To overcome the environmental impacts of releasing oil into the ocean for testing acoustic methods in field experiments using autonomous underwater vehicles (AUVs), environmentally friendly gas bubble plumes with low rise velocities are proposed in this research to be used as proxies for oil. An experiment was conducted to test the performance of a centrifugal-type microbubble generator in generating microbubble plumes and their practicability to be used in field experiments. Sizes of bubbles were measured with a Laser In-Situ Scattering and Transmissometry sensor. Residence time of bubble plumes was estimated by using a Ping360 sonar. Results from the experiment showed that a larger number of small bubbles were found in deeper water as larger bubbles rose quickly to the surface without staying in the water column. The residence time of the generated bubble plumes at the depth of 0.5 m was estimated to be over 5 min. The microbubble generator is planned to be applied in future field experiments, as it is effective in producing relatively long-endurance plumes that can be used as potential proxies for oil plumes in field trials of AUVs for delineating oil spills.
Gravity Based Structures (GBSs) are commonly used in the offshore oil and gas industry for storage during the production of hydrocarbons. The GBS sits on the sea bed, but it is subjected to forces and moments caused by waves. Obtaining accurate predictions of the magnitude of wave induced forces and bending moments is essential input to the structural design. Other key operational factors are the wave field in the vicinity of the structure and the amount of green water that will come onto the deck of the GBS. Scale model experiments can be used to obtain these predictions but this is an expensive option, especially in the early stages of a design, when many different concepts may be considered. An alternative method is to use Computational Fluid Dynamics (CFD). One CFD approach that is particularly suited to the challenges of predicting the required performance parameters for a GBS is the Volume of Fluid method, which is available in the commercial code Flow-3D. This paper presents the numerical simulation of the wave field around a surface piercing cylinder using the Volume of Fluid approach and compares it to published results. It also presents the simulations of the forces, moments and wave field around a proposed Gravity Based Structure, for which model data was available. The results show that predictions made using the Volume of Fluid method agree well with the observed wave patterns close to the structure in both cases, and that the method also gives good predictions of the observed forces and bending moments acting on the GBS.
This paper presents the development of a motion simulator for a moored FPSO, which includes numerical prediction of the FPSO motions in wind, waves and current. It also presents the resulting mooring line tension, 3-dimensional visualization of the FPSO motion, and summary analysis of the resulting motion parameters. The FPSO motion in waves was simulated using an in-house seakeeping code, MOTSIM. A spread mooring line routine, based on catenary theory, was developed and added to MOTSIM to calculate the restoring force of each mooring line. The visualizer (or animator) was developed in-house from open source software, including Ogre, Hydrax and Skyx. It can playback a 3-dimensional view of the simulation (above and below water). The user can view the results in a movie-like format, and change viewing position during the play-back. The user can also run a new simulation from the animator by inputting the required parameters.
The program for analyzing the time dependent responses generated by MOTSIM was developed as a stand-alone program using MATLAB. The analyzer can conduct statistical analysis of time-domain response signals.
A heading control system and a DP control system were also developed in the simulator and can be activated to help control the FPSO motion if required.
A validation of the ship motion prediction and mooring tension was conducted against model experiments using 100-year return period environments with different combinations of wave, wind and current directions.
The simulator was developed as a forecasting tool to help operators predict platform performance based of forecast weather conditions.
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