A time history simulation of tethers during towing-out, as well as static or time domain simulations of tethers once installed, can be performed in Aqwa for both regular and irregular waves.
Tethers are considered by Aqwa as flexible cylindrical tubes whose diameters are small compared to the wavelength of the waves, and are described by a series of elements along each tether. Each element may have different geometric and/or material properties.
The analysis of towed tethers is an independent process, and requires no hydrodynamic database from an Aqwa hydrodynamic diffraction analysis. As all tethers are regarded as a mooring capability in Aqwa, a nominal structure must be input to define the position of the axis system, in which the towed tether displacements are output and the eigenvalue solution is performed. The nominal structure plays no other part in the analysis.
An installed tether system is often applied for anchoring a tension leg platform (TLP) to the sea bed, as it prevents vertical motion but allows lateral motion due to environmental loadings. With a sufficient tether axial stiffness, the tether-hull system keeps the heave, pitch and roll natural frequencies above the wave energy frequencies. For an installed tether system, the hydrodynamic database from an Aqwa hydrodynamic diffraction analysis is required for the diffracting structures. For non-diffracting structures, however, a tube model can be used.
The tether modeling techniques are based on the following limitations and assumptions.
No axial deformation
Bending and lateral motion only are considered, omitting translation and rotation in the axial direction.
Zero axial tension in towed tether
Both the wall and effective tensions in a towed tether are assumed to be zero, and hence the bending stiffness is purely structural.
Small motions
It is assumed that the lateral and rotational deformations of the tether from the defined tether axes are small. This means that the analysis is unsuitable for large rotations about the transverse axes, for example for the upending of a structure.
Mass/stiffness
The mass/stiffness ratio of any element must not be too small. Very short elements inherently have small mass/stiffness ratios, which gives rise to very high natural frequencies. These high frequencies may cause stability problems and roundoff errors in the analysis. As a general rule, natural periods of less than 1/100th of a second are not allowed.
Time step width
In a time domain analysis, the time step width must be small enough to resolve the response motion of the tether. This includes any transients that may be present either initially or, more importantly, throughout the analysis. A good rule of thumb is that the time step width should be less than 1/10th of the period of any response.