*Article published in CRANE BRASIL Ed. 73
Offshore activities, that is, those that are carried out off the coast, involve port construction, dredging, oil platform operations, construction and maintenance of these platforms, fishing, mineral exploration and many others, where lifting operations are fundamental.
Although frequent, these lifts bring challenges in terms of ensuring the safety of the people involved, integrity of the lifted goods, preservation of the environment and operational efficiency.
As with other critical engineering activities, these challenges are overcome through adequate rigging plans (lift projects), advance planning, execution by trained and qualified personnel and continuous supervision of the lift.
Recently, monitoring technology has helped with the challenges, which can inform in real time all the forces, velocities, accelerations and stresses involved, through optical, electronic and laser sensors.
Below are exemplified 3 challenges to be overcome in offshore lifting, in the area of construction, maintenance and offshore operation.
1) Amplification of forces and movements during lifting
Invariably, in offshore lifts, there are wave, wind and current effects, which can occur alone or in combination. In addition, you can have the relative movement of the platform crane and the vessel with the load, for example, a floating platform removing a part from a raft or lifting a container from a support boat.
All of this will amplify the forces that act on the equipment involved, for example, a steel cable sling of a container, which would be hoisted on land, with traction of 10 tons, in offshore hoisting it can reach up to 15 tons. A 5-ton container on land, on the other hand, can experience up to 10 tons of force when withdrawing the support vessel.
In addition to the amplification of the forces, the movements are amplified, with the load having greater amplitudes, with the possibility of loss of control causing a collision.
To overcome the amplification of forces, the offshore lifting standards, in a simplified way, use a dynamic amplification factor, which multiplies the static weight lifted. This procedure, although direct, can lead to the undersizing of the elements, especially when there is resonance.
The solution to this challenge is to carry out, for more complex cases, a dynamic analysis of the lifting, taking into account the wave, vessels, crane and load, where the result shows the expected forces and displacements, allowing the dimensioning of the elements. With this, it is possible to increase safety and expand the meteoceanographic window available for the operation, avoiding unnecessary waits.
2) Lifting without using the crane
One of the constant activities of platforms is maintenance and repair, which usually involves changing equipment. Most of them are positioned under decks, being inaccessible to the crane.
One of the solutions is the construction of temporary platforms or monorails on the edge of the deck, where the crane transfers the load and it is taken horizontally to the interior of the platform.
For horizontal movement, trolleys, “turtles” or hoists supported by the platform structure can be used.
3) Force Monitoring During Lifting
With the dynamic effects and consequently greater probability of overloading equipment, accessories and cargo, a challenge is to identify occurrences that may indicate risks to lifting.
This can be done by installing sensors on the cargo, slings, structure and other points of interest. These sensors can measure dynamic pulling forces on the crane’s slings and cable, tensions, accelerations, displacements and other parameters that allow control, based on design values, all in real time.
These sensors are capable of taking thousands of readings per second, a fundamental requirement to identify peaks caused by dynamic actions, unlike sensors normally installed on cranes, which only take a few readings per second.
Therefore, it is possible to identify anomalies during the lift, interrupt the operation, solve the problem and proceed with the lift, increasing safety in critical lifts, bringing excellent cost/benefit to the project.
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