| dc.description.abstract | Offshore wind is an increasing sector within renewable energy, and WindBarge aims to fill a gap in the offshore development, with a simple and cost efficient design targeted towards intermediate water depths between 40-60 m. The floater has a barge shape with straight edges, and shallow draft. Mooring configuration consists of a single anchor leg mooring and suction anchor.
The use of single point mooring is anticipated to reduce costs due to less material use and simpler installation procedures. In addition, it may entail reduced motions and loads, since the floater is allowed to rotate with the current environmental condition. This is called weathervaning.
This master thesis is divided into two parts: Initial design of barge structure and assessment of fatigue damage. Initial barge design is governed by natural periods and stability in pitch and roll.
Hydrostatic and hydrodynamic analyses in HydroD are used to assess design criteria and determine hydrodynamic properties. Time domain analyses of barge, wind turbine and mooring line is performed at 60 m water depth with coupled SIMO-RIFLEX analyses in SIMA.
Steel costs of the presents design is compared to steel costs of monopiles at 35 m water depth. Decay tests are used to verify natural periods and damping levels for the floating system. Test with constant wind and approximately no wave load are used to verify the behavior of the wind turbine.
A number of environmental cases are used to assess fatigue damage under normal power production and normal environmental conditions. Each case is performed for five seeds, with duration of one hour, co-aligned turbulent wind and irregular waves. To investigate the effect of misaligned loading, additional analysis are performed with misaligned wind and waves.
Fatigue damage is reviewed separately for load cases with co-aligned and misaligned loading. Only load cases with co-aligned loading is included in consideration of lifetime fatigue damage.
Effect of aerodynamic damping is briefly examined by comparing the change in loading on turbine and floater as exposed to irregular waves, with and without turbulent wind.
Constant wind tests confirmed normal turbine behavior after the controller was tuned to avoid negative damping, and static pitch angle was improved.
Response motions and loading on wind turbine are for most cases seen to be well below critical limits. Transverse motion is lower than anticipated for both co-aligned and misaligned loading.
Exceptions from expected behavior are: A computational error during an analysis of the 50 year sea and strange surge translation when only exposed to beam sea. It was not possible to identify the cause for the computational error, but it might be due to excessive mooring line loading.
Instability effects in sway and yaw occurred during initial analysis, and a simplified approach of increased hydrodynamic stiffness in sway and yaw is adopted to continue with further analysis. This clearly limits floater motions in transverse direction, such that the obtained results do not capture the real floater behavior.
It is likely that increased stiffness causes the strange behavior described for analysis without wind. Analyses without wind, confirmed that in particular barge pitch moment is clearly dependent on aerodynamic damping, and barge pitch moment is approximately 8.5 times larger without wind.
Assessment of lifetime fatigue damage gives failure after approximately 3.3 years. The number of environmental cases used is not sufficient to give a good representation of fatigue life. It is hard to draw a firm conclusion about whether fatigue damage is under- or over estimated, but it is likely that the tower requires enforcement to survive 20 year. Analyses with misaligned loading showed a clear tendency of decreasing fatigue with increasing angle of misalignment. It would be expected to see a certain increase in damage for beam sea, and it is assumed that the increased stiffness also alters these results.
To sum up, the criteria related to natural periods and stability are satisfied and the amplitude of loading and response is not exceeding critical limits. Rough cost estimates of steel weight indicates that WindBarge is capable of competing with monopiles. However, increased stiffness is too high, such that the real floater behavior not is captured. Requirements regarding fatigue failure are neither satisfied.
Instability problems must be solved, possibly by adjusting the mooring line and changing distribution of ballast so that longitudinal center of gravity is located in front of the tower. The internal barge structure should be designed, and tower enforced. Fatigue damage should be examined with sufficient number of environmental cases, and include the effect of misaligned loading. Investigating the effect of blade failure and assessment of load reduction due to weathervaning, should also be done. | en |
