appendix a - assumptions register s as -...

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REPORT NO./DNV REG NO.: 2013-4091 / 17TLT29-4

REV 1, 11.06.2013

APPENDIX A - ASSUMPTIONS

REGISTER

SKANGASS AS

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Report for Skangass AS

Appendix A - Assumptions Register

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DNV Rep. No.: 2013-4091 Revision No.: 1

Date : 11.06.2013 Page 1 of 1

Table of Contents

Assumption no. Subject

Description and Background Data

1-A Manning Level

2-A Meteorological Data

3-A Meteorological Parameters

4-A Ignition sources – Equipment

5-A Ignition sources – Traffic

6-A Ignition Sources – People

7-A Ignition sources – Hot work

8-A Bunkering installation – Base case design and inventory

9-A Escape and Evacuation of Passengers and Personnel

LNG accidents

Representative Scenario Assumptions

1-C Release Location / Height

2-C Release Sizes

Frequency Analysis Assumptions

3-C Leak frequencies

Event Tree Modelling Assumptions

4-C Detection and Isolation Times

5-C Isolation Failure

6-C Immediate Ignition Probability

7-C Event Tree Framework

8-C Event Tree Probabilities

Consequence Modelling Assumptions

9-C Dispersion Parameters

10-C Consequence Modelling Parameters

Storage & loading – Specific

1-D Bunkering Frequency

Impact Criteria

1-H End Point (Impact) and Vulnerability (Fatality) Criteria

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DNV Reg. No.: 17TLT29-4 Revision No.: 1

Date :11.06.2013 Page 1 of 32

LNG Bunkering Terminal, Risavika Harbour Date: 20.05.2013

Assumption No.: 1-A Revision: 1

Category: Description and Background Data

Subject: Manning level and distribution

Specifications:

Manning levels are defined for the following time periods:

‘Day’: Morning shift (08:00 to 16:00) and Afternoon shift (16:00 to 00:00).

‘Night’: 00:00 to 08:00.

The risk analysis is based on the onsite population and off-site population :

- 1st party: Personnel in the plant; Fjordline and Skangass personnel involved in the bunkering

operation; Skangass personnel.

- 2nd party: Risavika harbour personnel, Fjordline personnel not involved in the bunkering

operation, Ferry terminal workers.

- 3rd party: Population (workers and public) working/evolving in the Container ara,

Ernegiveien + Risavika, Rest Companies areas. Fjordline’s Passengers, public evolving around

the plant, Tananger residents.

The LNG bunkering operation of Fjordline which are not to be present 24 hours a day, all days

throughout the year, are taken into account. The bunkering operations are planned to last 1.5 hours

every day. 1 hour for LNG cool down and transfer and ½ an hour for connection and disconnection of

loading arm to ship manifold. Hence, possible leakage, from downstream ESD valve, may occur only

during 1 hour per day.

In order to obtain a good representation of the risk picture towards the ferry passengers and to take a

conservative approach, it has been decided that the same population is present for 30 % of the totality

of the LNG bunkering operations.

A total number of 4 workers (1st party) will be present at the bunkering terminal. 1 to 2 Fjordline

personnel at the ship manifold and 1 to 2 Skangass personnel at the jetty. This figure is used to

calculate the average individual risk for 1st party population.

The distribution of on-site (inside the ISPS area of Risavika jetty 38) and off-site people is summarised

in Table 1 and Table 2. Indoor and outdoor factors used in the analysis for the different parties are

presented in Table 3 and Table 4.

Implication of assumption:

Societal risks are directly influenced by the numbers of personnel exposed to hazardous events and

hence the results are sensitive to the manning assumptions.

Key influence on societal risk / FAR.

Reference: /1/ LNG Bunkering of Fjordline ferries Project Design Basis 17.02.2012, Draft Version

Prepared by: Sign: J-B, Berthomieu Date: 21.05.213

Internal Verification: Sign: Date:

Comment from Skangass AS:

Approved by Skangass AS: Sign: Date:

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Attachment to Assumption 1-A

Table 1 Base case manning within each area for 1st party

Area/Category During Bunkering, i.e. afternoon

Fjordline personnel involved in bunkering activity 2

Skangass Bunkering personnel 2

Table 2 Base case manning and population

Area/Category ‘Day’

‘Night’ 24 hrs

average ‘Morning’ ‘Afternoon’

Peninsula 16*1

16*1

0 2

Hiking Track 8*1

8*1

0 1

Skangass LNG plant*2

9 4 4 6

Ferry Terminal – Office Workers 100 10*3

10*3

40

Ferry Terminal – Industry Workers 10 10 14 11

Ferry Terminal – Passengers incl. ferry crew 0 1500 0 500

Energiveien+Risavika – Office Workers 400 40*3

5 148

Energiveien+Risavika – Industry Workers 559 56*3

0 205

Container Area – Office Workers 10 10 1 7

Container Area – Industry Workers 50 50 0 33

Rest Companies – Office Workers 1 139 114*3

10 421

Rest Companies – Industry Workers 715 72*3

0 262

Living Quarters 60 60 60 60

Tananger*4 5 964 5964 5 964 5 964

*1 In a non-working day

*2 24 hours average will be used when no bunkering is on-going. ‘Afternoon’ will be used when bunkering is

on-going.

*3 Assuming that 10% are working overtime.

*4 Population updated at the 1

st of January 2011, source: Statistisk sentralbyrå.

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Table 3 Indoor/outdoor fraction – 1st party

Area/Category

Indoor

fraction

Outdoor

fraction

Administration Building 1 0

Operator/Maintenance 0.8 0.2

Truck Loading (1 person per truck per 1.2h) 0 1

Ship Loading (Jetty- only during connection and disconnection) 0 1

Ship Deck (during loading only) 0 1

Ship Bridge (during loading only) 0.75 0.25

Table 4 Indoor/outdoor fraction - 2nd

and 3rd

party

Location/Category Indoor fraction Outdoor fraction

Peninsula 0 1

Hiking Track 0 1

Ferry Terminal – Office Workers 1 0

Ferry Terminal – Industry Workers 0 1

Ferry Terminal – Passengers 0.75 0.25

Ship passengers 0.75 0.25

Energiveien+Risavika – Office Workers 1 0

Energiveien+Risavika – Industry Workers 0 1

Container Area – Office Workers 1 0

Container Area – Industry Workers 0 1

Rest Companies – Office Workers 1 0

Rest Companies – Industry Workers 0 1

Living Quarters 0.75 0.25

Tananger Population 0.75 0.25

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Figure 1 Risavika harbour and population location

Skangass LNG

anlegg

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Subject: Meteorological Data

Specifications:

Data on the wind direction, wind speed and atmospheric stability are combined to form a set of

representative weather categories in the surroundings of Sola, which are taken from the QRA for Lyse

LNG Base Load Plant (ref A). Table 5 shows the wind data and Figure 2 shows the wind rose for Sola.


The weather conditions have a key influence on flammable cloud dispersion and fore heat loads, hence

the consequences associated with any release. The influence of any specific weather category and

direction will vary for each and every release, where on balance the resulting influence of any changes

in the meteorological assumptions will have a negligible influence on the risk results.

Relevant to specific consequences – risk is not sensitive to individual meteorological assumptions.

Reference:

QRA for Lyse LNG Base Load Plant – Train 1 (R100-LE-S-RS0003)





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Attachment to Assumption 2-A

Table 5: Rationalised representative weather categories for Sola

Wind Direction (o)

% Occurrence of Weather Classes (Pasquill Stability, Wind Speed)

D1.5 (day) / F1.5 (Night) D6 D12 Total

1.5m/s 6 m/s 12 m/s

292.5 – 337.5 1.99 14.71 2.79 19.49

337.5 – 22.5 0.961 7.09 1.346 9.397

22.5 – 67.5 1.012 7.47 1.417 9.899

67.5 – 112.5 1.633 12.04 2.293 15.966

112.5 – 157.5 1.335 9.89 1.878 13.103

157.5 – 202.5 0.501 3.69 0.702 4.893

202.5 – 247.5 0.807 5.96 1.13 7.897

247.5 – 292.5 1.977 14.57 2.76 19.307

All 10.216 75.42 14.316 100

Figure 2 Wind rose data for Sola, Rogaland

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Subject: Meteorological Parameters

Specifications:

In addition to the weather categories, certain meteorological constants are defined as inputs to the

consequence modelling. These values are summarised below:

Parameter Value Notes and References

Atmospheric temperature 10ºC Range is 5º to 45ºC.

Atmospheric pressure 101 325

N/m2

Average sea level pressure.

Relative humidity 68% Range is 50% to 85%.

Surface temperature 10ºC Taken to be the same as atmospheric

temperature.

Surface roughness parameter 0.3 for land

0.05 for

water

Land value appropriate for terrain with varying

geometry, water value for coastal waters.

Solar flux 100 W/m2 The maximum solar flux (i.e. midday

midsummer) is about 1320 W/m2. However, the

solar flux varies diurnally, annually and with

cloud amount. Hence the annual mean value will

be less than half the maximum. 100 W/m2 is a

representative value.

Wind speed reference height 10 m Standard for meteorological measurements.


The dispersion and consequences associated with LNG and other dense gas releases are relatively

sensitive to assumptions affecting the heat transfer to the cloud. Hence, the above values are relatively

conservative representative conditions, but will not necessarily correspond to the worst-case dispersion

conditions that may occur.

Representative conditions used – relevant to consequences, with relatively minor influence on

subsequent risks.

Reference:

QRA for Lyse LNG Base Load Plant – Train 1 (R100-LE-S-RS0003)





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Subject: Ignition Sources – Equipment

Specifications:

The basis for defining ignition source probabilities for equipment items within the plant is taken as the

JIP Ignition Modelling study /1/.

The values given in the JIP Ignition Modelling study are summarised below, together with

modification factors proposed within the study and the value adopted for this analysis. It is considered

that with respect to ignition sources associated with electrical and rotating equipment, the Skangass

LNG Plant and the LNG bunkering facility is consistent with a modern, best-practice offshore facility.

Hence the recommended modification factors are adopted for all except the ‘Other’ category, which is

considered to be less practical to control for land based facilities.

Equipment Type

Base Ignition

Probability, per

second of exposure

JIP

Modification

Factor

Modification

factor Used

Ignition Probability

Used, per second of

exposure

Electrical 2.7E-08 per m2 0.49 0.5 1.4E-08 per m

2

Other 2.1E-09 per m2 0.65 1 2.1E-09 per m

2

Pump 2.1E-07 per item 0.61 0.6 1.3E-08 per item

Compressor 5.1E-06 per item 0.61 0.6 3.1E-06 per item

Generator / Turbine 6.2E-06 per item 0.61 0.6 3.7E-06 per item

Ignition Source Ignition

probability

Time of

exposure (s)

Flare 54-FC-101 0.251)

10

Fired Heater for Hot Oil 52-FA-101 0.12)

60

H2S Converter 0.12)

60

Electrical Substation1)

0.13)

600

Ferry terminal and surroundings (e.g. lighting poles) 0.7 60

1) The flare is located 70m above the ground level, and only large leaks from segment 1 and 2 are

assessed to reach the flare. For other scenarios the ignition probability will be 0.

2) 60 seconds of exposure with an operating probability of 0.5

3) It is assumed that if the air intake of a substation or building is exposed to gas for 600 seconds,

which then enters the building, ignition will occur (i.e. ignition probability of 1). It is, however,

assumed that the gas detection and automatic closure of the HVAC intake dampers is effective in

isolating the sources within the building in 90% of such cases, i.e. the activation frequency for each

substation / building ignition source is taken as 0.1.

In case of gas detection, the bunkering activity will be automatically stopped


Key influence in determining the likelihood of flash fire, pool fire and explosion hazards and the extent

of each (i.e. time of ignition relative to size of cloud). Note, however, that the overall effect is that

there are a significant number of very low ignition probabilities.

Overall effect is a key influence on the risks, but not sensitive to any particular ignition source.

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Reference:

/1/ JIP, 1998. Ignition Modelling, Time Dependent Ignition Probability Model, Joint Industry Project –

DNV, Scandpower, et al. DNV Report No. 96-3629, Revision 4, February 1998

/2/ QRA for Lyse LNG Base Load Plant – Train 1 (R100-LE-S-RS0003)





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Subject: Ignition sources – Traffic

Specifications:

Bunkering will only take place when southbound ferry lay at port. Boarding will take place

either by passenger tube or cars/trucks by the ro-ro ramp. Consequently, vehicles do not board

through ISPS area, where bunkering station is to be located. Vehicles board from the parking

area south/southwest to the ISPS area, at a minimum distance of 40 m from the bunkering

station.

Bunkering is planned to take place at the same time as cars are boarding. However, no

passengers are allowed in the tube during this operation.

Unloading of vehicles only takes place when northbound ferry lay at port. Vehicles are then

routed through the ISPS area, i.e. directly past the bunkering station. However, bunkering is

not taking place when unloading vehicles.

The jetty is assumed closed for traffic except for when boarding/unloading takes place.

There are 150 parked cars and trucks waiting to board, with the possibility to have their

engines running.

150 cars and trucks are assumed to be boarding the ferry by the south gate, planned LNG

bunkering operations is allowed during this step.

Ignition Source Ignition

probability, per vehicle/vessel

Time of

exposure (s) Traffic density, per day

Trucks/cars loading 0.4

60 150

Maintenance traffic 0.4 60 1

Parking area traffic 0.4 60 20

Ferry 0.5 60 1


Probability of ignition in case of release.

Influences societal risk result.

Reference:

/1/ LNG Bunkering of Fjordline ferries Project Design Basis 17.02.2012, Draft Version

/2/ Unloading Loading routes Layout Draft

/3/ PGS 3, Guideline for quantitative risk assessment (“Purple book”), Ministry of VROM, 2005





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Figure 3 Loading/unloading routes; tube and bunkering station (‘manifold’) location; parking areas

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Subject: Ignition Sources – People

Specifications:

The default value assigned within PhastRisk for the ignition source associated with people

corresponds to 1.68E-4 per person per second of cloud exposure. This value has been derived to

account for the probability of ignition associated with people in general, and includes an allowance for

smoking and general human behaviour associated with residential areas.

The value assigned to personnel working offshore by the JIP Ignition Study (Reference /2/) is

significantly lower, being almost 4 orders of magnitude lower even for dense populations.

The value applicable to Skangass personnel at the bunkering station (i.e. a trained workforce, with no

smoking and with traffic and hot work ignition sources accounted for separately) would be much

closer to that recommended by the JIP study in practice; however the default PhastRisk value will be

conservatively applied in the analysis.






Reference: /1/ PhastRisk version 6.7., 2011

/2/ JIP, 1998. Ignition Modelling, Time Dependent Ignition Probability Model, Joint Industry Project –

DNV, Scandpower, et al. DNV Report No. 96-3629, Revision 4, February 1998.





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Subject: Ignition sources – Hot work

Specifications:

The analysis is based on no hot work taking place within the bunkering area.






Reference: /1/ LNG Bunkering of Fjordline ferries Project Design Basis, 17.02.2012, Draft Version





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Subject: Bunkering station – Base case design and inventory

Specifications:

The base case design is described by Skangass in Design Basis document /1/ and drawings /2/, /3/ and

/4/.

The ferry bunkering system relevant for the analysis corresponds to /2/ :

- 1 LNG booster pump

- 1 8” LNG pipeline

- 1 4” Vapour Return Line

- 1 flowmeter package

- 1 LNG loading arm

- Small bore fittings, manual and actuated valves.

The details of equipment are available in the appendix D of this study.

The LNG booster pump, the fiscal metering package and the first part of the pipeline is located inside

the plant area. The pipeline within the plant will be routed in the existing pipe rack /1/. Outside the

plant, the pipeline will be routed in an underground tunnel up to jetty 38 following the route indicated

in reference /3/. Part of this section will be alongside water pipes. The LNG pipeline will run

underground between the ferry terminal building and the jetty (together with power cables) before

going vertically up. The pipe will then go above ground the last 7 meters before the bunkering station.

The design of the piping has the following characteristics:

- Double-wall (pipe-in-pipe) stainless steel pipe

- Full containment

- Routed in an underground tunnel

- Carried on pipe supports

- Leak detection between the double walls

The loading arm is assumed to be equipped with a break-away coupling according to SiGGTO

standard.

The inventory of a given section is defined as the isolatable mass within that section under normal

operating conditions and in addition to that, the inventory released prior to the segment isolation.

The total inventory is according to Skangass 21,8 m3. /4/

Pressure used in the analysis (leak frequency and release rate), when bunkering is on-going, is 10 barg

downstream the booster pump /4/. This is according to Skangass based on pump vendor’s

specifications.

Pressure used between bunkering operations is the settle out pressure of 7 barg, ref. /1/. Note that all

the pipes and equipment downstream the ESD valve will be filled with LNG during the standby mode.


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Reference: /1/ LNG Bunkering of Fjordline ferries Project Design Basis, 17.02.2012, Draft Version

/2/ Cryonorm Project BV documents, P&IDs number 1301-1100-100, sheets TA01, TB01, TC01,

received 06.05.2013

/3/ Suggested pipeline_trase_24.2.2012.pdf

/4/ Eivind Anfindsen, Skangass, 06.05.2013





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Subject: Escape and Evacuation of Passengers and Personnel

Specifications:

It is assumed that the escape and evacuation of passengers and personnel are following the LNG plan

evacuation of Fjordline /1/


The above assumptions each have a qualitative influence on risks to 1st, 2

nd and 3

rd parties.






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Assumption No.: 1-C Revision: 1

Category: Representative Scenario Assumptions

Subject: Release Location / Height

Specifications:

Release location for each section is derived from the plot plan/drawings of the respective area. The

location is generally selected as that of the vessel containing the main inventory of the section or,

where a number of vessels apply, as the centre of the section.

The representative release height for the plant is 1.5 meter.

The representative release height for the underground pipelines is 0 meter.

The representative release height for the bunkering station is 1 meter.


Dispersion is based on those inputs, and location and height contributes significantly for the

consequence modelling.

Reference:





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Category: Representative Scenario Assumptions

Subject: Release Sizes

Specifications:

To define the hazardous release events applying to each Process Accident release scenario (QRA

section), representative hole sizes are modelled. The selection of the hole sizes is made based on the

need of defining different leak categories, and for each leak size a leak rate is associated. It depends on

how refined the assessment can be, meaning that as more leak categories chosen, a better distribution

of the frequency per leak size is obtained. Based on previous experience, Skangass proposes the

selection of three different leak categories ranging from the following sizes:

Range of Leak Sizes: Representative Hole Size / Equivalent Diameter:

Small 1 to 10 mm To be calculated by LEAK

Medium 10 to 50 mm To be calculated by LEAK

Large greater than 50 mm To be calculated by LEAK

Skangass assessed that the maximum volume LNG released due to leak from the loading arm was

estimated to be 3 m3 /1/.

Cryonorm assessed that the inventory in the system is 21.8 m3. /2/

Skangass assessed that the main LNG line up to the ESV valve at the ferry terminal jetty is 15 m3. /3/


The release size taken as representative is a key factor in the release parameters and subsequent

consequences in each case. However, the use of representative releases is inherent in QRA and the

frequencies are assigned according to each of the defined leak size ranges, such that the overall risks

should not be sensitive to the specific values selected. Nevertheless, the representative nature of each

release size should be recognised.

Reference: /1/ Assumption 4-C

/2/ Email received from Eivind Anfindsen 08.05.2013

/3/ Email from Eivind Anfindsen received 06.05.2013





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Category: Frequency Analysis Assumptions

Subject: Leak frequencies

Specifications:

The generic failure data used as the basis of the frequency analysis of valves, flanges and pipes is the

UK HSE’s Hydrocarbon Release Database from 2010, or HCRD 2010 /1/, /2/. Although the leak

frequency data refer to offshore leaks, it is commonly applied for onshore installations. The leak

frequencies for LNG piping are considered as having a failure frequency of 10% of regular process

pipes.

Equipment count is based on P&ID (Cryonorm Project BV documents, P&IDs number 1301-1100-

100, sheets TA01, TB01, TC01), received 06.05.2013.

Equipment downstream ESV valve for ferry bunkering is assumed to only be in use during bunkering.

Loading arm leak frequency is calculated based on ACDS data covering both connection failures and

ranging failures (leading to disconnection) /2/. The ACDS data is considered to be the most

representative data for liquefied gas loading arms but can be considered a conservative estimate for

LNG.

The table below gives a breakdown of contributors to the loading arm failure frequency from ACDS

data, predicted by DNV /2/:

Cause / Type of failure Failure Frequency (per visit)

Connection Failures Failure of arm 5.7E-05

Failure of quick release connection 5.7E-06

Failure of ship's pipework 6.1E-06

Operator error 6.1E-06

Ranging Failures Mooring fault 6.7E-07

Passing ships 2.3E-07

All 7.6E-05

Leak frequency per visit in the ACDS data /2/ is for filling of LNG tankers, which typically lasts for

18-24 hours, whereas the ferry bunkering duration is significantly shorter, see Assumption no. 1-A and

1-D. The total generic frequency above is this thus reduced accordingly.

It should also be noted that the generic frequency data is not modified to account for dropped objects,

this should be considered. The generic data includes leaks from all causes, including dropped objects,

such that additional dropped object risks should only be included where identified as a particular

hazard or potential leak cause. (The passengers have access to the sun deck, however due to the design

of the ship it is assumed that it will not be possible to drop any objects from the deck and onto the

bunkering station).


Key influence on the risks (i.e. risk is directly proportional to frequency).

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Reference: /1/ HSE, 2010. Offshore Hydrocarbon Release Statistics, 2010 (until march 2010)

/2/ DNV Guideline 16, LNG QRA Guideline, 09.11.2011





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Category: Event Tree Modelling Assumptions

Subject: Detection and Isolation Times

Specifications:

The times required to detect a release and then to initiate isolation are summarised in the tables below,

which give the representative times assumed for Process Barge and ‘Other’ process events,

respectively.

F&G Detection

The fire and gas detection depends on the location and magnitude of the event, the number, location of

detectors and their PFD (probability of failure on demand). However, the basic design of the LNG

bunkering terminal is considered to have enough gas detectors.

The F&G system is automatic activated upon gas detection.

Automatic shutdown of the ESV valve at the bottom of the loading arm:

Manual activation of the emergency shutdown and isolation push-buttons by the operator in the

CCR. The F&G detection system is the basis of ESD duration time.

ESD system – release duration

The initial release rate [in kg/s] is calculated within the PHAST RISK discharge model and set

constant during the representative release duration. In reality, the internal pressure is reduced and this

reduces the release rate (please refer to assumption 11-C), meaning that the release rate should drop in

time. Larger release rates have shorter duration than smaller scenarios for the same segment, as the

inventory after the ESD closure is the same.

At the jetty, DNV recommends to use ESD total time of 90 seconds when the operation is

continuously supervised by operators (60 seconds for detection and initiation, 30seconds for isolation).

The main study will be based on this time at the jetty. For the other areas, the detection and response

time are in compliance with the table provided by Skangass below.

The line will constantly have a flow of 330 m3/h during bunkering operation and no flow during no

bunkering operation. The line will then be liquid filled with stagnant LNG.

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Representative detection and response times

Based on the experience of Skangass and the use of fast responsive gas detectors, ensuring automatic

closure of ESV upon detection, the following response times have been estimated:

Leak Size

Response Time (min) Cumulative Time to Initiation

(min)

Detection Isolation Isolation

Small (<10 mm) 2 2 4

Medium (10-50 mm) 0.2 0.3 0.5

Large (>50 mm) 0.2 0.3 0.5

The ESV valve is based on Skangass input, able to be closed in 6 seconds. /1/

Note: Skangass will use assumptions for response times as requirements for selection of

designer/vendors. DNV recommends that equipment is qualified for compliance with these

requirements.


The detection and isolation assumptions are key influences on the release duration and impact on the

selection of representative release rates. On balance, any specific inventory assumption will have a

limited influence on the overall risks, although the inventory is a key parameter with respect to the

detailed modelling of each scenario.

Reference: /1/ Eivind Anfindsen email received on the 06.05.2013





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Subject: Isolation Failure

Specifications:

To account for the possibility of failure to isolate occurring either due to failure of the relevant ESDs

(PESD) or due to human error (Phuman), the probability of isolation failure is determined as:

Pisolation failure = 1 – (1-Phuman)*(1-PESD)

Where:

PESD = 1 – (1-PFDESD)N

And:

PFDESD is the probability of failure on demand of the ESD(s)., and is the sum of technical

failure rate of logic (0.15%) , valve (2% to 5%) and actuator (0.5%), ref. A and B.

N is the number of ESDs required for isolation, and On average 2 valves are assumed to be

required to isolate a section, hence N = 2.

Phuman = Probability of human failure, and is set to 10%

The general rule-set adopted is that two ESD valves are required for isolation of a section. For liquid

and gas sections, a probability of failure on demand of 2% is assumed.

As a result, the probability of isolation failure applied within the study is calculated as follows:

Gas QRA Sections: Pisolation failure = 0.15

Liquid QRA Sections: Pisolation failure = 0.15

Reference: A: TD0096, Akseptkriterier og beskrivelser av svikt for utvalgte sikkerhetskomponenter (DRAFT)

B: Anbefalte sviktrater – risikoanalyser, Skangass AS 29.03.05.


The probability of isolation failure has a key influence on the frequency of release events that have

sufficient duration to lead to escalation.

Reference:





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Subject: Immediate Ignition Probability

Specifications:

The probability of immediate ignition is derived as a function of the release rate and release phase

using the framework set out below. This immediate ignition probability model is the same as derived

for the corresponding Kårstø QRA study conducted by DNV /1/.

It should be noted that the basis of the derived ignition probabilities is the energy associated with each

release. As such it is important to note that the “liquid” category, in this context, corresponds to the

phase of the material at standard pressure and temperature. Hence, in this study, only the condensate

release scenarios are treated as liquid in terms of ignition probability (i.e. all other liquid releases are

actually gas under cryogenic, or pressurised, conditions).

Leak Size

Category

Size Interval, Release Rate (kg/s) Immediate Ignition

Probability Gas Liquid

Small < 1 < 1.2 0.01%

Medium 1 – 10 1.2 - 25 0.1%

Large > 10 > 25 1%


The immediate ignition probability has a direct influence on the risks associated with jet and pool fire

risks to personnel (and to assets), which contribute around a quarter of the overall risks.

Reference: /1/ “Kårstø Plant”. DNV Report No. 98-3090, Rev. 01.





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Subject: Event Tree Framework

Specifications:

Figure 4 shows the framework for the modelling of a release within the PHAST RISK risk model, as

summarised below:

Immediate ignition has a defined probability for each release, detailed in Assumption 6-C.

Given that immediate ignition occurs, the majority of release scenarios will be modelled as a jet

fire, for gas releases. Where rainout occurs (i.e. where some liquid is present in the release) a

similar event tree applies where the equivalent outcome will be a pool fire (liquid only), or both

pool and jet fires (where liquid rains out from the initial discharge).

However, the event tree structure enables a proportion of short duration releases (defined as less

than 20 seconds, in this study) to be modelled separately. The event tree enables the user to define

the proportion of these short duration events that are fireballs, flash fires or explosions (conditional

probabilities A, B and C in Figure 4, which are defined in Assumption 8-C). As above, where

liquid is present, the event tree enables pool fires to be either neglected, modelled as the only

outcome, or modelled in addition to the gas impacts (where the latter option is applied within this

study).

Delayed ignition is calculated within the risk model for each release, as described in the Appendix

A assumptions.

Where delayed ignition occurs, the outcome is split into flash fire and explosion scenarios

(conditional probabilities D and E in Figure 4, which are defined in Assumption 8-C). This applies

equally to vapour clouds arising from gas releases or clouds flashed from liquid releases, where

delayed ignition of liquid releases will have an additional (“late”) pool fire outcome.


The event tree framework is a key aspect of the QRA model, although the main influence on the risk

results is the probabilities applied within the framework, as described elsewhere within this appendix.

Reference:





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Attachment to Assumption 7-C

Source

Immediate

Ignition ?

Evaluate

short release ?

Short release

?

Consequence

(model used) ?

Residua

l Pool

Fire?

Y Y Y y p=F

Releas

e

Defined

for each

release

User

defined

(p=1)

Determined

by PHAST

RISK

according to

release

properties

p=A Firebal

l n

y p=F

p=B Flash

Fire n

y p=F

p=C Explosi

on n

N N

Jet

Fire

Delayed

Ignition ?

Consequence

(model used) ?

N Y y p=G

Determined by PHAST RISK

according to dispersion,

duration, ignition sources

p=

D Flash

Fire N

y p=G

p=E Explosi

on N

N

No

ignitio

n

Figure 4 Example risk model event tree structure

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Subject: Event Tree Probabilities

Specifications:

The development of a release is largely defined by the stage at which ignition occurs, where the

immediate and delayed ignition parameters are described elsewhere within this appendix.

Immediate ignited releases with pool formation are likely to develop pool fires. Thus in case of a

release with liquid drop which ignites immediately, the pool fire is considered as one of the

consequences.

Process areas: % fireball, % flash fires, % explosion, % pool fires (i.e. A=, B=, C=, F= 1)

Delayed ignition events are split between flash fire and explosion outcomes (probabilities D and E,

respectively, in Figure 4) which may result or not in pool fires as follows:

Process areas: % flash fire, % explosion, % pool fires (i.e. D=, E=, G= 0.15)

Those probabilities will come from the explosion assessment.


Short duration events (in the context of PHAST RISK, i.e. less than 20 s) are very limited, and the

difference in risks to personnel associated with flash fire and explosion events are not major. Hence,

the above values do not have a major influence on the overall risks, although the influence will

accumulate for all release scenarios for which each parameter set is applied.

Reference:





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Category: Consequence Modelling Assumptions

Subject: Dispersion Parameters

Specifications:

The key inputs into the dispersion modelling are the release / discharge parameters. Additional

assumptions that influence the dispersion are:

Weather. The wind speed, direction and stability have a key influence on the downwind dispersion

distance of vapour clouds (and to a lesser extent the radiation contours associated with fires).

These parameters are defined within Assumption 2-A.

Ambient conditions. The air and surface temperature, together with other local parameters such as

atmospheric pressure and relative humidity, will also have an influence on dispersion. These

parameters are defined within Assumption 3-C.

Congestion / impingement. The dispersion parameters are derived for an idealised release, with no

consideration of potential obstructions. It is likely that a release will impinge on equipment;

therefore the release is treated as impinged releases.


The above assumptions each have key influences on the consequence results.

Reference:





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Category: Consequence Modelling Assumptions

Subject: Consequence Modelling Parameters

Specifications:

The key inputs into the consequence modelling are taken directly from the discharge and dispersion

modelling inputs and results. A wide range of additional parameters are applied within the models,

where in general the widely accepted PHAST Risk default values are applied. The key parameters that

are specific to the above consequence models are summarised below.

Jet fire – maximum surface emissive power (SEP): 250 kW/m2

Jet fire – rate modification factor (the mass of vapour that remains in cloud calculated by PHAST is

multiplied by this factor – determines the proportion of the liquid fraction that contributes to the jet

fire for 2-phase jets): 3

Pool fire – minimum duration – 10 seconds

Fireball / BLEVE – maximum SEP: 300 kW/m2

Fireball / BLEVE – mass modification factor (the mass of vapour that remains in cloud calculated

by PHAST is multiplied by this factor – determines the proportion of the liquid fraction that

contributes to the fireball/BLEVE): 3

Flash fire – The size is calculated based on mass between LFL and UFL (for ignition probabilities,

the 50% LFL is used)

Explosion – minimum explosion energy: 5 x 106 kJ

Explosion – explosion efficiency: 10%


The above assumptions each have key influences on the consequence results.

Reference:





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Assumption No.: 1-D Revision: 1

Category: Frequency Assumptions

Subject: Bunkering Frequency

Specifications:

Planned frequency of LNG bunkering is 1 per day.

Bunkering duration is 1 hour for LNG cool down and transfer and ½ an hour for connection and

disconnection of loading arm to ship manifold.

The ferry is scheduled to arrive at 8 in the afternoon. Fjordline /2/ has estimated the following ferry

arrival delays:

Delay less than 3 hours: 4 %

Delay more than 3 hours: 2 %

According to Fjordline /2/ 1 % of scheduled ferry travels are cancelled.


The above assumptions each have key influences on the frequency estimates.


/2/ Email from Larsen/Fjordline to Gautestad/Skangass, 24.02.2012, subject: “SV: Forsinkelser av

anløp pga vær etc.”





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Assumption No.: 1-H Revision: 1

Category: Assumptions

Subject: End Point (Impact) and Vulnerability (Fatality) Criteria

Specifications:

Human

Impact

(End Point)

Impact

Criteria

Event

Location

Vulnerability

Parameters Notes

Outdoor Indoor

Explosion

(Heavy

Blast)

Heavy blast

damage –

350 mbarg

Other areas 0.3 1 Default – building collapse is

main impact potential

Explosion

(Light

Blast)

Light blast

damage –

100 mbarg

Other areas 0.1 0.3 Default – building damage is

main impact potential

Flash fire 50% LFL All 1 0.1 Fatality assumed if outdoors;

shielded if indoors

Fireball /

BLEVE

250 kJ/m2

thermal dose All 0.7 0.1

High fraction killed if outdoors;

shielded if indoors

Jet fire

12.5 kW/m2

radiation

level

Storage &

Loading 0.5 0.1

LP – spray rather than jet fires;

open area - good escape prospect

Process

Areas 0.7 0.1

High fraction killed if outdoors,

but some escape possible

Pool fire 12.5 kW/m

2

radiation

level

Storage &

Loading 0.5 0.1

Open area - good escape

prospects for pool fires

Process

Areas 0.5 0.1

Open area - good escape

prospects for pool fires


The risks are directly influenced by the impact and fatality assumptions, which quantify the severity of

the consequences. The above assumptions include some allowance for differing escape characteristics

in different areas of the facility, but remain consistent with established, conservative best-practice.

Reference: - DNV expert judgement – using PHAST Risk defaults and DNV Technical data

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Det Norske Veritas:

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appendix a - assumptions register s as -...

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