3 October 2005
CATENARY BEHAVIOR. It is not desirable or practical to moor a ship
rigidly. For example, a ship can have a large amount of buoyancy, so it usually must be
allowed to move with changing water levels. Another problem with holding a ship too
rigidly is that some of the natural periods of the ship/mooring system can become short,
which may cause dynamic problems.
A ship can be considered a mass and the mooring system as springs.
During mooring design, the behavior of the mooring `springs' can be controlled to fine
tune the ship/mooring system behavior to achieve a specified performance. This can be
controlled by the weight of chain or other tension member, scope of chain, placement of
sinkers, amount the anchor penetrates the soil, and other parameters. The static
behavior of catenaries can be modeled using the computer program CSAP2 (NFESC
CR-6108-OCN, Anchor Mooring Line Computer Program Final Report, User's Manual
for Program CSAP2). This program includes the effects of chain and wire rope
interaction with soils, as well as the behavior of the catenary in the water column and
above the water surface.
As an example, take the catenary shown in Figure 6-6. This mooring leg
consists of four sections. The segment next to the anchor, Segment 1, consists of wire
rope, followed by three segments of chain. Sinkers with the shown in-water weight are
located at the ends of Segments 2 and 3. In this example, a plate anchor is driven 55
feet (16.8 meters) into mud below the seafloor. The chain attachment point to the ship
is 64 feet (19.5 meters) above the seafloor. The mooring leg is loaded to its design
horizontal load of H = 195 kips (8.7 E5 newtons) to key and proof load the anchor soon
after the anchor is installed. The keying and proofing corresponds to a tension in the top
of the chain of approximately 210 kips. Figure 6-6 shows the shape of the chain
catenary predicted by CSAP2 for the design load.
The computed load/deflection curve for the design water level for this
mooring leg, after proofing, is shown in Figure 6-7. The shape of this and the other
mooring legs in this mooring, which are not shown, will strongly influence the static and
dynamic behavior of the ship/mooring system during forcing.