Study on Probabilistic Risk Assessment Model for Crossing Situation in Sunda Strait

Sunda Strait is a busy channel where cargo vessels could probably have a crossing situation with roro ferries. Due to a very limited record of the actual crossing collisions in Sunda Strait, this study performs a Probabilistic Risk Assessment (PRA) of near miss crossing situations in Sunda Strait due to the Traffic Separation Scheme (TSS) that has been set since July 1st, 2020. The analysis is based on the Automatic Identification System (AIS) data during three time-intervals (TI), the first two TIs represented the condition before the TSS came into force, while the last TI was taken after the TSS has been set. The traffic in Sunda Strait was categorized to eight vessel courses, two conditions and seven crossing zones. We proposed a new perspective for the evaluating the TSS by looking at the crossing situation with three different bases, namely crossing zone basis, course basis, and vessel type basis. The probability of a crossing situation was calculated based on the hour basis for each time interval. The UK HSE standard for individual risk is utilized and it is found that the TSS effectively reduced the frequency level of crossing situation from unacceptable to ALARP in CZ 1, 2, and 4. While in CZ 3, the frequency is decreased dramatically from unacceptable to acceptable level.


Introduction
These courses represent the IASL 1 that has been set as the course for cargo vessels that are going to pass through Indonesian territory. These courses are dominated by vessels engaged in a trip from Australia, Europe, or West Asia going to East Asia (Vietnam, Japan, etc.) and most of them are general cargo and bulk carriers. Course 1 is going to northern direction, while Course 2 is the opposite. These courses are important passageways in Sunda Strait as big vessels are using both courses alternately.

Course 3 and Course 4 (C3 and C4).
The other important courses in Sunda Strait are Course 3 and 4, which crossed by roro/passenger ferries that sail from Port of Merak to Port of Bakauheni and vice versa. Based on the actual schedule published by the Port of Merak, there are 50 trips per day commenced from the Merak, which make the traffic in the Sunda Strait vicinity becomes very crowded. Course 3 is from Port of Merak going in western direction and Course 4 is on the opposite direction from Port of Bakauheni to the eastern direction.
2.1.3. Course 5 and 6 (C5 and C6) These two courses are parallel with the Course 1 and Course 2, but a little denser and diverge traffic is observed in the eastern part of Sunda Strait. Similar with C1 and C2, these courses are dominated by merchant vessels. Most of the vessels are sailing in the domestic route, from the Java Sea to the industrial ports close to the Port of Merak, although several vessels also engaged in the international voyage similar to those in Course 1 or Course 2. The TSS implementation does not affect the traffic pattern in C5 and C6 as no routeing measure is set in this area.

Course 7 and 8 (C7 and C8).
These courses are dominated by domestic cargo vessels with smaller dimension sailing from the west part of Sumatera to the Java Island and return. Both courses are spanned diagonally and intersect with Course 1, Course 2, Course 3, and Course 4. Bulk carrier, tanker and general cargo are still found to be the most frequently passing vessel in these courses. Course 7 is going to the northeast direction, while Course 8 is going to the southwest direction.

Crossing zone 1 (CZ 1), Crossing zone 2 (CZ 2), and Crossing zone 3 (CZ 3).
The CZ 1, CZ 2, and CZ 3 are in the west area of Sunda Strait. The CZ 1 is an intersection between the course that follow IASL 1 (Course 1 and 2) with Course 7 and 8 which follow a diagonal course, and with Course 3 and 4 which designated for roro/passenger ferries. It is expected that crossing zone 1 would have the highest number of crossing situation, due to this crossing zone is made by 6 courses that intersect each other. The crossing zone 2 is the intersection between Course 1 and 2, the passage for cargo vessels, and Course 3 and 4, the courses for roro/passenger ferries. Hence, crossing situation is expected between ocean going cargo vessel and roro ferries. The crossing zone 3 (CZ 3) is located a little bit to the east side of the CZ 2, which is the potential location of crossing situation made by Course 7 and 8, which dominated by domestic cargo vessels with Course 3 and 4 for roro/passenger ferries.

Crossing zone 4 (CZ 4) and Crossing zone 5 (CZ 5).
The CZ 4 and CZ 5 are crossing zones that involving Course 5 and 6 dominated by cargo vessels and Course 3 and 4 for roro/ferries. Both locations are in the east area of Sunda Strait close to the Port of Merak. The CZ 4 is made between Course 5 and 6 with Course 3, which accommodating the roro/passenger ferries going from Port of Merak to the Port of Bakauheni. While the CZ 5 is also involving Couse 5 and 6 together with Course 4, which is the passage for crossing roro/passenger ferries going from Port of Bakauheni to the Port of Merak.

Crossing zone 6 (CZ 6) and Crossing zone 7 (CZ 7).
These crossing zones are made due to the the extension of Course 5 and 6 that meet with Course Course 7 and 8. As the Course 7 and 8 are dominated by domestic cargo vessels that going to or from the eastern part of Indonesia, so the vessel engaged in those courses might encounter a crossing situation in CZ 5 or CZ 6. The difference is, the vessel engaged in Course 7 and 8 would meet vessel engaged in Course 5 and 6 that sail to the north direction in CZ 6, whereas in CZ 7 the vessel sailing in Course 7 and 8 might encounter with vessel sailing in Course 5 and 6 that is going to the eastern direction.
This study is utilizing vessel trips as the basis for conducting the probability assessment, which make the development of the trips database is important. The trips database is arranged to contain information including timestamp, MMSI, vessel name, vessel type, length, width, trip count, course number, and crossing zone. The IWRAP software not only automatically connects the adjacent coordinates of one vessel to be considered as one trip, but also terminate the trip if the distance between one point to the next one is too far. However, due to the missing AIS data in between, it makes sometimes the software considers 1 trip as 2 or more trips. Hence, we need to conduct a visual observation to validate the trips resulted from the IWRAP. Another reason to conduct the visual observation is the inconsistency of the direction that we might find on some trips which make it hard to categorize the trip into the course number. The example of trips resulted by the IWRAP is depicted by Figure 6. Figure 6(a) shows the trip list of a vessel which consists of two trips. If we see the duration of both trips, the time gap between two trips is less than 1 hour, but the IWRAP separated it to be two different trips as shown in Figure 6(b) and (c). Thus, by doing the visual observation we conclude the trip count in the trip database for this vessel is only one trip instead of two.

Probability of crossing situations
The importance of analyzing the crossing situations probability in this paper is because we want to understand the likelihood of a trip is involved in a crossing situation. As mentioned in the previous chapter, this probability assessment is performed under three point-of-views. A vessel must be engaged in a trip and belong to one vessel type to enable it to be analyzed. The detailed equations and explanations for the probability of crossing situations are outlined as follows: 4.1.1. Crossing zone basis. The probability of crossing situation in crossing zone is: The number of is multiplied by 2, because one crossing situation in a crossing zone is made by two trips of two vessels. Hence, to calculate the probability of it, the number of crossing situation is doubled.
Where is the probability of crossing situation in crossing zone and is the total number of trips in a day (24 hours). The frequency of the crossing zone basis in this study is translated as the number of crossing situation occurred in the for one year when there are trips a day. The result of this calculation is the daily frequency. Thus, to make it becomes an annual frequency, the result is multiplied with 365.
Where is the probability of crossing situation in course .

Vessel type basis.
The annual frequency of crossing collision using the vessel type basis is: (10) Where is the probability of crossing situation of vessel type .

Acceptance Criteria
The annual frequency of the crossing situations in the Sunda Strait shall be categorized under the acceptance criteria to understand whether the TSS is improving the frequency or make it even worse. It is supported by the International Maritime Organization Maritime Safety Committee (IMO MSC) 72/16 about the Formal Safety Assessment (FSA) decision parameters including risk acceptance criteria [16]. Hence, an acceptance criteria from the UK HSE [17] for individual risk is used to categorized the level of frequency of crossing situations and it is shown in Figure 10. The figure separates the reversed triangle into three parts: unacceptable, As Low as Reasonably Practicable (ALARP), and acceptable. The situation falls under the unacceptable level when the frequency is more than 1.00E-03 per year. All efforts shall be made despite of the cost when the situation is on this level. Right below the unacceptable is the ALARP, when the frequency of 1.00E-03 until 1.00E-06. The mitigation action might be carried out to keep the risk at that level or even reduce it, not increase it. The last one is the acceptable level that can be achieved when the frequency is less than 1.00E-06. If the risk falls in this region, a good practice is needed to keep the situation. is