Marine operasjoner i havbruksindustrien med spesielt fokus på brønnbåtoperasjoner

Sammendrag

The thesis highlights how planning and execution of marine operations in the aquaculture industry are implemented in Norwegian regulations and standards. The study identifies a lack of regulations, thus proposals for improvements are emphasized. As an example, specifications for the overall duration of the operation including a time buffer that considers unforeseen events is one of the measures that should be included in the regulations. Furthermore, operational limits must be determined for what is acceptable levels of significant wave height, wind and current strength without compromising on the safety of personnel, fish and equipment. With the stipulated total duration of the operation and the required operational limits, a sufficient weather window can be defined for the operation to take place within safe limits. DNV-OS-H101 "Marine Operations, General" can be used as a basis for adopting the principles of the petroleum industry and a supervisory authority will be necessary to ensure that the industry complies with the legislation.
Based on the lack of regulations and standards for planning and execution of marine operations in the aquaculture industry, the objective of this thesis is to show a methodology for how operational limits can be established for a typical operation in the industry. When determining operational limits, the uncertainty of the weather forecast should be accounted for by using alpha factors. As alpha factors are not established for the aquaculture industry, this uncertainty is not accounted for in this thesis. In cooperation with the supervisors, the operation was decided to be a wellboat operation, where the vessel is moored to a single cage system with the use of four mooring lines. To determine the operational limits, the axial force in the vessel’s mooring lines was chosen to be the design parameter of interest. The maximum allowed axial force was set to be the breaking strength of the mooring line with a safety factor of three.
A coupled SIMO-RIFLEX model was established to simulate the operation. The sensitivities of the operational limits to changes in weather exposure was examined by varying the following parameters; weather directions, wave amplitude and period, with and without applied current and wind. Three directions were examined, 180, 225 and 270 degrees relative to the wellboat’s coordinate system. Since the industry has no specified procedures for how much pre-tension the mooring lines should have, other than tightening the lines sufficiently, the sensitivities of the operational limits to the degree of pre-tension was examined.
The operational limits are sensitive to short wave periods due to the influence of large wave drift forces. The wave drift force is a function of the wave amplitude squared, which means that an increase in the amplitude will result in a significant increase in the contribution from the wave drift forces. To maintain sufficient safety when the amplitude is large, the wave period must be long, as the wave drift forces approach zero for long periods. The results from the simulations also showed that the system is sensitive to changes in the incoming load direction from waves, wind and current. For the three directions analysed in this thesis, bow sea (270 degrees) is the most exposed, while head sea (180 degrees) is the least exposed. This is because the wave drift forces are greater for 270 degrees, in addition to the wind and current coefficients which gives a significantly greater contribution in this direction compared to 180 degrees. When wind and current act on the system in addition to waves, the total mean forces acting on the system will increase, thus the operational limit will be reduced. Assessment of the system’s natural periods is necessary, as corresponding wave periods may cause a significant increase in the axial forces in the vessel’s mooring lines. Results from the sensitivity analysis of pre-tension suggest that the system is not particularly sensitive to the amount of pre-tension applied.