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Ensuring integrity of crane safety systems

Andrew Garvie, equipment and integrity technical authority, examines how to ensure the integrity of crane safety systems.

With all equipment and supplies offshore lifted on to the asset via the crane, cranes are often regarded as the main artery of a platform and disruption to operations can impact heavily on other activity on the platform. Failures can also have catastrophic consequences, for the asset and personnel, so it’s vital that cranes are safe, reliable and avoid unnecessary downtime.

Incidents have occurred where a crane is operated beyond the safe limits of its design, raising questions about the integrity of crane safety systems.

As safety systems are designed to activate outwith the normal operations of the crane they are not expected to be used on a daily basis, so the problem lies in how operators can demonstrate that they are working.

Ensuring safe operational limits of cranes is only one aspect of crane safety, there are several other safety systems on the crane so it was important that we examined how they can be tested and verified as well.

While the design and types of components are different across the various crane types, the systems largely fall in to the following categories:

  • Overload protection
  • Rated capacity indicators
  • Motion limits
  • Engine overspeed.

The first step is to understand the system operation and what it’s designed to do. For example, is it fully automatic, does the operator have an activation button, is it mechanical, what is measuring the system parameters, does the control system monitor the health of the safety system?

On some systems there is secondary protection, where for example, two switches are designed to work in parallel so that either can be triggered. However, if this isn’t realised there’s potential for the same switch to be tested each time without the other being checked.

After the systems are understood it’s also important to recognise what failures may occur in the cranes that could potentially overcome these systems. If these failures aren’t explored, the danger is that it’s assumed the safety limits will work regardless.

Where we’ve identified cranes that are at risk we’ve developed a new design to upgrade the crane safety systems, with the maintenance routines and testing procedures also amended to reflect the changes.

It’s important to document changes and differences as no two cranes are alike, even sometimes across one crane manufacturer. Often the manufacturer makes one standard model of crane and provides a generic manual - but when the crane is purchased it is manufactured to the owners specifications which can differ from the OEM basic machine.

Once it’s understood how the safety system is designed to work and any failures that might occur, then a testing regime can be developed. Frequently, although checklists ascertain that the crane operator should test limits, they don’t explain what should happen during the test or how the test shows the system is working correctly.

Making it easy for tests to be carried out is crucial. One example of this is to lay out markings on the deck for testing the boom limits at various radius'.

The operator booms up, making sure that when the limit activates the hook block is within the square on the deck. If this happens then he knows that the limit has worked in the right place, if not then there’s an issue and there needs to be some calibration.

Different systems will need to be tested in different ways, doing a full function test on your overload protection system would potentially put the crane at risk so consideration should be given to an appropriate simulated test. For example, putting a test curve in the rated capacity indicator will act like a load multiplier, the crane can then lift a load which is within its limits, but the control system thinks that it’s being overloaded and activates the protection system – demonstrating the safety limits work without any stresses being put on the crane.

There’s also a risk to doing a full function test on the ultimate boom minimum radius limit, so we need to fully understand how the system works to create a suitable and safe testing regime. For example, on some cranes it has been identified that both the working radius limit switch and the ultimate radius limit switch work electrically in series, they’re both operating the same contactor which operates the same brake valve and hoist pump controller, so testing either switch shows the overall system is working.

Combining a full function test on the normal radius limit switch, with a simulated test on the ultimate switch where it is triggered manually, will demonstrate that both the system and the ultimate limit switch are working. This can then be supplemented by verifying the alignment of the ultimate limit activation components to confirm that they are correctly located and therefore should activate at the correct point.

Across the North Sea there are many differences in how the cranes operate, making developing a single policy or testing regime unfeasible. Understanding each individual crane, how it is set up and what systems are in place are the first steps to developing a testing regime which will ensure the integrity of the safety systems.

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