Investigation of Hydrodynamic Aspects of the Design of the Universal Buoyancy Concept (UBC) Loading Buoy
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- Institutt for marin teknikk 
A new loading and discharge concept for ships, referred to as the Universal Buoyancy Concept (UBC), is proposed as an effective alternative to land based infrastructure for distribution of primarily LNG. The UBC system consists of a slack moored stepped spar buoy equipped with pads for shipside vacuum attachment. The buoy is connected to shore based tank facilities with a flexible cryogenic riser, thus providing opportunity for offshore loading and discharge of cryogenic liquids. In the following masters thesis, some hydrodynamic aspects of the UBC has been investigated with experimental methods. The model tests have been done in collaboration with fellow classmate Andreas Nilsen. A model test specification was prepared, where the objectives of the test and a detailed test plan was formulated. A model of the UBC was produced partially by MARINTEK and partially by Andreas and myself. A ship model was provided by MARINTEK and ballasted to a light loading condition for conservative ship motions. A rigid coupling arrangement between the ship and the buoy with possibility for measurement of interaction forces were installed in the ship model. Static pull-out tests for adjustment of a simple spring mooring system and decay tests were performed. Subsequently 4 headings were tested for regular waves and current. Afterwards, post-processing and analysis of the results has been conducted from scratch in Matlab. Results in this masters thesis consist of RAO s of first order motions and interaction forces between UBC and the ship for coupled condition, RAO s for independent and relative motion, estimation of current drag forces and moments on UBC (including ship shadow effects), current forces on the coupled system, mean wave drift forces, and vortex induced vibrations on UBC. More information can be worked out from the results if needed. Head sea current gives the largest drag forces and moments on UBC in coupled condition. Transverse current moments on UBC will be smallest with UBC placed downstream of the ship. Drag coefficients from measured mean drag moments are lower than those for a smooth circular cylinder in the same Reynolds number regime, though existing experimental results for stepped cylinders and cylinders with free end reports an increase in drag coefficient for these shapes compared to a smooth circular cylinder. It is speculated if the transducer is not sufficiently sensitive to the small force regime.Wave drift forces are considerable for beam sea conditions, nearly the same magnitude as current forces. For longer waves, the head sea condition is expected to be most critical with regard to wave drift depending on the ship size. Experimental values of drift forces are generally larger than conservatively predicted with Maruo s formula (total reflection).It is crucial to model exact mass distribution for accurate results of linear motions of UBC. A reduction in mass radius of gyration from 12.9 m to 7.12 m gives order of magnitude tripled linear roll/pitch motions of UBC, and the natural roll/pitch period is reduced is reduced with 10 seconds, from 25.8 to 15.8 seconds.For the considered ship size, ship motions dominate UBC motions, especially in heave. Both horizontal and vertical relative motions are sharply increasing with wave period. Relative motion between UBC and ship is significantly smaller with UBC positioned on the leeward side of the ship, with horizontal motion halved for wave frequencies of 1.3 rad/s. Moderate crossflow excitation forces at the theoretical vortex shedding frequency of the top cylinder are registered for current speeds of 0.2 m/s (model scale). Hence top cylinder vortex shedding is assumed to dominate the crossflow forces. For model scale current speeds of 0.3 m/s, crossflow excitation forces become powerful, and seem to lock on to the roll frequency of the ship. For lower current velocities, the natural roll period of UBC will be excited. Scaling of crossflow forces is difficult due to uncertainty in both Strouhals number and wake development for a stepped cylinder with free end at full scale Reynolds numbers.Beam sea conditions are the most critical condition with regard to first order ship-buoy reaction forces, and of the two, UBC on the windward side will be the worst. On the leeward side UBC is protected against both incoming and reflected waves. Ship roll motions are larger with UBC on the leeward side, suggesting that the dominating phenomenon contributing to reaction forces are inertia forces from the surface waves, not drag forces from ship-induced forced linear motions of UBC. It is acknowledged that forces and especially moments are over-estimated due to the rigid coupling.Fourier analysis of time series has revealed second order effects at twice the wave frequency in a majority of both interaction force and motion measurements. The wave profile is investigated and found to display similar, but less significant second order effects. It is believed that a second order wave profile develops from a first order as it travels down the tank. In retrospect, a Fourier analysis of the wave profile at the location of the test setup and closer to the wavemaker could be conducted as part of the calibration of environments.