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Method for assessing reliability requirements of a safety instrumented control function

  • US 7,480,536 B2
  • Filed: 09/21/2006
  • Issued: 01/20/2009
  • Est. Priority Date: 09/21/2006
  • Status: Active Grant
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1. A method for analyzing a system for safety to personnel, said method comprising:

  • a) segmenting a product into subcomponents for hazard review;

    b) identifying at least one operating parameter of a first subcomponent of said product;

    c) identifying an inherent hazard of said first subcomponents based on an analysis of the at least one operating parameter;

    d) identifying features of a structure or operation of the subcomponent corresponding to the inherent hazard;

    e) identifying modifications or controls for the identified features which would mitigate the inherent hazard;

    f) prioritizing the identified features with respect to an effect that each of said features has on safety of the product;

    g) identifying current documentation that defines the structure or operation of the subcomponent;

    h) including in the current documentation, a safety audit procedure that identifies one or more of said prioritized features for inspection, and i) determining whether an unsafe condition could result from the inherent hazard after step (e); and

    wherein an unsafe condition has been determined, further conducting a hazardous operation review comprising;

    j) identifying at least one contributing factor to the unsafe condition, where said factors are selected from a group comprising at least one of;

    a design deviation of the subcomponent, an operating mode of the subcomponent, and a mode of personal interaction with the subcomponent;

    k) generating a matrix correlating the identified features and the contributing factors, wherein the matrix identifies the at least one contributing factor corresponding to each of the identified features;

    l) creating a hazardous operation table that identifies for each of said identified features a cause of the corresponding contributing factor and the modifications and controls to mitigate the hazard;

    m) determining a risk of the hazard based on a severity level of the unsafe condition corresponding to the hazard and a likelihood of an occurrence of the hazard;

    n) if the risk exceeds a predetermined level, identifying further modifications or controls for the identified features which would mitigate the inherent hazard, and then repeating the determination of risk step until the risk is no greater than the predetermined value or no further modifications or controls are identifiable; and

    further comprising an accident scenario review (ASR), if after step (n) the severity exceeds the predetermined level, said accident scenario review comprising;

    o) identify one or more of the inherent hazards contributing to the unsafe condition;

    p) generate a logical path of nodes in the ASR leading from the identified inherent hazards to an accident occurring due to the unsafe condition, wherein the logical path is generated using the hazardous operations table;

    q) identify the nodes of the logical path that, if avoided, would prevent the accident;

    r) for each identified node, assign a likelihood level of a probability that the event will occur, if the node includes a safety instrumented control function, set the failure probability of the safety instrumented control function equal to 1, thereby disabling a risk reduction for the safety instrumented control function; and

    s) if the likelihood level for proceeding through the scenario to a final unsafe condition exceeds a predetermined tolerable risk threshold, then identifying at least one of modifications or additional safety instrumented control functions that would mitigate the inherent hazard;

    t) for each safety instrumented control function assess the risk reduction factor requirements to achieve the predetermined tolerable risk by calculating a ratio between a tolerable risk threshold value and said level for proceeding through the scenario to the final unsafe condition, wherein the ratio represents a required risk reduction factor for the safety instrumented function, thereby defining a probability of failure requirements for the safety instrumented control function.

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