hawsers connected, the TLP was kept in the zero position by two hold back lines and a Fluid Transfer Line.


Model tests were conducted in wind, waves and current in a variety of setup configura- tions: with the semi-submersible alone, TLP alone, TLP + semi-submersible coupled and TLP + semi-submersible decoupled. During the test campaign the orientation of the TLP alone was changed once and the orientation of the coupled setup (TLP + semi- submersible) was changed twice, allowing for different basin current incident angles, in combination with a variety of wave directions.


Complex setup This complex model test setup required thorough planning and prepa- ration. The mooring lines of the semi-sub- mersible, as well as the hold back lines of the TLP, had to be truncated in such a way that the stiffness is represented properly and the footprint of all anchor blocks fitted on the 10 m deep basin floor in all three coupled orientations.


With the proper truncation at hand all anchor blocks were placed in the correct position at the beginning of the project making use of the moveable basin floor. With the anchor blocks in place divers could easily adjust any change of orientation.


During these tests a total of 87 channels were measured. Video cameras were positioned above and underwater to capture the coupled motions of the TLP and semi-submersible. The combination of measured signals and video images gave a lot of important insight into the understanding of the complex hydrodynamics of the coupled test setup.


VIM tests in Depressurised Wave Basin An extensive tow test campaign was carried out to investigate the VIM behaviour of the TLP alone and the TLP coupled to the semi-submersible by a hawser system. For the coupled system, tests were carried out with the two models inline with each other using two different hawser layouts and with the semi-submersible positioned at a sideways offset. Lastly, captive reference drag tests were performed for the TLP and semi- submersible. In the tow tests the TLP was mounted under an air-bearing system and vertical mooring springs. The air-bearings mean that the model can be tested at the


Inplace tests, TLP and semi-submersible coupled


correct mass/displacement ratio, which is important for the VIM response of TLPs. They have extremely low friction in the horizontal plane. This setup ensures a set of ‘clean’ tests, where the total damping and flow pat- terns originate from the structure’s hull alone. In addition, the tow heading of the model can be changed very quickly and efficiently. For the VIM tests an optimised scope was carried out for different headings and a range of reduced velocities. The two-body system proved to be particularly challenging


as the coupled models had to be rotated in the basin for different headings and two different hawser systems. The Depressurised Wave Basin, measuring 240m x 18m x 8m, again proved to be extremely suitable for such complex tests.


These tests show that VIM is particularly relevant for fatigue issues and that it remains an important phenomenon that should be taken into account during the design stage.


VIM tests, TLP and semi-submersible coupled


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