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

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.

What are the assumptions in load calculation of ship impact
on marine structure?

As discussed earlier kinetic energy approach is used to
calculate loading of ship impact on marine structure; same assumptions taken
for these are
  • Selections of type and size of ships producing
    design collision force.
  • The maximum speed of collision
  • The mode of contact between ship and structure.
Interesting thing is that when a marine vessel approaching
to the berthing facility is decelerated from certain speed, a certain amount of
water moving along with the vessel is also decelerated. So the mass that are
used to determine the kinetic energy not only sole mass of the vessel, but also
an addition of mass is required which is often called hydrodynamically added

How are the loads encountered in design?

The usual case is to consider a constant mass of water or forces from other sources in addition to mass of vessel while designing marine structure. Thus additional massed are considered independent of all other factors controlling it (Saurine, 1963 & Vasco costa, 1964 and 1973). Other designer take into account for possible eccentricities (Thoresen, 1977) while designer like Giraudet (1966) considered only under keel clearance. Special attention was given to accidental collision between a vessel and an offshore structure in current design approach (Donegan, 1982, Foss & Edvardsen, 1982, Oliveria 1981, Petersen and Pedersen, 1981 and pettersen and johnsen, 1981) which basically deals with structural aspects. 

Netherlands Ship Model Basin (NSMB) a laboratories of Maritime Research Institute Netherlands (MARIN) has developed an advanced computer program which can receive input of all factors discussed earlier influencing added mass. At first Van Oortmerssen (1974) was first presented this at the Offshore Technology Conference. This program is now further developed by refining and necessary validation.
Available codes

To obtain secure and safe berth of a vessel, certain acceptable limits are imposed over motion and minimum design strength of marine structure. Special attention is provided during transferring fluids from or into vessel like gas, oil and similar petrochemical products. All types of forces incorporated in these process should be encountered in design as exactly as possible. Notional design codes, European and several international codes are available; all are considering mainly reliability method as in the EN 1900 (European Norm). In EN 1990 standard, design is done by establishing a safe structure by providing partial safety factor.

The loads and their combination used in this case are quite different from that used for building and bridge structure and require different partial factor. British standard includes deterministic calculation approach for near-shore loads but the partial factors used in these approaches have not been yet evaluated in line with partial safety factor used in EN 1990. BS 6349-part-2 provided Annex A to fill the limitation of it as much as possible but still not based on probabilistic formulas or calculations. This Annex A of BS 6343 provide us as a guide for ship berthing action, accidental ship impact action and mooring action which are not detailed in EN 1990.

Behavior of berthed vessel

For a large vessel, tugs are required to keep it moving for better control and they tow the vessel to desired location declared by port authority. How many tugs are required depends on

• Size of the vessel
• Power to weight ratio of it
• Direction and speed of the winds and currencies to be encountered for berthing

Tugs are often called tow boat or tug boat


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