IADC/SPE 128477 20KSI BOP Stack Development Melvyn F. Whitby, SPE, John E. Kotrla, SPE; Cameron Drilling Systems
Copyright 2010, IADC/SPE Drilling Conference and Exhibition This paper was prepared for presentation at the 2010 IADC/SPE Drilling Conference and Exhibition held in New Orleans, Louisiana, USA, 2–4 February 2010. This paper was selected for presentation by an IADC/SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the International Association of Drilling Contractors or the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the International Association of Drilling Contractors or the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the International Association of Drilling Contractors or the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of IADC/SPE copyright.
Abstract After the introduction of the Cameron 18-3/4” 20,000 psi EVO Drilling BOP at OTC, 2009, this paper will focus on identifying and completing the GAP technologies necessary for design and testing of a Deepwater 20,000 psi subsea BOP stack. This paper will identify design methodologies employed in design and testing of the BOP and outline GAP technologies that must be addressed for completing a Deepwater Subsea BOP stack. Taking a bottom up approach, the discussion will focus on the Wellhead connector and necessary loads, the Drill Through column, Choke and Kill systems and packaging to allow for use on existing rigs with minimal upgrade intervention. Background Industry leading BOPs notwithstanding, a Stack is much more than the sum of its parts. It is a complete, integrated system package. It is synergistic combinations of numerous design disciplines and dissimilar products and components that all must work effectively together, in the context of an offshore drilling vessel, to maintain well control at all times to safeguard the lives of everyone on board the rig, as well as the environment as a whole. Toward this end, we continue… Starting with the wellhead connector, one option for Cameron would be the 18-3/4” DWHC wellhead and profile. Already the strongest 15K connector in the industry and able to withstand the most stringent tension and bending requirements, as well as pressure loadings, it would seem an obvious starting point. There will undoubtedly be some tweaking to be done to the internal components of the connector to handle the added pressure loads, but those modifications can wait until the fully detailed engineering study and prototype testing can be done for the 20K model. Most assuredly, a part of the engineering study will also be to determine if our existing EVO-Con connector geometry is sufficiently robust to withstand the rigors of 20K service while interfacing with multiple wellhead locking profiles. While the DWHC connector is easily scalable up to 20K service, the operational flexibility of the EVO-Con architecture cannot be overlooked, or underestimated. The Choke and Kill valves needed to be upgraded to a 20K pressure rating. While Cameron has existing 20K gate valves, they are not packaged in a dual cavity block body with spring-assist subsea operators. So, this “re-packaging” effort was carried out to ascertain the basic configuration, size, and weight of the “new” gate valves before they could be included in the overall Stack layout. Until the C/K valve configuration and size was determined, the overall Stack frame envelope could not be finalized. As it turned out, while all the 20K pressure containing equipment was larger than the 15K equipment it replaces, it was not so
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large that we could not fit it into our existing large frame envelope of 216.5” by 189” (4.8M X 5.5M). Obviously, there would be less maintenance access inside the frame compared to 15K BOPs and valves, but not as restrictive as with TL BOPs inside our existing small frame envelope of 149.6” by 177.2” (3.8M X 4.5M). Design Methodology Whilst there is much discussion and an on-going effort to provide guidance for Equipment greater than 15K, in the interest of expediency, it was decided internally within Cameron to apply current design codes and practices. The 20K “EVO” BOP was designed, tested and qualified to API 16A 3rd edition. Although initial testing was conducted at ambient temperatures using standard temperature packers, the final qualification will take place at high temperature (350F continuous service). The selected material yield strength, for the purpose of analysis and stress criteria was derated for temperature in accordance with API. ASME section VIII, Division II, was used for the analysis and comparative Finite Element Analysis. Accordingly, a 1.5 times working pressure hydrostatic shell test was performed in accordance with API 16A. End Connections The design basis for the end connection selected for the BOP and Drill Through column is an API style bolted flange connection. The initial calculations for the flange configuration were performed using the Taylor Forge standard. This was then optimized using Finite Element Analysis and finally validated using strain gage data at hydrostatic and working pressure conditions. There has been much discussion regarding the selection and use of a BX style ring gasket for sealing the connection. The obvious choice was to utilize a typical 18-3/4” BX-164 profile and gasket. Any deformation that might occur, would be as a direct result of connection separation. To this end, it was decided that this would be overcome by increasing the bolting preload. Instead of the typical 52% of yield bolting preload, this was increased to 75% of bolt yield. At 30,000 psi hydrostatic test pressure, no flange separation took place. Therefore, the gasket and connection was able to withstand the applied pressures with no deformation. The strain gage data validated the initial analysis and FEA by way of the fact that after application of pressure, the strain gage readings returned to zero verifying that no plastic deformation had taken place. Wellhead Connector The Drilling industry has already begun to introduce wellhead profiles designed for use on wells greater than 15,000 psi. A competitive 20K wellhead profile was introduced at OTC 2009 at the same time as the 20K EVO BOP. For the drilling wellhead connector interface, one proposition would be to adopt the Cameron EVO connector principle that has been successfully employed on recent 15M subsea BOP stacks. The design basis is centered upon a 30” industry wellhead profile. As discussed in the background, another solution would be to employ the successful Cameron 18-3/4” DWHC wellhead profile. In either case, a modification to the body center section will be required to accommodate the 18-3/4” 20K connection. A similar case study was undertaken on the Cameron 15K DWHC wellhead connector and profile. Ram type ‘EVO” BOP The design basis for the 20K BOP was centered about the successful introduction of the 15K EVO BOP. Using the design principles established, a 20M double ram type BOP was found to be comparable in size and weight to the existing Cameron TL 15M BOP. This became the foundation for the development of a 20K subsea BOP stack that could be incorporated within the envelope and framework of an existing TL BOP stack. Obviously, the number of cavities required may significantly impact the outcome, but a 6 cavity 20K stack would be comparable to an existing 6 cavity TL stack. As with the end connections, the design stress criteria were derated for temperature in accordance with API. Using the existing T/TL/EVO cavity geometry, this became the design foundation for the BOP body envelope. Stress analysis and supporting FEA of the ram body geometry revealed that at 20K working pressure, the stresses in the ram block and cavity were within API allowable, even with the reduction in material yield strength allowable. The BOP was successfully hydrostatic shell tested to 30,000 psi. As with the end connections, strain gage data validated the initial design analysis and supporting FEA.
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After initial API 16A factory acceptance testing had taken place, the BOP undertook a full API 16A fatigue test, consisting of 546 closures and including 78 pressure cycles to 20,000 psi. The 20K EVO BOP successfully passed this test with no deformation to the cavity or ram blocks taking place. This test was concluded after the 546 cycles but was not continued until such time as a pressure test could not be obtained. At no time during the test, a closing pressure of greater than 1500 psi was required to establish and maintain a seal against 20,000 psi wellbore pressure. Inspection of the ram packer sealing elements at the conclusion of the test revealed no serious packer degradation had taken place. The next phase of testing for the ram type BOP will be focusing on the high temperature aspect (350F continuous). 18-3/4” Annular BOP There will be much discussion in the future about the selection of the appropriate working pressure of the annular BOP for a 20K stack. Historically, a working pressure for the annular was accepted at 50% of the rated stack working pressure, with the progression toward 15K and higher methodology has derived a 10K annular to be suitable. Obviously in the short term a 50% rating for the annular would allow use of existing equipment that is field proven, however if well control edicts demand that a 15K annular be developed then this will entail significant engineering to be accomplished. Stack Frame design Obviously, there would be less maintenance access inside the same frame with a 20K BOP Stack and valves compared to 15K BOPs and valves, but not as restrictive as with Cameron’s TL BOPs inside the previously designed small frame envelope of 149.6” by 177.2” (3.8M X 4.5M). That frame size would simply be too small to consider, so the larger envelope size of 189” by 216.5” (4.8 M X 5.5M) was selected. This larger frame size should allow reasonable maintenance access to all the BOPs and valves, as well as all the Controls panels and assemblies that will need to be installed onto the frame. An additional consideration in the specific design of any frame is structural strength adequate to withstand the loads imparted by the total weight of the BOP Stack. Since the 20K pressure containing components will be heavier than their 15K pretecessors, there is a high probability that the frame will need to be strengthened for the increased loads. However, in this case, since we were able to start with a frame size and design that was already complete, with full FEA models and results, making an educated determination as to the amount of increased strength needed will be less of a design effort than the original development of the geometry of the frame. Further considerations for the frame are locations and mounting arrangements for all the various components, panels, assemblies, accumulators and Pods of a suitable Control System enhanced for the rigors of a 20K BOP Stack. Reason would dictate that the increased fluid volumes required for the new 20K equipment will require additional emergency system accumulators (acoustics, Autoshear circuits, etc.). Plus, if the 20K fields are in water depths in excess of 10,000 ft, the increase in required accumulator volume will be compounded. And there are specific design considerations for the overall Choke and Kill “system” that will be installed on the first 20 Ksi BOP Stack that are atypical from what has been considered “normal” design issues on 15 Ksi Stacks. One of these is pressure end load growth. This is the term used to describe the phenomenon whereby the Choke and Kill spools “stretch” under pressure at a different rate than the BOP column. Since the BOP column is significantly more substantial than the Choke and Kill spools, it restricts the amount that the entire Stack assembly is allowed to expand when under pressure, or, when only the choke and Kill system is under pressure and the BOP comumn is not. This phenomenon manifests itself by exerting an unrestrainable bending moment on the lowest Choke and Kill valves’ connection to the BOP outlets. This bending moment sometimes, but not always, results in a leak, and is exeserbated if the BOP outlet to Choke and Kill valve connection is undertorqued. While this pressure end load growth was never reported as a problem on 10 KSI Stacks, it is only rarely a problem on 15 Ksi Stacks, but is expected by many “skilled in the art” to be more of a problem on 20 Ksi Stacks. Thus, the Choke and Kill valves being developed for this project will have oversized inlet flanges utilized for their connection to the BOP outlets. Specifically, the 3-1/16” 20 Ksi Choke and Kill valves will have 4-1/16” 20 Ksi inlet flanges to match the 41/16” 20 Ksi outlets on the BOPs. This “up-sizing” on the BOP outlet geometry yields a connection with significantly greater bending strength to specifically resist the pressure end load growth induced bending moments. With further study there may be more atypical design considerations discovered that can only be associated with the new 20 Ksi BOP Stacks. In these cases, simply extrapolating 15 Ksi equipment designs will not be adequate, but in many other cases, a reasonable extrapolation will surely suffice. Determining where the extrapolations work and where they do not will be the single largest determination in the effort to achieve a workable 20 Ksi “system”. To that end, we will continue to strive.
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• Height ƒ. With the EVO-BOP 20K, the 20K BOP Stack maximizes the height for upgrades on legacy 15K systems • Footprint ƒ. The footprint of the EVO BOP 20K Stack remains consistant with current 15K designs due to the compact EVO BOP design
Legacy 15K TL System versus EVO BOP20K System
