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Application of the suggested ice strength coefficients in ISO 19906 to intermittent crushing
No full textFor offshore wind turbines on monopile or jacket foundations without ice cones, one of the relevant design load cases is that of ice floes or level ice crushing against the structure resulting in ice-induced vibrations. In relation to that design load case, a relevant question is which ice strength coefficient to use in the crushing formula in ISO 19906 for determining design peak loads during intermittent crushing. Despite the guidelines in the standard being relatively clear on this matter, there often exists uncertainty regarding if and how to account for velocity effects and compliance effects when defining the ice strength coefficient CR. Ice tank tests were recently conducted to investigate the dependence of global peak loads on far-field ice speed for both rigid and compliant structures. Those tests revealed that the compliance effect and velocity effect on the global loads originate from the same strengthening effect in the ice. As a consequence, the absolute global loads on the rigid structure and compliant structure did not differ significantly. Applying these results to the challenge of defining the ice strength coefficient for intermittent crushing, it can be stated that if the velocity effect is accounted for in the ice strength coefficient, then there is no need for further increase due to compliance of the structure. ISO 19906 provides some suggested values for the ice strength coefficient which include provisions for the velocity effect and can therefore be directly applied to determine the peak loads during intermittent crushing, as the standard also suggests.Offshore Engineerin
Reliability, Limit States and Action Factors for ISO 19906
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Abstract
The new international code ISO 19906 "Petroleum and natural gas industries - Arctic offshore structures" [1] was published in December 2010 after more than nine years of development. ISO 19906 uses the limit states design approach. Compliance is based on ensuring that the action effects arising from each factored action combination do not exceed factored resistances. The partial action factors have been calibrated to reliability targets. This paper describes the philosophy behind the quantification and use of the reliability targets, the definition of the limit states and action combinations, the philosophy of principal and companion environmental actions, and how the calibrated values of the action factors were incorporated in the action combinations.
Introduction
The new ISO 19906 has been developed in Working Group 8 (WG8) of ISO Technical committee TC67/SC7 "Offshore structures". Experts from all arctic nations and from most other nations involved in the design of offshore structures in arctic and cold regions participated in WG8. Consequently, all the arctic nations and all European nations have undertaken to adopt ISO 19906 into their sets of national standards, and ISO 19906 is expected to be adopted worldwide for design of arctic offshore structures. This paper explains the design approach and underlying structural reliability levels in order to assist with application of the code in different ice covered environments and regulatory jurisdictions.
Reliability targets for arctic offshore structures have been used to calibrate the values of partial action factors for use in limit states action combinations, with particular focus on environmental actions arising from sea-borne ice, see Maes and Thomas [2] and OGP report 422 [3]. If these partial action factors are applied to Ultimate Limit State (ULS) and Abnormal (Accidental) Limit State (ALS) action combinations and the resulting action effects do not exceed factored resistances, the reliability targets are deemed to be achieved
Reliability Targets
"Reliability Target" as used in ISO 19906 is expressed as an annual probability of failure which is associated with global structural failure or with component failure leading to global structural failure. In common with the other ISO codes for offshore structures, namely ISO 19900 [4] and its associated standards, commonly known as the ISO 19900-series, ISO 19906 considers both life-safety and environmental protection through specifying three exposure levels, L1, L2 and L3, as defined in ISO 19902. [5].
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Conical structures in ice: Relevant relationships for ISO 19906
No full textConical shapes are often chosen for structures that are exposed to floating ice, such as offshore drilling platforms, bridge piers and offshore wind turbine foundations. The flexural ice failure promoted by the shape of a cone can lead to lower forces on the structure than compressive failure, which would take place if ice is to encounter a vertical surface. In spite of the wide use of conical structures, many aspects of their performance in ice covered waters remain poorly understood. For example, the ISO 19906 Arctic Offshore Structures standard does not provide guidance with respect to ridge keel loading on cones, or the height to which ice could ride up on a conical structure. With the planned revision of that document presently beginning, it is important to address the guidance gaps in order to include new insights in the revised standard. Previous work by the authors employed a numerical model of ice dynamics in order to predict ice failure patterns and forces on various conical structures. The present work extends those studies and predicts the extent of ice ride-up on the structures; the roles of the waterline width of the structure and ice thickness are discussed. The resulting ice force estimates are comparable to those predicted by an approach recommended in ISO 19906. Both the plastic and elastic beam-bending methods recommended by ISO 19906 for determining ice actions on conical structures require an initial assumption of the maximum height of accumulated rubble. Little guidance is given in ISO 19906, except to note that this height depends on the structure geometry and ice regime. For the present calculations, the chosen rubble height for each ice load calculation is based on the results of the corresponding numerical model run. Previous numerical studies by the authors have also been reviewed and the estimates of ice ride-up heights summarized. Numerical simulations could be expanded to produce a comprehensive range of ride-up values to support users of ISO 19906.Peer reviewed: YesNRC publication: Ye
Overview of Case Studies for Draft ISO 19906
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Abstract
The new international code ISO 19906 "Petroleum and natural gas industries - Arctic offshore structures" [1] was published in December 2010 after more than nine years of development, see Spring et al. [2]. Under the ISO process for standards development, ISO/DIS 19906 (the draft for public comment) was published in January 2009 and distributed to all ISO member countries for review and comment by June 2009. During this five-month period the opportunity was taken to perform case studies to ensure that the provisions of the code and its references were sufficiently complete for an experienced and knowledgeable engineer to design the structures within its scope. The objectives were to identify errors and gaps, ensure that diverse structures were adequately addressed, and to collate comments to the ISO draft for consideration by the Technical Panels with delegated responsibility for the provisions as they prepared the Final Draft. This paper sets out the case studies performed and the improvements arising in the final document resulting from comments provided by the organisations performing the case studies.
Introduction
The new ISO 19906 has been developed in Working Group 8 (WG8) of ISO Technical committee TC67/SC7 "Offshore structures". Experts from all arctic nations and from most other nations involved in the design of offshore structures in arctic and cold regions participated in WG8. Consequently, all the arctic nations and all European nations have undertaken to adopt ISO 19906 into their sets of national standards, and ISO 19906 is expected to be adopted worldwide for design of arctic offshore structures. WG8 and its supporting industry therefore decided to initiate a programme of case studies both to validate the standard and to start the process of industry engagement.
WG8 established TP10 "Case studies and calibration" in June 2008 to execute a calibration study and to perform case studies on ISO/DIS 19906. The members were volunteers from around the world, many from the oil and gas companies who would be future owners and operators of arctic offshore structures..
TP10 defined eight cases for study. Each case was for a specific structure type in a specified arctic region, with key parameters defined but otherwise it was left to the performer of the study to bring their experience and knowledge to complete the scenario. Proposals were sought from organizations around the world, including consultants, design contractors, universities, research institutes and classification societies. Forty seven offers were received, from North America, Europe and Russia, of which fifteen were contracted using industry funding and one was offered at no cost TP10 held five meetings and corresponded extensively by email and telephone conference calls. The OGP (the International Association of Oil and Gas Producers) facilitated funding from oil companies participating in WG8 and TP10 to fund TP10's activity. The OGP also acted as the administrator for the contracts required to execute the work directed by TP10.
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Ice Actions in ISO 19906 - Overview
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Abstract
Eighteen ice experts from nine countries were actively involved in the development of new guidelines for ice actions that are an essential part of the new standard ISO 19906. This paper provides an overview of the methods that are recommended for ice action determination.
This standard is based on the limit states design method. Therefore, guidelines are provided for ice actions in four different categories of limit states. Characteristic values of ice actions depend on the limit state concerned and also on the life-safety and consequence categories that are applicable to a structure.
As a general principle, design shall be carried out separately for global ice actions, local ice actions and for dynamic ice actions. Operational procedures may be used to reduce ice actions. Ice action scenarios need to be selected by taking account of the specific conditions in the ice regime for which a structure is designed.
Data from full scale measurements on instrumented structures were the basis for developing methods for calculating ice actions. These methods are recommended for the practical determination of design ice actions on offshore structures. Both probabilistic and deterministic methods can be used. For many action scenarios, it is useful to consider limit-stress, limit-energy and limit-force mechanisms as well as specific ice failure modes. ISO 19906 provides a variety of methods and formulas that consider these limiting mechanisms and ice failure modes for the determination of global ice actions. Three methods for determining local ice actions in different conditions are provided. Basic modes of dynamic ice actions are described and simple models are provided for vertical and conical structures.
1. Introduction
Several surveys of predicted ice actions have been carried out in recent years (Shkhinek et al. (1994); Croasdale (1994) and Timco and Croasdale (2006)). International experts were invited in these surveys to determine ice actions for a few basic scenarios. These studies have revealed little consensus and a wide spread in the predicted values of ice actions. This situation has been a good reason for the development of the new standard ISO 19906, which should significantly reduce the discrepancies in the prediction of ice actions.
The new guidelines of ice action determination are provided in the sub-clauses 8.2 and A.8.2 of the new standard. These clauses were written by an international group of eighteen experts from nine countries, as shown in Table 1. The resulting clause is the consensus of these experts.
It is well known that formulas on ice action can not be derived directly from first principles. Empirical factors that are obtained from experiments play a very central role in all ice load prediction. The new standard emphasizes the importance of full-scale data.
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Ice actions for hydraulic structures of primary flood defense Afsluitdijk - “Applying ISO 19906 at home”
No full textThe Afsluitdijk is one of the Dutch Delta programme landmarks. Afsluitdijk is a 32 km long primary flood defense, separating lake IJsselmeer from the Wadden Sea and protecting large areas of the Netherlands against flooding. Over the period spanning 2018 - 2022 the levee will be upgraded by the consortium Levvel (BAM, Van Oord and Rebel). Ice resistance is one of the aspects that is part of the engineering scope of the project. A detailed study on ice actions that apply to the various exposed structural elements is performed, specifically for the hydraulic structures that are integrated in the levee. Dutch regulations with respect to ice actions do specify some levels for ice actions but are not very specific regarding their range of applicability. Ice engineering experience from projects abroad is integrated with Dutch temperate winter conditions, to derive fit-for-purpose ULS and ALS ice actions. The analysis procedure is scenario-based, following the ISO 19906 (2nd ed) limit-stress, limit-force, limit-momentum based approach. This paper describes the study approach and the main results that are obtained
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Ice properties in ISO 19906's second edition
Get PDFThe second edition of ISO 19906 Arctic Offshore structures was issued in 2019. In this paper, we describe the changes that were made to sections dealing with ice properties and discuss the relationship between them and ice actions. The changes can be divided into five groups: 1) Physical properties (temperature, density and porosity), 2) modulus of elasticity, 3) ice friction, 4) mechanical properties in level ice (uniaxial and multi-axial compressive strength, flexural strength and borehole jack strength), and 5) the keel properties of first-year ridge (Mohr-Coulomb, macro-porosity). The standard is written in such a way so that simple approaches in design guidelines complement more elaborate models. Both currently face at least three challenges, namely, the lack of full-scale data, a complicated physical environment, and a lack of understanding of the deformation mechanisms taking place in the ice
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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