EM 1110-2-1100 (Part II)
30 Apr 02
(3) Mooring lines.
(a) Mooring lines are made from steel, natural fibers, and synthetic materials. Steel ropes may be made
of different strength grades and galvanized for protection against corrosion. Natural fibers include manilla,
sisal, and coir. Commonly used synthetic materials are nylon, dacron, polypropylene, Kevlar, and Karastan
(b) Synthetic lines are easy to handle, do not corrode, and have excellent strength-to-weight ratios.
Different construction types include stranded, plaited, braided, and parallel yarns. Stranded is the least
satisfactory for mooring lines because it tends to unlay under free end conditions. Each has different
mechanical properties that make it appropriate for different applications. These include strength, weight,
stretch, endurance, and resistance to abrasion and cuts.
(c) Care should be taken not to mix different materials and lengths in mooring arrangements (Oil
Companies International Marine Forum (OCIMF) 1978). Elasticity is a measure of a mooring line's ability
to stretch under load. It is a function of material, diameter, and length. The ultimate breaking strength has
been related to the square of the nominal rope diameter (Wilson 1967). If two lines of different elasticity but
similar lengths and orientations are combined at the same point, the stiffer one will assume more of the load.
Also, lines of different length will carry different amounts of the total load. Thus, one of the lines may be
near breaking while the other one is carrying almost no load.
(4) Fenders. Fenders are like bumpers on cars, designed to protect vessels and piers during berthing and
mooring against forces due to winds, waves, and currents. They are designed to absorb impact energy of the
vessel through deflection and dissipation. Some respond very fast and violently and others more slowly. The
latter are the more desirable because they produce smaller fender forces. Highly elastic, recoiling type
fenders should be replaced with the non-recoiling type if possible. Fenders can be continuous or placed in
certain areas where vessels land. The mooring system should be designed based on the combined response
of mooring lines and fenders to ensure that resonance effects with the environmental forces are minimized.
The interested reader should consult Bruun (1989) for additional information on types, materials,
characteristics, selection, forces, deformation, and energy absorption of fenders.
(5) Surge natural period.
(a) For moored vessels at a dock or quay, surge is one of the most important parameters to consider.
Ranges of allowable movements for different vessels have been given by Bruun (1989).
(b) The motion of a moored ship in surge can be described by the motion of a linear system with a single
degree of freedom. Restoring or reaction forces due to the change in position, velocity, and acceleration of
the ship from equilibrium are assumed linear. The exciting force is due to the drag force of the water flowing
past the ship. The motion of the ship in surge is assumed to be independent of other directions of motion.
Damping is assumed to be small for the low-frequency motions of a ship in surge. Solving for the undamped
natural period in surge Ts
Tn ' 2 π
(c) The virtual mass of the ship mv is the sum of the actual mass or displacement of the ship m and the
added mass ma due to inertial effects of the water entrained with the ship. For a ship in surge, ma is
approximately 15 percent of the actual mass m, which is based on the ship's displacement