Wave Crest Heights and Deck Damage in Hurricanes Ivan, Katrina, and Rita
Authors: Forristall George Z. 
Published: October 2008

The hurricanes of 2004 and 2005 caused substantial damage even to relatively new deepwater facilities such as Petronius and Pompano. Crest heights calculated using standard theories are unlikely to have caused such damage. Several possible reasons for the discrepancy between crest height calculated by standard methods and observed deck damage ere considered. Freak waves due to unusual wave spectra were not observed at sites where individual waves were recorded. Much of the damage occurred on structures with small members so diffraction was not an issue. Calculations of the maximum crest height over the area of a deck were able to explain the damage. If the an entire deck is inundated, thelateral force on a structure increases greatly. Our calculations how that while local inundation is more likely than previously suspected, the inundation is usually local and does not necessarily threaten the integrity of the structure.

Introduction

In September 2004, Hurricane Ivan generated the largest waves ever recorded in the Gulf of Mexico. In 2005, Hurricanes Katrina and Rita added to the devastation. Over 100 platforms were destroyed in these storms. Most of them were old, designed to standards that are now obsolete. But the lower decks of many large new structures also suffered damage, casting doubt on present design standards. Figure 1 shows the bent plate girders under the south end of the cellar deck of the Petronius compliant tower after Hurricane Ivan. This damage must have been caused by one or more wave crests striking the girders. The bottom of steel on the cellar deck is at 16.75 m above mean water level and the deck was set down approximately 0.3 m during the storm. The wave crest near the plate girders must have exceeded 16.45 meters above mean water level.

Cardone et al. [1] found that the Oceanweather wave hindcast model did a good job of matching significant wave height measurements in Ivan. The purpose of this paper is to determine whether crest heights calculated on the basis of those hindcasts can explain the observed deck damage. The maximum hindcast significant wave height at the Oceanweather grid point closest to Petronius was 14.76 m. According to Forristall's [2] empirical wave height distribution, the highest wave in 1000 would be 24.95 m. The crest height of a Stokes fifth order wave of this height is 13.96 m, far below the height needed to produce the observed damage. As shown in the next section, second order crest height statistics give an estimated maximum crest height about two meters higher, but still too low to explain the damage to the deck.

There are several possible explanations for discrepancies between predicted crest heights and observed damage. One suggested mechanism is that unusual directional spectra can increase the chance of freak waves larger than predicted by second order theory. Two inherently contradictory causes of freak waves have been suggested in the literature. The first is crossing seas with very wide directional spreading. The second is related to the Benjamin-Feir instability.