2                                           Hazard Register and List of Modeled Scenarios

This appendix analyzes the potential hazards identified in the HAZID session documents as part of the MQRA. This analysis was conducted by the DNV team following the MQRA HAZID session using the HAZID log sheets as well as research on specific topics as required

 

Hazard Registers are presented for the potential hazards. Each Hazard Register begins with a description of the potential hazard, causes, consequences and safeguards. Then a typical risk assessment potential scenario is developed. In this potential scenario development, the frequency of the scenario is considered. Where the frequency is less than the lower bound of the Hong Kong EIAO criteria (1x10-9 /year), the scenario is not considered credible. The conclusion on the credibility of the scenario is provided in the risk evaluation section of the Hazard Register. Those scenarios that are credible will then be considered in the MQRA risk analysis. The Hazard Registers are not listed in order of priority.

 

2.1                                     High-Energy Ship Collision

A powered high-energy ship collision of a large ship at near right angles into the side of the LNG carrier is estimated to be a worst-case scenario.  Breach of containment is possible if the collision energy is sufficiently high.  Such a scenario could possibly lead to fire, potentially affecting populations on the water as well as onshore. The fire would be fought by existing fire fighting equipment that is standard on an LNG carrier. These include, but are not limited to, automatic fire detection and suppression, manual fire fighting equipment together with the water deluge that coats the entire deck and outer hull of a carrier in the event of an emergency.

 

This potential scenario will be evaluated along the entire transit route to both Black Point and South Soko. Three areas of particular concern were identified for the Black Point route.

 

 

2.2                                     Collisions between Gas Carriers

Along the route of the LNG carrier, it has been considered whether it is possible for two gas carriers to be in the channel at the same time.  This situation is not anticipated for South Soko.

Considering the volumes of LNG to be delivered, it is not anticipated that two LNG carriers (one inbound, one outbound) would be transiting Hong Kong waters at the same time.   In the unexpected event that this did occur, the transit of the inbound carrier would be delayed until the outbound carrier had cleared the southeast entry to the Ma Wan channel.

 

Considering a collision scenario between an inbound LNG carrier and an LPG vessel, traffic management safeguards are in place to assist in the minimization of vessel collisions.

 

2.3                                     LNG Carrier Grounding 

The Hong Kong waters involved in this study have a relatively low incidence of grounding.  This is due to the presence of very wide channels and deep water.  Nonetheless, the bottom of the transit route can be rocky and there is heavy population along some portions of the route, in particular along the west and north sides of the Ma Wan Fairway (Node 4, Black Point route), on Castle Peak Road, where the LNG carrier must make a greater than 90 degree turn to the west. There is also some concern in Node 5 of the Black Point route where the LNG carrier must pass through the narrowest point of the channel (310 meters) through buoys CP4 and CP5. The use of tugs are anticipated to greatly reduce this risk.

 

Grounding will be addressed in the detailed analysis in the MQRA.

 

2.4                                     Tsing Ma Bridge

At Node 4 of the Black Point route, the LNG carrier will be passing under the Tsing Ma Bridge, a heavily traveled bridge connecting Tsing Yi Island to Lantau and linking Chep Lap Kok Airport to the population centers of Hong Kong.  It is understood that the supports of the bridge are set on islands, meaning that a scenario where an LNG Carrier drifts toward the supports would lead to a grounding scenario, prior to the LNG Carrier colliding with the bridge or the supports. 

 

When an LNG carrier is nominated for discharge in Hong Kong it will be known that there is an air draft restriction under the Tsing Ma bridge.  Information on air draft will be part of the precharter exchange on carrier particulars between the owner/operator and the charterer and will be determined prior to approving the LNG carrier for the intended voyage

 

This scenario will be included in the detailed analysis of grounding scenarios as part of the MQRA.

 

2.5                                     Hazard Register – Black Point

Black Point Hazard No. H1:  Grounding

Black Point Hazard No. H2:  Ship Collision - Powered Collision of LNG carrier into another vessel

Black Point Hazard No. H3:  Ship Collision – Drifting Collision

 

Black Point Hazard No. H4:  Ship Collision - Powered Collision of another vessel into the LNG carrier

Black Point Hazard No. H5:  Ship Collision (Striking) – Powered Collision of LNG vessel into obstructions

Black Point Hazard No. H6:  Sinking / Foundering

Black Point Hazard No. H7:  Breach of Cargo Containment

Black Point Hazard No. H8:  Fire and Explosion

Figure 2‑1 – Event Tree for Fire and Explosion


Black Point Hazard No. H9:  Aircraft Impact into LNG Carrier During Takeoff/Landing from Chep Lap Kok Airport

2.6                                     Hazard Register – South Soko

South Soko Hazard No. H1:  Grounding

South Soko Hazard No. H2:  Ship Collision - Powered Collision of LNG carrier into another vessel

South Soko Hazard No. H3:  Ship Collision – Drifting Collision

 

South Soko Hazard No. H4:  Ship Collision - Powered Collision of another vessel into the LNG carrier

South Soko Hazard No. H5:  Ship Collision (Striking) – Powered Collision of LNG vessel into obstructions

South Soko Hazard No. H6:  Sinking / Foundering

South Soko Hazard No. H7:  Breach of Cargo Containment

South Soko Hazard No. H8:  Fire and Explosion

                       

                        Figure 2‑2 – Event Tree for Fire and Explosion

 

 

 

South Soko Hazard No. H9:  Aircraft Impact into LNG Carrier During Takeoff/Landing from Chep Lap Kok Airport

 

 

2.7                                     Events Excluded from the Analysis

The following section details the scenarios that have been excluded from the analysis.  The scenarios have been judged to either not result in a breach of cargo containment, have a frequency of less than 1x10-9 per year, or do not increase the consequence (hazard to life).  Further analysis (performed as needed) is also discussed.  

 

2.7.1                               Events without Breach of Cargo Containment

1.    750mm and 1500mm Grounding Events

For the grounding scenario, it was decided to only consider a small release (250mm inner cargo tank hole size), given that large and medium releases (750mm and 1500mm inner cargo tank hole sizes, respectively) related to grounding were considered unlikely, and are below the frequency criteria. 

The Sandia report conducted an analysis for the penetration of a double-hulled tanker, and determined that the penetration would need to be approximately three meters before generating a hole in the inner hull (ref. 03).  The Sandia report does not consider an accidental grounding event to result in cargo tank breach.  To date there has not been a loss of containment of LNG due to grounding.  The two grounding events that have occurred at 12 and 17 knots did not lead to a breach of containment.  As the speed of the LNG carrier in Hong Kong waters will not exceed 12 knots, the possibility of a medium or large breach of containment is remote.  To include grounding events in the analysis, the most credible scenario was determined to be a small hole in the cargo tank (250mm).     

 

2.   Drifting Collision between LNG carrier and another vessel

The drifting collision between an LNG carrier and another vessel is described in the earlier sections related to Black Point and South Soko Hazard No. H3.  The speed of the current in the Hong Kong harbor is not expected to exceed 1-3 knots.  A carrier will assume roughly 75% of the current strength, leading to a drifting speed of 0.75-2 knots.  The safe berthing speed is not greater than 4 knots.  At this speed, the design of the LNG carrier must be able to withstand collision with a fixed object or other vessel.  A drifting collision would involve such low impact energy that there would not be a breach of containment. 

 

3.    Breach of Containment from Pipework/Equipment on LNG Carrier

Breach of containment from pipework or equipment on the LNG carrier would occur due to spontaneous failures (material defects, construction defects, fatigue, corrosion, etc.) and/or operation errors.  Any release from the carrier equipment would be localized to the carrier and would be mitigated by the safety systems on board, such as the fire water protection systems.  As detailed in the Black Point and South Soko Hazard No. H8, in the 40 year history of LNG carrier transit there has not been a fire/explosion on board that resulted in a breach of containment. 

 

2.7.2                               Events with Frequency of < 10-9 per year

1.    Breach of Cargo Containment

A cargo containment breach due to material defect, fatigue, extreme environmental conditions, overfilling, etc. is described in the earlier sections related to Black Point and South Soko Hazard No. H7.  The tank construction materials are very highly corrosion resistant.  Forecasts will be available to assist in making any decision for avoidance of adverse weather to limit hull stresses from sloshing and potential structural fatigue.  Onboard operational procedures will describe how to avoid overfilling of tanks.  If there would be a potential for failure, this would have occurred in the open seas given the increased load to the carrier in these conditions and not in Hong Kong Harbour.  The frequency of a double containment cargo tank failure (2.6x10-10 per year based on onshore failure frequency) falls below the EIAO criteria and thus was not considered further in the MQRA analysis. The postulate of instantaneous tank failure has also been raised. The mechanisms for instantaneous tank failure do not exist for the transit of LNG in LNG carriers. 

 

2.    Potential Aircraft impact into the LNG Carrier

The hazard of an aircraft impacting the LNG carrier is described in the earlier sections related to Black Point and South Soko Hazard No. H9. The frequency of this event (estimated at 4.7x10-10) showed that the likelihood of aircraft impact on an LNG carrier is below the EIAO criteria and was thus not considered further in the MQRA analysis.

 

2.7.3                               Events without Increase to Consequence

 

1.    Potential LNG spill / fire / explosion in the carrier void space

The potential to have an LNG spill in the carrier void space that would lead to breach of containment from a second LNG tank due to embrittlement or that would lead to a fire or explosion that may also lead to a breach of containment from a second LNG tank is addressed more fully in Appendix III, Section III.2.  The Sandia Report (ref. 03) considers several incident scenarios and assesses the potential for and impacts of LNG carrier damage.  Sandia draws its conclusions from “embrittlement scoping analyses (that) were conducted to assess the potential damage to an LNG carrier from small and large LNG spills based on available fracture mechanics data and models”.  Sandia concludes:

 

·       While accidental incidents could lead to minor to moderate damage to a LNG carrier, they would not lead to severe structural damage and the potential for cascading damage to other tanks. 

 

·       Should there be a secondary event, “This cascading release is not expected to increase significantly the overall fire size or hazard ranges, but the expected fire duration would increase.”

 

2.    Sinking / Foundering

The sinking / foundering of an LNG carrier is described in the earlier sections related to Black Point and South Soko Hazard No. H6.  Capsize or sinking of the LNG carrier would most likely result from extreme weather.  In the 40 years of LNG carrier operations, a sinking event has never occurred.  According to the Stolt Nielsen Transportation Group (ref. 04), the Draught Fore at Normal Ballast for Ship Stolt Aquamarine (whose length is 176.8 meters) is 7.8 meters.  This is higher than the maximum wave height in Hong Kong Harbour in a typhoon, which is 5 meters.  This ship is almost the same size as LNG cargo transiting through Hong Kong harbor, Thus, it is almost impossible that the LNG cargo could be submerged by water even in extreme weather like a typhoon in the Hong Kong Harbour, as the sea states that the LNG carrier is designed for are of the order of 10 meters in normal operation.  Should wave conditions in the order of 10 meters arrive in Hong Kong harbor, there would be much more serious consequences for the harbor and surrounding area such that the LNG carrier events become inconsequential.

 

The carrier would avoid operating in severe weather conditions that exceeded its design criteria.  Thus the low likelihood of such an event deemed the scenario not credible to include in the analysis. 

 


References