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Yutao Liu 1 Probability Analysis of Damage to Offshore Pipeline by Ship Factors 1 2 Yutao Liu 3 Ph.d. Candidate 4 School of Naval Architecture, Ocean and Civil Engineering 5 Shanghai Jiaotong University 6 800 Dongchuan Road, Shanghai, P.R. China 7 TEL: 021- 54748155, FAX: 8621-62933163 8 Email: [email protected] 9 10 11 Hao HU 12 School of Naval Architecture, Ocean and Civil Engineering 13 Shanghai Jiaotong University 14 800 Dongchuan Road, Shanghai, P. R. China 15 TEL: 8621-62933091, FAX: 8621-62933163 16 E-mail: [email protected] 17 18 19 Di Zhang 20 School of Naval Architecture, Ocean and Civil Engineering 21 Shanghai Jiaotong University 22 800 Dongchuan Road, Shanghai, P. R. China 23 TEL: 86-13472652951, FAX: 8621-62933163 24 E-mail: [email protected] 25 26 Submitted to the Transportation Research Board 92 th Annual Meeting 27 for Presentation and Publication 28 Submission date: July 31, 2011 29 30 Word Count: 4247(text) + 3250(13 figures) = 7497 words 31 32 Corresponding Author TRB 2013 Annual Meeting Paper revised from original submittal.

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Page 1: Probability Analysis of Damage to Offshore Pipeline …docs.trb.org/prp/13-0923.pdfYutao Liu 1 1 Probability Analysis of Damage to Offshore Pipeline by Ship Factors 2 3 Yutao Liu 4

Yutao Liu                                                                                                                          1 

Probability Analysis of Damage to Offshore Pipeline by Ship Factors 1 

Yutao Liu 3 

Ph.d. Candidate 4 

School of Naval Architecture, Ocean and Civil Engineering 5 

Shanghai Jiaotong University 6 

800 Dongchuan Road, Shanghai, P.R. China 7 

TEL: 021- 54748155, FAX: 8621-62933163 8 

Email: [email protected]

10 

11 

Hao HU ∗ 12 School of Naval Architecture, Ocean and Civil Engineering 13 

Shanghai Jiaotong University 14 

800 Dongchuan Road, Shanghai, P. R. China 15 

TEL: 8621-62933091, FAX: 8621-62933163 16 

E-mail: [email protected] 17 

18 

19 

Di Zhang 20 

School of Naval Architecture, Ocean and Civil Engineering 21 

Shanghai Jiaotong University 22 

800 Dongchuan Road, Shanghai, P. R. China 23 

TEL: 86-13472652951, FAX: 8621-62933163 24 

E-mail: [email protected] 25 

26 

Submitted to the Transportation Research Board 92th Annual Meeting 27 

for Presentation and Publication 28 

Submission date: July 31, 2011 29 

30 

Word Count: 4247(text) + 3250(13 figures) = 7497 words 31 32 

                                                              ∗ Corresponding Author

TRB 2013 Annual Meeting Paper revised from original submittal.

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Yutao Liu                                                                                                                          2 

ABSTRACT 33 

The transport of hydrocarbons by offshore pipeline is threatened by the rapid expansion of pipe 34 

networks and the increasing frequency of maritime activities. Risk management is thus necessary to 35 

manage and prevent ship-related hazardous events that may damage offshore pipelines. Probability 36 

analysis is the key to assessing the risk associated with ship operations on offshore pipelines, and 37 

decision making in managing that risk. Bayesian Network (BN) models are proposed in this paper to 38 

determine the probability of anchor damage and trawling damage to subsea pipelines. The BN 39 

models are developed by integrating directed acyclic graphs, and three computational methods 40 

(Boolean operation, standard and historical statistical analysis, and fuzzy set theory) to elicit 41 

marginal probability tables and conditional probability tables. A case study illustrates the utilization 42 

of two BN-related functions – probability prediction and probability updating – to determine final 43 

probabilities of damage to a subsea pipeline. The results of the analysis support risk ranking and risk 44 

reducing decisions associated with maritime operations in the area of offshore pipelines. 45 

46 

KEY WORDS 47 

Offshore Pipeline; Ship Factor; Bayesian Network; Damage Analysis; Probability Analysis 48 

TRB 2013 Annual Meeting Paper revised from original submittal.

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Yutao Liu                                                                                                                          3 

INTRODUCTION 49 

The transportation of hydrocarbons by subsea pipelines is highly efficient and convenient, while 50 

requiring minimal cost. As a result, offshore pipelines are gradually becoming the primary mode of 51 

transportation of oil and gas at sea. China currently has 3,000 kilometers of installed offshore 52 

pipelines, and is planning to increase that length threefold in the next decade. Some hazards 53 

associated with operating offshore pipelines include leakage, rupture and even bursts that result in 54 

interruption to transportation and production of hydrocarbons, require clean-up operations, and can 55 

cause catastrophic health, environment and safety accidents (1, 2). Consequently, maintaining the 56 

integrity of offshore pipeline transportation networks is of vital importance to a nation’s economy 57 

and peoples’ lives. Historical data (3) illustrate that a large number of accidents to offshore pipelines 58 

were caused by impact, ship anchoring and corrosion, as shown in Fig.1. 59 

60 

FIGURE 1 Accident breakdown of offshore steel pipeline. 61 

Obviously, the majority of anchoring and impact accidents are associated with passing 62 

ships, which can be categorized as damage to offshore pipeline by “ship factors”. With the rapid 63 

extension of offshore pipeline networks and the increasing frequency of maritime activities, it can 64 

be reasonably expected that accidents to offshore pipelines by passing ships will become more 65 

frequent. Therefore, risk assessment for ship factor hazards is necessary, and the results from such 66 

assessments could support risk ranking and then serve as the main basis to judiciously divide 67 

resources for inspection, maintenance and protection among different pipeline networks, pipeline 68 

segments, or related assets. 69 

In addition to the scoring-type algorithm method (4), many qualitative or 70 

semi-quantitative methods, such as the analytic hierarchy process (AHP), fuzzy logic, and neural 71 

networks, have been utilized in risk assessment models for onshore and offshore pipelines (5-9). 72 

These models give relative values of the assessment results, which could support risk ranking but 73 

fail in judging whether the risk is acceptable to the local community. Therefore, current research 74 

ranges from qualitative schemes to quantitative probabilistic systems. Also, physical models in 75 

association with probabilistic methodology are utilized to analyze failures of offshore pipelines 76 

caused by accidental external loads in research and incorporated into standards (10-13). These 77 

models offer absolute assessment results but require complex analytical procedures. In addition, the 78 

parameters used in these models are not frequently updated when new data become available for 79 

statistical analysis of the operation period of offshore pipelines. Fault tree (FT) analysis has also 80 

been shown to be an effective method in probabilistic failure analysis and has been employed in 81 

21%

30%26%

7%

6%5%

1%1% 1% 1% 1%

AnchoringImpactCorrosionStructuralMaterialNatural HazardConstructionMaintenanceHuman errorOperation problemsOther

TRB 2013 Annual Meeting Paper revised from original submittal.

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Yutao Liu                                                                                                                          4 

pipeline engineering (14, 15); however, FT analysis is best limited to modeling simple static 82 

systems, with complex systems being modeled through other procedures. 83 

Bayesian network (BN) has a much more flexible structure than FT, and can fit a broad 84 

range of accident scenarios. This paper presents a BN model for analyzing the probability of 85 

damage to offshore pipelines by ship factors. The remainder of this paper is structured as follows: 86 

Section 2 describe offshore pipelines and the hazards of anchor damage and trawling damage. Next, 87 

the establishment of BN models is presented in Section 3. Section 4 introduces the case study of the 88 

offshore pipeline from Pinghu Oil-Gas Field to Shanghai. Finally, conclusion and comparison of 89 

BN and FT are provided in Section 5. 90 

91 

DESCRIPTION OF OFFSHORE PIPELINE AND ACCIDENT SCENARIO 92 

As shown in Fig.2, the section of offshore pipelines considered in this paper is the middle portion of 93 

a subsea pipeline, viz., the section of pipeline away from both the platform and the shoreline 94 

(e.g., >500 m from the platform and >300m from the coastline). This middle section of pipeline is 95 

minimally affected by offshore platform operations and onshore activities. 96 

97 FIGURE 2 Illustration of a typical offshore pipeline. 98 

Anchor and trawling accidents (i.e., “ship factor”) to offshore pipelines occur frequently 99 

and, furthermore, previous studies (3) show that the frequency of containment loss caused by anchor 100 

and trawling accidents were 37% (19/52) and 44% (23/52) respectively. Therefore, this paper is 101 

limited to modeling anchor and trawling accidents. 102 

In order to establish a complete model for probability analysis, the accident scenarios 103 

may consider the following factors. 104 

Passing ship: engineering ship (supply boat, crane ship, etc.); transport ship (tanker, 105 

commercial ship, cruise ship, etc.); fishing vessel 106 

Hazardous activities: emergency anchoring (anchor weight, anchor shape); bottom trawling 107 

(type of fishing net, depth of casting net, draw force) 108 

Offshore pipeline characteristics: type (steel pipeline, flexible or umbilical); water depth; 109 

embedment depth; diameter, wall thickness, coating thickness 110 

Possible consequence to pipeline: impact damage, hooking damage 111 

TRB 2013 Annual Meeting Paper revised from original submittal.

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Yutao Liu                                                                                                                          5 

For this paper, the energy (in kilojoules) that a “ship factor” accident imposes onto a 112 

subsea pipeline is the hazard to be analyzed, and “damage” is the release of hydrocarbon from a 113 

distressed pipeline exposed to hazard. As pipeline characteristics (e.g., diameter, thickness, and 114 

material properties) vary significantly among pipelines and, therefore, energy was selected to define 115 

damage to facilitate application of the method to a wide range of situations. 116 

117 

ESTABLISHMENT OF BN MODELS 118 

Bayesian Network 119 

The representation of BN is a directed acyclic graph, in which the nodes represent variables, and 120 

arcs signify direct causal relationships between the linked nodes. If a node doesn’t have any parents 121 

(i.e. root node), the node contains a marginal probability table which expresses the prior probability. 122 

Otherwise, the node contains a conditional probability table that specifies how strongly the linked 123 

nodes influence each other (16). 124 

A BN methodology is used either to predict the probability of unknown variables or to 125 

update the probability of known variables. The processes of probability prediction and probability 126 

reasoning are all based on Bayes’ theorem. In the predictive analysis, conditional probabilities of the 127 

form | are calculated, indicating the occurrence probability of a 128 

particular accident given the occurrence or non-occurrence of a certain primary event. According to 129 

the conditional independence and the chain rule, BN finally represents the joint probability 130 

distribution of variables , … , included in the network as 131 

| 1

where are the parents of in the BN, and reflects the properties of the BN. In the 132 

updating analysis, those of the form | are evaluated, showing the 133 

posterior probability of a particular event given new information, called evidence E. The evidence is 134 

usually operational data including occurrence or non-occurrence of the accident or primary events 135 

(17): 136 

|, ,

∑ , 2

137 

Directed Acyclic Graphs for Anchor Damage and Trawling Damage 138 

As shown in Fig.3, anchor damage is defined as damage to an offshore pipeline by anchor impact, 139 

where the anchor impact occurs when a ship passes above an offshore pipeline, the ship’s engine 140 

fails and an anchor is deployed under emergency conditions. Two primary factors affecting impact 141 

energy are 1) the water depth which greatly influences the impact probability of the anchor, and 2) a 142 

pipeline’s coating which often reduces the severity of the impact (18). Both impact probability and 143 

impact energy are integrated to determine anchor damage to a pipeline. 144 

As shown in Fig.4, trawling damage to offshore pipelines is mainly caused by fishing 145 

nets hooking on to a pipeline. Fishing boats cast nets to perform bottom trawling, and the nets 146 

become caught on subsea pipelines. The nets entangled on the pipeline are dragged by the fishing 147 

boats resulting on large tensile loads being placed on the pipeline and causing damage to the 148 

TRB 2013 Annual Meeting Paper revised from original submittal.

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Yutao Liu                                                                                                                          6 

pipeline. Entanglement of a fishing net on a pipeline is a function of net-casting depth, water depth, 149 

and the pipeline coating (which mitigates the attachment of nets to a pipeline) (19). 150 

151 FIGURE 3 Directed acyclic graph of anchor damage. 152 

153 

FIGURE 4 Directed acyclic graph of trawling damage. 154 

155 

Elicitation of Marginal Probability Table (MPT) and Conditional Probability Table (CPT) 156 

The elicitation of MPT and CPT is complex due to the large amount of judgments required to fully 157 

quantify these relationships in a BN model. They are elicited in the following three ways in this 158 

study. 159 

Table 1 illustrates the conversion of Boolean operations (disjunction and conjunction 160 

) into CPTs, which is applied only to the events are considered binary (with two states: 0 and 1). 161 

This conversion process has been introduced in some researches about the comparison between 162 

fault tree and BN (20, 21). 163 

Historical data and standards (12, 19) offer an alternative method to educe MPT and 164 

CPT. One example is the marginal probability of engine. According to an investigative report (22), 165 

the failure rate of engines is 2E-05 per hour. As engines have rapidly improved and their reliability 166 

increased we choose 2E-06 as a conservative estimate of engine failure rate as a ship passes over an 167 

offshore pipeline thereby requiring an anchor to deployed under emergency conditions. In addition, 168 

TRB 2013 Annual Meeting Paper revised from original submittal.

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Yutao Liu                                                                                                                          7 

Table 2 shows the example of the CPT of anchor damage, which refers to the proposed damage 169 

classification used for bare steel pipes given by DNV-RP-F107 (13). 170 

TABLE 1 CPTs of Ship Passing and Anchor 171 

Boolean operation

engineering ship passing no transport ship passing no passing no fishing vessel passing no passing no passing no passing no ship passing

(passing = 1, no = 0)1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 1

Boolean operation

ship passing passing no engine fail work fail work anchor

(anchor = 1, no = 0)1 0 0 0 0 1 1 1

TABLE 2 CPT of Anchor Damage 172 

impact energy no

(0 kJ) low

(< 2.5 kJ) medium

(2.5 – 10 kJ)high

(10 – 20 kJ) tremendous

(>20 kJ) impact impact no impact no impact no impact no impact no

Dam

age no 1 1 0 1 0 1 0 1 0 1

minor 0 0 1 0 0 0 0 0 0 0 moderate 0 0 0 0 0.5 0 0.25 0 0 0 major 0 0 0 0 0.5 0 0.75 0 1 0

Minor damage: Damage neither requiring repair, nor resulting in any release of hydrocarbons. Moderate damage: Damage requiring repair, but not leading to release of hydrocarbons. Major damage: Damage leading to release of hydrocarbons, water, etc.

Finally, expert judgment is another important way of eliciting MPT and CPT. However, 173 

experts often prefer to linguistic judgment than probabilistic description, like “safe” and “unsafe”. 174 

The integration of fuzzy set theory can help domain experts to elicit MPT and CPT in an efficient 175 

manner. For example, in this paper we define the buried depth of the offshore pipeline using three 176 

fuzzy numbers, , defined over universe of discourse where each subset represents an depth grade; 177 

, , , as illustrated in Fig.6. According to experts’ opinion, 178 

the triangular membership function for each fuzzy number is represented by the 179 

following set of equations, 180 0, 0 ,

,, ,

, , ,

,, ,

, , ,

0, ,

3

In this example, it can be seen that for a pipeline with buried depth 0.75 meters the 181 membership values are 0.75, and 0.25 and zero for . The fuzzy set 182 

representing the buried depth can be written as a MPT, as shown in Fig. 5. 183 

TRB 2013 Annual Meeting Paper revised from original submittal.

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Yutao Liu                                                                                                                          8 

184 

FIGURE 5 MPT of buried depth educing by fuzzy set theory. 185 

Water depth can similarly be modeled as a fuzzy set, and buried depth and water depth 186 

are modeled as fuzzy sets to complete the evaluation of anchor (Fig. 3) and trawling (Fig. 4) 187 

damage. 188 

The elicitation of CPT could also be carried out from expertise by integrating fuzzy set 189 

theory with the AHP method. This method has been described extensively by (21), and is beyond 190 

the scope of this paper. 191 

192 

CASE STUDY 193 

Offshore Pipeline from Pinghu Oil-Gas Field to Shanghai 194 

An offshore pipeline with a length of 386.2 km transports natural gas from Pinghu Oil-Gas Field to 195 

Shanghai. The pipeline is divided into 6 segments in order to get evaluation results with significant 196 

difference. The essential data to create the BN model are listed in the Table 3. As shown in Fig.6, the 197 

1st and 2nd segments of the pipeline are located in the Zhoushan fishing ground, where fishing 198 

vessels appear frequently. A national coastal shipping line with busy transport ships runs through the 199 

3rd segment, and the 5th and 6th segments are mainly threatened by engineering ships servicing the 200 

Pinghu Oil-Gas field. In addition, the offshore pipeline is set on the continental shelf of the East 201 

China Sea, and thus water depth gradually increases from west to east. Finally, among all the 202 

pipeline sections, only the 6th one enjoys a covering of submarine soil with a depth of about one 203 

meter. 204 

TABLE 3 Basic Data of the Offshore Pipeline 205 

Pipeline segment

length (km)

water depth (m)

buried depth (m)

frequency of passing ship (per year) engineering transport fishing

1 25.2 0-10 0 50 50 1500 2 75.2 10-20 0 50 150 1000 3 70 30-50 0 50 2000 500 4 105.6 50-80 0 100 300 300 5 48.5 80-100 0 700 150 100 6 61.7 100-110 1 1000 100 50

TRB 2013 Annual Meeting Paper revised from original submittal.

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Yutao Liu                                                                                                                          9 

206 FIGURE 6 Offshore pipeline from Pinghu Oil-Gas Field to Shanghai. 207 

208 

Probability Prediction for the Pipeline Segments 209 

The procedure for estimating the probability of anchor damage, , is described by the equation: 210 

· 4

In the Eq. (4), (1, …, n) means the type of ship. is the frequency of the type of ships 211 

passing by per year. And calculated by the BN model reflects the probability of anchor damage 212 

once an -type ship passing through. In this paper, we only consider engineering ship, transport 213 

ship and fishing vessel. The probability of trawling damage, , is described by the equation: 214 · 5

where is the frequency of fishing vessels running through the offshore pipeline per year, and 215 

expresses the probability of trawling damage by one fishing vessel. 216 

In this paper, the BN model is analyzed using HUGIN 7.6 (23). By Eqs. (4) and (5), we 217 

could obtain probability prediction results of anchor damage and trawling damage, as shown in 218 

Table 4. Both types of damage are classified by the damage degrees of minor damage, moderate 219 

damage and major damage, which have been briefly introduced in the Table 2. 220 

TABLE 4 Probability Prediction Results of Anchor Damage and Trawling Damage 221 

Pipeline segment

anchor damage trawling damage minor moderate major minor moderate major

1 0.00E+00 3.58E-04 1.59E-03 1.50E-01 4.37E-02 1.79E-03 2 0.00E+00 2.01E-04 9.03E-04 4.38E-02 1.11E-02 4.52E-04 3 0.00E+00 2.80E-04 1.36E-03 5.00E-02 1.46E-02 5.95E-04 4 0.00E+00 6.80E-05 3.05E-04 1.07E-03 2.66E-04 1.08E-05 5 0.00E+00 7.44E-05 2.77E-04 4.71E-05 1.17E-05 4.76E-07 6 1.83E-04 8.25E-05 1.09E-04 9.80E-08 2.43E-08 9.90E-10

222 

Analyses of Main Causes 223 

The main causes of accident scenarios could be analyzed by the probability updating process, as 224 

TRB 2013 Annual Meeting Paper revised from original submittal.

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Yutao Liu                                                                                                                          10 

introduced in the section 2. Here the example of anchor damage to the 1th pipeline segment is 225 

utilized to illustrate the analysis process. After inputting the MPTs elicited from the Table 3 (prior 226 

probabilities of primary events), then given major anchor damage occurs (top event occurs), the 227 

updating of the marginal probabilities of primary events ( | ) could be 228 

calculated by HUGIN 7.6. As shown in Table 5, it can be concluded that “engine fail”, “buried depth 229 

is shallow”, “fishing vessel passes” and “water depth is shallow” generally have the greatest 230 

contribution to the occurrence of the top event, i.e., they are the main causes of the “major anchor 231 

damage” accident scenario. 232 

TABLE 5 Prior and Posterior Probabilities of Major Anchor Damage to the 1th Pipeline 233 

Segment 234 

Primary event (per hour) Prior Posterior Primary event Prior Posteriorengineering ship passes 0.0057 0.0249 buried depth is shallow 1 1 transport ship passes 0.0057 0.0268 buried depth is medium 0 0 fishing vessel passes 0.1712 0.9485 water depth is shallow 0.7 0.8033 engine fails 2.0E-05 1 water depth is medium 0.3 0.1967 235 

Risk Ranking and Risk Reducing Measures 236 

In order to compare the damage probability and the risk of the relevant hazards, an individual 237 

ranking from 1 (very low probability) to 5 (very high probability) is proposed (24), as shown in 238 

Table 6. Note, however, that the limits given in the Table 6 may be adjusted to comply with case 239 

specific requirements. Then the probability analysis results of the offshore pipeline (Table 4) could 240 

be ranked. For example, Table 4 shows the probability of major anchor damage to the 1st segment 241 

is 1.59E-03, which intervenes between 1E-03 and 1E-02. As a result, we consider it is high 242 

according to the ranking in Table 6. Then the probability ranking for major damage and 243 

minor/moderate damage are provided, with the analysis results of the main causes for several risky 244 

pipeline segments (see Table 7). 245 

Balance between safety and cost requires using different strategies for different damage: 246 

the occurrence of major damage should be eliminated, while the minor and moderate damage 247 

should be reduced as low as reasonably practicable (ALARP). Therefore, the following risk 248 

mitigation measures are proposed to address the results presented in Table 7. 249 

Risk reducing measures (reduce probability or reduce damage degree) must be taken for the 1st 250 

and 3rd segments, so as to practically eliminate major damage to these segments during the 251 

pipeline’s lifetime. 252 

Risk reducing measures (introduce safe distance or safety areas, introduce extra chaser tug or 253 

anchor chain buoys, etc.) are better suited for the 1st segment. As a result, the high probability 254 

of major damage and the very high probability of minor/moderate trawling damage could be 255 

both decreased by these measures. 256 

Since it is impractical to control the frequency of passing ships or the emergency anchoring of 257 

transport ships, measures to reduce the damage intensity, such as increasing a pipeline’s 258 

concrete coating are most appropriate for the 3rd segment. 259 

Furthermore, to evaluate the economic effects of any risk reducing measures, a cost-benefit 260 

calculation should be performed. It will be a combination of engineering principles and sound 261 

business practices based on economic theory (25), and the combination of BN model with 262 

cost-benefit value (CBV) is the major purport of our ongoing research. 263 

TRB 2013 Annual Meeting Paper revised from original submittal.

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Yutao Liu                                                                                                                          11 

TABLE 6 Damage Probability Ranking for One Pipeline 264 

Ranking Description Probability (per year) 1 (very low) So low probability that event considered negligible. <1E-05 2 (low) Event rarely expected to occur. 1E-04>1E-05 3 (medium) Event individually not expected to happen, but when

summarized over a large number of pipelines have the credibility to happen once a year.

1E-03>1E-04

4 (high) Event individually may be expected to occur during the lifetime of the pipeline.

1E-02>1E-03

5 (very high) Event individually may be expected to occur more than over during lifetime.

>1E-02

TABLE 7 Probability Ranking and Main Causes analysis for the Offshore Pipeline 265 

Segment major damage minor/moderate damage main causes of risky segments anchor trawling anchor trawling

1 high (1) high (2) medium very high (1) “buried depth is shallow”, “fishing

vessel passes”, “water depth is shallow”

(2) “fishing vessel passes”, “buried depth is

shallow”, “water depth is shallow”

(3) “buried depth is shallow”, “transport

ship passes”, “water depth is shallow”

2 medium medium medium very high 3 high (3) medium medium very high 4 medium low low medium 5 medium very low low low 6 medium very low low very low 266 

CONCLUSION 267 

This paper analyzed the probability of damage to offshore pipelines by passing ships using Bayesian 268 

Network. Firstly, anchor damage and trawling damage are identified as the main accident types to be 269 

described and analyzed. This is then followed by the creation of BN models. Three methods 270 

(Boolean operation, standard and historical statistic, fuzzy set theory) were used to elicit marginal 271 

probability table and conditional probability table. Subsequently, the Pinghu-Shanghai offshore 272 

pipeline is evaluated by BN models. Two important functions of BN – probability prediction and 273 

probability updating - were used to analyze damage probability and find the main cause of damage, 274 

respectively. Finally, the evaluation results were combined to support risk ranking and risk 275 

reduction measures. 276 

The risk assessment framework presented in this paper is of use to oil and gas operators 277 

so that the risk of anchor and trawling damage can be effectively managed. The quantification of 278 

risk allows an engineered design of pipelines and adjustment of maritime operations so that risk is 279 

controlled. The largest concern of operators, related to offshore pipelines, is the disruption of 280 

hydrocarbon delivery to the evacuation point. Anchor and trawling damage has previously caused 281 

interruptions in hydrocarbon deliveries, and operators could use the BN model to quantify damage 282 

frequency, consequences, mitigation measures, and cost of mitigation to achieve a specified risk of 283 

trawling and anchor damage. By doing so, the expected loss of hydrocarbon and expected costs of 284 

construction (depending on acceptable risk level to the operator) can be determined for establishing 285 

budgets for design, construction and installation, and also for operations and maintenances. 286 

287 

288 

TRB 2013 Annual Meeting Paper revised from original submittal.

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REFERENCES 289 

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TRB 2013 Annual Meeting Paper revised from original submittal.