27.3. Concept design#
In order to understand how a concept design of a mooring system is performed, it is illustrating to follow the quasi-static approach. Using the force excursion diagram including the tension in the highest loaded mooring line, the following steps are used:
Establish the mean position of the floater and the associated pre-tension.
Determine the offset caused by the mean wind and current load as well as the mean 2nd order wave drift force.
Using the stiffness of the mooring system at the resulting offset, the 2nd order wave drift motions are determined (function of the 2nd order wave drift spectra and the line/riser damping).
At the extreme excursion (mean offset + 2nd order motion) the 1st order wave motion is added.
The maximum tension is either Tension due to offset + significant 2nd order motion + maximum 1st order or Tension due to offset + maximum 2nd order motion + significant 1st order. In general based on a 3-hour storm.
In engineering practice often time domain analysis is used which can take into account all non-linear effects such as line dynamics and the wind and wave spectra.
The quasi static analysis shows that the 2nd order (difference frequencies) motions need to be analysed as they can cause significant motions of a floating body. The response of a floating body is hugely effected by its natural frequencies or periods.
The natural frequencies of heave, roll & pitch are often in the wave frequency range and drive first order motions.
The natural frequencies of surge, sway and yaw are often in the 2nd order difference range and attract second order motions. In this example paper on a FOW, the graphs are given of the wave spectrum and the surge response. The 1st order response in surge from the wave spectrum can be clearly identified.
The natural frequencies of pitch and surge are outside the wave period range, while there is still the largest motion response due to these two natural frequencies (note also the log scale in motion response). Due to the second order wave forcing, the resonant behaviour and low damping (some viscous damping from the mooring lines and structure), the surge and pitch motions are considerable. The reason is that the second order load spectrum has highest values near the zero frequency axis.
In designing a mooring system, the following aspects should also be considered:
The mooring line connection to a (suction or driven) pile for a catenary mooring is often below the seabed, to be most effective to generate the horizontal resistance from the soil. This creates an ‘inverse catenary’
The line dynamics, in particular near the floater are often covered in the safety factors. It is however important to verify that highly tensioned mooring lines do not experience sudden slack conditions. In shallow water this risk is high and is often caused because due to inertia or drag the line cannot follow the 1st order motions of the vessel.
Installation tolerances such as positioning inaccuracy of the anchors is a factor to take into account in the sensitivity analysis of a mooring.
In offshore installation the mooring line will not have the same length as on paper and in particular with long moorings the length tolerances need to be captured.
Tensioning cannot always be done accurately and tensioning tolerances need to be checked.
The effect of loads from additional appurtenances on the hull but also the effect of heavy intermediate connections in the mooring line.
When a mooring line is laid, there are often requirements to ensure the twist is acceptable.
As discussed earlier, what is the acceptable uplift at a drag anchor, 10 or 15 degrees?
In some soils there is potential for trenching of the lines. This can become substantial and changes the response of the line to sideways motion as it is difficult to determine where the line comes of the seabed.