- By Ajay Menon | In: Naval Architecture |
- Issue Time
The anchor chain is made up of multiple link segments that resemble closed loops.
What is anchor chain?
The anchor chain is made up of multiple link segments that resemble closed loops. Each loop is connected through to its neighbouring loops, creating a chain of any given length.
The major advantage with this type of anchor rode is that it is resistant to the wear and tear that is extremely common with anchor cables. Because it is generally made of metal, the rubbing action between two adjacent links will cause only small deformations over long periods.
Compared to the problem of chaffing with cables, the chain rode can withstand almost all types of friction-related deformations with the application of lubricants. Most lubricants eventually come off as a result of continuous exposure to water. However, this merely requires the anchor to be wound back up, and the required links to be coated in grease.
Although common lubricants such as WD-40 are used, their toxic effects on the marine flora and fauna because of chemical exposure have gradually resulted in them being switched out for grease and other types of lubricants.
Another advantage with the anchor chain is that it can compactly fit in the anchor chain storage locker, because of the individual weight of each link. Unlike rope or cable that tends to be elastic, and hence more difficult to coil up, the individual links are much easier to store.
However, the biggest problem with using chains, especially for large structures such as ships or offshore installations, is that the anchor rode becomes extremely heavy. When this is combined with the weight of the anchor itself, the total mass of the system comes to a few tons. To be able to haul up this weight, heavy-duty motors or cranes are required, that can complicate the operation.
Another issue is that metal does not absorb as much energy compared to cable or ropes. Because of this, it is often not able to redirect any jerks or other disturbances being generated from the ocean bed or seafloor. If the entire chain abruptly becomes taut because of natural wave motions, a massive shock can be transmitted to the vessel or structure, that can cause failure and metal fatigue over time.
An interesting aspect of anchor chains is the catenary action of weighted links. Since each link weighs a sizeable amount, the entire chain tends to sag or fall in the middle because of this weight. Catenary refers to the natural shape that a chain takes when it is suspended between two points.
The benefit of this catenary action is that it decreases the initial angle between the seafloor or ocean bed and the anchor rode. Because of this, a certain amount of forces and loads are absorbed and dissipated away from the structure and anchor. However, this only takes place as long as the angle remains small and there are acceptable ranges of loads.
This type of anchor rode is mainly used for large vessels and offshore installations. This is because these structures are considerably heavy, displacing several tens of thousands of cubic metres of water. If a cable rode is used to anchor these structures, it will eventually chaff and simply break apart.
Heavy-duty metal chain links are used that can withstand the enormous loads placed on the chain. Common materials that are used in manufacturing the individual metal links include high carbon steel and heat-treated high carbon steel. The latter is known as the transport chain or G70 and has an exceptional strength to weight ratio that makes it ideal for such purposes.