The Smedvig operated drilling rig West Alpha was in mid December 2000 successfully installed at the Snorre B field and started drilling production wells for Norsk Hydro in 350 metres water depth. The production risers were already installed at the field during the summer 2000 waiting for the Snorre B production semi to be installed early summer 2001. The mooring lines of West Alpha cross the riser arches that float up to 110 m over the sea floor, and a line breakage when using chain mooring lines would cause considerable damage and might have delayed the oil production by up to one year. This would have significant impact on total field economics. A special mooring system has therefore been designed to ensure that contact between a falling mooring line and the pre installed riser system is avoided for all likely breakage scenarios. The paper describes the design process and analyses of this system. In addition to standard dynamic analyses to document that the system complies with the required safety factors, extensive non linear FEM analysis have been performed to document the behaviour of a free falling mooring line for different breakage scenarios. Advanced plastic analysis of mooring line components has also been performed in order to document the ultimate load capacity under combined torsional and axial loading.
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