Descripción del Bloque II del cuestionario

Of the major HSE risks presented in Table 4.6, the release of reservoir fluids at D-Island presents the greatest societal HSE risk. This hazard has been subject to a Bowtie review as discussed in Section 4.5.1 and documented in Appendix E. In addition, there have been a number of risk assessments completed for well blowouts as listed in Table 4.2 and a SIMOPs review of concurrent Rig 401 and 402 drilling activities [51]. The key conclusions from these risk assessments and SIMOPs review are summarised below.

Shallow Gas Blowouts

Shallow gas blowouts are well control incidents that occur when drilling without the BOP installed.

The D-Island drilling programme was planned as a batch drilling and completions programme. The first phase was the Top Hole Drilling, which was subject to the potential risk of shallow gas blowouts, however, no shallow gas was encountered. The in-reservoir drilling phase follows on from top hole drilling, and during this phase the BOP is installed and therefore shallow gas blowouts require no further consideration.

Reservoir Blowout Frequency

The Kashagan Blowout Risk Assessment [19] has estimated blowout frequencies for wells drilled in the Kashagan field. The blowout frequencies for conventional drilling with negligible / low losses and Closed Hole Circulation Drilling (CHCD) in fractured zones are presented in Table 4.8. The in-reservoir drilling programme for D-Island is planned to utilise conventional drilling methods unless severe losses are encountered (i.e. fractured zones), when CHCD will be employed.

The expected narrow pressure margin conditions at the Kashagan field can be compared to conditions found while drilling High Pressure High Temperature HPHT wells. When drilling in (HPHT) conditions, utilizing traditional over-balanced drilling method, the first barrier, the mud column, is found to be highly unreliable, due to narrow pressure margins. In HPHT wells, an average of one kick experienced per well drilled is not uncommon.

Using CHCD, the mud column is less critical because drilling can commence while fluid is pumped in to the well. In case the equipment related to CHCD should fail, the BOP will have to be closed.

This is similar to drilling utilizing traditional drilling methods.

Experience shows that the reliability of the critical drilling equipment used as primary barriers during CHCD drilling is higher than the reliability of mud column while drilling under HPHT conditions, with the possibility for extensive mud losses.

Based on this, it is judged that using of CHCD equipment while drilling the Kashagan wells, reduces the risk of loss of the first barrier compared to the potential for loss of the highly unreliable first barrier using traditional drilling methods under the same conditions. Again, in case the equipment related to CHCD should fail, the second barrier, the BOP, will be activated, as in the case of HPHT drilling.

It is therefore judged that the Kashagan field can be drilled in a safer way using the CHCD equipment, than drilling the wells using only traditional drilling methods in high loss to formation conditions.

Table 4.8 : Estimated Blowout Frequencies

Estimated blowout frequency for Kashagan wells in areas Phase

where the potential for severe

losses is remote with potential for severe losses.

CHCD technique is applied (fractured zones)

Drilling 5.3 x·10^-5 1.0·x 10^-4 Per well

In both drilling conditions the frequency of blowouts indicates that this event is a remote possibility.

Furthermore, even given a blowout, the consequences vary depending on the flow path of the reservoir fluids through the well (see Toxic Gas Dispersion below), the warning time, and weather conditions at the time of the blowout.

Loss of Containment During Well Completions

For completions operations the cemented casing and liner provides the primary barrier against the reservoir fluids. As long as the casing and liner are intact there should be minimal risk of hydrocarbon exposure during completion operations [[64], [65]].

Warning Time

Simulations of kick propagation up the well bore have been performed in order to estimate a warning time for workers at the drill floor i.e. the time from when the kick is observed until the formation fluid can reach the surface [19]. Warning time is expected to be in the region of 35 minutes when mud is circulated at high rates (i.e. 1600 lpm) but is expected to be significantly longer when circulation rates are lower or the well is shut in (i.e. 500 to 700 minutes). The results demonstrate that there is a sufficient warning duration to enable mustering of non-essential personnel at temporary refuges (TR) and escape of vessels in the D-Island vicinity before gas is expected to reach the atmosphere.

Toxic Gas Dispersion

A range of credible Kashagan blowout scenarios have been identified [19] and toxic dispersion modelling using PHAST has been completed for a range of weather conditions, as summarised in Table 4.9. Medium range dispersion of toxic pollutants (shoreline impact) has been addressed under a separate section.

Table 4.9 : Summary of Toxic Dispersion Modelling from Well Release and Blowouts Distance to Concentration End Point (m) at Ground Level

(Min / Max) The significance of the different toxic concentrations is explained below:

• 10 ppm is the 8 hour Time Weighted Average (TWA) exposure limit for H₂S;

• 28 ppm for 60 minutes is the AEGL-2 equivalent dose. Acute Exposure Guideline Level 2 is the dose of chemical (in air) at, or above which there may be irreversible or other serious long lasting effects or impaired ability to escape;

• 100 ppm for 30 minutes is the IDLH equivalent dose. IDLH (Immediately Dangerous to Health

& Life] is deemed the dose at which escape may be made without injury or irreversible health effects and without deleterious / severe impediment to escape (e.g. irritation); and

• 500 ppm is the concentration beyond which filter hoods are not guaranteed to provide protection from the toxic H2S gas cloud. (Note: during in-reservoir drilling operations filter hoods are not being used at D-Island instead 22 minute SCBA sets are being provided, see Section 5.1.)

These results show that concentrations of H2S above 500 ppm are limited to the immediate vicinity of D-Island. Given the warning time (see above), non-essential personnel are expected to be mustered at enclosed TRs; the SCBA sets enable the safe evacuation of personnel from their work place to the TR. The Shapagat’s TR is not pressurised and personnel connect their SCBA sets to the cascade system, whereas the Karlygash’s TR is pressurised and personnel do not connect to a cascade system. The Karlygash provides cascade connection points on the outside of the TR to enable connection of SCBA sets while personnel wait to enter the airlocks. If essential personnel (i.e. the drilling crew) remain outside the TRs to manage the well control incident, they will be plugged into Drill Rig/Island Cascade breathing systems.

Neighbouring facilities (e.g. A-Island) are not expected to experience H2S concentrations in excess of 500ppm even under worst case conditions. However, IDLH concentrations (>100ppm) could reach A-Island under worst case conditions e.g. large blowouts in very stable conditions with low

wind speeds directed from D-Island to A-Island. Critically, personnel at A-Island will have sufficient warning time from the detection of the kick to a plume reaching A-Island to enable evacuation from the island (i.e. in excess of 1 hour).

The in-reservoir drilling programme at D-Island is scheduled for the ice season, when vessels in the immediate vicinity of D-Island are expected to be under the control of the Marine Coordinator i.e. third parties are not expected to be nearby. Even so, all Vessels and Securitee messages would be issued to notify any third party vessels of the incident on D-Island. On detection of the kick, vessels under the control of the Marine Coordinator would be instructed to escape via the primary escape route (managed ice channel), which is upwind of D-Island and therefore not vulnerable to wind-driven toxic gas dispersion. In any event, the warning time for a well control incident is considered to be sufficient for vessels to escape to a safe distance.

Medium Range Dispersion of Pollutants (Ignited and Unignited)

Agip KCO have undertaken medium range dispersion modelling of ignited and unignited worst case blowouts (i.e. through open well) from the Kashagan East location at 46.49° N, 52.24° E during the ice season and the ice free season [52]. The modelling was completed using CALPUFF to estimate the airborne concentrations at, and just beyond, the shoreline for pollutants expected from the worst-case blow-out, whether ignited or not. Indications are that the worst case non-ignited blow-out would create an offensive smell of H2S throughout the region. However the direct effects would be unlikely to be detrimental to health. While the effects of the ignited blow-out are to some extent concentrated on the land close to the shoreline, these are also unlikely to be directly detrimental to health.

Fire and Explosion Effects

The rig designer completed fire and explosion modelling for the effects of ignited BOP deck releases and ignited gas returns in the mud on the BOP system including control panels and lines [20]. Most of the scenarios modelled were based on the premise that hydrocarbons were brought to the surface, which is not the case for the in-reservoir drilling stage (but will be for future well testing, which is not addressed by the HSE Case). However, the impact of a delayed ignition blowout from one rig on the well control equipment on the other rig was assessed. The results of CAM2 explosion modelling indicates that a 13 well slots separation between Rigs 401 and 402 will minimise the blast overpressure to below the 300 mbar design withstand overpressure, and therefore no significant structural damage is be expected to occur [22].

The rig designer’s fire modelling also shows a maximum credible horizontal jet fire (i.e. through annulus blowout), which indicates that structurally damaging levels of thermal radiation (i.e.

>37.5kW/m²) are not expected to reach the Shapagat or Karlygash LQs grounded at the South edge of the drilling island. Heat flux is expected to be well below 5kW/m².

Agip KCO have separately conducted fire modelling of worst case vertical blowouts (i.e. through open well) developed on the basis of a database generated with a series of CFD calculations using FLUENT code and validated against experimental data [53]. The results are summarised in Table 4.10, which indicate that the structurally damaging thermal radiation levels are limited to the drilling rig and are not expected to impact on the LQ vessels where personnel will be mustered.

Table 4.10 : Radius of Radiation Flux End Points (m) for Vertical Jets Safety

3kW/m² 7.5kW/m² 12.5kW/m² 37.5kW/m² Block D Drilling Full Bore

Concurrent Drilling Activities

Agip KCO have comprehensively reviewed SIMOPs at A and D-Islands. The SIMOPs review drew on drilling QRA risk data and rule sets, the drilling CONOPs Manual, and site plot plans to develop a Matrix of Permitted Operations (MOPO) for concurrent activities on Rig 401 and 402. The MOPO describes drilling rules and constraints to minimise the risk of well control incidents e.g. concurrent CHCD drilling is not permitted (see Appendix D).

Concurrent Construction Activities

To minimize overlap between well operations and construction activity, Agip KCO has adopted the strategy of in-reservoir well operations being conducted during the winter months (nominally November to March), thus allowing for the majority of construction activity in the summer months to occur free of H2S risk. Where this is not practicable a detailed set of rules have been developed to govern the risk assessment process [66].