Mooring Lines Model
Mooring systems are central to the design and analysis of floating offshore structures, wave energy devices, and anchored marine infrastructure. FLOW-3D and FLOW-3D HYDRO include a compliant Mooring Lines Model that simulates the full three-dimensional dynamics of mooring lines and their interactions with floating or moving objects. Lines can be taut or slack, may rest partially or fully on the seafloor or riverbed, and can fail under excessive load, giving engineers a realistic picture of mooring system behavior under wave, current, and structural loading conditions.
How the Model Works
The model uses a Finite Segment approach to compute mooring line dynamics. Each line is divided into discrete segments, and the position and velocity of each segment’s mass center are calculated by solving the dynamic equation of motion. Forces acting on each segment include gravity, buoyancy, tension at both ends, and fluid drag in the normal and tangential directions computed using a quadratic drag law.
Mooring lines can be pinned to moving. The end-point of the mooring line moves with the object. The tension of the mooring line produces forces that act on the object. This two-way coupling allows the model to capture the feedback between structural motion and line dynamics. Line endpoints can be fixed to anchors, attached to other moving or stationary objects, or defined as free ends. Multiple lines can be present in a single simulation with an arbitrary connection configurations.
The mooring line model can be used for even when the line extends below the computational domain, as is common when anchoring to deep water seafloors. The submerged portion of the line outside the domain is treated with a uniform deep-water current assumption and drag forces are evaluated accordingly.
Line failure is also supported. At each time step the maximum tension in each line segment is compared against a user-defined Minimum Breaking Load, and the line severs at the point of maximum tension if that threshold is exceeded.
A floating structure restrained by four mooring lines anchored to the seabed. Mooring line extension from free state (top right) and vertical velocity of the structure center of mass (bottom right) are shown alongside the 3D simulation.
Applications
The Mooring Lines Model addresses a wide range of offshore and marine engineering applications:
- Floating Production Storage and Offloading (FPSO) vessels for analyzing mooring system response to wave and current loading in offshore oil and gas operations
- Wave energy converters (WECs) for simulating how mooring configurations affect device motion, power capture, and structural loads
- Floating wave breakers for evaluating mooring forces and breakwater performance under design wave conditions
- Anchored ships and marine vessels for assessing mooring line loads and vessel motion in port or offshore environments
- Floating infrastructure and equipment for modeling buoys, platforms, pontoons, and other moored structures subject to hydrodynamic loading
Wave action drives the displacement of a moored buoy, with mooring line tension and extension tracked throughout the simulation alongside the overall displacement of the floating structure.