ANATOMY OF A MUDSLIDE AND DAMAGE CAUSED BY HURRICANE IVAN

Transcription

1 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

2 THE MISSISSIPPI RIVER DELTA

3 Mississippi River New Orleans Mississippi River Delta

4 ~50 miles Mudslide-Prone Region of Delta

5 THE DELTA FRONT HAS A BAD REPUTATION CONSIDER SHELL S SOUTH PASS AREA PLATFORM SHORTLY AFTER ITS INSTALLATION IN 1969

6 SP-7OB Water Depth ~300 ft Seafloor slope Seafloor 16 Piles Driven to ~400 ft SP-70 PLATFORM 1969

7 SP-7OB Hurricane Camille Waves Seafloor Seafloor Failure Zone ~100 ft Seafloor sliding SP-70 SEAFLOOR FAILURE

8 SP-7OB Seafloor Soil Force Against Piles Seafloor SP-70 SOIL FORCES

9 SP-7OB Platform Motion Seafloor Piles Fail at Base of Jacket Seafloor Soil Motion SP-70 PLATFORM MOTION

10 SP-70 PLATFORM FAILURE 1969

11 ??? WHY IS THE SEAFLOOR UNSTABLE IN THE DELTA? WHAT WAS SO SPECIAL ABOUT HURRICANE IVAN? HOW DO SEAFLOOR FAILURES DAMAGE STRUCTURES?

12 THE STORY STARTS WITH THE RAPID GROWTH OF THE MISSISSIPPI RIVER DELTA

13 THE BIRDSFOOT DELTA Head of Passes Southwest Pass South Pass Pass A Loutre NE Pass SE Pass

14 Year ~ mi GROWTH OF THE PASSES

15 THE MAJOR PASSES HAVE BEEN GROWING SEAWARD AT RATES OF 100 TO 200 ft/yr FOR MORE THAN 300 yr.

16 THE SUBMERGED DELTA FRONT HAS ALSO BEEN GROWING SEAWARD

17 Consider this profile line across the seafloor GROWTH OF THE DELTA FRONT

18 Depth Below Sea Level, Feet Deposition in shallow water Fails and slides down the slope Stops in deeper water and becomes a mudflow lobe Flat top Steep front slope Distance from South Pass, Miles SEAWARD GROWTH OF THE DELTA

19 From Coleman et al, 1980 Southwest Pass South Pass Mudflow lobes BATHYMETRY MAP

20 Depth Below Sea Level, Feet Pre-Delta ( YBP) Mudlobe in Distance from South Pass, Miles SEAWARD GROWTH OF THE DELTA

21 From Coleman et al, 1980 Southwest Pass South Pass Mudflow lobes BATHYMETRY MAP 1979 BATHYMETRY

22 Depth Below Sea Level, Feet Mudlobe Advance = 2.7 mi Pre-Delta ( YBP) Mudlobe in 1874 Mudlobe in Distance from South Pass, Miles SEAWARD ADVANCE OF THE DELTA

23 THIS CALCULATED RATE OF ADVANCE IS 135 ft/yr ( ).

24 THE PASSES ADVANCE. THE MUDFLOW LOBES SIMILARLY ADVANCE. THE MUDFLOWS MOVE THE DELTA FRONT SEAWARD.

25 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.

26 SEDIMENT STRENGTH

27 Depth Below Sea Level, Feet Strength, PSF % Common of typical South Pass Area Profile Worst cases strength Typical strength in Gulf of Mexico DELTA FRONT STRENGTH PROFILES

28 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).

29 MOST OF THE TIME, HURRICANE WAVES CAUSE THE SERIOUS MUDSLIDES.

30 MAJOR HURRICANE HISTORY 10 major hurricanes (Category 3 or greater) passed over or near the delta

31 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).

32 Delta Waves in advance of the storm

33 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.

34 WAVE-TRIGGERED SEAFLOOR FAILURES

35 Wave Length Height Water Depth Pressure increase seafloor Pressure Decrease SEAFLOOR PRESSURE

36 Moving Moving Waves Moving Waves Waves seafloor seafloor DYNAMIC SEAFLOOR PRESSURE

37 Downward Motion Upward Motion seafloor Lateral Motion SEAFLOOR MOTION

38 ~10 ft seafloor ~10 ft ~10 ft (EXAMPLE CALCULATIONS) HOW MUCH MOTION DURING SEAFLOOR FAILURE?

39 Cyclic sediment movements caused by waves Time Lateral Distance moved Net downslope motion WAVE-ACTUATED MOVEMENTS

40 seafloor Zone of High Shear Stresses That Can Cause Seafloor Failure FAILURE STRESSES

41 ~2 L L Ivan s Largest Waves Normal Largest Hurricane Waves seafloor ~2 Z Z IVAN S FAILURE-LEVEL STRESSES

42 Depth Below Sea Level, Feet Strength, PSF Normal large hurricanes ft failure depths Ivan-size storms ft failure depths FAILURE DEPTHS 300

43 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.

44 FACILITIES EXPOSED TO MUDFLOWS

45 Western Limit of Hurricane Force Winds Ivan.s path REGIONAL PLATFORMS AND PIPELINES

46 REGIONS THAT EXPERIENCED SEAFLOOR MOVEMENTS DURING IVAN

47 From Coleman et al, 1980 Region of Mudflow Gullies Region of Mudflow Lobes SEAFLOOR GEOMORPHOLOGY

48 SEAFLOOR GEOMORPHOLOGY Mudflow Gully system 7-miles long Mudflow Lobes are fed by the gullies From Coleman et al, 1980

49 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.

50 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.

51 Depth Below Sea Level, Feet 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

52 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

53 Water Depth ~500 ft Seafloor 35 FT OVERRUN + 10 FT SUBBOTTOM FAILURE Seafloor Piles Driven to ~500 ft PLATFORM DESIGN CRITERIA

54 45-ft thick overflow Old Seafloor 150-ft thick Sub- Bottom Failure Motion PLATFORM FAILURE DURING IVAN

55 THE COMBINED LOADS OF THE 45-FT THICK OVERFLOW AND 150 FT THICK SUB- BOTTOM SEDIMENT FAILURE TOPPLED THE PLATFORM

56 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.

57 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.

58 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 ~ Wave-bottom pressures generated seafloor failures in shallow & deep water on the delta front, causing the loss of numerous pipelines.

59 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.