Assessment of offshore wind turbines support structures within a reliability based framework 1 Dr Athanasios Kolios Research Fellow, Risk and Reliability of Offshore Structures Dr Amir Chahardehi Research Fellow of Structural Integrity Prof Feargal Brennan Head of Offshore, Process and Energy Engineering Dept. European Wind Energy Association Annual Event March 211 Brussels, Belgium

Introduction & Context Offshore structures are design for a service life around 20 years (eg. DNV) Experience of Oil & Gas industry, has already faced the issue of structures that have exceeded their intended service life and operate for more than three decades API RP 2A - Section 17, for the assessment of existing structures, is applied to an average of 50 platforms per year by the Gulf of Mexico Operators 1 This study aims to point out benefits of assessment of existing support structures towards: Optimization of their inspection and maintenance management Setting up a framework for the evaluation of their ability to sustain their operation further from their intended service life 1 D. J. Wisch, F. J. Puskar, T. T. Laurendine, O'Connor, P. E. Versowsky, J. Bucknell, “An Update on API RP 2A Section 17 for the Assessment of Existing Platforms, OTC 16820

Reassessment of Offshore Oil & Gas Structures Norwegian Petroleum Safety Authority has invested extensively in research in the area. Many offshore oil & gas installations are now in a life extension stage worldwide. Successful management of ageing plant requires an understanding of the applicable ageing mechanisms, and a recognition that not all of those ageing mechanisms are in action all of the time. [Galbraith et al 2005] “Ageing is not about how old your equipment is; it’s about what you know about its condition, and how that’s changing over time.” Plant Ageing, HSE Book, Typical Offshore Jacket Structure

Motivation and Requirements for Assessment of Offshore Structures Motivation: – Structural Deterioration – Accidental Damage due to impact, accidents etc. – Changing demands – Change in regulations – Life extension Requirements: – Development and application of a widely accepted framework for the adequate evaluation of the condition of a structure through out its service life, considering updated data for each case individually Parallel Technologies

Optimization of Inspection and Maintenance Before optimisation After optimisation (redundancy) 20 years 1.0 Cumulative Cost of Inspection Planning Before and After Optimisation T. Onoufriou, “Risk Based Integrity Management of High Value Assets, University of Surrey, Infrastructure Reliability and Management Centre

Structural Integrity Management Process 1: high level requirements and strategy remaining reservoir life; life extension potential; net present value (NPV) and scope for enhancing NPV; planned platform removal or abandonment date; requirements for increased or modified equipment to meet future development of the platform; health, safety and environmental requirements. Process 2: Inspection and reporting Divers; ROVs; small-scale1 and, more recently, large-scale acoustic emission (AE) Process 3: Assessment (Evaluation) Process 4: Repair and mitigation Process 5: Management of information Process 6: Formal integrity reporting and evaluation of SIM programme Process 7: Checking and audit D.N. Galbraith, J.V. Sharp, and E. Terry, “Managing Life Extension in Ageing Offshore Installations”, SPE 96702

Design According to Standards Design standards, evolved over the last decades include general information regarding the design, maintenance and operation of structures. Based on the provisions of API RP-2A Section 17, the key aspects of reassessment should include: – Inspections, surveys and data management – Environmental conditions and forces – Structural elements, systems and analysis – Foundation elements, systems and analysis – Operational considerations – Policy considerations and consequences Component Based-Design Analysis vs Design Generic, standalone standard for assessment of structures does not exist Limitations of Standards

Aging Mechanisms in Structures Fatigue, which is more significant in the North Sea and in similar environments around the world; Loss (usage) of anodes and/or corrosion, both of which are more significant in the Gulf of Mexico and similarly warmer waters in the rest of the world; Accumulated damage, due for example, to impacts from objects dropped from the platform or attendant vessels; Scour, which can increase the height of the structure subjected to hydrodynamic loading; Changes in effective water depth which can increase both the hydrodynamic loading on the structure and the probability of the deck being inundated during extreme weather conditions; effective water depth can increase due to settlement, subsidence, or vertical movements of the tectonic plates. Of these mechanisms, most are involved in aging of offshore wind support structures

Context of Structural Reliability Deterministic vs Stochastic Modelling Classifications of Uncertainty Modelling of Uncertainty Safety Margin (Supply vs Demand): Stochastic Design Calculation of Reliability

Reliability Based Inspection and Maintenance Optimised Intervals Regular Intervals Reliability Index β Target β Year in service Target β Reliability Index β Year in service T. Onoufriou, “Risk Based Integrity Management of High Value Assets, University of Surrey, Infrastructure Reliability and Management Centre

Target Reliability Levels Target (acceptable) Reliability Index Classes of Structural Reliability Types of Structural Reliability Analysis Levels of Reliability Source Allowable system failure probability Risk Analysis (analytical Assessment)10 -6 /yr CSA10 -5 /yr DNV /yr ISO-1000 people10 -7 /yr Professional recommendations10 -5 life time Social Criteria /yr Existing Structures /yr Table US Army Corps of Engineers 1997, Table B-1 B. Bhattacharya, R. Basu, and K. Ma. Developing target reliability for novel structures: the case of the Mobile Offshore Base. Marine Structure. 13; 37-58, 2001 Class of failureConsequence of failure Less seriousSerious I - Redundant structure P F =10 -3 P F =10 -4 ( β t =3.09)( β t =3.71) II - Significant warning before the occurrence of failure in a non-redundant structure P F =10 -4 P F =10 -5 ( β t =3.71)( β t =4.26) III - No warning before the occurrence of failure in a non-redundant structure P F =10 -5 P F =10 -6 ( β t =4.26)( β t =4.75) DNV Classification (CN 30.6)

Reliability Assessment of Offshore Support Structures INITIAL DESIGN RELIABILITY ASSESSMENT β o INSTALLATION CALLIBRATION BASED ON REAL DATA β 1 MONITORING/DATA COLLECTION RELIABILITY ESTIMATION β i EVALUATION AND ACTION COSTING OPTIMIZATION Optimization for service life performance DEFINITION OF STOCHASTIC VARIABLES GEOMETRICAL MODEL NUMERICAL SIMULATIONS LIMIT STATES SELECTION AND CALCULATION RELIABILITY ESTIMATION Structural Reliability Assessment Data update Reliability Assessment, FORM/SORM, Stochastic Methods Design Standards Provisions

Numerical Example Thickness Deterioration Models

Conclusions Evaluation of performance of a structure through out its service life is an issue of great importance as application in wind turbines yields for optimized design and operation Structural health monitoring systems allow appropriate planning of inspection and maintenance, as well as provide all the required information for its integrity assessment, towards extension of its service life Structural Reliability can stand as a comprehensive indicator for the evaluation of the safety of a structure Offshore Oil & Gas installations can provide significant experience, having already treated similar problems adequately

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