It almost never is…
I used the captain’s overriding authority once, refusing to do a “really necessary” crane job in open sea. The ship was diverted to a sheltered anchorage, despite a huge loss of time. But we did the crane job, in safe conditions.
The important thing is: to make an assessment, and stick to it.
Marc Van de Velde