FOUNDATIONS FOR OFFSHORE STRUCTURES DELHI CHAPTER Pradeep Hora It takes less time to do a thing right than explain why you did it wrong. - Henry Wadsworth Longfellow 2 1
Site Investigations (Survey) What do you need site surveys for? Just about anything you need to put on or in the seabed Exploration Surveys, Rig Site Surveys, Platform Site Surveys, Pipeline Route Surveys, Cable Route Surveys, Field Development Surveys, Clearance, Abandonment and Inspection Surveys 3 The data required - site-specific information Seabed topography and morphology such as rock outcrops and boulders Nature of the soils and rocks, their stratification and variability Soil strength, deformation and consolidation characteristics The influence of specific factors such as cyclic loading, rate of loading, soil sensitivity and thixotropy The possibility of scouring 4 2
The type and quantity of data required will depend upon factors that include: Type of structure planned. Installation/construction methods proposed. Water depth. Existing site data. Project phase, e.g. feasibility study or final design. Acceptable risk to people and the environment. Type of foundation loads 5 The sequence for the investigation campaign Desk study Topographical and geophysical survey Geotechnical survey and laboratory testing Additional geophysical and/or geotechnical surveys and/or laboratory testing as required Depending on the size of a project and/or the complexity of the geotechnical context and associated risks (geohazards), additional intermediate stages may be necessary. 6 3
Geophysical Survey A geophysical survey is required to understand the nature or characteristics of the seabed. Establishing bathymetry. Identification and location of significant seabed features, obstructions and hazards. Determination of the geometry of the subsurface layers. Extrapolation of local geotechnical data across the entire site. 7 Geophysical survey vessel 8 4
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Geophysical Survey Basic Principles Use of geophysical remote sensing techniques, calibrated by some limited ground truth sampling. Virtually always use acoustic methods In operation, an acoustic energy source generates a pulse of sound that travels through the water column and, where powerful enough, penetrates into the seabed. The sound energy is reflected back as an echo to a receiver system and the lapse in travel time from transmission to reception is converted into ranges. 15 Geophysical Techniques The sensors tend to fall into three categories: Seabed measuring sensors, e.g. echo sounders, single beam, multi beam sounders (Water depth measurement, seabed topography) Imaging sensors, e.g. side-scan sonar, laser-scan, acoustic scanning systems (for generating a scaled image of the seabed morphology and features, Detection/identification of seabed obstructions) Sub-bottom profilers, e.g. pingers, boomers etc. (Interpretation of soil lithology, stratification, Shallow Gas Detection) 16 8
Side scan sonar and images 17 18 9
GAMBAS operating principles 19 20 10
Jack-up Rig Foot Prints 21 JUR Imprints 22 11
Buried Pipelines 23 Pockmarks Acoustic record of pockmarks North Sea Shallow depressions formed in soft clay sediments by escaping gas bubbles. - a long axis of around 57m - with a mean depth of 2m to 8m deep 24 12
Autonomous Underwater Vehicles (AUV) AUVs have mission endurance ranging from 12 to 60 or more hours and some can reach depths of 6000 m. 25 HUGIN 3000 (High Precision Untethered Geosurvey and Inspection System) Semi-autonomous vehicle independently powered and controlled (cable free). Depth rated to 3000 metres. Inertial Navigation System Positional accuracy of the AUV is less than 5 m, significantly better than towed systems Sensors Multibeam Bathymetry and Imagery System Chirp Side Scan Sonar Chirp Sub-bottom Profiler Magnetometer (optional) 26 13
Gravity Corer Vibro Corer 27 Geotechnical Investigation 28 14
Self Elevating Platforms >4.0m & < 20.0m Water Depth Techniques: All onshore drilling, sampling and in-situ testing methods 29 Specialized Geotechnical Investigation Vessels > 20.0m Water depth Techniques: Wire line sampling and insitu testing (CPTU, seismic cone, vane) ONGC GTV- Samudra Sarvekshak 30 15
Geotechnical Investigation Inter tidal Zone < 4.0m Water Depth Techniques: All onshore drilling, sampling and in-situ testing methods 31 32 16
DRILLING SYSTEM FOR DOWNHOLE SAMPLING & IN-SITU TESTING Geotechnical Drilling Systems - Drilling Rigs - Reaction Systems - Motion Compensation Systems 33 34 17
35 PROD Portable Remotely Operated Drill 2000-m water depths 150-m exploration depths bsf 36 18
PROD on deck 37 Launch and recovery 38 19
Remotely operated & automated drilling Samples offloaded on deck 39 Scope of a Geotechnical Investigation. One or more soil borings depending on: - Type and size of structure - Soil conditions - Spatial soil variability potential. Supplemented by in-situ testing: - Cone (CPT) soundings - Remove vane tests - Piezo probe tests - Hydraulic fracture tests 40 20
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Sampling and In-Situ Testing Three Approaches:. Down hole mode. Seafloor mode. Over the Side mode Down hole Mode 43 Seabed In Situ Systems Seafloor Mode Deepwater Wheel drive SEACALF 44 22
45 Push Samples 46 23
47 Downhole Vane 48 24
WISON Mk-III 49 Downhole Piezo-Cone 50 25
Hydraulic Fracture Test 51 Field Laboratory Testing Soil Classification Unit Weight Moisture Content 52 26
Field Laboratory Testing Shear Strength Determination Torvane & Pocket Penetrometer tests UU Triaxial Miniature vane test 53 Required Basic Soil and Rock Parameters 54 27
Additional Parameters for Specific Design Issues 55 56 28
Standard Design requirements Axial Pile capacity vs depth of pile penetration Lateral Pile Load-Deflection Data (p-y) Axial Pile Load-Settlement Data (t-z,q-z) Mud-mat Bearing Capacity Pile Driving Recommendations 57 Engineering Analyses Static and Dynamic Axial Behavior - Ultimate Capacity - Load Settlement (t-z, q-z analysis) Static and Dynamic Lateral Behavior - p-y data 58 29
Additional Engineering Analyses Pile-Spud can Interaction Studies MODU Foundation Stability Assessment Mud-mat Bearing Capacity Pile Monitoring and Drivability Studies Mudflow Studies (Wave induced) Pile Driving Analysis Deepwater Anchor Analysis 59 Non-Routine Engineering Analyses Finite Element Studies Conductor Studies - Jetted - Drilled & Grouted - Suction Pile Setup Studies 60 30
A representative schedule for deepwater site investigation 61 Breakdown of project cost 62 31
COST One bore hole (Year 2005 ) Mobilization : 600,000 USD Borehole : 130,000 USD Lab testing : 8,000 USD Report : 15,000 USD Total : 753,000 USD Present cost 1.3~2.0 million USD 63 Questions 64 32
These presentations are for the purpose of generating awareness in the field of Marine Geotechnology and Foundations for Offshore structures with no commercial intent. Reference: The list below is not exhaustive. API (2000) & Errata & Supplement 1, 2 & 3 (2002, 2005 & 2007) Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms Working Stress Design, API RP 2A, American Petroleum Institute, Washington, USA. API (2005) Recommended Practice for Design and Analysis of Station Keeping Systems for Floating Structures, API RP 2SK, American Petroleum Institute, Washington, USA. API (2011) Geotechnical and Foundation Design Considerations, API GEO, American Petroleum Institute, Washington, USA. Herbich, J.B (1991) Handbook of Coastal and Ocean Engineering, Gulf Publishing Co. McCarron, W.O. (2011) Deepwater Foundations and Pipeline Geomechanics, Joss Publishing. OTC (1969 to 2013) Proceedings Annual Offshore Technology Conference, OTC Volumes, Houston Texas, USA Le Tirant, P. (1979) Seabed Reconnaissance and Offshore Soil Mechanics for the Installation of Petroleum Structures, Ed. Technip, Paris Poulos, H.G. (1988) Marine Geotechnics. Unwin Hyman, London. Rendolph, M. & Gourvenec, S. Offshore Geotechnical Engineering, Spon Press, New York, USA. Part-I Foundation Concepts 65 Various Internet Sites: http://www.fugro.com/ http://www.ihchydrohammer.com http://www.intermoor.com/ http://www.km.kongsberg.com http://www.menck.com/ http://www.ngi.no/en/ http://www.vryhof.com/ http://vulcanhammer.com/ Part-I Foundation Concepts 66 33
Thank you for your attention 67 34