*Article for Crane Brasil Magazine – Edition 56/2018
Since the beginning, in the last century, of oil and gas exploration through offshore platforms, cranes have played a fundamental role in their daily operation. Since 95% of the provision, equipment and inputs are taken by sea through supply boats and loaded by cranes to the platforms, the lifting and cargo handling operations have a notable influence on the operational efficiency of oil exploration and production and gas.
In addition to routine operations, which are common day-to-day lifts, where offshore containers, cargo baskets, pipelines and people are moved, there are also special, non-routine activities, which involve assembling new modules, replacing equipment, various maintenance and lifting different loads. Also, offshore cranes are used in even more specialized activities, such as lifting new platforms, launching subsea equipment thousands of meters deep and removing deactivated platforms.
Cranes installed on platforms are generally of the “crane on pedestal” type where the superstructure is supported by a steel tube welded to the structure or directly to the hull, depending on the type of platform, and face more adverse conditions during operation than land cranes. , due to the action of waves, current and wind, which cause movements of the support boat, platform, crane and cargo. Even on fixed platforms there is the relative movement of the load on the support vessel and the tip of the crane’s boom, amplifying the forces on the crane and load.
With this, it is to be expected that there is a difference between land cranes and offshore cranes. Based on the cranes installed on exploration and production platforms, such as drillships, semi-submersibles and FPSO, there are some fundamental differences compared to mobile land cranes.
The offshore crane load table takes into account the dynamic effects of the movement of the support vessel, the load and the platform, with different capacities for each sea wave height, also differing according to the type of lifting, with a table for offboard lifting (outside the platform), when the load will be retained or placed on the support vessel and onboard lifting, or when the load is lifted inside the platform and placed on the platform itself, this being a more favorable situation than the first. In land mobile cranes, the dynamic effect factors must be part of the load weight, through dynamic amplification factors.
Obviously, the dynamic effects incorporated in the tables do not include lifts where there may be resonance, that is, the wave may have a natural period such that it amplifies the movements of the platform or support boat, also amplifying the forces applied to the crane, which may reach to three times the static value, which can cause catastrophic overloads or loss of load stability. Hence the dynamic analysis during the project is fundamental for the safety of the operation.
Since movements caused by waves cannot be avoided, offshore cranes have a much more robust structure than land mobile cranes, as in addition to conventional efforts, they must withstand horizontal forces in the boom plane (offlead) and forces outside the boom plane. 9sidelead), which cause a large increase in bending and torsion moment, occurring due to the great movement that the load can experience when removed from or placed on the support vessel. These lateral and horizontal efforts are not foreseen in operations with land cranes.
Another factor that leads to the robustness of offshore cranes is the possibility of the load being attached to the support boat, which can also overload the crane when the boat makes the downward movement in the wave and the platform simultaneously makes the upward movement, with a great impact.
There are currently two main standards for design, manufacture and testing of offshore cranes, an American one, API Specification 2C, and a European one, EN 13852 parts 1 and 2. Cranes designed by the American standard tend to be more structurally robust as a way of increasing the safety. The cranes designed according to the European standard tend to be lighter, but safety is increased through electronic devices that prevent risky situations, such as overloads and cargo trapped in the support boat, with the disadvantage being a greater probability of involuntary detachment of the load .
From the brief description of some characteristics of cranes and offshore lifting, one can see the complexity of these operations in relation to lifting carried out on land, which always requires a specialized lifting project (rigging plan), effective advance planning, detailed prior inspection and execution by a qualified team, in order to guarantee the safety and continuity of offshore operations.
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