Use of Weight Elements to Improve the Dynamic Performance of Umbilicals

The development strategy in the North Sea has over the last years been towards tie in of smaller fields to existing infrastructure. Detailed engineering is required to obtain an optimal design for each application. This can be challenging for systems with multiple design constraints e.g. interference with adjacent structures combined with strict floater interface design requirements. Flexible risers and umbilicals are usually arranged in compliant configurations, where floater motions are absorbed by large changes of the configuration geometry in the 3D space. A vital design issue for such systems is to avoid overbending and fulfill fatigue life design criteria at floater supports. Another design challenge often encountered in design of such systems is to avoid/limit compressive effective tension in extreme load cases. In this paper it is proposed to introduce weight elements along parts of the riser/umbilical to improve the overall dynamic behavior and thereby address these design challenges. This principle was applied when designing the configuration for Morvin DEH riser. Morvin DEH riser is connected to A˚sgard B semi-submersible platform in the North Sea at 300m water depth. For this particular problem the design challenge was to limit the angle response in extreme load cases and fulfill fatigue design requirements in the bellmouth interface. Another constraint was at the same time to avoid interference between adjacent mooring lines and umbilicals. A feasible solution was found by introducing weight elements in the upper part of the umbilical. This had a minor impact on the cost schedule of the project. The Morvin DEH riser was installed successfully in the summer of 2009.