Appendix E
Calculation of Mooring Restraint
and Loads
Details of Hand Calculation Method
Example
Comparison with
Computer Method Results
E.1 INTRODUCTION
Reference should be made to' Figure E I which illustrates the general mO'O'ring layout and nomenclature.
E.2 BASIC PRINCIPLES OF ANALYSIS
- MODring line behaviDur under load is elastic, i.e., tension is prO'pO'rtiDnal ID extensiDn and vice
versa. In the absence O'f pretensiO'ning, the impDsition Df an external lDad Dn a vessel means
that the ve~sscl must move away from the fLxed shore pDints fO'r the necessary restraining loads
to' be developed in the mDDring lines. The distribution Df tensiDns between the various lines in a
mO'Dring layout is determined by tbe geDmetry of the whO'le layout. Movements Df the different
pans Df the ve.ssel must be cDmpatible, as must be the resultant Df the line tensiDns with tbe
external loads. Individual lines cannDt be cDnsidered in iSDlatiDn.
cO's a - [n lateral restraint the tension developed in any line is propDrtiDnal to'
L
where a is
defined as Ihe hDrizDntal angle of a line relative to' a true breast and L as the tDtal length Df
line between jelly hDOk and vessel winch. The line length is relevant as relative extensiDns and
tensiDns are required in cDnsidering multi-line laYDuts. The cDmponent of this tensiDn effective. .. .. cos^2 a
IJ] reslstlng the lateral IDad IS propofllonal to --
L
- The max.imulll restrai.nt capability of a mooring laYDut is reacbed when the most heavily IO'aded
line is at its limiling tension, in design cases 55 Dia (MBL). BeYDnd this poinl it would be assumed
lhis line had failed. The impDsed load would then have 10 be resisted by fewer lines. 10 mDst
cases they wDuld not be a.ble to sustain it and a sequeotiaJ failure would ensue. The maximum
resiraint capability therefore includes a factor for tIle tensiDn in the critical line indicated by
subscript He". - Vertical line inclinations defined by 0 the angle Df a line above hDrizontal, affect tensions and
restraint capabilities in the same manner as horizontal angles delined by Cl. - By cDrDllary the efficiency of any lille in resisting an imposed load is related to the geDmetry of
the critical Line as well a its Dwn. These concepts may be illustrated by study of an apparently
paradoxical example. For a jetty with all main mDoring points on a single line parallel to the
vessel's heading, the lateral reslraint provided by two Irue breast lines, one angled head and one
angled stern line is less than that of two angled head and IWO' angled stem lines.