Section 4.0
Design Loads, Safety Factors
and Strength
4.1 General
These guidelines are intended to assist ship operators, designers and equipment suppliers in outfitting
sbips with mooring equipment designed to afely accommoda te the expected loads.
Mooring equipment means those pieces of equipment mounted onboard a srup to handle the loads
needed to temporarily attach the ship to a pier, the ocean bottom or another ship. Mooring equip-
ment includes bollards, mooring winches, anchor windlasses , chain stoppers, fairleads and capstans.
Anchoring equipment is not included in these guidelines, since it is adequately covered in Classification
Society rules.In order to define the strengtb of a filli ng or piece of equipment, the following factors must be
known or assumed:- the magnitude, positIon and dircctjon of application of the most severe serv ice load; Ihis is
the safe working load (SWL) of the fitting. - I.he safety factor associated with the above load.
- the 'de' ign basis' for the fitting.
The safe working load is the maximum load that should normally be applied in service conditions.
The 'design b a i. ' of a fitting provides an additional margin of safety agai,nst tbe rupture of any
ponion of the fillings or attachments. The stress LeveJs on which the 'design basis' is calculated
are in excess of the SWL aDd represent the load level at which any part or component reaches the
minimum yield point of tbe material. The safety factor may therefore be defined as the ratio of
yield load to safe working load, where yield load is the load at which the fitting or equipment will
start to deform permanently. Allerna tjvely, the safety factor may be defined as the ratio of breaking
load to SWL. The lalter definition would be applicable 10 ilems such as fibre or wire ropes where
the yield load is difficult to establish.Safety factors account for uncenaimies such as addiLional dynamic loads, normal wear or corrosion
of fittings or equjpmenl, small materia.l or welding defects, locked-in weld stress, etc. Tbe choice of
tbe safety factor is also influen.ced by the consequence of a failure. For example, safety factors must
be very high for ropes used to hoist personnel.Tbe load location and direction, in combination with the line load, determines Ihe tOlal load and
stress on a filling. A lioe led J 80° around a bollard or pedestal fairlead subject the fitting to twice
the rope load; and a line attached to a bollard near the top of the barrel produces a higher stress
than one attached close Lo the base.4.2 Basic Strength Philosophy
Since the wire rope, synthetic rope or chain with a specific breaking strength is used as tbe Link
between the ship and the berth, it is convenient to relate the strength of equipment and fittings to
the strength of tbe associated lines or chains.