Prediction of the response from ice forces to a lighthouse structure: Reanalysis of the Nygrån lighthouse collapseincident in the light of new design codes

Abstract

Ice interacting with offshore structures is one of the main concerns for engineering activities in arcticand sub-arctic areas. As new offshore structures are built or planned in these areas, the need to developmore accurate calculation methods of ice loads and the corresponding structural response is critical. Asignificant work regarding the level of ice forces and prediction methods have led to a new design codeISO/DIS 19906 (2009), trying to meet a consensus in arctic engineering. Several lighthouse structureslocated in the Gulf of Bothnia have experienced damage or failed due to loads from ice. Nygrånlighthouse was constructed in 1958 and collapsed in the spring, April 1969. In order to evaluatethe guidelines given ISO/DIS 19906 (2009), a re-investigation of the collapse was performed. Theseguidelines was compared to the guidelines given by IEC 61400-3 (2009) and API RP 2N (2009).A finite element model of Nygrån lighthouse was developed based on the actual geometry. It wasfound that the global static design load according to ISO/DIS 19906 (2009) was about equal to thedesign load of the structure, while IEC 61400-3 (2009) and API RP 2N (2009) gave loads that were84 % and 250 % higher, respectively. According to the guidelines on dynamic ice action given inISO/DIS 19906 (2009), the structure was susceptible to resonant ice induced vibration caused byfrequency locked-in ice crushing. While according to IEC 61400-3 (2009) the structure should neverexperience frequency locked-in ice crushing. It was thought that the onset criterion in IEC 61400-3(2009) was not conservative, compared to recent full-scale findings. If frequency locked-in ice crushingwould occur on the structure, the dynamic amplification of the displacement response would beup to a factor of 3.4, while von Mises stress in the critical cross-section was amplified with a factorof 10. This was thought to be a result of inertial forces from the mass located above the waterline. Itis concluded that the highest stresses could not be calculated by simply amplifying the static loadsfrom ice crushing. In the light of new design codes, an alternative explanation of the collapse hasbeen derived. It was found that resonant ice induced vibrations may lead to a bending moment at thesurface of rupture 4.5 times higher than the static collapse load.