Incorporating human and organizational factors in risk analysis for offshore installations

This paper presents a methodology for quantitative risk analysis on oil and gas production platforms that incorporates both technical and operational conditions. The basic building blocks of the methodology are barrier block diagrams, event trees, fault trees, and influence diagrams. Barrier block diagrams are used to illustrate the event scenarios and the effect of barrier systems on the scenarios. Event trees are used in the quantitative analysis of the scenarios, while fault trees are used to analyze the performance of the different barrier systems. Influence diagram are used to analyze the effect of risk influencing factors on the initiating events in the event trees and the basic events in the fault trees. The intention of the analysis is to reflect installation specific factors both with respect to technical systems, operational conditions as well as human and organizational factors. 1.3 Delimitations and assumptions Due to the extensive scope and complexity of the total BORA project, some delimitations and assumptions are made; a) This paper deals primarily with the containment barrier function, b) The BORA methodology is illustrated for a part of the release scenario “Release due to valve(s) in wrong position after maintenance” (called case example), c) Corrective action is performed when failures are revealed since only the detection is illustrated in the barrier block diagrams. 2 THE BORA METHODOLOGY The BORA approach for incorporation of human, operational and organizational factors in quantitative risk analysis is described in the following subsections. The BORA approach consists of the following steps: 1 Development of a basic risk model. 2 Assignment of industry average frequencies/probabilities of initiating events and basic events. 3 Identification of risk influencing factors (RIFs) and development of risk influence diagrams. 4 Assessment of the status of RIFs. 5 Calculation of industry average frequencies/probabilities of initiating events and basic events. 6 Calculation of installation specific risk, incorporating the effect of technical systems, technical conditions, human factors, operational conditions, and organizational factors. 2.1 Development of a basic risk model The basic building blocks of the BORA model are barrier block diagrams, event trees, fault trees, and influence diagrams. Barrier block diagrams are used to illustrate the event scenarios and the effect of barrier systems on the event sequences and consist of initiating events, barriers aimed to influence the event sequence in a desired direction, and possible outcomes of the event sequence. Event trees are used in the quantitative analysis of the scenarios. The performance of the safety barriers are analyzed by use of fault trees. These fault trees are linked to the event trees by use of the computer package RiskSpectrum. Influence diagrams are used to analyze how the RIFs affect the initiating events in the event trees and the basic events in the fault trees. Existing models from present QRAs have been taken as a starting point for the BORA models. However, the existing QRA models need to be extended and refined. Traditionally, the event modeling in QRA’s has started with loss of containment as the initiating event, and the barriers aimed to limit the consequences of the release have been modeled. Causal analysis of hydrocarbon releases has normally not been included, since the assessment of hydrocarbon (HC) leaks has been related to generic frequency analysis. As mentioned earlier, this paper mainly deals with modeling of the containment function (or "prevent release of hydrocarbons"). However, in the overall BORA risk model, the accident scenarios are further developed, and the effect of the consequence reducing barriers are taken into account (Vinnem et al, 2004). Sklet & Hauge (2004) describes some 20 representative release scenarios that have been modeled by use of barrier block diagrams. Each barrier block diagram comprises the following: − An initiating event, i.e. a deviation from the normal situation which may cause a release of hydrocarbons. − Barrier systems aimed to prevent release of hydrocarbons. − The possible outcomes of the event sequence, which depend upon the successful operation of the barrier system(s). The barrier block diagram for the release scenario “Release due to valve(s) in wrong position after maintenance” is illustrated in Figure 1. Figure 1. Barrier block diagram for one release scenario. As seen in Figure 1, several of the barriers are non-physical by nature, thus requiring human and operational factors to be included in the risk model. For a complete description of each release scenario, reference is made to Sklet & Hauge (2004). In order to perform a quantitative risk analysis, three main types of events need to be quantified: 1 The frequency of the initiating event, i.e. in the example case: “The frequency of valve in wrong position after maintenance”. 2 The probability of failure of the barrier systems, which for the example case includes: i) Failure to reveal valve(s) in wrong position after maintenance by self control/use of checklists, ii) Failure to reveal valve(s) in wrong position after maintenance by 3 party control of work, and iii) FailValve(s) in wrong