Pile Design and Construction Practice, Fifth edition

400 Piling for marine structures
(7) Compressive and uplift forces induced by overturning movements due to loads 1 to 5 above
(8) In some parts of the world piles may also have to carry vertical and lateral loads from
floating ice, and loading from earthquakes.
The above forces are not necessarily cumulative. Whereas wind, wave, and current forces
can occur simultaneously and in the same direction, the forces due to berthing impact and
mooring rope pull occur in opposite directions. Berthing would not take place at times of
maximum wave height, nor would the thrust from ice sheets coincide with the most severe
wave action. Where containers are stored on the deck slab the possibility of stacking them
in tiers above a nominal permitted height must be considered.
8.1.1 Loading on piles from berthing impact forces
The basic equation used in calculating the force on a jetty or independent berthing structure
due to the impact of a ship as it is brought to rest by the structure is
kinetic energy Ek � msV 2
2g
(8.1)
where m s is the displacement of the ship and the mass of water moving with the ship, and
V is the velocity of approach to the structure.
The whole of the energy as represented by equation 8.1 is not imparted directly to the jetty
piles. Kinetic energy is also absorbed by the deformation of the hull of the ship and by the
compression of the fenders and of the cushioning between the fenders and their supporting
structure. Ships normally approach the jetty at a narrow angle to the berthing line and the
kinetic energy in the direction parallel to this line is generally retained in kinetic form but
a part may be lost in overcoming the resistance of the water ahead of the ship’s bows, in friction
against the fenders, and in the pull on the mooring ropes if these are used to restrain
longitudinal movement. A full consideration of the complexities involved in calculating the
magnitude and direction of berthing forces cannot be dealt with adequately in this book, and
the reader is referred to Part 4 of the British Standards Code of Practice (BS 6349-1: 2000)
for maritime structures (8.1) for guidance on these problems.
On the assumption that the kinetic energy of the ship transverse and parallel to the
berthing line has been correctly calculated the problem is then to assess the manner in which
the energy is absorbed by the fenders and their supporting piles. Taking the case of a vertical
pile acting as a simple cantilever from the point of virtual fixity below the sea bed, and
receiving a blow from the ship with a force H applied at a point A (Figure 8.3a), the distance
moved by the point A can then be calculated by the simple method shown in equation 6.20
and repeated here for convenience, namely:
distance moved y � H(e � z f) 3
3EI
(8.2)
If the ship is brought to rest by the vertical pile as it moves the pile head over the distance
y, then the work done by the force H over this distance is given by
work done � 1
2 Hy � H 2 (e � zf) 3
6EI
(8.3)