3 October 2005
across the pier to improve mooring efficiency and increase the number of mooring lines
that are used.
Detailed structural analyses show that ship's double bitts have the
maximum safe working capacity when equal load is applied to each of the two barrels of
the bitts. It is recommended that methods be used to provide for equal loading to the
barrels in Heavy Weather mooring. This can be accomplished by various methods:
Using two parts of mooring line. Put an eye over the barrel of a set of bitts
and run to the shore fitting. Repeat for the second barrel. Carefully tend the lines so
both parts of line have the same pretension.
Using one line in two parts (Figure 11-2). Put an eye over one barrel and
run to the shore fitting. Run the line around a sheave, equalizer or tie down the loop of
line going around the shore fitting. Then tie off the end of the line to the second barrel
of the ship's double bitts.
Detailed Heavy Weather mooring designs need to be prepared for each
ship at each berth. These designs need to consider the site-specific design criteria and
special circumstances. As a general rule-of-thumb:
Provide adequate numbers of breasting lines to safely secure the ship.
Place the lines towards the bow and towards the stern so they resist both lateral loads
Breasting lines should be approximately perpendicular to the ship's
centerline (to resist loads and moments), be of similar length (to help improve load
sharing) and should all have low vertical angles (to improve mooring efficiency and help
account for water level changes).
Provide adequate numbers of spring lines, which are approximately
parallel to the ship's centerline. Equal numbers of spring lines should be run towards
the bow and stern. These lines should have rather low vertical angles to account for
water level changes.
It is recommended that preliminary designs be developed with quasi-static
designs and factors of safety of 2.5 or higher on all components. Dynamic methods can
then be used to refine and verify the designs. The various references cited in this
section provide examples of Heavy Weather mooring designs. Chapter 8 of this UFC
also provides Heavy Weather mooring examples.