Transcript Document
The Gulf Oil Spill Whose to Blame as well as a reflection of the science literacy problem in the US Deep Water Drilling: The Dangers of High pressure Geometry of the BOP How an Oil Well Works The oil entering at the bottom requires an internal pressure source to push it to the top – this is usually supplied by associated gas deposit Also Pressure by Direct Injection Reservoir Pressure is key to driving the oil out Simple Physics ► Without any additional source of pressure oil by itself will not be squeezed up the tube. Thus crude oil without an internal pressure sure is usually a Dry Well ► The deeper you drill the higher the internal pressure you likely to tap. This is why you need a pressure regulator a.k.a The Blow Out Preventer 1. The top of the BOP is the bottom of the seafloor about 5000 feet below the surface. The confining pressure from 5000 feet of water is 2250 PSI (or 150 times higher than sea level air pressure). 2. Deep Reservoir Pressure was estimated by Coast Guard to be 8000 PSI 3. When this 8000 psi reservoir was tapped there was obviously enough pressure to allow oil to flow up vertically through the 4000 meter long pipe – in turn this reduces the reservoir pressure as its now been tapped. One can calculate the pressure in this 4000 meter column of oil to be (in round numbers) 5000 psi. 4. This now means that the pressure at the bottom of the BOP is 8000 -5000 = 3000 psi. Recall that the confining pressure by the sea water is 2250 psi. This means the differential pressure is 750 psi, which is less than the confining pressure or NO FLOW 5. Geological data suggested that average reservoir pressures in these deep marine pockets would be 20,000 – 40,000 psi. If you use the 20,000 psi (which then creates a differential pressure of about 18,000 psi) then you calculate a flow rate of 65,000 barrels a day for a 21 inch diameter pipe 6. The initial estimate of 1000 barrels a day was therefore impossibly low. By the way, if you leave your bathtub on for 24 hours the volume flow rate would be equivalent to 500 barrels a day. 7. In addition, BOPs are rated at 15,000 PSI. The mere fact that it failed tells you the pressure must be at least that Also, a typical deep marine rig will process about 20,000 barrels a day. The fact that BP got away for at least a week by stating a figure of 1000 barrels a day is a sad testimony to the science illiteracy of this country and especially congress! 8. For pipe flow, the flow rate goes as the square root of the pressure differential so if you double the pressure differential (40,000 PSI) then the flow rate increase to 1.4 x 65,000 = 91,000 barrels a day. Flow rate from Velocity Estimates If you don’t know (or pretend not to know) the actual pressure differential there is another method that can be used to estimate the flow rate. The flow velocity comes from video camera measurements only – no one every installed a velocity meter because it was too difficult. Various techniques can be used to estimate the flow velocity from video frame rate data. Those estimates come in at around 1 meter/sec (about 2 mph) Velocity based flow estimate ► ► ► ► ► Diameter of pipe is 21 inches = 0.53m; pipe cross section is pR2 = .2 square meters Volume flow rate is then 1 m/sec * .8 = .2 cubic meters per second .2 * 3600 seconds per hour = 720 cubic meters per hour = 720,000 liters/hour One bbl = 159 liters. Thus 720,000/159 = 4500 barrels/hour (in round numbers) 4500 x 24 = 108,000 barrels per day Sources of Uncertainty There is considerable confusion about which pipe diameter to use. Some suggest that the 21 inch pipe is merely the outlet pipe and that the interior pipes are 14 inches in diameter. If you use 14 inches as the diameter this will reduce the flow rate determined by the pressure differential method by about a factor of 2. On the other hand, it’s also possible that the internal reservoir pressure was as high as 100,000 PSI there by getting back your factor of 2. Sources of Uncertainty But the biggest uncertainty by far is the oil/gas mixture in the flow. The previous calculations assume 100% crude oil. The effect of mixing gas is two fold: a) the effective density of the fluid lowers and could lower greatly: pressure differential flow rates scale directly with fluid density – if the effective density is down by a factor of 10, the flow rate will be down as well. b) The fluid velocity at exit is strongly determined by the gas oil mixture. Pure crude will have an outflow velocity considerably less than oil/gas mixture. The video always shows methane bubbles present but the bubbling rate changes this implies variable oil to gas mixture with time. Bottom Line ► Because of the presence of gas in the effluent it is scientifically impossible to precisely know the volume flow rate. ► Worse case: 50,000 – 100,000 bbl/day ► Best case: 10,000 -25,000 bbl day Note that the official Federal Task Force flow committee arrives at a rate of 12,000 – 19,000 bbl/day but that took an entire federal task force to do we just did in twenty minutes. Total Official Oil Budget Whose to Blame – 3 Options • BP as Corporate Greedy Assholes that punctured a deep wound in Mother Earth and let her bleed out • Consumerism demand for the product drives the industry to premature extraction techniques where accidents happen and become the cost of doing business • The regulatory environment (MMS) that didn’t understand simple physics. The Price of Business