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Springs & Ropes and Sinusoidal Motion — Additions to the General Moving Object Model

This Development Note highlights additions to the General Moving Object model to be released in FLOW-3D version 9.4.

The current General Moving Object model allows users to prescribe forces and torques acting on moving objects. In many applications, however, some types of external forces and torques are not known a priori. For example, springs exist extensively in machinery, and ropes are used broadly in offshore mooring systems. Their forces and torques on moving objects are functions of their deformation determined by location and orientation of the moving objects. In the next release of FLOW-3D, users can define springs and ropes that exert forces and torques on the moving objects.

Animation 1: An oil platform fastened to the seabed by 12 ropes.

 

Animation 2: The same oil platform, but floating freely on the water.

Modeling Multiple Springs & Ropes

The model allows multiple springs and ropes. Both ends of each spring or rope can be attached to any moving object or can be fixed in space. All springs and ropes are assumed to have elastic deformation. A spring can experience compression and/or extension depending on whether it is a compression spring, extension spring, compression and extension spring or torsion spring. A rope, however, only allows extension. At each time step of computation, compression/extension of each spring or rope as well as its corresponding force and torque acting on the moving object are calculated. Inertia of the springs and ropes is not considered, thus a rope's shape is not calculated when it is relaxed.

Simulating Offshore Platforms

Animations 1 and 2 show an oil platform pushed about by waves under hurricane conditions. In animation 1, a total of twelve wire ropes are anchored at seabed 500 m below sea surface and are fastened tautly on two opposite sides of the platform. In animation 2, no ropes exist—the platform floats freely on the water. In both animations, the waves have a 100 m length and a 10 m height (measured from trough to crest) and are simulated using a fifth-order Stokes wave model (also to be in the next release of FLOW-3D). The wave period corresponding to the wave length is 7.62 s. The deck size of the platform is 90 m by 87 m. Comparison of the two animations clearly demonstrates how the ropes restrict and stabilize the platform's motion under severe wave conditions.

Prescribing Sinusoidal Motion

In addition to the spring and rope capability, users can also prescribe sinusoidal motion to a moving object. Input parameters include amplitude, frequency and initial phase for each sinusoidal velocity component. Animation 3 shows a piston pump moving water from a low level to a water tank. Sinusoidal motion is prescribed to the piston. The opening and closing of the two check valves are driven by fluid flow.

Animation 3: FLOW-3D users will be able to model sinusoidal motion, as in this example of a piston pump moving water into a tank.

Conclusion

These additions to the General Moving Object model give FLOW-3D users an even greater breadth of applications that they can model. In addition to the examples above, spring forces and torques can be easily applied to valves and other moving mechanical parts in numerical simulations. In ocean engineering, users can study the stability of many kinds of Floating Production, Storage, and Offloading (FPSO) systems. Motions of floating wave power generation systems, mooring of ship and marine measurement equipment and boat towing can also be simulated.