Size matters

With the 2012 Atlantic hurricane season well under way, Weather Research Center's Jill Haslings discusses key learnings from the super storms of the past decade and how the Freeman/Hasling Damage Potential Scale can help the oil industry better understand the threat from the Gulf hurricanes to come.

The US National Oceanic & Atmospheric Administration has forecast a less active hurricane season this year, putting the chances at 70% that the Atlantic will see nine to 15 named storms with top winds of 39mph or higher during the six-month season, which began 1 June. Of those storms, one to three could become major hurricanes, with top winds of 111mph or higher, NOAA said.

 Ivan was a warning of the cyclones to come. Jill Hasling

Weather Research Center's seasonal hurricane outlook indicates that the Gulf of Mexico coast from the Texas/ Louisiana boarder to the tip of Florida has the highest risk of experiencing a hurricane or tropical storm landfall. Other significant hurricanes in this phase of the Orbital Cyclone Strike Index [OCSI] are Hurricane Allen in 1980, a severe hurricane which made landfall in Alabama in 1882, and a severe hurricane in 1893 which made landfall in Louisiana. So there is a 46% chance that the Gulf of Mexico could experience a severe hurricane.

Between 2004 and 2008, hurricanes Ivan, Katrina, Rita and Ike demonstrated how vulnerable Gulf of Mexico offshore operations are to these monsters of the summer. Hurricane Ivan in 2004 was a very large hurricane that, fortunately, made landfall east of most of the oil leases.

However, an offshore buoy near the center of the hurricane measured a wave of 91ft. Researchers suspect that there were waves of 132ft during the hurricane that the instruments could not capture. Ivan was a warning of the cyclones to come.

In 2005, hurricanes Katrina and Rita in the Gulf of Mexico both reached Category 5 intensity on the Saffir-Simpson scale. The last time the offshore oil industry had experienced this type of devastation was with hurricane Andrew in 1992, which was a comparatively small storm. After the devastating hurricane season of 2005, the offshore industry started to review its practices and procedures to see if changes were needed.

Since severe hurricanes are such a rare event in the Gulf of Mexico, it can be easy to forget how devastating they can be. What have we learned from past hurricanes, and what lessons have we learned from these most recent hurricanes?

OE's Ivan cover in November 2004 and (right) Gulf of Mexico hurricance victims.

Through years of study, Weather Research Center (WRC) has developed a hurricane wind and wave model to hindcast and study the characteristics of tropical cyclones. With the development of GIS software, WRC meteorologists were able to plot a hurricane's wind field the total area of high winds generated by a storm--and its track, and to map the locations of offshore platforms. Hurricanes come in all sizes. After the 2004 and 2005 hurricane seasons, WRC meteorologists wanted to demonstrate to the community the correlation between the size of the hurricane wind field and potential wave heights.

Figure 1: Wind fields of significant Gulf of Mexico hurricanes.

Figure 2: Hurricane wind fields of Katrina, 2005.

The impacts of hurricane Ike started WRC meteorologists to wonder how the size of past Gulf of Mexico wind fields would compare. The wind fields of significant Gulf of Mexico hurricanes were hindcast and plotted using GIS software. Figure 1 shows the varying sizes of hurricane wind fields. Hurricane Carla in 1961 and hurricane Ike in 2008 were the largest. The yellow areas indicate Category 1 maximum sustained winds.

Figure 3: Cross-section of Gulf of Mexico hurricane wind fields.

The best demonstration of the impact of the size of the hurricane's wind field to the damaging impact of the hurricane's waves is Ike in 2008. The size of Ike's wind field was second only to the size of Carla's in 1961.

The size of a hurricane can change during its life cycle. Figure 2 shows this with the wind fields of Katrina in 2005.

Figure 4: Cross-section of Gulf of Mexico hurricane significant wave fields.

To better see the different sizes of the hurricane wind fields, WRC meteorologists created hurricane wind and wave profiles for past Gulf of Mexico hurricanes.

Figure 5: Tracks of the major hurricanes of the 1910s (on left) and 2000s.

The orange line in Figure 3 represents the cross-section of Ike's wind fields. The line across the bottom is how far from the center of the hurricane the wind extends, in nautical miles, on either side. Note how far from the center the maximum sustained winds greater than or equal to hurricane force (winds greater than or equal to 64 knots) extend. The only known past hurricane with a larger swath of winds greater than or equal to hurricane force than Ike was Carla, indicated by the green line.

Figure 4 shows a crosssection of hurricane significant wave fields. Note that Ike, represented by the orange line, had wave crest of 45ft or more (significant waves of 34ft or more) for over 160 nautical miles. A 34ft wave would result in at least a 45ft wave crest in deep water.

What these graphs demonstrate is that size matters when it comes to hurricane wind fields and wave fields. The size of the hurricane is sometimes more important than the intensity, especially when considering the number of offshore properties that lie in a hurricane's path. As the data show, a hurricane with winds of only Category 2 on the Saffir-Simpson scale could generate waves that usually are associated with a Category 4 hurricane.

The Saffir-Simpson scale does not take into account the size of the hurricane wind field. With this in mind, WRC developed a scale that would take into account the size of the hurricane and would be a better indicator of the type of damage that could impact offshore oil & gas assets.

Table 1: Number of Gulf platforms destroyed by hurricanes.

Our research led to the development of the Freeman-Hasling Damage Potential Scale, which ranks the severity of the damage that could be caused to offshore properties on a scale of 1 to 5.

Table 2: Number of major Gulf oil lease hurricanes by decade.

Table 1 shows the number of offshore structures destroyed by Gulf of Mexico hurricanes versus the number of platforms installed, as well as the percentage of installations destroyed by a particular hurricane. As seen, Andrew in 1992 and Rita in 2005 destroyed the most platforms.

WRC meteorologists then plotted hurricane wind fields and the number of offshore platforms by decade. Figure 5 compares two especially active decades, the 1910s and 2000s.

After comparing all decades going back to the 1850s, a pattern appeared suggesting that the Gulf of Mexico has experienced two consecutive decades with a high number of Category 3 or greater hurricanes followed by a decade with fewer significant hurricanes. Table 2 shows these decades and, if the pattern holds true, what can be expected over the next few decades. If history is any indication, the 2010s should be a relatively quiet decade, with the 2020s and 2030s seeing a return to more active hurricane seasons. But one must remember that it only takes one major hurricane on the right path to cause significant damage to oil & gas operations. OE