A Comparative Study Between Surface and Subsea Bop Systems in Offshore Drilling Operations

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BRAZILIAN JOURNAL OF PETROLEUM AND GAS ISSN 1982-0593 TSUKADA, R. I.; MOROOKA, C. K.; YAMAMOTO, M. “A COMPARATIVE STUDY BETWEEN SURFACE AND SUBSEA BOP SYSTEMS IN OFFSHORE DRILLING OPERATIONS”. Brazilian Journal of Petroleum and Gas. v. 1, n. 2, p. 88-94, 2007. A COMPARATIVE STUDY BETWEEN SURFACE AND SUBSEA BOP SYSTEMS IN OFFSHORE DRILLING OPERATIONS 1 R. I. Tsukada*, 1C. K. Morooka, 1M. Yamamoto 1 Universidade Estadual de Campinas – Departamento de Engenharia de Petróleo * To whom all corre
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  BRAZILIAN JOURNAL OF PETROLEUM AND GAS ISSN 1982-0593 TSUKADA, R. I.; MOROOKA, C. K.; YAMAMOTO, M. “A COMPARATIVE STUDY BETWEEN SURFACE AND SUBSEA BOP SYSTEMS IN OFFSHORE DRILLING OPERATIONS”. Brazilian Journal of Petroleum and Gas . v. 1, n. 2, p. 88-94, 2007. 88 Downloaded from World Wide Web http://www.portalabpg.org.br/bjpg   A COMPARATIVE STUDY BETWEEN SURFACE AND SUBSEA BOP SYSTEMS IN OFFSHORE DRILLING OPERATIONS 1 R. I. Tsukada * , 1 C. K. Morooka, 1 M. Yamamoto 1  Universidade Estadual de Campinas – Departamento de Engenharia de Petróleo * To whom all correspondence should be addressed.  Address:  R. Mendeleiv s/n – Campinas – São Paulo – Brazil – CEP 13083-970 Telephone number:  +55 19 3521-3356  E-mail:  tsukada@dep.fem.unicamp.br   Abstract. The high demand for petroleum, associated with its high price, has motivated many major petroleum companies to operate in deep and ultra-deep waters. This trend  brings about many technical and economical challenges. One alternative to drilling operations in ultra-deep water is Surface Blow-Out Preventer (SBOP), a technique that has proven to be very promising from many technical and scientific works. The present study introduces a comparative analysis between the surface and subsea BOP system installation in offshore drilling operations. We have focused on results for riser displacement behavior and stresses. Advantages and disadvantages for the each system are discussed, particularly for offshore deepwater drilling operations.  Keywords :  drilling riser; SBOP; BOP; drilling system 1. INTRODUCTION The pursuit of petroleum to satisfy the growing demand, associated with petroleum high prices, has motivated many major  petroleum companies to operate in deep and ultra-deep waters. Many technical and economical challenges are brought about by such trend, and must be effectively accounted for. In ultra-deep water drilling operations, the drilling platform is connected to the Blow-Out Preventer (BOP), installed at the wellhead on the seabed by the drilling riser. The drilling riser is a steel tube containing the drill string which enables the flow of drilling fluids. In most drilling systems, the drilling fluid is  pumped into the well flowing through the drill string and returns to the surface by flowing up through the annular space between the drilling riser’s internal wall and the outer circumference of the drill string. The BOP is a  piece of safety equipment used to circulate kicks and control the pressure of the well while the kick is being circulated. The word ‘kick’ is commonly used to describe a phenomenon that occurs during the drilling operation when a high-pressure formation is reached, generating an unfavorable pressure gradient between the  petroleum formation and the well. This  pressure gradient ultimately causes an influx of fluid from the formation to the well, which increases the pressure at the bottom of the well. If this phenomenon continues, an uncontrolled fluid flow to the surface may form, which is usually referred to as the Blow Out. The traditional drilling operation approach is to lower a subsea blow-out preventer (BOP) within a large-diameter riser and connect it to the well head. The heavy submersed BOP and accompanying riser require appropriate riser tensors with even higher capacity. Therefore, deeper water further exacerbates this problem, requesting high-capacity riser tensors, which demands the use of expensive and scarce 5th-generation semi-submersibles. A new concept of drilling system, still in development, is a possible BOP arrangement called the Surface BOP (SBOP) which utilizes a lighter surface BOP at or near deck level, a higher-pressure riser and a Subsea Disconnected System (SDS) for quick disconnections. This arrangement greatly  BRAZILIAN JOURNAL OF PETROLEUM AND GAS TSUKADA, R. I.; MOROOKA, C. K.; YAMAMOTO, M. “A COMPARATIVE STUDY BETWEEN SURFACE AND SUBSEA BOP SYSTEMS IN OFFSHORE DRILLING OPERATIONS”. Brazilian Journal of Petroleum and Gas . v. 1, n. 2, p. 88-94, 2007. Downloaded from World Wide Web http://www.portalabpg.org.br/bjpg   89  reduces the capacity requirement of the riser tensors enabling the use of cheaper and more  plentiful 3rd-generation semi-submersibles. The SBOP concept is used in fixed  platforms. However, the first use of the SBOP technology applied in deepwater operations was reported in 1967 at the Nigeria’s EA field (Brunt et al, 2004). The SBOP was already used in ultra-deep water drilling operations with a dynamic positioned (DP) drilling  platform, as presented by Azancot et al. (2004), Brander et al. (2004) and Taklo et al. (2004). The SBOP system also offers more advantages than just being economical. Firstly, the SBOP system employs a casing riser, which reduces environmental loads and top tensions  by more than 50%. Furthermore, the casing riser joints can be used as a traditional casing, which allows the more frequently renewal of the riser, refreshing the fatigue life of the riser  joints. Therefore, the discharge of drilling fluid caused by riser failure is reduced by more than 50% (Taklo et al., 2004). Due to the SBOP placement, the reliability, downtime and maintenance are also improved. Usually, traditional offshore drilling systems use a flex-joint above the BOP, which causes riser wear induced by flexible movements and motion. The SBOP system uses a stress joint in  both extremities of the riser. In the SBOP system, the SDS is installed over the wellhead. In this situation, this equipment can be considered redundant if used to close the well, since the SBOP is attached to the system at the surface. Also, the SDS can be closed, even if a riser failure occurs. The items listed above are some of the advantages of the system as far as safety is concerned, presented in scientific and technical works. One of the main disadvantages of the SBOP system is the need for a high-pressure riser. If safe high-pressure risers are available, the SBOP system becomes an extremely attractive option when drilling in deep waters. In this context, comparative analyses  between an offshore drilling system using a submersed BOP and a system using a SBOP were carried out with a focus on the mechanical  behavior of the drilling riser for the operation of BOP or SDS installation. The results were obtained by numerical simulation in the time domain. 2. DESCRIPTION OF THE OFFSHORE DRILLING SYSTEMS In this work, the offshore drilling system Tensioning Cable x y z Diverter Kill / Choke lines Ball Joint Telescopic Joint Rotary Table LMRP BOP Flex Joint Drill String Drilling Riser Figure 1.  Traditional drilling system’s layout.    BRAZILIAN JOURNAL OF PETROLEUM AND GAS TSUKADA, R. I.; MOROOKA, C. K.; YAMAMOTO, M. “A COMPARATIVE STUDY BETWEEN SURFACE AND SUBSEA BOP SYSTEMS IN OFFSHORE DRILLING OPERATIONS”. Brazilian Journal of Petroleum and Gas . v. 1, n. 2, p. 88-94, 2007. 90 Downloaded from World Wide Web http://www.portalabpg.org.br/bjpg  using a submersed BOP will be called the traditional offshore drilling system and will follow the API norm (1993), which is used in the design of drilling systems. Figure 1 illustrates the main equipment that composes this type of drilling system. The interface between the riser and the drilling platform is made by the telescopic  joint, which is used to avoid the transmission of heave motion (vertical motion) of the platform to the riser, which could greatly reduce the service life of the riser and its associated components. To increase the riser’s rigidity a tensioning system is used to apply tension to the riser, thus increasing it’s bending stiffness. This is done by cables that are installed at the telescopic joint, which is part of the tensioning system. A ball joint is installed above the telescopic joint but below the diverter, and is a component used to avoid bending moment concentrations at the interface between the telescopic joint and the diverter. The diverter allows the flow of drilling fluid from the riser to the drilling fluid treatment system, and if necessary this equipment can transport fluids generated by the kick away from the platform. The rotary table is used to transmit rotation to the drill string. At the seabed, the riser is connected to a flex  joint, which has the same finality as the ball  joint, but with controlled rotational stiffness. Below the flex joint is the LMRP (Low Marine Riser Package), to allow the disconnection of the riser and the BOP in the case of emergencies. The kill and choke lines assist the circulation of a kick. In the traditional drilling system these are connected on the outer surface of the riser along with other auxiliary lines. In the operation of the BOP installation at the seabed, the interface between the drilling  platform and the riser is made by a component called ‘spider’. This component is installed in the drilling platform deck, where the riser will  be clamped, allowing the assembly of the riser until complete installation of the BOP. Until now, there is no drilling norm that contemplates the use of a SBOP. In view of this, the system will be herein described according to Brander et al. (2004). They described a drilling system with a SBOP used in a real drilling operation with a DP drilling  platform. The layout of the drilling system and the main components are presented in Figure 2. To make the SBOP system as safe as the traditional offshore drilling systems, proper equipment must be installed to the subsea wellhead in order to allow the disconnection of the riser at the seabed in the case of emergency. Tensioner    Tensioning Ring   Telescopic   Joint   Stress Joint   SBOP   Diverter    Flex Joint   Strakes   Spool   extension   Stress Joint   SDS   Riser    Wellhead    x y z   Figure 2.  Layout of ultra-deep water drilling system using a SBOP.    BRAZILIAN JOURNAL OF PETROLEUM AND GAS TSUKADA, R. I.; MOROOKA, C. K.; YAMAMOTO, M. “A COMPARATIVE STUDY BETWEEN SURFACE AND SUBSEA BOP SYSTEMS IN OFFSHORE DRILLING OPERATIONS”. Brazilian Journal of Petroleum and Gas . v. 1, n. 2, p. 88-94, 2007. Downloaded from World Wide Web http://www.portalabpg.org.br/bjpg   91  This equipment is called the Subsea Disconnected System (SDS), as mentioned above. The SDS is not so different from the submersed BOP: it is constituted of a set of rams that permit shearing of the drill string and closes the well, with or without the drill string accommodated inside the well. A stress joint is installed on top of the SDS to minimize the variation of stiffness at the interface between the SDS and the riser, thereby avoiding stress concentration points. At the top riser region another stress joint is installed to avoid stress concentrations between the SBOP and the riser. This joint is connected to the tensioning ring where riser tensors normally used to transmit tension to the riser are connected. Between the SBOP and tensioning ring, a spool extension is installed to lift the SBOP in order to increase the gap  between the tensor and the SBOP, thus decreasing the chance of collisions. Vortex Induced Vibration (VIV) strakes are installed to avoid vortex shedding that induce these vibrations, which reduce the riser’s life due to fatigue. The diverter, telescopic joint and flex  joint in the SBOP system have the same function as in the traditional offshore drilling system. 3. RISER MODEL EMPLOYED The vertical riser can be structurally modeled as an extensive beam element under axial tension, environmental loads and pressure effects due to internal and external fluid  pressure (Morooka et al., 2006). The riser’s Axial-Flexural Equation for the in-line and transversal directions was given by Chakrabarti and Frampton (1982): ( )( ) [ ] f dzdxAAAdzxd PAPAT dzxd EIdzd  00iiss 22ii00 2222 =g-g+g- -+-÷÷ ø öççè æ   (1) where:  EI   = Bending Stiffness T   = Axial Tension  A 0   = Outer Area  A i  = Inner Area  A S   = Sectional Area P 0   = External Pressure P i  = Internal Pressure γ s  = Specific weight of riser material γ i  = Specific weight of inner fluid γ 0  = Specific weight of outer fluid  x  = Displacement  f = Force The environmental loads consist of wave and current forces. The waves and currents are considered to be srcinating from the same direction, referred to as the in-line direction. The VIV is considered to be acting in the transverse direction, which is perpendicular to the in-line direction. In order to estimate in-line hydrodynamic forces, a modified Morison Equation is used considering the relative velocity (equation 2), as presented by Morooka et al. (2005). Wave kinematics is obtained by applying the Stokes 5 th -Order Theory. ( ) ( )  xu AC  xU uV  AC u A f   I  Acr  D D I  x  -+-++=  4 20  D A  I   rp  =  2 0  D A  D  r  =  (2) 22 )(  y xU uV  C r   +-+=  where:  r   = Outer fluid density 0  D  = Outer diameter  D C   = Drag coefficient  A C   = Added mass coefficient u  = Water particle velocity u  = Water particle acceleration c U   = Current velocity  x  = In-line riser velocity  y  = Transverse riser velocity  x  = In-line riser acceleration In extreme stress analyses, the von Mises stress is employed in order to assess whether the stress throughout the riser exceeds the admissible stress levels recommended by the API (1993). In this work, the von Mises stress (equation 3) is calculated based on the API (1993).
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