ANATOMY OF A MUDSLIDE AND DAMAGE CAUSED BY HURRICANE IVAN by Jim Hooper, Fugro-McClelland Marine Geosciences Presented at the Houston Marine Insurance Seminar Westin Galeria Hotel, Houston Texas September 19, 2005
THE MISSISSIPPI RIVER DELTA
Mississippi River New Orleans Mississippi River Delta
~50 miles Mudslide-Prone Region of Delta
THE DELTA FRONT HAS A BAD REPUTATION CONSIDER SHELL S SOUTH PASS AREA PLATFORM SHORTLY AFTER ITS INSTALLATION IN 1969
SP-7OB Water Depth ~300 ft Seafloor slope Seafloor 16 Piles Driven to ~400 ft SP-70 PLATFORM 1969
SP-7OB Hurricane Camille Waves 60-70 Seafloor Seafloor Failure Zone ~100 ft Seafloor sliding SP-70 SEAFLOOR FAILURE
SP-7OB Seafloor Soil Force Against Piles Seafloor SP-70 SOIL FORCES
SP-7OB Platform Motion Seafloor Piles Fail at Base of Jacket Seafloor Soil Motion SP-70 PLATFORM MOTION
SP-70 PLATFORM FAILURE 1969
??? WHY IS THE SEAFLOOR UNSTABLE IN THE DELTA? WHAT WAS SO SPECIAL ABOUT HURRICANE IVAN? HOW DO SEAFLOOR FAILURES DAMAGE STRUCTURES?
THE STORY STARTS WITH THE RAPID GROWTH OF THE MISSISSIPPI RIVER DELTA
THE BIRDSFOOT DELTA Head of Passes Southwest Pass South Pass Pass A Loutre NE Pass SE Pass
Year ~1300-1400 1684 1840 2005 18 mi 1840 1684 1684 1840 2005 2005 GROWTH OF THE PASSES
THE MAJOR PASSES HAVE BEEN GROWING SEAWARD AT RATES OF 100 TO 200 ft/yr FOR MORE THAN 300 yr.
THE SUBMERGED DELTA FRONT HAS ALSO BEEN GROWING SEAWARD
Consider this profile line across the seafloor GROWTH OF THE DELTA FRONT
Depth Below Sea Level, Feet 0 100 200 300 400 Deposition in shallow water Fails and slides down the slope Stops in deeper water and becomes a mudflow lobe Flat top Steep front slope 0 1 2 3 4 5 6 Distance from South Pass, Miles SEAWARD GROWTH OF THE DELTA
From Coleman et al, 1980 Southwest Pass 100 200 300 South Pass Mudflow lobes 400 500 600 1979 1874 BATHYMETRY MAP
Depth Below Sea Level, Feet 0 200 400 600 800 1874 Pre-Delta (600-700 YBP) Mudlobe in 1874 0 2 4 6 8 10 12 Distance from South Pass, Miles SEAWARD GROWTH OF THE DELTA
From Coleman et al, 1980 Southwest Pass South Pass Mudflow 100 200 lobes 300 1979 BATHYMETRY MAP 1979 BATHYMETRY 400 500 1000
Depth Below Sea Level, Feet 0 200 400 600 800 Mudlobe Advance = 2.7 mi 1979 1874 Pre-Delta (600-700 YBP) Mudlobe in 1874 Mudlobe in 1979 0 2 4 6 8 10 12 Distance from South Pass, Miles SEAWARD ADVANCE OF THE DELTA
THIS CALCULATED RATE OF ADVANCE IS 135 ft/yr (1874-1979).
THE PASSES ADVANCE. THE MUDFLOW LOBES SIMILARLY ADVANCE. THE MUDFLOWS MOVE THE DELTA FRONT SEAWARD.
DEPOSITION IN THE DELTA Deposition rates 1-2 ft/yr in front of the Passes. Sediments are relatively impermeable, plastic (sticky) clay. Results in weak strength profiles up to 300-ft thick.
SEDIMENT STRENGTH
Depth Below Sea Level, Feet 200 400 600 Strength, PSF 0 2000 4000 6000 0 15% Common of typical South Pass Area Profile Worst cases strength Typical strength in Gulf of Mexico DELTA FRONT STRENGTH PROFILES
THIS THICK PILE OF WEAK SEDIMENT CAN FAIL WHEN: 1. HURRICANE WAVES SHOAL INTO THE DELTA. 2. THE SEAFLOOR BECOMES TOO STEEP (DUE TO DEPOSITION OR EROSION).
MOST OF THE TIME, HURRICANE WAVES CAUSE THE SERIOUS MUDSLIDES.
MAJOR HURRICANE HISTORY 10 major hurricanes (Category 3 or greater) passed over or near the delta 1900-2005.
MAJOR HURRICANE HISTORY Camille passed over the east side of the delta as a Category 5 storm (1969). Ivan passed east of the delta as a Category 4 & 5 storm (2004). Katrina passed just west of the delta as a Category 4 & 5 storm (2005).
Delta Waves in advance of the storm
HURRICANE WAVES Wave size increases with the intensity of the storm. Large storm waves start arriving hours (or even days) before the center of the storm. Storm waves trigger the seafloor failures.
WAVE-TRIGGERED SEAFLOOR FAILURES
Wave Length Height Water Depth Pressure increase seafloor Pressure Decrease SEAFLOOR PRESSURE
Moving Moving Waves Moving Waves Waves seafloor seafloor DYNAMIC SEAFLOOR PRESSURE
Downward Motion Upward Motion seafloor Lateral Motion SEAFLOOR MOTION
~10 ft seafloor ~10 ft ~10 ft (EXAMPLE CALCULATIONS) HOW MUCH MOTION DURING SEAFLOOR FAILURE?
Cyclic sediment movements caused by waves Time Lateral Distance moved Net downslope motion WAVE-ACTUATED MOVEMENTS
seafloor Zone of High Shear Stresses That Can Cause Seafloor Failure FAILURE STRESSES
~2 L L Ivan s Largest Waves Normal Largest Hurricane Waves seafloor ~2 Z Z IVAN S FAILURE-LEVEL STRESSES
Depth Below Sea Level, Feet Strength, PSF 0 0 100 200 1000 2000 3000 Normal large hurricanes 50-70 ft failure depths Ivan-size storms 140-160 ft failure depths FAILURE DEPTHS 300
IVAN VS. NORMAL HURRICANES Cyclic stresses were greater in magnitude under Ivan s maximum waves, and reached deeper below the seafloor. Result was more seafloor failures, and larger seafloor failures, during Ivan.
FACILITIES EXPOSED TO MUDFLOWS
Western Limit of Hurricane Force Winds Ivan.s path REGIONAL PLATFORMS AND PIPELINES
REGIONS THAT EXPERIENCED SEAFLOOR MOVEMENTS DURING IVAN
From Coleman et al, 1980 Region of Mudflow Gullies Region of Mudflow Lobes SEAFLOOR GEOMORPHOLOGY
SEAFLOOR GEOMORPHOLOGY Mudflow Gully system 7-miles long Mudflow Lobes are fed by the gullies From Coleman et al, 1980
MUDFLOW GULLIES Filled with semi-mobile sediment typically 40 to 80 feet thick. Gully lengths >6 miles. Major mudflow activity occurs several times a year in some gullies, and every few years in others.
MUDFLOW LOBES Individual flow thickness ranges from a few feet to ~50 feet. Stacked mudflows are several hundred feet-thick in some areas. Mudflow lobes tend to remain stable until triggered by large storm waves.
Depth Below Sea Level, Feet 0 200 400 600 Mudflow Gully Transport Zone Slow accumulation zone Overflow Zone 0 10,000 20,000 30,000 40,000 Distance from Arbitrary Origin, Feet THE FATE OF MUDFLOW SEDIMENTS
Mudflows in Gullies Overflow of older lobes Platform & Pipeline Failures SOUTH PASS REGION FAILURES MAP OF SEAFLOOR INSTABILITY FEATURES OFFSHORE SOUTH PASS AREA DURING IVAN
Water Depth ~500 ft Seafloor 35 FT OVERRUN + 10 FT SUBBOTTOM FAILURE Seafloor Piles Driven to ~500 ft PLATFORM DESIGN CRITERIA
45-ft thick overflow Old Seafloor 150-ft thick Sub- Bottom Failure Motion PLATFORM FAILURE DURING IVAN
THE COMBINED LOADS OF THE 45-FT THICK OVERFLOW AND 150 FT THICK SUB- BOTTOM SEDIMENT FAILURE TOPPLED THE PLATFORM
PIPELINE FAILURES Pipelines failed when the seafloor moved from beneath it. This happened to pipelines located in gullies. Pipelines that were overrun by mudflows experienced high lateral loading causing them to rupture.
SUMMARY 1. Rapid deposition along the delta front has created a great thickness of very weak sediments. 2. The sediments are marginally stable at best, and seafloor failures are triggered by hurricane waves.
SUMMARY 3. Hurricane Ivan s waves created failure pressures deep below the seafloor, deeper than predicted by platform design criteria. This caused the loss of one platform in a water depth of ~500. 4. Wave-bottom pressures generated seafloor failures in shallow & deep water on the delta front, causing the loss of numerous pipelines.
SUMMARY (cont.) 5. Large-scale seafloor failures are the primary geologic process for seaward growth of the delta. 6. Past rates of seaward growth of the delta front will likely be maintained, and seafloor failures will continue to occur.