The definition of a mooring is to “fix firmly, and secure”, so why does this seemingly simple task often go wrong?
There are numerous incidents of buoys coming loose from their moorings, for example in 2012 a £40,000 scientific buoy used to collect data off the coast of Weston-Super-Mare went missing. It came loose of its moorings and was found 20 miles away having been battered by tides and rocks, causing damage to the sensitive and expensive monitoring equipment.
The costs involved in having an incorrect mooring system can be considerable. Damage to the buoys itself may result in repair bills or replacement costs. Sinkers and chains may need to be replaced if they can’t be recovered, and there are the significant costs involved in retrieving, inspecting and redeploying the buoy. It all adds up.
Although moorings can fail due to particularly strong storms, most of the issues that cause failures can be mitigated against using the correct mooring configuration.
A mooring system is more than just a length of chain and a heavy weight, and requires a bit of thought and planning to prevent failures occurring.
So why do moorings fail?
The chain is too short – The simple rule that a mooring chain should be 3x the charted depth of water may work in some conditions, but this formula can’t be a golden rule. Variables such as the different forces exerted on the moorings (wind, waves and currents) must all be taken in to account. In many locations a longer chain acts as a dampener against these forces. The additional weight of a longer chain balances the buoy and keeps excessive motion in check.
If the chain is too short it can impose shock loads on the sinker, buoy and any electronic systems on board. This can cause wear, tear and premature failure of the components. A costly mistake.
The sinker is too big – although a large sinker won’t cause a problem on its own (only to your wallet!), combined with a short chain puts further strain on the components. Although the buoy won’t get dragged along, the chain and shackles take the full shock of the forces and are more likely to fail – causing your buoy to go on an expensive walkabout.
So how do you work out what the correct, most cost effective mooring for your location is?
Mobilis have created the IALA approved CALMAR software, which allows the user to calculate the correct mooring, not just of their own buoys, but also that of other manufacturers. The software uses more load parameters than other programme, and Mobilis have been using this full analysis method to calculate moorings for over 20 years and “have not experienced a single buoy mooring line breakage over several thousand buoy mooring specifications.”
The software also provides a minimum sinker mass required. With the correct length of chain, the upwards pull on the sinker is reduced, meaning sinker sizes can also be reduced – a bonus in terms of costs and transport and handling.
It may feel wrong to have a longer chain and many people might have concerns about the buoy rotating around the sinker. However this is not necessarily the case. Prevailing weather and seas usually come from one direction; the longer chain spends most of the time buried in sand or mud on the sea floor, and only comes into action in the worst of conditions. This results in comparatively little movement of the buoy and the mooring settles into the seabed. It could mean the difference between the buoy staying on station or breaking free.
Our advice is to try out the free CALMAR software for yourself and see if a longer chain and a lighter sinker could be the answer to your mooring issues. It could end up saving you time and money in the long run, and avoid your buoys going wandering off.
