OS-C501

Recommendations

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Offshore Standard DNV-OS-C501, November 2013 Sec.2 Design philosophy and design principles – Page 18 1 General 1.1 Objective SECTION 2 DESIGN PHILOSOPHY AND DESIGN PRINCIPLES 1.1.1 The purpose of this section is to identify and address key issues which need to be considered for the design, fabrication, and operation of FRP components and structures. Furthermore, the purpose is to present the safety philosophy and corresponding design format applied throughout this Standard. 2 Safety philosophy 2.1 General 2.1.1 An overall safety objective is to be established, planned and implemented covering all phases from conceptual development until abandonment of the structure. 2.1.2 This Standard gives the possibility to design structures or structural components with different structural safety requirements, depending on the Safety Class to which the structure or part of the structure belongs. Safety classes are based on the consequence of failures related to the Ultimate Limit State (ULS). 2.1.3 Structural reliability of the structure is ensured by the use of partial safety factors that are specified in this Standard. Partial safety factors are calibrated to meet given target structural reliability levels. Note that gross errors are not accounted for. Gross errors have to be prevented by a quality system. The quality system shall set requirements to the organisation of the work, and require minimum standards of competence for personnel performing the work. Quality assurance shall be applicable in all phases of the project, like design, design verification, operation, etc. 2.2 Risk assessment 2.2.1 To the extent it is practically feasible, all work associated with the design, construction and operation shall ensure that no single failure is to lead to life-threatening situations for any persons, or to unacceptable damage to material or to environment. 2.2.2 A systematic review or analysis shall be carried out at all phases to identify and evaluate the consequences of single failures and series of failure in the structure such that necessary remedial measures may be taken. The extent of such a review is to reflect the criticality of the structure, the criticality of planned operations, and previous experience with similar structures or operations. Guidance note: A methodology for such a systematic review is the Quantitative Risk Analysis (QRA) which may provide an estimation of the overall risk to human health and safety, environment and assets and comprises (i) hazard identification, (ii) assessment of probability of failure events, (iii) accident development and (iv) consequence and risk assessment. It should be noted that legislation in some countries requires risk analysis to be performed, at least at an overall level to identify critical scenarios, which may jeopardise the safety and reliability of the structure. Other methodologies for identification of potential hazards are Failure Mode Effect Analysis (FMEA) and Hazardous Operations studies (HAZOP). 2.3 Quality assurance ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 2.3.1 The safety format of this Standard requires that gross errors (human errors) shall be controlled by requirements to the organisation of the work, competence of persons performing the work, verification of the design and Quality Assurance during all relevant phases. 3 Design format 3.1 General principles 3.1.1 The basic approach of the Limit State Design method consists in recognising the different failure modes related to each functional requirement and associating to each mode of failure a specific limit state beyond which the structure no longer satisfies the functional requirement. Different limit states are defined, each limit state being related to the kind of failure mode and its anticipated consequences. 3.1.2 The design analysis consists in associating each failure mode to all the possible failure mechanisms (i.e. the mechanisms at the material level). A design equation or a failure criterion is defined for each failure mechanism, and failure becomes interpreted as synonymous to the design equation no longer being satisfied. DET NORSKE VERITAS AS
Offshore Standard DNV-OS-C501, November 2013 Sec.2 Design philosophy and design principles – Page 19 3.1.3 The design equations are formulated in the so-called Load and Resistance Factor Design (LRFD) format, where partial safety factors (load factors and resistance factors) are applied to the load effects (characteristic load values) and to the resistance variables (characteristic resistance values) that enter the design equations. 3.1.4 The partial safety factors, which are recommended in this Standard, have been established such that acceptable and consistent reliability levels are achieved over a wide range of structure configurations and applications. 3.1.5 This section discusses the limit states that have been considered relevant for the design of structures made of FRP materials, presents the underlying safety considerations for the recommended safety factors and finally introduces the adopted LRFD format. 3.1.6 As an alternative to the LRFD format a recognised Structural Reliability Analysis (SRA) may be applied. The conditions for application of an SRA are discussed at the end of this section. 3.2 Limit states 3.2.1 The following two limit state categories shall be considered in the design of the structure: — Ultimate Limit State (ULS) — Serviceability Limit State (SLS). 3.2.2 The Ultimate Limit State shall be related to modes of failure for which safety is an issue. The ULS generally corresponds to the maximum load carrying capacity and is related to structural failure modes. Safety Classes are defined in accordance with the consequences of these failure modes on safety, environment and economy. The ULS is not reversible. 3.2.3 The Serviceability Limit State should be related to failure modes for which human risks or environmental risks are not an issue. The SLS is usually related to failure modes leading to service interruptions or restrictions. Service Classes are defined in accordance with the frequency of service interruptions due these modes of failure. The SLS is usually reversible, i.e. after repair or after modification of the operating conditions (e.g. interruption of operation, reduction of pressure or speed) the structure will again be able to meet its functional requirements in all specified design conditions. Guidance note: Ultimate Limit States correspond to, for example: - loss of static equilibrium of the structure, or part of the structure, considered as a rigid body - rupture of critical sections of the structure caused by exceeding the ultimate strength or the ultimate deformation of the material - transformation of the structure into a mechanism (collapse). - loss of stability (buckling, etc…) Serviceability Limit States corresponds to, for example: - deformations which affect the efficient use or appearance of structural or non-structural elements - excessive vibrations producing discomfort or affecting non-structural elements or equipment - local damage (including cracking) which reduces the durability of the structure or affects the efficiency or appearance of structural or non-structural elements. 3.3 Safety classes and service classes ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 3.3.1 Safety classes are based on the consequences of failure when the mode of failure is related to the Ultimate Limit State. The operator shall specify the safety class according to which the structure shall be designed. Suggestions are given below. 3.3.2 Safety classes are defined as follows: — Low Safety Class, where failure of the structure implies low risk of human injury and minor environmental, economic and political consequences. — Normal Safety Class, where failure of the structure implies risk of human injury, significant environmental pollution or significant economic or political consequences. — High Safety Class, where failure of the structure implies high risk of human injury, significant environmental pollution or very high economic or political consequences. 3.3.3 Service classes are based on the frequency of service interruptions or restrictions caused by modes of failure related to the Serviceability Limit State. These modes of failure imply no risk of human injury and minor environmental consequences. The operator shall specify the service class according to which the structure shall be designed. Suggestions are given below. DET NORSKE VERITAS AS
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OS-C501

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