The effect of the mooring loads on floator motions can be significant for small water plane are floaters like CALM buoys. Not only does the mooring system contribute to the static restoring force components, but the dynamic behaviour of the mooring lines also affects the inertia and damping of the moored CALM buoy. The results from model tests with a moored CALM buoy were compared with the results from two series of time-domain computer simulations. First, fully dynamic coupled simulations were carried out, in which the interaction between the floater motions and the dynamic mooring line loads was modelled for all 6 modes of motion. Second, quasi-static simulations were carried out, in which only the (non-linear) static restoring force characteristics of the mooring system were taken into account. The comparison of results from the simulations and the model tests clearly indicates that the fully dynamic coupled simulations show a much better correspondence with the model test results than the quasi-static simulations. It is concluded that for the simulation of the behavior of a moored CALM buoy in waves a fully dynamic coupled mooring analysis is essential.
This paper describes a series of model tests aimed at gaining insight in the tension variations in the export risers and mooring lines of a CALM buoy. The test result were therefore not only analysed carefully, but were also used as input and to validate a numerical tool that computes the coupled motions of the buoy and its mooring system. The tests were carried out at a model scale of 1 to 20. Captive tests in regular and irregular waves were carried out to investigate non-linearities in the wave forces on the buoy for example from the presence of the skirt. Decay tests were carried out to determine the damping of the buoy’s motions and to obtain the natural periods. Finally, tests in irregular waves were carried out. The dynamics of the mooring system and the resulting damping have a significant effect on the buoy’s motions. A numerical tool has been developed that combines the wave-frequency buoy motions with the dynamical behaviour of the mooring system. The motions of the buoy are computed with a linearised equation of motion. The non-linear motions of the mooring system are computed simultaneously and interact with the buoy’s motions. In this paper, a comparison is shown between the measurements and the simulations. Firstly, the wave forces obtained with a linear diffraction computation with a simplified skirt are compared with the measured wave forces. Secondly, the numerical modelling of the mooring system is checked by comparing line tensions when the buoy moves with the motion as measured in an irregular wave test. Thirdly, the decay tests are simulated to investigate the correctness of the applied viscous damping values. Finally, simulations of a test in irregular waves are shown to validate the entire integrated concept. The results show that: 1. The wave-exciting surge and heave forces can be predicted well with linear diffraction theory. However, differences between the measured and computed pitch moment are found, caused by a simplified modelling of the skirt and the shortcomings of the diffraction model. 2. To predict the tension variations in the mooring lines and risers (and estimate fatigue) it is essential that mooring line dynamics are taken into account. 3. The heave motions of the buoy are predicted well. 4. The surge motions of the buoy are predicted reasonably well. 5. The pitch motions are wrongly predicted.
The wake flow behind a ducted azimuthing thruster was investigated. The thruster wake is an important factor in thruster interaction effects. Model tests were carried out for 3 different configurations; a thruster in open water conditions, a thruster under a flat plate and a thruster built into a barge. Two different thrusters were considered, a ‘normal’ thruster with a horizontal propeller axis and a ‘tilted’ thruster with a propeller axis and nozzle oriented 7 deg down-wards. In the tests the propeller thrust and torque were recorded, as well as the nozzle thrust and unit thrust. The velocities in the wake of the thruster were measured using a PIV (particle image velocimetry) system, for down-stream locations up to x/D = 19. The influence of the thruster tilt, the plate above the thruster and bilge radius on the thruster wake flow were investigated. Detailed PIV measurements were carried out on the wake flow behind the thruster in open water conditions. The PIV system used can measure 3D velocities in large set of points in a 2D plane, which is illuminated by a laser light beam. The flow velocities were measured in a large number of cross sections at different distances from the thruster. The PIV measurements provide a detailed image of the flow velocities in the thruster wake, showing the axial velocities, as well as the rotation and divergence of the wake. Subsequently, PIV measurements were carried out for the thruster under a flat plate and the thruster under a barge. The measurement results show a thruster wake that is deformed by the presence of the plate and the barge. The plate and the bottom of the barge form a flat plane above the thruster, clearly flattening the cross section of the thruster wake. Furthermore, the wake flow at the side of the barge, near the bilge radius, results in a low pressure region, causing the wake flow to diverge up as it flows from under the barge into the open water. This phenomenon is known as the Coanda effect and is strongly dependent on the bilge radius and the distance between the thruster and the side of the barge. The effect of both these parameters was confirmed in the model test results presented. The typical flow patterns observed as a result of the Coanda effect are illustrated in Figure 1 below. The results of the present model test research are used to further improve the understanding of the physics of thruster interaction effects. Furthermore, the results will serve as validation material for CFD calculations.
Thruster-interaction model tests were carried out in MARIN’s Deepwater Towing Tank. Detailed PIV measurements were performed of the wake flow behind the azimuthing thrusters on two different DP vessels, a Semi-submersible and a Drill Ship. The flow velocities were measured in a large number of cross sections at different distances from the thrusters. The PIV measurements provide a detailed image of the flow velocities in the thruster wake, showing the axial velocities, as well as the transverse and vertical velocity components. First, measurements were carried out on a DP Semi-submersible (scale 1:40), which was equipped with 8 azimuthing thrusters. The results of the PIV measurements show the wake flow, interacting with nearby thrusters and the opposite pontoon of the semi-submersible. An example is shown in Figure 1 below. Deflection of the thruster wake, caused by the Coanda effect, was observed. The results for thrusters with a 7 deg downward tilt were compared with the results for thrusters with a horizontal propeller axis. Furthermore, the effect of ambient current was investigated. Second, measurements were carried out on a DP Drill Ship (scale 1:40), which had 6 azimuthing thrusters. The results of these PIV measurements also gave insight in the wake flow behind the azimuthing thrusters and the interactions between neighbouring thrusters. An example is shown in Figure 2 below. In this case, special attention was paid to the development of the thruster wake along the vessel length, up to a distance of more than 40D downstream. The results of the present research are used to further improve the understanding of the physics of thruster interaction effects. Furthermore, the results will serve as validation material for CFD calculations that are foreseen in the near future.
This paper discusses thruster interaction effects for a DP shuttle tanker, equipped with two main propellers and rudders, as well as two bow tunnel thrusters. Thruster-interaction model tests were carried out in MARIN’s Deepwater Towing Tank. Detailed PIV measurements were taken of the wake flow behind the main propellers and rudders. Furthermore, PIV measurements were taken of the wake flow of one of the two bow tunnel thrusters. The flow velocities were measured in a large number of cross sections at different down-stream positions. The PIV measurements provide a detailed image of the velocities in the thruster wake, showing axial velocities, as well as transverse and vertical velocity components. The results of the first set of measurements showed in detail the wake flow behind the main propeller of the DP shuttle tanker. The wake flow pattern was determined at rudder angles of 0 deg and 10 deg. Since the research is related to DP performance, bollard pull conditions (zero forward speed) were considered in the model tests. The results of the second set of measurements showed in detail the wake flow of one of the bow tunnel thrusters. The wake flow pattern was investigated in zero speed conditions, as well as for the vessel at forward speed. The observed flow patterns helped to explain the reduced bow tunnel performance at forward speed. The results of the present research are used to further improve the understanding of the physics of thruster interaction effects. Furthermore, the results will serve as validation material for CFD calculations that are currently being performed.
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