Gudfinnur Sigurdsson

Hot Spot Stress S-N Curve for Fatigue Analysis of Plated Structures

Finite element analysis is being used by designers for fatigue assessment of structures. It is therefore important that a proper link between calculated hot spot stress and fatigue capacity is established. The fatigue capacity is expressed as a hot spot stress S-N curve. This paper presents a derivation of a hot spot stress S-N curve to be used when the hot spot stress is derived from finite element analysis of plated structures. The hot spot S-N curve is linked to the methodology used for finite element analysis of plated structures that is being included in design recommendations for fatigue assessment of welded structures.

Recommended Design Fatigue Factors for Reassessment of Piles Subjected to Dynamic Actions From Driving

Calculated probabilities of fatigue failure depend on the analysis procedure used for design. Calculated probabilities of a fatigue failure also depend on long term stress ranges due to loading and uncertainties associated with this. In order to ensure the consistent safety level for assessment of fatigue failure, the design fatigue factors (DFFs) to be used for fatigue design should be dependent on the analysis procedure and premises used. In the present paper, an assessment of appropriate DFFs for piles subjected to dynamic actions from pile driving has been performed by probabilistic analysis based on: uncertainty with respect to dynamic cyclic stress during pile driving, and fatigue capacity of circumferential welds in piles. Accumulated probabilities of fatigue failures in pile butt welds are presented. An assessment of uncertainties involved in calculation of stress ranges during pile driving has been performed. It is shown that the uncertainty in loading when driving records are known is lower than that estimated on the basis of soil data. Thus, in order to obtain consistent safety levels, different DFFs should be used when calculated stress ranges are derived based on soil data only, as compared with the actual stress ranges and number of blows determined from driving records. The results from probabilistic analyses together with recommended design fatigue factors are presented in this paper.

Fracture Control — Offshore Pipelines: Probabilistic Fracture Assessment of Surface Cracked Ductile Pipelines Using Analytical Equations

Since modern pipelines usually display ductile fracture behaviour, fracture assessments accounting for ductile tearing should be used. In this work we use a simplified strain-based fracture mechanics equation in the probabilistic fracture assessments. Furthermore, we use the traditional tangency criterion between the crack driving force and the crack growth resistance, in calculation of the onset of critical ductile tearing. Additionally, two types of external load on the line-pipe are considered, namely strains due to external bending moments and internal pressure. We establish the probability of fracture for line-pipes with relevant diameter to thickness ratios, and thicknesses, for J-laid or S-laid offshore pipelines. The distinction between system effects, in which all defects are likely to be subject to the same loading, and cases where only a small part of the pipeline will experience high loading, is also discussed.Copyright

Probabilistic Modelling for Reliability Analysis of Jackets

Experience from recent reliability analyses of jacket platforms is used to discuss selected aspects of probabilistic modelling in more detail. These modelling details can have a significant effect on the computed reliabilities. An overview of basic considerations and failure modes in jacket reliability analysis is included to set the various details into context. Ultimate limit states for jackets in relatively shallow water are emphasised; i.e. quasi-static structural response is applicable. The following topics are considered: (a) Failure modes and some requirements to load and resistance analysis. (b) Directionality in loading and resistance. (c) Random periods of individual extreme waves. (d) Foundations — axial and lateral capacity modelling for multiple piles and model uncertainty for pile capacity.Copyright

Evaluation of Free Spanning Pipeline Design in a Risk Based Perspective

The Ormen Lange subsea pipeline shall be designed to meet a specified risk acceptance criterion, established by consideration of failure probability and consequences of failure. Traditional design for Vortex Induced Vibrations (VIV) of free spans limits the allowable free span length and implies that interventions work may be required. Through a risk based approach the probability of fatigue failure of free spanning pipelines is quantified, and the governing uncertainties identified. A sensitivity analysis of different risk control options is performed. The outcome facilitates to focus in the design process such that a preferred design solution can be identified and implemented via testing campaign in the design stage, prelaid rectification activities and inspection programs. The aim is to obtain a cost efficient design that comply with the given acceptance criterion. Best practices as reflected in DNV-RP-F105 “Free Spanning Pipelines” and updated field specific design guidelines form the basis for the analysis. A probabilistic module is implemented on top of DNV-RP-F105 methodology, which allows application of a dedicated uncertainty modeling for a specific project. Parameters considered include: Pipeline properties, effective axial force, span length and gap, soil properties, ocean current (distribution, depth and directional variation), multiple mode response analysis (VIV response models, natural frequencies, damping, effect of concrete, static deflection), different SN curves, strakes and monitoring. Both the As Laid phase and the Operational phase are considered for different locations along the pipeline route.Copyright

Assessment of Ageing Structures: Case Studies

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).Copyright

Coating Degradation Models: Cost Optimisation of Inspection, Maintenance and Repair of FPSO’s

Corrosion control design and management for a new build Floating Production Storage and Offloading installation (FPSO) operating in certain benign regions, such as West Africa, China and Brazil, can provide significantly increased challenges compared to their North Sea counter parts. Even though there are over 100 FPSO’s operating worldwide, designing and implementing a cost optimal Inspection, Maintenance and Repair (IMR) system for a 20 year service still remains a major challenge. Primarily this is due to the fact that there is limited information available to facilitate the corrosion control design for a 20 year continuous service. Therefore, it is difficult to select a cost effective corrosion control design that addresses both the fabrication and operational aspects. This paper describes a probabilistic model for coating degradation and its application for implementing IMR for long year service of FPSO’s. The model can be utilized as a tool for planning of inspection and maintenance of the corrosion protective coatings and also estimate the associate costs. This enables the operator to model various scenarios for future inspection and maintenance work and thereby select cost optimal solutions for the given FPSO requirements. The paper will also demonstrate the proposed model through a realistic case study.Copyright

Reliability Reassessment of a Jacket Platform With Gas Seepage in the South China Sea

A structural reliability analysis is carried out on a jacket platform in 75 m water depth, in the South China Sea. A platform collapse failure mode is considered, with emphasis on uncertain soil conditions around the pile foundations, due to gas seepage while the platform has been in service. Random environmental conditions due to wind, waves and current are taken into account, based on observed data. Allowance for the short duration of environmental measurements is included and has a marked effect on the results. Two response surfaces are applied in the reliability calculation, to model the loads and the system capacity.Copyright

Probabilistic Inspection Optimization of Free-Span Surveys for Subsea Gas Pipelines

Probabilistic methods have been used to develop the basis for free-span inspection of a gas pipeline system in the South China Sea. The objective of the probabilistic analysis was to study the probability of fatigue failure associated with postulated planar flaws in the HAZ of repair welds performed on some of the girth welds. The impact of flaws on the fatigue life under different free-span conditions were studied. Conventional free-span analysis involves computation of allowable free-span lengths based on onset of in-line vibrations and does not normally consider fatigue crack growth. To consider the effect of the weld flaws on the failure probability a combined probabilistic fatigue and fracture model is required. For the particular pipelines analysed automatic ultrasonic testing (AUT) was used replacing the conventional radiography of the girth welds. Conservatism in the free-span assessment can then be significantly reduced by taking into account detailed flaw sizing information from the AUT. The inspection records provide distribution of flaw height, length and position. Combined with information on current distribution, material strength and fracture toughness distribution, a detailed probabilistic fatigue crack growth and unstable fracture assessment can be conducted as per the Det Norske Veritas (DNV) 2000 Rules for Submarine Pipeline Systems [1] using the response models of the DNV Guideline 14 for free-span analyses [2]. The objective of this analysis is to estimate the critical free-span lengths and the time for fatigue cracks to penetrate the pipe wall.Copyright

A New Recommended Practice for Inspection Planning of Fatigue Cracks in Offshore Structures Based on Probabilistic Methods

During the last 30 years a methodology for planning in-service inspection of fatigue cracks based on probabilistic methods has been developed. Due to the nature of the fatigue phenomena it is well known that minor changes in basic assumptions can have significant influence on the predicted crack growth lives. Calculated fatigue lives are sensitive to input parameters using standard design analysis procedures. Calculated probabilities of fatigue failure using probabilistic methods are even more sensitive to the analysis methodology and to input parameters to the analyses. Fracture mechanics analysis is required for prediction of crack sizes during service life in order to account for probability of detection after an inspection event. Analysis based on fracture mechanics needs to be calibrated to that of fatigue test data or S-N data. Thus, use of these methods for planning inspection requires considerable education and engineering skill. Therefore the industry has asked for guidelines that can be used to derive reliable inspection results using these methods. DNV has during the last years performed a joint industry project on use of probabilistic methods for planning in-service inspection for fatigue cracks in offshore structures. The recommendations from this project are now being included in a DNV Recommended Practice. The main background for this document is presented in this paper.Copyright