Vortex Induced Motion (VIM) of a Deep Draft Semi® with four square columns has been observed in model tests and in the field for a prototype configuration. The model test observations were developed into design guidance procedures. Some of the assumptions based on the model test results are validated with the field observations. A quantitative description of the field measurements is presented. A comparison between the field observations and the design guidance is provided.

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Vortex Induced Motion (VIM) response of a deep draft semisubmersible with square columns has been observed various model test conditions and in the field. A good understanding of the VIM phenomenon is desired because VIM can lead to significant SCR fatigue, especially for the larger diameters SCRs. A VIM design guidance was developed from several previous experimental investigations. Those experimental investigations are characterized by hulls with limited detail, and a simplified mooring / SCR system where typically only the stiffness is taken into consideration. A comparative experimental study is performed to identify the differences in VIM response under the absence and presence of hull appurtenances. Further, the VIM responses of a semisubmersible restrained by a mooring system without and with an SCR system are compared. An evaluation of the experimental results leads to an improved understanding of the VIM response at model test scale.

Field measurements were made of the vertical distribution of impact pressures exerted by breaking waves. Four distinct types are recognized and compared. These are near-breaking wave, plunging breaker, spilling breaker and post-breaking bore. The measurements were obtained by placing a 6 foot aluminum flat plate, backed by a cylinder in the surf zone, so that the fiat faced the approaching breakers. Five sensors were placed at one foot intervals on the flat. The sensors consisted of strain gage mounted aluminum diaphragms. Results indicated that impact pressure is significantly influenced by breaker type. The bore generated the largest impact pressures, followed in decreasing order by plunging breaker, spilling breaker and near breaking wave. In the vertical array, the largest impact pressures were recorded at or near the top, except for the bore where the reverse occurred. A qualitative explanation is given of various phenomena associated with impact pressures, by considering breaker mechanics.