May 16, 2023
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Since childhood, we have been intrigued by this question: How are ships ‘parked’ upon their arrival in ports, jetties, and piers? Unlike cars, they can’t simply be put off gear and parking brakes! Ships don’t have brakes in the first place.
Ships need to be fastened and fixated soundly for conducting all kinds of shore operations such as cargo loading/unloading, refuelling, bunkering, ballasting/deballasting, boarding/deboarding, maintenance, repairing, and often for idle times based on voyage schedules and berth or workforce availability.
So, mooring or the system of securing a vessel soundly for the purposes mentioned above is indispensable in studying ships and offshore structures.
It is common to get confused with the terms mooring, docking and anchoring and may use them interchangeably. But there are stark differences between them.
Anchoring is the system of securing a vessel amid the sea when the ship is not near the vicinity of a permanent structure. In other words, when a vessel requires to be fastened or stranded for various purposes in the deep waters, anchoring is used so that the vessel does not drift away in the action of hydrodynamic forces present in various forms. For anchorage or anchoring, anchors, those heavy weights as old as the history of ships, have been used for a very long time.
However, in recent times, vessels can be secured anywhere in the seas with the help of in-situ dynamic positioning systems (DPS), which are automated systems that can keep the vessel affixed to a particular coordinate without the help of anchors.
In the traditional systems of anchors, the heavyweight is suspended from the vessel and is allowed to settle onto the seabed. This fixity of the weight in the seabed and the resultant high degrees of tension created in the strong anchoring lines, which are heavy-built lock chains, helps keep the vessel in its position. The size of the anchors depends on the size of the vessel.
Docking, on the other hand, alludes to the hauling of the vessel entirely away from the water to a dry area adjacent to the surrounding waters for various purposes such as maintenance, repair, refitting, or even disposing of an old vessel. Here, for all practical purposes, the vessel is suitably shifted to an enclosed area, and the water is then drained off, leading to a dry surface. The term ‘dry docking’ is often used for repair and maintenance work.
Now, mooring again is the fastening of a vessel to any shore or land-based structure with the help of suitable mechanisms such that the vessel is not subjected to free motion. This land-based structure may include berths, jetties, piers, wharves, quays, etc.
The basic components of a typical mooring system are as follows:
These are the main components of any mooring system. In earlier times, ropes were mainly used as mooring lines. However, steel or high-grade synthetic materials have been widely used. The main requirements of any mooring line should be high strength and elasticity. The forces transmitted from the ship structures are transmitted directly to these mooring lines.
The net effect on the lines becomes manifold from this static load coupled with the random dynamic behaviour from the tendency of motions or environmental loads acting on the ship. So, these mooring lines’ material and sizes are carefully chosen based on the vessel and the type of mooring arrangement.
All mooring lines are characteristic of a Safe Working Load or SWL based on their properties. Like all other structures, these mooring lines have a definite breaking strength with a safety factor margin. Steel hybrids of high grades are often used, and common materials such as Polyamide or HMPE are used for synthetic.
Generally, lower elastic or higher rigid but greater strength lines are used for larger vessels, and higher elastic or less rigid materials are used for smaller ships. The reason is that heavier vessels, due to their greater inertia, have a lesser tendency to respond to external forces and, thus, are less subjected to random motion parameters than smaller vessels. Therefore, when these lines have higher flexibility, they can cater to smaller ships more prone to motion when moored. HMPE and steel have low elasticity, whereas other materials, such as polyamide, have greater elasticity.
On the shoreside, the mooring lines are joined to the winches. These winches act as end supports for these lines and help in handling and directing the lines as per requirements. The winch system controls the tension and extension of the mooring lines. The mooring lines are commonly operated by electro-hydraulic power that uses hydraulic pressure to control the tension and traction forces on the lines. The main component of the winches is cable drums, either oriented horizontally or vertically. The size of the drum once again depends on the ropes.
The winches are operated by electrical and hydraulic mechanisms. These are mainly dedicated engines or motors that supply the power to the winch through torque. In turn, the power supply is from the shore supply main lines. These systems’ capacity is as per the capacity and configuration of the winch and related mooring lines.
At the other end of the mooring lines, they must be tightly secured to the vessel structure. These attachments are usually on the main deck but can sometimes be placed in subsequent decks. Once again, based on the type of vessel, the number, size, type, and configuration of these fittings are present. The fittings associated with these mooring are all designed and constructed per standard IACS guidelines based on the vessel and the related mooring system. Usually, the fitting used are chocks, bollards, fairleads, bitts, etc.
Now, despite the mooring lines, vessels often tend towards motion and, as a result, can be prone to hit on the adjacent shore or dock structure to which it is secured. Such impacts can result in damage to the shore structure as well as the vessel. So, for the same reason, the vessel and the shore structure are padded with a securing arrangement known as fenders, which absorb impact energy in the event of a collision or strike. The design of the fenders is once again based on the vessel type and size. They are usually made of rubber, wood, or high-grade synthetic polymers.
Now, for a mooring system, the main job is to secure a vessel in its position when berthed safely and also ensure that the vessel is kept sound from collisions or damages due to vessel-structure interactions due to external dynamic forces. Moreover, the mooring arrangement should be such that for various port or dock operations, there is never a problem. So, for a good mooring system for a given vessel, the main characteristics can be listed as follows:
Mooring can be of various types:
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