Ageing and Life Extension of Offshore Structures
Description
A significant proportion, estimated at over 50%, of the worldwide infrastructure of offshore structures and pipelines is in a life extension phase and is vulnerable to ageing processes. This book captures the central elements of the management of ageing offshore structures and pipelines in the life extension phase. The book gives an overview of: the relevant ageing processes and hazards; how ageing processes are managed through the life cycle, including an overview of structural integrity management; how an engineer should go about assessing a structure that is to be operated beyond its original design life, and how ageing can be mitigated for safe and effective continued operation.
Key Features:
* Provides an understanding of ageing processes and how these can be mitigated.
* Applies engineering methods to ensure that existing structures can be operated longer rather than decommissioned unduly prematurely.
* Helps engineers performing these tasks in both evaluating the existing structures and maintaining ageing structures in a safe manner.
The book gives an updated summary of current practice and research on the topic of the management of ageing structures and pipelines in the life extension phase but also meets the needs of structural engineering students and practicing offshore and structural engineers in oil & gas and engineering companies. In addition, it should be of value to regulators of the offshore industry.
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Persons
Gerhard Ersdal has had an interest in existing structures and especially the safety of older structures for most of his engineering career. He received his MSc in structural engineering in 1991 and then worked for eight years in a major engineering company in Norway, (Multiconsult) designing primarily offshore structures and bridges and also working on the restoration of many historic buildings in Norway. In 1999, he joined the Norwegian Petroleum Directorate and conducted research on the safety of older offshore structures in a PhD programme at the University of Stavanger. He received his PhD on life extension of ageing offshore structures in 2005. He is the project manager for the Norwegian Petroleum Safety Authority's Ageing and Life Extension research programme, with responsibility for several workshops, conferences and papers on the topic. In 2013, he was awarded a professorship at the University in Stavanger on ageing and life extension of structures.
Prof. John V. Sharp has over 35 years' experience in offshore & marine engineering, with particular interests in offshore technology, safety, life extension, structural integrity, risk management and renewable energy. He was responsible for the UK Health & Safety Executive's £6M offshore health & safety research programme between 1993 and 1996, with particular interests in structural integrity and risk management. He has been a Visiting Professor at Cranfield University since 1996, which includes lecturing and teaching on Master's Courses on offshore engineering and renewables (offshore wind, wave and tidal). Sharp is also a Commissioner for Alderney Commission for Renewable Energy (since 2010), with specific interests in tidal energy. He has also undertaken consultancy work for a number of organisations, which has included assessment and management of ageing offshore installations, life extension, performance indicator measures for organisational capability for both structural integrity and asset maintenance.
Dr. Alexander Stacey is a Structural Integrity Specialist Inspector in the Energy Division of the Hazardous Installations Directorate of the UK Health & Safety Executive. He graduated from the University of London's Imperial College with a degree in Mechanical Engineering and a Ph.D. on research in fatigue and fracture mechanics. He was subsequently employed as a Fracture Mechanics Specialist in the Offshore Division of Lloyd's Register. In his current role as a Structural Integrity Specialist in the Energy Division of the Health and Safety Executive, his primary interest is the structural integrity management of offshore installations throughout the lifecycle. Principal activities include the inspection of duty holders' structural integrity management systems, the assessment of safety cases, the development of guidance, codes and standards and supporting R&D. A key area of interest is the management of ageing and life extension of the UK's offshore infrastructure.
Content
Preface xi
Definitions xiii
1 Introduction to Ageing of Structures 1
1.1 Structural Engineering and Ageing Structures 1
1.2 History of Offshore Structures Worldwide 4
1.3 Failure Statistics for Ageing Offshore Structures 8
1.3.1 Introduction 8
1.3.2 Failure Statistics of Offshore Structures 8
1.3.3 Experience from Land Based Structures 9
1.3.4 Experience from Offshore Fixed Steel Structures 10
1.3.5 Experience from the Shipping and Mobile Offshore Unit Industries 14
1.4 The Terms 'Design Life' and 'Life Extension' and the Bathtub Curve 15
1.5 Life Extension Assessment Process 18
References 20
2 Historic and Present Principles for Design, Assessment and Maintenance of Offshore Structures 23
2.1 Historic Development of Codes and Recommended Practices 23
2.1.1 US Recommended Practices and Codes 23
2.1.2 UK Department of Energy and HSE Guidance Notes 24
2.1.3 Norwegian Standards 26
2.1.4 ISO Standards 27
2.2 Current Safety Principles Applicable to Structural Integrity 28
2.2.1 Introduction 28
2.2.2 Application of Safety Principles to Structures 29
2.2.2.1 General 29
2.2.2.2 Partial Factor and Limit State Design Method 30
2.2.2.3 Robustness 32
2.2.2.4 Design Analysis Methods 34
2.2.2.5 Management of Structures in Operation 35
2.2.3 Managing Safety 35
2.2.4 Change Management 38
2.3 Current Regulation and Requirements for Ageing and Life Extension 38
2.3.1 Regulatory Practice in the UK for Ageing and Life Extension 38
2.3.2 Regulatory Practice in Norway Regarding Life Extension 40
2.3.3 Regulatory Practice in the USA 41
2.3.4 Regulatory Practice Elsewhere in the World 42
2.4 Structural Integrity Management 43
2.4.1 Introduction 43
2.4.2 The Main Process of Structural Integrity Management 45
2.4.3 Evolution of Structural Integrity Management 47
2.4.3.1 The Early Years 47
2.4.3.2 The Introduction of Structural Integrity Management into Standards 47
2.4.4 Current SIM Approach 47
2.4.5 Incident Response and Emergency Preparedness 51
2.4.6 SIM in Life Extension 52
References 53
3 Ageing Factors 57
3.1 Introduction 57
3.1.1 Physical Changes 59
3.1.2 Structural Information Changes 59
3.1.3 Changes to Knowledge and Safety Requirements 60
3.1.4 Technological Changes 61
3.2 Overview of Physical Degradation Mechanisms in Materials 62
3.3 Material Degradation 63
3.3.1 Introduction 63
3.3.2 Overview of Physical Degradation for Types of Steel Structures 64
3.3.3 Steel Degradation 65
3.3.3.1 Hardening Due to Plastic Deformation 65
3.3.3.2 Hydrogen Embrittlement 66
3.3.3.3 Erosion 68
3.3.3.4 Wear and Tear 68
3.3.4 Concrete Degradation 68
3.3.4.1 Concrete Strength in Ageing Structures 68
3.3.4.2 General 70
3.3.4.3 Bacterial Induced Deterioration 71
3.3.4.4 Thermal Effects 72
3.3.4.5 Erosion 72
3.4 Corrosion 73
3.4.1 General 73
3.4.2 External Corrosion 73
3.4.3 Various Forms of Corrosion 74
3.4.3.1 CO2 Corrosion 74
3.4.3.2 Environmental Cracking Due to H2S 74
3.4.3.3 Microbiologically Induced Corrosion 74
3.4.4 Special Issues Related to Corrosion in Hulls and Ballast Tanks 75
3.4.5 Concrete Structures 75
3.4.5.1 Corrosion of Steel Reinforcement 75
3.4.5.2 Corrosion of Prestressing Tendons 77
3.5 Fatigue 77
3.5.1 Introduction 77
3.5.2 Factors Influencing Fatigue 80
3.5.3 Implications of Fatigue Damage 81
3.5.4 Fatigue Issues with High Strength Steels 83
3.5.5 Fatigue Research 84
3.6 Load Changes 85
3.6.1 Marine Growth 85
3.6.2 Subsidence andWave in Deck 86
3.7 Dents, Damages, and Other Geometrical Changes 86
3.8 Non-physical Ageing Changes 88
3.8.1 Technological Changes (Obsolescence) 88
3.8.2 Structural Information Changes 89
3.8.3 Knowledge and Safety Requirement Changes 90
References 91
4 Assessment of Ageing and Life Extension 95
4.1 Introduction 95
4.1.1 Assessment versus Design Analysis 96
4.2 Assessment Procedures 97
4.2.1 Introduction 97
4.2.2 Brief Overview of ISO 19902 99
4.2.3 Brief Overview of NORSOK N-006 101
4.2.4 Brief Overview of API RP 2A-WSD 102
4.2.5 Brief Overview of ISO 13822 102
4.2.6 Discussion of These Standards 103
4.3 Assessment of Ageing Materials 104
4.4 Strength Analysis 107
4.4.1 Introduction 107
4.4.2 Strength and Capacity of Damaged Steel Structural Members 108
4.4.2.1 Effect of Metal Loss andWall Thinning 109
4.4.2.2 Effect of Cracking and Removal of Part of Section 110
4.4.2.3 Effect of Changes to Material Properties 110
4.4.2.4 Effect of Geometric Changes 110
4.4.2.5 Methods for Calculating the Capacity of Degraded Steel Members 110
4.4.3 Strength and Capacity of Damaged Concrete Structural Members 111
4.4.4 Non-Linear Analysis of Jacket of Structures (Push-Over Analysis) 113
4.5 Fatigue Analysis and the S-N Approach 115
4.5.1 Introduction 115
4.5.2 Methods for Fatigue Analysis 116
4.5.3 S-N Fatigue Analysis 117
4.5.3.1 Fatigue Loads and Stresses to be Considered 117
4.5.3.2 Fatigue Capacity Based on S-N Curves 119
4.5.3.3 Damage Calculation 121
4.5.3.4 Safety consideration by Design Fatigue Factors 122
4.5.4 Assessment of Fatigue for Life Extension 122
4.5.4.1 Introduction 122
4.5.4.2 High Cycle/Low Stress Fatigue 123
4.5.4.3 Low Cycle/High Stress Fatigue 124
4.6 FractureMechanics Assessment 126
4.6.1 Introduction 126
4.6.2 Fatigue Crack Growth Analysis 128
4.6.3 Fracture Assessment 131
4.6.4 Fracture Toughness Data 132
4.6.5 Residual Stress Distribution 132
4.6.6 Application of Fracture Mechanics to Life Extension 132
4.7 Probabilistic Strength, Fatigue, and Fracture Mechanics 134
4.7.1 Introduction 134
4.7.2 Structural Reliability Analysis - Overview 135
4.7.3 Decision Making Based on Structural Reliability Analysis 136
4.7.4 Assessment of Existing Structures by Structural Reliability Analysis 138
References 139
5 Inspection and Mitigation of Ageing Structures 143
5.1 Introduction 143
5.2 Inspection 144
5.2.1 Introduction 144
5.2.2 The Inspection Process 145
5.2.3 Inspection Philosophies 147
5.2.4 Risk and Probabilistic Based Inspection Planning 148
5.2.5 Inspection of Fixed Jacket Structures 150
5.2.6 Inspection of Floating Structures 154
5.2.7 Inspection of Topside Structures 155
5.2.8 Structural Monitoring 158
5.3 Evaluation of Inspection Findings 160
5.4 Mitigation of Damaged Structures 161
5.4.1 Introduction 161
5.4.2 Mitigation of Corrosion Damage 163
5.4.3 Mitigation of the Corrosion Protection System 163
5.4.4 Mitigation of Fatigue and Other Damage 166
5.5 Performance of Repaired Structures 168
5.5.1 Introduction 168
5.5.2 Fatigue Performance of Repaired Tubular Joints 168
5.5.3 Fatigue Performance of Repaired Plated Structures 170
References 171
6 Summary and Further Thoughts 173
6.1 Ageing Structures and Life Extension 173
6.2 FurtherWork and Research Needs Related to Ageing Structures 174
6.3 Final Thoughts 176
A Types of Structures 177
A.1 Fixed Platforms 177
A.2 Floating Structures 177
Reference 179
B InspectionMethods 181
B.1 General Visual Inspection 181
B.2 Close Visual Inspection 181
B.3 FloodedMember Detection 181
B.4 Ultrasonic Testing 182
B.5 Eddy Current Inspection 182
B.6 Magnetic Particle Inspection 182
B.7 Alternating Current Potential Drop 182
B.8 Alternating Current Field Measurement 182
B.9 Acoustic Emission Monitoring 183
B.10 Leak Detection 183
B.11 Air Gap Monitoring 183
B.12 Strain Monitoring 183
B.13 Structural Monitoring 184
C Calculation Examples 185
C.1 Example of Closed Form Fatigue Calculation 185
C.2 Example of Application of Fracture Mechanics to Life Extension 186
Index 191
1
Introduction to Ageing of Structures
It is the destiny of the man-made environment to vanish, but we, short-lived men and women, look at our buildings so convinced they will stand forever that when some do collapse, we are surprised and concerned.
Levy and Salvadori (2002)
1.1 Structural Engineering and Ageing Structures
How long can a structure last? Historically we have seen structures failing before they were ready to be used.1 Others, such as historical monuments, have lasted for centuries and millennia.2 The life span of a structure will depend on its design, its construction, and fabrication, the material used, the maintenance performed, the challenging environment it has been exposed to, the accidental events it has experienced, and whether it is possible to repair and replace any damaged or deteriorated structural parts. Metallic structures from the 1700s are still carrying their intended loads. Such evidence may lead us to believe that structures may last forever. However, only one of the Seven Wonders of the Ancient World is still standing, namely the Great Pyramid of Giza (constructed around 2500 BCE).
Changes start to appear in structures from the moment they are constructed. The material in structures will degrade (mainly by corrosion and fatigue) and accumulate damage (such as dents and buckles). The environment the structures are placed in will change, and that will influence the degradation mechanism. The loads on a structure will change with changes in use. The foundations of the structure may experience settlement and subsidence, which implies additional stresses in the structures and may introduce changes to the loading. Furthermore, technological developments may lead to materials, equipment, and control systems related to the structure being outdated and spare parts for these systems becoming unavailable (obsolescence). Compatibility between new equipment and the equipment that is already in place on the structure (e.g. to control stability and ballast on a floating structure) may prove to be difficult. Ultimately we may face the problem of changing to a new technological solution with possible issues concerning safety and functionality, or continue to use the old technological solutions with their limitations. All of the above may make a structure less safe.
The assessment of an ageing structure for possible further use has to be based on the available information. Ideally, information about the original design and fabrication of the structure, its use and the inspections performed over the years are required to determine whether a structure is fit for further use. This assessment needs to be based on an understanding of the current safety of the structure. However, for older structures, the necessary information required to show that they are sufficiently safe may be lost or impossible to obtain. Lack of information, new knowledge, and new requirements may change our understanding of the safety of a structure, and may force us to regard the structure as unsafe requiring further mitigation.
However, new knowledge, methods and requirements may provide information that leads to a better understanding of the integrity of an existing structure, including the possibility that the integrity is better than expected and sufficient for safe operation in the life extension phase.
Finally, as time passes since the design of a structure, the evolution of technical knowledge normally leads to society developing more stringent requirements for safety.3 This improved understanding will increase expectations for the safe operation of structures, including older ones designed to lesser criteria.
Offshore structures are continuously exposed to all of the above types of change. They operate in an environment that causes corrosion, erosion, environmental and functional loads, incidents and accidents that deteriorate, degrade, dent, damage, tear, and deform the structure. In addition to the changes to the structures themselves, the loads and corrosive environments in which they operate will change over time. Further, the way these are used may change, which as a result will alter the loading, the environment these are exposed to and possibly the configuration of the installation. In addition, our knowledge about the structures will change, e.g. the type of information that we have retained from design and inspection of the structure. Further, the physical theories, mathematical modelling and engineering methods used to analyse the structures may change, typically as new phenomenon are discovered. Finally, our evaluation of offshore structures is also influenced by societal changes and technological developments. This may result in changes to the requirements that are set for offshore structures, taking into account obsolescence, lack of competence, and the availability of spare parts for old equipment.
These changes may be grouped into four different types:
- Physical changes to the structure and the system itself, their use, and the environment they are exposed to (condition, configuration, loading, and hazards).
- Changes to structural information (the gathering of more information from inspections and monitoring, but also potential loss of information from design, fabrication, installation, and use).
- Changes to knowledge and safety requirements that alter our understanding of the physics and methods used to analyse the structure, and the required safety that the structure is supposed to have.
- Technological changes that may lead to equipment and control systems used in the original structure being outdated, spare parts being unavailable, and compatibility between existing and new equipment and systems being difficult.
These groups of changes are illustrated in Figure 1.1, where it is indicated that the physical and technological changes impact the safety and functionality of the structure directly, while structural information changes and changes to knowledge and safety requirements primarily change how we understand the safety and functionality of a structure. Further, it is indicated that physical changes and structural information changes apply to one specific structure, while technological changes and changes to knowledge and safety requirements are a result of societal and technological developments, and are applicable to all structures.
Figure 1.1 The four main elements of ageing of a structure.
These issues are highly relevant for a structural engineer; as we will show in Section 1.2, as the early offshore structures in the oil and gas industry are getting rather old. Many offshore structures from the 1990s are now passing their planned life expectance. However, there is a need for many of these structures to remain in service as there is still oil and gas remaining in the reservoirs. Further, many fixed and floating structures provide an important hub for the increasing number of subsea installations. The continued use of these older structures has the potential to save substantial costs and minimise environmental damage by avoiding the building of new structures.
In Section 1.3 we will show that failure statistics for structures indicate that structures in the oil and gas industry have a significant failure rate, particularly for floating structures. Further, older structures fail more often compared with newer structures. This is not surprising taking into account that structures will degrade and accumulate damage, that their use may change in unfavourable ways, that systems related to the structures may experience obsolescence and that newer structures may be designed according to improved methods and more stringent regulations and standards.
Facing the challenge of having relatively many older structures in the oil and gas industry, and at the same time knowing that older structures fail more often than newer ones, structural engineers need to:
- Understand how structures change as they get older (Chapter ).
- Develop methods to assess these structures properly so that the structures that are unfit for further service are decommissioned, either because they are unsafe or they cannot be proved to be safe due to lack of important information (Chapter ).
- Manage these older structures properly in their life extension phase (Chapter ).
This book is generally about these items, but in order to understand older structures it is important to know about early designs and maintenance practices, as these will have an impact on our understanding of older structures. Similarly, it is important to know about the present requirements, because older structures will in many regions of the world be measured to the same safety standards as new structures. Further, the design of early structures was based on the knowledge and experience at that time and the methods often resulted in safe designs. In the intervening period there have been significant improvements in knowledge and experience which can be applied to the management of these older structures. These topics are covered in Chapter .
1.2 History of Offshore Structures Worldwide
Over the years, several types of offshore platforms have been used to produce oil and gas. One of the earliest successful fixed platforms was a wooden platform used by Pure Oil (now Chevron) and Superior Oil (now ExxonMobil) 1 mile from the coast in a water depth of 4.3 m in 1937 (Offshore 2004). The first floating production was from...
