Mooring Design
7. Design Calculations
For a spar, currents will usually control the design. Unlike wind and wave, which results in mean and dynamic loading on the structure, currents loads tend to produce a large mean load on the hull that is in-line with the current flow plus transverse hull motions due to vortex induced motions.
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Basics
A mooring design can either be the result of a quasi-static mooring analysis, or of a full dynamic response analysis. The latter is normally carried out using proprietary software on a medium sized computer (e.g., Marintek, 1987). Which is better depends largely on the range of errors of the techniques, and on the amount and accuracy of input data. A well analyzed quasi-static analysis will be much better than a dynamic study performed with limited data. Any study based on measured data, or tank trials will also in general be more accurate. If either is subject to model calibration, post-monitoring and analysis, then the probability of survival must be enhanced. The general steps in quasi-static mooring analysis are as follows:
(i) The mooring geometry and mooring excursion/force equations are
defined.
(ii) The mean environmental force is applied to the system, and the
excursion (offset) calculated.
(iii) The periodic wave forces, and response amplitude is now
applied to the system.
(iv) The line tensions resulting from this maximum excursion are now
calculated.
(v) These line tensions are now compared with the minimum breaking
load of the riser components.
(vi) The maximum peak anchor loads are calculated for each line,
and direction.
(vii) A safety factor (generally 2) is introduced in to the line
strengths.
(viii) The maximum peak line loads are recalculated with 1 line
broken, or after a line failure.
(ix) If the proposed mooring specification fails the safety factor
test, then a new specification is tried.
This type of mooring analysis is most common, and documented by many
sources. In addition, it is endorsed by a number of insurance
classification societies. However, it is open to errors through the
lack of sufficient, or accurate data, and particularly through
under-estimating wave climate. Also, a mooring system which may
survive, could still be so "stiff" that fish stocks are killed
through abrasion, or nets torn, during violent storms.