A Study on FSA Application to PRS for Safe Operation of Dynamic Positioning Vessel

2.1. Research Premise

The cumulative data on human injury, property damage and environmental pollution in the incident reports of DP vessels are necessary for the FSA implementation(IMO, 2008).

The FSA is used to recommend the methods to mitigate hazards such as human injury, environmental pollution, property damage, etc. resulting from marine incidents based on the cumulative data. One of incident reports on DP vessels is the IMCA’s DP Station Keeping Incidents. However, aforementioned hazards resulting from DP LOP incidents are not classified and analyzed in the data, these data cannot be regarded as risk factors. Therefore, in this study, it is considered the causes of DP LOP incidents and the severity by incident types as risk factors in conducting the FSA as in Table 1 with reference to previous studies. Still, since drift off in Table 1 was caused by engine problems rather than by PRS errors, we excluded drift off from the incidents caused by PRS errors in the FSA implementation.

Table 1

Categorization of DP LOP Incidents(Shi, 2005).

2.2. Tools Used in the FSA

3.1. Hazard Identification(FSA Step 1)

The risk is assessed by multiplying the probability of incident by the consequence of damage. Quantified in the index, the Risk Index (RI) equals the Frequency Index (FI) added by the Severity Index (SI), and the notion is as follows.

Table 2 classifies the severity of DP LOP incidents into drive off, operation abort and time loss, and quantifies it into the SI of LOP incidents based on the Table 1. Table 3 shows the FI of LOP incidents caused by PRS errors, which was determined in brainstorming based on the data of past incidents. The appropriate scope of FI standard can be determined based on incident reports. The RI based on the SI and the FI is shown in Table 4. Drive off (SI 3) in case of FI 7 has the highest risk of 10 and time loss (SI 1) in case of FI 1 has the lowest risk of 2.

Table 2

Severity Index of LOP Incidents

Table 3

Frequency Index of PRS Error DP LOP Incidents

Table 4

Risk Index of DP LOP Incidents(Chae, 2016)

3.2. Risk Analysis(FSA Step 2)

It is necessary to identify the frequency of LOP incidents in order to identify the hazard of DP LOP incidents caused by PRS errors. According to the IMO Guidance, the frequency of DP LOP incidents caused by PRS errors equals the number of LOP incidents that occurred for a certain period of time divided by the duration in which a certain DP vessel was exposed to risks, or the DP vessel year(IMO, 2008). DP vessels do not operate 365 days a year and do not always use the DP system during operation. Thus, such aspects should be taken into account in the calculation of the DP vessel year.

To this end, this study referred to previous studies on the DP vessel year, or the DP system operation period of a DP vessel in a year. Previous studies set the operation period of DP vessel and the period of DP system operation as 70%, respectively, and determined as follows (Shi, 2005).

The number of the DP vessels exposed to risks was estimated based on the Clarkson (2012) and KMI (2012) reports. Since it was difficult to find data that classified the number of DP vessels separately, the average annual growth rate of 9% presented in the Clarkson (2012) report was reversely applied based on the total number of 6,052 DP vessels as of 2011 presented in the KMI (2012) report. Accordingly, it was found that 38,840 DP vessels were operated for the decade from 2001 to 2010.

The total number of 38,840 DP vessels multiplied by the number of days a DP vessel was exposed to risks throughout the specified year, or 178.85 (days), equals the total number of days DP vessels were exposed to risks for a decade, which amounts to 6,946,534 (days). If the said figure is divided into 365 (days), it results in 19,032 DP vessel year. The frequency of DP LOP incidents related to the PRS for the decade based on such data is shown in Table 5. It is found that LOP incidents caused by PRS errors fall under the category of time loss, drive off and operation abort.

Table 5

Frequency of DP LOP Incident Scenario Caused by PRS

In Table 6, the incidents that resulted in drive off, operation abort and time loss were classified into PRS types and the frequency was calculated. The frequency is calculated by dividing the number of PRS errors by the fleet at risks. It is found that drive off and time loss caused by DGPS errors demonstrate the highest frequency.

Table 6

Error frequency of each PRS(Chae, 2016)

The RI in Table 7 was calculated based on Table 6 by applying the SI and the FI in Tables 3 and 4. It shows that drive off (RI 8) caused by the DGPS has the highest risk.

Table 7

RI of each PRS for DP LOP Incidents(Chae, 2016)

In the FSA, the hazard of incidents is expressed in the two-dimensional F-N curve(IMO, 2013). In the F-N curve of the IMO’s FSA, the horizontal axis “N” is the number of casualties in incidents and the vertical axis “F” is cumulative frequency(IEC, 2009).

However, since the DP LOP incidents analyzed in this study do not include human injuries, this study set the aforesaid frequency of drive off, operation abort and time loss, caused by PRS errors as the horizontal axis “N” and the cumulative frequency as the vertical axis “F.”

To this end, the frequency and cumulative frequency of LOP incidents with regard to drive off, operation abort and time loss were analyzed as in Tables 8 to 10. The cumulative frequency is a value calculated by consecutively adding the frequency of each PRS.

Table 8

Cumulative Frequency of Drive Off(Chae, 2016)

Table 10

Cumulative Frequency of Time Loss(Chae, 2016)

Table 9

Cumulative Frequency of Operation Abort(Chae, 2016)

The F-N curve was created based on the cumulative frequency indicated in Tables 8 to 10 as in Fig. 1. As shown in Fig. 1, parts of time loss and drive off are beyond the boundary of As Low As Reasonably Practicable (ALARP). In Fig. 1, in general, Intolerable refers to the area where hazard should be reduced regardless of costs, Tolerable if ALARP refers to the area where hazard should be reduced to the extent that it is reasonably probable, and Broadly Acceptable refers to the area tolerable without further measures.

Fig. 1

F-N Curve of Drive off, Operation abort and Time loss(Lee et al, 2011), (IMO, 2007)

Various studies were carried out by industry on the permissible standard of ALARP boundary, which serves as the standard of such classifications(IMO, 2000). While the ratio of average casualties to the total production, the average death rate acceptable in the activities of DP vessels, the figure of economic value, etc. of certain activities are necessary, no research has been conducted on the permissible standard of social hazard and personal hazard in the operation of DP vessels.

Based on the RI by PRS in Table 6 and the F-N curve in Fig. 1, the areas where it is necessary to control risks of LOP incidents caused by PRS errors were identified by the RI as in Table 11. Drive off caused by DGPS errors demonstrated the highest RI, followed by drive off caused by the HPR.

Table 11

Drive Off, Operation Abort & Time Loss Risks Caused by PRS(Chae, 2016)

Next, the Bayesian Network was applied to quantitatively examine the probability of PRS errors. In order to classify 117 PRS-related DP LOP incidents by the occurrence of failure in the MS Excel file, this study created a flow chart, ran the Excel file on the GeNIe, and examined the prior probability of DP LOP incidents as in Fig. 2. The leftmost factors indicate 10 kinds of errors by the PRS, and six factors represent the types of PRS used on the DP vessels.

Fig. 2

Prior probability of PRS errors and LOP incidents

As a result, the probability of LOP incidents caused by PRS errors is shown to be 48% for time loss, 41% for drive off and 14% for operation abort. In addition, in case of PRS, DGPS errors turned out to be the highest at 54%, the causes thereof were shown to be a signal weak or fail at 21% and hardware failure of PRS devices at 19%.

Fig. 3 shows, when the probability of occurrence of drive off is set at 100% (conditional probability), what would the probability of occurrence of causes be. According to Fig. 3, it is found that drive off would be caused by DGPS at 57%, by HPR at 31.7%, and by Artemis at 19.6%. In addition, the main causes of PRS errors are found to be signal weak or fail at 28.2%, hardware failure of the PRS at 25.0%, and the influence by external circumstances such as vessels surrounding the main vessel or other structures at 12.3%, etc.

Fig. 3

Conditional probability of Drive off

3.3. Risk Control Options(FSA Step 3)

Up to this point, this study have confirmed the PRS-related risk elements which mainly affect DP LOP incidents. In this paragraph, we would like to suggest Risk Control Options (RCOs) for the confirmed risk elements as above. We performed the identification of RCOs as per the following procedures.

This study have proposed the RCOs to control risks of LOP incidents caused by PRS errors through experts’ brainstorming as provided in Table 12 based on the standards mentioned in the IMO’s FSA. In regard to brainstorming, we listed up the RCOs for each of the PRS device in advance and checked whether the experts agreed with each of the items listed therein.

Table 12

Risk control options(Chae, 2016)

To be granted the experts’ consent through brainstorming and determining priority of the proposed items, this study applied the expert concordance matrix as stated in MSC 83/INF. 2 and the equation is as follows(IMO, 2007).

When W becomes “0”, it means that no agreement was made on the proposed scenario by experts; when W becomes “1”, it means that every expert agreed with the proposed scenario 100%. According to the Guidance, W > 0.7 shows a relatively high degree of expert concordance, 0.5 < W < 0.7 shows a middle degree of expert concordance, and W < 0.5 shows an unreliable degree of expert concordance(IMO, 2007). Table 12 is the result of brainstorming on the RCOs and the expert concordance stood at W = 0.91.

Since it is inefficient to reduce the risk by applying all of the RCOs proposed in Table 12, this study determined the priority for application of RCOs as in Table 13 through another brainstorming.

Table 13

Priority of RCOs(Chae, 2016)

Basically, the concept of redundancy is critically applied to DP vessels. Accordingly, experts determined that installation of more than two types of PRS on the Class 1 DP vessel would be the RCO with the highest priority. In addition, the experts also agreed that providing training to manufacturers of PRS and related devices to ensure that they have a clear understanding of how to use equipment and limitations could become the RCO with highest priority.

In order to confirm the reduced risk level through the application of RCOs, this study checked the reduced risk level by applying DP simulation to A1, B1, C1 and E1. As D1 and F1 were not installed in the model DP vessel of a simulator, this study were not able to apply D1 and F1. The DP simulator used in this case is the MT’s DP simulator of the United States installed in Korea in 2014.

In order to confirm the frequency of reduced occurrence of drive off, operation abort and time loss in the DP simulation upon application of each of the RCOs, the DPO was set to make an Offshore Supply Vessel (OSV) approach a semi-submersible drilling rig. The approaches were made, by 25 times each, from places 250m apart from each side of the drilling rig as shown Fig. 4(Chae, 2016). In case more than two types of PRS are used on the DP vessel, there was a case where an error occurred actually in one of the PRS in the simulation. However, when the above was not the case, we triggered an artificial error in PRS in the course of the operation, made the DPO responses to such error and checked the frequency of occurrence of drive off, operation abort and time loss.

Fig. 4

Concepts of DP vessel approach simulation

As a result of the simulation performance, in case two PRS are installed and used as in Table 14, Drive off went down by 11% on average. Operation abort and time loss also went down by 10% and 15%, respectively .

Table 14

Frequency of Drive Off Caused by PRS during Simulation(Chae, 2016)

Based on the opinion of DPO instructors that about 30% of those operating DP vessels do not receive any training on manufacturers, experts agreed through brainstorming that the reduced risk level caused by training of manufacturers about PRS equipment (A2, B2, C2, D2, E2 and F2) as in the RCOs of Table 12 would be 30%.

Based on the simulation result and 30% of the reduced risk level caused by training of manufacturers, this study applied the reduced risk level and drew F-N curve, and the result was shown as in Fig. 5.

Fig. 5

F-N curve after RCOs applied(Chae, 2016)

It is found that both time loss and drive off, which went beyond the boundary line in Fig. 1, moved to the Tolerable if ALARP part. Through the above, this study was able to confirm that installation of more than two types of PRS on every DP vessel and training about the relevant equipment would be able to bring resonable amount of reduction in the risk of incidents of drive off, operation abort and time loss on DP vessels.

3.4. Cost-benefit Assessment(FSA Step 4)

According to the IMO Guidance, Gross Cost of Averting a Fatality (GCAF) and Net Cost of Averting a Fatality (NCAF) are used as the standards to measure the cost-benefits of RCOs for reduction of risk.

GCAF measures the cost benefits of RCOs by checking the ratio of additional expenses of RCOs spent to reduce the risk to human life, whereas NCAF measures the cost benefits of RCOs by checking the ratio of additional expenses of RCOs spent to reduce the risk to human life in comparison with the economic benefit thereof(IMO, 2013). The equations are as follows.

This study calculated the expenses incurred to reduce the risk of LOP incidents of DP vessels. In addition, in case of NCAF, expenses incurred for protection of human life, values of vessel and cargo at the time of total loss, crew and repair expenses, etc. which are included in the economic benefits need to be quantified. However, since there is a limit to secure the relevant materials necessary. therefor, we applied GCAF only (Lee et al, 2011). Based on such premise, this study performed the cost-benefit assessment as per the following procedures.

In this case, the PLOP refers to the reduced risk of LOP incidents per DP vessel years and △PLOP refers to the potential risk reduction of LOP incidents per DP vessel after the application of RCOs. The NPV in the equation (9) refers to the net present value, and the meaning of each factor is as follows.

The data on the initial installation costs of the PRS were provided by DP equipment manufacturers as in Table 15(Chae, 2016).

Table 15

PRS Installation & Maintenance Costs (Unit: USD)

The cost-benefit analysis on RCOs A1, B1 and C1 was carried out accordingly as in Tables 16 to 18(Chae, 2016).

Table 16

Cost-benefit assessment for RCO A1

Table 18

Cost-benefit assessment for RCO C1

Table 17

Cost-benefit assessment for RCO B1

The GCAF of RCOs A1, B1 and C1 was found to be USD 275,772 (A1), USD 862,043 (B1), and USD 1,176,723 (C1), respectively. The IMO Guidance indicates the GCAF which costs USD 3,000,000 or less has cost benefits(IMO, 2006), (IMO, 2008). Therefore, RCOs A1, B1 and C1, which install more than two types of PRS on every DP vessel, are more reasonable in terms of cost-benefit assessment.

3.5. Recommendations(FSA Step 5)

So far, this study conducted the FSA from Steps 1 to 4, and based on the FSA on the PRS of the DP vessels in Step 5, this study make the following recommendations to mitigate risks of DP LOP incidents caused by PRS.