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What is the ‘Swing Factor’?
All forms of transfer to offshore installations involve risk. Analysis of past incidents relating to traditional basket transfers has highlighted one specific risk area that warrants particular focus.
The dynamic motions of a vessel make it difficult to ensure that a crane is centred directly over a load prior to making a lift. An ‘off-centred’ lift will result in a sudden swinging motion (or pendulum effect) which can lead to a violent collision. Such collisions are part of the everyday concerns of offshore crane operators and deck crews, who, to a degree, are trained to anticipate, manage and sometimes control such motions. However, given the number of variables involved (vessel motion, crane motion, timing of lifts etc) it is unrealistic to expect to completely avoid such collisions.
Collisions involving traditional personnel carriers that offer no peripheral protection are of particular concern. A lifted load will swing twice the distance of the offset of the vessel from the crane boom tip i.e. for an offset of 3m the load will swing 6m. The highest velocity is reached at the lowest point (half way through the swing) and a collision at this point would result in the maximum damage. There is also little time to react, for example for a 3m vessel offset and a 30m crane line an impact with an obstacle 3m away will occur in just 2.8 seconds (and at a speed of 1.5m/s).
Reflex Marine has analysed a database of 62 crane transfer incidents (including 48 injuries and 7 fatalities). The number of incidents involving ‘swing factor’ was found to be as high as 32%, clearly indicating that this is one of the key risks to be addressed in personnel transfer operations.
Landing area – Ideally, to avoid any risk of collision, the landing area on a vessel should have a radius of twice the maximum anticipated vessel offset. For example, if the vessel is expected to hold station within 3m the statistics would be as follows:
a. Maximum vessel offset – 3m (10ft)
b. Maximum swing distance (2 × a) – 6m (20ft)
c. Clear landing area radius (b.) – 6m (20ft)
Diameter of clear landing area (2 × c.) – 12m (40ft)
To completely avoid the risk of a collision, with even a modest vessel offset, a large landing area (free of cargo and other obstructions) is required. Often it is impractical (or even impossible) to provide this amount of space. Also, even on a flat deck, the combination of the descent of the transfer device during the swing and vessel heave can result in a re-collision with the deck and, serious incidents have resulted from this scenario.
Passenger protection – Another approach is to provide sufficient protection to avoid harm to passengers when collisions do occur. The Frog transfer capsule, for example, has been designed to protect passengers in collisions of up to 2m/s (6.7ft/s) or 7.2km/hr (4.5 mph); a design verified through biomechanical studies and crash tests.
Regardless of the transfer device used, the ‘swing factor’ can effect your operations. Each operation has a unique risk profile and operators should be aware of the key factors effecting transfer risk. These risks can be managed through training, awareness, careful selection of transfer equipment and appropriate operational controls. Reflex Marine is available to help and advise if you have specific concerns about your operation.
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