2.5 MOORING RESTRAINT REQUIREMENTS
Having determined the environmental forces acting on the ship it is necessary to calculate the strength
and number of mooring lines required to balance these forces.
Complller programmes are widely available to carry out this task for any combination of wind and
current speeds and angles. A hand calculation method is described in Appendix D. For either
method the three-dimensional coordinates of all ship and tenninal mooring points must be known
or assumed, ru; well as the elastic characteristics of the mooring lines. Computer calculations are
especially suitable to explore the adequacy of the mooring system of an existing or planned ship at
a tenninal known to have unusual environmental conditions or mooring geometry.
Hand calculations arc not always adequate for use by the terminal des.igner in evaluating either
mooring limitations or the need for, and amount of, shore augmentation which may be required
for a given vessel at a given benh for given operating environmental conditions. In this case it is
necessary to evaluate various mooring arrangements which cater for the particular combinations of
forces in order to determine that arrangement which is the best for that berth. The simplifying
assumptions (or the laborious, time-consuming and costly effort to avoid thero) may in this case
render a hand calculatIon method impractical. A more detailed discussion of the basic principles of
mooring calculations is provided later.
Static and dynamic computer programs are available. Thus, in designing tenninal facilities, the
engineer is advised 10 use tbe tools and techn.ology which will aid in the design of a safe berlh.
2.5.1 Basic Principles Dj Mooring Calculations
A tanker at a jetty or sea island is held in tbe berth by a combination of mooring lines and
brcasling dolphins which resist the forces applied to tbe tanker by wind. current, waves and otller
environmental factors. There are multiple mooring lines which connect from several locations on the
ship to several mooring points on the jetty or sea island. It is a system in which the forces in the
lines cannot be calculated solely by the principle of static equilibrium, so the solution must consider
the elasticity of the components. Basically, the tanker and mooring system together may be considered
as a two-dimensional elastic system such that when a load is applied to tbe tanker, the I'anker will
move a small but determinable amount. By determiniog the amount of movement of the tanker, the
forces in tbe mooring lines and breasting dolphins can be detennined.For steady-state forces on the Lanker (cuHent and wind forces are considered steady-state forces for
the purposes of the analysis), the forces in (he mooring Lines are determined using the following basic
principles or characteristics:2.S.1.1 The Principle oj Sialic Equilibrium
The sums of the compODeocs of tbe forces in the mooring lines and breasting dolphins in eacb
principal direction and [be mOment of forces about the centre of the tanker are equal and opposite to
the sums of the components of the applied forces (current and wind) and the moment of these forces.
The principal directions are ahead (or ru;tem) and abeam, and the momeut is a yawing moment.2.5.1.2 The Load/Defleclion Characteristics Dj each Mooring Line and Breasling DolphinFor each there is a relationship bet weeD its elongation (for mooring lines in tension) or inward
deflect.ion (for breasting dolphins in compression) and the force in the member.For mooring lines, the load/deflection characteristic is dependent on the material and construction of
the line, its diameter and the loaded length (i.e., from tbe ship's winch to the mooring point on the
jetty or sea island). Mooring lines become stiffer (less stretch for a gjven load) witb use, and the
cbaracteristic for used line is normally employed for calculatiT18 loads rather tban the characteristic
for new line. The characteristics can be obtained from line manufacturers or suppliers.