Offshore wind decommissioning: an assessment of the risk of operations

Figures & data

Table 1. Decommissioned offshore wind farms (4C Offshore 2020).

Country	Wind farm	Actual year of decommissioning	Estimated year of decommissioning based on design life
Denmark	Vindeby Offshore	2017	2011
Germany	Hooksiel	2016	2028
Netherlands	Lely	2016	2017
Sweden	Utgrunden I	2018	2020
	Yttre Stengrund	2015	2021
UK	Blyth	2019	2020
	Beatrice Demonstration	2016	2026

Figure 1. FTA symbols.

Figure 2. Fault tree diagram for top event (suspended/failed decommissioning operations).

Table 2. The rankings for the likelihood of occurrence.

Index	Probability
1	p < 0.00001
2	0.00001 ≤ p ≤ 0.0001
3	0.0001 ≤ p ≤ 0.01
4	0.01 ≤ p ≤ 0. 1
5	p ≥ 0. 1

Figure 3. Fault tree diagram for collision event (intermediate event).

Figure 4. Fault tree diagram for vessel damage event (intermediate event).

Figure 5. Fault tree diagram for crane failure event (intermediate event).

Figure 6. Collision event tree.

Figure 7. Vessel damage event tree.

Figure 8. Crane failure event tree.

Table 3. Frequency ranking of lifting and loading incidents.

Frequency Index	Intermediate events/top event	Probability
3	Collision	8.46 × 10^−4
3	Vessel damage	3.94 × 10^−3
4	Crane failure	6.42 × 10^−2
	Top event	6.9 × 10^−2

Figure 9. Importance ranking of basic events.

Figure 10. Sensitivity of basic events on top event.

Figure 11. Collision consequence scenario 2.

Figure 12. Vessel damage consequence scenario 3.

Figure 13. Crane failure consequence scenario 3.

Table 4. Mitigation and control measures for the intermediate events.

Intermediate event	Personnel	Asset (Vessel/WF asset)	Procedures/Regulations	Lifting configuration	Contingency planning	Reporting and investigation
Collision	Use of appropriate PPE	Choosing appropriate vessel size for WF access (perhaps standardised criteria)	Safety zone standards should be assessed for decommissioning operations. Where applicable it should be different from WF installation safety zones.	Lifting configuration should be selected based on age of structure and number of removal and transportation vessels available on site.	Financial contingency provisions should be made to address issues that may be caused by hazards.	Collision risks during decommissioning (and installation) operations should be adequately documented, recorded and investigated.
Vessel damage	Depending on causes of event, personnel trainings must incorporate improved methods to improve competence.	Determination of an appropriate accidental limit state. Vessel hardening by design.	Limiting operations during a threshold of environmental conditions.	More research into vessel response during different lifting configurations.	Number of emergency/evacuation vessels should be assessed based on the number of removal vessels and wind farm size/layout.	Investigation reports are made easily accessible for improved learning and development for different stakeholders.
Crane failure	Use of appropriate PPE. Promptness in response to move personnel into safe zone.	Replace any failed parts considering the cost and safety optimal level. Inspection records digitised to prevent errors in manual entries.	Ensure the appropriate certifications are obtained before each set of lifts.	Removing components in smaller modules depending and factoring the wind farm size into the chartered lift vessel.	Ensuring the right contract is drawn up for any financial liabilities.	Investigation reports are made easily accessible for improved learning and development for different stakeholders.

Table

Operation	Guideword(s)	Deviation/Hazards	Possible causes	Possible consequences
Planning and regulatory approval	Delay	Schedule delays	Poor weather conditions. Failures or breakdown of decommissioning equipment without backup.	Cost overrun risks.
	Delay	Delay in documentation submission/approval	Delay in obtaining the appropriate permits. Incomplete environmental impact assessment.	Schedule delays and cost overruns.
Preparation activities for removal (removal of hydraulic fluids, marine growth)	Confined space risks	Asphyxiation	Inadequate ventilation in confined spaced.	Personnel harm/fatalities.
		Injuries caused by accreted ice fragments	Extreme weather (cold)	Personnel injuries.
	Electrical hazards	Electrocution	Poor disconnection procedures. Personnel error.	Electrical burns. Other personnel injuries/fatalities.
	Spills	Spill of hydraulic fluid	Poorly secured hydraulic fluid receptacle. Poor procedures. Personnel error.	Additional cost/duration for clean up if above allowable limit.
	Dropped object	Dropped loads	Poorly fastened equipment	Additional cost to retrieve dropped load.
Vessel positioning	Impact	Vessel collision with other assets	Tug impact. Manoeuvre collision.	Vessel damage. Vessel flooding. Vessel capsize/sink. Collision with other assets. Structural damage of other assets.
	Drift	Loss of station-keeping	Faulty dynamic positioning (DP) system. Loss of power in vessel. Extreme environmental conditions (typhoon).	Structural failure. Vessel flooding. Loss of towline. Personnel injury.
	Stability	Loss of stability	Differential sticking of cut monopile. Loss of structural integrity.	Vessel capsizes or sink. Personnel injury/fatality.
	Ascent/Descent	Sinking of removal or transport vessel	Damaged vessel hull. Impact from other vessels. Dropped objects.	Loss of unit. Personnel injury/fatality.
	Ballasting	Ballast system failure	Electrical or mechanical fault. Centrifugal pump failure. Failure of ballast valves.	Delay in ballasting operations. Delay in removal operations.
	Operational fault	Failure of support equipment (tugs)	Mechanical failure. Electrical failure. Barge/tug operational failure.	Schedule delays. Cost overruns.
Personnel transfer/hoisting	Fall	Fall from height. Man overboard.	High winds. Broken hoist cable. Vessel impact. Exposure to dropped object. Spinning of blade when removed.	Personnel injury/fatality.
	Dropped load	Dropped tools/equipment	Poor attachment/fastening of tools.	Additional costs for retrieval.
Lifting and loading operations (blades, nacelle, hub, tower)	Overloading	Lifting crane overload	Inaccurate estimation of payload weight. Presence of significant amount of marine growth on payload.	Structural damage. Loss of vessel stability.
	Structural damage	Damaged lifting point. Hook stabilisation tool failure. Lifting clamp damage. Tugger wire failure.	Inadequate structural integrity inspection. Inaccurate estimates of residual anode thickness. Corrosion thinning at crevice of lifting point. Cyclic loading over the course of the asset life. Inadequate/poor inspection of lifting points.	Dropped objects. Personnel injury/fatality. Collision with nearby vessels or installation.
	Collision	Navigation failure	Loss of control. Faulty procedure. Violation of navigation procedures. Poor use of navigation equipment.	Loss of removal vessel positioning. Financial losses.
		Manoeuvring failure	Loss of control. Poor management of vessel manoeuvre. Failure of tug, barge or other transport vessel.	Collision can lead to damage of nearby assets.
		Detection failure	Poor personnel observation/attention.	Personnel injuries/fatalities.
	Impact	Dropped/swung objects and loads	Extreme weather (high winds). Structural failure. Loss of vessel station keeping. Fastening system not properly installed. Vessel movement during lift. Limited work area on vessel. Other vessel(s) on site. Compromised wire ropes.	Removed component(s) fall over. Personnel injuries/fatalities.
	Stability	Loss of stability	Uneven/poor deadweight distribution during lifting operations. Differential sticking of cut pile.	Impact. Personnel injury.
	Operational fault	Vessel or handling equipment. breakdown. Improper fastening of components.	Mechanical/electrical failure of generator and prime mover assembly. Human error.	Delay in operations.
	Operational fault	Mechanical hazards.	Contact with moving parts.	Personnel injury or fatality.
	Extreme weather conditions	High winds/waves.	Working in winter season.	Fall from height. Collision with tower or other machinery. Personnel injury/ fatality. Dropped objects. Delay in project schedule.
Lifting and loading (foundation, offshore substation (OSS) and masts)	Structural hazards	Stuck pile	Wrong vessel positioning. Soil compaction above estimations.	Vessel damage. Man overboard. Personnel injury/fatality.
	Dropped load	Dropped tools/equipment.	Poorly secured load. Sea fastener failure. Equipment failure. Poor adherence to loading/unloading plan.	Additional cost of retrieval.
Lifting and loading (miscellaneous structures like scour protection)	Dropped load	Dropped structure on nearby assets.	Equipment failure during loading operations. Dropped within dangerous zone.	Additional cos of retrieval. Personnel injury/fatality if in dangerous zone.
Transportation to port	Collision	Collision with other assets/passing vessels	Poor navigation system. Poor weather. Failure of engine.	Vessel damage.
	Drift	Vessel drift	Poor weather. Vessel operational fault.	Collision with other assets.
	Operational fault	Barge operation fault	Electrical/mechanical fault.	Schedule delay.
Waste management	Spills	Fluid spills	Poorly sealed hydraulic fluid receptacles.	Environmental pollution. Additional cost to clean up.
Site clean-up and surveys	Lost equipment	Lost ROV	Murky waters. Poor weather.	Additional cost to charter new ROV.
Post-decommissioning	Monitoring error	Data misinterpretation	Insufficient Environmental hazard identification (ENVID) analysis.	Poor decision-making based on faulty data.

Table

MCS	Probability	MCS	Probability
$E5$	5.02E-04	$E1.E2.E3$	6.4491E-12
$E6$	2.34E-04	$E2.E3.E4$	4.8145E-12
$E7$	6.71E-05	$E10.E13.E14$	4.9891E-11
$E8$	2.09E-05	$E10.E13.E15$	1.0511E-10
$E9$	2.23E-05	$E10.E13.16$	4.9891E-10
$E17$	1.20E-03	$E11.E13.E14$	1.6148E-11
$E18$	6.40E-04	$E11.E13.E15$	3.4021E-11
$E19$	1.00E-04	$E11.E13.E16$	1.6148E-10
$E20$	2.00E-03	$E12.E13.E14$	3.2056E-10
$E29$	7.00E-03	$E12.E13.E15$	6.7535E-10
$E30$	2.11E-02	$E12.E13.E16$	3.2056E-09
$E31$	3.30E-02	$E21.E22.E24$	2.394E-11
$E32$	3.10E-03	$E21.E23.E24$	5.7456E-12
		$E25.E27.E28$	8.34E-16
		$E26.E27.E28$	3E-11

Table

Intermediate event	Frequency rank	Consequence safety/hazard promoting factors	Consequence rank				Risk index
			P	A	E	EN	P	A	E	EN
Collision/contact	3	Hull damage	0	1	2	0	–	3	6	–
		Loss of stability	2	1	2	0	6	3	6	–
		Detached/dropped load	1	4	3	3	3	12	9	9
		Man overboard	4	0	4	0	12	–	12	–
		On collision course with another vessel	0	5	5	0	–	15	15	–
Vessel damage	3	Water ingress	0	2	3	0	–	6	9	–
		Flooding	0	3	3	0	–	9	9	–
		Loss of stability	2	1	2	0	6	3	6	–
		Detached/dropped load	0	2	3	3	–	6	9	9
		Sink/capsize	4	5	5	3	12	15	15	9
Crane failure	4	Loss of crane/load integrity	0	3	2	0	–	12	8	–
		Swinging/dropped load	0	4	3	2	–	16	12	8
		Within dangerous zone	4	4	0	0	16	16	–	–
		Loss of crane strength	4	3	4	0	16	12	16	–
		Ineffective remedial/corrective action	3	5	5	0	12	20	20	–

P - Personnel consequence

A - Asset consequence

E - Economic consequence

EN - Environmental consequence

Personnel consequence, P, refers to the crew health- related impacts such as injuries requiring first aid, medical attention or fatalities. Asset consequence, A, refers to damages to the lifting vessel and the WT if retrieved parts are intended for reuse. Economic consequence, E, refers to financial losses. Environmental consequence, EN, relate to environmental damages caused by incidents like spills, unrecovered dropped objects, etc.

Offshore, C. 2020. https://www.4coffshore.com/windfarms/.

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