Today's modern ships require deeper drafts to move goods more economically. Removal of the sediment material from the navigation channels and berths by dredging allows more fully loaded ships safe passage into and out of berthing facilities. Shallow draft clearances (shallow depths) in navigation channels and berthing facilities forces shippers to carry less cargo increasing the effective shipping cost of the delivery. In the case of tanker ships carrying petroleum products, costly and environmentally hazardous lightering may be required before the tanker can enter the shallow port.
Lightering is the act of transporting cargoes from ship to shore via a lighter vessel. Lightering involves the open water transfer of fuel from the tankers to several smaller vessels to distribute the load and reduce the draft of the tanker to an allowable entry depth. Dredging minimizes costly "lightering" missions and maximizes offloading at docks, thus reducing overall transportation costs. This lightering, increased the overall number of ships coming into a harbor. It also creates a situation where oil is transferred in mid-bay.
When a lightering tanker approaches a fully-loaded VLCC or ULCC, it may have 50' of freeboard, compared to approximately 20' for the VLCC or ULCC. As the VLCC or ULCC transfers cargo to the lightering vessel, these freeboard heights essentially reverse. When the lightering tanker uncouples from the VLCC or ULCC, it may have a 20'freeboard, and the VLCC or ULCC may have a 50' freeboard.
Primary fenders are the large fenders that are positioned along the parallel midbody of the ships. They are either foam or pneumatic. Both foam and pneumatic fenders work by compressing air, although in foam fenders the air is contained within small closed foam cells. Foam fenders may be manufactured with Standard, Low Reaction, or High Capacity foam, corresponding to the different pressures of pneumatic fenders, but with a somewhat greater range of pressures. Primary fenders are positioned at both ends of the parallel mid-body of the ship, with suitable fenders in between. The quantity of fenders used depends upon the sizes of the two vessels, as well as whether multiple vessels are anticipated. For many operations where the length of the smaller vessel does not vary (such as on a dedicated lightering vessel), a quantity of four fenders is commonly used.
Primary fenders are commonly used in the floating mode (i. e., not suspended). They are also commonly outfitted with chain and tire nets. The chain and tire nets help stabilize the fenders in the seaway, they provide protection against abrasion of the ship hull by the chain net, and they act as a longitudinal shock absorber. They also slightly increase the standoff between the vessels.
Secondary fenders may be either pneumatic or foam filled. They are normally somewhat smaller than the primary fenders and are suspended well above the waterline. (Some operators refer to them as "baby fenders".) Their purpose is to protect bow and stern plating from inadvertent contact during mooring and unmooring. They are normally located forward and aft of the parallel midbody.
Use of a dedicated vessel for lightering versus a vessel of opportunity lets operators optimize their equipment for the operation. A dedicated lightering vessel, for example, may be equipped with fender davits. This allows the fenders to be lifted out of the water and stored on deck during transit to and from the lightering operation. If the vessel is not equipped with fender davits, the fenders must either be towed out to the lightering area by the lightering tanker or brought out by a support vessel. Having the fenders out of the water for transit significantly increases the speed of transit.
When a dedicated vessel is used, rigging for the fenders can also be dedicated and optimized for the operation. Quick-release hooks for the lines that connect the two vessels can be permanently mounted. Strain gages can be employed on the quick release hooks, giving a real-time readout of the line loads during the operation.
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