Foundation, Concrete and Earthquake Engineering

How to Determine Ship Impact Loads on Marine Structure?

Nowadays the size of vessels are increasing resulting greater risk of damage due to impact. Thus in some important marine structures consideration for ship impact is essential to provide a way to imbibe energy and to minimize damage. The usual countermeasure is to achieve diaphragm action by increasing mass, prestressing and well distribution of reinforcement.

Importance of determining ship impact loads

A significant volume of gas and oil are now transported by shipping and near future this transportation is expected to be increased. A usual way to reduce cost of transportation is to use large vessels and new ports are constructing in new locations may often be situated hostile environment seeking more attention to structural design as loads on marine structure and berthed vessel are increased. We are using conventional approach for decades which considered marine vessels used from 1980 and requires to upgrade for current large vessels.

How are loads exerted on marine structures?

The vessels approaching to a jetty is berthed as softly as possible. Even with utmost care it is inevitable to minimize load transferred to marine structure like dolphins or jetty. Unfortunately the force exerted by a hundred thousand DWT ship in this process is not small. After berthing is done and every mooring line is attached to dolphins certain other forces act on the fenders or dolphins or jetty and on the vessel.

Dolphins are supported by several piles founded on riverbed or seabed leveled above water and connected to each other to establish a fixing point or a platform often constructed apart from shore. The purpose of dolphin is to facilitate dry access platform by providing a fixed structure. When it is not possible to extend shore, dolphin is introduced as an example when limited length of berth is not enough to accommodate larger number of ships.

Mooring lines are cables used to secure a vessel at a berth. The arrangement of lines should kept symmetrical about midship point of the vessel to establish stability.

Vessel loads are caused by the ambient weather like winds and currents which exert kinetic energy on fenders and subsequent transferring through mooring line and dolphin respectively. At the moment of contact between berthing structure and vessel, through fendering, the energy interaction generates reaction force until the vessel is completely anchored to rest. Kinetic energy of marine vessel together with motion of water followed by it, produces forces that are transferred to marine structure which is finally transferred to the subsoil through the pile. The way of distribution and to absorb this energy within the structure depends on structural stiffness and location of loads exerted.

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