| With the decline of fishery resources in China and increasing demand on sea foodtogether with growing marine economic, the development of deep-water anti-wave cages isan effective way to the sustainable development in domestic marine aquaculture industry.Studying on the hydrodynamic performance of deep-water cage facilities is an importanttechnical support to protect human lives and property for fishermen. The present dissertationis based on the researches of the National863High Technology Development Plan Projectand the National Natural Science Fund Project.HDPE sea-cage with double floating tubes is the most popular cages used in China,sharing more than70%of over6000cages totally. In the present work, model tests andfull-scale measurement on site are performed on the circular sea cages with perimeter40meters and height10meters, which are commonly used in China. The measured data areprocessed by means of FFT. The experimental results are then analyzed by applying fish gearmechanics and theory on the interaction of marine structures with waves. The followingfindings are obtained:1. To inspect the differences between the Tauti rules and Dickson rules, which arepopularly used in the hydrodynamic tests of fishery gears, model tests of single cage withcircular shape are carried out in the tank under these two rules respectively. The resultsindicate that both rules can be used as the rules of cage model tests with an exceptional littledifference in certain cases of flow. The resistance of the cage body or the whole cage systemis found to be a power function of the flow velocity. Similarly, the drag coefficient can beexpressed as a power function of the Reynolds number.2. Tensions in each cable that is mooring a single circular net-cage system sufferingcurrent, waves or a combination of current and waves are measured. The cage system isfacing or oblique in45degrees relative to the coming waves or (and) current. It is found thatthe mooring forces are mainly acting on the front-line (both in the cases of head and obliqueseas), the rear side-line (in head seas) or the front side-line (in oblique sea).3. Fourier analysis is applied to the maximum cable tension to obtain the components ofsteady force, the linear and nonlinear wave forces. It is helpful to understand thecharacteristics of tensions when suffering from action of waves or wave-current.4. By comparing to the record of a fixed wave height gage, the corresponding position of the wave crest (or trough) relative to the cage system can be deduced at the moment when aspecial value appears in the time history of tension for the cage models in head seas.5. The relationship between the cable tension and wave height, wave period or theother parameters is obtained through data analysis. The sum of the Fourier coefficients up tothe fourth-order is defined as the maximum tension of cables. The relationship between themaximum cable tension and wave height is investigated (both in the cases of head and obliqueseas).6. Through dimensionless analysis, the relationship between non-dimensional maximumtension and wave length normalized by cage diameter or wave steepness is obtained. Theregression formulae relating the maximum cable tension of the leading-line and back-line tothe wave height for the prototype of sea cages in actual head sea conditions, as well as therelationship between the non-dimensional maximum tensions and wave steepness, arededuced. Comparing both results showes that the non-dimensional maximum tension can beused to reckon the maximum cable tension of prototype cages in real sea under various seaconditions.7. By comparing the maximum cable tension under the action of both waves and currentin head and oblique seas, it is found that the mooring system facing the waves is better thanthe one oblique to the waves.8. The relationship between the maximum cable tensions and flow parameters is obtainedfor a group of cages moored in both waves and currents. The maximum cable tensions ofprototype grouped cages are evaluated under a series of real sea conditions., It is found thatthe maximum cable tension is in the range of48KN-199KN when actual wave height is2m-5m. On the other hand, the maximum cable tension is in the range of81KN-402KN whenvelocity of current is31.6m/s together with wave height of2m-5m.9. In addition, an underwater stress auto-recorder is independently developed, which canmeasure the tension of underwater cables in sea for a long time. Tests under severe seaconditions are carried out and which verify the reliability of the stress recorder. Besides, thevariation of tensions in floating ropes under bad sea conditions is obtained, which will be thefirst-hand data in the future force analysis for the sea cages system.The non-dimensional analysis of maximum cable tension, the development ofunderwater stress auto-recorder and full scale measurements on site are the main points of thepresent research. The results of the present research will enrich the hydrodynamic studies ofnet-cages and the findings will provide reference for theoretical calculations and designs ofthe HDPE sea-cages with double floating tubes. |
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