ISMS2014

Collision Risk Assessment of Vessel and Offshore Platform: Case Study of Platform Construction Project at Bintuni Bay West Papua

Muhammad Habib Chusnul Fikri*, Ketut Buda Artana**, Made Ariana**, Dinariyana D.P.**, Kriyo Sambodho ***

* Student at the Department of Marine Engineering ITS Surabaya**Lecturer at the Department of Marine Engineering ITS Surabaya*** Lecturer the Department of Ocean Engineering ITS SurabayaEast Java, Indonesia.Mhabibcf93@gmail.com, ketutbuda@its.ac.id

Proceedings of the 3rd International Symposium of Maritime ScienceNov. 10-14, 2014 Kobe, Japan

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AbstractOne leading oil and gas company operates Tangguh gas field in Bintuni Bay, West Papua. There are also two refineries and a gas liquefaction facility operating there. In addition, the company currently has a many facilities consisting of two offshore platforms, gas pipelines, and fourteen locations of wellhead. Currently, the company initiated a project to build two new platforms, and a new refinery to increase production. As a one of requirements to issue a permit from the government of Indonesia, the risk will be assessed based on the level of consequence or impact of a collision on the platform and the level of frequency of collisions between ships and platforms. This Research describes how much risk impact due to collisions between vessels and platforms. The analysis is based on three main variables; platform geometry to the shipping channel, the strength of the structure in reducing the impact energy of the ship, and the seabed soil 's ability to absorb the impact energy of collision and hold the platform in order to remain upright. From this research, it will be evaluated whether the risk is acceptable or not, some certain steps that need to be done should risk is unacceptable.

Keywords Bintuni Bay, Marine Engineering, Risk Assessment, Ship Platform Collision.

I. FREQUENCY CALCULATION Visiting vessels, which approach the platform on legitimate business under their own power, including: Passing merchant vessels, which pass close to the platform because it lies close to their route between ports Fishing vessels, which may pass close to the platform repeatedly if it lies within fishing grounds. Naval vessels, which may conduct exercises near to platforms. Offshore tankers, which may load at offshore moorings near to other platforms. Other support vessels which are anchored beside platforms for long periods.

Figure 1 shows the design position of platform and the pipeline connecting those platform to receiving terminal. As shown that the facilities are located in a distance from the shipping lane. The distance between platforms to the center of the shipping lane is around 3000 m.

Platform BPlatform A

Figure 1. Platform and Shipping Lane Position

I.1 Head-on Passing Vessel CollisionThe collision frequency (i.e. the predicted number of collisions per year) is calculated for each shipping lane which passes the platform as: [1]

FCP = N x Fd x P (1)Where:N=Total traffic in the lane (vessel movements/year).

Fd=Proportion of vessels that are in the part of the lane directed towards the platform.

P=Probability of collision per passing vessel

Figure 2. Fault Tree Analysis of Head-on Collision

Fault tree analysis is used to calculate probability of collision per passing vessel. Few factors are included in calculation in order to know the probability of collision per passing vessel based on each developed collision scenario. Figure 3 shows the collision frequency is proportional to the size of the platform and the ship. The combined size is known as the collision diameter. The collision diameter is defined as the width of that part of the shipping lane crosssection from which the ship would hit the platform unless it changed course. If the traffic across the lane follows a normal (or Gaussian) distribution, the probability can be determined accurately by integration of the appropriate part of the distribution, which is usually carried out using published tables. If the collision diameter is small compared to the lane width, a more convenient analytical approximation is: [1]

Figure 3. Head-on Passing Vessel Collision Geometry [1]

Fd = D x f (A) (2)

For the normal distribution, the probability density is: [1]

f (A) = [exp (-k2/2)]/ (2) (3)

A = distance from platform to lane centerline at closest point of approach = standard deviation of traffic distribution across the lane, normally 50% of Ak = A/ i.e. the number of standard deviations that the platform is from the lane centerline

I.2 Drifting Passing Vessel CollisionThe developed scenario for drifting passing vessel collision is as follow: while there are any possible passing vessel attempt to collide with platform, there are some possibility of ships machinery or propulsion system breakdown. The possibility of drifting passing vessel collision then is influenced by wind and current direction at the present time. The mathematical equation is presented as follow [1] FCD = Nb x P x D/BL (4)

Where:Nb=total traffic in the box (vessels/year)

P=Probability of collision per passing vessel

DBL==collision diameterbox length perpendicular to wind direction

Figure 4. Fault Tree Analysis of Drifting Collision

For drifting calculation, developed fault tree analysis has a little different. A factor of wind/current blow is included as a cause factor that lead into collision while ship in a dead-ship condition Most merchant ships have a single diesel engine driving a single propeller. Breakdowns (i.e. loss of power, propulsion and/or steering) are not comprehensively reported, because most are repaired by the ship itself before any damage is caused. The frequency of breakdowns depends on the severity/duration of events which are included.

Figure 5. Drifting Passing Vessel Collision Geometry [1]

I.3 Visiting Vessel CollisionTankers which load offshore, or are permanently moored offshore as floating production or storage vessels, may suffer several types of collisions while approaching the platform. Same cases also applied on approaching supply vessel. An offshore tanker may become adrift if: It suffers machinery breakdown while approaching or departing from its mooring. It suffers machinery breakdown or major DP failure while stationed using dynamic positioning.

It may then collide with nearby platforms if: It drifts towards the platform It is unable to restart its machinery (if applicable) It is unable to use thrusters to alter its track It is unable to use anchors to stop

A probability distribution of wave directions may be obtained from wave climate data for the area. In a simple study, it is often assumed similar to the wind rose, which is more readily available, or even more simply assumed to be uniform. The angle subtended by the platform is estimated as: [1]A = arctan [(D1 + D2)/2L] (5)

where:

A=angle subtended by platform (rad)

D1D2L===width of tanker normal to drift trackwidth of platform normal to drift track initial distance of tanker from platform

For a uniform distribution of wave directions, the probability of drifting on a collision course is then /2.

II. CONSEQUENCE ASSESSMENTThere are two criteria of impact level as follow: Global failure; very large impact collision, resulting in a very massive deformation that led to failure of structure and facility shutdown Local failure; produced impact has exceeded the power of material elasticity, resulting in permanent deformation. However, failure of structure still can be avoided so as facility shutdown is not necessary. Any impact absorbed by pile would give various effect depend on impact energy given by collision. Ship impact energy is calculated as follows:

Ek= k(mv2)/2 (6)

Where: m= ships displacement (weight) v= ships velocity k= 1.1 for head-on collision = 1.4 for drift collision

Beam deflection is the deflection due to the influence of external force against a column. In addition to the deflection of the beam, there are other deformations in the column due to the impact force, which is dent. Dents per diameter ratio indicates the possibility of a tear in the column due to the impact force. Collision energies capable of being held by a cylindrical tubular column is as follows [3]:

(7) Where: E= absorbed energy D= outer diameter OD mp= plastic moment capacity (=0.25 x SMYS x t2) = dent depth t = column thickness

Where the dent per diameter ratio exceeds 5%, damage repairing is needed. [3] The denting of a tubular is described by the equation below. This equation for impact energy (E), obtained from integration of the impact force as a function of the dent depth, are[13]

(8)

The result shows that equation (7) and equation (8) give similar value. In other word, those equations are verified.

III. ANALYSIS OF COLLISION RISKIII.1 Frequency CalculationFrequency analysis is conducted by estimating annual ships call per year. Considering the geometry location of platforms and ships lane which can came across by any directions, variation of standard deviation value is putted into calculation as shown from table 1. However, conservative values of hazard probability are taken into account to make sure that every possible hazards are covered into calculation. The analysis is conducted within this scenario:Scenario:1. Human error by standby watching officer2. Failure of platform location identification by navigation system3. Failure of propulsion system-dead ship4. Failure of determining the shipping lane, causing the ship take voyage lane near platform (within 500 m prohibited radius)

Table 1. Frequency Calculation for Head-on Collision

As shown from table above, result of annual frequency calculation indicate the hazard of being hit by ship is safe. Basically, as long as the annual probability of collision is less than one, we could assume that is safe.

III.2 Consequences AnalysisVarious kind of possible ships are used in calculation of consequences. For each ship classes based on displacement and engine power, each consequence level is calculated by various speed (4-10 knot for head-on collision and 1-4 knot for drifting collision). Size of the single leg is obtained by data provided from BP Indonesia [Ref no.25, 26], having 1600 mm of diameter and 60 mm wall thickness. By this simulation, it will be known the limit of structure resistance from impact energy

Table 2. Consequence Calculation

Finite element analysis also conducted as a verifying method of consequence analysis

Figure 6. Results of FEM Analysis

Figure 7. Results of Empirical Analysis

From this analysis, it can be concluded that estimate value of impact energy absorbed by platform structure is about 37% of total impact energy (kinetic energy from ship)

IV. Conclusion and MitigationAs shown from table above, the structure is not strong enough to resist the impact given by ships collision. This is show that the design of structure is not made to withstand impact collision. Therefore, to reduce the risk of collision the probability of collision itself must be reduced by do some mitigation efforts.Mark of Restricted Area. Under the law of Republic of Indonesia number one (1) year 1973 about Continental Shelf of Indonesia, to carry out exploration work in natural resources, Indonesian government establishes a prohibited area in a radius of 500 meters from the outermost point of the installation of the exploration, and a limited area with a radius of 1250 meters from the outermost point of the restricted areas, where all the ships of third parties are prohibited from entering or doing activity in the area. A set of buoys can be used as mark of restricted areas.

Figure 8. Illustration of border of restricted area around platforms.

Radio communication between offshore installations. The Health and Safety Executive has become aware of several recent incidents where installations, or their attendant standby vessels, have been unable to establish radio communication with a vessel on a collision course with the installation. Further examination of these incidents has shown that there were a number of reasons why difficulty was experienced in establishing communications. However, it has become clear that radio procedures used to establish communications with the approaching vessels are often not correct in that the initial calling was made on VHF Channel

Figure 9. Marine VHF radio.

Under the new GMDSS procedures, distress, urgency and safety alerts are made in a significantly different manner than before. Such alerts are not made by voice, but digitally via digital selective calling (DSC) equipment, on different frequencies (on VHF the alerting frequency is now channel 70). After communication is established the parties would change to a distress working frequency, which on VHF is channel 16.Ship-to-ship communication is to be conducted on VHF channel 13. By further amendment, the IMO has determined that ships subject to SOLAS are to maintain a continuous watch where practicable on VHF channel 16.Automatic Identification Systems. TheAutomatic Identification System is an automatic trackingsystemused onshipsand byvessel traffic servicesfor identifying and locatingvesselsby electronically exchangingdatawith other nearbyships, AIS base stations, and satellites. When satellites are used to detect AIS signatures then the term Satellite-AIS) is used. AIS information supplementsmarine radar, which continues to be the primary method of collision avoidance for water transport. AIS was developed by the IMO technical committees as a technology to avoid collisions among large vessels at sea that are not within range of shore-based systems.

The technology identifies every vessel individually, along with its specific position and movements, enabling a virtual picture to be created in real time. The AIS standards include a variety of automatic calculations based on these position reports such as Closest Point of Approach (CPA) and collision alarms. As AIS is not used by all vessels, AIS is usually used in conjunction with radar. However, this recommendation is mean to install AIS system inside platform for the purpose of two ways communication with passing vessel, in conjunction with the use of radio communication.

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