This paper provides a comprehensive finite element analysis to investigate the ultimate capacity of corroded members under concentric axial loads. The paper investigates previous experimental work on stocky and slender tubular columns with simulated patch corrosion and provides a numerical model that can estimate the columns capacities. Further, a parametric study is performed to investigate the effect of geometric parameters such as location, height, and width of corrosion patch on the ultimate capacity of corroded columns. Finally, the paper presents a comparison between laboratory tests to the formulae of superseded standards and numerical analysis using nonlinear finite element methods. The numerical model proposed in this paper show good agreement with the results from the experimental work.

Experimental and numerical studies of the effect of crack deflecting holes in steel plates under high cycle fatigue are presented in this paper. The experimental studies show that with the careful location of the holes, crack propagation can be arrested. A numerical model is provided and validated against the experimental work. The numerically predicted crack propagation direction and crack growth rate were in good agreement with the crack propagation obtained in the experimental work.

FPSOs and FSOs have been used on the Norwegian continental shelf for petroleum production, storage and offloading since the 1980’s and have been increasingly common since the end of the 1990’s. During these years a significant amount of experience has been gathered. On the Norwegian Continental Shelf (NCS) shipowners and operators are obliged to report damages and incidents to the Petroleum Safety Authority Norway (PSA). This article gives an overview of reported incidents related to the structural and maritime systems on offshore ship shaped units in the petroleum activity (FPSOs and FSOs) on the Norwegian Continental Shelf. Reported incidents are summarised with a focus on the period 2000 to 2019. The incidents include cracks, dents, corrosion, turret incidents, green water, position keeping systems, stability and ballasting, collisions and incidents related to the offloading systems as reported by operators and shipowners to the PSA. A summary of the reported damages and incidents are discussed with respect to common causes based on available data. The importance and the possible consequences of such incidents are further discussed.

Safety regulation and standards have developed intensively since the startup of the petroleum industry in Norway in the early 1970’ties. Prescriptive standards where used in the early days. In the 1980’ties risk analysis and the 10 −4 criteria were introduced, along with accidental loads and the progressive collapse limit state. Since 2002 barriers have been a general part of the safety regulation in Norway. This paper aims at reviewing the development of safety regulation of structures and marine systems in Norway, present the barrier regulation as it is described in the Norwegian regulations, and finally evaluate the relevance of the barrier regulation for structures and marine systems.

An increasing number of offshore installations are in the life extension stage of life, resulting in a growing concern among structural engineers about ageing related issues. However, structural engineers are often asked if there is experience of any ageing accidents or if there are examples of such. A safety conscious person would then reply that theory points towards a growing failure rate and hence that ageing accidents will eventually occur. Good engineering practice would then imply that there is no point in waiting for the accident to occur. As responsible engineers we should investigate the hazards and avoid accidents without having to wait for a potentially fatal accident to lead to action. The other possible answer is to question the term “ageing accident”. What is the definition of an “ageing accident”? This paper will investigate into the term “ageing accident” and will propose a definition of this term. Further, this paper will describe some examples of ageing accidents, in particular the many structural failures that have occurred in the Gulf of Mexico hurricanes.

As part of its regulatory duties the Norwegian Petroleum Safety Authority (PSA) periodically undertakes audits of the Structural Integrity Management (SIM) functions of the operators of facilities within the Norwegian offshore jurisdiction; typically one or two audits of reasonable detail are undertaken each year, but in addition PSA have also tried a series of mini-audits based purely on interviews without examination of evidence. In many cases certain aspects of the SIM cause the auditors some concern and there is a degree of consistency about those aspects of SIM causing such concern, and on occasion become formal “findings” of the audits. In general the actual execution of SIM activities are performed adequately, but the full understanding of the purpose of SIM and realistic intent and expectation are missing. A particular concern of the auditors is the anticipation of structural form-specific accidental and abnormal events and their incorporation into the SIM. This paper will provide a summary of the more important lessons learnt over several years of auditing, in an attempt to provide industry with a better understanding of the regulator’s expectations of SIM, particularly for installations that are ageing or have unique features.

A large number of the Norwegian gas and oil production facilities have exceeded their initial design life but the respective fields are still producing substantial levels of hydrocarbons. In order to ensure technical and operational integrity of these ageing facilities, the Norwegian oil industry Association (OLF) has initiated a project to establish the necessary standards and guidelines for assessing and ensuring safe life extensions. This paper presents this project and the headlines of these standards and guidelines.

In 2005, the Petroleum Safety Authority Norway (PSA), gave special priority to a programme of ageing and life extension. The programme comprised technical investigations into how ageing will reduce the safety of the facilities and how this can be managed, plus cooperation with the petroleum industry to develop standards and guidelines for assessment of ageing facilities for life extension. In addition, audits have looked into the way operators are dealing with the implications of ageing facilities. This paper summarizes the outcome of the programme and the experience gained from it.

In all phases of the service life of a structure, uncertainties will occur, hence probability based methodologies may be an important and valuable tool in order to verify structural integrity and the corresponding expected safety level. This paper gives a brief description of an approach for how to demonstrate the safety of facilities in the life extension phase by the use of risk based inspection (RBI) plans. This is illustrated with four separate cases used for assessment and life extension of FPSOs operating in harsh environments on the Norwegian continental shelf (NCS). The assessments are partly based on the methodology introduced in NORSOK (2009).

A considerable number of the structures in the Norwegian part of the North Sea have passed or are close to their design life. Material degradation will play an important role in the ageing of these structures and the evaluation of their safety. An overview of research work initiated by the Petroleum Safety Authority (PSA) is presented. The paper focuses on various material aspects of ageing related to offshore facilities, the risks they represent to the integrity of a facility and how to deal with them in a life extension process. The paper presents and discusses expectations towards the industry with respect to evaluation of ageing materials in life extension.

A large number of facilities and parts of the infrastructure on the Norwegian Continental Shelf are approaching or have exceeded their original design life. Many fields, however, have remaining recoverable oil and gas reserves which may be profitable if the field’s life is extended. From a safety point of view, the condition of systems, structures and components may not be acceptable for extended operation. Ageing and life extension have been a top priority for the Petroleum Safety Authority Norway (PSA) and PSA has asked SINTEF to conduct a study of various aspects of ageing and life extension. The paper presents main results from the study, including how to document the safety of an ageing facility and how to uphold the safety level by means of a maintenance programme balancing three aspects of ageing: 1) Material degradation, 2) Obsolescence, i.e. operations and technology being “out of date” and 3) Organisational issues. The paper presents six main steps of the life extension process and discusses important issues to consider for operators in a life extension process.

An increasing number of platforms in the Norwegian continental shelf are reaching their design life. For various reasons these platforms will require an assessment of their structural integrity. When performing these assessments the engineer is faced with tasks where little guidance is found in design standards, for several reasons. The two most important being: 1) The analyses that is performed in a typical assessment of existing structure is often applying very advanced techniques and methodology that seldom is used in design of new structures, as the cost of doing advanced analysis is relatively low compared to replacement of an existing structure, but relatively high compared to moderate additions of e.g. steel in the design of a new structure. 2) Design standards are based on theories, methods and experience for structures in a given design life (e.g. fatigue design and corrosion protection design). When this design life is extended, sound methods for ensuring that the structures are still sufficient safe is needed. Such methods will normally be “condition based design”, where inspection, maintenance and repairs are included in the assessment in integrated way. Such methods are not given in normal design standards. For these reasons a new NORSOK standard is developed that gives recommendation on how to deal with the specific aspects that engineers meet when performing assessments of structures in general, but also specifically for assessment for life extension. The standard is named “Standard for Assessment of Structural Integrity for Existing Load-bearing Structures” and is issued as a NORSOK standard and given the number N-006 [1]. The topics that are covered in the standard include: Shut down and unmanning criteria for platforms not meeting ordinary requirements, specific issues for determination of ultimate capacities by use of non-linear methods, cyclic capacity checks, fatigue life extension, requirements to in-service inspection etc. The paper describes the background and the content of the new standard and it presents examples of recommendations given. The role of the new standard in the Norwegian regulatory system is shown.

To monitor the development of the risk of major accidents of structures and maritime systems on the Norwegian Continental Shelf a set of incident indicators has been established. These indicators are given a weight related to the fatality risk predictions in order to develop a total risk indicator. Aggregated and weighted indicators as well as individual indicators are presented and evaluated. In addition indicators related to barriers are established giving a method to monitor the performance as a function of time, and to benchmark the participants in the industry. The present paper describes the method used in the risk level project to monitor the risk related with emphasis on structures and maritime systems, including hazards as wave in deck, vessel collisions and cracks on the structures. The paper further presents the actual development using the methodology, and the identified major contributors to the risk level.

There are an increasing number of older installations in use on the Norwegian Continental shelf. Inspections are a key issue in ensuring the safety of an older installation, and the inspection intervals, inspection methods and its reliability are clearly influencing the safety of the installations. Different approaches for updating inspection plans for older installations are considered in order to achieve decreased inspection intervals as the structure are ageing. The most promising method consists in increasing the rate of defects / crack initiation at the end of the expected lifetime. Different system aspects are considered incl. assessment of the acceptable annual probability of failure for one component dependent on the number of critical components. Information obtained from inspection of one component can be used not only to update the inspection plan for that component, but also for other nearby components. An example indicates that a high degree of correlation between the uncertain parameters in different components is needed in order to obtain substantial information which can be used in inspection planning. Fatigue failure of one (or more) components does not necessary imply total collapse of the structure. The importance of each component can be measured by the RIF (Residual Influence Factor) for each component and is illustrated by examples. When an installation becomes older then the number of fatigue critical components can be expected to increase. If the maximum acceptable annual system probability of failure is the same as for a new installation, this implies that the acceptable annual fatigue probability of failure decrease, which again implies more inspections.

A considerable part of the structures on the Norwegian continental shelf have passed or are close to the design service life. Several of these structures are planned to be used further in an extended life. In most cases, traditional analyses in accordance with design methods are used for evaluating the structures for life extension. The present regulation and standards have so far had emphasis on design of new facilities, and a need for updated regulations and standards also taking into account ageing facilities and life extension is well overdue. This paper will discuss aspects of ageing that may reduce safety of offshore facilities, maintenance needs for ageing facilities, and propose general principles of assessment of ageing facilities for life extension. The paper is a summary of the research performed by and for the Petroleum Safety Authority - Norway.

A considerable part of the structures in the Norwegian part of the North Sea have passed or are close to their design lifetime. Degradation of materials will play an important role in the ageing of these structures and the evaluation of their safety. Hence, Petroleum Safety Authority Norway has initiated several research projects to investigate into the ageing of materials, and how to ensure robust materials also for life extension. The present paper gives a summary of these research projects.

In order to ensure the safety of an offshore structure it is important to identify and maintain the barriers preventing hazardous events. Also, when monitoring the safety, the monitoring should be regarding how well these barriers are functioning, and utilise these to reassess the safety of the structure over time. The purpose of this paper is to apply a well-known method in risk assessment, Haddon’s energy and barrier model, to a new area; structural safety. The purposes of this exercise are to look at the structural safety from a risk assessment point of view, and to use this to identify and give an overview of the existing barriers. Furthermore, the purposes are to evaluate the efficiency and redundancy of these barriers, and to use this to evaluate the safety of offshore structures. This paper will analyse the safety of a fixed offshore structure through a qualitative approach. A possible event chart for a fixed offshore installation during operation in storms is established and analysed. Some of the root causes for potential structural failure are identified. These root-causes are kept on a general level, but considered in more detail than often seen in risk analysis. Hazards that are normally included in risk analysis, like boat collisions, fire, explosions, and dropped objects are not evaluated. Hazards that are evaluated are structural failure due to wave loading, fatigue damage, aging, and gross errors in design, fabrication, installation and operation. In order to identify the barriers (hazard reduction strategies, physical barriers and vulnerable target protection strategies), the different failure paths in the event chart are then analysed using Haddon’s ten preventive strategies for reducing damage from hazards. As an example a fixed offshore steel structure is used. A list of proposed barriers that influence the safety of such a fixed offshore installation are presented, and methods to measure these barriers are discussed.

This paper uses historic accidents to identify important barriers for the structural and marine safety of offshore drill rigs. Several accidents and near misses have been evaluated and barriers that failed, did not exist at the time of the accident or that stopped the accident are identified. Based on these analyses and review of anticipated failure modes in standards and regulations, a simplified generic barrier diagram is established. The generic barrier diagram can be used to evaluate current practice and provide input to general safety management. The possible degradation of the barriers with increasing rig age is also evaluated, and the possible effect the degradation might have on the integrity of aging rigs.