B.5.1 List of RRMs
The full range of techniques listed in Section B.4 is employed in the search for risk reducing measures. This has resulted in the following list of RRMs (for illustration purposes the list has been reduced to a manageable length):
1. Limited explosion relief area increase 2. Protective shielding on escape ways 3. Additional escape way
4. Reduction of the number of leak sources 5. Installation of freefall lifeboats
6. Removal of all weather cladding on process modules.
B.5.2 Evaluation of RRMs
Limited Explosion Relief Area Increase
The scope of this proposal is a minor improvement of explosion relief areas around the process modules on the installation, in order to compensate for increased risk due to new equipment, but no further reduction. The proposal will increase ventilation rates slightly and reduce peak explosion overpressures somewhat. This is in accordance with good practice as well as codes and standards.
One benefit of the limited extent of this change is that the working environment conditions (chill factor) for the personnel working in the process areas should not be made dramatically worse. The downside is that the improvement in ventilation is very slight.
The engineering assessment of this proposal is that it should be implemented as a compromise between the need to increase ventilation and the physical working environment conditions. But this measure is not sufficient to solve the critical issue of the lack of protection against smoke for personnel escaping from a fire.
Table B.1 presents the risk results for the calculation of changes resulting from an increase in explosion relief areas, presented against the base case risk results.
Table B.1. Risk values for explosion relief increase
Alternative
Annual impairment
frequency (escape ways) FAR
PLL (/yr)
ǻPLL (/yr)
Base case 3.76ǜ 10-4 4.2 0.0147
Increase of explosion
relief areas 3.75ǜ 10-4 4.4 0.0154
- í0.0007 The annual impairment frequency for escape ways for both alternatives should be stated as 3.8 ǜ 10-4. The frequencies are presented with a higher number of decimals in this and subsequent tables, in order to show the difference.
Protective Shielding on Escape Ways
This option involves the installation of protective shielding on existing escape ways together with overpressure protection in order to avoid smoke ingress into the enclosed escape ways. With respect to good practice, this option certainly fulfils relevant requirements: shielding and overpressure protection of exposed escape ways is a common solution. Typically on FPSOs, this is the common way to protect an escape way running the full length of the vessel, past the process areas.
On the other hand, this topic is not addressed in the present codes and stan- dards, being regarded as a novel aspect, and therefore not yet reflected in standards.
From an engineering point of view, the judgement should be that protective shielding may be a good solution to high heat loads, but it needs to be combined with overpressure protection in order to ensure that the escape ways are not made unusable due to smoke ingress.
This proposal has been discussed with workforce representatives who support this proposal fully but have questioned whether it is sufficient as a good total solution for safe escape from the installation in the event of serious fire.
The effect on risk results has been calculated, and the results are as shown in Table B.2. It is shown by the results that a substantial reduction in the frequency of impairment of escape ways is due to this improvement, and also a clear reduction in FAR value. The escape ways’ impairment frequency is slightly above the limit of 10-4 per year (see further description in Section 5.1, page 101), but not signifi- cantly above, when uncertainties are taken into consideration.
Table B.2. Risk values for heat shielding on escape ways
Alternative
Annual impairment
frequency (escape ways) FAR
PLL (/yr)
ǻPLL (/yr)
Base case 3.76ǜ 10-4 4.2 0.0147
Protective shielding 1.25ǜ 10-4 3.4 0.0118 0.0029
A workshop in order to discuss possible options with all parties involved has been conducted, with representation from workforce, management, operational manage- ment, HES specialists, engineering personnel, etc. The consensus in the workshop was that an extra escape way might be the ultimate solution, if heat shielding of escape was not sufficient to reduce the frequency of escape ways impairment. It was noted that this was a borderline issue.
Additional Escape Way
Provision of an additional escape way with sufficient shielding is the ultimate solution to the issue of safe escape in the event of critical fire and/or explosion. It is often the case that the best solution may be found when building a new construction, rather than trying to alter an existing design. A further advantage of the additional escape way is that redundancy is built into the design: it would be unlikely for both escape ways to be impaired at the same time, especially when one of these is thoroughly protected.
It is therefore obvious that this solution satisfies good practice and that enginee- ring judgement gives this proposal a good score. Workforce representatives also favour this proposal, whereas management will probably find it very expensive.
The effect on risk results has been calculated, and the results are as presented in Table B.3. It is shown by the results that a substantial reduction of the frequency of impairment of escape ways is due to this improvement, and also a substantial reduction in FAR value. The escape ways’ impairment frequency is slightly below the limit of 10-4 per year, but not significantly, when uncertainties are taken into consideration.
Table B.3. Risk values for additional escape way installation
Alternative
Annual impairment
frequency (escape ways) FAR
PLL (/yr)
ǻPLL (/yr)
Base case 3.76x 10-4 4.2 0.0147
Additional escape way 9.40x 10-5 2.5 0.0088 0.0059
The workshop referred to above observed that there is strong opposition to this proposal because of the excessive cost involved in constructing a new escape way.
Reduction of the Number of Leak Sources
The workshop referred to also put forward additional proposals for risk reduction.
One of these is a proposal to reduce the number of leak sources. This would entail removing the majority of flanged connections in the process area and replacing them with welded connections. This would be particularly important in the crude oil export pumping area, which is the main source of fire risk with effect on the escape ways.
Removal of leak sources is in accordance with good practice, as well as codes and standards. But an evaluation of this proposal will focus on the side effects due to the fact that this will involve very extensive modification of the process systems.
There will be a lot of hot work in the process area, not all of which will be conduc- ted in a shut down and depressurised condition. It is therefore considered that the increase of risk during modifications is substantial, and will probably be consi- derably higher than the risk reduction per year.
It is further noted that a reduction of the impairment risk by 75% is needed in order to come below the limit of 10-4 per year; this means that leak frequencies will also need to be reduced by 75%. It is further noted that export pumps are important as leak sources, and that the relevant leak scenarios are almost impossible to elimi- nate.
The conclusion from the engineering judgement of this proposal is that it is unlikely to provide the necessary risk reduction and will increase risk substantially during implementation of modifications. This proposal is therefore not recommen- ded for further consideration.
Installation of Freefall Lifeboats
Installation of freefall lifeboats (two boats for redundancy purposes) may reduce the need to use escape ways in case of fire. This is not usually a preferred solution, but may act as a compensatory measure if no other solution can be found. Obvi- ously, the protection of escape ways is unchanged, but the need for escape to the shelter area will be formalistically reduced. On the other hand, this is more a formal solution than a practical, operational one. For escape purposes, escape over bridges to a shelter area on a separate installation is distinctly preferable to using lifeboats, including freefall lifeboats.
Consultation with stakeholders is likely to result in strong opposition from the workforce, and also some reluctance from operational management.
Table B.4 shows the results for the option to install freefall lifeboats. It is shown that the reduction of impairment frequency is substantial, whereas the reduction in FAR (and PLL) is more limited. The reduction is not sufficient in relation to the limit of 10-4 per year.
Table B.4. Risk values for installation of freefall lifeboats
Alternative
Annual impairment
frequency (escape ways) FAR
PLL (/yr)
ǻPLL (/yr)
Base case 3.76ǜ 10-4 4.2 0.0147
Install freefall lifeboats 1.71ǜ 10-4 3.7 0.0130 0.0018
It should further be noted that installation of two freefall lifeboats will be very expensive, probably the most expensive of all the proposals considered.
Remove all Weather Cladding on Process Modules
The removal of all weather cladding on process modules will increase natural ventilation in the process areas. The engineering evaluation of this proposal has concluded that it will have the following effects:
x Reduce ignition probability through reducing gas concentrations and extent of gas cloud within flammable region.
x Reduce explosion overpressure in case of delayed ignition, as a result of having more explosion relief areas.
x Possibly reduce smoke production slightly in the event of fire, due to improved ventilation.
x Working environment in the process area will suffer deterioration due to an increase in wind chill factors.
The increased wind chill factor in the process area is a severe negative factor, which is likely to cause strong opposition from the workforce. In this regard, the proposal is not in line with working environment standards. But improvement of ventilation rates is in line with technical safety principles designed to reduce the effects of hydrocarbon leaks. It is quite common that a compromise has to be found between these two opposing objectives.
Table B.5 presents the revised results for the option to remove all weather clad- ding. It is noted that some reductions are shown, but the reduction in impairment frequency for escape ways is not at all sufficient to reduce the value below the limit of 10-4 per year. This, together with the working environment aspect, means that this proposal will not receive much support from some of the stakeholders.
Table B.5. Risk Values for Removal of all Weather Cladding in Process Area
Alternative
Annual impairment
frequency (escape ways) FAR
PLL (/yr)
ǻPLL (/yr)
Base case 3.76ǜ 10-4 4.2 0.0147
Remove all weather
cladding 2.37ǜ 10-4 3.4 0.0118 0.0029