Whereas the process of material selection in engineering field can be a very complex and iterative process, it is important to note that it has a greater impact on the overall design and workability of a machine [3]. The material selected would define and predict the workability of the device and should be taken into keen consideration. Hence to improve the performance and prevent future failures in a machine it is equally critical. To address this a systematic approach known as material failure analysis is applied. This process will help to examine and test materials thereby enabling the determination of cause of failure in a component, structure or a product [7].
The methodology that has been used is a qualitative and quantitative analysis of the annular which is a sealing material for the blowout preventer.
During the drilling process, the pressure in the borehole controls all formations high pressure fluids emanating from the earth. If for any reason the borehole pressure falls below the formation pressure thereby failing control these formation pressure, they will enter the borehole and reach the surface that is being drilled hence resulting to a situation known as blow out [6]. The borehole pressure consists of pump pressure , the hydrostatic of the drilling mud and finally the friction pressure loss in the annulus which is our main area of concern .Therefore in order to prevent this , an annulus cap is used which when activated it automatically close the bore hole and seal it, which in some situations the sealing pressure is approximately 20,000Psi.
An annulus cap/seal is usually positioned above the ram preventer and will form a seal between the pipe and wellbore, it forma s a seal on the wellbore itself in cases where pipe is missing[6]. It is recorded that this operation can be automatically controlled using the hydraulic control system with sensors as it involves the application of pressure [3]. Upon taking a command, fluid containing high pressure will be directed to the lower sides where closing parts of hydraulic system are located, thereby moving the operation of the piston upwards and consequently the packer gets pressed [7].
The presence of the cap material at the top will prevent the packer from moving in another direction except towards the center of the borehole in order to either seal it or pack off a drill pipe .since it lacks blocking mechanisms , it cannot be closed over a longer period of time since its packing unit can be damaged. Similarly, to avoid damaging the rubber core when relieving pressure, it is advisable not to open the cap .since it requires a lot of hydraulic fluid to close it, it will require a lot of time [4].
Annular is made or elastomeric material that has the chemical properties of rubber and plastic [4]. It is stated that they are chosen based on the conditions that they will satisfy including the operating temperature of the downhole, produced fluids, solvents and completed fluids such as oil based mud[]. The difference in thermal temperatures between the surrounding environment and the temperature of the well bore affects the material [properties of the cap to change thereby creating thermal stress on the pipe[7] .Despite lacking the best elastomer for capping to suit these conditions , hydrogenated nitrile is used due to its higher temperature rating and also good chemical resistance to corrosion and chemical compounds .
In order to form a seal , the pressure in the elastomer must be greater than the differential pressure across the packer and this elastomer pressure is generated by the setting force which is applied to the packer[3] . High pressure forced gas and oil to spill off the wellbore in Macondo blowout .the caps designed to seal the drill pipe failed due to leakages of hydraulic fluid and multiple failures in critical systems [2].
Wendy Zukerman on the daily news reports that one of the major causes of oil spillage was pressure test failure. On more than two occasions, real time monitoring data in the rig indicated that the pressure in the pipe had increased when it was supposed to drop.suprisingly these indicators went unrecognized for a period of forty minutes .this means that the annulus cap failed to operate thereby the pressure from increasing the formation pressure beyond the borehole pressure [8].
According to A federal report on the guardian in April 2010, it shows that the oil spillage was due to factors of multiple failures and poor testing [1]. The blowout preventer experienced very enormous problems .i.e. they had holes and in the first place the company never tested it on the safety systems .also there was screw ups with fluid pressure, the drilling mud, cement and poor decisions [5].
In regards to that, applying material selection strategy approach where all the chemical and physical properties of materials are keenly considered we can allude that the rubber material used for making the cap is not the most appropriate , a more refined material with best chemical properties will help to solve the issue of pressure blow out . Through chemical property analysis, the bore should be enhanced with a mixture of barium sulphate and mud, also cement slurry should be added to the annulus cap to prevent downward hardening of the bore [4].
Conclusion
In order to extend the life span of the seal assembly and also humans it is recommended that the movement of the seal be restricted as much as possible. Although the most preferred material which is hydrogenated nitrile tends to meet the requirements for sealing, too much exposure to high concentrations of bromides and hydrogen sulfide may end up destroying the cap. Therefore the locator seal assembly should be landed so that when the well is producing it gets into constant compression thus limiting the movement of the elastomeric.
Thus it is the duty of every engineer to ensure that his / her design performs operations for the time period that it is mean for. To achieve this, every engineering designer should evaluate and recognize all the potential failure modes that relate to his/ her design. Thus a clear understanding of the analytical techniques required in predicting the failure modes so that in the event of any failure it may be detected and prevented.
Bea, B. B., 2002. deepwater Horizon blowout. 1st ed. mexico: sarah yang[1]. pp 10
abstract: This article talks about the explosion that occurred in mexico on april ,20, 2010 killing 11 lives and leaving a wide range of Gulf of Mexico spilled by oil.it focuses on the possible causes of the massive blowout and the steps being put in place to prevent future occurrence. in addition , the article records that adding a fliud comprising of barium sulphate which is enhanced with mud will help to maintain the required pressure thereby preventing the fliuds from seeping.
Borchardt, J. K., 2010. Executive Summary of Explosion and Fire at the Macondo Well. 1st ed. new orleans: Transocean[2]. pp 7-9
Abstract; the article presents a report on an explosion that occurred in Macondo 2003 due to failures. It records that the blowout preventer had been concluded to be very safe and the cause of the explosion seemed to be mysterious .basically it is thought that the major cause of the failure was leakage of hydraulic fluid
Collins, J. A., 2000. Failure of Materials in Mechanical Design: Analysis, Prediction, Prevention. 2nd ed. new york: John Wiley & Sons[3]. pp 367-371
Kutz, M., 2002. Handbook of Materials Selection. 1st ed. edinburgh: John Wiley & Sons[4]. pp 277-278
Mulcahy, M. B., 2010. BP oil spill. 1st ed. carlifonia: US chemical safety board zeroes[5]. pp 03
abstract; This article presents a report provided by the US chemical safety board zones on the cause of BP oil spillage in Washington dc. The board provided the failure that resulted into the spillage. When examined, the pressure test that could have revealed the problems in the drill were unrecognised and if they would have been then the explosion woiuld have been prevented .
Preben Hempel Lindøe, M. B. O. R., 2014. Risk Governance of Offshore Oil and Gas Operations. 2nd ed. london: Cambridge University Press[6]. pp 887-890
Stewart Lockie, D. A. S. D. R. F., 2013. Routledge International Handbook of Social and Environmental Change. 2nd ed. london: Routledge[7]. pp 05
Zukerman, W., 2010. Avoiding the Blowout. 2nd ed. carlifonia: National Academy of Engineering[8]. pp 12
abstract; This article presents a report on the explosion that occurred at the Gulf of Mexico .
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