OCIMF_Mooring_Equipment_Guidelines_(Second_Edition)

E.3 ASSUMPTIONS

. The problems may be fairly analysed in two separate parts by resolution of the loadings into
lateral and longitudinal components.
Generally moorings are arranged in accordance with Ihis concepl as discussed in Sections 1.3
and 1.5. Longitudinal restraint should be provided by springs and lateral restraint by breast
lines. While angled head and Slern lines are not recommended, they can be taken into account
in the lateral restraint case, if they are tended like breast lines.

Angled head and stern lines by vinue of their orientat.ion and length take up lesser tensions

[ban well placed springs and breasts. Provided they are arranged more as laleral restraint

lines, i.e., (J less than 45°, their effects on tbe case of longitudinal loading are not generally

significant.

Provided that the mooring line layout is reasonably symmetrical aboul the jelty centre-line,

in tbe lateral loading case the longitudinal components of head and stern line tensions cancel

out.

2. Mooring Line tension is proportional to extension.

In fact, load/extension characteristics are not linear al very small or very large loads, but

with a small pre-tension applied they do not deviale significantly from linearity in the .zone

of interest up to 55 % (MBL).

A small pre-Iension, genenlUy less than 1011!0 (MBL). effectively removes the sag frolD a line

and allows tbis complication 10 be ignored.

3. When tbe imposed loads are zero, the tensions iD the mooring lines are assumed to be zero,
   although the lines are jusl taut. For the purposes of the calculat.ion, no funller tending of lines
   takes place as external loads are imposed.

While some pre-tension i required 10 remove sag and it is good practice to apply a ignific.ant

pretension to reduce vessel movemenl under moderate load fluctuali.ons, uneven application

or poor line lending do nullify the benefits as far as ultimate restraint capacity is concerned.

In practice, values of pre-Icnsioll are seldom more than 2011!0 (MBL), and mea urements

indicate that they are usually s.ubstantially lower.

The assumption is slightly conservative in its results but is probably reasonably representative

of the majority of situations.

4. The problem of lateral restraint calculation may be further split iDlO two parts by considering
   the bow and stern of Ihe vessel separately.

Lateral restraint lines are usuaJly grouped near the bow and stem of the vessel.

GeneraJly, imposed loadinlls at bow and slcrn are nOI tOO dissimilar, so angular displacements

are not very signifit-anl.

5. In "mixed" mooring layouts synthetic lines are ignored.

By comparison with wires (hey take up insignificant tensions due to (beir much g'reater

elasticity.

Any benefi1 from pre.tensioning is excluded by assumption 3.

E.4 LIMITATIONS OF METHOD

In general, tbe method is applicable with reasonable accuracy to effective mooring layouts, in

parI icul ar:

• Lateral and longitudinal restraint functions separated.