Integrating Computational Fluid Dynamics and Multibody Model

of an Oil Production Adapter Base for Disconnection Analysis

Gerson BrandGraduate Student – Department of Mechanical Engineering

School of Engineering of São Carlos - University of São Paulo - Brazil

Marcelo Prado / John Hough

MSC Brasil Software e Engenharia Ltda.

Cesar Lima

Petróleo Brasileiro S.A. - Petrobrás

ABSTRACT

A methodology of Computation Fluid Dynamic

(CFD) and Multibody System (MBS) integration was

introduced to analyze a Production Adapter Base (PAB)

disconnection. Steady state CFD analyzes in different PAB

positions were done to get preliminary data considering

both fluid and mechanical dynamic as uncoupled. Then the

mechanical dynamics of PAB was investigated using a

multibody model and the steady state CFD results. Finally

a CFD analysis with moving mesh and mesh element

addition was done to simulate the coupling of both

dynamics and to validate the methodology.

INTRODUCTION

Production adapter base (PAB) is a sub sea

structure used in offshore platforms to extract the oil using

a high pressure natural gas injection into the well. The

PAB is attached to the wellhead through a dog house lock

system, which is a very confident component. Even though

the probability of fail of this component is remote, its

occurrence could be very dangerous for the sea

environment due to the possibility of PAB disconnection

and consequent oil leakage, what justifies this analysis.

In order to understand the dynamics of this

structure when the lock system fails computational fluid

dynamic (CFD) analyzes of the natural gas leakage inside

the chamber were run and a multibody model (MBS) of the

whole system were built. Four software were used to

perform these analyzes: Star-Design, Star-CCM+ and Star-

CD from CD-Adapco and MSC.Adams from

MSC.Software. The methodology used to integrate both

kinds of simulation considered first the fluid and

mechanical phenomenon as uncoupled, then studied them

separately and finally verified the coupling influences

through a complete CFD transient analysis using mesh

element addition to simulate the PAB movement.

Figure 1: Representation of the disconnection region to be analyzed.

According to Anderson (1995), computational

fluid dynamics constitutes a new “third approach” in

philosophical study and development of whole discipline

of fluid dynamics. In the seventeenth century, the

foundations for experimental fluid dynamics were laid in

France and England. The eighteenth and nineteenth

centuries saw the gradual development of theoretical fluid

dynamics, again primarily in Europe.

As a result, throughout most of the twentieth

century the study and practice of fluid dynamics involved

the use of pure theory on the one hand and pure experiment

on the other hand.

However, the advent of high-speed digital

computer combined with the development of accurate

numerical algorithms for solving physical problems on

these computers has revolutionized the way we study and

practice fluid dynamics today.

Anderson affirms that computational fluid

dynamics is today an equal partner with pure theory and

pure experiment in the analysis and solution of fluid

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dynamics problems. However, computational fluid

dynamics provides nothing more than just a third new

approach. It nicely and synergistically complements the

other two approaches of pure theory and pure experiment,

but it will never replace either of these ones.

Figure 2: The “three dimensions” of fluid dynamics.

According to Ferziger and Perić (2002), in some

cases experimental studies are very difficult if not

impossible. For example, the measuring equipment might

disturb the flow or the flow may be inaccessible. Some

quantities are simply not measurable with present

techniques or can be measured only with an insufficient

accuracy.

Experiments are an efficient means of measuring

global parameters, like drag, lift, pressure drop or heat

transfer coefficients. In many cases, however, it may be

essential to know whether flow separation occurs, whether

the wall temperature exceeds some limit or whether

compressible effects have considerable influences on the

system.

These advantages of CFD, however, are

conditional on being able to solve the Navier-Stokes

equations accurately, which is extremely difficult for most

flows of engineering interest. Accurate numerical solutions

for high Reynolds number flow are particularly difficult.

According to Ferziger and Perić if we are unable

to obtain accurate solutions for all flows, we have to

determine what we can produce and learn to analyze and

judge the results. First of all, we have to bear in mind that

numerical results are always approximate. The reason for

differences between computed results and “reality” is that

errors arise from each part of the processes used to

produce numerical solutions. Some examples are:

• The differential equations may contain

approximations or idealizations;

• Approximations are made in the discretization

process;

• Turbulence phenomenon require a very fine space

and time discretization to the calculation, which

makes necessary the use of assumptions or

idealizations to the modeling;

• In solving the discretized equations, iterative

methods are used. Unless they are run for a very

long time, the exact solution of the discretized

equations is not produced.

The application of CFD as a tool for the study of

the PAB disconnection phenomenon was choose because

of the great difficulties to simulate its operational

conditions in an experimental research.

How the main objective of the analysis was

verifying the possibility of a complete disconnection of the

PAB in case of a fail in the dog house lock, it was found

that CFD would give sufficient accurate results, even with

all limitations imposed by assumptions and simplifications,

to analyze and judge which configurations of water depth

and internal gas pressure would be favorable to the

occurrence of the complete disconnection.

METHODOLOGY

An initial analysis of the phenomenon conditions

leads us to make assumptions about the interaction

between the fluid flow and the mechanical dynamics. The

main characteristic is that the pressure difference between

the gas inside the chamber and the external ambient is very

high, while the inertial properties of the PAB are also very

high.

From this characteristic, some estimation was

made and it was found that the fluid dynamics is much

faster than the mechanical dynamics. It means that the fluid

flow stabilize after a change in the volume of control much

faster than a significant displacement of the PAB can

occur.

The assumption of the possibility of studying both

phenomenon uncoupled lead us to the follow approach: a

series of steady state CFD analysis were done at different

PAB positions and to different internal pressure of the gas,

covering all the configurations possible.

Some hypothesis and simplifications were studied

before the effective start of the steady state simulations.

The first one was the evaluation of the influence

of the gas injection through the system. This gas injection

keeps the pressure high inside the chamber during the oil

exploration. However, it was found that the mass flow

being injected in the system is much lesser than the leakage

mass flow when the lock fails. Then, this gas injection was

not considered in the simulations and this hypothesis

provided a great reduction in the number of steady state

CFD analysis required to cover all configurations.

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The second hypothesis was the consideration of

the control volume as symmetric along the revolution axis.

It was found little three-dimensional variations along this

axis, however some simulations comparing both three-

dimensional and symmetric results showed variations

lesser than 1% in the parameters of interest. The adoption

of this hypothesis had great impact in the number of

elements of the model, simplifying a 600000 model

elements to a 30000 model.

The last hypothesis was the consideration of a

single-phase fluid system. It means that the parameters of

interest, force acting at PAB and gas mass flow due the

leakage, do not have significant changes when considering

both phases (sea water and natural gas) or just considering

gas phase. The comparison between both cases were run

and it was found variations lesser than 3% in the

parameters measured. For the multi-phase case evaluation

it was used Volume of Fluid methodology (VOF) to solve

the free surface problem. The adoption of this

simplification reduced the computational efforts, mainly

because of the finer time discretization required by the

simulation of free surface.

After these considerations, the methodology

chosen for the first part of the analyzes was running a

series of axisymmetric steady state CFD simulations

considering the PAB at different positions to different

internal gas pressure.

The software Star-Design was used to the mesh

generation. The geometry has been prepared to be a slice

part with little thickness of the three-dimensional volume

of control, as shown in Figure A-1 in appendix A. The

mesh element type chosen was the polyhedral because of

its properties of faster solution using less memory and

great accuracy.

Star-CCM+ was used as the pre-processor, solver

and post-processor for the steady state analyzes. The mesh

was imported from Star-Design and was converted to a

two-dimensional mesh. The boundary regions were set as

shown in Figure A-2. The boundary types were set

according to Table 1.

Boundary Name Bounday Type

Leakage_Inlet Stagnation Inlet

Housing No-Slip Wall

Line No-Slip Wall

Conector No-Slip Wall

Outlet Pressure

Table 1: Description of the boundary types.

According to Star-CD User Guide (2004), a

stagnation boundary is typically used on a boundary lying

in a large reservoir where the fluid properties are not

significantly affected by flow conditions in the solution

domain. It normally appears in compressible flow

calculations. This was the exactly definition of the

Leakage_Inlet boundary and that is way stagnation inlet

was used.

According to the same reference, pressure

boundary specifies a constant static pressure or

piezometric pressure on a given boundary. This is the

definition of the outlet boundary, which represent a

constant pressure due the sea depth.

Due convergences problems to the steady state

analyzes, in some configurations it had been used a

transient approach until the fluid system found the

equilibrium, which usually took less than 0.1s.

Some visualizations of the fluid flow at these

analyzes are shown in Figures A-3 and A-4.

The force acting on the connector, which is

directly linked to the PAB, and the mass flow leaking from

inside the chamber were monitored for each configuration.

Over 200 configurations were analyzed and the

results were plotted on three-dimensional graphs, as shown

in the examples in Figures A-5 and A-6. This surface data

plotted represent the response of the monitored parameters

to all the possible conditions to occur in the system

studied.

From this data, the multibody model calculated

the mechanical dynamic of the PAB. The Figure 3 shows

the interactive process of the calculation.

Figure 3: Interactive process of PAB dynamic calculation.

At each time step, the multibody model read the

information from CFD data matrix according to state of the

system, which means instant internal pressure and PAB

position, and calculates the new state for the next time

step. The multibody model is shown in Figure A-7.

A similar methodology can be seen in Ruiz et al

(2002), in which the aerodynamics forces for a vehicle

were found using steady state CFD analyzes and the

calculation of the influence of these forces on the vehicle

dynamics was calculated by the MBS model using the

same integration procedure suggested here.

As it was commented previously, this procedure is

based on the consideration that both the fluid dynamics

and the mechanical dynamics are uncoupled. To validate

this methodology, it was necessary a transient CFD

analysis using mesh motion to simulate the variations on

the volume of control and to compare the results.

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How the total displacement of the PAB was about

0.7m, which was very big comparing to the elements size,

the use of cell-layer addition was necessary to maintain a

coherent mesh during all simulation. Star-CD was the

software used for this analysis.

According to the Star-CD User Guide (2004), a

cell is removed by collapsing intervening faces between

two opposite sides in a given direction. This is done by

moving together the vertices making up the faces.

The procedure chosen for this analysis was to

build the model with all the cells, which represent the PAB

in the highest position, and divide the region where the cell

addition would be in cell layers with different cell type

each.

After this procedure and after the setup of the

simulation and the boundaries conditions, events were set

up to remove all the cell layers in a negative time and to

start the cell addition at time zero, simulating the

disconnection of the PAB. The Figure A-9 shows the

model in Star-CD and the colored region represents the

cell layers to be removed/added.

Due the movement of the mesh, the connections

between the moving mesh region and the stationary mesh

region changes all the time during the simulation. A sliding

interface was set between these regions and then there

were no restriction about the relative vertices position of

each region.

The velocity of disconnection was set 1m/s, which

corresponded to the highest velocity obtained in MBS

analyzes. The highest velocity favors the coupling of fluid

and mechanical dynamic and so this was the most critical

situation against the methodology chosen.

The parameters monitored were the same of the

steady state analyzes and they were plotted as a function of

the PAB position. The comparison between the results

obtained in the multibody model using the steady state

CFD data and CFD transient analysis with moving mesh is

shown in Figure A-10 and A-11.

The highest difference of the results of force

acting on PAB and gas mass flow for both methodologies

was respectively 1.05% and 7.87%. One important

characteristic of this comparison is that the force obtained

in this validation analysis was lesser and gas mass flow

was higher, which means that the variation favors the

security.

Considering that the evaluated situation is the

most critical condition, this variation is acceptable to

validate the steady state approach to analyze and judge

which conditions of water depth and gas internal pressure

would be favorable to the occurrence of complete

disconnection of the PAB in case of fail in the dog house

lock.

CONCLUSIONS

This paper presented an approach for solving a

fluid-structure interaction with great displacement to

analyze the possibility of complete disconnection of oil

Production Adapter Base (PAB) in case of fail of the main

lock. The methodology used considered both fluid and

mechanical dynamics uncoupled and an integration of a

series of steady state CFD data and a multibody model was

established. The methodology was validated by a transient

CFD analysis with moving mesh and cell layer addition to

simulate the coupling between both phenomenons.

ACKNOWLEDGMENTS

Part of the work related to the transient CFD

analysis with mesh element addition was supported by

Gerald Schmidt and John Rogers from CD-Adapco USA.

The comments of the referees are gratefully

acknowledgments.

REFERENCES

1 – Anderson Jr., John D., Computational Fluid Dynamic

The Basis with Application, McGraw-Hill, Inc., New

York, USA, 1995.

2 – Ferziger, J. H., Perić, M., Computational Methods for

Fluid Dynamics, Springer-Verlag Berlin Heidelberg

New York, 2002.

3 – Ruiz, S., Català, A., Punset, A., Arbiol, J., Marí, R.,

Multi Body System – Computational Fluid Dynamics

(CFD) integration, 1st MSC.ADAMS European User´s

Conference, London, 2002.

4 – Star-CD Version 3.24 User Guide, CD-Adapco Group,

2004.

5 – Star-CD Version 3.24 Methodology, CD-Adapco

Group, 2004.

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Appendix A – List of Pictures

Figure A-1: Mesh generation at Star-Design.

Figure A-2: Boundary regions used in the model.

Figure A-3: Transonic gas flow visualization – Star-CCM+.

Figure A-4: Absolute Pressure plot – Star-CCM+.

Figure A-5: Force acting on PAB as a function of pressure difference

and PAB position.

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Figure A-6: Mass flow as a function of pressure difference and PAB

position.

Figure A-7: Multibody model at MSC.Adams.

Figure A-8: CFD model at Star-CD.

Figure A-9: Representation of the sliding interface.

Figure A-10: Comparison of the force acting on PAB data between both

methodologies.

Figure A-11: Comparison of the gas mass flow data between both

methodologies.

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