Research on Operation Safety of Offshore Wind Farms

Journal of Marine Science and Engineering

Article Research on Operation Safety of Offshore Wind Farms

Junmin Mou 1,2, Xuefei Jia 1, Pengfei Chen 1,2 and Linying Chen 1,2,*

1 School of Navigation, Wuhan University of Technology, Wuhan 430063, China; [email protected] (J.M.); [email protected] (X.J.); [email protected] (P.C.) 2 Hubei Key Laboratory of Inland Shipping Technology, Wuhan University of Technology, Wuhan 430063, China * Correspondence: [email protected]

Abstract: The operation of offshore wind farms is characterized by a complicated operational environment, long project cycle, and complex vessel traffic, which lead to safety hazards. To identify the key factors affecting the operational safety of offshore wind farms, the risk characteristics of offshore wind farm operations are analyzed based on comprehensive identification of hazards and risk assessment theory. A systematic fault tree analysis of the offshore wind farm operation is established. The assessment shows that the key risk factors that induce offshore wind power collapse, corrosion, fire, lightning strikes, blade failure, personal injury, ship collision, and submarine cable damage accidents are gale, untimely overhauling, improper fire stopping methods, high average number of thunderstorm days, the loose internal structure of fan, working at height, collision avoidance failure, and insufficient buried depth of cables.

Keywords: offshore wind farm; fault tree analysis; operational safety; hazard identiﬁcation

Citation: Mou, J.; Jia, X.; Chen, P.; 1. Introduction Chen, L. Research on Operation Safety of Offshore Wind Farms. J. Globally, the use of wind energy for power generation gains importance due to en- Mar. Sci. Eng. 2021, 9, 881. https:// vironmental benefits and the possible contribution to a safe energy provision [1]. The doi.org/10.3390/jmse9080881 operation of offshore wind farms is characterized by a complicated operational environment, long project cycle, and complex vessel traffic, which lead to safety hazards. In 2019, Academic Editor: María Dolores 865 accidents occurred in offshore wind farms, which increased by 22.3% compared with Esteban Pérez 707 accidents in 2018 [2]. Therefore, it is essential to identify the risks and hidden dangers of offshore wind power to water traffic and personnel safety. Received: 10 July 2021 At present, most studies on risk assessment for offshore wind farms start from a Accepted: 14 August 2021 single object, such as infrastructure [3–6], offshore wind power equipment and personnel Published: 15 August 2021 safety [7–10], and navigation waters of offshore wind farms [11–13]. For infrastructure risks, the majority of coating damages on offshore wind power platforms can be attributed to Publisher’s Note: MDPI stays neutral unsuitable constructive design and mechanical loading [3]. Seawater is corrosive compared with regard to jurisdictional claims in to drinking water and temperature affects corrosion processes [4]. Price and Figueira [5] published maps and institutional affil- found that corrosion has different types. In addition, waves and wind can initiate stress cor- iations. rosion cracking and corrosion fatigue. For offshore wind power equipment and personnel risks, literature [6] pointed out that strong wind and fractured bolts during construction may affect the offshore wind turbine’s life. Fire is also the main accident of focus. An electrical fire prevention system for an offshore wind turbine in [7] was implemented Copyright: © 2021 by the authors. according to the risk characteristics of the fire accident. A method to evaluate the lightning Licensee MDPI, Basel, Switzerland. strike rate of the entire offshore wind farms was proposed in [8]. Literature [9] pointed out This article is an open access article that people working in offshore wind farms encountered sleep quality problems. Chao distributed under the terms and et al. [10] established a time-varying analytical model for the WT failure rate affected by conditions of the Creative Commons wind speed and lightning. For navigation waters of offshore wind farms’ risks, collision is Attribution (CC BY) license (https:// a common event in the navigation risks of offshore wind farms. The Automatic Identifi- creativecommons.org/licenses/by/ cation System is used to analyze the risk of ship collision with offshore platforms in [11]. 4.0/).

J. Mar. Sci. Eng. 2021, 9, 881. https://doi.org/10.3390/jmse9080881 https://www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2021, 9, 881 2 of 32

Literature [12] described the impact of the operation and maintenance ship on the fan at different speeds in detail. The submarine power cable of the offshore wind farm, used for connecting power generation devices to onshore equipment, may have a significant impact on navigation safety [13]. Both qualitative and quantitative technologies have been used to measure the risk of wind farm operations. Representative qualitative analysis methods are Fault Mode Analysis Method [14], Tree Chart Analysis [15], SWOT (Strengths Weaknesses Opportunities Threats) [16]. Representative quantitative methods are Bayesian method [17–19], Monte Carlo Analysis [20], and reliability-based design optimization tools [21,22]. However, the operation of the offshore wind farm is systematic. All the components and factors are closely connected. Focusing on a certain object may lead to underestimation of the risk of the whole system. Therefore, in this paper, we carry out a systematic analysis on the operation safety of offshore wind farms from the perspectives of infrastructure, offshore wind power equipment and personnel safety, and navigation waters. To identify the key factors affecting operational safety, the risk characteristics of offshore wind farm operations are analyzed based on comprehensive identification of hazards and risk assessment theory. The fault tree analysis of the offshore wind farm operation is established. Qualitative and quantitative analyses on the operation risk are carried out according to questionnaires and expert judgment. Instead of the probability of occurrence of the basic event, a risk index is proposed to illustrate the contribution of each basic event to the top event. The risk index is a quantitative assessment of the opinions of the experts according to the questionnaires responded by 50 experts who have worked in offshore wind farms for more than 2 years. Finally, the risk of infrastructure, offshore wind power equipment, and personnel safety, as well as navigation waters to the operation of offshore wind farms are obtained. The remainder of this paper is arranged as follows: Section2 provides the mythology of this article. Sections3–5 provides the risk analysis for offshore wind farm infrastructure risks, equipment and personnel risks, and navigation risks, respectively. In Section6, we discuss the main findings and measures to improve the safety of the operation of offshore wind farms. Section7 concludes the article and provides future research directions.

2. Methodology Fault tree analysis (FTA) is applied to carry out a top-down, deductive failure analysis for the offshore wind farm operation. The undesired state of the wind farm and related events are identified and connected using Boolean logic to understand how systems can fail, and how to reduce the risk. The methodology of this paper is shown in Figure1. First, the operation data of offshore wind farms are collected by literature reading and field research. The data are analyzed to find out the characteristics of wind farm accidents and the causes of risks. An overview of accidents and risk factors in related research is provided in AppendixA. According to the natural environment and navigation environment, the risks are divided into three categories: offshore wind power infrastructure risks, including collapse, corrosion, and other risks; wind power and electrical equipment risks, including fire, lightning, blade failure, personnel injury, and other accidents, and navigable waters risks, including collision, and cable accidents. Secondly, based on the Fault Tree Analysis, the logical analysis of the risk causes is carried out to determine the top events, intermediate events, and basic events based on the collected data. After the basic events are identified, the risk source questionnaire of the offshore wind farm is carried out, and the questionnaire is distributed to the people who have worked in offshore wind farms for more than 2 years. The questionnaire is provided in the AppendixB; 60 questionnaires are collected and 50 are valid. The respondents are the staff of the offshore wind farms located in Hangzhou Bay. The wind farms are about 20 km from shore, and the water depth is 8–12 m. A risk index which is the mean value of the answers is used to represent the contribution of each basic event to the top event. The risk of the top events, i.e., infrastructure, offshore wind power equipment, and personnel safety, and navigation waters during the operation of offshore wind farms are then obtained. J. Mar. Sci. Eng. 2021, 9, 881 3 of 33

J. Mar. Sci. Eng. 2021, 9, 881 3 of 32 personnel safety, and navigation waters during the operation of offshore wind farms are then obtained.

Offshore wind farm operation data collection

Wind power and Offshore wind power electromechanical Navigable waters risks infrastructure risks Natural equipment risks Navigation environment environment of offshore of offshore Foundation collapse, Fire, lightning, blade wind farm Collision accidents, wind farm corrosion and other failure, personnel injury cable accidents risks and other accidents

Risk index calculation

Logical analysis of risk causes

Accident Tree Analysis

Qualitative analysis Quantitative analysis Minimal Cut Set, Structural Probability importance degree important degree

Important impact factors during offshore wind power operation

Suggestions on risk control of offshore wind farm operation

Figure 1. Methodology.

Thirdly,Thirdly, thethe riskrisk of the wind farm operation operation is is assessed assessed using using the the Isograph Isograph Reliability Reliability WorkbenchWorkbench (x64,(x64, 13.013.0).13.013.0). The minimum cut set, structural important degree, degree, and and prob- proba- bilityability importance importance degree degree are are provided. provided. Identiﬁcation Identification of minimalof minimal cut cut sets sets is one is one of the of mostthe importantmost important qualitative qualitative analyses analyses of a faultof a fault tree. tree. The The top top event event occurs occurs if one if one or moreor more of of the minimalthe minimal cut setscut sets occur. occur. The The structural structural importance importance degree degree analyzes analyzes the importancethe importance of each of basiceach basic event event from from the structure the structure of the of fault the faul tree.t tree. Quantitative Quantitative analysis anal ofysis the of probabilitythe probabil- im- portanceity importance degree degree of the basic of the events, basic whichevents, analyzes which analyzes the impact the of impact changes of in changes the probability in the ofprobability basic events of basic on top events events. on The top Fussell–Veselyevents. The Fussell–Vesely importance degree importance [23] is degree applied [23] in thisis manuscript.applied in this The manuscript. Fussell–Vesely The Fussell–Vesely importance is importance the probability, is the given probability, that a critical given that failure a hascritical occurred, failure that has atoccurred, least one that minimal at least cut one set minimal containing cut aset particular containing element a particular contributed ele- toment that contributed failure, which to that can failure, be approximated which can bybe approximated by

mi ˇ i mi ˇ i 1 − ∏j=1(1 − (Qj(t)) ∑j=1 Qj(t) IFV (i|t) ≈ ≈ (1) Q0(t) Q0(t)

ˇ i where Qj(t) denotes the probability that minimal cut set j among those containing component i is failed at time t. J. Mar. Sci. Eng. 2021, 9, 881 4 of 32

In the end, the important inﬂuencing factors that affect offshore wind power operation are identiﬁed. Accordingly, safety management measures for each typical event are formulated.

3. Infrastructure Risks 3.1. Fault Tree for Infrastructure Risk This section describes the fault tree analysis of collapse (see Figure2) and corrosion (see Figure3). The use of the logic gate, i.e., AND gate and OR gate, is based on existing related research and expert judgment. For example, ‘Poor foundation stability’ and ‘External force’ are connected by an AND gate in Figure2. Theoretically, a significant earthquake should be able to cause the collapse of a wind turbine no matter if the foundation stability is good or poor. However, on the one hand, in reality, the offshore wind turbine is usually established more than 20 km away from the shore and as well as avoiding choosing the water area with the most possible significant earthquakes. On the other hand, the influence of earthquakes cannot be underestimated. Accordingly, ‘Poor foundation stability’ and ‘External force’ are connected by an AND gate.

3.2. Risk Index of the Basic Events According to the questionnaires, the risk index of the basic events in the fault tree for collapse and corrosion are provided in Tables1 and2.

Table 1. Collapse risk factors of offshore wind farms.

Number Risk Factor Risk Index

cA1 Tight deadlines 0.5104 cA2 Unsafe construction 0.491 cA3 Imperfect maintenance 0.5004 cA4 Ship that lose stability 0.523 cA5 Ship that is anchored near the wind turbine 0.423 cA6 Inappropriate parameter settings for anti-lift of pile foundation 0.529 cA7 Inappropriate parameter settings for anti-overturning 0.5278 cA8 Unreasonable structure design 0.5416 cA9 Loose bolts 0.5184 cA10 Insufﬁcient bolt strength 0.548 cA11 Insufﬁcient load bearing capacity of the foundation 0.5926 cA12 Inaccurate hydrographic survey 0.4682 cA13 Inaccurate geological survey 0.528 cA14 Inappropriate corrosion prevention methods 0.4756 cA15 No corrosion prevention methods for the infrastructure 0.5408 cA16 Tide 0.3402 cA17 Gale 0.5312 cA18 Earthquake 0.4462

Table 2. Corrosion risk factors of offshore wind farms.

Number Risk Factor Risk Index

cB1 Untimely overhauling 0.5146 cB2 Improper anti-corrosion measures 0.539 cB3 Improper equipment selection 0.423 cB4 Salt spray 0.5384 cB5 Tide 0.3702 cB6 Marine organisms attached to the equipment structure 0.39 cB7 Scouring 0.4012 cB8 Metal structure of steel 0.4416 J. Mar. Sci. Eng. 2021, 9, 881 6 of 33

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Collapse

CA Q=0.8287

Poor foundation External force stability

CA1 CA2

Unqualified Foundation design Loose foundation Tide Gale Earthquake component defects

cA cA cA CA3 CA4 CA5 16 17 18 FR=0.3402 FR=0.5312 FR=0.4462

Inappropriate Inappropriate Unreasonable Insufficient load Insufficient Tight Unsafe Imperfect Ship parameter settings parameter settings Insufficient Foundation structure bearing capacity foundation deadlines construction maintenance collision for anti-lift of pile for anti- strength settlement design of the foundation durability foundation overturning

cA cA1 cA2 cA3 CA6 cA6 cA7 cA8 CA7 CA8 CA9 11 FR=0.5104 FR=0.491 FR=0.5004 FR=0.529 FR=0.5278 FR=0.5416 FR=0.5926 Inappropriate No corrosion Ship that is Inaccurate Inaccurate Insufficient corrosion prevention Ship that lose anchored near Loose bolts hydrographic geological bolt strength prevention methods for the stability the wind survey survey turbine methods infrastructure

cA cA cA cA cA cA4 cA5 cA9 10 12 13 14 15 FR=0.523 FR=0.423 FR=0.5184 FR=0.548 FR=0.4682 FR=0.528 FR=0.4756 FR=0.5408

Figure 2. Fault tree analysis model for collapse risk of the offshore wind farm (‘FR’ refers to the risk index of the basic events to the top event. ’Q’ refers to the risk index of the top event to Figure 2. Fault tree analysis model for collapse risk of the offshore wind farm (‘FR’ refers to the risk index of the basic events to the top event. ’Q’ refers to the risk index of the top event tothe the offshore offshore wind wind farm farm during during operation). operation).

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Corrosion

CB Q=0.2679

Untimely Corrosion overhauling damage

cB1 CB1 FR=0.5146 Improper anti- Corrosive corrosion measures environment

cB2 CB2 FR=0.539

Improper Marine organisms Metal equipment Salt spray Tide attached to the Scouring structure of selection equipment structure steel

cB3 cB4 cB5 cB6 cB7 cB8 FR=0.423 FR=0.5384 FR=0.3702 FR=0.39 FR=0.4012 FR=0.4416

FigureFigure 3. Fault 3. Fault tree tree analysis analysis model model for for corrosion risk risk of of the the offshore offshore wind wind farm. farm. 3.3. Risk Assessment 3.2. Risk Index of the Basic Events (1) Risk assessment of collapse AccordingWith the to help the of questionnaires, Isograph Reliability the Workbench, risk index the of minimumthe basic cutevents set, structural in the fault im- tree for collapseportance and degree, corrosion and probabilityare provided importance in Tables degree 1 and of the 2. collapse accidents are analyzed as follows. TableThere 1. areCollapse 42 minimum risk factors cut sets of offshore of collapse wind accidents: farms. • Second-order minimum cut sets: {cA11, cA17}, {cA10, cA17}, {cA8, cA17}, {cA15, cA17}, Number {cA6, cA17}, {cA Risk13, c AFactor17}, {cA 7, cA17}, {cA9, cA17}, {cA1, cA17}, {cA3, cA17}, Risk {cA 11,Index cA18}, cA1 {cA2, cA17},Tight {cA14, deadlines cA17}, {cA12, cA17}, {cA10, cA18}, {cA8, cA18}, {cA15, c0.5104A18}, {c A6, cA18}, {cA13, cA18}, {cA7, cA18}, {cA9, cA18}, {cA1, cA18}, {cA3, cA18}, {cA2, cA18}, {cA14, cA2 Unsafe construction 0.491 cA18}, {cA12, cA18}, {cA11, cA16}, {cA10, cA16}, {cA8, cA16}, {cA15, cA16}, {cA6, cA16}, cA3 {cA13, cImperfectA16}, {cA7, cmaintenanceA16}, {cA9, cA16}, {cA1, cA16}, {cA3, cA16}, {cA2, cA16}, {c 0.5004A14, cA 16}, cA4 {cA12, cA Ship16}; that lose stability 0.523 • Third-order minimum cut sets: {cA4, cA5, cA17}, {cA4, cA5, cA18}, {cA4, cA5, cA16}. cA5 Ship that is anchored near the wind turbine 0.423 The structural importance degree of collapse accidents is as follows: cA6 Inappropriate parameter settings for anti-lift of pile foundation 0.529 I(cA18) = I(cA17) = I(cA16) > I(cA15) = I(cA14) = I(cA13) = I(cA12) = I(cA11) = I(cA10) = A c 7 InappropriateI(cA9) = I(c parameterA8) = I(cA7) =settings I(cA6) = for I(cA anti-overturning3) = I(cA2) = I(cA1) > I(cA5) = I(cA4). 0.5278 cA8 The Unreasonable Fussell–Vesely Importancestructure design of the basic events of wind turbine collapse 0.5416 in offshore wind farms is shown in Figure4. The order of probability importance is listed as follows: cA9 Loose bolts 0.5184 cA10 Insufficient bolt strength 0.548 cA11 Insufficient load bearing capacity of the foundation 0.5926 cA12 Inaccurate hydrographic survey 0.4682 cA13 Inaccurate geological survey 0.528 cA14 Inappropriate corrosion prevention methods 0.4756 cA15 No corrosion prevention methods for the infrastructure 0.5408 cA16 Tide 0.3402 cA17 Gale 0.5312 cA18 Earthquake 0.4462

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Untimely overhauling(cB1) > Improper anti-corrosion measures(cB2) > Salt spray(cB4) J. Mar. Sci. Eng. 2021, 9, 881 7 of 32 > Metal structure of steel(cB8) > Improper equipment selection(cB3) > Scouring(cB7) > Ma- rine organisms attached to the equipment structure(cB6) > Tide(cB5).

Figure 4. Fussell–Vesely ImportanceImportance ofof eacheach basicbasic eventevent ofof collapsecollapse accident.accident.

Gale(cA17) > Earthquake(cA18) > Tide(cA16) > Insufﬁcient bearing capacity of the foundation(cA11) > Insufﬁcient bolt strength(cA10) > Unreasonable structural design(cA8) > No corrosion prevention methods for the infrastructure(cA15) > Inappropriate parameter settings for anti-lift of pile foundation(cA6) > Inaccurate geological survey(cA13) > Inappropriate parameter settings for anti-overturning(cA7) > Loose bolts(cA9) > tight deadlines(cA1) > Imperfect maintenance(cA3) > Unsafe construction(cA2) > Inappropri- ate corrosion prevention methods(cA14) > Inaccurate hydrographic survey(cA12) > Ships anchored near the wind turbine(cA5) > Ships that lose stability(cA4). (2) Risk assessment of corrosion With the help of Isograph Reliability Workbench, minimum cut set, structural importance degree, and probability importance degree of the corrosion accidents are as follows. There are 6 minimum cut sets of corrosion accidents:

• Third-order minimum cut sets: {cB1, cB2, cB4}, {cB1, cB2, cB8}, {cB1, cB2, cB3}, {cB1, cB2, cB7}, {cB1, cB2, cB6}, {cB1, cB2, cB5}. The structural importance degree of corrosion accidents is as follows: I(cB2) = I(cB1) > I(cB8) = I(cB7) = I(cB6) = I(cB5) = I(cB4) = I(cB3). The Fussell–Vesely Importance of the basic events of wind turbine corrosion in offshore wind farms is shown in Figure5. The order of probability importance is listed as follows: Figure 5. Fussell–Vesely Importance of each basic event of corrosion accident.

4. Equipment and Personnel Risk 4.1. Fault Tree for the Equipment Failure and Personnel Injury This section describes the fault tree analysis of fire (see Figure 6), a lightning strike (see Figure 7), blade failure (see Figure 8), and personnel injury (see Figure 9).

J. Mar. Sci. Eng. 2021, 9, 881 9 of 33

Untimely overhauling(cB1) > Improper anti-corrosion measures(cB2) > Salt spray(cB4) > Metal structure of steel(cB8) > Improper equipment selection(cB3) > Scouring(cB7) > Ma- rine organisms attached to the equipment structure(cB6) > Tide(cB5).

J. Mar. Sci. Eng. 2021, 9, 881 8 of 32 Figure 4. Fussell–Vesely Importance of each basic event of collapse accident.

Figure 5. Fussell–VeselyFigure 5. Fussell–Vesely Importance Importance of each basic of each event basic of corrosion event of accident.corrosion accident.

Untimely overhauling(c4. EquipmentB and1) > Personnel Improper anti-corrosionRisk measures(cB2) > Salt spray(cB4) > Metal structure of steel(cB8) > Improper equipment selection(cB3) > Scouring(cB7) > 4.1. Fault Tree for the Equipment Failure and Personnel Injury Marine organisms attached to the equipment structure(cB6) > Tide(cB5). This section describes the fault tree analysis of fire (see Figure 6), a lightning strike 4. Equipment and(see PersonnelFigure 7), blade Risk failure (see Figure 8), and personnel injury (see Figure 9). 4.1. Fault Tree for the Equipment Failure and Personnel Injury This section describes the fault tree analysis of ﬁre (see Figure6), a lightning strike (see Figure7), blade failure (see Figure8), and personnel injury (see Figure9).

4.2. Risk Index of the Basic Events According to the questionnaires, the risk index of the basic events in the fault tree for ﬁre, lightning, blade failure, and personnel injury are provided in Tables3–6.

Table 3. Fire risk factors of offshore wind farms.

Number Risk Factor Risk Index f1 Improper ﬁre stopping methods 0.4756 f2 Lightning strike 0.5272 f3 Improper electrical operation 0.4698 f4 Mechanical damage to electrical devices 0.457 f5 Salt spray corrosion 0.484 f6 Overload 0.5292 f7 Vulnerabilities in device’s manufacturing 0.4728 f8 Violation of safety regulations 0.5586 f9 Improper selection and installation of electrical devices 0.5166 f10 Irrational design of electrical circuit 0.5066 f11 Welding 0.475 f12 Small space 0.5052 f13 Overspeed 0.5052 f14 Mechanical friction 0.4904 f15 Increases in Electrical Contact Resistance 0.4572 f16 Poor ventilation 0.4074 f17 Oil leakage 0.483 f18 Oily cotton and other materials 0.4582 J. Mar. Sci. Eng. 2021, 9, 881 10 of 33

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Fire

FQ=0.3423

Improper fire stopping Ignition source Combustible materials methods

f1 F1 F2 FR=0.4756 Oily cotton and Spark Heat Oil leakage other materials

F3 F4 f17 f18 FR=0.483 FR=0.4582 Lightning Spark caused Spark caused Local heat Electrical spark Overspeed strike by fault by welding generation

f2 F5 F6 F7 f13 F11 FR=0.5272 FR=0.5052

Electrical Improper Violation Improper selection Irrational design Increases in Small Mechanical Poor short electrical of safety and installation of of electrical Welding Electrical Contact space friction ventilation circuit operation regulations electrical devices circuit Resistance

F8 f3 f8 f9 f10 f11 f12 f14 f15 f16 FR=0.4698 FR=0.5586 FR=0.5166 FR=0.5066 FR=0.475 FR=0.5052 FR=0.4904 FR=0.4572 FR=0.4074 Insulation Device aging deterioration

F9 F10

Mechanical Vulnerabilities damage to Salt spray Overload in device's electrical corrosion manufacturing devices

f4 f5 f6 f7 FR=0.457 FR=0.484 FR=0.5292 FR=0.4728 Figure 6. FaultFault tree tree analysis model for fire ﬁre risk of the offshore wind farm.

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Lightning strike

LQ=0.9406

Improper lightning Lightning protection

L1 L2

Poor contact No regular High average Failure of lightning Limited scope of between the flange Insulation inspection of number of Large device sizes protection lightning of the tower and deterioration lightning thunderstorm days measures protection the electrode protection facilities l1 l2 l3 L3 l6 L4 L5 FR=0.4704 FR=0.4658 FR=0.3926 FR=0.4898

Widespread use Decrease of Unreliable Unqualified Wrong location Errors in design and exposure of thermal Poor design of Salt spray conductive grounding of the lightning of equalizing composite stability of lightning rod pathway resistance rod network materials grounding grid l4 l5 l7 l8 L6 l12 l13 l14 FR=0.4276 FR=0.4106 FR=0.5494 FR=0.527 FR=0.5388 FR=0.5526 FR=0.4966

Limited thermal Problems in Local corrosion of capacity of the grounding grid grounding devices grounding devices construction

l9 l10 l11 FR=0.5182 FR=0.4678 FR=0.4768 Figure 7. Fault tree analysis model for lightning strike risk of the offshore wind farm.

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Blade failure

BQ=0.9986

Blade aging

B1 Blade loss

Erosion by Hyperthermic Effect of chemical Ultraviolet Corrosion by salt atmospheric environment medium irradiation spray particles

b1 b2 b3 b4 b5 B2 FR=0.446 FR=0.4826 FR=0.3534 FR=0.4448 FR=0.4866

Improper operation Fracture failure Blade cracking Lightning strike Erosion Crack and maintenance of fan

B3 B4 B5 B6 B7 B8

Defects in Erosion Erosion Invasion of Over- Unqualified blade Harmful Inherent Surface Brake Premature blade Gale Crack by by water impurities such as power material vibration Lightning defects crack failure failure design particles vapor particulate matter operation

b6 B9 B10 b11 B11 b13 B12 b18 b19 b20 B13 b23 b24 B14 FR=0.538 FR=0.506 FR=0.4728 FR=0.4458 FR=0.4792 FR=0.4458 FR=0.5052 FR=0.5758 Immature Loose internal No Corrosion Unqualified Decrease Cracks in the connection Long Repair Inexperienced Imbalance High humidity manufacturing structure of timely by salt grounding in blade between lightning Fatigue service Gale Defects not in manufacturers impeller blade surface process fan repair spray resistance insulation attractor and blade time of fan time

b7 b8 b9 b10 b12 b5 b14 b15 b16 b17 b21 b22 b11 B15 b12 FR=0.5206 FR=0.5194 FR=0.5578 FR=0.6132 FR=0.5778FR=0.4866 FR=0.4774 FR=0.4326 FR=0.425 FR=0.4718 FR=0.5264FR=0.4798 FR=0.506 FR=0.5778 Fouling on Oil spilling Crack the surface of blades b25 b26 b27 FR=0.4686 FR=0.541 FR=0.4006 FigureFigure 8. 8.Fault Fault treetree analysisanalysis modelmodel for blade failu failurere risk of of the the offshore offshore wind wind farm. farm.

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Personnel injury

P Q=0.9795

Mechanic Falls from Fall into Noise Electroshock wound height water

p1 P1 P2 P3 P4 FR=0.404 Improper Inefficient Contacting live Landing and Ships that lose Equipment leakage equipment personnel Protection failure Working at height parts of equipment leaving the fan stability management management P5 P6 P7 P8 P9 p2 p3 P10 FR=0.5532 FR=0.4786

Wrong Poor Improper Violation Wrong Improper Violation No professional Residual Fracture of operation Lightning Live mechanical Mechanical personnel of the fixed use of of the operation and Maneuvering charge Misoperation protection of power strike inspection parts equipment protection operation position of protection operation maintenance error inline safety rod switch assembly equipment procedure safety ropes devices procedure ships

p4 p5 p6 p7 p8 p9 p10 p11 p12 p13 p14 p15 p12 p16 p17 FR=0.486 FR=0.5514 FR=0.4948 FR=0.5352 FR=0.5694 FR=0.5414 FR=0.4886 FR=0.5344 FR=0.5744 FR=0.5472 FR=0.5582 FR=0.5708 FR=0.5744 FR=0.5284 FR=0.4866

Figure 9. Fault tree analysis model for personnel injury risk of the offshore wind farm. Figure 9. Fault tree analysis model for personnel injury risk of the offshore wind farm.

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Table 4. Lightning strike risk factors of offshore wind farms.

Number Risk Factor Risk Index l1 High average number of thunderstorm days 0.4704 l2 Poor contact between the ﬂange of the tower and the electrode 0.4658 l3 Large device sizes 0.3926 l4 Salt spray 0.4276 l5 Widespread use and exposure of composite materials 0.4106 l6 No regular inspection of lightning protection facilities 0.4898 l7 Unreliable conductive pathway 0.5494 l8 Unqualiﬁed grounding resistance 0.527 l9 Limited thermal capacity of the grounding devices 0.5182 l10 Problems in grounding grid construction 0.4678 l11 Local corrosion of grounding devices 0.4768 l12 Poor design of lightning rod 0.5388 l13 Wrong location of the lightning rod 0.5526 l14 Errors in design of equalizing network 0.4966

Table 5. Blade failure risk factors of offshore wind farms.

Number Risk Factor Risk Index b1 Hyperthermic environment 0.446 b2 Effect of chemical medium 0.4826 b3 Ultraviolet irradiation 0.3534 b4 Erosion by atmospheric particles 0.4448 b5 Corrosion by salt spray 0.4866 b6 Defects in blade design 0.538 b7 Immature manufacturing processes 0.5206 b8 Inexperienced manufacturers 0.5194 b9 Imbalance impeller 0.5578 b10 Loose internal structure of fan 0.6132 b11 Gale 0.506 b12 No timely repair 0.5778 b13 Lightning 0.4728 b14 Unqualiﬁed grounding resistance 0.4774 b15 High humidity blade surface 0.4326 b16 Decrease in blade insulation 0.425 b17 Cracks in the connection between lightning attractor and blade 0.4718 b18 Erosion by particles 0.4458 b19 Erosion by water vapor 0.4792 b20 Invasion of impurities such as particulate matter 0.4458 b21 Fatigue 0.4798 b22 Long service time of fan 0.5264 b23 Over-power operation 0.5052 b24 Brake failure 0.5758 b25 Oil spilling 0.4686 b26 Crack 0.541 b27 Fouling on the surface of blades 0.4006

Table 6. Personnel injury risk factors of offshore wind farms.

Number Risk Factor Risk Index p1 Noise 0.404 p2 Working at height 0.5532 p3 Landing and leaving the fan 0.4786 p4 Residual charge inline 0.486 p5 Wrong operation of the power switch 0.5514 J. Mar. Sci. Eng. 2021, 9, 881 14 of 32

Table 6. Cont.

Number Risk Factor Risk Index p6 Lightning strike 0.4948 p7 Live inspection 0.5352 p8 Misoperation 0.5694 p9 Poor mechanical parts assembly 0.5414 p10 Mechanical equipment 0.4886 p11 Improper personnel protection equipment 0.5344 p12 Violation of the operation procedure 0.5744 p13 Fracture of protection safety rod 0.5472 p14 Wrong ﬁxed position of safety ropes 0.5582 p15 Improper use of protection devices 0.5708 p16 No professional operation and maintenance ships 0.5286 p17 Maneuvering error 0.4866

J. Mar. Sci. Eng. 2021, 9, 881 4.3. Risk Assessment 16 of 33

(1) Risk assessment of fire With the help of Isograph Reliability Workbench, the minimum cut set, structural importancef17}, {f1, degree, f10, f18}, and {f1, probability f13, f18}, {f1, importance f7, f17}, {f1, degree f3, f17}, of {f1, the f14, fire f18}, accidents {f1, f5, f18}, are {f1, presented underneath.f15, f17}, {f1, f4, f17}, {f1, f7, f18}, {f1, f3, f18}, {f1, f15, f18}, {f1, f4, f18}, {f1, f16, f17}, {f1, Theref16, aref18}; 28 minimum cut sets of fire accidents:  Fourth-order minimum cut sets: {f1, f11, f12, f17}, {f1, f11, f12, f18}. • Third-order minimum cut sets: {f1, f8, f17}, {f1, f6, f17}, {f1, f8, f18}, {f1, f2, f17}, {f1, f9, The structural importance degree of fire accidents is as follows: f17}, {f1, f10, f17}, {f1, f13, f17}, {f1, f6, f18}, {f1, f2, f18}, {f1, f14,f17}, {f1, f9, f18}, {f1, f5, I(f1) > I(f18) = I(f17) > I(f16) = I(f15) = I(f14) = I(f13) = I(f10) = I(f9) = If8) = I(f7) = I(f6) = I(f5)f17}, = I(f4) {f1, = f10, I(f3) f18}, = I(f2) {f1, > I(f12) f13, = f18}, I(f11). {f1, f7, f17}, {f1, f3, f17}, {f1, f14, f18}, {f1, f5, f18}, {f1, f15,The f17}, Fussell-Vesely {f1, f4, f17}, Importance {f1, f7, f18}, of {f1, the f3,basic f18}, ev {f1,ents f15,of wind f18}, turbine {f1, f4, fire f18}, in {f1, offshore f16, f17}, {f1, windf16, farms f18}; is shown in Figure 10. The order of probability importance is listed as follows: • Fourth-orderImproper fire minimum stopping methods(f1) cut sets: {f1, > Oil f11, leakage(f17) f12, f17}, {f1, > Oily f11, cotton f12, f18}. and other materials(f18)The structural > Violation importance of safety regulations(f8) degree of fire > accidentsOverload(f6) is as> Lightning follows: strike(f2) > Im- proper selection and installation of electrical devices(f9) > Irrational design of electrical I(f1) > I(f18) = I(f17) > I(f16) = I(f15) = I(f14) = I(f13) = I(f10) = I(f9) = If8) = I(f7) = I(f6) circuit(f10) > Overspeed(f13) > Mechanical friction(f14) > Salt spray corrosion(f5) > Vul- = I(f5)nerabilities = I(f4) =in I(f3)device’s = I(f2) manufacturing(f7) > I(f12) = I(f11). > Improper electrical operation(f3) > Increases ElectricalThe Fussell-Vesely contact resistance(f15) Importance > Mechanical of the basic damage events to electrical of wind turbinedevices(f4) fire > inPoor offshore ven- wind farmstilation(f16) is shown > Small in Figure space(f12) 10. The > Welding(f11). order of probability importance is listed as follows:

FigureFigure 10. 10.Fussell–Vesely Fussell–Vesely Importance Importance of each each basic basic event event of the of thefire ﬁreaccident. accident.

(2) Risk assessment of lightning With the help of Isograph Reliability Workbench, the minimum cut set, structural importance degree, and probability importance degree of the lightning accidents are presented underneath. There are 45 minimum cut sets of lightning accidents:  Second-order minimum cut sets: {l1, l13}, {l1, l7}, {l2, l13}, {l2, l7}, {l1, l12}, {l2, l12}, {l1, l8}, {l2, l8}, {l1, l9}, {l2, l9}, {l4, l13}, {l4, l7}, {l1, l14}, {l2, l14}, {l1, l6}, {l4, l12}, {l2, l6}, {l5, l13}, {l4, l8}, {l1, l11}, {l5, l7}, {l2, l11}, {l4, l9}, {l1, l10}, {l5, l12}, {l2, l10}, {l5, l8}, {l3, l13}, {l3, l7}, {l4, l14}, {l5, l9}, {l3, l12}, {l4, l6}, {l3, l8}, {l4, l11}, {l5, l14}, {l3, l9}, {l5, l6}, {l4, l10}, {l5, l11}, {l3, l14}, {l5, l10}, {l3, l6}, {l3, l11}, {l3, l10}. The structural importance degree of lightning accidents is as follows:

J. Mar. Sci. Eng. 2021, 9, 881 15 of 32

Improper ﬁre stopping methods(f1) > Oil leakage(f17) > Oily cotton and other materials(f18) > Violation of safety regulations(f8) > Overload(f6) > Lightning strike(f2) > Improper selection and installation of electrical devices(f9) > Irrational design of electrical circuit(f10) > Overspeed(f13) > Mechanical friction(f14) > Salt spray corrosion(f5) > Vul- nerabilities in device’s manufacturing(f7) > Improper electrical operation(f3) > Increases Electrical contact resistance(f15) > Mechanical damage to electrical devices(f4) > Poor ventilation(f16) > Small space(f12) > Welding(f11). (2) Risk assessment of lightning J. Mar. Sci. Eng. 2021, 9, 881 With the help of Isograph Reliability Workbench, the minimum cut set, structural17 of 33

importance degree, and probability importance degree of the lightning accidents are presented underneath. I(l5)There = I(l4) are 45= I(l3) minimum = I(l2) =cut I(l1) sets > I(l14) of lightning = I(l13) = accidents: I(l12) = I(l11) = I(l10) = I(l9) = I(l8) = I(l7) =• I(l6).Second-order minimum cut sets: {l1, l13}, {l1, l7}, {l2, l13}, {l2, l7}, {l1, l12}, {l2, l12}, {l1, Thel8}, {l2,Fussell–Vesely l8}, {l1, l9}, {l2,Importance l9}, {l4, l13}, of {l4,the l7},basic {l1, events l14}, {l2, of l14},wind {l1, turbine l6}, {l4, lightning l12}, {l2, in l6}, off- {l5, shore l13},wind {l4, farms l8}, {l1,is shown l11}, {l5, in Figure l7}, {l2, 11. l11}, The {l4, or l9},der {l1, of probability l10}, {l5, l12}, importance {l2, l10}, {l5, is l8},as follows: {l3, l13}, High{l3, l7}, average {l4, l14}, number {l5, l9}, of {l3,thunderstorm l12}, {l4, l6}, days(l1) {l3, l8}, > {l4, Poor l11}, contact {l5, l14}, between {l3, l9}, the {l5, flange l6}, {l4, of l10},the tower{l5, and l11}, the electrode {l3, l14}, {l5, point(l2) l10}, {l3, > salt l6}, spray(l4 {l3, l11},) > {l3, Widespread l10}. use and exposure of composite materials(l5)The structural > Large importancedevice sizes(l3) degree > Wrong of lightning location accidents of the lightning is as follows: rod(l13) > Unreliable conductiveI(l5) = pathway(l7) I(l4) = I(l3) => I(l2)Poor = design I(l1) > I(l14)of lightning = I(l13) rod(l12) = I(l12) => I(l11)Unqualified = I(l10) grounding = I(l9) = I(l8) re- = sistance(l8)I(l7) = I(l6). > Limited thermal capacity of the grounding devices(l9) > Errors in design of equalizing network(l14)The Fussell–Vesely > No regular Importance inspection of the of basic lightning events protection of wind turbine facilities(l6) lightning > Local in offshore corro- sionwind of farmsgrounding is shown devices(l11) in Figure > Proble 11. Thems order in grounding of probability grid construction(l10). importance is as follows:

FigureFigure 11. 11. Fussell–VeselyFussell–Vesely Importance Importance of of each each basic basic event event of of a a lightning lightning accident. accident.

(3)High Risk average assessment number of blade of thunderstorm failure days(l1) > Poor contact between the ﬂange of the towerWith the and help the electrodeof Isograph point(l2) Reliability > salt Wo spray(l4)rkbench, > Widespreadthe minimum use cut and set, exposure structural of importancecomposite materials(l5)degree, and >probability Large device importance sizes(l3) >degree Wrong of location the blade of thefailure lightning accidents rod(l13) are presented> Unreliable underneath. conductive pathway(l7) > Poor design of lightning rod(l12) > Unqualiﬁed groundingThere are resistance(l8) 24 minimum > cut Limited sets of thermal blade failure capacity accidents: of the grounding devices(l9) > Er- rors in design of equalizing network(l14) > No regular inspection of lightning protection  First-order minimum cut sets are as follows: {b10}, {b24}, {b9}, {b6}, {b7}, {b8}, {b23}, facilities(l6) > Local corrosion of grounding devices(l11) > Problems in grounding grid construction(l10).{b5}, {b2}, {b19}, {b1}, {b18}, {b4}, {b3}, {b25}, {b26}, {b27};  Second-order minimum cut sets are {b12, b26}, {b12, b25}, {b22, b20}, {b12, b27}, {b13, b14}, {b11, b20}, {b13, b17}, {b21, b20}, {b13, b15}, {b13, b16}. The structural importance degree of blade failure accidents is as follows: I(b24) = I(b23) = I(b22) = I(b21) = I(b20) = I(b19) = I(b18) = I(b11) = I(b10) = I(b9) = I(b8) = I(b7) = I(b6) = I(b5) = I(b4) = I(b3) = I(b2) = I(b1) > I(b13) > I(b12) > I(b27) = I(b26) = I(b25) > I(b17) = I(b16) = I(b15) = I(b14). The Fussell–Vesely Importance of the basic events of wind turbine blade failure in offshore wind farms is shown in Figure 12. The order of probability importance is listed as follows: Lightning(b13) > Loose internal structure of fan(b10) > Brake failure(b24) > Imbalance impeller(b9) > No timely repair(b12) > Defects in blade design(b6) > Immature manufacturing processes(b7) > Inexperienced manufacturers(b8) > Over-power operation(b23) >

J. Mar. Sci. Eng. 2021, 9, 881 16 of 32

(3) Risk assessment of blade failure With the help of Isograph Reliability Workbench, the minimum cut set, structural importance degree, and probability importance degree of the blade failure accidents are presented underneath. There are 24 minimum cut sets of blade failure accidents: • First-order minimum cut sets are as follows: {b10}, {b24}, {b9}, {b6}, {b7}, {b8}, {b23}, {b5}, {b2}, {b19}, {b1}, {b18}, {b4}, {b3}, {b25}, {b26}, {b27}; • Second-order minimum cut sets are {b12, b26}, {b12, b25}, {b22, b20}, {b12, b27}, {b13, b14}, {b11, b20}, {b13, b17}, {b21, b20}, {b13, b15}, {b13, b16}. J. Mar. Sci. Eng. 2021, 9, 881 18 of 33 The structural importance degree of blade failure accidents is as follows: I(b24) = I(b23) = I(b22) = I(b21) = I(b20) = I(b19) = I(b18) = I(b11) = I(b10) = I(b9) = I(b8) = I(b7) = I(b6) = I(b5) = I(b4) = I(b3) = I(b2) = I(b1) > I(b13) > I(b12) > I(b27) = I(b26) = I(b25) Corrosion> I(b17) = by I(b16) salt = spray(b5) I(b15) = I(b14). > Effect of chemical medium(b2) > Erosion by water vapor(b19)The > Fussell–VeselyInvasion of impurities Importance such ofas theparticulate basic events matter(b20) of wind > Hyperthermic turbine blade environ- failure in ment(b1)offshore > wind Erosion farms by isparticles(b18) shown in Figure > Erosion 12. The by order atmospheric of probability particles(b4) importance > Ultraviolet is listed irradiation(b3)as follows: > Crack(b26) > Oil spilling(b25) > Long service time of fan(b22).

FigureFigure 12. 12. Fussell–VeselyFussell–Vesely Importance Importance of ofeach each basic basic event event of blad of bladee failure failure accident accident (The (Theevent event whose whose Fussell–Vesely Fussell–Vesely importanceimportance degree degree is less is less than than 0.01867 0.01867 is ignored). is ignored).

(4)Lightning(b13) Risk assessment > Loose of personnel internal structureinjury of fan(b10) > Brake failure(b24) > Imbalance impeller(b9)With the >help No timelyof Isograph repair(b12) Reliability > Defects Workbench, in blade the design(b6) minimum > Immature cut set, structural manufac- importanceturing processes(b7) degree, and > Inexperiencedprobability importance manufacturers(b8) degree of >the Over-power personnel injury operation(b23) accidents > areCorrosion presented by underneath. salt spray(b5) > Effect of chemical medium(b2) > Erosion by water vapor(b19) > InvasionThere are of impurities17 minimum such cut as sets particulate of personnel matter(b20) injury accidents: > Hyperthermic environment(b1) > Erosion by particles(b18) > Erosion by atmospheric particles(b4) > Ultraviolet irradia-  First-order minimum cut set is as follows: {p1}; tion(b3) > Crack(b26) > Oil spilling(b25) > Long service time of fan(b22).  Second-order minimum cut sets are as follows: {p12, p2}, {p15, p2}, {p5, p8}, {p12, p9}, (4) Risk assessment of personnel injury {p14, p2}, {p13, p2}, {p5, p7}, {p11, p9}, {p6, p8}, {p12, p10}, {p4, p8}, {p6, p7}, {p11, With the help of Isograph Reliability Workbench, the minimum cut set, structural p10}, {p4, p7}, {p3, p17}, {p3, p18}. importance degree, and probability importance degree of the personnel injury accidents are presentedThe structural underneath. importance degree of personnel injury accidents is as follows: I(p1)There > I(p2) are 17 > minimumI(p8) = I(p7) cut = sets I(p3) of > personnel I(p12) > I(p11) injury > accidents: I(p10) = I(p9) > I(p17) = I(p16) = I(p6) = I(p5) = I(p4) > I(p15) = I(p14) = I(p13). The Fussell–Vesely Importance of the basic event of wind turbine personnel injury in offshore wind farms is shown in Figure 13. Thus, the order of probability importance is as follows: Working at height(p2) > Violation of the operation procedure(p12) > Misopera- tion(p8) > Live inspection(p7) > Noise(p1) > Wrong operation of power switch(p14) > Poor mechanical parts assembly(p9) > Improper personnel protection equipment(p11) > Light- ning strike(p6) > Mechanical equipment(p10) > Residual charge inline(p4) > Landing and leaving the fan(p3) > Improper use of protection devices(p15) > Wrong fixed position of safety ropes(p14) > Fracture of protection safety rod(p13) > No professional operation and maintenance ships(p16) > Maneuvering error(p17).

J. Mar. Sci. Eng. 2021, 9, 881 17 of 32

• First-order minimum cut set is as follows: {p1}; • Second-order minimum cut sets are as follows: {p12, p2}, {p15, p2}, {p5, p8}, {p12, p9}, {p14, p2}, {p13, p2}, {p5, p7}, {p11, p9}, {p6, p8}, {p12, p10}, {p4, p8}, {p6, p7}, {p11, p10}, {p4, p7}, {p3, p17}, {p3, p18}. The structural importance degree of personnel injury accidents is as follows: I(p1) > I(p2) > I(p8) = I(p7) = I(p3) > I(p12) > I(p11) > I(p10) = I(p9) > I(p17) = I(p16) = I(p6) = I(p5) = I(p4) > I(p15) = I(p14) = I(p13). J. Mar. Sci. Eng. 2021, 9, 881 The Fussell–Vesely Importance of the basic event of wind turbine personnel injury in 19 of 33

offshore wind farms is shown in Figure 13. Thus, the order of probability importance is as follows:

Figure 13. Fussell–Vesely Importance of each basic event of personnel injury accident. Figure 13. Fussell–Vesely Importance of each basic event of personnel injury accident. Working at height(p2) > Violation of the operation procedure(p12) > Misoperation(p8) 5. Navigation> Live inspection(p7) Risks of Wind > Noise(p1) Farm > Waters Wrong operation of power switch(p14) > Poor me- 5.1. Faultchanical Tree parts for assembly(p9)Navigation Risk > Improper personnel protection equipment(p11) > Lightning strike(p6) > Mechanical equipment(p10) > Residual charge inline(p4) > Landing and leavingThis section the fan(p3) describes > Improper the fault use tree of protection analysis devices(p15) of collision >(see Wrong Figure ﬁxed 14) position and submarine of cablesafety (see ropes(p14)Figure 15). > Fracture of protection safety rod(p13) > No professional operation and maintenance ships(p16) > Maneuvering error(p17).

5. Navigation Risks of Wind Farm Waters 5.1. Fault Tree for Navigation Risk This section describes the fault tree analysis of collision (see Figure 14) and submarine cable (see Figure 15).

J. Mar. Sci. Eng. 2021, 9, 881 18 of 32 J. Mar. Sci. Eng. 2021, 9, 881 20 of 33

Collision

CC Q=0.7194

Collision No anti-collision tendency measures

CC1 CC2

Improper anti- No timely Collision with Collision caused Arbitrary sailing collision facilities emergency propulsion by drifting in the wind farm of fan foundation response

CC CC3 CC4 cc11 CC9 10 FR=0.5456 Collision Ships in No effective reminder in Failure in Customary Bad Out of Equipment near avoidance conflict the late period of collision remote routes in the weather control the waterways failure direction avoidance process monitoring wind farm

cc1 CC5 cc7 CC8 cc12 cc13 cc14 cc15 FR=0.5044 FR=0.5448 FR=0.5366 FR=0.4828 FR=0.5774 FR=0.5748

Mooring Navigation Steering Propeller Unattended system failure failure failure failure

CC6 CC7 cc8 cc9 cc10 FR=0.5028 FR=0.5186 FR=0.4774

Electromagnetic Failure of Lack of Low Operation interference from navigation lookout visibility error wind farms equipment

cc2 cc3 cc4 cc5 cc6 FR=0.5308 FR=0.568 FR=0.5174 FR=0.4544 FR=0.5156 FigureFigure 14. 14.Fault Fault treetree analysisanalysis modelmodel for collision risk of of the the offshore offshore wind wind farm. farm.

Submarine cable

S Q=0.7171

Shallow location Dragging anchor of submarine cables

S1 S2

Anchor in the wind Insufficient buried Exposed Bad weather farm without depth of cables submarine cable choice

s1 S3 s7 S4 FR=0.4928 FR=0.5626

No filed No warning Fishing in Damage of information sign in the Ship equipment Cable near the No timely submarine cable Bad weather submarine about the submarine cable failure waterways maintenance laying area cables submarine cable laying area

s2 s3 s4 s5 s6 s1 s8 S5 FR=0.5388 FR=0.5446 FR=0.5092 FR=0.5106 FR=0.5178 FR=0.4928 FR=0.4914 No protection Wave wash for submarine cables

s9 s10 FR=0.5276 FR=0.4926 FigureFigure 15. 15.Fault Fault treetree analysisanalysis modelmodel for submarine ca cableble risk risk of of the the offshore offshore wind wind farm. farm.

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5.2. Risk Index of the Basic Events According to the questionnaires, the risk index of the basic events in the fault tree for collision and submarine cable are provided in Tables7 and8.

Table 7. Collision risk factors of offshore wind farms.

Number Risk Factor Risk Index

cC1 Collision avoidance failure 0.5044 cC2 Lack of lookout 0.5308 cC3 Low visibility 0.568 cC4 Operation error 0.5174 cC5 Electromagnetic interference from wind farms 0.4544 cC6 Failure of navigation equipment 0.5156 cC7 Bad weather 0.5448 cC8 Mooring system failure 0.5028 cC9 Steering failure 0.5186 cC10 Propeller failure 0.4774 cC11 Improper anti-collision facilities of fan foundation 0.5456 cC12 No effective reminder in the late period of the collision avoidance process 0.5366 cC13 Failure in remote monitoring 0.4828 cC14 Customary routes in the wind farm 0.5774 cC15 Equipment near the waterways 0.5748

Table 8. Submarine cables’ risk factors of offshore wind farms.

Number Risk Factor Risk Index s1 Bad weather 0.4928 s2 No ﬁled information about the submarine cable 0.5388 s3 No warning sign in the submarine cable laying area 0.5446 s4 Ship equipment failure 0.5092 s5 Fishing in submarine cable laying area 0.5106 s6 Cable near the waterways 0.5178 s7 Insufﬁcient buried depth of cables 0.5626 s8 No timely maintenance 0.4914 s9 Wave wash 0.5276 s10 No protection for submarine cables 0.4126

5.3. Risk Assessment (1) Risk assessment of collision With the help of Isograph Reliability Workbench, the minimum cut set, structural importance degree, and probability importance degree of the collision accidents are presented underneath. There are 40 minimum cut sets of collision accidents:

• Third-order minimum cut sets: { cC1, cC3, cC14}, { cC1, cC3, cC15}, { cC7, cC9, cC14}, { cC7, cC9, cC15}, { cC7, cC8, cC14}, { cC7, cC8, cC15}, { cC1, cC3, cC11}, { cC1, cC2, cC14}, { cC7, cC9, cC11}, { cC1, cC2, cC15}, { cC1, cC3, cC12}, { cC7, cC9, cC12}, { cC1, cC4, cC14}, { cC1, cC6, cC14}, { cC1, cC4, cC15}, { cC7, cC10, cC14}, { cC1, cC6, cC15}, { cC7, cC8, cC11}, {cC7, cC10, cC15}, {cC7, cC8, cC12}, {cC1, cC2, cC11}, {cC1, cC2, cC12}, {cC1, cC4, cC11}, { cC1, cC6, cC11}, {cC7, cC10, cC11}, {cC1, cC4, cC12}, {cC1, cC6, cC12}, {cC7, cC10, cC12}, {cC1, cC3, cC13}, { cC7, cC9, cC13}, {cC7, cC8, cC13}, {cC1, cC5, cC14}, {cC1, cC5, cC15}, {cC1, cC2, cC13}, {cC1, cC4, cC13}, {cC1, cC6, cC13}, {cC7, cC10, cC13}, {cC1, cC5, cC11}, {cC1, cC5, cC12}, {cC1, cC5, cC13}. The structural importance degree of collision accidents is as follows: I(cC1) > I(cC7) > I(cC10) = I(cC9) = I(cC8) > I(cC15) = I(cC14) = I(cC13) = I(cC12) = I(cC11) > I(cC6) = I(cC5) = I(cC4) = I(cC3) = I(cC2). J. Mar. Sci. Eng. 2021, 9, 881 22 of 33

I(cC1) > I(cC7) > I(cC10) = I(cC9) = I(cC8) > I(cC15) = I(cC14) = I(cC13) = I(cC12) = I(cC11) > I(cC6) = I(cC5) = I(cC4) = I(cC3) = I(cC2). The Fussell–Vesely Importance of the basic event of wind turbine collision in offshore wind farms is shown in Figure 16. The order of probability importance is listed as follows: Collision avoidance failure(cC1) > Bad weather(cC7) > Customary routes in the wind farm(cC14) > Equipment near the waterways(cC15) > Improper anti-collision facilities of C J. Mar. Sci. Eng. 2021, 9, 881 fan foundation (c 11)> No effective reminder in the late period of collision avoidance20 of 32 process(cC12) > Failure in remote monitoring(cC13) > Low visibility(cC3) > Steering failure(cC9) > Mooring system failure(cC8) > Lack of lookout(cC2) > Operation error(cC4) > Failure of navigation equipment(cC6) > Propeller failure(cC10) > Electromagnetic interference from The Fussell–Vesely Importance of the basic event of wind turbine collision in offshore windwind farms(c farmsC is5). shown in Figure 16. The order of probability importance is listed as follows:

Figure 16. Fussell–Vesely Importance of each basic event of a collision accident. Figure 16. Fussell–Vesely Importance of each basic event of a collision accident.

Collision avoidance failure(cC1) > Bad weather(cC7) > Customary routes in the wind farm(c(2) RiskC14) assessment > Equipment ofnear submarine the waterways(c cable C15) > Improper anti-collision facilities ofWith fan foundationthe help of (c CIsograph11)> No effectiveReliability reminder Workbench, in the late the period minimum of collision cut set, avoid- structural importanceance process(c degree,C12) and > Failure probability in remote importance monitoring(c degreeC13) >Low of the visibility(c submarineC3) > Steeringcable accidents are presentedfailure(cC9) >underneath. Mooring system failure(cC8) > Lack of lookout(cC2) > Operation error(cC4) > Failure of navigation equipment(c 6) > Propeller failure(c 10) > Electromagnetic interfer- There are 18 minimum cut setsC of submarine cableC accidents: ence from wind farms(cC5).  Second-order(2) Risk assessment minimum of submarine cut sets: cable {s3, s7}, {s2, s7}, {s6, s7}, {s5, s7}, {s4, s7}, {s1, s7};  Third-orderWith the help minimum of Isograph cut Reliabilitysets: {s3, s8, Workbench, s9}, {s2, s8, the s9}, minimum {s6, s8, cut s9}, set, {s5, structural s8, s9}, {s4, s8, importances9}, {s3, s8, degree, s10}, {s2, and probabilitys8, s10}, {s1, importance s8, s9}, {s degree6, s8, s10}, of the {s5, submarine s8, s10}, cable {s4, accidents s8, s10}, {s1, s8, are presented underneath. s10}.There are 18 minimum cut sets of submarine cable accidents: •TheSecond-order structural importance minimum cut degree sets: {s3, of s7}, su {s2,bmarine s7}, {s6, cable s7}, {s5, accidents s7}, {s4, s7},is as {s1, follows: s7}; •I(s7)Third-order > I(s8) > I(s10) minimum = I(s9) cut sets:> I(s6) {s3, = s8, I(s5) s9}, = {s2, I(s4) s8, s9},= I(s3) {s6, s8,= I(s2) s9}, {s5, = I(s1). s8, s9}, {s4, s8, s9}, The{s3, Fussell–Vesely s8, s10}, {s2, s8, Importance s10}, {s1, s8, s9}, of the {s6, s8,basic s10}, event {s5, s8, of s10},submarine {s4, s8, s10}, cable {s1, in s8, offshore s10}. wind farms is Theshown structural in Figure importance 17. The degree order of of submarine probability cable importance accidents is asis follows:listed as follows: InsufficientI(s7) > I(s8) buried > I(s10) depth = I(s9) of > I(s6)cables(s7) = I(s5) > = No I(s4) timely = I(s3) =maintenance(s8) I(s2) = I(s1). > Wave wash(s9) > No protectionThe Fussell–Vesely for submarine Importance cables(s10) of the basic > No event warning of submarine sign in cable the in submarine offshore wind cable lay- farms is shown in Figure 17. The order of probability importance is listed as follows: ing area(s3) > No filed information about the submarine cable(s2) > Cable near the waterways(s6) > Fishing in submarine cable laying area(s5) > Ship equipment failure(s4) > Bad weather(s1).

J. Mar. Sci.J. Mar.Eng. Sci.2021 Eng., 9,2021 881, 9, 881 21 of 32 23 of 33

FigureFigure 17. Fussell–Vesely 17. Fussell–Vesely Importance Importance of each basic basic event event of submarineof submarine cable cable accident. accident. Insufficient buried depth of cables(s7) > No timely maintenance(s8) > Wave wash(s9) 6. Discussion> No protection for submarine cables(s10) > No warning sign in the submarine cable layingAccording area(s3) to > the No previous filed information Fault Tree about Analysis, the submarine during cable(s2) the operation > Cable period, near the the risks of offshorewaterways(s6) wind >power Fishing are in submarineinfrastructure, cable layingequipment area(s5) and > Ship personnel, equipment and failure(s4) navigation. > The resultsBad show weather(s1). that: (1) 6.Collapse Discussion and corrosion are the main focuses of infrastructure risks of the offshore windAccording farm operation. to the previous Fault Tree Analysis, during the operation period, the risks of offshore wind power are infrastructure, equipment and personnel, and navigation. The (a) The top three risks for infrastructure collapse are gale, earthquake, and tide. results show that: (b) The top three risks for infrastructure corrosion are untimely overhauling, im- (1) Collapse and corrosion are the main focuses of infrastructure risks of the offshore windproper farm anti-corrosion operation. measures, and salt spray. (2) Fire,(a) lightning, The top blade three failure, risks for and infrastructure personne collapsel injury are are gale, the earthquake, main events and of tide. equipment and(b) personnelThe top safety three accidents. risks for infrastructure corrosion are untimely overhauling, improper anti-corrosion measures, and salt spray. (a) The main causes of equipment fire are improper fire stopping methods, oil leak- (2) Fire,age, lightning, oily cotton, blade and failure, other and materials. personnel injury are the main events of equipment and personnel safety accidents. (b) The main risks of lightning strikes are the high average number of thunderstorm (a) The main causes of equipment fire are improper fire stopping methods, oil days, poor contact between the flange of the tower and the electrode, and salt leakage, oily cotton, and other materials. (b)spray.The main risks of lightning strikes are the high average number of thunder- (c) The mainstorm causes days, poor of turbine contact between blade failure the flange are of the the loose tower internal and the electrode,structure of the fan, brakeand salt failure, spray. and imbalance impeller. (d) (c)The mainThe main causes causes of ofpersonnel turbine blade injury failure are are working the loose at internal height, structure misoperation, of the and live inspection.fan, brake failure, and imbalance impeller. (d) The main causes of personnel injury are working at height, misoperation, and (3) Ship collisionslive inspection. and submarine cable accidents are the main navigation risks of off- (3)shore Ship wind collisions farms. and submarine cable accidents are the main navigation risks of offshore (a) windThe farms. risks of ship collisions are collision avoidance failure, bad weather, and cus- (a)tomaryThe routes risks of in ship the collisions wind farm. are collision avoidance failure, bad weather, and (b) The riskscustomary of submarine routes in the cable wind accidents farm. are insufficient buried depth of cables, no timely maintenance, and wave wash. The following measures are proposed to improve the safety of the operation of offshore wind farms. (1) Measures for infrastructure safety: (a) To reduce the risk of collapse, firstly, the design of the wind turbines should be well considered. Secondly, the manufacturing processes need to be improved.

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(b) The risks of submarine cable accidents are insufficient buried depth of cables, no timely maintenance, and wave wash. The following measures are proposed to improve the safety of the operation of offshore wind farms. (1) Measures for infrastructure safety: (a) To reduce the risk of collapse, firstly, the design of the wind turbines should be well considered. Secondly, the manufacturing processes need to be improved. Thirdly, the qualified connection between the components should be guaranteed. Besides, periodic inspection and maintenance are indispensable. (b) To reduce the risk of corrosion, proper anti-corrosion measures for different parts of the turbines in different environments are needed. Moreover, corrosion monitoring and periodic maintenance should be well carried out. (2) Measures for equipment safety: (a) For fire prevention, fully functioning, maintenance, and inspection of the Fire Protection Systems are the keys. Refinements of related management regulations are also needed. (b) For lighting strikes, qualified and complete lighting protection devices are essential. (c) For blade failure, using proper blade design according to the wind farm location and environment is important. Keeping the smoothness of the surface of the blames is necessary to avoid flutter. Regular wind turbine inspections should be carefully conducted. (3) Measures for personal safety: (a) Improvement of the safety management systems and training to strengthen personnel safety awareness are needed. (b) Professional operation and maintenance ships should be equipped, and early warning systems and First Aid Center are also needed. (4) Measures for navigation safety: (a) Optimizing the planning and design before constructing the offshore wind farms is an efficient way to avoid significant impacts on existing navigation routes. Besides, competent traffic management is needed for efficient and safe navigation in and near wind farms. (b) The precautionary zones should be set where submarine cables lay. The placement of the submarine cables should be fielded in time. Besides, timely repairments are needed once any small faults occur.

7. Conclusions In this paper, the risk characteristics of offshore wind farm operations are analyzed based on the identification of hazards and risk assessment theory. A systematic fault tree analysis of the offshore wind farm operation is carried out. Based on the experience of offshore wind power operation and accident data statistics, and combined with the fault tree analysis method, this paper proposed a top-down system risk modeling method for offshore wind farm operation safety from the perspectives of infrastructure, offshore wind power equipment, and personnel safety, and navigation waters. Based on the risk index of each basic event that is calculated according to experts’ judgment, quantitative and qualitative analysis, including the minimum cut set, structural importance degree, and probability importance degree of each risk, are carried out. Consequently, the key factors and importance of offshore wind farm operations are analyzed. According to the analysis, the main risks to offshore wind farms are collapse, corrosion, fire, lightning strikes, blade failure, personal injury, ship collision, and submarine cable damage accidents. The main causes are gale, untimely overhauling, improper fire stopping methods, and high average number of thunderstorm days, the loose internal structure of J. Mar. Sci. Eng. 2021, 9, 881 23 of 32

fan, working at height, collision avoidance failure, and insufﬁcient buried depth of cables. Moreover, suggestions to reduce the operation risk of offshore wind farms are proposed. The results presented in this manuscript are based on 50 questionnaires, and the risk index for each basic event is the mean value of the answers. For future research, weighting factors to include the impact of the job position and work seniority can be taken into account. Moreover, various data resources can be combined to identify the risk, such as operation data, and AIS data of nearby ships are needed. Besides, validation is important for risk assessment research which should be considered in the next step. Furthermore, other data mining techniques and risk measures can be applied for comparison.

Author Contributions: Conceptualization, X.J. and J.M.; methodology, X.J. and L.C.; formal analysis, X.J. and P.C.; investigation, X.J. and L.C.; writing—original draft preparation, X.J.; writing—review and editing, L.C., J.M. and P.C.; supervision, L.C., J.M. and P.C.; project administration, J.M.; funding acquisition, J.M. and L.C. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Fundamental Research Funds for the Central Universities (WUT: 2021IVA051). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data available on request due to restrictions of privacy. Conﬂicts of Interest: The authors declare no conﬂict of interest.

Appendix A

Table A1. An overview of accidents and risk factors in related research.

Event Risk Factors Ref. The failure of a primary steel member Exposed to harsh and complex stresses (corrosion, physical loads, biological attack, and Infrastructure mechanical damage) [5] risks Environment (violent winds, large waves, temperature changes, infrared radiation, and ice and snow loads) Fractured bolts Human factors (including both “ethical” failures and accidents) Design ﬂaws (many of which are often the result of unethical practices) [6] Collapse Material failures Extreme conditions or environments Collisions between vessels and offshore wind turbines [19] Scouring [20] Unsuitable constructive design Coating failure (coating deterioration) and corrosion (metal loss) [3] Mechanical loading Stability and function of the steel structures of offshore wind constructions Corrosion The mechanical variables include: loading frequency, stress intensity factor, loading waveform, load interaction effects (variable amplitude loading), residual/mean stresses, material type and geometry

The metallurgical variables are microstructure and material composition, mechanical properties, heat [4] treatment, etc. The environmental variables include: temperature, pH, level of cathodic protection, coating type, oxygen concentration, etc. J. Mar. Sci. Eng. 2021, 9, 881 24 of 32

Table A1. Cont.

Event Risk Factors Ref. Oxygen and humidity Site-specific factors (water temperature, salinity, chlorinity, water depth, and current speed) The structures have long-term exposure to humidity with high salinity Intensive influence of UV light Wave action Bird droppings [5] Mechanical load (e.g., ice drifts or floating objects) Irregular inspection intervals Maintenance and repair costs Design life Chemical attack Abrasive action of waves and other substances in suspension (ice drift or floating objects) The attack of microorganisms Equipment and ignition source concentration All offshore wind turbines are unattended Wind turbine electrical equipment failure Overload Short circuit Fire Grounding fault [7] Technical defects Improper selection of electrical and electronic components Bolt loosening leads to high contact resistance Excessive exposure to humidity, salt fog, and other environmental conditions Limited operating experience Thunderstorm activity The topographical conditions Lightning [8] The height of the wind turbines The number of tall structures around Lightning [4] Lightning strikes Strong winds Large waves [8] Maintenance crew and spare parts cannot reach the wind farm immediately Lightning Blade failure Fatigue cracks Environmental condition such as the free corrosion conditions Loading conditions [10] Microstructure Welding procedure Residual stresses J. Mar. Sci. Eng. 2021, 9, 881 25 of 32

Table A1. Cont.

Event Risk Factors Ref. Noise Personnel Sleeping troubles and poorer sleep quality injury [9] Poor air quality External environment (wind, wave, tide, current, foggy) Navigational failure (watchkeeping, radar, voyage planning, mechanical, breakdown, etc.)

Ship (traffic, size and type, mode of operation, etc.) [11] Collision mitigation measures (ARPA, SBV, RACON, rotation of platform, tug boat assistance, anchoring failure, etc.) Offshore platform (geographical location, type, size, age, etc.) Collision Offshore wind farm is close to the traffic lanes for commercial and passenger ships [12] Maneuvering and drifting collisions Ship categories Minimum passing distance [14] Season and day/night time Courses Caused by fishing or emergency anchoring by ships Ships that are in an emergency may take immediate anchoring Submarine Shipping activities in water depths lower than 200 m cable [13] Seawater corrosion Seafloor sediment High/low wind Installation defects Calibration error Aging Environmental shocks Manufacturing and material defect Connection fault Failure in Corrosion [14] wind turbine Icing systems Maintenance errors Overload Fatigue Lightning strike Mechanical overload Insulation failure Insufficient lubrication Software failure J. Mar. Sci. Eng. 2021, 9, 881 26 of 32

Appendix B J. Mar. Sci. Eng. 2021, 9, 881 28 of 33 The questionnaire of identification of risk factors of operation of offshore wind farms are as follows. Fields marked with an asterisk (*) are required fields and must be filled.

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