7. QUANTITATIVE RISK ASSESSMENT
7.12 D ROPPED O BJECTS (P IPES )
7.12.1 During pipe lay in close proximity to existing lines there is a possibility that a dropped pipe joint could damage the existing lines. The horizontal separation between pipelines is defined in the design basis are as follows:
7.12.2 Separation between NSP1 and NSP2 lines
• Anchored lay vessel - 1000 m (WD<30 m), 1200 m (30 m<WD<100 m) and 1400 m (WD>100 m)
• DP lay vessel – 500 m
With a minimum separation of 500 m it is considered that the probability of a dropped pipe joint contacting the NSP1 lines is extremely low.
Operational procedures will define that no pipe transfer operations at pipeline crossings will be permitted until the minimum separation of 500m is achieved.
7.12.3 Separation between the two NSP2 lines
• Anchored lay vessel – 55 m (assumed to be a monohull vessel)
• DP lay vessel – 75 m (WD<100 m), 105 m (WD>100 m)
With these separations there is a possibility of a dropped pipe joint contacting the adjacent line and these risks have been considered further.
7.12.4 The probability of a pipe falling overboard and hitting the operating pipeline has been estimated using the methodology described in DNV-GL Recommended Practice DNV-GL-RP-F-107, Risk Assessment of Pipeline Protection (reference 6.11). The calculations have been carried out on an Excel spreadsheet and were first checked against the DNV-GL example given in the RP to confirm correct use of the various formulae. The methodology is described Appendix I.
QUANTITATIVE RISK ASSESSMENT PIPELINE CONSTRUCTION RISK ASSESSMENT – INCLUDING NORTH OF BORNHOLM OPTION
7.12.5 The calculations are based on pipeline separations indicated above and an angular deviation angle of 5° (reference 6.11 table 10). An example of the inputs and results are attached in Appendix I and the results summarised in the following tables.
7.12.6 The following table summarises the total annual frequency of hitting the pipe and the energy distribution for anchored operations. These results are based on loading from the side nearest the pipe.
Energy Band (kJ) – Anchored (separation 55 m)
Water Depth 100-200 200-400 400-800 >800 Total Fr Hit
50 m 1.3E-16 1.9E-16 3.8E-16 5.7E-16 2.4E-13
100 m 1.7E-08 2.6E-08 5.2E-08 7.7E-08 3.4E-05 150 m 2.2E-07 3.3E-07 6.5E-07 9.8E-07 4.3E-04 200 m 9.6E-07 1.4E-06 2.9E-06 4.3E-06 1.9E-03
7.12.7 The following table summarises the total annual frequency of hitting the pipe and the energy distribution for anchored operations. These results are based on loading from the side on the opposite side of the pipe.
Energy Band (kJ) – Anchored (separation 55 m – ‘far’ side)
Water Depth 100-200 200-400 400-800 >800 Total Fr Hit
50 m 0.0E+00 0.0E+00 0.0E+00 0.0E+00 0.0E+00
100 m 6.6E-20 9.9E-20 2.0E-19 3.0E-19 6.6E-19 150 m 2.2E-12 3.3E-12 6.6E-12 1.0E-11 2.2E-11 200 m 4.5E-10 6.7E-10 1.3E-09 2.0E-09 4.5E-09 7.12.8 The following table summarises the total annual frequency of hitting the pipe and
the energy distribution for DP pipe lay operations with a 75m separation.
Energy Band (kJ) DP (separation 75 m, wd < 100 m)
Water Depth 100-200 200-400 400-800 >800 Total Fr Hit
50 m 0.0E+00 0.0E+00 0.0E+00 0.0E+00 0.0E+00
100 m 6.0E-11 9.0E-11 1.8E-10 2.7E-10 6.0E-10
7.12.9 The following table summarises the total annual frequency of hitting the pipe and the energy distribution for DP pipe lay operations with a 105m separation.
Energy Band (kJ) DP (separation 105 m, wd >100m)
Water Depth 100-200 200-400 400-800 >800 Total Fr Hit 100 m 0.0E+00 0.0E+00 0.0E+00 0.0E+00 0.0E+00 150 m 2.3E-12 3.4E-12 6.8E-12 1.0E-11 2.3E-11 200 m 3.1E-09 4.6E-09 9.2E-09 1.4E-08 3.1E-08
7.12.10 The following table shows the energy for different damage classes for pipe with wall thickness of 34.6 mm and 30.9 mm. These values have been taken from Saipem Pipeline Damage Assessment (reference 7.6).
Damage Class
Energy (kJ)
Energy (kJ)
Damage Description
Wall thickness
34.6 mm 30.9 mm
D0 77.5 65.4 No damage
D1 219.2 185 Minor damage
D2 402.7 339.8 Major damage – no release of
hydrocarbons
D3-R0 620 523.2 Major damage – minor release of hydrocarbons
D3-R1 866.4 731.2 Major damage – major release of hydrocarbons
7.12.11 The Saipem calculation indicates that hydrocarbon release is likely for the energy bands of 400 kJ and above and this corresponding frequency is as follows:
Pipe lay Mode and Separation Annual Pr > 400 kJ Anchored – 55 m ‘near’ side 9.0 x 10-6
Anchored 55 m ‘far’ side 3.4 x 10-9
DP 75 m 4.5 x 10-10
DP 105 m 2.3 x 10-8
QUANTITATIVE RISK ASSESSMENT PIPELINE CONSTRUCTION RISK ASSESSMENT – INCLUDING NORTH OF BORNHOLM OPTION
7.12.12 If the first line is not operational when the second line is installed the consequence would be major damage and would require extensive repair. If the first line is operating the consequence would be the release of hydrocarbons and this is discussed in the following paragraphs.
7.12.13 It can be seen that the frequency of hydrocarbon release as a result of a dropped pipe joint is very low for all cases except the anchored vessel loading pipe on the side nearest the existing pipeline.
7.12.14 It is evident that the probability of hitting the pipe increases as the horizontal separation decreases. The anchored case is based on a vessel with a beam of 32.2m. However, the loading operations of the anchored pipe lay vessel will need to be managed to ensure risks are maintained at ALARP levels.
7.12.15 Saipem (references 7.7 – 7.11) have considered the consequence of gas leakage following pipe damage and this has been referred to in this assessment. In the event of a subsea gas release the vapours will form a gas cloud when they reach the surface and if no immediate ignition occurs will dilute in the air. If the cloud encounters an ignition source before it is diluted below its flammable limit a flash fire will occur. If the release at the surface has sufficient velocity a jet fire will occur.
7.12.16 The Saipem report concludes that the surface velocity will be such that a gas cloud will form and that ignition probability will be 1.0 for any vessel located within the cloud. Calculations following a full bore rupture of the pipeline indicate the following limits:
Distance of Flammable Limits at 10 m height Upper Flammable
Limit (UFL) in metres Lower Flammable Limit
(LFL) in metres LFL/2 – metres
79 136 193
7.12.17 It therefore evident, that in the event that a dropped pipe joint strikes the pipe and there is a full bore release, the pipe carrier and the pipe lay vessel will be within the flammable limits. It is also possible that an AHT and survey vessel could also been in close proximity depending on the phase of the operation.
Saipem indicate that flash fires generally have a short duration and do less damage to equipment and structures than personnel. It is assumed that anyone caught in a flash fire would probably be killed, and the fatality rates are based on the number of crew on the open deck at the time of the flash fire.
7.12.18 It has been conservatively estimated that 20% of vessel crew would be on deck and exposed to the flash fire. This corresponds to 50 on the pipe lay vessel, 10 on the survey vessel and 3 on the pipe carrier and anchor handler tug.
7.12.19 The individual risks for various pipeline separations are estimated in the following
Individual risk per year 9.0E-07 9.0E-07 9.0E-07 9.0E-07
55 metre Separation
(far side of pipe) 3.4E-09 3.4E-09 3.4E-09 3.4E-09 Frequency of gas
release per year 1.84E-07 1.02E-08 1.02E-08 3.4E-08 Potential loss of life per
Individual risk per year 4.5E-11 4.5E-11 4.5E-11
105 metre
Individual risk per year 2.3E-09 2.3E-09 2.3E-09 7.12.20 It is noted that in areas where the separation distance is 55 m pipe loading may
need to be carried out on the ‘far’ side of the existing line. This restriction only applies to the installation of the second NSP2 line.
7.12.21 There is also a possibility that vessels located near to the gas plume could experience loss of buoyancy and start sinking. This was considered in a recent HSE research report (reference 7.12) into the stability of semi-submersible drilling rigs and only two loss of buoyancy incidents were reported over a period 1974 to 1998. These related to shallow water blowouts where one rig caught fire and the other developed a major list but did not catch fire. It concluded that the risk of fire or explosion was higher than the risk of loss of buoyancy.
QUANTITATIVE RISK ASSESSMENT PIPELINE CONSTRUCTION RISK ASSESSMENT – INCLUDING NORTH OF BORNHOLM OPTION
7.12.22 Laboratory experiments (reference 7.13) on a small scale indicate that in a rising plume of small bubbles (smaller than the vessel), the vessel is more likely to be pushed aside rather than sink provided the fluid is unconfined (i.e. the fluid can move away from the bubble source area) and this effect was observed by GM in a drill ship incident some years ago.
7.12.23 In the dropped pipe situation both the pipe carrier and the pipe lay vessel would be some way from the centre of the gas plume and it is more likely that the vessels would be pushed aside rather than sink. Based on the above comments it has been assumed that the probability of sinking is 0.2. Vessel abandonment in this situation would be more difficult and the conditional probability of unsuccessful rescue derived in the section on sinking has been increased from 0.04 to 0.1, this corresponds to a combined probability of 0.02.
7.12.24 The risks associated with gas release and loss of buoyancy are indicated on the following table:
Individual risk per year 9.0E-09 9.0E-09 9.0E-09 9.0E-09 55 metre Separation
Individual risk per year 3.40E-12 3.4E-12 3.40E-12 3.40E-12 DP Vessel
Individual risk per year 4.5E-13 4.5E-13 4.5E-13 105 metre
Individual risk per year 2.3E-11 2.3E-11 2.3E-11
7.12.25 As would be expected the anchored pipe lay case presents the highest risk as has been discussed previously.
7.12.26 In the event that a vessel did sink there is likely to be some oil spillage and the risks associated with sinking have been addressed previously in section 6.7 of this report.
7.12.27 Ramboll have concluded that the toxicity of released gas to the environment is negligible as only a limited amount of gas will dissolve in the water and the major part will escape to the atmosphere. The impact on marine life will be local and due to:
1) The depletion of dissolved oxygen, and
2) The possible super-saturation of the sea with dissolved gas.
The Saipem risk assessment reports (reference 7.7 – 7.11) conclude that consequences of gas release to the environment are moderate.