FOUNDATION ISSUES: OFFSHORE WIND FARMS Indian Context R.K. Ghanekar, Head - Geotechnical Section, INSTITUTE OF ENGINEERING AND OCEAN TECHNOLOGY (IEOT), ONGC, PANVEL, NAVI MUMBAI
OFFSHORE WIND ENERGY IN INDIA Nascent Stage ONGC s Perspective Plan 2030: Attention to Alternative energy sources - a component of 6.5 GW of alternative energy by 2030 IEOT: Review of current know-how about foundations
Offshore Petroleum vs. Offshore Wind What is different for offshore wind turbines? Loading: High H/V ratio, 50 years storm, more uncertainty in hydrodynamic loading due to shallower waters Dynamic Response: dynamic sensitivity, time domain analyses instead of frequency domain Risk Scenario: Low risk / low consequence of failure categories Soil Investigation Strategy: regional approach with increased importance of geophysical survey Foundation Cost: 20 to 45% of total cost based on type of foundation and soil conditions
Support Structure concepts Malhotra (2007)
Support Structure concepts (After Plocan, 2012) www.windplatform.eu/fileadmin/ewetp.../hernandez-brito.pdf
Support Structure concepts Hybrid monopile-shallow footing system (Arshi and Stone, 2012)
Summary of common concepts (Gavin and Doherty, 2012) Malhotra (2007)
Indian Offshore conditions Focus on areas - up to 30 m water depth Soil conditions Western Offshore: Most of the areas have top soft marine clay Exceptions in Northern (Tapti) area
Indian Offshore conditions Eastern Offshore: Mainly soft clayey profile to a significant depth Under-consolidation in some deltaic areas Some surface sands e.g. in Porto Novo
Most relevant Support structure concepts Monopile or hybrid system Tripod or four legged jacket Gravity base structures in some select areas Focus on monopile or piled jacket structures
Applicable International Codes DNV-OS-J101 2010 / Rev. 3: Design of Offshore Wind Turbine Structures: principles, technical requirements and guidance for design, construction and in-service inspection GL-Wind 2005 / Ed. 2: Germanischer Lloyd Rules and Guidelines, IV Industrial Services, Part 2 Guideline for the Certification of Wind Turbines: design, assessment and certification of offshore wind turbines and farms (also, Type Certification and Project Certification). IEC 61400-3 Feb. 2009 / Ed. 2: Design requirements for offshore wind turbines: engineering and technical requirements during design of offshore wind turbines, inclusive of their support structures
SERVICEABILITY Common Foundation design Issues Foundation performance tolerance based on operational requirement of wind turbine: Generally rotation at Monopile pile head (defined at seabed): Installation Tolerance Accumulated permanent rotation due to history of SLS loads applied during design life (cyclic wave & wind) Installation tolerance generally ranges from 0.2 to 0.25 o and total tolerance 0.5 o Scour prediction and protection
Common Foundation design Issues Dynamic response: Wind Turbine structures: more sensitive to dynamic loading Foundation stiffness governs the natural frequency of monopile structures For monopile structures the natural frequency is kept between wave and rotor frequency (P) & blade passing frequency (3P) Any under or over estimation of foundation stiffness can be problematic For stiffer structures natural frequency is kept above 3P but cyclic degradation of foundation stiffness needs to be checked Fatigue: 5 to 8% reduction in foundation stiffness can increase fatigue susceptibility by ~ 100% for monopile
Foundations for Monopile & Jacket Structures Lateral capacity and dynamic sensitivity govern the design of Monopile foundations Axial tensile capacity generally governs the pile design for Jacket structures Operational tolerances is another important design issue
Pile Foundations design Issues- General Major limitations of common pile design methods (generally as per API RP 2A): both for axial and lateral loading Main text method for sands over or under-predicts axial pile capacity depending on soil state CPT based methods too individually predict quite differently and FOS is not recommended The underlying databases have data from much slender and lower diameter piles especially compared to monopiles (both for API Main text method & CPT based methods) Lateral capacity too involves similar problems. Especially important for monopiles.
Research Endeavours Proposed JIP DYNDATA: A dynamic pile load test data pool for the offshore German wind industry Cathie & Associates JIP on Reliability of API and CPT-based axial pile capacity design methods Norwegian Geotechnical Institute (NGI) A new constitutive model called UnDrained Cyclic Accumulation Model (UDCAM) for clays by NGI for FEM analysis of monopiles
Conclusions There are important differences in foundation design requirements between offshore petroleum and wind turbines Most likely applicable support structure concepts appear to be monopiles or piled jackets Codal provisions exist but many design issues are unresolved / inadequately resolved Initially, expert support will be required for the design and installation With the existing offshore oil and gas experience, the expertise can be quickly developed indigenously
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