Subsurface towing of heavy module
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- Institutt for marin teknikk 
The offshore industry is expanding, new contracts are offered all over the world and new oil fields are discovered deeper and further offshore. With this expansion follows great business opportunities for existing and new subsea companies. With this kind of competition it is crucial for the companies to be innovative and offer competitive solutions to a wide range of problems. This thesis provides a description of different methods of transporting a subsea structure to its installation site. Traditionally the structure is transported on deck of a heavy lift vessel and lowered through the splash zone at the installation site. This is a weather sensitive method and provides dynamic challenges when lowering the structure through the splash zone. A different option is to perform a subsurface towing of the structure. This is still considered as a relatively new concept even though it has been conducted several years with success. A subsurface towing is safer for the personnel, more cost efficient and less weather sensitive. However transportation on deck is often preferred due to its short transportation time. The subsea 7 method of performing a subsurface towing was used in the Tyrihans project and this method and project will form the basis of the analysis done in this thesis. A simple 1-DOF program was developed in the project thesis written by the author the fall of 2010. This program has been subject to extensive improvement to make it suitable to do an analysis of the Tyrihans project. The results provided from the 1-DOF program will be compared with the results provided from Orcaflex in terms of time histories and FFTs of the time histories. In addition to these comparisons; a parametric study of the templates heave motion with varying added mass coefficients as well as a parametric study of the offset angle with varying drag coefficients are conducted.