A11102 L55 5 3 4 NATLINSTOFSTANDARDS&TECHR.I.C. All102655534 Yokel,FelixY/Matfoundationsforoffsh QC100.U56NO.86-34191987VI19MC.1UNOBS-dP S) 86-3419 Mat Foundations for Offshore Structures in Arctic Regions Felix Y. Yokel DEPARTMENT OF COMMERCE U.S. National Bureau of Standards Center for Building Technology Gaithersburg, MD 20899 Robert G. Bea PMB Systems Engineering, Inc. 500 Sansome Street, Suite 400 San Francisco, CA 94111 May 1986 Issued February 1987 Prepared for: U.S. Department of the Interior Minerals Management Service 100 Technology Assessment and Research Branch .1156 647 National Center -3419 Reston, Va 22091 1986 1 NBS RESEARCH INFORMATION NBSIR 86-3419 CENTER MAT FOUNDATIONS FOR OFFSHORE STRUCTURES ARCTIC REGIONS IN mi Felix Y. Yokel 4 ^ DEPARTMENT OF COMMERCE U.S. National Bureau of Standards Center for Building Technology Gaithersburg, MD 20899 Robert G. Bea PMB Systems Engineering, Inc. 500 Sansome Street, Suite 400 San Francisco, CA 9411 May 1986 Issued February 1987 Prepared for: U.S. Department of the Interior Minerals Management Service Technology Assessment and Research Branch 647 National Center Reston, Va 22091 U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Director . ABSTRACT Design limit states for offshore gravity structures in the Alaskan Beaufort and Eastern Chukchi continental shelves are discussed. The report contains a description of geological conditions, design loads, and type of structures used. Three foundation types are considered: foundations for artificial islands; foundations for caisson retained island with sand cores; and rigid foundations for various types of gravity structures which are positioned on the ocean floor with a minimum of preparation. Design limit states for these foundations are identified and the required reliability against the occurence of these .limit states is discussed. Our ability to determine founda- tion resistance is assessed. Key Words: artificial islands; geotechnical engineering; ice forces; mat foundations; ocean engineering; offshore platforms; oil production; sand and gravel berms; soil exp1 orat1 on soil ; testi ng i i i TABLE OF CONTENTS A1.BSTRACT i i i 2. LIST OF FIGURES v INTRODUCTION 1 SUBSURFACE CONDITIONS |» 2. 1 General .................................. C--I 2.2 Geological Character st cs of Unconsol dated Deposits i i i 2.2. Surficial Geology cn.tn 3. 1 2.2.2 Evidence of Instability cn 2.2.3 Seismicity and Faults si 4. 2.2.4 Phenomena Associated with the Arctic Region vi . 23..32 Engineering Characteri sti cs of Unconsolidated Deposits *-o 2.3.1 Soil Types Encountered *>o 2.3.2 Strength Character st cs of Soils ......... * i i 2.3.3 Summary of Engineering Character sti cs .... mo i DESIGN LOADS w sj 3. 1 General wvj Loads Acting on Structures si cj DESIGN LIMIT STATES cnw 4. 1 General uiw 4.2 Requirements for Safety and Servi ceabi 1 i ty ........ 4. 3 Structural Concepts >c . 4.3.1 Background Information o 4.3.2 Gravity Structures Used or Considered ...... 'O 4.4 Design Limit States for Foundations ^ 4.4.1 Definition of Limit States ................. s1- 4.4.2 Practical Limits to Load and Resistance w 4.4.3 Design Strategies ^W'0'0<i<icococO'sivi>sjvivicjiLnt.nc.n 4.4.4 Failure Probabi 1 i ti es Assoc, with Limit States cn 4.4.5 Design Limit States for Artificial Islands o 4.4.6 Design Limit States for Caisson Retained Isl. cn 4.4.7 Design Limit States for Rigid Mat Foundations. <! 4.4.8 Design Limit States for Sand and Gravel Berms. i> 4.5 Resistance to Loads and Di spl acements co 4.5.1 General od 4.5.2 Characteri zati on of Soils vO 4.5.3 Augmentation of Soil Resistance 1 >o 4.6 Reliability of Predictions 1 h 5. SUMMARY 133 6. NEEDED STUDIES AND RESEARCH 137 7. REFERENCES 139 v ' ... . LIST OF FIGURES Figure Seismic Re-flection Profiles Taken by U.S.G.S. in 1 The Alaskan Beaufort and Eastern Chukchi Seas ... Fi gure 2. Thickness of Holocene Marine Sediments on the Alaskan Beaufort and Chukchi Shelves F gure 3. Youthful Landslide Terranes i Fi gure 4. Data on Scour and Erosion F gure 5 Recorded Earthquake Epicenters i . F gure 6 Areas of Shallow Gas Deposits and Hydrated Gas i F gure 7. Schematic Sketch of Ice Regime in Early Spring i Figure 8. Ice Zonation in the Alaskan Beaufort and Eastern Chukchi Seas F gure 9. Idealized Sketch of Ice Gouge i Figure 10. Ice Gouge Intensities in the Alaskan Beaufort and Eastern Chukchi Seas Figure Distribution of Bottom Sediments on the Beaufort 1 1 Sea Shelf Figure 12. Sand and Gravel Resources on the Beaufort Sea Shelf Figure 13. Soil Profiles in the Prudhoe Bay Area Figure 14. Idealized Schematic Profile of Unconsolidated Deposits in the Beaufort Sea Shelf Figure 15. Normalized Shear Strength and Excess Pore Water Pressures vs. Shear Strain Obtained in Undrained Consolidated Triaxial Tests of Beaufort Sea Silts F gure 16. Undrained Shear Strength Profile for Beaufort Sea i Silts Fi gure 17. Overconsol dat on Ratio Profile for Beaufort Sea i i Silts Figure 18. Water Content vs. Undrained Shear Strength for Beaufort Sea Silts Figure 19. Pro-file of Expected Range of (Jndrained Shear Strengths for Normally Consolidated Alaska OCS Silts 31 Figure 20. Soil Profiles in Harrison Bay 32 Figure 21. Provisional Map of Subsea Permafrost Distribution 34 Figure 22. Current Patterns Recorded in the Beaufort and Eastern Chukchi Seas 39 Figure 23. Typical Strength Character st cs of Sea Ice .... 43 i i Figure 24. Correlations Between Ice Pressures and Contact Area 44 Figure 25. Global Ice Forces as a Function of Mean Recurrence Interval 46 Figure 26. Estimated Global Ice Forces Against a 120-180m Wide Exploration Structure as a Function of Water Depth 47 Figure 27. Estimated Global Ice Forces Against a 120-180m Wide Production Structure as a Function of Water Depth 48 Figure 28. Winter Season Ice Pressure Record for the Canadian Beaufort Sea 49 Figure 29. Calculated Force-Time History for Large Ice Feature Impact 50 Figure 30. Typical Winter Ice Loading Histogram 51 Figure 31. Risk for Selected Engineering Projects 56 Figure 32. Probability of Failure Obtained from Cost Benefit Analysis 57 Figure 33. Typical Shallow Dredged Island Cross Sections .. 62 Figure 34. Tarsuit Island 63 Figure 35. The Gulf Mobil Arctic Caisson (MAC) 64 Figure 36. Exxon's Super CIDS 65 Figure 37. The BWACS Floating Caisson 67 Figure 38. The Arctic Cone Exploration Structure (ACES) ... 68 vi i i