SHIP ТО SHIP TRANSFER GUIDE (PETROLEUM)
(Third Edition 1997)
For Use with Crude
Oil and Petroleum Products
APPENDIX 3
Fender Selection
Calculation
In order to select a suitable fender arrangement it is
necessary to calculate the kinetic energy of a ship and the energy to be
absorbed at the point of contact.
The following is only a guide and it is recommended that
individual fender manufacturers, fender rental or STS agencies are approached
for advice when planning an STS operation.
Determining the kinetic energy of a ship:
The displacement of the ship should be ascertained at
the intended draft prior to the berthing operation; this can be designated W1.
Determining the additional weight of the ship:
This is considered to be equal to the weight of water
represented by a cylindrical vessel where
(diameter) D = Draft of the vessel in metres
(length) L = Length between perpendiculars of vessel in metres and can be designated as W2 and expressed as
W2 = 0.805 (D)2 (L).
The total weight to be considered for Ship A is WA = Wl + W2.
A similar calculation should be carried out for the
second ship (Ship B) to determine the value of WB.
Approach Velocity:
The approach velocity of the ships can have a dramatic
effect on the energy absorption requirements of the fender system. The
allowance for velocity should take into consideration the effects of local
weather, sea, swell conditions and the physical size and weight of the ships
involved. It is common to work within a range of between 0.1 to 0.3 metres per second (0.2
to 0.6 Knots) and it should be noted that
an increase of about 0.02 m/sec in
velocity can result in approximately 20
percent increase in energy absorption requirements.
During berthing operations between two ships it is
rare that they make parallel contact with each other and it is more likely that
one fender will absorb the initial contact. Under this condition some of the
energy is absorbed by the action of the ship pivoting around the fender and it
is generally accepted that about half of the energy is absorbed in this manner.
Example: for a single point contact the value of E
(energy) can be calculated as follows:
E = 0.051(W)(v)2(0.5) tons/metres
|
were |
W = |
WA x WB |
|
WA + WB |
and V = approach velocity in metres per second.
Energy absorption requirements in the event of a
parallel berthing are given by the formula:
E = 0.051(W)(V)2 tons/metres.
It should be noted that in the event of a parallel contact the load will be spread over the remaining fenders in the system.
Calculations should be made for both conditions of
berthing and unberthing.
EXAMBLE:
|
|
Displacement at Draft (tons) |
Draft (metres) |
Length B.P. (metres) |
|
Ship A |
65,000 |
7.5 |
260 |
|
Ship B |
312,000 |
21.0 |
320 |
Planned approach velocity of a maximum of 0.2 m/sec and allowing for a single point landing at the forward fender:
Ship A
|
Displacement at Draft |
W1 = 65,000
tons |
|
Additional Weight =
0.805 (7.5)2 (260) |
W2 = 11,773
tons |
WA = W 1 + W2
WA = 76,773 tons
Ship B
|
Displacement at Draft |
W1 = 312,000
tons |
|
Additional Weight =
0.805 (21)2 (320) |
W2 = 113,602
tons |
WB = Wl + W2 WB = 425,602 tons
Where
|
W = |
WA x WB |
= |
76733 x 425602 |
= 65,040 tons |
|
WA + WB |
76733 + 425602 |
Energy (E) = 0.025(65040)(0.2)2 E = 65 tons/m
On completion of the calculations for berthing and unberthing, reference should be made to the relevant fender
manufacturer's performance tables in order to select a fender system that will
provide energy absorption capability in excess of that indicated by the
calculations.
Should the
approach velocity be higher than planned, for example 0.3m/sec, then in the
case given above, the Energy (E) will be increased to 146 tons/m as worked
example below, with a subsequent possible change in fender requirement.
Energy (E) = 0.025 (65040) (0.3)2 E = 146 tons/m