Probabilistic analysis of risk and mitigation of deepwater well blowouts and oil spills

Abstract

The development of robust risk assessment procedures for offshore oil and gas operations is a major element for the assessment of the potential feedback between planned activities and the environment. We illustrate a methodological and computational framework conducive to (1) a quantitative risk analysis of deepwater well barrier failures and subsequent hydrocarbon release to the environment and (2) the analysis of the value of the deployment of conventional and/or innovative mitigation measures. Our methodological framework is grounded on historical records and combines the use of Dynamic Event Trees and Decision Trees from which we estimate probability of occurrence and impact of post-blowout events. Each sequence of response actions, which are undertaken immediately after the event or in the subsequent days, is considered within the context of appropriately structured event paths. This approach is conducive to an estimate of the expected value of key decisions and underlying technologies, with an emphasis on their potential to reduce the oil spill volume, which can critically impact the environment. Our study yields an original comparative analysis of diverse intervention strategies, and forms a basis to guiding future efforts towards the development and deployment of technologies and operating procedures yielding maximum benefit in terms of safety of operations and environmental protection.

Appendices

Appendix 1

Duration of activities and interventions (Part 1)

Interventions and activities	Mean ≡ Mode [d]	Min [d]	Max [d]
Site Preparation	11	5	17
Mobilize Intervention Workover Control System Hardware	3	1	5
Mobilize IWOCS Vessel	3	1	5
IWOCS System and Vessel in Field	8	4	12
Debris Removal	16	8.5	23.5
Mobilize Debris Removal Hardware	5	3	7
Transport Debris Removal Hardware to Field	8	4	12
Mobilize Vessel with ROV and Construction Support	3	1	5
Vessels in Field	8	4	12
Debris Removal Plan developed	2	2	2
Debris Removal Activities	5	3.5	6.5
Rapid CUBE	17	11	23
Mobilize CUBE and Necessary Equipment	3	1	5
Transport CUBE by Airplane	1	1	1
Mobilize Support Vessels and Well Testing Equipment	5	3	7
Preparation for Offshore Transportation	1	1	1
Transport CUBE on Site	8	4	12
Mobilize required consumables (chemicals)	2	0	4
Mobilize MODU and arrival to Location	11	5	17
Make Up and Run CUBE on Riser	1	1	1
Position Acquisition and Flow Rate Adjustment	1	1	1
Early Capture to the Intervention Vessel (Drilling Rig)	1	1	1

Duration of activities and interventions (Part 2)

Interventions and activities	Mean ≡ Mode [d]	Min [d]	Max [d]
Well Capping (Shut Only Scenario)	24	18	30
Mobilize Tested Well Cap System	7	5	9
Transport Well Cap System by Airplane	1	1	1
Transport Well Cap System to Dock	2	2	2
Mobilize Intervention Vessel	5	3	7
Preparation for Offshore Transportation	1	1	1
Transport Well Cap to Site	8	4	12
Offload Well Cap and Prepare for Deployment	1	1	1
Deploy Well Cap in Safe Zone	2	2	2
Position, Land and Test Well Cap	1	1	1
Shut In Well	1	1	1
Monitor Well for Stability	1	1	1
Preparatory activities for Well Kill (BSSI, CS)	18	10	26
Mobilize Pumping Services and Equipment	3	1	5
Mobilize MODU and arrival to Location	11	5	17
Mobilize Test package to the drilling rig and Test	4	2	6
Approach the well head with the auxiliary line	2	2	2
Debris Removal (BOP choke/kill line inlet)	2	2	2
Connect the Auxiliary Line with Pumping Unit	1	1	1
Blowstop (Secondary Intervention)	21	13	29
Wait for the auxiliary line	0	0	0
Debris Removal (BOP choke/kill line inlet)	2	2	2
Mobilize Blowstop Equipment	5	3	7
Mobilize Blowstop equipment to dock	5	3	7
Mobilize Blowstop equipment on site	8	4	12
Prepare the Injection System on MODU	1	1	1
Bullets Injection	0	0	0
Coating Swelling	1	1	1
Static Well Killing (Well Capped)	28	22	34
Mobilize Pumping Services and Equipment	3	1	5
Mobilize MODU and arrival to Location	11	5	17
Mobilize Test package to the drilling rig and Test	4	2	6
Perform Well Kill after Capping Stuck Shut In	4	4	4
Relief Well and Dynamic Killing	71	49.5	92.5
Mobilize Drilling Equipment	9	7	11
Mobilize MODUs and arrival to Location	13	7	19
Drilling Relief Well	45	31.5	58.5
Perform Well Kill and Abandonment Scope	4	4	4

Appendix 2

2.1 Flow chart for risk evaluation methodology and factors contributing to expected blowout risk

We illustrate here the main steps leading to expected blowout risk evaluation with the aim of evidencing the contribution of the various model components.

We do so by considering in Fig. 11 an exemplary setting corresponding to the decision of deploying all intervention techniques in case of occurrence of the top event (well blowout). We illustrate the example of the Event Tree for the Annulus flow-path in case of dynamically positioned rigs. We consider one given path inside the event tree for which the applicability of all techniques (at the moment of their application) is consistent with the actual conditions of the spill and the rig. Note that probability values shown in the grey circles of Fig. 11 are solely to be considered as representative of the actual statistics associated with given events.

Fig. 11

Flow chart for risk evaluation methodology and factors contributing to expected blowout risk in an exemplary setting

All of the events, each with its probability of occurrence, are listed on the left hand side of the chart in Fig. 11. Cause-effect relationships between events, applicability of interventions and their success are displayed following the assumptions considered in Sects. 2.3.1 and 2.3.2.

The right hand side of the diagram is devoted to the calculation of the impact in terms of blowout flow rates, their duration and of their final product, leading to quantifiable Oil Spill Volumes (OSV). Figure 11 only shows the events that cause a modification of blowout flow rate, either increasing or decreasing it (i.e., post blowout interventions). Oil spill volumes are then summed up to obtain an overall final spill volume for the considered branch of the Event Tree. The resulting value of the expected blowout risk for the considered decision is hence calculated as a sum of probabilities associated with each branch, multiplied by the corresponding oil spill impact. Note that the value included in the diagram necessarily coincides with the expected value for Long Term Future scenario shown in Fig. 9a with a blue bar.

Appendix 3

Assumptions considered on mode, minimum and maximum values of times of activities

Interventions and activities	Mean ≡ Mode [d]	Min [d]	Max [d]
Mobilization of equipment or VoO (fast)	1	3	5
Mobilization of equipment or VoO (medium)	3	5	7
Mobilization of equipment or VoO (slow)	5	7	9
Arrival of VoO to incident site	4	8	12
Debris removal activities on site	3.5	5	6.5
Relief well drilling activities	31.5	45	58.5