Environmental statement - Flyndre and Cawdor - Maersk Oil

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Flyndre and Cawdor Environmental Statement Section 6 Hydrocarbon Releases diesel, and the other 36% relate to condensates, hydraulic oils, oily waters and other unknown types of oil. Spilled amount (tonnes) 900 800 700 600 500 400 300 200 100 0 Figure 6‐1 Volume of spilled oil and number of spills in UKCS waters. 6.2. POTENTIAL SOURCE OF HYDROCARBON SPILLS AT THE FLYNDRE AND CAWDOR DEVELOPMENT 6.2.1. DIESEL SPILL FROM DRILLING RIG Historical data collected since 1975 indicate that mobile offshore drilling units (MODU) account for only 23% of all diesel spills. The majority of these spills are caused by hose failure and drain overflows and the releases are relatively small, predominantly less than 0.1 tonnes (Oil and Gas UK, 2009). Table 6‐1 presents data submitted to DECC from the period 2001‐2007. Maintenance/operation activities Total amount spilled (tonnes) Total number of oil spill reports 10‐100 tonnes
Flyndre and Cawdor Environmental Statement Section 6 Hydrocarbon Releases Period 1990‐1999 2000‐2007 Number Frequency Number Frequency 160 0.246 78 0.172 Table 6‐2 Number and frequency of spills from MODUs in UKCS waters (Oil and Gas UK, 2009). Apart from blowouts the worst case spill scenario from MODUs would results from incidents such as vessel grounding, collision or explosion leading to a total loss of fuel. The results from modelling such a scenario during the drilling of the Cawdor wells are presented in Section 6.3.1. 6.2.2. UNCONTROLLED OIL SPILL FROM WELL BLOW‐OUT A well blowout refers to the uncontrolled release of hydrocarbons from a well after the pressure control systems have failed. Primary well control is achieved by maintaining a hydrostatic pressure in the wellbore greater than the pressure of the fluids in the formation being drilled, but less than the formation fracture pressure. If the pressure of the fluid in the wellbore is less than the formation pressure, the well will flow and an influx will enter the wellbore. If fluid pressure is greater than the formation fracture then the formation fractures and the wellbore fluid is lost into the formation (losses); in a worst case scenario there can be insufficient wellbore fluids left in the wellbore to resist formation pressure and an influx occurs. Flow of reservoir fluids into the well is stopped by closing the Blow Out Preventer (BOP) which is the initial stage of secondary well control. Secondary well control is completed by circulating the well to kill weight fluid and displacing the influx out of the well. If primary and secondary well control fails a blow out can occur. Based on revised guidance issued by DECC (DECC, 2010), drilling engineers considered six potential scenarios when determining the worst‐case scenario for a well blow‐out. These are: 1. The first time the reservoir is penetrated is while drilling 12 1 /4” hole. If the well were to flow at this stage (for instance due to the fact that a large hydrocarbon influx has entered the well), it is possible hydrocarbons may reach the surface as the wellbore pressures in the 12 1 /4” open hole will be significantly below what is required to keep the shales back. Hole collapse and plugging is expected to occur within a couple of days or weeks. 2. Similarly, during the drilling of the 8‐ 1 /2” reservoir hole section it is possible hydrocarbons may reach the surface. Hole collapse and plugging would be expected within a couple of weeks if a blow‐out were to occur while drilling the reservoir. 3. Once the sand screens have been installed, before the completion is run, there is a full steel‐ lined conduit in place from reservoir to surface, with an 8.5” internal diameter from the top of the sandscreens. If the well is live and the drilling rig would somehow lose station with the BOP not holding pressure and all hydraulic control lost, the well will have an unrestricted flow through the 9 5 /8” production casing with an internal diameter of 8½”. This will present a lower pressure loss conduit to the environment than when the well is completed. This scenario constitutes the worst case blow out event for the spill modelling. 4. In the completion phase hydrocarbons are purposely introduced into the wellbore during the production clean‐up. Although this would appear to introduce more blow‐out risk than in any drilling scenario, there are no less than 4 barriers in place: The sub‐surface safety valve (SSSV), the subsurface test tree, lubricator valves above the subsurface test tree and the surface test tree. Moreover, the SSSV, the subsurface & surface test trees have valves which fail closed. If the well is live and the rig would somehow lose station with the BOP not holding pressure and all hydraulic control lost, the subsurface test tree and SSSV will close D/4114/2011 6‐3
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Environmental statement - Flyndre and Cawdor - Maersk Oil

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