a comparative study between surface and subsea bop systems in offshore drilling operations

8/12/2019 A Comparative Study Between Surface and Subsea Bop Systems in Offshore Drilling Operations

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BRAZILIA N JOURNAL OF PETROLEUM AND GAS ISSN 1982-0593TSUKADA, 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.

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A COMPARATIVE STUDY BETWEEN SURFACE AND SUBSEA BOP

SYSTEMS IN OFFSHORE DRILLING OPERATIONS

1R. I. Tsukada*, 1C. K. Morooka, 1M. Yamamoto

1

Universidade Estadual de Campinas Departamento de Engenharia de Petrleo

* To whom all correspondence should be addressed.

Address:R. Mendeleiv s/n Campinas So Paulo Brazil CEP 13083-970

Telephone number:+55 19 3521-3356

E-mail:[email protected]

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 systeminstallation 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 stringwhich 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

risers 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 thatoccurs 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 risertensors 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 quickdisconnections. This arrangement greatly


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BRAZILIAN JOURNAL OF PETROLEUM AND GASTSUKADA, R. I.; MOROOKA, C. K.; YAMAMOTO, M. A COMPARATIVE STUDY BETW EEN SURFACE AND SUBSEA BOP SYSTEMS IN OFFSHORE DRILLING OPERATIONS.Brazilian Journal of Petroleum and Gas. v. 1, n. 2, p. 88-94, 2007.

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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 operationswas reported in 1967 at the Nigerias 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 casingriser 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 beclosed, 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 analysesbetween 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

yz

Diverter

Kill / Choke lines

Ball Joint

Telescopic Joint

Rotary Table

LMRP

BOP

Flex Joint

Drill String

Drilling Riser

Figure 1.Traditional drilling systems layout.


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BRAZILIAN JOURNAL OF PETROLEUM AND GASTSUKADA, 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.


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 risers rigidity a

tensioning system is used to apply tension to

the riser, thus increasing its bending stiffness.

This is done by cables that are installed at the

telescopic joint, which is part of the tensioningsystem. 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 thecirculation 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 thatcontemplates 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

TelescopicJoint

Stress Joint

SBOP

Diverter

Flex Joint

Strakes

Spool

extension

Stress Joint

SDS

Riser

Wellhead

x

yz

Figure 2.Layout of ultra-deep water drilling system using a SBOP.


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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 stringaccommodated 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 andtensioning 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 risers 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 risers

Axial-Flexural Equation for the in-line and

transversal directions was given by Chakrabartiand Frampton (1982):

( )

( )[ ] fdz

dxAAA

dz

xdPAPAT

dz

xdEI

dz

d

00iiss

2

2

ii002

2

2

2

=g-g+g-

-+-

(1)

where:

EI= Bending Stiffness

T = Axial TensionA0 = Outer Area

Ai= Inner Area

AS= Sectional Area

P0= External Pressure

Pi= Internal Pressure

s= Specific weight of riser material

i= Specific weight of inner fluid0= 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 originating 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-linehydrodynamic 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 5th-Order Theory.

( ) ( )xuACxUuVACuAf IAcrDDIx -+-++=

4

2

0DAIrp

= 2

0DADr

= (2)

22)( yxUuV Cr +-+=

where:

r = Outer fluid density

0D = Outer diameter

DC = Drag coefficient

AC = Added mass coefficient

u = Water particle velocity

u = Water particle acceleration

cU = 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 (equation3) is calculated based on the API (1993).


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( ) ( ) ( )2prpz

2

pzp

2

ppreq2

1s-s+s-s+s-s=s qq (3)

where:

prs = radial stress

qsp = hoop stress

pzs = axial stress

4. RESULTS AND DISCUSSION

Following the riser model presented above a

preliminary analysis of the structural behavior

of a riser during the operation of the SDS or

BOP installation was studied. The analysis was

made considering the boundary condition in the

top as clamped and the bottom as free. Figure 3

illustrates the BOP and SDS installation

operation, where L represents the riser length

and U is the current velocity.

Table 1 presents the riser geometry and

configuration used for the numerical

simulation. The riser of the drilling system

using the SBOP is presented by Brander et al.

(2004). For the traditional drilling system, the

values were assumed based on drilling normal

practices.

The dynamic behavior was calculated by

taking a constant riser element length (9.5 m),

for both systems. The SDS and the SBOP were

considered as a very rigid and heavy element

(SDS mass = 35000 kg and BOP mass =

300000 kg). Table 2 presents the otherparameters used in the simulation, where CL,

CD and CM are the lift, drag and inertia

coefficient.

Figure 4 presents the results for the BOP and

SDS in-line displacement, which is the

positions of riser maximum displacement, and

the top von Mises stress, which is the most

critical region. These results were obtained by

varying the riser length (L) in many

simulations.

From Figure 4a, it can be observed that the

SDS permits larger displacement than the

SBOP. In this operation the riser is tensioned

only by the weight of the SBOP or SDS.

U

WAVE

L

BOP

Subsea

BOP System

WAVE

U L

SDS

Surface

BOP System

Figure 3.A comparative scheme for BOP and SDS installation operation.

Table 1.Riser geometry and configuration.

Nominal Size [m] 0.34 0.533

Material Grade P110 X80

Density [kg/m] 8006 7860

Inner Diameter [m] 0.313 0.486Yield Strength [MPa] 759 551

Table 2.Riser Parameters in the simulation.

Sea water density [kg/m3] 1025

Drilling fluid density [kg/m3] 1200

Structural damping 0.003

Strouhal number 0.2

CL 1.0CD, CM 1.2, 2.0


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Therefore, since the SDS is lighter than the

BOP, the riser of the SBOP system will present

a greater displacement.

From Figure 4b, it is observed that both

systems have a very near von Mises stress and

both are in accordance with the API (1993),

which states that the von Mises stress must be

less than 67% of the yield strength.

5. CONCLUSION

A comparative study between subsea and

surface BOP systems was presented. Riser

maximum displacement and von Mises stresses

along the system length were evaluated

considering BOP and SDS installation,

respectively, in deepwater offshore drilling

operations.

After considerations made throughout the

study, it was possible to confirm some

interesting results presented in the literature

showing how promising the operation with

SBOP drilling system can be.

This study has also highlighted some

important concerns regarding the application of

the SBOP system. In particular, specialattention is required in a deepwater drilling

scenario. In this case, numerical simulations

have been demonstrated as a powerful

procedure to analyze the complex non-lineardynamic behavior of the riser under hanging

conditions. Besides, the axial tension behavior

needs special care in the analysis.

In the present study, the results confirmed

that drilling risers can support the operation for

installation of both BOP and SDS systems.

However, higher riser displacement levels have

been observed in the SDS case than in the BOP

case.

ACKNOWLEDGEMENTSThe authors would like to thank CNPq and

Finep (CTPetro), Petrobras and PRH/ANP 15

(The Brazilian National Petroleum Agency),

for supporting the present study.

REFERENCESAMERICAN PETROLEUM INSTITUTE,

Washington. API Recommended Practice

16Q, Design, Selection, Operation and

Maintenance of Marine Drilling Riser

Systems, Washington, 1993, 48p.

AZANCOT, P., MAGNE, E., ZHANG, J.,

Surface BOP Management System &

Design Guidelines. In: IADC/SPE DrillingConference, 2002, Texas (U.S.A.), Houston:

International Association of Drilling

Contractors, Richardson: Society of

Petroleum Engineers, 2002, IADC/SPE74531.

BOP or SDS In-line Displacement

BOP or SDS Vertical Position [m ]

In-lineDisplacem

ent[m]

0

0.05

0.1

0.15

0.2

0.25

0 500 1000 1500 2000 2500

SDS

BOP

BOP or SDS In-line Displacement

BOP or SDS Vertical Position [m ]

In-lineDisplacem

ent[m]

0

0.05

0.1

0.15

0.2

0.25

0 500 1000 1500 2000 2500

SDS

BOP

0

50

100

150

200

250

300

350

0 500 1000 1 500 2 000 2500

SDS

BOP

BOP or SDS Vertical Positi on [m]

VonMisesStress

[MPa]

Top von Mises Stress

0

50

100

150

200

250

300

350

0 500 1000 1 500 2 000 2500

SDS

BOP

BOP or SDS Vertical Positi on [m]

VonMisesStress

[MPa]

Top von Mises Stress

(a) (b)

Figure 4.BOP and SDS in-line displacement (a) and top von Mises stress(b) as a function of BOP or SDS vertical position (riser length).


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BRANDER, G., MAGNE, E., NEWMAN, T.,

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a comparative study between surface and subsea bop systems in offshore drilling operations

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