2.3. Evaluation of consequences and making decision

The effects on the system attributable to hazardous event are defined, and Markovian process is used to model the probability of changes during offloading operation that could cause changes in the risk profile developed in step 2. The decision-making theory is used to evaluate the possibility of emergency disconnection during the operation given the result of Markovian process.

The probability of the system on "i state" is calculated as an approximate discrete model, based on that for small steps (Δθ toward zero) with recurrent algorithm. Assumed two states,

Decision-Making Model for Offshore Offloading Operations Based on Probabilistic Risk Assessment

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2. Declare probability distribution of the initial state. In this case it is assumed that shuttle tank begins the offloading operation in operative zone: P1(θ<sup>k</sup> = 0) = 1 and

The Markovian process shows the probability that the position of shuttle tanker will change from operational zone to alert zone in each environmental condition. That change affects the decision of continuing the offloading operation. The decision-making theory can be used to evaluate the need for disconnection in the case of occurrence of an environmental change

The method is applied on the analysis of the offloading operation, when the crude oil is transported to shore by shuttle tankers through an offloading arrangement with the use of a shuttle tanker with dynamic positioning systems (DP). From the point of view of the shuttle tanker, tandem offloading operation can in principle be summarized into the following five operational stages [15]: (1) approach, tanker approaches FPSO and stops at a predefined distance; (2) connection, messenger line, hawser, and loading hose are connected; (3) loading, oil is transferred from FPSO to tanker; (4) disconnection, manifold is flushed, and loading hose and hawser are disconnected; and (5) departure, tanker reverses away from FPSO while sending back hawser messenger line and finally sails away from oil field. In the first stage, the shuttle tanker approaches FPSO, at a maximum speed of 1.5

the basic steps of the procedure are:

P2(θ<sup>k</sup> = 0) = 0.

3. Select time steps (Δθ).

5. Calculate θk= θk-1+ Δθ.

9. Pð Þ¼ θ<sup>k</sup> Mk∙Pð Þ θ<sup>k</sup>�<sup>1</sup>

1. Declare initial variable counter k=0, θ<sup>k</sup> = 0, andθ end.

4. Save tk, P1(θk), P2(θk), and increment counter: k = k + 1.

6. Calculate state transition rates (pijk(θ)) for θ=(θk-1 + θk)/2.

coupled to a critical component failure in the shuttle tanker.

4. Application of the methodology

8. Calculate probability of the system state i at tk as:

7. Calculate transition matrix Mk for transition rates of step 4 using Eq. (5).

3. Return to step 4: The procedure continues until t = tend

Consequences of hazardous events or abnormal incidents on the shuttle tanker and offloading operation are described and explained. A severity numerical scale is defined for hazardous event classification. This scale was defined for three sets—safety of personal, facilities, and environment—the first is related to the damages or the lesions that can be caused to the employees and others, the second refers to damages in equipment or installations in shuttle tanker or FPSO, and the third is associated with the damages on fauna, flora, and ecosystem. That classification is presented in Table 2.

The risk is the combination between the failure probability and the severity magnitudes [20]. The decision-making part is related with accepting a certain risk scenario. The decisionmaking theory is used to evaluate the possibility of emergency disconnection during the operation. The risk is associated with an uncertain event or condition that, if it occurs, has a negative effect on system operational condition.
