Load pins for critical load measurements

Paul Boughton

Steve Hewitt looks at the uses of load measuring pins wth the oil industry

Within the oil industry load measuring pins are widely used – from drill rigs, supply boats and sub-sea assemblies. They can also be used to replace an existing joint, where clevis pins, are used in cranes, winches, anchor chain stops, wedge sockets and shackles.
The material from which pin is manufactured is usually a high strength stainless steel and sometimes a super alloy (Fig. 1).
Strain gauges are used to measure the shear strain at the maximum shear section through the pin and arranged into a whetstone bridge circuit. By using the shear stress, the measurement is not dependant on load position and the supports as bending effects have very little effect on the measurement.
To help with this 3D modelling is used that is then linked to a finite element analysis (FEA) package. With the use of the FEA, the stresses in the pin can be seen and this allows for modifications to be carried out before the material is cut. The 3D modelling also allows the product to be visualised before final drawings are completed (Fig 2).
The load pin can also measure loads in more than one direction and so can measure the loads in two plains X and Y. The output from the strain bridge generally varies as the angle of the applied load rotates around the central axis by the cosine law and so, with a single applied bridge at 90 degrees, the output has reduced to zero.
By installing a dual bridge offset by 90 degrees, it is possible to have two signals from the pin, which can be used to determine the resultant force and angle.
This format of pin can be used on variable angle jib cranes without adding extra steelwork to give a constant effective angle over the pin.
By having the loads in both plains the resultant load can be calculated and by referring one of the plains to a known angle the direction of load can be calculated.
We also know that strain gauges can be classified as simple apparatus and this makes it easier to use in hazardous area with the use of the correct electronics.
Applications for load pins

With anchor handling using the winches, load pins can be installed in the brake mechanism for static load measurement or in the base of the winch for dynamic measurements.
There are so many variation with winch design and the requirements required from the operator that each case is considered individually.
Mooring buoys are another application where load pins are used to measure the hawser tension. This application is critical for safety as well as to try to predict the life of the hawser.
Chain stoppers for tankers and tugs include pins that can also be monitored to control forces. Diving equipment that needs load monitoring often use load pins as a practical way to monitor load. Drill string tension and heave compensation use load pins as method to capture the load.
Future developments

As knowledge of materials and designs develop, the load measuring pin improves in accuracy and predicted life. The associated electronics are also making it easier to measure loads that were, in the past difficult, but can now be continually monitored
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Steve Hewitt is Director, Active Load Limited, Reading, England. www.activeload.co.uk
Fig. 1. A typical load pins.
Fi. 2. 3D modelling also allows load pins to be visualised before final drawings are completed.

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