On a jack-up barge, cranes serve several purposes and are involved in various jobs offshore, such as loading and cargo handling to and from supply vessels, barges, platforms, personnel transfer, support for drilling and oil production, well service and maintenance, etc. Combined with the elevating capability of a jackup, cranes on such barges can achieve a substantial hook height and reach tall offshore objects. The cranes on a jackup barge are used to lift equipment and materials from the barge to the offshore platform and vice versa.
Leg Encircling Cranes (LECs), also known as „fully rotating crane“, are specifically designed for use on jack-up vessels. These cranes are built around the leg, and their design allows for better use of free deck space. Their advantages are lightweight design, minimal deck space requirements, and long boom lengths.
The biggest Leg Encircling Crane was installed on the offshore installation vessel Aeolus by Van Oord. LEC allows for the installation of larger and heavier foundations and turbines for offshore wind farms.
However, cranes, including LECs, can be costly, require skilled labor to operate, and may not be feasible for use at all sites.
On the picture we can see the simple version of such crane, on a jackup vessel constructed in China.
CRANE RADIUS
The distance from the crane at which the load can be lifted is inversely proportional to the capacity of the crane. As the boom radius increases, the allowed capacity (SWL) reduces.
LOAD CHARTS
Load charts specify the maximum weight that a crane can lift (SWL) at various boom lengths, radii, and angles. This information is essential for ensuring that the crane is not overloaded.
By referring to the load chart, operators can determine the maximum weight that can be lifted at different boom lengths.
Offshore lifting is dynamic in nature due to waves, boat movement, wind forces, etc. It is important to include the dynamic effects into the calculation as these can significantly reduce the crane capacity.
Crane capacity is also downgraded when transferring personnel, to increase the safety margin for such lifts.
If you want to learn more about the operation of the jacking system, watch the videos below.