Based upon the studies described earlier, the following conclusions were derived:
7.2.1 Comparison with laboratory test results
The numerical simulations were compared to a number of laboratory tests for two experimental phases. Based on the results, the following conclusions were drawn:
1. The developed numerical model “TRSPAR” was able to predict the natural frequencies of the truss spar platforms in both intact mooring and mooring line failure conditions with acceptable accuracy. The surge, heave and pitch differences between the predictions and the measurements were ranged from 5.06% to 24.3% for the two conditions.
2. The numerical model “TRSPAR” developed for assessment of the truss spar wave frequency responses was able to predict the platform motions due to regular waves obtaining good agreement with experimental results. This was verified in the case of intact mooring (maximum RMSD was 0.061) and mooring lines failure (maximum RMSD was 0.08) conditions.
3. The numerical scheme developed for the evaluation of the floating offshore structure slow drift motions successfully estimated the low frequency responses of the truss spar platform when connected to its mooring and when mooring line failure occurred. For the two conditions, the RMSD values for the simulation and the experiment show that the predicted WFR trend was relatively agreed well with the measurements compared to the LFR. However,
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WFR and LFR have been fairly well predicted by the developed numerical model (RMSD ranged from 107.4 to 1074). The peak responses were predicted with good accuracy as the maximum difference between the predictions and measurements for the two structure conditions was 17.3%.
4. Mooring line failure altered the system natural frequencies. The most significant effect was for the surge natural frequency.
5. In the wave frequency responses, mooring line damage has insignificant effect on the heave and pitch responses. However, the mooring line failure surge RAOs were almost same as in the intact mooring condition surge except for relatively low frequency wave components where mooring damage condition gave lower results. For random waves, mooring line failure affected resonant surge response (increased by 32.5%) more than the peak heave (increased by 19.8%) and pitch (increased by 14.4%) responses.
6. The major contribution of mooring line failure to the structure was causing the migration surge distance. This migration distance occurred due to the unbalanced upper and downstream mooring line forces. In addition, a noticeable transient surge response followed the failure.
7.2.2 Second order difference frequency forces
The total forces acting upon truss spar platform included nine linear and nonlinear forces. These nine sources of forces were analyzed and their equivalent effects were discussed. Some important conclusions are mentioned here:
1. Calculation of the wave forces (inertia and drag forces) in the mean position of the structure resulted in linear surge and pitch motions only.
2. The displaced geometry for the inertia forces significantly increased the surge and pitch resonant responses. With respect to surge response, the surge displaced geometry effect was greater than pitch effect while for pitch response, these two effects were almost equal.
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3. Additional of the second order difference frequency drag forces had insignificant effect on surge and pitch motions, while inclusion of the viscous damping significantly decreased the resonant surge and pitch responses.
4. Axial divergence increased the resonant surge and pitch motions.
5. An interesting relationship between the free surface fluctuation and convective acceleration forces was observed that these forces were almost equal in magnitude and opposite to each other.
6. In addition to the earlier second order forces, temporal acceleration can be considered as one of the important second order difference frequency forces.
Addition of this force increased the slow drift surge and pitch motions.
7.2.3 Current and wind forces
1. The main contribution of current and wind forces was a significant increase in the structure surge mean offset.
2. Presence of current substantially decreased the surge and pitch resonant responses as it increased the structure damping.
3. Wind force reduced the second order surge response more than pitch response as the mooring line stiffness increased.
4. The numerical model “TRSPAR” developed for the analysis of truss spar platform subjected to combined wave, current and wind forces gave same trend of results when compared to a fully coupled dynamic analysis code
“WINPOST”. However, the resonant surge and pitch results were higher than the corresponding WINPOST results.
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1. Strengthening the mooring line system of the structure by adding additional lines increased the mooring line restoring forces.
2. The major effect of adding mooring lines to the structure was the decrease in the second order surge and pitch responses. The surge response was affected more than the pitch response.
7.2.5 Quasi-static and dynamic mooring line analysis
1. The MATLAB code developed for quasi static analysis of deepwater mooring lines was able to predict the mooring line system restoring force obtaining good agreement with the experimental measurements.
2. The numerical results obtained from the MATLAB code, which was developed for dynamic analysis of deepwater mooring line, were compared to experimental results. These numerical model results demonstrated the importance of the mooring line dynamic effect in the design and analysis of deepwater mooring lines.
7.2.6 Investigations on the taut deepwater mooring line design parameters 1. For multi-component deepwater mooring line, the restoring force is directly
proportional to the line pretension. The effect of pretension on the restoring forces is relatively high for small pretensions.
2. For relatively small horizontal tensions, the line elongation has insignificant effect on the mooring line restoring force. However, its effect became significant for relatively large mooring line tension.
3. The cable unit weight is inversely proportioned to the horizontal mooring line tension at fairlead. The force-excursion relationship becomes linear for relatively light cable and nonlinear for the heavy one.
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