STUDY OF SLUG FLOW CHARACTERISTICS AND

PERFORMANCE OF CORROSION INHIBITORS, IN MULTIPHASE

FLOW, IN HORIZONTAL OIL AND GAS PIPELINES

A Thesis Presented to

The Faculty of the

Fritz J. and Dolores H. Russ College of Engineering and Technology

Ohio University

In Partial Fulfillment

of the Requirement for the Degree

Master of Science

Ashwini Kaul

March, 1996

5 . CONCLUSIONS ................................................. 78

6 . REFERENCES .................................................. 81

7 . APPENDIX A ................................................... 86

8 . APPENDIXB ................................................... 88

9 . APPENDlXC ................................................... 90

LIST OF TABLES

Table 3.4.1 Table 3.4.2 Table 3.4.3 Table 4.2.1

Table 4.2.2

Table 4.2.3 Table 4.2.4 Table 4.2.5 Table 4.2.6 Table 4.2.7 Table 4.2.8 Table 4.3.1

. . . . . . . . . . . . . . . . . . . . . Test matrix for the shear stress experiments 3 5 . . . . . . . . . . . . . . . . . . . Test matrix for the slug frequency experiments 35

. . . . . . . . . . . . . . . Test matrix for testing the performance of inhibitors 36 Corrosion rate data in full pipe flow for imidazoline. 1iq.velocity = 1.5 d s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 Corrosion rate data in full pipe flow for polyamine salt.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1iq.velocity = 1.5 d s 53 . . . . . . . . . . . . . . . . . . Corrosion rate data for imidazoline in slug flow 54

. . . . . . . . . . . . . . . . . . . Pressure drop data for imidazoline in slug flow 55 Average void fraction data for imidazoline in slug flow . . . . . . . . . . . . . 56

. . . . . . . . . . . . . . . . Corrosion rate data for polyamine salt in slug flow 64 . . . . . . . . . . . . . . . . . Pressure drop data for polyamine salt in slug flow 65

. . . . . . . . . . . Average void fraction data for polyamine salt in slug flow 66 Corrosion rate data for various slug frequencies . . . . . . . . . . . . . . . . . . . 76

LIST OF FIGURES

Flow patterns in gas-liquid flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Flow regime map for water-carbon dioxide system . . . . . . . . . . . . . . . . . . . . . . . 9 Idealized slug unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Comparison between a real moving slug (top) and a stationary hydraulic jump(bottom) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Hydraulic jumps in open channel flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Layout of the experimental system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 Testsection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 Data acquisition system for the wall shear stress : . . . . . . . . . . . . . . . . . . . . . . 39 Variation of instantaneous shear stress for 40% water cut. 20 cm from the

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . slug front. Froude number 12 42 Variation of instantaneous shear stress for 40% water cut. 40 cm from the slug front. Froude number 12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 Variation of instantaneous shear stress for 40% water cut. 80 cm from the slug front. Froude number 12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Variation of instantaneous shear stress for 40% water cut. 20 cm from the slug front. Froude number 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Variation of instantaneous shear stress for 40% water cut. 40 cm from the slug front. Froude number 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Variation of instantaneous shear stress for 40% water cut. 80 cm from the slug front. Froude number 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Variation of instantaneous shear stress for 40% water cut. 20 cm from the slug front. Froude number 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Variation of instantaneous shear stress for 40% water cut. 40 cm from the slug front. Froude number 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Variation of instantaneous shear stress for 40% water cut. 80 cm from the

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . slug front. Froude number 6 46 Corrosion rate Vs . Froude number for 80% water cut. with imidazoline . . . . . 57 Corrosion rate Vs . Froude number for 40% water cut. with imidazoline . . . . . 57 Pressure drop Vs . Froude number for 80% water cut. with imidazoline. 30 cmfrom the slug front . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 Pressure drop Vs . Froude number for 80% water cut. with imidazoline. 60 cm from the slug front . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 Pressure drop Vs . Froude number for 40% water cut. with imidazoline. 30 cmfrom the slug front . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 Pressure drop Vs . Froude number for 40% water cut. with imidazoline. 60 cm from the slug front . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 Average void fraction Vs . Froude number for 80% water cut. with imidazoline.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 cm from the slug front 60 Average void fraction Vs . Froude number for 80% water cut. with imidazoline.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 cm from the slug front 60 Void fraction Vs . Froude number for 80% water cut. with imidazoline.

at the bottom ofthe pipe, 30 cm from the slug front . . . . . . . . . . . . . . . . . . . . . 61 4.2.10 Void fraction Vs. Froude number for 80% water cut, with imidazoline,

at the bottom ofthe pipe, 60 cm from the slug front . . . . . . . . . . . . . . . . . . . . . 61 4.2.1 1 Average void fraction Vs. Froude number for 40% water cut, with imidazoline,

30 cm from the slug front . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 4.2.12 Average void fraction Vs. Froude number for 40% water cut, with imidazoline,

60 cm from the slug front . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 4.2.13 Void fraction Vs. Froude number for 40% water cut, with imidazoline,

at the bottom ofthe pipe, 30 cm from the slug front . . . . . . . . . . . . . . . . . . . . . 63 4.2.14 Void fraction Vs. Froude number for 40% water cut, with imidazoline,

at the bottom of the pipe, 60 cm from the slug front . . . . . . . . . . . . . . . . . . . . . 63 4.2.15 Corrosion rate Vs. Froude number for 80% water cut, with polyamine salt . . . 67 4.2.16 Corrosion rate Vs. Froude number for 40% water cut, with polyamine salt . . . 67 4.2.17 Pressure drop Vs. Froude number for 80% water cut, with polyamine salt,

30 cm from the slug front . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 4.2.18 Pressure drop Vs. Froude number for 80% water cut, with polyamine salt,

60 cm from the slug front . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 4.2.19 Pressure drop Vs. Froude number for 40% water cut, with polyamine salt,

30 cm from the slug front . . . . . . , . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . 69 4.2.20 Pressure drop Vs. Froude number for 40% water cut, with polyamine salt,

60 cm from the slug front . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 4.2.21 Average void fraction Vs. Froude number for 80% water cut, with polyamine salt,

30 cm from the slug front . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 4.2.22 Average void fraction Vs. Froude number for 80% water cut, with polyamine salt,

60 cm from the slug front . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 4.2.23 Void fraction Vs. Froude number for 80% water cut, with polyamine salt,

at the bottom ofthe pipe, 30 cm from the slug front . . . . . . . . . . . . . . . . . . . . . 71 4.2.24 Void fraction Vs. Froude number for 80% water cut, with polyamine salt,

at the bottom ofthe pipe, 60 cm from the slug front . . . . . . . . . . . . . . . . . . . . . 71 4.2.25 Average void fraction Vs. Froude number for 40% water cut, with polyamine salt,

30 cm from the slug front . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 4.2.26 Average void fraction Vs. Froude number for 40% water cut, with polyamine salt,

60 cm from the slug front . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 4.2.27 Void fraction Vs. Froude number for 40% water cut, with polyamine salt,

at the bottom of the pipe, 30 cm from the slug front . . . . . . . . . . . . . . . . . . . . . 73 4.2.28 Void fraction Vs. Froude number for 40% water cut, with polyamine salt,

at the bottom of the pipe, 60 cm from the slug front . . . . . . . . . . . . . . . . . . . . . 73 4.3.1 Corrosion rate Vs. slug frequency, Froude number 12 . . . . . . . . . . . . . . . . . . . 77 4.3.2 Corrosion rate Vs. slug frequency, Froude number 6 . . . . . . . . . . . . . . . . . . . . 77 C. 1 Layout of the slug control system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 C.2 Circuitry of the valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

CHAPTER 1

INTRODUCTION

One of the frequent and major problems encountered in the long distance

transportation of oil and gas is the internal corrosion of the pipelines. The corrosion related

problems result in losses due to repairs, loss in production, replacing the damaged equipment

and drop in efficiency. These losses run into millions of dollars each year.

In the early stages of a well, the flow is mostly oil and natural gas. But, as the well

gets older, the pressure inside the well decreases and enhanced recovery techniques have to

be used to maintain the production. The most common method involves the addition of sea

water and carbon dioxide to the reservoir. These fluids are also produced along with oil and

gas from the well. Many oil fields are located in remote areas such as Alaska or subsea.

Hence, it is usually uneconomical for each well to have its own separator. It is a common

practice to transport the multiphase mixture (oil, water, gas) from several wells, through

large diameter pipelines which can be several hundred kilometers long to a central gathering

station. Here it is separated into single phases and transported m h e r . Carbon steel is the

most economical construction material for the pipelines.

This multiphase mixture causes many problems in the pipelines. Carbon dioxide

dissolves in water to form a weak carbonic acid. The reactions involved are discussed in the

next chapter. This acid is corrosive in nature and can lead to high corrosion rates in carbon

steel pipelines. The resulting corrosion product is mainly iron carbonate, which is formed

by the reaction of ferrous ions with carbonate ions (from the dissociation of carbonic acid).

2

Depending on the composition of the water used, other products such as chlorides and

sulphides may also be formed. The extent of corrosion of the pipe walls is governed by the

concentration of the various reacting species and factors such as pH, pressure, temperature,

and flow conditions @e Waard and Milliams, 1975, De Waard and Lotz, 1993).

As mentioned earlier, many oil fields are located in remote areas such as Alaska or

subsea. Therefore the pipelines are, in general, in deep water or covered with snow, thus

making the maintenance, replacement, and clean up of the pipelines expensive. Also, there

is an extensive use of carbon and low alloy steels for the manufacture of pipelines because

using stainless steel or other expensive corrosion resistant materials is not economical. The

low alloy carbon steels for many reasons are ideal materials of construction but usually

exhibit poor corrosion resistance.

All corrosion processes involve interaction between a metal and a fluid. Relative

motion between the fluid and the metal surface also affects the corrosion rate (Sydberger

1986).

The corrosion problem has triggered the consideration of many corrosion control

programs in various oil fields around the world. These programs include methods such as,

selection ofthe pipe-wall thickness with sufficient corrosion allowance, the use of corrosion

resistant alloy materials, internally coating the pipe wall, dehydration of the oil-water

mixture and the injection of corrosion inhibitors into the pipelines. Of all the methods, the

injection of corrosion inhibitors is most widely adopted. Corrosion inhibitors play a vital

role in controlling corrosion associated with oil and gas production and transportation.

There is a strong dependency on the inhibitor deployment for achieving cost effective

3

corrosion control. The inhibitors have organic polar molecules that adsorb to the metal

surface or react with the corrosion products there to form a protective layer on the inside of

the pipe wall. The inhibitor is injected into the pipeline either by a continuous process or

in a batch mode. In each process, it is hoped that the existing flow in the pipeline will

redistribute the inhibitor around the pipe and along its length.

To facilitate the right choice of the inhibitor, laboratory testing has become a critical

step. The success of the laboratory tests depends on having a clear understanding of the

operating conditions such as temperature, pressure, fluid properties, solution pH, and the

flow conditions under which the inhibitor is expected to perform. There is also a need to

understand what conditions can be reproduced in a test facility and how well they relate to

the actual field conditions. A limited understanding of the field conditions can lead to gross

errors in estimations.

The effectiveness of inhibitor films is usually tested in the laboratory using bubble

tests, rotating cylinder electrodes (RCE), autoclaves, film persistence wheel test, and small

diameter flow loops (Mercer, 1985). Only single phase systems consisting usually of water

are used in the tests. These techniques do not simulate the actual field conditions involving

the multiphase flow of oil, water, and gas, and also do not take into account the various flow

regimes associated with it. Hence, the effectiveness of the corrosion inhibitors from the

above tests can be grossly under-estimated. When extrapolated to field conditions, many

failures of pipelines have occurred due to the poor performance of corrosion inhibitors that

were effective in the above tests. McMahon et al. (1995) carried out single phase tests with

Inhibitors in the rotating cylinder electrodes and have shown that the inhibitors reduced the

4

corrosion rates to negligible values at all concentrations. They also performed experiments

in small diameter flow loops and found that the RCE overestimates the performance of the

inhibitors.

Co-current flow of liquid and gas in a pipe results in various flow patterns. These

patterns depend upon the flow rates of the liquid and gas phases. Figure 1.1 shows a

schematic of the flow patterns of a two-phase gas-liquid flow in a horizontal pipe 10 cm in

diameter. At low liquid and gas velocities, a stratified flow regime exists. In this regime the

gas flows as a stratified layer over the liquid. At very low liquid and gas velocities, the

interface between the two layers is smooth and this regime is called smooth-stratified flow.

With further increase in the fluid velocities, two kinds of transitions can occur. At higher

gas velocity, regular two-dimensional waves begin to form on the liquid-gas interface. This

regime is called wavy-stratified flow. With further increase in the gas velocity, the two-

dimensional waves grow further in height and the front of the wave begins to roll over. This

regime is called rolling wave.

At higher liquid velocity, there is a transition from stratified to an intermittent flow

pattern. The waves on the liquid layer now begin to grow and bridge.the pipe. Intermittent

flow is of two types, plug flow and slug flow. At low gas velocities the waves form lumps

of liquid called plugs. The plugs flow over the liquid film intermittently between elongated

gas bubbles, with very little turbulence. At higher gas velocities, the fiont of the plug begins

to overrun the liquid film and in the process assimilates it into its structure. This results in

acceleration of the slug front and a highly turbulent slug flow is formed. As the gas velocity

is increased even further, 1 a . e three-dimensional roll waves start to appear on the liquid film

5

between slugs. The slugs become highly aerated at this point. This regime is called pseudo-

slug flow. As the gas velocity is further increased, the slugs are no longer able to hold the

gas. At this point the gas starts to flow in the central core of the pipe with a thin layer of

liquid flowing in the annulus around it. This regime is called annular flow. Figure 1.2

shows a flow regime map for a two phase water-carbon dioxide system in a 10 cm diameter

pipe in which the various flow regimes are plotted as functions of gas aid liquid velocities.

The flow patterns become more complicated when oil is added to the system. There is

currently very little understanding of three phase flow. Recently, research has been initiated

to extend the mechanistic knowledge of two-phase flow to three phase gas-liquid-liquid

flows (Jepson, 1990).

Of all the flow regimes, slug flow causes severe corrosion problems. Oil and gas

pipelines often operate in the slug flow regime when high production rates are required. The

mechanisms involved in slug flow are very different from those in plug flow.

An idealzed slug unit is shown in Figure 1.3. It consists of four zones. Ahead of the

slug is a slow moving stratified liquid film with gas flowing above it. Waves form on the

gas-liquid interface that grow to bridge the pipe. This causes the liquid to be accelerated by

the gas. The front of the slug overruns the slow moving liquid film ahead of it and

accelerates it to the velocity of the slug. A mixing vortex is created in this process. When

the front of the slug scoops up the slow moving liquid film, it also entrains considerable

amounts of gas, thus creating a highly frothy turbulent region behind the slug front, which

is called the mixing zone. The gas is released into the mixing zone at in the form of pulses

of bubbles (Jepson, 1987). Visual observations show the release of gas to be a closely

6

periodic phenomenon. These gas bubbles are forced towards the bottom of the pipe and can

impact and collapse there. The turbulent mixing and the impact of the collapsing bubbles

produce high shear at the bottom of the pipe. In the slug body, the level of turbulence is

reduced and the gas starts to move towards the top of the pipe due to buoyancy. Eventually

a point is reached where the liquid velocity is no longer sufficient to sustain the bridging of

the pipe, and the slug falls off. This is called the slug tail. The liquid is shed from the slug

tail to a trailing film which moves slower than the slug body. This liquid mixes with more

incoming liquid to form a film. The film height increases, waves are formed on the surface,

and the next slug is initiated.

Slug flow can drastically reduce the effectiveness of corrosion inhibitor films in the

pipelines. This is due to the highly turbulent nature of the mixing zone of the slug. It has

been shown by Jepson (1989) that there are regions of high shear forces in the slug which

can destroy the boundary layer close to the wall. This makes the formation of a stable

inhibitor film difficult, and any protective layer present is also removed by the slug. This

leads to increase in the corrosion rates. The exact mechanism for the removal of the film is

still unclear. Very little data is available on the slugs. Hence, an understanding of the cause

and subsequent treatment of the corrosion problems in multiphase flow is of extreme

importance.

This work utilizes a stationary slug or hydraulic jump to carry out experiments under

slug flow conditions. Jepson (1987) suggested that the slugs are hydraulic jumps

propagatimg along a pipe. He hrther showed that stationary slugs are similar to true moving

slugs of the same Froude number. Due to high velocities and transient nature of the slug

7

flow regime, it is dfficult to acquire accurate experimental data from real moving slugs. The

flow in the case of stationary slug is not intermittent. Hence, they are very usefbl in studying

various aspects of slug flow. With the hydraulic jumps it is easier to measure the wall shear

stress, pressure drop, liquid fractions, and corrosion rates at different locations in the slug

body.

This work is divided into three parts. The first part involves the study of fluctuations

in wall shear stress at the bottom of the pipe, in slug flow. A hot film sensor is used to

measure the wall shear stress. In the second part of the work, two generic inhibitors, one oil

soluble and one water soluble are tested for their performance in multiphase flow. Tests are

performed in full pipe and slug flows. In each test, measurements of corrosion rate, pressure

drop across the slug, and gas fractions inside the slug are taken. The measurements of

pressure drop and gas fractions show the effect of the inhibitors on the physical properties

of the fluids. In the third part, experiments are performed to study the effect of slug

frequency on the corrosion rates.

- FLOW DIRECTION SMOOTH

STRATIFIED

' WAVY STRATIFIED

ROLLING

0

SLUG FLOW

ANNULAR

Figure 1.1 Flow patterns in gas-liquid flow

1

SUPERFICIAL GAS VELOCITY (d)

FSgurr! 1 3 Flow regime map for watewarbon dioxide system

CHAPTER 2

LITERATURE REVIEW

This chapter discusses the previous works carried out by the researchers on corrosion and

its inhibition in multiphase flow in oil and gas pipelines.

2.1 Flow regimes and slug flow

Earlier, the various flow patterns were discussed for a two-phase liquid-gas mixture

flowing in a horizontal pipe. These patterns depend upon the velocities of the different

phases.

One of the very first studies carried out was by Baker (1954). He presented the maps

of various flow regimes occurring in two phase gas-liquid flow. The central position in

Baker's map is occupied by slug flow. Later, Govier (1962), Mandhane (1974), and Acikoz

(1992) studied the characteristics of gas-liquid flow in horizontal tubes and provided various

flow regime maps for flow in horizontal pipes. Each showed that slug flow occurs over a

wide range of gas and liquid velocities.

The first realistic model of slug flow was put forward by Dukler and Hubbard (1975).

They performed experiments in a 3.75 cm horizontal pipe with air and water as the working

fluids. They were able to determine the physical mechanisms involved in slug formation and

formulate a mathematical model to predict slug flow characteristics. The model was

illustrated as a fast moving slug which overruns a slow moving liquid film. This process

creates a mixing vortex at the front of the slug. A new film is shed behind the slug which

12

decelerates with time. The gas pocket flows in a stratified layer over the liquid film between

slugs.

Jepson (1989) suggested that slugs are formed as a result of hydraulic jumps

propagating along a pipe. He introduced the concept of 'stationary slugs'. He showed that

a mixing vortex was present in the fiont of a stationary slug, and the flow of gas-liquid inside

the slug body is similar to that in true moving slugs. Figure 2.1 shows the comparison

between a stationary slug and a true moving slug. The front of a moving slug has a

translational velocity V,, the slug body moves with a velocity V, and the liquid film in front

of the slug has a velocity of V, A stationary slug can be visualized as a moving slug seen

from a frame of reference moving with a velocity equal to the translational velocity (VJ

of the slug. Therefore, the translational velocity is zero for a stationary slug.

Jepson and Kouba (1989) showed that there are different kinds of slugs, and the

'strength' of the slug depends on the Froude number calculated in the liquid film ahead

of the slug. The Froude number is calculated as follows:

where,

Vt = translational velocity of the slug.

V f = velocity of the liquid film.

h,,, = effective height of the liquid film, defined as

the wetted area of flow divided by the width of the film.

g = the acceleration due to gravity.

They also showed that the stationary slugs are similar to the moving slugs of the same

Froude number. Chow (1959) showed that the strength of a hydraulic jump is governed

by the film Froude number as shown in Figure 2.2. It is seen that as the film Froude

number is increased, the jump entrains more gas. Higher Froude numbers produce

steady and strong jumps. The increase in gas entrainment increases the mixing and also

the level of turbulence inside the jump. Jepson (1987, 1989), through experimental

measurements and observations, showed that the profiles of hydraulic jumps in open

channels and circular pipes, at the same Froude numbers, are similar.

Fan, Jepson, and Hanratty (1992) confirmed from pressure drop measurements that

the front ofthe slug is a hydraulic jump. They also developed a simple model for stationary

slugs.

Lee and Jepson (1993) performed experiments for oil-water-gas flows using two oils

of viscosities 2 cp and 18 cp, and carbon dioxide as the gas. They showed that flow patterns

and flow regime transitions in three phase oil-water-gas are different from those obtained

for gas-liquid and oil-water systems. The models for two phase flow did not predict the

three phase flow transitions. Further, the liquid film heights for both oil and water could not

be calculated from these models since they do not predict the effect of a second liquid phase.

Neogi, Lee, and Jepson (1994) presented a model to predict the liquid film thickness

for three phase oil-water-gas stratified flow in the horizontal pipelines. The model was

found to be in good agreement with the experimental data. Also, the gas had a significant

effect on the thickness of the oil film.

2.2 Corrosion mechanisms

Corrosion processes involve interaction between a metal and a fluid.

Sydberger (1987) showed that the relative motion between the fluid and the metal surface

also affects the corrosion rate. Ellison and Wen (198 1) proposed three different corrosion

mechanisms, namely, convective mass transfer, phase transport, and erosion-corrosion. In

convective mass transfer controlled corrosion, the corrosion rate is affected by either the

convective transport of corrosive materials to the metal surface and/or the rate of transport

of the dissolved corrosion products away fiom the surface. The phase transport controlled

corrosion depends on the wetting of the metal surface by the phase containing the corrosive

material. The phase distribution is strongly affected by the multiphase flow. Erosion-

corrosion occurs when high velocity, highly turbulent fluid flow and/or flow of abrasive

materials prevent the formation of protective film, allowing fiesh material to be continuously

exposed to the corrosive environment.

King (1981) outlined the possible types of mechanisms affecting the internal

corrosion of pipelines. These include carbon dioxide corrosion, sulphide corrosion,

microbial corrosion, acid corrosion, and erosion corrosion. Corrosion is an electrochemical

process in which water behaves as a conducting medium. Thus, one factor common to all

the cases is the quantity of water present in the system. Carbon dioxide corrosion may occur

separately or in combination with oxygen or sulphide. Carbon dioxide dissolves in water

to form a corrosive carbonic acid. Hydrogen sulphide also provides a strong corrosive

environment even when present in small traces. This is referred to as 'sour corrosion'. In the

15

absence of hydrogen sulphide, it is called 'sweet corrosion'.

De waard and Milliams (1975), Ikeda et al. (1985), Ogundale and White (1986),

Videm and Dugstad (1989), and De Waard and Lotz (1993) have investigated the

mechanisms of carbon dioxide corrosion on carbon steel under different conditions of pH,

temperature, pressure, and oil-water compositions. They also have proposed various models

to predict the corrosion rates. The types of products formed on the pipe wall depend on the

experimental conditions. Iron carbonate, iron bicarbonate, iron carbide, and a variety of iron

oxides are the primary corrosion products on the inside of the pipe wall. The various

reactions are listed below.

Carbon dioxide dissolves in water to form carbonic acid:

co, + &O * qco,

The carbonic acid dissociates to form a carbonate ion:

De Waard and Milliams (1975) performed experiments using brine in stirred beakers. They

used weight loss methods to determine the corrosion rates. They suggested that the

undissociated acid molecule in Equation 2.2 is adsorbed on the metal surface and reduced

there. Based on the results obtained they proposed the following mechanism for the cathodic

reaction:

Reduction &C03 + e- * H + HC0,-

16

This is the rate determining step and the corrosion rate is directly correlated with the

concentration of the undissociated carbonic acid in the solution.

The anodic reaction is given by:

The anodic dissolution reaction was found to be the same as that of Bockris, Drazic, and

Despic (1962). They showed that the rate controlling step depends on the pH. The overall

reaction is given by:

De Waard and Milliams (1975) hrther showed that the solubility of iron carbonate decreases

with an increase in temperature. They also indicated that most of the dissolved species is

not iron carbonate but iron bicarbonate. Iron bicarbonate decomposes at higher

temperatures.

The iron carbonate dissolves into the solution until it reaches the solubility limit at that

particular temperature and thereafter precipitates on the metal wall. Iron carbonate may

form a protective film on the pipe wall depending upon the pH of the solution, temperature,

pressure, and flow rate. Different products may be formed if there is sulphide, oxygen, or

chlorine present in the system.

17

Dugstad (1992) found that precipitation rates increased at higher temperatures, thus

resulting in low corrosion rates. He reported fiom his solubility experiments that the amount

of iron carbonate in the solution when steel corrodes may be five to ten times the value

calculated using the thermodynamic data. The level of supersaturation of iron carbonate was

found to be dependent upon the water volume to steel ratio and temperature. Also, under

supersaturated conditions, it takes 20 to 40 hours to cover the surface with a protective iron

carbonate layer.

Efird and Janiski (1989) carried out studies in autoclaves and showed that although

crude oil does not participate in the corrosion of steel, it does have a significant effect on the

corrosion of steels in crude oil-brine mixtures. Janiski (1986) observed that the degree of

protectiveness of the corrosion product films in crude oil-brine mixtures is dependent on the

crystalline size of the corrosion product. The results obtained fiom the experiments carried

out in only brine environments can therefore lead to gross errors when extrapolated to the

field environments.

2.3 Measurement techniques and experimental systems

Various techniques are available to monitor corrosion rates. These include weight

loss measurement using coupons, electrical resistance probes, linear polarization resistance

probes, electrochemical impedance spectroscopy, and electrochemical noise techniques. The

electrochemical impedance spectroscopy, and electrochemical noise techniques are better

suited to provide information on the type of corrosion. The linear polarization resistance

method can be used to measure corrosion rates in a short time. The disadvantage of

electrochemical techniques is that they cannot be used in the environments where high

18

concentrations of oil are used. This is due to the low conductivity of oil over the surface of

the probes.

McKenzie and Vassie (1985) suggested the use of weight loss coupons and electrical

resistance probes for the measurement of corrosion rates. The use of electrical probes is a

simple method and is best suited for short term measurements, while the use of coupons is

best suited for long term measurements. The coupons can also be used for the surface

characterization using scanning electron microscopy (SEM) and auger spectroscopy

techniques. The disadvantage of the weight loss coupons technique is that they cannot be

used for continuous in-situ monitoring of the corrosion rates.

Various systems have been used by the researchers in the past to estimate the

corrosion rates obtained in the fields. These include the bubble test, stirred beakers, rotating

cylinder electrode, jet impingement, and small diameter recirculating flow loops. De Waard

and Milliams (1975) measured corrosion rates in stirred beakers in carbon dioxide systems.

Nesic and Lunde (1993) performed similar experiments in recirculating flow loops. Their

results on the effects of temperature and carbon dioxide pressure were similar to De Waard,

Lotz, and Milliams (1991). They also showed that the corrosion rate increased with an

increase in the liquid velocity. Efird et al. (1993) performed experiments using flow loops,

rotating cylinder electrodes, and jet impingement systems. They found that the corrosion

rates obtained from the rotating cylinder were much lower than those obtained from the flow

loop.

Sun and Jepson (1992), Zhou (1993), Kanwar and Jepson (1994), Vuppu and Jepson

(1994) and Menezes and ~ e ~ s o n (1994) showed the importance of large diameter flow loops

19

to study corrosion in pipelines. They conducted experiments in a 10 cm internal diameter

flow loops under several full pipe flow and slug flow conditions. They also showed that the

results fiom small diameter (2.5 cm and 5 cm) flow loops cannot be extrapolated to the large

diameter pipelines. This is because the flow mechanisms differ in both the systems.

2.4 Corrosion rates

Sun (199 1) and Menezes (1 994) performed experiments using 15 cp and 96 cp oils,

respectively, under several full pipe flow and slug flow conditions. They found that

increasing the liquid velocity increased the corrosion rate. Also, under slug flow conditions,

the corrosion rate increased with increase in the Froude number. Further, they showed that

the corrosion rate increases with decrease in water cut. Sun (1991) also showed that the

corrosion rates at the top of the pipe were lower than that at the bottom of the pipe. This was

due to higher shear stresses at the bottom than at the top. Menezes (1994) found that the

corrosion rate increases with decrease in water cut, up to 40%, and then decreases with

further decrease in the water cut.

Zhou (1993) performed experiments using a 2 cp oil under full pipe flow and slug

flow conditions. The results obtained were similar to those of Sun except that the corrosion

rate decreased with decrease in water cut. It was found that the average shear stress

increases with decrease in water cut. Also, the average and maximum shear stress increase

with the increase in Froude number and decrease with increase in distance into the slug.

2.5 Predictive models

De Waard and Milliams:

logi, = -(1.3)(pH) + B

where,

- 1,

- corrosion rate (mm/yr)

pH = pH of the solution

B - - constant

The pH term incorporates the effects of temperature and carbon dioxide partial pressure.

This model predicts an increase in corrosion rate with increase in temperature and carbon

dioxide partial pressure. This model does not take into account the influence of flow rates,

oil and, the presence of corrosion products on the pipe wall.

De Waard et al. (1991) later modified the model to predict 'worst case' corrosion

rates for systems saturated with the ~ e ~ ' corrosion product. They applied correction factors

to account for the various environmental parameters and the formation of iron corrosion

product on the wall.

1% v,,, = 5.8 - - ''lo + 0.67 log (P,) T

Where,

- v n m o

- corrosion rate (rnrnlyr)

T = temperature (K)

Pco, = partial pressure of carbon dioxide (bar)

The temperature at which protective scales begin to form on the metal surface is given by:

where,

'scale = temperature at which scales begin to form (K)

fco2 = hgacity of carbon dioxide (bar)

If the operating temperature is more than T,,, then corrosion rate obtained from Equation

(2.9) was multiplied by a correction factor F,,,,, given by:

FSc,, has a minimum value of one.

The De Waard et al. (1991) model had a major drawback. It did not account for the

flow and predicted the same corrosion rate at different velocities. De Waard and Lotz (1993)

revised the model and introduced the effect for the effect of flow velocity. The new model

had a mass transfer term that accounted for the flow velocity and a reaction term that

accounted for the chemistry of the system.

Where,

- VCOR

- corrosion rate (mm/yr)

- Vmact

- corrosion rate due to the chemistry of the system ( d y r )

- V,,& - corrosion rate due to the flow

C - - constant

The mass transfer term is given by:

where,

D - - diffusion coefficient (m2/s)

u - - liquid velocity (mls)

[H,CO,I - - concentration of carbonic acid

d - - hydraulic diameter (m)

v - - kinematic viscosity (m2/s)

The reaction term is given by:

logv-, = 5.8 - - + 0.67 log (f,) T (2.14)

Efkd et al. (1993) in their model showed that corrosion rate can be calculated using

the wall shear stress, as follows:

where,

&OR = corrosion rate (mm/yr)

T w - - shear stress at the wall (N/m2)

a, b = constants

The results fi-om Equation (2.15) for small diameter pipes can be extended to larger diameter

pipes. However, the constants are for brine only and hence, different values of a and b are

required for other systems.

Kanwar and Jepson (1994) performed full pipe flow experiments in a 10 cm diameter

pipe. They used a 2 cp oil and a 18 cp oil in the experiments. They modified Equation

(2.15) and came up with a relationship, given below:

where,

CR = corrosion rate (mm/yr)

P = partial pressure of CO, ( m a )

t - - shear stress (N/m2)

b, c = constants

This model does not take into account the protective scale formations at temperatures greater

than 60 C and therefore cannot be used above 60 C.

24

2.6 Corrosion inhibition

The corrosion inhibitors adsorb to the pipe wall or react with the corrosion products

on the wall to form a protective layer.

King (1981) suggested that the thickness and quality of the protective layer depends

upon the concentration of the corrosion inhibitor in the bulk phase. Below a certain critical

concentration, the metal surface is not adequately covered, and a poor inhibitor film is

formed which is not able to inhibit the corrosion reactions.

Harrop (1993) classified the different corrosion inhibitors available in the market

based on the mechanism, environment to which they are added, and the metal to be

protected. Some of th.e commonly used inhibitors are classified as anodic, cathodic,

passivating, oxidizing, film forming, organic, vapor phase, and volatile. Depending upon

the application, the basic constituent in the inhibitor can be a chromate, nitrite, phosphate,

sulphite, amine, imidazoline and quaternary.

Harrop (1993) also outlined the various methods available to study the performance

of corrosion inhibitors in oil and gas pipelines. These include the bubble test, rotating

cylinder electrode, jet impingement, and recirculating flow loops. The bubble test, rotating

cylinder electrode, and jet impingement are used for the initial screening of inhibitors. The

main disadvantage ofthese tests is that they don't imitate the actual flowing conditions in the

existing pipelines, and therefore, the results from these tests can lead to gross errors in

estimating the effectiveness of the inhibitor.

Pettus (1974) suggested that the amount of oil-water present in the system and the

type of flow are the primary factors for proper selection of inhibitors for the fields. These

25

factors play an important role in deciding whether to use an oil soluble, water soluble, oil

dispersible, or a water dispersible mhibitor. He showed that the presence of as much as 10%

oil in the system can reduce the effectiveness of a water soluble inhibitor by as much as

80%. The drag reduction phenomenon in turbulent flow was examined by Virk (1 975). He

showed that the skin fiction in turbulent flow of a liquid is reduced significantly by the

additives.

Menezes (1994) used a 10 cm internal diameter flow loop to examine the

perfbrmance of five commercial inhibitors under slug flow conditions. A 2 cp oil and ASTM

standard sea water and carbon dioxide were the working fluids. Visual observations showed

that the inhibitors changed the color of the oil-water mixture, forming emulsions. They also

affected the physical properties, such as pressure drop and void fraction. The results show

that the inhibitors reduced the corrosion rates to less than 0.13 rnmlyr at a Froude number

of 6. However, only three inhibitors were effective at a Froude number of 12.

Vuppu (1 994) performed experiments under slug flow conditions using a 2 cp oil in

the system. A drag reducing agent and corrosion inhibitors were tested in the experiments.

Coupons were also inserted into the system in all the tests for surface characterization. The

scanning electron microscopy (SEM) studies on the coupons showed thin inhibitor films in

all the cases where the inhibitor was effective. In the cases where the inhibitor or the drag

reducing agent was ineffective, circular bubble impact regions were found on the surface of

the coupons. These impacts tear holes in the inhibitor films, thus making the inhibitor

ineffective. The analysis showed evidence of corrosion inside the impact regions, even when

low corrosion rates were recorded.

m zone

Figure 2.1 Comparison between a real moving slug (top) and a stationary hydraulic jump (bottom)

4 vt - vs

Mixing zone

vt - vg - - vt - vo

T e

u - d - - -C

F, = 1-1.7 Undulor jump

- s,

C- - - 7/////////////m//////////m//////m

F, = 1.7- 2.5 Weak jump

F,=2.5-4.5 Oscilbtinq jump

w - --- -///-1 F, = 4.5-9.0 S teody jump

F, >9.0 Strong jump

Figure 2.2 Hydraulic jumps in open channel flow

CHAPTER 3

EXPERIMENTAL SETUP AND PROCEDURE

The description of the experimental setup, procedure and the techntques used are

discussed in detail in this section.

3.1 Description of the flow loop

The schematic layout of the system is shown in Figure 3.1. It is similar to the one

designed by Jepson (1987). The system consists of a 1.4 m3 3 16 stainless steel tank and a

10 m long, 10.16 cm internal diameter plexiglass pipeline. A 2 cp Oil, ASTM artificial sea

water, and carbon dioxide are the working fluids. The liquid from the tank is pumped into

7.6 cm PVC pipe D by a centrfigal pump. The flow rate of the liquid is controlled by a

bypass line and is measured by calibrated orifice meter E. The liquid is then forced under

a gate into the plexiglass pipe where it forms a fast moving liquid film. The carbon dioxide

gas is introduced into the system at port H shown in Figure 3.1. The gas-liquid mixture from

the plexiglass pipeline then enters the tank. Here it is separated by a de-entrainer plate inside

the tank, the gas being vented to the atmosphere through the exhaust I. The carbon dioxide

gas is also used to pressurize the system. The pressure inside the tank is indicated by gauge

B installed on the top of the tank. All the measurements are taken in the test section F

located downstream from the gate. The hydraulic jump generated inside the test section is

controlled by manipulating the gas flow at the inlet. This is done by using a needle valve

in conjunction with a flow control system. The liquid inside the tank is heated by two 1.5

kW heaters positioned at J.

3.2 Description of the test section

Figure 3.2 shows a schematic of the test section. The probes used to measure the

corrosion rate are flush mounted with the pipe wall at ports A and B. At port E, the shear

stress probe is flush mounted with the wall to measure the wall shear stress at the bottom of

the pipe.

The pressure taps located at points D are connected to a U-tube manometer to

measure the pressure drop across and within the slug body. The manometer is filled with

water and meriam blue colored fluid, having a specific weight of 1.75.

C is a 0.95 cm diameter sampling tube used to withdraw fluid samples from the slug

body. The samples are then used to determine the voidlliquid fractions within the slug.

3.3 Measurement techniques and procedure

This section discusses the methods and apparatus used to perform the

experiments.

3.3.1 Measurement techniques

3.3.1.1 Corrosion rate measurement

The corrosion rates are measured using an electrical resistance (ER) technique. This

is a very direct and simple method and works well in oil-water mixtures. The principle

behind the working of an ER probe is that the electrical resistance of a metal sheet is

inversely proportional to its thickness. Measuring the change in resistance over an adequate

30

period of time gives the change in thickness of the metal during that time period. The

corrosion rate can then be calculated. Since, these changes in resistance are of very small

magnitude, it is difficult to measure them directly. The ER probe is therefore constructed

in such a way that it forms two arms of a bridge network. The arms are constructed using

1018 carbon-steel. One arm of the probe is protected from corrosion by a suitable insulation

and is used as a reference, while the other arm is exposed to the corrosive environment. This

makes it possible to measure the resistance of the exposed arm by measuring the the ratio

of the resistances of the two arms (McKenzie and Vassie, 1985). Another reason for using

a two arm probe is that the fluctuations resulting from temperature changes are compensated.

The following relationship is used to calculate the corrosion rate with the readings obtained

from the probes. This method has been described in detail by Zhou (1993).

CR = A probe reading x24 x 365 x probe span A time (hours ) x 1000

where,

CR = corrosion rate (mm/yr)

3.3.1.2 Shear stress measurement

The wall shear stress is measured by using hot-film sensors flush mounted at the

bottom of the pipe. The hot-film sensor works on a heat transfer technique which gives a

correlation between the wall shear stress and the rate of heat transfer from the sensor to the

3 1

h. Extreme care has to be taken to ensure that the probe is not exposed to gas pockets at

any time during the experiments, because this causes the probe to burn-out. The voltage

signals from the probe are passed to an F A 100 anemometer system which converts and

stores these analog signals in digital form using a MetraByte's Model DAS20 AD converter.

This data is then processed using the TSI Anemometry Software Package and an IBM

computer. Figure 3.3 shows the schematic description of the data acquisition system for

measuring the wall shear stress at the bottom of the pipe. At a given location in the slug, the

probe takes about 1000 data points per second. The rate of data collection can be increased

or decreased as per requirement. By moving the slug to different axial locations the wall

shear stress at the respective locations is measured. During processing, the voltage data is

converted into shear stress data (Appendix A). Then, the change in shear stress with time

at each location, and the change in average shear stress with distance along the slug are

plotted. The probe is calibrated for each set of experiments (for each oil-water composition).

The method of calibration is described in Appendix A.

3.3.1.3 Void fraction and oil-water concentration

The void fraction and oil-water concentration measurements are taken at three

circumferential positions (top, bottom, center) and two positions inside the slug body (30 cm

and 60 cm £?om the slug front). The samples are taken iso-kinetically and passed into a one

meter long tube having a total volume of 220 ml. The gas-liquid mixture in the tube is

allowed to separate and the volume of oil and water inside the tube are measured. The

volume of the gas is then determined by subtracting the total volume of the liquid from the

volume of the tube. The volume of the gas is then used to determine the void fraction.

220 - VL Void Fraction =

220

VO Oil Fraction = - VL

where,

VL = total volume of liquid inside the tube (ml).

VO = volume of the oil phase (ml).

A weighted average of the void fraction is then calculated at each position.

3.3.1.4 Pressure drop measurement

The pressure drop is measured at 30 cm and 60 cm from the slug front. The

calculations are carried out as follows:

Pressure drop = h x g x @, - p,,)

where,

h - - difference in height of the manometer fluid

g - - acceleration due to gravity

P b - - density of blue fluid

- Pw

- density of water

3 3

3.3.2 Procedure

The tank is filled with predetermined volumes of synthetic sea water and oil. The

sea water is prepared using ASTM standard sea salt. The composition of this salt is shown

in Table 3.3.2

The pump is turned on and carbon dioxide is circulated to purge oxygen from the

system. The level of dissolved oxygen inside the system has to be maintained below 20 ppb.

The presence of oxygen in excess of 20 ppb inside the system changes the corrosion

chemistry due to the formation of oxides. Also, the level of dissolved iron inside the system

is maintained below 30 ppm because it has been shown that high iron concentration inside

the system leads to supersaturation and reduced the corrosion rates. The oxygen and iron

levels are periodically monitored using CHEMetsa dissolved oxygen and iron test kits. The

fluids inside the system are changed if the iron level shoots above 30 ppm. The

deoxygenation process is essential for the corrosion experiments. However, for the other

measurements such as the pressure drop, void fraction, and shear stress, the deoxygenation

process is not necessary. After one set of experiments is complete, a known volume of water

is drained fiom the tank and more oil is added to increase the oil-water concentration to the

next desired level.

After the deoxygenation process is complete, the corrosion rate experiments are

started. For this purpose two ER probes are flush mounted at points A and B inside the test

section shown in Figure 3.2. The system is then pressurized to 0.136 MPa using carbon

dioxide. The corrosion rate experiments are usually started with the full pipe flow first.

These are carried out at different liquid velocities. Each full pipe flow experiment is carried

34

out for 15-20 hours. For slug flow experiments, the liquid flow rate is adjusted to a value

required to give the desired Froude number and the liquid is forced through the gate. At this

point gas is introduced and a hydraulic jump is created. The jump is positioned stationary

inside the test section by a control system on the gas line. This regulates the gas flow into

the system. Subsequently, measurements of pressure drop, void fraction and shear stress are

carried out. The control system is also used for the slug frequency experiments, where, the

the effect of slug frequency on the corrosion rates is examined. The details about the

working of the control system are provided in Appendix C.

3.4 Test matrix

The test matrices for the various experiments are given in Tables 3.4.1, 3.4.2 and

3.4.3. The density and viscosity of the oil at 40 C are 800 ~ g / m ~ and 2 cp, respectively.

Table 3.4.1 Test matrix for the wall shear stress experiments

Table 3.4.2 Test matrix for the slug frequency experiments

Watercut

80%

40%

* The corrosion rates are measured at the bottom of the pipe

Temperature [=I C

40

Water cut

80%

40%

Pressure [=I MPa

0.136

Temperature [=] C

40

Froude number

6, 9, 12

6, 9, 12

Pressure [=I MPa

0.136

Froude number

6

12

6

12

Frequency [=I mim-'

7, 13

7, 13, 19, 30

7, 13

7, 13, 19, 30

Parameter

,o,o,;o, rate*

A.

Liq

uid

tank

G

. Fl

ow h

eigh

t con

trol

gate

B

. P

ress

ure g

auge

H

. C

02

Fee

d L

ine

C.

Liq

uid

recy

cle

I. V

ent w

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D.

Liq

uid

feed

- 7.6

cm

PV

C

J. H

eate

r E

. O

rlfi

ce p

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man

omet

er

K.

Safe

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alve

F.

T

est s

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.16

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Bac

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re r

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p

Fig

ure

3.1

Lay

out o

f the

exp

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enta

l sys

tem

CHAPTER 4

RESULTS AND DISCUSSIONS

This chapter examines the results obtained from the shear stress, inhibitor and slug frequency

experiments.

4.1 Wall shear stress

The wall shear stress measurements at the bottom of the pipe were carried out in slug

flow for 40% and 80% water cuts. The measurements were taken at Froude numbers of 6,

9 and 12, and at three locations (20 cm, 40 cm and 80 cm) into the slug body. At each

location the average, maximum and minimum shear stress were calculated and the variation

of instantaneous shear stress was plotted as a fbnction of time.

4.1.1 Instantaneous shear stress fluctuations

The wall shear stress at the bottom of the pipe fluctuates with time and shows large,

prominent peaks at regular intervals. The peaks may be attributed to the pulses of bubbles

at the bottom of the pipe, which give bubble impacts and possible collapse there.

Figures 4.1.1,4.1.2 and 4.1.3 present the variations in instantaneous shear stress for

a Froude number of 12, at 20 cm, 40 cm, and 80 cm from the slug front, respectively. Figure

4.1.1 shows substantial fluctuations with a regular frequency. The frequency of the peaks

is about 2-3 peaks every 0.1 seconds. The average value of the shear stress is about 95 Pa,

and the maximum value is approximately 130 Pa. At 40 cm into the slug, Figure 4.1.2 shows

that the intensity of the fluctuations has decreased. Also, the frequency of the peaks has

decreased to one every 0.2 seconds. The average and maximum values of shear stress are

4 1

now about 65 Pa and 95 Pa, respectively. At 80 cm from the slug front (Figure 4.1.3) there

are no fluctuations. The average shear stress is 25 Pa. Zhou (1993) found similar results for

the average shear stress.

Figures 4.1.4, 4.1.5 and 4.1.6 show the variations in instantaneous shear stress for a

Froude number of 9, at 20 cm, 40 cm and 80 cm, respectively. At 20 cm, the intensity of

fluctuations and frequency are comparable to that for a Froude number of 12. However, the

average (78 Pa) and maximum (93 Pa) shear stresses are lower than those for a Froude

number of 12. At 40 cm and 80 cm, the average values are 30 Pa and 14 Pa, respectively,

which are again lower than those for a Froude number of 12.

Figures 4.1.7, 4.1.8 and 4.1.9 show the variation in instantaneous shear stress for a Froude

number of 6. The Figures show a considerable decrease in the level of fluctuations and

fiequency of the occurrence ofthe peaks. This is due to the lower turbulence level in Froude

number of 6. The frequency decreases from one every 0.2 seconds at 20 cm to almost zero

thereafter. Also, there is decrease in the average shear stress to 29 Pa at 20 cm, 22 Pa and

9.5 Pa at 80 cm into the slug body. The maximum values decreased to 56 Pa, 35 Pa and 10

Pa, at 20 cm, 40 cm and 80 cm respectively.

S~milar fluctuations in shear stress were observed for 80% water cut. However, the

average and maximum values were lower than those for 40% water cut.

Video images of the slugs were also taken during the experiments. On close

observation of the images in a super slow motion mode, the movement of the pulses of

bubbles can be very easily traced. The frequency of these pulses at different Froude

numbers, approximately match the frequency observed in the shear stress plots.

0 0 .2 0 . 4 0 .6 0 . 8 1 TIME (see)

4.11 VARIATION OF INSTANTANEOUS SHEAR STRESS FOR 40% WATER CUT

20 em FROM THE SLUG FRONT

I ' ' ' i " ' I 1 ' ' i l l '

I Froude number 12

- -

- -

Figan 41.2 VARIATION OF INSTANTANEOUS SHEAR STRESS FOR 40% WATER CUT

40 em FROM THE SLUG FRONT

0

0 0 . 2 0 . 4 0 . 6 0 . 8 1

TIME (see)

43

l ' " l ' " / " J I I

- Froude number 12

- -

- -

- -

1 1 1 , 1 1 , , 1 / , , I , , ,

Figore 41.3 VARIATION OF INSTANTANEOUS SHEAR STRESS FOR 40% WATER CUT

80 cm FROM THE SLUG FRONT

0 0 . 2 0 .4 0 . 6 0 . 8 1 TIME (see)

Fignre 41.4 VARIATION OF INSTANTANEOUS SHEAR STRESS FOR 40% WATER CUT

20 cm FROM THE SLUG FRONT

1 " ' 1 ' 1 ' / ' " ' ~ '

I Froude number 9

- -

- -

I 8 8 8 1 , , , l k , , l , , ,

0 0 . 2 0 .4 0 .6 0 . 8 1 TIME (see)

Figure 41.5 VARIATION OF INSTANTANEOUS SHEAR STRESS FOR 40% WATER CUT

40 ern FROM THE SLUG FRONT

0 0 . 2 0 . 4 0 . 6 0 . 8 1 TIME (see)

Figure 4.1.6 VARIATION OF INSTANTANEOUS SHEAR STRESS FOR 40% WATER CUT

80 em FROM THE SLUG FRONT

I Froude number 6

TIME (see)

Figure 4.1.7 VARIATION OF INSTANTANEOUS SHEAR STRESS FOR 40% WATER CUT

20 em FROM THE SLUG FRONT

I " ' / " ' I ( ( '

Froude number 6

- -

- -

- -

1 1 1 1 1 , 1 , 1 , 1 1 1 , 1 1

0 0 . 2 0 . 4 0 . 6 0 . 8 1 TIME (sec)

Figure 41.8 VARIATION OF INSTANTANEOUS SHEAR STRESS FOR 40% WATER CUT

40 em FROM THE SLUG FRONT

1 1 ' ' 1 ' ' ' 1 ' ' 1 ' ' ~

Froade number 6

- -

- -

- -

I , 8 8 1 1 3 1 1 1 8 1 1 , b

0 0 . 2 0 . 4 0 . 6 0 . 8 1 TIME (see)

F i p v 4.1.9 VARIATION OF INSTANTANEOUS SHEAR STRESS FOR 40% WATER CUT

80 ern FROM THE SLUG FRONT

47

Vuppu (1994) found evidence of circular bubble impacts on the coupons during the inhibitor

tests. These impacts were found on the inhibitor films formed on the coupons. Also,

corrosion products were observed inside the area damaged by the impacts. The corrosion

rates measured in the area of the bubble impacts have been found to be much higher when

compared to those obtained from the other locations inside the slug (Zhou, 1993).

4.2 Effectiveness of corrosion inhibitors

In this section, the results of the experiments carried out with two generic inhibitors,

irnidazoline and a polyamine salt, are presented. Both of the inhibitors were tested at

concentrations of 25 ppm and 75 ppm, at a temperature of 40 C and a pressure of 0.136 MPa

with water cuts of 40% and 80%. Experiments were carried out under full pipe and slug

flow conditions.

4.2.1 Performance of imidazoline

4.2.1.1 Corrosion rate

Irnidazoline is an oil soluble inhibitor, and is expected to adsorb to the metal surface

better, at higher oil compositions.

4.2.1.1.1 Full pipe flow

All the full pipe flow tests were carried out at a liquid velocity of 1.5 mls. Table

4 2.1 shows the corrosion rates obtained in full pipe flow. For 80% water cut, a

concentration of 25 ppm of the inhibitor reduced the corrosion rate from an uninhibited value

of 1.0 d y r to 0.3 d y r ; a reduction of 70%. A concentration of 75 ppm of the inhibitor

4 8

reduced the corrosion rate to negligible values. For 40% water cut, the corrosion rate was

virtually unchanged from the uninhibited value of 0.8 d y r , at a concentration of 25 ppm.

However, a hrther addition of 50 ppm reduced the corrosion rate to 0.1 d y r .

For an oil-water mixture flowing in a pipe, if the oil and water separate, the velocity

of the water phase is much higher than the mixture velocity, especially at low water cuts

(Malhotra, 1995). Consequently, at low concentrations, the inhibitor may not disperse well

through the fast moving water phase.

4.2.1.1.2 Slug flow

The change in corrosion rates for slug flow for different inhibitor concentrations for

80% water cut is shown in Figure 4.2.1. It is observed that the inhibitor does not decrease

the corrosion rate at a concentration of 25 ppm for both the Froude numbers of 6 and 12.

This is probably due to the high level of turbulence in slug flow. At 25 ppm concentration

the inhibitor may not be sufficient to form a stable film and any film formed may be getting

removed by the slug. However, at a concentration of 75 ppm the corrosion rate is decreased

from an uninhibited value of 0.8 d y r to 0.4 d y r at a Froude number of 6 and, from 1.7

d y r to 0.9 d y r at a Froude number of 12. The reduction is less than 50% for both the

Froude numbers.

Figure 4.2.2 shows a similar plot for 40% water cut. The results obtained are similar

to the 80% water cut, at an inhibitor concentration of 25 ppm. The inhibitor works well at

a concentration of 75 ppm. It decreased the corrosion rate from an uninhibited value of 0.5

d y r to 0.1 d y r at a Froude number of 6 and from 1.1 d y r to 0.8 mm/yr at a Froude

number of 12. The inhibition is better (80%) at a Froude number of 6. This is due to the less

49

turbulent nature ofFroude number 6 and, since there is more oil in the system at 40% water

cut than at 80% water cut, the inhibitor, being oil soluble, has better chances of forming a

film. At a Froude number of 12, even though there is more oil, the highly turbulent nature

and the strong impact of the pulses of bubbles may be restricting the formation of a stable

inhibitor film. The reduction in corrosion rate is therefore only 30%. The corrosion rates

are listed in Table 4.2.3.

4.2.1.2 Pressure drop

The addition of imidazoline had a very small effect on the pressure drop across the

slug. The effect of the Inhibitor on the pressure drop is presented in Figures 4.2.3 and 4.2.4,

for 80% water cut. The pressure drop measurements were taken at 30 cm and 60 cm from

the slug fiont. At 30 cm fiom the slug front, Figure 4.2.3 shows little change in the pressure

drop at the Froude numbers of 6 and 12, due to the addition inhibitor. For 60 cm into the

slug, Figure 4.2.4 shows similar results.

For 40% water cut, little changes in the pressure drops are again noted. The pressure

drops for the Froude numbers of 6 and 12 are 1.4 and 2.8 KPa, respectively. This is shown

in Figures 4.2.5 and 4.2.6. The pressure drops are listed in Table 4.2.4.

4.2.1.3 Void fraction

The average void fiaction for 80% water cut is presented in Figures 4.2.7 and 4.2.8.

At a Froude number of 6, Figure 4.2.7 shows the average gas content was about 20% and

changed very little with the addition of the inhibitor. However, at 30 cm from the slug front,

5 0

for a Froude number of 12, the inhibitor decreased the gas holdup by 45%. At 60 cm into

the slug, Figure 4.2.8 shows a similar decrease. Figures 4.2.9 and 4.2.10 show the variation

in the gas holdup at the bottom of the pipe for 80% water cut. Again small changes are

noted at a Froude number of 12.

Figures 4.2.11,4.2.12,4.2.13, and 4.2.14 show similar trends in gas holdup for 40%

water cut.

4.2.2 Performance of polyamine salt

4.2.2.1 Corrosion rate

4.2.2.1.1 Full pipe flow

For a liquid velocity of 1.5 mls, the corrosion rates are listed in Table 4.2.2. It is

observed that, for both 80% and 40% water cuts, an inhibitor concentration of 25 ppm was

insufficient to reduce the corrosion rates. However, for 80% water cut, a concentration of

75 ppm reduced the corrosion rate from an uninhibited value of 1.0 d y r to 0.3 rnmlyr.

For 40% water cut a concentration of 75 ppm of the inhibitor reduced the corrosion rate from

0.8 d y r to 0.1 rnmlyr.

4.2.2.1.2 Slug flow

Figures 4.2.15 and 4.2.16 show the corrosion rates at different inhibitor

concentrations, for 80% and 40% water cuts, respectively. It is observed that, for 80% and

40% water cuts, a concentration of 25 ppm of the inhibitor does not reduce the corrosion

rates significantly.

It is seen in Figure 4.2.15 that, at a Froude number of 6, a concentration of 75 ppm

of the inhibitor reduces the corrosion rate from an uninhibited value of 0.8 d y r to 0.5

mrnlyr. This is a reduction of only 37%. However, for a Froude number of 12, the same

concentration of the inhibitor reduces the corrosion rate from 1.7 d y r to 0.6 mmlyr (a

reduction of 65%).

For 40% water cut, Figure 4.2.16 indicates that the inhibitor did not reduce the

corrosion rate at both the Froude numbers. This is expected because of the low .

concentration of the water phase. This inhibitor, being water soluble, has better chances of

inhibition in 80% than in 40% water cut.

4.2.2.2 Pressure drop

For 80% water cut, Figures 4.2.17 and 4.2.18 show the effect on pressure drop inside

the slug at each Froude number.

At 30 cm from the slug front, Figure 4.2.17 shows that the pressure drop changes

very little with the addition of inhibitor. However, at 60 cm into the slug body, at a Froude

number of 6, there is a decrease in pressure drop from 1.8 KPa to 1.0 KPa. At a Froude

number of 12 there is a decrease from 4.2 KPa to 1.5 KPa. The increase in the inhibitor

concentration from 25 ppm to 75 ppm does not hrther affect the pressure drop.

For 40% water cut, little changes in the pressure drop at 30 cm and 60 cm from the

slug front are noted in Figures 4.2.19 and 4.2.20.

5 2

4.2.2.3 Void fraction

For 80% water cut, the average void fractions at 30 cm and 60 cm are presented in

Figures 4.2.21 and 4.2.22, respectively. It is noticed that higher concentration of the

inhibitor decreased the average gas holdup inside the slug at a Froude number of 12. The

effect of the inhibitor on the average gas holdup is not very significant at a Froude number

of 6 .

Figures 4.2.23 and 4.2.24 show similar trends in the gas concentration at the bottom

of the pipe, for both the Froude numbers.

For 40% water cut, Figures 4.2.25 and 4.2.26 show that higher concentration of the

inhibitor in most cases increased the average gas holdup at both the Froude numbers.

Figures 4.2.27 and 4.2.28 show similar trends for the gas concentration at the bottom of the

pipe.

Visual observations showed that the addition of the inhibitor turned the oil-water

mixture rmllcy. Also, liquid samples taken from the slug showed the formation of emulsions.

At 40% water cut, the emulsions were stronger at higher inhibitor concentrations and the oil-

water mixtures took a long time to separate into individual phases.

4.3 Slug frequency

The slug frequency experiments were carried out for 40% and 80% water cuts. For

a Froude number of 6, the corrosion rate measurements were taken at the frequencies of 7

Table 4.2.1 Corrosion rate data in full pipe flow for imidazoline Liquid velocity = 1.5 m/s

Table 4.2.2 Corrosion rate data in full pipe flow for polyamine salt Liquid velocity = 1.5 m/s

Water cut

8 0%

40%

Inhibitor concentration [=I PPm

----

25

7 5

----

25

7 5

Water cut

80%

40%

Equilibrium corrosion rate

[=] mm/yr

1 .o

0.3

0.1

0.8

0.7

0.1

Inhibitor concentration [=I PPm

----

25

75

----

25

75

Equilibrium corrosion rate

[=I m d y r

1.0

0.9

0.3

0.8

1.1

0.1

Table 4.2.3 Corrosion rate data for imidazoline in slug flow

Water cut

80%

40%

Inhibitor concentration

- - 1 ppm

----

25

7 5

----

25

75

Froude number

6

12

6

12

6

12

6

12

6

12

6

12

Equilibrium corrosion rate

[=I mmlyr

0.8

1.7

1.4

2.5

0.4

0.9

0.5

1.1

0.6

1.5

0.1

0.8

Table 4.2.4 Pressure drop data for imidazoline

Water cut

80%

40%

Inhibitor concentration

[=I PPm

----

----

2 5

75

2 5

7 5

Froude number

6

12

6

12

6

12

6

12

6

12

6

12

Pressure drop across the slug [=I KPa

30 cm

1.4

3.5

1.1

1.9

1.5

4.2

1.3

3.5

0.8

1.7

1.0

2.2

60 cm

1.8

4.2

1.4

2.8

1.6

4.6

1.5

4.6

1.2

2.9

1.6

2.9

Table 4.2.5 Average void fraction data for imidazoline

Water cut

80%

40%

80%

40%

Inhibitor concentration

[=I PPm

----

----

25

75

25

75

Froude number

6

12

6

12

6

12

6

12

6

12

6

12

Average void fraction across the cross

section, in the slug [=] Yo

30 cm

17.4

57.0

13.0

23.4

20.4

37.5

19.3

31.3

18.0

25.0

13.0

20.0

60 cm

11.3

39.2

10.5

17.0

13.4

21.5

15.8

25.0

8.4

13.0

8.4

10.5

6 1 2

FROUDE NUMBER

Fignre 4.21 CORROSION RATE V s FROUDE NUMBER FOR 80% WATER CUT WITH IMIDAZOLINE

6 1 2

FROUDE NUMBER

ELgnre 4 2 2 CORROSION RATE Vs FROUDE NUMBER FOR 40% WATER CUT WITH IMIDAZOLINE

6 1 2

FROUDE NUMBER

Flgnre 423 PRESSURE DROP Vs FROUDE NUMBER FOR 80% WATER CUT WITH IMIDAZOLINE

30 w FROM THE SLUG FRONT

6 1 2

FROUDE NUMBER

Flgnre 4 2 4 PRESSURE DROP Vs FROUDE NUMBER FOR 80% WATER CUT WITH IMIDAZOLINE

60 an FROM THE SLUG FRONT

6 1 2

FROUDE NUMBER

Fignre 4.25 FROUDE NUMBER Vs PRESSURE DROP FOR 40% WATER CUT WITH IMIDAZOLINE

30 em FROM THE SLUG FRONT

FROUDE NUMBER

Figure 426 FROUDE NUMBER Vs. PRESSURE DROP FOR 40% WATER CUT WITH IMIDAZOLINE

60 an FROM THE SLUG FRONT

6 1 2

FROUDE NUMBER

F1giu-e 4.27 AVERAGE VOID FRACTION V s FROUDE NUMBER FOR 80% WATER CUT, WITH IMIDAZOLINE

30 cm FROM THE SLUG FRONT

FROUDE NUMBER

Flgnre 428 AVERAGE VOID FRACTION V s FROUDE NUMBER FOR 80% WATER CUT, WITH IMIDAZOLINE

60 an FROM THE SLUG FRONT

6 12

FROUDE NUMBER

Flgure 4.29 VOID FRACTION V s FROUDE NUMBER FOR 80% WATER CUT WITH IMIDAZOLINE, AT THE BOTTOM OF THE PIPE

30 an FROM THE SLUG FRONT

6 12

FROUDE NUMBER

FIgare 4.210 VOID FRACTION V s FROUDE NUMBER FOR 80% WATER CUT WITH IMIDAZOLINE, AT THE BOTTOM OF THE PIPE

60 an FROM THE SLUG FRONT

6 1 2

FROUDE NUMBER

F l g m 4.211 AVERAGE VOID FRACTION V s FROUDE NUMBER FOR 40% WATER CUT WITH IMIDAZOLINE

30 an FROM THE SLUG FRONT

6 1 2

FROUDE NUMBER

Figure 4212 AVERAGE VOID FRACTION V s FROUDE NUMBER FOR 40% WATER CUT WITH IMIDAZOLINE

60 an FROM THE SLUG FRONT

6 1 2

FROIJDE NUMBER

Flgon 4.213 VOID FRACTION V s FROUDE NUMBER FOR 40% WATER CUT WITH IMIDAZOLINE, AT THE BOTTOM OF THE PIPE

30 an FROM THE SLUG FRONT

6 1 2

FROUDE NUMBER

Figore 4.2.14 VOID FRACTION V s FROUDE NUMBER FOR 40% WATER CUT WITH IMIDAZOLINE, AT THE BOTTOM OF THE PIPE

60 an FROM THE SLUG FRONT

Table 4.2.6 Corrosion rate data for polyamine salt in slug flow

-

Water cut

80%

40%

Inhibitor concentration

- - ppm

----

2 5

75

----

25

75

Froude number

6

12

6

12

6

12

6

12

6

12

6

12

Equilibrium corrosion rate

[=I mmlyr

0.8

1.7

0.7

1.6

0.5

0.6

0.5

1.1

0.7

1.6

0.5

0.6

Table 4.2.7 Pressure drop data for polyamine salt

Water cut

80%

40%

Inhibitor concentration

[=I PPm

----

----

25

75

25

75

Froude number

6

12

6

12

6

12

6

12

6

12

6

12

Pressure drop across the slug [=I KPa

30 cm

1.4

3.5

1.1

1.9

1.8

3.6

2.0

4.0

1 .O

1.9

1 .O

2.0

60 cm

1.8

4.2

1.4

2.8

1.0

1.5

1 .O

1.6

1.8

4.2

1.4

3.3

Table 4.2.8 Average void fraction data for polyamine salt

Water cut

80%

40%

Inhibitor concentration

[=I PPm

----

----

2 5

7 5

25

75

Froude number

6

12

6

12

6

12

6

12

6

12

6

12

Average void fraction across the cross

section, in the slug [=I

30 cm

17.4

57.0

13.0

23.4

24.6

62.3

17.8

19.8

23.7

22.2

29.3

20.2

Yo

60 cm

11.3

39.2

10.5

17.0

23.4

34.0

18.0

22.4

17.7

12.6

27.1

32.9

6 1 2

FROUDE NUMBER

Fignre 4.215 CORROSION RATE Vs FROUDE NUMBER FOR 80% WATER CUT WITH POLYAMINE SALT

6 1 2

FROUDE NUMBER

Figure 4.216 CORROSION RATE Vs FROUDE NUMBER FOR 40% WATER C W WITH POLYAMlNE SALT

6 1 2

FROUDE NUMBER

FIgnre 4.217 PRESSURE DROP Vs FROUDE NUMBER FOR 80% WATER C W WITH POLYAMINE SALT

SO an FROM THE SLUG FRONT

Finnre 4 2 1 8 PRESSURE DROP Vs. FROUDE NUMBER FOR 80% WATER CUT 0

WITH POLYAMINE SALT 60 an FROM THE SLUG FRONT

6 1 2

FROUDE NUMBER

Figure 4.2.19 PRESSURE DROP Vs FROUDE NUMBER FOR 40% WATER CUT WITH POLYAMINE SALT

30 an FROM THE SLUG FRONT

6 1 2

FROUDE NCiMBER

Ftgore 42.20 PRESSURE DROP Vs FROUDE NUMBER FOR 40% WATER CUT WITH POLYAMINE SALT

60 an FROM THE SLUG FRONT

6 1 2

FROUDE NUMBER

FIgnre 4.221 AVERAGE VOID FRACTION V s FROUDE NUMBER FOR 80% WATER CUT WITH POLYAMINE SALT

30 an FROM THE SLUG FRONT

FROLBE NUMBER

F@re 4.222 AVERAGE VOID FRACTION Vs FROUDE NUMBER FOR 80% WATER CUT WITH POLYAMINE SALT

60 an FROM TFIE SLUG FRONT

6 1 2

FROUDE NUMBER

Elgore 4.223 VOID FRACTION Vs FROUDE NUMBER FOR 80% WATER C W WITH POLYAMINE SALT, AT THE BO'ITOM OF THE PIPE

30 an FROM THE SLUG FRONT

6 1 2

FROUDE NUMBER

Fignre 4.224 VOID FRACTION V s FROUDE NUMBER FOR 80% WATER C W WITH POLYAMINE SALT, AT THE BO'lTOM OF THE PIPE

60 cm FROM THE SLUG FRONT

6 1 2

FROUDE NUMBER

Flgwe 42.25 AVERAGE VOID FRACTION V s FROUDE NUMBER FOR 40% WATER CUT WITH POLYAMINE SALT

30 an FROM THE SLUG FRONT

6 1 2

FROUDE NUlCWER

gore 42.26 AVERAGE vom FRACTION vs FROUDE NUMBER FOR 40% WATER CUT WITH POLYAMINE SALT

60 an FROM THE SLUG FRONT

6 1 2

FROUDE NUMBER

Flgwe 4.227 VOID FRACTION V s FROUDE NUMBER FOR 40% WATER CUT WITH POLYAMINE SALT, AT THE BO'ITOM OF THE PIPE

30 an FROM THE SLUG FRONT

6 12

FROUDE NUMBER

FLgnre 4.228 VOID FRACTION V6 FROUDE NUMBER FOR 40% WATER CUT WITH POLYAMINE SALT, AT THE BOTTOM OF THE PIPE

60 an FROM THE SLUG FRONT

74

and 13 per minute. For a Froude number of 12, the measurements were taken at frequencies

of 7, 13, 19 and 30 per minute. A slug with a Froude number of 6 has a slow movement and

therefore it is not possible to obtain high slug frequencies in a given section of the pipe. Due

to this restriction, the experiments were carried out only at two frequencies for a Froude

number of 6. However, for a Froude number of 12, the maximum frequency that could be

obtained was about 30 slugdmin. Jepson and Taylor (1988) found similar results in a 30 cm

internal diameter pipeline. They observed that the frequency of slugs for a Froude number

of 12 that could be obtained in a given section of the pipe is in the range of 30-40 slugs/min.

The corrosion rate data is listed in Table 4.3.1. Here, the corrosion rates from the

slug frequency experiments are compared with that obtained in the stationary slug. It is

observed that an increase in slug frequency increases the corrosion rate. At higher

frequencies, the mixing zone of the slug passes over the probe more frequently. This

enhances the corrosion process and causes an increase in corrosion rates.

For 80% water cut, the corrosion rates obtained for a Froude number of 6 were 0.4

d y r and 0.6 W y r , at the frequencies of 7 and 13 per minute, respectively. For a Froude

number of 12 the corrosion rates obtained were 0.6 d y r , 0.7 mmlyr, 0.8 d y r and 1.1

d y r for the frequencies of 7, 13, 19 and 30 per minute, respectively.

For 40% water cut, similar increase in corrosion rates was noted at both the Froude

numbers.

Figure 4.3.1 is a plot of the corrosion rate as a hnction of slug frequency for a

Froude number of 12. It is observed that for 80% and 40% water cuts, the corrosion rate

increases linearly with slug frequency, the rate of increase being similar in both the cases.

7 5

Also, it is seen that at these rates of increase, the stationary slug corrosion rates will be

reached at about 58 and 44 slugslmin for 80% and 40% water cuts, respectively. These are

shown by the dotted lines.

Figure 4.3.2 shows the corrosion rate-slug frequency relationship for a Froude

number of 6 . For 80% water cut, the rate of increase of corrosion rate from 7 slugslrnin to

13 slugslmin is similar to the rate of increase for a Froude number df 12, for the same

frequency range. Assuming that the increase in corrosion rate with slug frequency is linear,

it is seen that the stationary slug corrosion rates will be reached at about 19 slugslmin for

80% water cut and 25 slugslmin for 40% water cut.

Table 4.3.1 Corrosion rate data for various slug frequencies

Equilibrium corrosion rate

[=I mmlyr

0.8

0.4

0.6

1.7

0.6

0.7

0.8

1.1

0.5

0.2

0.3

1.1

0.3

0.5

0.6

0.8

Slug frequency [=] min-'

stationary

7

13

stationary

7

13

19

3 0

stationary

7

13

stationary

7

13

19

30

Water cut

80%

-

40%

Froude number

6

-

12

6

12

0 1 0 2 0 3 0 4 0 5 0 6 0

SLUG FREQUENCY (&I)

t u l r ~ l ~ ~ ~ t l ~ l l ~ l ~ l ~ I " " 1 " " - .

Figan 43.1 CORROSION RATE Vs. SLUG FREQUEN,CY FROUDE NUMBER 12

-

0 1 0 2 0 3 0 4 0 5 0 6 0

SLUG FREQUENCY (miri1)

\

- - stationary slug (80% water cut)

- - 0 0 - -.-- 1: 0.-

I - stationary slug (40% water cut) - -. - * - 1.

- I .

-.----; I - I I . I I - I I ' I I ' I I - I I ' I I - ! , , I > , , , .

1 .

--+- 80% water cut

-+- 40% water cut

I " " I " " 1 " '

Figare 43.2 CORROSION RATE Vs. SLUG FREQUENCY FROUDE NUMBER 6

: .

I " " I " " ~

-0- 80% water cut - 40% water cut L

- -

- - stationary slug (80% water cut)

a #* 1 * I

I stationary slug (40% water cut) - -+---; 5-0 : I I I I

I , , , , # I , . I . , I , I , , I , , , . I , , , ,

CHAPTER 5

CONCLUSIONS

It was found that slug flow severely affects the corrosion rates in multiphase systems.

Wall shear stress

In slug flow, the instantaneous shear stress at the bottom of the pipe fluctuates with

time and shows large prominent peaks at regular intervals. The peaks may be attributed to

the pulses of bubbles at the bottom of the pipe, which give bubble impacts and possible

collapse there.

The intensity of the shear stress fluctuations and the frequency of the occurrence of

the peaks increases with increase in Froude number and decreases with increase in distance

into the slug. This is due to the decrease in the level of turbulence at lower Froude numbers

and also with increase in distance into the slug.

The maximum values of shear stress are much higher than the average values. These

values increase with increase in Froude number and decrease with increase in distance into

the slug.

Due to higher turbulence and higher fiequency of the pulses of bubbles, the corrosion

rates also increase with increase in Froude numbers.

Effectiveness of corrosion inhibitors

Inhibitors behave differently in rotating cylinder eiectode, full pipe and slug flows. The

inhibitors are more effective in full pipe flow than in slug flow, the effectiveness in full pipe

79

flow being close to 100%.

The high levels of shear stress and turbulence in slug flow reduce the effectiveness

of the inhibitors. The reduction in corrosion rates in most cases was less than 50%. The

turbulence and the impact of the pulses of bubbles may be restricting the formation of a

stable inhibitor film.

In slug flow, imidazoline works well at a Froude number of 6, for 40% and 80%

water cut. This is due to low level of turbulence at a Froude number of 6. The polyamine

salt is ineffective in most of the cases.

Very little changes in gas holdup and pressure drop were observed after the addition

of imidazoline.

Higher concentration of the polyamine salt increased the gas holdup at 40% water

cut. This is due the formation of strong emulsions in the oil-water mixture.

Little changes were observed in pressure drop within the slug, after the addition of

the polyamine salt.

Slug frequency

The corrosion rate increases linearly with in slug frequency for both the Froude

numbers. The rate of increase is similar for 40% and 80% water cuts. The increase in

corrosion rate is because, the mixing zone of the slug passes over the probe more frequently

at higher slug frequencies, thereby enhancing the corrosion process.

It is seen that for a Froude number of 12, the stationary slug corrosion rates will be

reached at slug frequencies of about 58 and 44 slugslmin for 80% and 40% water cuts,

80

respectively. For a Froude number of 6, the stationary slug corrosion rates will be reached

at 19 and 25 slugslmin for 80% and 40% water cuts, respectively.

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APPENDIX A

Calculation of Froude number in a hydraulic jump

The Froude number is defined as

Fr, = vt - Vf

Jgh,

where,

vt = translational velocity of the slug

v , = velocity of the liquid film

hEFF = effective height of the liquid film, defined as the wetted area

(A3 divided by the width of the liquid-gas interface (T)

g = the acceleration due to gravity

A stationary slug or hydraulic jump can be visualized as a moving slug seen fiom a frame

of reference moving with the translational velocity (V,) of the slug. Hence, substituting

V, equal to 0, Equation 2.1 gives the Froude number for a stationary slug. The Froude

number for a stationary slug is therefore given by the equation:

Fr, = - vr ,I-

The negative sign indicates that the direction of flow of the film is opposite to that of the

slug front. If r is the radius of the pipe and h is the film height (equal to the gate height),

then W T gives the effective height (h,,) of the film, where

Using the mass balance we have

where, A, V, and Q are the cross-sectional area of the pipe, velocity of the fluid in the pipe,

and the volumetric flow rate, respectively, the film velocity is calculated. Then, using

Equations A. 1, the Froude number for a stationary slug is calculated.

APPENDIX B

Calibration of the hot film sensor to measure the wall shear stress

The hot film sensor is calibrated under fill pipe flow conditions for each oil-water

composition. During the operation of the hot film sensor, the sensor temperature is

maintained constant by a standard anemometry circuit. A laminar thermal boundary layer

grows on the probe in the direction of flow. The instantaneous average heat transfer

coefficient for the probe is proportional to the square of the voltage drop across the sensor.

In an ideal developing boundary layer, the heat transfer coefficient for the probe is

proportional to one-third of the wall shear stress. Since the temperature of the sensor is high

enough, the variations in the ambient temperature have a small effect on the calibration. So,

knowing the values of the constants A and B and the bridge voltage V, in the relationship

where,

- 7 ,

- mean wall shear stress

v - - anemometer bridge voltage

the wall shear stress can be calculated. These constants change only with the fluid

composition.

For a hlly developed flow within a smooth pipe

where,

- vs - superficial liquid velocity

P - - density of the fluid

f - - moody friction factor - - 0.0791

the fiction factor is valid for 2100 < N, < lo5

T, is calculated in full pipe flow for different liquid velocities and then substituted in

Equation B. 1. A linear plot of (2,)'l3 with V2 determines the values of the constants A and

B.

After calculating the values of the A and B, the changes in the bridge voltage are

recorded and substituted in Equation B. 1 to calculate the wall shear stress in slug flow.

APPENDIX C

Principle and working of the slug control system

The control system comprises of two pressure transducers, an on-off solenoid valve,

two proportional valves and a manually controlled needle valve. These valves are used to

regulate the supply of gas into the system. Figure C. 1 shows how the valves and transducers

are connected to the system. PI, P2 and P3 are the pressure taps connected to the pressure

transducers TI and T2. TI and T2 have a range of 0-2.8 KPa and 0-9.0 KPa, respectively.

C 1 and C2 are the proportional valves, S is the on-off solenoid valve, and N is the needle

valve. C1 and C2 require a dc input power while, S is operated by an ac power supply. PI

is connected to the high side of TI, and P2 is connected to the high side of T2 and the low

side of TI. P3 is connected to the low side of T2. X is the ,gas inlet into the system.

Working of the proportional valves

The valves C1 and C2 are powered by a 24 volts dc source and their response is

controlled by a 0-10 volts input. The input to.the valves is controlled by the pressure drop

across the slug. Figure C.2a shows the schematic of the circuitry. T1 has an input pressure

range of 0-3.0 KPa and an output range of 4-20 mA current. The current fiom T1 is passed

through a variable resistor R1. The resistance of R l is used to fine-tune the gas flow at X

to maintain the slug position. The voltage drop across R1, given by V1, is sent as an input

signal to C1 and C2 and the valves open accordingly.

9 1

Working of the on-off valve

Figure C.2b shows the schematic of the circuitry. T2 has an input pressure range of

0-9.0 KPa and an output range of 4-20 mA current. The current from T2 is passed through

a fixed resistor R2. The voltage drop across R2, which is V2, is sent into a comparator. This

is used to compare with V3 described below.

Figure C.2b also shows a pressure sensitivity selector for the on-off valve. It

consists of a variable resistor R3, which receives its input power supply from the mains. The

voltage range across R3 is 3.5-8.5 volts. The resistance of R 3 is can be set manually using

a knob. The voltage drop across R3, given by V3, is sent to a comparator. The comparator

compares the voltages V2 and V3. Whenever V2 becomes equal to V3, the comparator

sends a digital signal to a solid state relay, which then triggers the on-off valve S to be open.

The valve S is either totally open or totally closed.

Stationary slug control

Initially, the slug is moved into position between the pressure taps P1 and P2 using

the needle valve N. The control system is then switched on and the control valves are

automatically opened. The variable resistance R1 is set to a value required to keep the slug

between P1 and P2. When the slug drifts upstream, the pressure drop into T1 increases, thus

increasing V1. An increase in V1 causes the valves C1 and C2 to open some more, thus

increasing the amount of gas flow. This pushes the slug downstream. The inverse is also

true. When the slug drifts downstream, V1 decreases and the valves C1 and C2 start to close

thus, causing the slug to move upstream. The movement of the slug is restricted to within

5-10 cm from the original position of the slug.

Moving the slug back and forth

For the slug fiequency experiments the slug has to be moved across the probes A and

B, within the test section. For this purpose the slug is initially moved into the test section.

The resistance R1 is set to zero so that there is no gas flowing through'valves C1 and C2.

The control system is then switched on. The resistance R3 on the pressure sensitivity

selector is set to the value at which S is supposed to switch on. The needle valve N is totally

closed. The slug now begins to drift upstream. When the slug crosses P2 and is moving

towards P3, the pressure drop into T2 increases, thus increasing V2. When V2 equals V3,

the comparator turns the relay on, which in turn triggers the solenoid valve totally open.

This pushes the slug downstream. The timer circuit controls the time for which valve S is

open.

The value of R3 is high for low frequency slugs and vice-versa. For the high

frequency slugs, the valve S is open for a very short interval. For low slug frequencies,

valve S opens when the slug just reaches P2, and remains open for a longer time period, thus

driving the slug into the pipe section beyond the test section. This is achieved by

manipulating R3 and the timer. In some cases, such as for higher Froude numbers, to

achieve a lower fiequency, C1 and C2 are also used. This is because, in case of high Froude

numbers the slug has a tendency to come back faster as soon as S is closed. Maintaining a

small voltage across R1 keeps the slug from coming back fast because, as soon as the slug

crosses P2, C 1 and C2 open slowly thus, slowing down the movement of the slug.