Assessment of the Horn Mountain Spar in Hurricane Ivan

The Horn Mountain Spar experienced the full strength of Hurricane Ivan, a virtually 2500-yr wave and 600-yr wind in the Gulf of Mexico [1], in 2004. The measured and hindcast sea state reached a 53ft significant wave height and 88 knot wind speed (1-hr average at 10m elevation), which is significantly higher than the spar’s maximum design criteria of 41.7 ft wave height and 78.2 knot wind speed. Both the environment and spar responses in the hurricane were measured. However, 6 hours prior to the peak of the storm, the generator which powered the instrumentation stopped, preventing further measurements. In order to estimate the spar’s actual responses at the peak of the storm and to evaluate the accuracy of the existing spar motion analysis tools, a blind analysis test was performed using the measured and hindcast environmental conditions as input. The analysis results were then compared to the last available measurements of the spar responses in the hurricane. Since the analysis from the design phase was known to be conservative, a fully coupled spar motion analysis model was used in this blind analysis test in order to achieve a higher level of accuracy. The environmental condition was also modeled to a higher level of detail than in the design phase, including wave spreading, dual-peak wave spectrum, etc. Some minor adjustments to the fully coupled model were necessary to achieve the desired accuracy. From the blind test and the subsequent motion analysis for the peak of the storm, the following conclusions can be drawn: 1) The result from the fully coupled analysis model generally agrees very well with the measured spar motion responses early in the storm (when measurements were still available). 2) The spar’s pitch, roll and heave responses were estimated to be within the design limit even though the environment measured at a nearby platform significantly exceeded the maximum design condition at the peak of the storm. There was no drilling rig on the spar at the time of the storm. The findings confirm both the robustness of the current spar design practice and the accuracy of the spar analysis tools.