25.2. Purpose of Mooring System Design#
Different structures have vastly different demands. A temporary mooring for a supply ship has lighter requirements than a permanent mooring for an FPSO (Floating Production, Storage and Offloading). Understanding why you need a mooring is the first step to deciding which mooring approach fits.
A mooring system is defined as the complete setup of interconnected lines, connections, and anchor points between one (or several) floating and/or submerged body(-ies) and the seabed or other fixed points.
Often a mooring is used to connect a single floater to the seabed but sometimes it can be just the connection between two freely floating bodies. Key thought of this definition is to be aware that there can be multiple connections which should all be considered when designing a system. Reason is that they form an intricate dynamic system of masses and stiffnesses.
The main purpose can be summarised to the following:
Keep the bodies within a pre-defined area. Most likely these are areas in the waterline but thinking of a submerged structure it could also be a volume.
Limit the motions. First order motions caused by waves on floaters, such as heave, roll, & pitch are very difficult to restrain using a mooring system, but the mooring can be used to restrain or take account of second order motions such as surge, sway or yaw.
Limit the structural displacements within the bodies. In some cases, an elongated structure under influence of wave loading can be designed with a mooring system to limit internal structural displacement such as in a submerged tunnel or pipeline.