EXAMPLE (SI Units): 100-year conditions for a site at 90.25° W in 100 m water depth.
As the site is in the transition region between the West Central and Central regions, conditions will need to be derived using inter- polation between West Central and Central values. In addition, currents will need to be interpolated between the 70 m and 150 m regimes.
As the site is 0.25 deg from the West Central boundary, and 0.75 deg from the Central boundary, West Central values should be weighted by (1 – 0.25) = 0.75 and Central values by (1 – 0.75) = 0.25 when deriving conditions at the site.
Independent extremes:
1-hour 10 m WS:
Central WS, from Table 4.5.3-1: 48.0 m/s West Central WS, from Table 4.5.2-1: 38.1 m/s WS = 0.75(38.1) + 0.25(48.0) = 40.6 m/s Hmax:
Central Hmax, from Figure 4.5.3-2: 25.4 m West Central Hmax, from Figure 4.5.2-2: 19.8 m Hmax = 0.75(19.8) + 0.25(25.4) = 21.2 m
THmax:
Central THmax, from Table 4.5.3-1: 13.9 s West Central THmax, from Table 4.5.2-1: 13.0 s THmax = 0.75(13.0) + 0.25(13.9) = 13.2 s Surge and Tide:
Central Surge and Tide, from Figure 4.5.3-4: 1.45 m West Central Surge and Tide, from Figure 4.5.2-4: 1.22 m Surge and Tide = 0.75(1.22) + 0.25(1.45) = 1.28 m
Currents must first be derived for each depth regime, i.e., the 70 m contour and 150 m water depth or greater, prior to interpolation to the site water depth.
70 m:
Central current, from Table 4.5.3-1: 2.07 m/s West Central current, from Table 4.5.2-1: 1.31 m/s Uniform current = 0.75(1.31) + 0.25(2.07) = 1.5 m/s Current direction, from Figure 4.3.1-1: 235°
150 m:
Central surface speed, from Table 4.5.3-1: 2.40 m/s West Central surface speed, from Table 4.5.2-1: 1.91 m/s Surface speed = 0.75(1.91) + 0.25(2.40) = 2.03 m/s Central speed at mid-profile, from Table 4.5.3-1: 1.80 m/s West Central speed at mid-profile, from Table 4.5.2-1: 1.43 m/s Speed at mid-profile = 0.75(1.43) + 0.25(1.80) = 1.52 m/s Central 0-speed depth, from Table 4.5.3-1: 100.8 m West Central 0-speed depth, from Table 4.5.2-1: 80.0 m 0-speed depth = 0.75(80) + 0.25(100.8) = 85.2 m Mid-profile depth = 85.2 m / 2 = 42.6 m
Once the 70 m and 150 m currents have been calculated, they can be used to define the current profile and direction at the 100 m depth site. First, the new profile is estimated. This is done by approximating the 70 m as a 3-point profile like the 150 m current, and then interpolating between the 70 m and 150 m currents for the surface, mid-profile depth and 0-speed depth points of the profile. In this case, the 70 m value would be weighted by (150 – 100)/(150 – 70) = 0.625 and the 150 m value by (100 – 70)/(150 – 70) = 0.375.
Approximate 3-point profile for 70 m current:
Surface speed = 1.5 m/s
Mid-profile speed and level = 1.5 m/s at 70 m 0-speed depth = 70 m
100 m profile:
Surface speed = 0.625(1.5) + 0.375(2.03) = 1.7 m/s Mid-profile speed = 0.625(1.5) + 0.375(1.52) = 1.51 m/s Mid-profile level = 0.625(70) + 0.375(42.6) = 59.7 m 0-speed depth = 0.625(70) + 0.375(85.2) = 75.7 m
The profile at 100 m would thus be defined by the following three (speed, depth) points:
(1.7 m/s, 0 m), (1.51 m/s, 59.7 m), (0 m/s, 75.7 m)
Second, the direction of the profile must be resolved. For peak current cases the current in the transition region may be considered omni-directional (although realistically would tend to align with local bathymetry in depths closer to 70 m).
However, the example will instead assume a 100-year peak wave case is being analyzed, for which the shallow and deepwater currents both have proscribed headings. Table 5-1 shows the deepwater current will have a heading +15° from the wave heading.
The shallow water current has a heading of 235° as noted above. Assume a wave heading of 282° is selected (note, as 282° is within ±22.5° of 290°, the wave height is not factored); thus the deepwater current component has a heading of (282° + 15°) = 297°.
As this is a peak wave case, the associated current must first be derived. The process for estimating the 100 m current pro- file above is repeated, with the addition of load case factors from Tables 5-1 and 5-2. From Table 5-2 a factor of 0.7 should be applied to the 70 m current (speed only), while from Table 5-1 a factor of 0.75 must be applied to the 150 m current (speed and depth levels).
100 m peak-wave associated profile:
Surface speed = 0.625(1.5)0.7 + 0.375(2.03)0.75 = 1.23 m/s Mid-profile speed = 0.625(1.5)0.7 + 0.375(1.52)0.75 = 1.08 m/s Mid-profile level = 0.625(70) + 0.375(42.6)0.75 = 55.7 m 0-speed depth = 0.625(70) + 0.375(85.2)0.75 = 67.7 m
The associated profile at 100 m would thus be defined by the following three (speed, depth) points:
(1.23 m/s, 0 m), (1.08 m/s, 55.7 m), (0 m/s, 67.7 m)
The associated current direction is the heading of the resultant of the 70 m current and the average current speed over the upper 70 m of the 150 m profile. First, find the average speed over the upper 70 m of the 150 m profile (applying the factor from Table 5-1):
150 m profile:
Surface speed = 0.75(2.03) = 1.52 m/s Speed at mid-profile = 0.75(1.52) = 1.14 m/s 0-speed depth = 0.75(85.2) = 63.9 m Mid-profile depth = 0.75(42.6) = 32.0 m
The average speed over the upper 70 m of the 150 m profile is thus:
[(1.52+1.14)/2](32/70) + (1.14/2)(32/70) = 0.87 m/s With the factor from Table 5-2, the 70 m current is:
0.7(1.5) = 1.05 m/s
Now find the direction of the resultant of the 70 m current and the average current over the upper 70 m of the 150 m profile, both weighted to 100 m. For convenience, resolve to east-west, north-south components (east and north are positive):
Average 70 m current, 150 m profile: 0.375(0.87) = 0.33 m/s, heading 297°
East = 0.33 sin(297) = –0.29 North = 0.33 cos(297) = 0.15
70 m current: 0.625(1.05) = 0.66 m/s, heading 235°
East = 0.66 sin(235) = –0.54 North = 0.66 cos(235) = –0.38 Resultant direction:
East = –0.29 – 0.54 = –0.83 North = 0.15 – 0.38 = –0.23
Heading = arctan(–0.83/–0.23) = –105° or (360–105) = 255°
Thus, the associated current for the 100-year peak wave case for this 100 m site would be given by a profile defined by:
(1.23 m/s, 0 m), (1.08 m/s, 55.7 m), (0 m/s, 67.7 m) with a heading of 255°.
To complete the peak case, the wind and surge for this peak wave case would then be adjusted using the factors from either Table 5-1 or Table 5-2 (the wind and surge factors are the same for shallow and deep when evaluating peak wind or peak wave cases).
The final associated values would be:
Associated 1-hour 10 m WS = 0.95(40.6) = 38.5 m/s Heading = 282 – 15 = 267°
Associated surge and tide = 0.7(1.28 – 0.42) + 0.42 = 1.02 m
EXAMPLE (U.S. Customary Units): 100-year conditions for a site at 90.25°W in 328 ft water depth.
As the site is in the transition region between the West Central and Central regions, conditions will need to be derived using inter- polation between West Central and Central values. In addition, currents will need to be interpolated between the 230 ft and 492 ft regimes.
As the site is 0.25 deg from the West Central boundary, and 0.75 deg from the Central boundary, West Central values should be weighted by (1 – 0.25) = 0.75 and Central values by (1– 0.75) = 0.25 when deriving conditions at the site.
Independent extremes:
1-hour 32.8 ft WS:
Central WS, from Table 4.5.3-1: 157.5 ft/s West Central WS, from Table 4.5.2-1: 125.0 ft/s WS = 0.75(125.0) + 0.25(157.5) = 133.2 ft/s Hmax:
Central Hmax, from Figure 4.5.3-2: 83.3 ft West Central Hmax, from Figure 4.5.2-2: 65.0 ft Hmax = 0.75(65.0) + 0.25(83.3) = 69.6 ft THmax:
Central THmax, from Table 4.5.3-1: 13.9 s West Central THmax, from Table 4.5.2-1: 13.0 s THmax = 0.75(13.0) + 0.25(13.9) = 13.2 s Surge and Tide:
Central Surge and Tide, from Figure 4.5.3-4: 4.8 ft West Central Surge and Tide, from Figure 4.5.2-4: 4.0 ft Surge and Tide = 0.75(4.0) + 0.25(4.8) = 4.2 ft
Currents must first be derived for each depth regime, i.e., the 230 ft contour and 492 ft water depth or greater, prior to interpola- tion to the site water depth.
230 ft:
Central current, from Table 4.5.3-1: 6.8 ft/s West Central current, from Table 4.5.2-1: 4.3 ft/s Uniform current = 0.75(4.3) + 0.25(6.8) = 4.9 ft/s Current direction, from Figure 4.3.1-1: 235°
492 ft:
Central surface speed, from Table 4.5.3-1: 7.9 ft/s West Central surface speed, from Table 4.5.2-1: 6.3 ft/s Surface speed = 0.75(6.3) + 0.25(7.9) = 6.7 ft/s
Central speed at mid-profile, from Table 4.5.3-1: 5.9 ft/s West Central speed at mid-profile, from Table 4.5.2-1: 4.7 ft/s Speed at mid-profile = 0.75(4.7) + 0.25(5.9) = 5.0 ft/s
Central 0-speed depth, from Table 4.5.3-1: 331 ft West Central 0-speed depth, from Table 4.5.2-1: 262 ft 0-speed depth = 0.75(262) + 0.25(331) = 280 ft Mid-profile depth = 280 ft / 2 = 140 ft
Once the 230 ft and 492 ft currents have been calculated, they can be used to define the current profile and direction at the 328 ft depth site. First, the new profile is estimated. This is done by approximating the 230 ft as a 3-point profile like the 492 ft current, and then interpolating between the 230 ft and 492 ft currents for the surface, mid-profile depth and 0-speed depth points of the profile. In this case, the 230 ft value would be weighted by (492–328)/(492–230) = 0.625 and the 492 ft value by (328–230)/(492–
230) = 0.375.
Approximate 3-point profile for 230 ft current:
Surface speed = 4.9 ft/s
Mid-profile speed and level = 4.9 ft/s at 230 ft 0-speed depth = 230 ft
328 ft profile:
Surface speed = 0.625(4.9) + 0.375(6.7) = 5.6 ft/s Mid-profile speed = 0.625(4.9) + 0.375(5.0) = 5.0 ft/s Mid-profile level = 0.625(230) + 0.375(140) = 196 ft 0-speed depth = 0.625(230) + 0.375(280) = 248 ft
The profile at 328 ft would thus be defined by the following three (speed, depth) points:
(5.6 ft/s, 0 ft), (5.0 ft/s, 196 ft), (0 ft/s, 248 ft)
Second, the direction of the profile must be resolved. For peak current cases the current in the transition region may be considered omni-directional (although realistically would tend to align with local bathymetry in depths closer to 230 ft).
However, the example will instead assume a 100-year peak wave case is being analyzed, for which the shallow and deepwater currents both have proscribed headings. Table 5-1 shows the deepwater current will have a heading +15° from the wave heading.
The shallow water current has a heading of 235° as noted above. Assume a wave heading of 282° is selected (note, as 282° is within ±22.5° of 290°, the wave height is not factored); thus the deepwater current component has a heading of (282° + 15°) = 297°.
As this is a peak wave case, the associated current must first be derived. The process for estimating the 328 ft current profile above is repeated, with the addition of load case factors from Tables 5-1 and 5-2. From Table 5-2 a factor of 0.7 should be applied to the 230 ft current (speed only), while from Table 5-1 a factor of 0.75 must be applied to the 492 ft current (speed and depth lev- els).
328 ft peak-wave associated profile:
Surface speed = 0.625(4.9)0.7 + 0.375(6.7)0.75 = 4.0 ft/s Mid-profile speed = 0.625(4.9)0.7 + 0.375(5.0)0.75 = 3.5 ft/s Mid-profile level = 0.625(230) + 0.375(140)0.75 = 183 ft 0-speed depth = 0.625(230) + 0.375(280)0.75 = 222 ft
The associated profile at 328 ft would thus be defined by the following three (speed, depth) points:
(4.0 ft/s, 0 ft), (3.5 ft/s, 183 ft), (0 ft/s, 222 ft)
The associated current direction is the heading of the resultant of the 230 ft current and the average current speed over the upper 230 ft of the 492 ft profile. First, find he average speed over the upper 230 ft of the 492 ft profile (applying the factor from Table 5-1):
492 ft profile:
Surface speed = 0.75(6.7) = 5.0 ft/s Speed at mid-profile = 0.75(5.0) = 3.7 ft/s 0-speed depth = 0.75(280) = 210 ft
Mid-profile depth = 0.75(140) = 105 ft
The average speed over the upper 230 ft of the 492 ft profile is thus:
[(5.0+3.7)/2](105/230) + (3.7/2)(105/230) = 2.9 ft/s With the factor from Table 5-2, the 230 ft current is:
0.7(4.9) = 3.4 ft/s
Now find the direction of the resultant of the 230 ft current and the average current over the upper 230 ft of the 492 ft profile, both weighted to 328 ft. For convenience, resolve to east-west, north-south components (east and north are positive):
Average 230 ft current, 492 ft profile: 0.375(2.9) = 1.1 ft/s, heading 297°
East = 1.1 sin(297) = –1.0 North = 1.1 cos(297) = 0.5
230 ft current: 0.625(3.4) = 2.1 ft/s, heading 235°
East = 2.1 sin(235) = –1.7 North = 2.1 cos(235) = –1.2 Resultant direction:
East = –1.0 – 1.8 = –2.7 North = 0.5 – 1.2 = –0.7
Heading = arctan(–2.7/–0.7) = –105° or (360 – 105) = 255°
Thus, the associated current for the 100-year peak wave case for this 328 ft site would be given by a profile defined by:
(4.0 ft/s, 0 ft), (3.5 ft/s, 183 ft), (0 ft/s, 222 ft) with a heading of 255°.
To complete the peak case, the wind and surge for this peak wave case would then be adjusted using the factors from either Table 5-1 or Table 5-2 (the wind and surge factors are the same for shallow and deep when evaluating peak wind or peak wave cases).
The final associated values would be:
Associated 1-hour 32.8 ft WS = 0.95(133.2) = 126.3 ft/s Heading = 282 –15 = 267°
Associated surge and tide = 0.7(4.2 – 1.4) + 1.4 = 3.3 ft
7 Sudden Hurricane Conditions
A “sudden” hurricane is one which forms locally in the Gulf of Mexico, and due to speed of formation and proximity to infra- structure at time of formation may not allow sufficient time to evacuate manned facilities. The exact population of storms used to derive sudden hurricane conditions at a given site may be based on where storms form and how quickly storms move and inten- sify after formation, in comparison to the accuracy of storm forecasts and how quickly personnel and/or facilities may be removed from the site.
A set of sudden hurricane conditions is provided below in Table 7-1A and Figures 7-1A to 7-4A; those marked “A” are in SI Units while those marked “B” are in U.S. Customary Units. These conditions have been derived from a subset of storms defined by those whose center crossed 26°N within 60 hours of becoming named storms. In operational terms, this subset bounds those storms which generate 10 m (32.8 ft) 1-hour wind speeds of 15 m/s (49 ft/s) or greater at locations at or above 28°N within 24 hours of becoming named storms. As the conditions from this population of storms are essentially uniform across the Gulf of Mexico, only one set of conditions is provided, applicable to all regions within the limits of Section 3.
Load cases for sudden hurricane conditions may be developed in accordance with the guidelines in Section 5, using the following modifications:
• 100-year sudden hurricane load cases should be developed using the combination factors in the 10-year column. The 100- year sudden hurricane wave condition should be considered omni-directional.
• 1,000- and 10,000-year sudden hurricane load cases should be developed using the combination factors in the 100- year column. The 1,000- and 10,000-year sudden hurricane directional wave conditions may be approximated using Figure 4.2.2-1
Table 7-1A—Independent Extreme Values for Sudden Hurricane Winds, Waves, Currents and Surge (All Regions)
Return Period (Years) 100 1000 10000
Wind (10 m Elevation)
1-hour Mean Wind Speed (m/s) 29.1 39.3 48.0
10-min Mean Wind Speed (m/s) 32.0 44.0 54.5
1-min Mean Wind Speed (m/s) 35.7 49.9 62.8
3-sec Gust (m/s) 40.5 57.7 73.7
Waves, WD > = 1000 m
Significant Wave Height (m) 8.0 10.8 13.2
Maximum Wave Height (m) 14.0 19.1 23.3
Maximum Crest Elevation (m) 9.8 12.8 15.6
Peak Spectral Period (s) 12.2 14.2 15.7
Period of Maximum Wave (s) 11.0 12.8 14.1
Currents, WD > = 150 m
Surface Speed (m/s) 1.46 1.97 2.40
Speed at Mid-profile (m/s) 1.10 1.48 1.80
0-Speed Depth (m) 61.0 82.3 100.8
Currents, WD < = 70 m
Uniform Speed at 10 m Depth (m/s) 1.09 1.98 2.67
Uniform Speed at 70 m Depth (m/s) 0.76 1.39 1.87
Water Level, WD > = 500 m
Storm Surge (m) 0.42 0.60 0.75
Tidal Amplitude (m) 0.42 0.42 0.42
Notes:
Wind speeds for a given return period are applicable to all water depths throughout the region.
Crest elevation includes associated surge and tide.
See Figures 7-1A, 7-2A and 7-3A for wave and crest elevation values for water depths between 10 m and 1000 m.
The peak spectral period and period of maximum wave apply to waves in all water depths.
Currents in water depths between 70 m and 150 m should be estimated as described in 4.3.3.
See Figure 7-4A for surge and tide in water depths less than 500 m.
.
Figure 7-1A—N-Year Hs, All Regions
Sudden Hurricane N-YearHs
100 Year 1,000 Year 10,000 Year
0.0 5.0 10.0 15.0 20.0 25.0
10 100 1000
Water Depth, MLLW (m) Hs(m)
Figure 7-2A—N-Year Hmax, All Regions
Sudden Hurricane N-YearHmax
100 Year 1,000 Year 10,000 Year
5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0
10 100 1000
Water Depth, MLLW (m) Hmax(m)
Figure 7-3A—N-Year Max Crest Elevation, All Regions
Sudden Hurricane N-Year Max Crest Elevation (including Surge and Tide)
100 Year 1,000 Year 10,000 Year
5.0 10.0 15.0 20.0 25.0 30.0
10 100 1000
Water Depth, MLLW (m)
MaxCrestElevation(m)
Figure 7-4A—N-Year Surge with Tide, All Regions
Sudden Hurricane N-Year Surge and Tide
100 Year 1,000 Year 10,000 Year
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0
10 100 1000
Water Depth, MLLW (m)
SurgeandTide(m)
Table 7-1B—Independent Extreme Values for Sudden Hurricane Winds, Waves, Currents and Surge (All Regions )
Return Period (Years) 100 1000 10000
Wind (32.8 ft elevation)
1-hour Mean Wind Speed (ft/s) 95.5 128.9 157.5
10-min Mean Wind Speed (ft/s) 105.0 144.4 178.8
1-min Mean Wind Speed (ft/s) 117.1 163.7 206.0
3-sec Gust (ft/s) 132.9 189.3 241.8
Waves, WD > = 3280 ft
Significant Wave Height (ft) 26.2 35.4 43.3
Maximum Wave Height (ft) 45.9 62.7 76.4
Maximum Crest Elevation (ft) 32.2 42.0 51.2
Peak Spectral Period (s) 12.2 14.2 15.7
Period of Maximum Wave (s) 11.0 12.8 14.1
Currents, WD > = 492 ft
Surface Speed (ft/s) 4.8 6.5 7.9
Speed at Mid-profile (ft/s) 3.6 4.9 5.9
0-Speed Depth (ft) 200 270 331
Currents, WD 33 ft – 230 ft
Uniform Speed at 33 ft Depth (ft/s) 3.6 6.5 8.8
Uniform Speed at 230 ft Depth (ft/s) 2.5 4.6 6.1
Water Level, WD > = 1640 ft
Storm Surge (ft) 1.4 2.0 2.5
Tidal Amplitude (ft) 1.4 1.4 1.4
Notes:
Wind speeds for a given return period are applicable to all water depths throughout the region.
Crest elevation includes associated surge and tide.
See Figures 7-1B, 7-2B and 7-3B for wave and crest elevation values for water depths between 33 ft and 3280 ft.
The peak spectral period and period of maximum wave apply to waves in all water depths.
Currents in water depths between 230 ft and 492 ft should be estimated as described in 4.3.3.
See Figure 7-4B for surge and tide in water depths less than 1640 ft.
Figure 7-1B—N-Year Hs, All Regions
Sudden Hurricane N-YearHs
100 Year 1,000 Year 10,000 Year
10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0
10 100 1000 10000
Water Depth, MLLW (ft) Hs(ft)
Figure 7-2B—N-Year Hmax, All Regions
Sudden Hurricane N-YearHmax
100 Year 1,000 Year 10,000 Year
20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 110.0 120.0 130.0
10 100 1000 10000
Water Depth, MLLW (ft) Hmax(ft)
Figure 7-3B—N-Year Max Crest Elevation, All Regions
Sudden Hurricane N-Year Max Crest Elevation (including Surge and Tide)
100 Year 1,000 Year 10,000 Year
20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0
10 100 1000 10000
Water Depth, MLLW (ft)
MaxCrestElevation(ft)
Figure 7-4B—N-Year Surge with Tide, All Regions
Sudden Hurricane N-Year Surge and Tide
100 Year 1,000 Year 10,000 Year
0.0 5.0 10.0 15.0 20.0 25.0
10 100 1000 10000
Water Depth, MLLW (ft)
SurgeandTide(ft)
8 Seasonal Hurricane Conditions
The regional conditions presented in this document have been derived assuming an exposure period to hurricane encounters over the full year. Should a facility operate in such a manner as to restrict its exposure to hurricanes in the Gulf of Mexico (or one of the regions in the Gulf of Mexico) to periods less than one year, i.e. a seasonal operation, it would be reasonable to consider the facil- ity subject to hurricane conditions derived from a limited exposure period. For example, most of the severe hurricanes which have affected the Gulf of Mexico occur in the months of August, September and October; conditions derived considering only these three months will be more severe than those derived from the three months of May, June and July. Should seasonal conditions be used, care should be taken to evaluate the increasing risk incurred by operating close to the transition date to a “severe” season.
9 Guidelines for Site-specific Metocean Studies
Performance of a site-specific metocean study is the preferred way of ensuring regional variations in storm climate and local topo- graphic and bathymetric effects are properly accounted for, as well as ensuring sufficient data is available to properly identify the phasing between wind, wave, current and surge and to serve as inputs to response-based analyses aimed at determining n-year forces. It is emphasized that the goal of a site-specific study is more accurate information on the metocean conditions at a site;
site-specific studies should not focus on determining the lowest set of design conditions possible.
A site-specific study of hurricane metocean conditions should be based on a hindcast database of winds, waves, currents and surge derived from models that have been validated against severe historical storms from 1950 through 2005 including Opal, Ivan and Katrina. Validation should show the wind, wave and surge models have a coefficient of variation (COV) no more than 15% when comparing model peak wind speed, wave height or surge height to their respective measured peak values. An accept- able COV for the current model validations can be as high as 30%. Any bias between the model and data should be removed with at least a simple linear fitting process. Use of numerical wave, current and surge models based upon discrete finite element or finite difference solutions of the governing partial differential equations is preferred; grid resolution for models should be equal to or finer than 15 km (8 nm) and the overall domain should be sufficient to prevent boundary conditions from affecting the solution. Parametric models of wave, current and surge should only be used if they have been extensively calibrated against major severe storms like Opal, Ivan and Katrina.
The hindcast period used should at a minimum consider all Gulf of Mexico cyclones of tropical storm strength or greater between 1960 and the present date. The metocean specialist performing the study may choose to use a database starting period as early as 1945; data prior to 1945 should not be used as storm characterizations from earlier periods are unreliable, particularly when a storm is far from land. Data used for storm wind field characterization should use as a starting point the National Hurricane Cen- ter “best tracks” data set. Additional storm parameters such as radius to maximum winds should be determined from surface mea- surements, aviation reconnaissance, and satellite observations.
Because of the low frequency of occurrence and relatively small diameter of hurricanes, estimates of extremes made from a lim- ited (in this case, 50 years) database can vary substantially over relatively small distances, even within a region that would be expected on the basis of physical arguments to be statistically homogeneous. Specifically, sites that are very near the tracks of one or more of the few historical Category 4 or 5 hurricanes will have much greater estimates of 100-year winds, waves, current and surge than sites that are not near one of those tracks. It is not reasonable to expect that extreme hurricanes in the next few centuries will have exactly the same tracks as historical hurricanes. Therefore, some means of smoothing site-specific conditions estimated from a limited database, accounting for track variability, should be used. Commonly used methods include simple spa- tial smoothing of site specific estimates, track shifting, and grid point pooling. With regard to grid point pooling, there is no uniquely “correct” way to do it. However, using three or more grid points, all lying within the region that is expected to be homogeneous on the basis of physical arguments, arranged in a curvilinear array oriented more or less perpendicular to the tracks of the most severe hurricanes in deep water, or oriented along a bathymetric contour in shallow water, with a spacing of at least 75 km (41 nm) between grid points to reduce the correlation among grid point statistics, generally provides reasonable results. Some deepwater locations may need a south-north and east-west arrangement of grid points (such as a five-point “cross”
centered on the site) to capture the influence of both south-to-north and east-to-west tracking storms near the site. The distance over which pooling is performed should generally not be less than 150 km (82 nm) or greater than 300 km (164 nm) wide, and should be selected with attention to local water depth, fetch limitations, proximity to the Loop Current or areas with frequency warm-core eddies, and orientation of major storm tracks.
For return periods greater than 200 years, extremes may be derived either using the methods above or through the use of deduc- tive models or Monte Carlo simulations of synthetic storms.