TABLE OF CONTENTS We conclude this rather long chapter with a brief discussion of mooring systems (basically a mooring system). You might argue that mooring systems do not really belong in a chapter about wave-induced forces; however the discussion will not be lengthy enough to merit a separate chapter. This is as good a place as any since the mooring forces are involved in the low frequency horizontal-plane motions just discussed.
We will focus here on a simple catenary mooring system, which is probably the most common type for offshore applications. In this system the restoring force comes primarily from the weight of the line (usuaily a heavy chain), the end of which rests on the sea bottom. If the body is moved away from its equilibrium position, more chain gets lifted up off of the bottom, thus inducing a force transmitted through the line that acts to pull the body back. The line usually terminates at some sort of anchor; however it is desirable that the line be long enough to avoid applying a vertical force to the anchor.
Most analyses of mooring systems begin with a free body diagram of an element of the line, showing the forces that act on it (Figure 5.35): Tension, weight, buoyancy, and drag are the most important ones. The figure shows an element with initial (unloaded) length ds, cross sectional area A, with a weight per unit length in water of w and elastic modulus E; T...
