28 July 2005
deformation coefficient varies from 0.9 for a non-resilient fender to nearly
1.0 for a flexible fender. For larger ships on energy-absorbing fender
systems, little or no deformation of the ship takes place; therefore, a
coefficient of 1.0 is recommended.
f. Configuration Coefficient (Cc). This factor has been introduced to take
into account the difference between an open pier or wharf and a solid pier
or wharf. In the first case, the movements of the water surrounding the
berthing ship are not (or hardly) affected by the berth. In the second case,
the water between the berthing ship and the structure is squeezed, which
introduces a cushion effect that represents an extra force on the ship
away from the berth and reduces the energy to be absorbed by the fender
system. Therefore, a reduction factor has to take care of this effect.
Experience has indicated that for a solid quaywall about one quarter of the
energy of the berthing ship is absorbed by the water cushion; therefore,
the following values for Cc appear to be justified:
For open berth and corners of solid piers, Cc = 1.0.
For solid piers with parallel approach, Cc = 0.8.
For berths with different conditions, Cc might be chosen somewhere
between these values.
g. Effective Mass or Virtual Mass Coefficient (Cm). When a ship
approaches a dock, the berthing impact is induced not only by the mass of
the moving ship, but also by the water mass moving along with the ship.
The latter is generally called the "hydrodynamic" or "added" mass. In
determining the kinetic energy of a berthing ship, the effective or virtual
mass (a sum of ship mass and hydrodynamic mass) should be used. The
hydrodynamic mass does not necessarily vary with the mass of the ship,
but is closely related to the projected area of the ship at right angles to the
direction of motion. Other factors, such as the form of ship, water depth,
berthing velocity, and acceleration and deceleration of the ship, will have
some effect on the hydrodynamic mass. Taking into account both model
and prototype experiments, the effective or virtual mass coefficient can be
Cm = 1 + 2D/B
D = Maximum draft of ship
B = Beam width of ship