OCIMF_Mooring_Equipment_Guidelines_(Second_Edition)

For breasting dolphins, tbe characteristics for manufactured fender units are available from the
manufacturer. Deflection of t.he dolphin structure, if significant, can be calclllat.ed from the properries
of the structure.

2.5.1.3 The Geometrical Relationship Between Ihe Paris oJ 'lte System

The elongation of each mooring line and deflection of each breasting dolphin can be calculated from

the amount of surge, sway and yaw at the centre of the tanker. Since the tanker is essent.ially a rigid

element, each chock through whicb a line passes effeclively moves in reJat"ion to the mooring point

I hus changjng the distance between (he chock and the mooring point on the jelly or sea island.

Using the above principles or system characteristics, t.he forces in the mooring lines for wind and

current forc.:es <Jre determined by the following general procedure:

I. Calculate the applied forces in the fore/aft direction and the beam dir ection and the yaw

moment for wind and current as described in Section 2.) on wind and current force predictions.

2. Determine the elasticity of the entire mooring system from the load/detlection characteristics
   of each component and the geometry of the system. The elasticity of tbe system is expressed
   in terms of amount of surge, sway and yaw p~r unit force in each prillcipaL direction and per
   unit yaw moment.


3. Calculate the total amount of surge, sway and yaw at tbe centre of the tanker by mult.iplying
   the amount per unit force and moment detennined in Step 2 by the applied forces and moment
   calculaLed i.n Step I. Then calculate the new localion of each chock point.


4. Determine the force in each mooring line and breasting dolphin by calculating the stretch in
   the line (or compression of lhe dolphin) for the movement ·of the tanker calculated in Step 3
   and IInding the corresponding force from the load/deflection characterislic.

The above calculations arc complicated by the fact [hat the elasticity of the system varies with the

actual position of the canker because for any pankular position some lines may not be in tension

and some brcasting dolphins nOl in compression. These must be omitted in l~alclllating the elasticity

of the system in that position. Also the load/deflection characteristics of tbe components may not

be linear. This is t.he case for synthetic fibre lines and most c1astomer rellder units. Conse.quently,

che solution involves an interactive process in which the elasticity of the sy tern is first determined

for an assunl ed position of the canker. A llCW position is then determined by the above procedure

Dod tbe elasticity adjusted. This proce,ss is repeated until the desired accuracy of the calculated line

forces is obtained.

2.5.2 Computer Calculations

Because of the complexity of the calculations, they are generally performed using a computer. A

computer program can be wriuen incorporating the above procedure and IIsing matrix techniques

for tbe solution. When a computer or computer program is not availa ble, mooring line loads

can be approximated by the hand-calculation metbod described later. Since the hand-calculation

method does oot incorporate all t.he principles noted above, the calculated values will only be rough

approximations.

The above;:· method can be used for calcuJa t in ' mooring line loads for other steady-state forces such

as drift due to waves and jce. Once the applied forces have been establiShed, the loads in the lines

can be determined using Step. 1 to 4.

For dynamic loadings such as the oscillillory forces from waves, swelJ, seiche and surges from

passiog vessels, the maximum forces in the Hnes and breastjng dolphins may be calculated from the

maximum movement of t.he tanker ill surge, sway and yaw as determined from model tests or analy-

tical technjques. Once the maximum movement of the tanker is determined, t.he ma.""(imuIll force in

the mooring lines are determined as in Step 4 above_