Corrosion under Insulation on Offshore Facilities
by
MIGUEL LAMSAKI
Submitted
in partial fulfillment of the requirements
for the degree of
MASTER OF ENGINEERING
Major Subject: Petroleum Engineering
at
DALHOUSIE UNIVERSITY
FACULTY OF ENGINEERING
Halifax, Nova Scotia September, 2007
Copyright by Miguel Lamsaki, 2007
ii
Dalhousie University Faculty of Engineering
Process Engineering and Applied Science
The undersigned hereby certify that they have examined, and recommend to the Faculty of Graduate studies for acceptance, the project entitled Corrosion under Insulation on Offshore Facilities by Miguel Lamsaki in partial fulfillment of the requirements for the degree of Master of Engineering.
Dated: Supervisor:
Georges J. Kipouros Co- supervisor: George Jarjoura Examiners: Stuart Pinks P. Carey Ryan
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Dalhousie University
Faculty of Engineering
DATE:
AUTHOR: Miguel Lamsaki.
TITLE: Corrosion under Insulation on Offshore Facilities
MAJOR SUBJECT: Petroleum Engineering
DEGREE: Master of Engineering
CONVOCATION: October, 2007
Permission is herewith granted to Dalhousie University to circulate and to have for non-commercial purpose, at its discretion, the above project upon request of individuals or institutions.
Signature of Author
The author reserves others publication rights and neither the project nor extensive extracts from it may printed or otherwise reproduced without the authors written permission. The author attests that permission has been obtained for the use of any copyrighted material appearing in this project (other than brief excerpts requiring only proper acknowledgement in scholarly writing), and that all such use is clearly acknowledged.
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TABLE OF CONTENTS
LIST OF TABLES viii
LIST OF FIGURES ix
LIST OF ABBREVIATIONS AND SYMBOLS xi
ACKNOWLEDGEMENTS xii
ABSTRACT xiii
1. INTRODUCTION 1
1.1 BRACKGROUND 1
1.2 CORROSION MECHANISM 3
1.3 TYPES OF CORROSION 7
1.3.1 Uniform Attack 8
1.3.2 Pitting 9
1.3.3 Crevice Corrosion 12
1.3.4 Stress Corrosion Cracking 14
1.3.5 Hydrogen Damage 17
1.3.6 Intergranular Corrosion 18
1.3.7 Galvanic Corrosion 20
1.3.8 Selective Leaching 21
1.4 SCOPE OF THE PROJECT 23
2. INSULATION SYSTEMS 24
2.1 HEAT TRANSFER PROPERTIES 26
2.1.1 Conduction 27
2.1.2 Convection 28
2.1.3 Radiation 28
2.2 THERMAL PROPERTIES 29
2.2.1 Thermal Conductivity 29
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2.2.2 Thermal Conductance 30
2.2.3 Thermal Transmittance 30
2.2.4 Thermal Resistance 30
2.3 MECHANICAL AND CHEMICAL PROPERTIES 31
2.3.1 Density 32
2.3.2 Moisture Resistance 32
2.3.3 Compressive Strength 33
2.3.4 Thermal Use Range 34
2.3.5 Fireproofing 35
2.3.6 Sound Attenuation 36
2.3.7 Chemical Neutrality 36
2.3.8 Other Properties 37
2.4 INSULATION MATERIALS 40
2.4.1 Calcium Silicate 40
2.4.2 Expanded Perlite 41
2.4.3 Glass and Mineral Fibers 41
2.4.4 Cellular Glass 42
2.4.5 Polyurethane and Polyisocyanurate Foams 43
2.4.6 Elastomeric Foams 43
2.4.7 Aerogels 44
2.5 PROTECTIVE COVERINGS AND FINISHES 44
2.5.1 Adhesives 45
2.5.2 Cements 45
2.5.3 Coatings and Mastics 45
2.5.4 Sealants and Caulks 46
2.5.5 Jacketing Systems 47
2.5.5.1 Aluminum Jackets 48
2.5.5.2 Stainless Steel Jackets 49
2.5.5.3 Plastic Jackets 49
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2.5.5.4 All Service Jackets 49
2.6 ISULATION FAILURE MECHANISM 50
3. OIL AND GAS OFFSHORE STRUCTURES 55
3.1 TOPSIDE FACILITIES 61
3.1.1 Processing Systems 62
3.1.2 Storage Systems 64
3.1.3 Piping Systems 66
3.2 INDUSTRY TREND 69
4. CORROSION UNDER INSULATION 71
4.1 CORROSION UNDER INSULATION MECHANISM 72
4.2 FACTORS PROMOTING CORROSION UNDER INSULATION 75
4.2.1 Marine Environment 75
4.2.2 Air Pollutants 77
4.2.3 pH Effect 80
4.2.4 Environmental Conditions 83
4.2.5 Service Temperature 84
4.2.6 Insulation Materials 87
4.2.7 Mechanical Design of Equipment and Insulation Installation 88
4.2.8 Mechanical Damage 89
4.3 SUSCEPTIBLE PLACES 91
4.4 INSPECTION METHODS 92
4.4.1 Pulsed Eddy Current Testing 94
4.4.2 Real Time Radiography 94
4.4.3 Magnetostrictive Technology 95
4.4.4 Infrared System 97
4.4.5 Neutron Backscatter 97
4.4.6 Long Range Ultrasonic 98
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4.5 RISK BASED INSPECTIONS 98
4.6 INDUSTRY TREND 100
5. PROTECTIVE COATINGS 104
5.1 PAINT COATINGS 105
5.2 METALLIC COATINGS 108
5.3 SURFACE PREPARATION 110
5.4 FAILURE MECHANISM 111
5.5. INDUSTRY TREND 112
6. CASE STUDIES 114
6.1 INDUSTRY TREND 121
7. DISCUSSION 122
8. CONCLUSIONS 134
9. RECOMMENDATIONS 137
10. REFERENCES 140
11. APPENDICES 146
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LIST OF TABLES
Table 1.1 Standard Potential Series of Metals 5
Table 1.2 Acceptable Corrosion Rates of Ferrous and Nickel Based Alloys 7
Table 1.3 Effect of Alloying on Pitting Resistance of Stainless Steel Alloys 11
Table 1.4 Common Metal- Environment Combinations Leading to Stress Corrosion Cracking 16
Table 2.1 Moisture Resistance Property of Various Insulation Materials 33
Table 2.2 Compressive Strength of Different Insulation Materials 34
Table 2.3 Recommended Thermal Temperatures by Du Pont Company 35
Table 4.1 Major Ions in Solution in an Open Sea Water at S/00 = 35.0 77
Table 5.1 Paint Coating Application Coverage Rate 107
Table 6.1 Results of the Corrosion Test 117
Table 6.2 Occurrence of Stress Corrosion Cracking on coiled 304 spring Specimens in Boling Saturated Sodium Chloride Solution at 108C 119
Table 6.3 Occurrence of Stress Corrosion Cracking on coiled 304 spring Specimens in Boling Saturated Calcium Chloride Solution at 138C 120
Appendix A - Basic Types of Insulation for Low Temperatures 147
Appendix B - Basic Types of Insulation for Intermediate Temperatures 148
Appendix C - Basic Types of Insulation for High Temperatures 149
Appendix D Protective Coverings and Finishes 150
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LIST OF FIGURES
Figure 1.1 Basic Corrosion Cell 4
Figure 1.2 Uniform Attack in an Insulated Pipe 9
Figure 1.3 Random Pitting 10
Figure 1.4 Crevice Corrosion of an Area of a Teflon Washer on a 316 Stainless Steel Plate 14
Figure 1.5 Cross Section of a 304 Stainless Steel Pipe Showing Stress Corrosion Cracking 15
Figure 1.6 Hydrogen Damage on a Steel Pipe 17
Figure 1.7 Intergranular Corrosion in a Fireplug Component 19
Figure 1.8 Galvanic Corrosion between a Carbon Steel Pipe and a Brass Valve 20
Figure 1.9 Removal of Zinc from a Brass Pipe Due to Selective Leaching Process 22
Figure 2.1 Heat Transfer Modes 27
Figure 2.2 Effect of pH on Corrosion Rate of Iron in Aerated Water 37
Figure 2.3 Typical Vessel Insulation Using Rigid Blocks 38
Figure 2.4 Typical Pre-Formed Pipe Insulation Multilayer Construction 39
Figure 2.5 Removable and Reusable Insulation System 40
Figure 2.6 Typical Insulation System Where Compounds Are Used 46
Figure 2.7 Rubberized Asphalt Vapor Barrier Membrane on an Ammonia System 47
Figure 2.8 Aluminum Jackets Secured with Screws 48
Figure 2.9 Improper Finishing of Jacketing System 51
Figure 2.10 Improper Sealing of an Insulation End Section 52
Figure 2.11 Lower Section of an Aluminum Jacketing System Installed Over the Upper Section 53
Figure 2.12 Aluminum Jacket Laps Installed Near the Top Section of Piping 53
Figure 2.13 Typical Vessel Attachments Where Water May Bypass Insulation 54
Figure 3.1 Areas of Corrosion and Types of Corrosion Control for Offshore Structures 57
Figure 3.2 Hibernia Gravity Base Structure 59
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Figure 3.3 The Thebaud Facility 60
Figure 3.4 Floating Production, Storage and Offloading Vessel 61
Figure 3.5 Application of Rigid Cellar Glass Blocks on a Storage Tank 65
Figure 3.6 Corrosion Above an Insulation Support Ring 65
Figure 3.7 Schematic Representation of a Typical Christmas Tree System 67
Figure 3.8 Potential Places Where Water May Bypass Insulation on Piping 68
Figure 3.9 Insulation Jacket Open at Vertical Beam 69
Figure 4.1 Corrosion Under Insulation Near the Bottom Part of a Carbon steel Storage Tank 73
Figure 4.2 Metal Loss of Carbon Steel in Three Different Environments 76
Figure 4.3 Canadian SO2 Emissions from Acid Rain Sources, 1980 2004 79
Figure 4.4 Effect of pH on Corrosion of Iron in Aerated water at Room Temperature 80
Figure 4.5 Five Year Mean pH of Acid Rain in Canada and United Sates 82
Figure 4.6 Effect of Temperature on Carbon Steel Corrosion in Water 86
Figure 4.7 Unsealed Insulation Penetrations Where Water Can enter the Insulation 89
Figure 4.8 Mechanical Damage of Jacketing Systems 90
Figure 4.9 Real Time Radiography System 95
Figure 4.10 Schematic Diagram of Magnetostrictive Technology 96
Figure 5.1 Schematic Representation of Sacrificial Zinc Coating over Steel Surface at a Void 109
Figure 6.1 Carbon Steel Pipe and Insulation Samples Installed on the Pipe 115
Appendix E Typical Oil and Associated Gas Production Process 151
Appendix F Typical Gas Production Process 152
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LIST OF ABBREVIATIONS AND SYMBOLS
ASJ = All Service Jackets
API = American Petroleum Institute
CCPUF = Closed Cell Polyurethane Foam
g = Grams
FPSO = Floating Production Storage and Offloading
GBS = Gravity Base Structure
kg = Kilograms
kPa= Kilo Pascal
m2 = Metre Square
mm = Millimetres
mm/yr = Millimetres per year
MPa = Mega Pascal
NACE = National Association of Corrosion Engineers
NDT = Nondestructive Testing
OCPUF = Open Cell Polyurethane Foam
PIF = Polyisocyanurate Foam
PP = Polypropylene
PU = Polyurethane
RBI = Risk based inspections
VIP = Vacuum Insulation Panel
W/m xC = Watts per metre degree Celsius
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ACKNOWLEDGEMENTS
I would like to express my sincere appreciation and special thanks to the members of the
thesis supervisory committee, Dr Georges J. Kipouros, Professor, Department of Process
Engineering and Applied Science; Dr George Jarjoura, Professor, Department of Mining
and Metallurgical Engineering; Mr. Carey Ryan, Vice President, Petroleum Research
Atlantic Canada (PRAC); and Mr. Stuart Pinks, Manager, Health, Safety and
Environment, Canada Nova Scotia Offshore Petroleum Board (CNSOPB) for their
invaluable guidance, support and outstanding contribution throughout the course of this
research project and for making possible the realization and culmination of this study. I
would also like to thank all the staff of Petroleum Research Atlantic Canada for providing
me the opportunity to work in their facilities.
I would like to acknowledge the effort of Mr. Stuart Pinks and Mr. Carey Ryan who
made possible the communication and interaction with staff members from the offshore
industry, who as well provided their personal experiences and comments about the
problem of corrosion under insulation on offshore facilities.
Finally, special thanks to my beloved wife, Ana Santana; my father, Miguel N. Lamsaki;
my mother, Ana Lamsaki; my twin, Sergio Lamsaki; my sister, Irene Lamsaki; and my
friends, Luis Perez and Geronimo Bendito for their patient and support throughout the
period of the Master program.
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ABSTRACT
This thesis provides a comprehensive study of the problem of corrosion under insulation on offshore facilities. It also studies whether the actual characteristics of the environment of the east coast of Canada have an important effect on the occurrence of corrosion beneath insulation. Additionally, there is a review of the capabilities and limitations of the latest nondestructive evaluation techniques commonly used to inspect for corrosion on insulated systems together with the identification of opportunities for new or for improvements to existing inspection techniques.
Corrosion under insulation is and has been a major problem for the oil and gas industry for more than 50 years. It is difficult to identify because it remains hidden beneath the insulation hardware, frequently until unexpected failures occur. Corrosion under insulation can take place under any class of insulating material. Intruding water is the principal problem. Special consideration must be given to equipment design in order to avoid irregular shapes that are difficult to insulate and may be, in the long term, source of water intrusion. Systems with multiple protrusions through the insulation are more likely to allow water to diffuse into the insulation because sealants and caulking compounds used to seal joints and protrusion tend to get damaged quickly. In general, the insulation material that holds the least quantity of water, such as closed cell cellular glass insulation, should be used from the initial design phase of any offshore facility in order to prevent corrosion of the underlying metal surface.
Carbon steel and austenitic stainless steel are the two main materials commonly used for offshore applications. However, during the last few years the oil and gas industry is using more duplex stainless steel and super austenitic stainless steel alloys due to their improved corrosion resistant properties. Carbon steel is more likely to suffer uniform corrosion or pitting corrosion under insulation systems while austenitic stainless steel is subjected to stress corrosion cracking and highly localized pitting corrosion. Corrosion rates under insulation depend upon two factors besides the presence of moistures and water. First, warm and hot temperatures, usually the temperature range of -4C to 150C will have an important impact on corrosion under insulation and second, external and internal water contaminants such as chlorides and sulphides that may decrease the pH of water below 4.0 where corrosion rates are more likely to increase dramatically. In this case, since the north Atlantic region of Canada is presenting pH levels of rain and coastal fog near 4.0, special consideration should be given to insulation systems used on the existing offshore facilities.
In conclusion, preventing corrosion beneath insulation can be achieved with the right selection of insulation material, proper installation and effective application of risk based inspection programs together with the use of combined nondestructive examination techniques such as long range ultrasonic and magnetostrictive technology. However, there is the need to overcome their limited use on straight runs of pipes. It is also required to review the corrosion resistant properties of the new generation of alloys under severe conditions and under different types of coating and insulation systems to establish the temperature limits at which corrosion is more likely to occur and also to identify the more suitable protective coating to be used under insulation systems.
1. INTRODUCTION
1.1 Background
Since the first mobile offshore platform was used to drill a well 12 miles from the
Louisiana shore in the Gulf of Mexico in 1947, the continental shelf areas of the ocean,
like the Scotian and Jean d Arc Basins located in the north Atlantic region, now supply
approximately 25 % of the world total oil and gas production. Additionally, there will be
new exploration and production developments in deeper ocean basin areas combined with
a general production decline of onshore oil and gas reservoirs that will result in a
continuous growth of offshore hydrocarbon production [1].
According to the study of the world offshore oil and gas production forecast
2007-2011 published by Douglas and Westwood in April 2007, offshore oil production
has risen by over a third since 1991 and is forecast to continue to rise at about the same
rate by the year 2011 [2]. Simultaneously to this increment, the industry has faced a
variety of technical issues like corrosion under insulation that affects the performance and
the integrity of the offshore facilities.
A study prepared by Exxon Mobile Chemical and presented to the European
Federation of Corrosion in September 2003 indicated that:
The main cause of leaks in the chemical and refining industries is due to corrosion under insulation.
81 percent of piping leaks happened in pipes with a nominal diameter smaller than 4 inch.
More than 40 percent of piping maintenance cost is associated to corrosion under insulation [3].
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Experience has revealed that as time passes, jackets lose their capacity to protect
the insulation from the atmospheric conditions and thereby insulation gets wet. Water,
oxygen, and other corrosive contaminants will be able to reach the insulated metal and
therefore severe corrosion may occur [4].
One of the principal chemical manufacturing companies in the world, E.I. DuPont
de Numours and Company calculated that the direct cost associated with corrosion under
insulation can go beyond $10 million per year without including preventative
maintenance costs and indirect costs [3].
The proper design and selection of coating systems that are applied to piping and
vessels prior to installing the insulation have been major components in controlling
corrosion under insulation. Another factor that has been an important element for the oil
and gas industry in preventing and controlling corrosion problems is the development of
timely and reliable inspection techniques to detect corrosion under insulation and to
detect deterioration to insulation and associated sealing materials.
The aim of the corrosion engineer is to slow the corrosion process with the
application of cost effective corrosion monitoring and maintenance programs throughout
the useful life of the offshore structure. Usually corrosion losses are divided into two
categories: direct and indirect economic losses. Direct losses consist of costs related to
the cost of parts and labor to replace corroded metal. Indirect losses are associated with
plant shutdowns, loss of product and environmental damage [5].
At the present time, corrosion under insulation represents an important problem
for the oil and gas industry. Detection and prevention of corrosion under insulation can
represent a significant portion of the operating cost of a project; therefore it must be
carefully studied in order to maintain effectively and efficiently the offshore facilities
during their planned life cycle.
3
1.2 Corrosion Mechanism
Corrosion is the natural process of deterioration or destruction of a material due to
a chemical or electrochemical reaction with its environment [4]. Basically all
environments are corrosive. The most common corrosive environments are: air and
moisture; fresh and salt water; gases such as sulfur dioxide, chlorine and hydrogen sulfide
[6]. Corrosion of iron can be explained as an electrochemical process. The following
reaction describes the corrosion process of iron when is immersed in oxygenated water
[5]:
2 Fe 2 Fe++ + 4 e- anodic reaction (1)
O2 + 2 H2O + 4 e- 4 OH- cathodic reaction (2)
The overall reaction:
2 Fe + O2 + 2 H2O 2 Fe++ + 4 OH- 2 Fe (OH)2 (3)
Corrosion is a major concern when metals are used. The native state of metal is
the oxidized state. When metals are mined and refined, their original energy level is
increased. In the existence of oxygen and moisture, processed metal will instantly start
the process to return to its lowest level of energy [5]. The accumulated energy throughout
the refining process is released when metals convert to corrosion products [6].
During the corrosion process, the cathodic and anodic reactions occur
simultaneously; therefore it is possible to control corrosion by slowing down the rates of
either reaction [6]. One of the methods to reduce the rates of the anodic and cathodic
reactions is by the application of protective coating materials over the metal surface.
Protective coatings control the access of moisture and oxygen to the metal surface,
therefore corrosion rates are reduced.
4
The corrosion mechanism can be illustrated with the basic corrosion cell shown in
figure 1.1. It is composed of four elements: an anode, a cathode, an electrical path and an
electrolyte. The anode and cathode could be the same metal but different regions. In
offshore facilities the electrolyte is water in some form; a thin film of water is sufficient
to create the electrolyte in a corrosion cell. The electrical path could be a steel pipe or any
steel equipment that connects the anode with the cathode. Corrosion will not occur with
the absence of any of the four components [4].
Figure 1.1: Basic corrosion cell [7]
Normally the corrosion cell is known as the cathode, anode and the electrolyte.
The anode is the region of the metal surface that deteriorates and produces electrons. The
anode reaction is also called oxidation which means loss of electrons [8]. The cathode is
the section of the metal that does not corrode and consumes electrons produced at the
anode [4].
5
During the corrosion process, electrons flow from the anode region to the cathode
region. The driving force that allows the electrical current to flow is the energy that is
accumulated in the metal, also known as the potential of the metal. Each metal has
different corrosion resistant characteristics due to the amount of energy that is required
during its refining process, therefore every type of metal has a different tendency to
deteriorate. Table 1.1 shows the standard potential of metals compared to the standard
hydrogen electrode whose potential is zero [4].
Table 1.1: Standard potential series of metals [4]
Energy Required for Refining Metal Volts Tendency to Corrode
Most energy required Magnesium -2.37 Greatest tendency
Aluminum -1.66
Zinc -0.76
Iron -0.44
Tin -0.14
Lead -0.13
Hydrogen 0.00
Copper 0.34 to 0.52
Silver 0.80
Platinum 1.20
Least energy required Gold 1.50 to 1.68 Least tendency
The offshore environment is considered by many as the most severe of the
environments. In the oil and gas industry the most common metals are carbon steel and
stainless steel, therefore the hundreds of offshore platforms and drilling rigs operating
around the world are affected by the extreme corrosive marine conditions. This
6
unavoidable factor associated directly with offshore activities often leads to costly and
extensive maintenance and repair programs.
The common expression to describe the capacity of corrosion resistance of metals
and nonmetals in different environments is the corrosion rate. Corrosion rates are
expressed in different ways such as: grams per square inch per hour, milligrams per
square centimeter per day and percent weight loss. Another expression widely used by
engineers and scientist to express the corrosion rate is millimeters and micrometers per
year. The following formula is used to calculate the corrosion rate from the weight loss of
metals during a corrosion test [6]:
mm = 87.6 x W (millimeters per year) (4) Yr D x A x T
Where:
87.6 = conversion factor from centimeters per hour to millimeters per year
W = weight loss, mg
D = density of specimen, g/cm3
A = area of specimen, cm2
T = exposure time, hr
Another useful way to measure the extent of corrosion of almost any form of
corrosion except stress corrosion cracking is the depth of penetration, especially if the
attack is localized. The penetration refers to the depth of the deepest pit found on the
corroded area [8]. Many factors determine the corrosion rate of pipes, vessels and
different equipment on offshore platforms and rigs. Some examples of these factors are:
the conductivity of the electrolyte, the pH of water, dissolved gases, temperature and air
pollution [4]. Table 1.2 shows reference values commonly used to describe the metals
corrosion resistance property [6].
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Table 1.2: Acceptable corrosion rates of ferrous and nickel based alloys [6]
Corrosion Rate (mm/yr) Relative Corrosion Resistance*
< 0.02 Outstanding
0.02 0.1 Excellent
0.1- 0.5 Good
0.5 1 Fair
1 5 Poor
>5 Unacceptable
*Based on typical ferrous and nickel based alloys
Covered areas as the case of an insulated pipe, where moisture and dust become
trapped, will have a higher rate of corrosion than uncovered areas. Conductivity of the
electrolyte is directly proportional to the rate of corrosion. Sodium chloride dissolved in
sea water increases the conductivity of the electrolyte and therefore increases the rate of
corrosion [9]. Another factor that can affect the corrosion rate is the solubility of
corrosion product. Usually when the corrosion product dissolves into the electrolyte, the
conductivity is increased and the corrosion rate will rise [10].
1.3 Types of Corrosion
Corroded metal appears in numerous forms depending on the corrosive
environment, the type of the metal, the nature of the corrosion product, the stress on the
metal and other variables. Corrosion is usually classified by the appearances on the
attacked metal [6]. Different types of corrosion have similar characteristics and therefore
can be classified into specific groups. Some of these types involve mechanisms that have
common characteristics that may contribute to the initiation of a specific class of
8
corrosion [10]. Every form of corrosion can be recognized by simple visual observation
and some of them can be identified just with the naked eye. The solution of a corrosion
problem can be achieved by cautious examination of the corroded equipment [6]. Eight
forms of corrosion are usually categorized by corrosion scientists and engineers and they
can be found on offshore insulated equipments. These types of corrosion are defined as:
uniform or general attack, crevice corrosion, pitting, intergranular corrosion, selective
leaching, galvanic corrosion, stress corrosion cracking and hydrogen damage [10].
1.3.1Uniform Attack
Uniform attack or generalized corrosion is a homogeneous chemical or
electrochemical reaction over a large area of a metal, characterized by uniform thinning
that proceeds without appreciable localized attack [6]. Uniform attack is the most
common type of corrosion, but at the same time the least risky [8]. From a technical
perspective, it is the form of deterioration that has the greatest damage of metal on a
tonnage basis [6]. Figure 1.2 shows an example of uniform attack on an insulated pipe.
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Figure 1.2: Uniform attack in an insulated pipe [11]
Carbon steels and copper alloys under the effect of atmospheric conditions are
good examples of materials that usually show signs of general attack, while materials,
such as stainless steels or nickel chromium alloys, are usually affected with localized
attack [10]. During the general corrosion process, the corroding metal acts at the same
time as the anode and the cathode. With uniform corrosion the engineer is able to
calculate the life of the equipment and thereby can program inspections and replacements
on a regular schedule [6].
1.3.2 Pitting
Pitting corrosion is known as the deterioration of metals at localized areas rather
than over its whole surface. The corrosion reaction is concentrated at the localized areas
where the corrosion rate will be greater than the average corrosion rate over the entire
surface [4]. Figure 1.3 shows a deteriorated steel pipe due to pitting corrosion.
10
Figure 1.3: Random pitting [11]
Usually, the word pit is used to express any mark on the surface of metals that has
a shape of a hole. Crevice corrosion, galvanic corrosion, failure of a metal coating, or
corrosion by water droplets are some of the factors that may give rise to the initiation of
pits. The way it manifests on the corroded metal is with the development of sharply
defined cavities. The holes could be large and shallow or deep and narrow, but usually
they are reasonably small. Depending on the characteristics of the corrosive environment
they may be almost completely round or elliptic or have irregular shape [4].
Pits are sometime apart from each other over the surface of metals or sometimes
they are close together and they look like an irregular surface [6]. Pits typically grow in
the direction of gravity. Pitting corrosion is not restricted to carbon steels; it may also
occur in diverse metals used in offshore facilities. From a practical point of view,
11
chloride solutions generally promote the occurrence of pitting [8]. Stainless steels used in
offshore facilities are very susceptible to this type of corrosion due to seawater and its
chloride content that induces the occurrence of pitting [6].
In general the stainless steels are more vulnerable to be attacked and deteriorated
by pitting corrosion than any other type of metals or alloys. A variety of alloy studies
have been done to improve the pitting resistance of stainless steels. The results are
summarized in Table 1.3 [6].
Table 1.3: Effects of alloying on pitting resistance of stainless steel alloys [6]
Element Effect on Pitting Resistance
Chromium Increases
Nickel Increases
Molybdenum Increases
Silicon Decreases; increases when present with molybdenum
Titanium and
Columbium
Decreases resistance in FeCl3; other media no effect
Sulfur and Selenium Decreases
Carbon Decreases
Nitrogen Increases
Pitting is one of the most dangerous forms of corrosion. It causes unexpected
failure by deep perforations with only a small percent of weight loss of the metal. Pits are
difficult to detect by simple visual examination, especially when they are very small and
covered with corrosion products that frequently mask them [10].
Sometimes this type of corrosion requires a long time before pits become visible
on the metal surface. The time to form could range from months to years depending on
12
the corrosive environment and the type of metal. Pitting can be much more serious than
uniform corrosion, because sometimes they occur after an unpredictable period of time
when the attacked area is penetrated in a very short time and failure occurs with extreme
suddenness [6].
Additionally, pitting is complicated to predict by laboratory test and also difficult
to measure quantitatively, because under identical conditions a variety of pits with
different depths may occur. A method of measuring pitting intensity is with the ratio of
the deepest metal penetration at the deteriorated area to the average metal penetration
obtained by the general weight loss. Another method is to calculate a pitting rate
equivalent, that measures the deepest pit and the exposure time during the lab test
converted to an annual penetration rate [4]. When pits are not many and are widely
separated and at the same time there is not general corrosion attacking the metal, there is
a high ratio of cathode to anode area. As a result the penetration rate is greater than when
pits are many and closer together [10].
1.3.3 Crevice Corrosion
Crevice corrosion is an intense localized corrosion caused by a concentration cell
in which some corrosive agent is depleted inside the crevice. Corrosion in crevices can be
reduced by a good design of the equipment. Many different sites in offshore equipment
that are covered with insulation materials may give rise to this type of localized corrosion
if moisture or water penetrates through the insulation and reach the metal surface. The
crevice can be produced in four different ways [8]:
1. Cracks, seams, or metallurgical defects could act as sites for corrosion initiation.
2. A gap between metal contacting another metal that could allow moisture to enter, such
as in the threads of nuts and bolts or between lapped joints.
13
3. Deposits over the metal surface, such as precipitated salts, dirt, corrosion product or
dust.
4. Metal contacting porous nonmetallic material, such as gaskets, insulation materials or
porous paint [8].
Usually, during the corrosion process, the crevice deteriorates evenly just as the
metal outside the crevice does [8]. Because crevice corrosion is found very often in metal
components, it is normally considered a form of corrosion by itself. Nearly all metals and
alloys are vulnerable to this type of attack [12].
In the presence of seawater, the deterioration of copper and its alloys at crevices
occurs outside of the crevice rather than within. In the case of stainless steel alloys the
deterioration occurs within crevices. In general, metals that are resistant to general
corrosion are susceptible to develop crevice corrosion [10]. Figure 1.4 shows an example
of crevice corrosion on a stainless steel plate.
14
Figure 1.4: Crevice corrosion at the location of a Teflon washer on a 316L
stainless steel plate [13]
Stainless steels are vulnerable to this type of corrosion because they become
anodic within the crevice and cathodic outside it, developing a large ratio of cathode and
anode area, resulting in an intense localized corrosion attack. Crevice corrosion often
causes the development of stress corrosion cracking or corrosion fatigue [8].
1.3.4 Stress Corrosion Cracking
Stress corrosion cracking manifests itself with fine fractures that penetrate deeply
through the metal, caused by the existence of tensile stress or plastic strain and a
corrosive solution. If tensile stress or plastic strain does not exist, the metal would not
corrode in a cracking way [6]. Usually during stress corrosion cracking, metal loss is
normally very low, while cracks penetrate into the metal. The cracks may be
15
intergranular or transgranular, but always perpendicular to the highest stresses [4]. Figure
1.5 shows a stainless steel cross section that suffered stress corrosion cracking.
Figure 1.5: Cross section through 304 stainless steel pipe showing stress corrosion
cracking [14]
All alloys are vulnerable to the development of stress corrosion cracking in some
few specific environments, and only pure metals seem to be resistant to it. Table 1.4
shows the typical metal environment combination where stress corrosion cracking
usually occurs. Although it is found frequently in metals, it can also occur in other type of
solid materials, such as ceramics and polymers. Any surface discontinuity such a
mechanical crack or pit created on the metal surface by crevice corrosion or from
localized attack may act as a stress raiser, and thereby serve as a site for initiation of
stress corrosion cracking [10].
16
Table 1.4: Common metal-environment combination leading to stress corrosion cracking [8]
ALLOYS ENVIRONMENT
Carbon steel, moderate strength Caustic; nitrates; carbonates; bicarbonates;
anhydrous liquid NH3; moist H2S
Carbon steels, high strength
Natural waters; distilled water; aerated
solutions Cl-, NO3-; SO42-; PO43-; OH-;
liquid NH3; many organic compounds
Stainless steels Chlorides, caustic; water + Oxygen
Nickel alloys
Hot caustic; molten chlorides; high
temperature water and steam contaminated
with O2, Pb, Cl-, F-, or H2S
Copper alloys Ammonia; fumes from HNO3; SO2 in air +
water vapour; mercury
Aluminum alloys
Aqueous solutions especially with halogen
ions; water; water vapour; N2O4; HNO3;
oils; alcohols; CCl4; mercury
Titanium alloys
Red fuming HNO3; dilute HCl or H2S4;
methanol and ethanol, chlorinated or
bicarbonated hydrocarbons; molten salt;
Cl2; H2; HCl gas
Zirconium alloys
Organic liquids with halides; aqueous
halide solutions; hot and fused salts;
halogen vapors
Magnesium alloys Water + oxygen; very dilute salt solutions
Experience has demonstrated that insulation materials, used in chemical plants
and in offshore facilities containing a few parts per million of chloride, give rise to stress
corrosion cracking, especially on stainless steel alloys, when water penetrates the
17
insulation and leaches out the chlorides [8]. The main factors affecting stress corrosion
cracking are temperature, solution corrosive concentration, stress intensity, metal
composition and structure. The incidence of stress corrosion cracking is greater at higher
temperatures and time to failure is shorter [4]. Stress corrosion cracks appear to be the
result of a brittle mechanical fracture, when in reality they are the consequence of
corrosion processes [6].
1.3.5 Hydrogen Damage
Hydrogen damage refers to mechanical damage of a metal that results from the
simultaneous action of hydrogen and residual or applied tensile stress. Hydrogen damage
appears on specific metals and alloys in different ways such as cracking, blistering and
embrittlement [10]. An example of a failed steel pipe due to hydrogen action is shown in
Figure 1.6.
Figure 1.6: Hydrogen damage on a steel pipe [7]
18
Atomic hydrogen is an element that can diffuse inside metals and initiate the
damage. Therefore, hydrogen damage is caused only by the atomic form of hydrogen.
Usually some of the hydrogen atoms form hydrogen gas and escape as gas bubbles, but at
the same time a fraction of the atoms may penetrate into the metal and once inside, they
can form gaseous molecular hydrogen and cause sudden and unexpected failures. Atomic
hydrogen can be produced by corrosion reactions, by high temperatures moist
atmospheres, by electrolysis process or during pouring of the molten metal [3].
Usually hydrogen embrittlement occurs when there is an applied tensile stress and
hydrogen is dissolved in the metal. Actually this type of corrosion is not well understood
and especially hydrogen embrittlement detection is one of the most difficult features of
the problem [15]. One of the best accepted theories that describes hydrogen
embrittlement is that hydrogen atoms disseminate ahead of a fracture tip and affect the
bonding between metal atoms, causing microcracks ahead of the principal crack, and
thereby the fracture will increase under tensile stress that is below the yield strength. [8].
Not all metals and alloys are affected by hydrogen embrittlement. The most susceptible
metallic materials to this type of corrosion are: medium and high strength steels, titanium
alloys and aluminum alloys [15].
Any macroscopic defect in the steel or even a void offers a region for hydrogen
atoms to combine, produce hydrogen gas, and build enough pressure to cause hydrogen
damage. Usually during the corrosion process there is a period of time when any
evidence of damage is appreciable, followed by abrupt and catastrophic failure [4].
1.3.6 Intergranular Corrosion
The microstructure of metallic materials is formed by grains, divided by grain
boundaries. This type of corrosion refers to the preferential attack at and adjacent to grain
19
boundaries, while the grains remain mostly unaltered [15]. Intergranular corrosion can
occur in the absence of stress. Impurities at the grain boundaries of metals is one of the
factors that can cause this type of corrosion. [6]
This class of localized attack is typically associated with the segregation of
specific components or the development of a compound in the grain boundary.
Intergranular corrosion typically manifests itself along a narrow path beside the grain
boundary. In extreme cases, the complete grains may be removed due to total
deterioration of their boundaries and thereby the mechanical properties of the structure
will be seriously affected [16]. Figure 1.7 shows an example of intergranular corrosion of
a fireplug component.
Figure 1.7: Intergranular corrosion in a fireplug component [16]
20
1.3.7 Galvanic Corrosion
Galvanic corrosion or bimetallic corrosion is the most known of all forms of
electrochemical corrosion. When two different metals are placed in contact in a corrosive
or conductive solution, the less corrosion resistant of the metals becomes anodic and will
corrode while the more corrosion resistant metal becomes cathodic and will remain
almost unaffected. This combination of dissimilar metals is known as a bimetallic couple
or galvanic cell [4]. Figure 1.8 shows an example of galvanic corrosion between a carbon
steel pipe and a brass valve.
Figure 1.8: Galvanic corrosion between a carbon steel pipe and a brass valve [11]
The cathode anode area ratio is an important factor in determining how fast the
corrosion process will be in a galvanic cell. The severity of damage in a bimetallic couple
21
is proportional to the total cathodic area exposed to the corrosive solution. In more
common terms, the cathode anode area principle can be described as follows [4]:
Large cathode and small anode = severe corrosion (5) Small cathode and large anode = minor corrosion (6)
Another factor that affects the intensity of galvanic corrosion is the composition and
amount of moisture present in the atmosphere. The corrosion process is more severe in an
offshore atmosphere than in a dry inland atmosphere. Moisture in offshore areas contains
salt and therefore is more corrosive and conductive than moisture in an inland location,
even under the same percentage of humidity and temperature conditions [6].
1.3.8 Selective Leaching
Selective leaching refers to the deterioration of one metal from an alloy by
corrosion processes while the other components remain unaffected [6]. This corrosion
process is a class of galvanic corrosion on a microscopic scale [8]. The most common
example is shown in Figure 1.9 where zinc is leached out of a brass pipe. Usually the
dimensions of the affected area do not change considerably when selective leaching
occurs and corrosion sometimes appears to be superficial [6]
22
Figure 1.9: Removal of zinc from a brass pipe due to selective leaching process [11]
Selective leaching is usually a very slow process that leaves the metal in a
weakened condition where stress corrosion cracking may occur in the presence of tensile
stress [8]. This type of corrosion does not occur with all types of alloys. Selective
leaching represents a very serious problem because of unexpected failures may occur due
to the poor strength of the attacked metal [6].
23
1.4 Scope of the Project
As was mentioned before, corrosion under insulation will continue to persist as world
offshore petroleum activity increases. The offshore exploration and production activities
in the east coast of Canada are not excluded from this fact. This work is focused on the
following objectives:
1- Develop a technical understanding of the problem of corrosion under
insulation on offshore facilities, and recognize the main factors that contribute to the
phenomenon. Additionally this work evaluates whether the natural environment of the
east coast of Canada creates a larger or lesser concern on the occurrence of corrosion
under insulation than that observed in other offshore areas such as the North Sea or the
Gulf of Mexico.
2- Industry practices for the inspection of corrosion under insulation will be reviewed
as well as the evaluation of the integrity of the insulation itself along with the associated
weather barriers such as metal jackets, sealing materials, and coating systems that are
applied to piping and vessels prior to installing insulation. Identification of the inspection
techniques and risk based management approaches that are currently in use, along with a
discussion on their capacity and limitation are also examined.
3- Identify opportunities for new, or for improvements to existing
inspection techniques and risk based management approaches to improve
the detection of corrosion under insulation, to detect deterioration to insulation,
sealing materials, coatings systems applied under insulation, and to appropriately manage
findings such that asset integrity is effectively and efficiently maintained for the planned
life span of the oil and gas offshore structures.
24
2. INSULATION SYSTEMS
The purpose of this chapter is to give a general description of the mechanism of
the insulation systems, the properties of insulation systems, types and forms of insulation
materials and related accessories, design and selection considerations, and failure
mechanisms.
Insulation systems are usually known as materials or combination of materials
that reduce heat transfer from a hot area such as the internal wall of a vessel to a colder
region. The movement of heat can occur in different modes: conduction, radiation,
convection or a combination of these [17]. These heat transfer modes are described in
Section 2.1. The term "thermal insulation" usually applies to insulation systems used on
equipment whose working temperature ranges from -75C to 815C. Insulation materials
that are used on equipment working at temperatures below -75C are termed cryogenic
and those above 815C are termed "refractory" [18].
In the recent years, the insulation industry has developed improved insulation
materials to ensure effective energy conservation. The use of insulation contributes in
reducing the energy requirements of any system. The majority of insulation materials can
reduce at least 90% of the undesired heat transfer as compared to bare surfaces. The
proper selection and the mode of installation of the insulation systems play an important
role in energy management [21].
Based on the purpose for which the insulation materials are used the following
four categories are recognized:
1. Reduction of heat loss: as was mentioned before, the main reason for using insulation
systems is to conserve energy by reducing heat loss or gain of vessels, piping, and
25
equipment. The direct benefit of this reduction is the cost savings in fuel required to meet
the operational or process requirements [20].
The selection of the type of insulation system as well as its optimal thickness for a
specific offshore process or equipment are important factors from the economic stand
point in order to find which will have the best performance in energy conservation over
the planned period of operation of the offshore structure [20].
Usually, for a given set of operating and economic variables there will be just one
insulation system that will cover the desired requirements. One of the main factors that is
considered during the selection of the insulation system for heat loss reduction is the
highest recommended temperature at which the properties of the insulation material will
not be affected. Sealants and caulking systems commonly used to seal gaps that result
from the insulation of irregular sections such as equipment support brackets or to seal end
sections are usually the weakest component in the insulation system [20].
Offshore facilities such as piping and vessels are insulated mostly to conserve
heat. Thermal insulation becomes an important factor for enhancing the product flow
properties, especially in the case of paraffinic crudes or wet gas where the product must
be maintained above the temperature at which paraffin crystals or gas hydrates start to
form and cause difficulties to the product flow [21]. Additional reasons of using
insulation in offshore production platforms are to increase cool down time of products
after shutting down and also to control the operational parameters of the systems [22].
2. Condensation Prevention: Condensation prevention is the second principal reason of
applying insulation systems on pipes and equipments carrying cold fluids after heat gain
prevention [22]. Since the operating temperature of cold systems can be below the dew
point at which moisture in the offshore atmosphere may condense and form an electrolyte
26
film over the metal surface of pipes and equipment, the use of insulation systems provide
the additional benefit of preventing the initiation of corrosion processes.
3. Personnel Protection: In the case of hot systems where energy conservation is not a
consideration, the control of surface temperature is necessary from the stand point of
personnel safety and comfort. Normally any hot surface such a hot pipe or vessel must be
insulated in order to maintain the surface temperature of the insulation below 48 C at
which the skin of a person will not burn [20].
4. Noise Reduction: The last consideration for applying insulation materials is noise
attenuation. In some particular cases it is desired to reduce the noise that may be
generated by equipments or piping systems, mainly for comfort reasons.
In addition to the previous four categories, insulation systems could also provide
additional benefits [17]:
Prevent damage to equipment from exposure to fire or corrosive environments Offer additional structural strength Reduce water vapor diffusion Enhance operating efficiency of heating and cooling systems
2.1 Heat Transfer Properties
Insulation materials are specially designed to reduce the three ways of heat energy
transfer: conduction, convection and radiation. Figure 1.10 shows a schematic
representation of the heat transfer modes. Contrary to what one may think, conduction is
not the only manner of heat propagation that takes place within insulation systems. Most
of insulation materials are porous and hold small pockets of air. Additionally, a thin film
27
of liquid or air may be present between the insulation material and the equipment on
which it is installed. Therefore conduction in not the only way of heat transfer [20].
Figure 2.1: Heat transfer modes [23]
Heat will continue to flow as long as a temperature difference exists between the
equipment to be insulated and the surrounding atmosphere [19]. In this section a brief
description of the various modes by which heat can flow is presented in order to have a
better understanding of the basic principles of heat flow on which insulation systems are
based.
2.1.1 Conduction
Conduction is defined as: the process by which heat flows from a region of
higher temperature to a region of lower temperature within a medium (solid, liquid or
28
gaseous), or between different media in direct physical contact [23]. The principal
process by which heat flows through insulation materials is conduction [20]. The heat is
transferred by molecular contact, where heated molecules vibrate and transmit the energy
to cooler molecules. Gas and solid conduction are the principal factors in insulation
technology [21].
2.1.2 Convection
Convection is the process by which heat flows through liquids or gases. It does
not occur in solids. The heated fluid becomes less dense and therefore will rise and take
the heat energy with it. Colder and heavier fluid will replace the empty space left by the
hot fluid [20]. Convection process is virtually eliminated within porous insulation
materials. The temperature difference within the cells is so small that the convection
process will not occur [19].
2.1.3 Radiation
Radiation is a process by which heat flows from a higher temperature body to a
lower temperature body when the two bodies are not in contact [23]. The heat is
transported by waves similar to radio waves emitted by the hot substance. The energy
transmitted in this way is called radiant heat. Any fluid or solid is able to radiate heat. As
the temperature of the radiating substance increases, the intensity of the emission will
also increase [20].
When radiation waves reach another body, the heat is either absorbed by its cold
surface, is transmitted through or absorbed. One of the methods to control the radiation
process is by inserting absorbers or reflectors within insulation materials. Another factor
that affects radiation is the density of the material. At higher density values the radiation
process is reduced but convection and material costs increase. Therefore it is very
important to understand the different modes of heat transfer during insulation design [19].
29
2.2 Thermal Properties
During the process of design and selection of particular insulating materials there
are four principal thermal properties that have to be taken in consideration in order to
cover the operational and safety requirements of any offshore and onshore processing
system.
In this section, a general description of the four main properties that must be
considered in the selection of an insulating material is presented.
2.2.1 Thermal Conductivity
The efficiency of insulation materials is measured by a property called thermal
conductivity which refers to the ability of a material to conduct heat [19]. It is denoted
with the letter k and is expressed in Watts per metre per degree Celsius (W / m x C).
This property measures the amount of heat that is transmitted in one hour through a
homogeneous material per unit thickness in a direction perpendicular to a surface [20].
The driving force for the flow of heat is the temperature difference between opposite
sides of the insulation material [21]. As the thermal conductivity increases, the heat flow
increases. Therefore this property is very important in selecting insulation systems.
One of the features related to the thermal conductivity is that it changes with
temperature and it is usually published on tables per mean temperature and not related to
operating temperature. The mean temperature is the average temperature of the insulation
and is calculated with the sum of the hot and cold surface temperatures and dividing the
value by two. Another factor that is important to know is that it also changes with time.
Some insulation materials have their cells filled with a special gas that decreases the
thermal conductivity, but usually after manufacture, some percentage of this gas diffuses
out of the insulation and thereby the thermal conductivity increases [19]. In the
30
appendices section there are several tables available of various types of insulation
materials with their thermal conductivity properties as a reference.
2.2.2 Thermal Conductance
Thermal conductance refers to the quantity of heat that is transmitted through a
homogeneous material of an arbitrary thickness [20]. It is denoted by the letter C and
expressed in Watts per metre square per degree Celsius (W / m2 x C). The following
formula is usually used to calculate the conductance of different materials:
C = k (7) t Where:
k = thermal conductivity (W / m x C)
t = Insulation thickness (metre)
2.2.3 Thermal Transmittance
Thermal transmittance is defined as the measure of heat energy transmitted by a
material or assembly including the boundary air films [23]. It refers to the amount of
heat that is transmitted through one square metre of a material. It is denoted with the
letter U and expressed in Watts per metre square per Celsius (W / m2 x C).
2.2.4 Thermal Resistance
Thermal resistance as the name indicates is the resistance of solid materials to the
heat flow. It is denoted with the letter R and expressed in metre degree Celsius per
Watts (m x C / W). The following formula can be used to calculate the thermal
resistance of materials [23]:
31
R = t/ k = 1/C = 1/U (8)
Where:
t = Insulation thickness
k = Thermal conductivity
C = Thermal conductance
U = Thermal transmittance
Heat flow can be reduced by increasing the thermal resistance of the insulation
system. In the case of various materials assembled together in series, the total thermal
resistance of the insulation system will be the sum of all the individual resistances of each
material [19].
2.3 Mechanical and Chemical Properties
In some specific applications, for example, offshore facilities, other properties
beside thermal properties are considered in the selection of an insulation material.
Depending on the characteristics of the geometry of the equipment to be insulated and
also additional factors such as: characteristics of the surrounding environment,
combustibility of the material, compressive strength and chemical composition of the
insulation, the type of insulation system will vary from one particular application to
another.
In this section some of these additional properties and factors are described in
order to explain the complexity in the selection of an insulation system that could cover
all the requirements of a specific system other than energy conservation.
32
2.3.1 Density
Density of the insulation material is an important property for calculating the
loads on the support structures. It also affects other properties such as compressive
strength and thermal conductivity. Sometimes the density of the insulation material will
be related to the ease of installation of the product; therefore for applications where there
is not too much space available to install the insulation system, a flexible and less dense
material may be considered [19].
2.3.2 Moisture Resistance
Insulation systems are most effective when they are dry. In the case of offshore
applications, the moisture resistance or the ability of the insulation material to resist
vapor moisture intrusion is very important in order to achieve the effectiveness of the
insulation and prevent further corrosion problems.
The moisture resistance capacity will vary depending on the type of material and
its cell structure. The quantity of moisture that can be absorbed by an insulation material
will be determined by the internal cell structure of the product. Closed cell insulations,
like cellular glass type, have the capacity to prevent the diffusion of water vapor into the
insulation [19]. However, most of the insulation systems are able to absorb, accumulate
and transmit water or water vapor throughout the insulation. It is common to combine
weather or vapor barriers such as metal jackets or mastics with the insulation material in
order to prevent the ingress of water into the insulation [17].
The moisture resistance effectiveness of insulation materials can be calculated by
measuring the flow of water vapor, also called permeance through the insulation material.
It is measured in perm-inch that refers to the weight of water, in grains, that is
transmitted through a 25 millimetre thickness or one inch of the material in question in
33
one hour and one foot square, having a pressure difference between faces of one inch of
mercury. The higher the value of permeance the higher amount of water vapor that is able
to diffuse into the insulation material [20]. Table 2.1 shows a list of different insulation
materials and their general moisture resistance.
Table 2.1: Moisture resistance property of various insulation materials [24]
Insulation material Permeance (perm-inch)
Cellular glass 0.00
Flexible elastomeric 0.09
Cellular polystyrene 1 to 3
Phenolic 1 to 3 Polyisocyanurate 1 to 3
Polyurethane 1 to 3 Fibrous glass 40 to 110
2.3.3 Compressive Strength
Compressive strength is an important property to be considered in the selection of
an insulating material if the insulation must support a load or will be subjected to
mechanical abuse such as climbing over and foot traffic [18]. Usually this property gives
an idea of how much deformation could occur under specific loads. A common reference
value at which compressive strength is reported and compared is five and ten percent of
deformation [19]. Table 2.2 shows a list of insulation materials and their compressive
strength value at five and 10 percent of deformation.
34
Table 2.2: Compressive strength of different insulation materials [22]
1. CCPUF = Closed cell polyurethane foam 2. OCPUF = Open cell polyurethane foam 3. PIF = Poly-isocyanurate foam 4. VIP = Vacuum Insulation Panels 5. PU = Polyurethane 6 PP = Polypropylene
2.3.4 Temperature Use Range
The expected operating temperature range of a pipe, vessel or any uninsulated
equipment is a very important factor in the selection of an insulating material. For a hot
35
or cold system, the maximum expected temperature will dictate the selection of the
product and the adhesive used to bond the insulation to the equipment and itself [20].
All insulation systems have a recommended temperature range at which the
system is designed to maintain its integrity and capability to perform its function. Usually
the insulation systems experience a physical change when the recommended service
temperature is exceeded. There are industry standards where the temperature range is
specified for every type of insulation material, but frequently the manufacturers provide
their own acceptance service temperature [19]. Table 2.3 shows a comparative list of
generic insulation materials with their recommended service temperature.
Table: 2.3: Recommended thermal temperatures by Du Pont Company [25] Generic Insulation Materials Recommended Service Temperature C
Polystyrene foam -73 to 60
Polyurethane foam rigid -73 to 82
Polyisocyanurate rigid -73 to 149
Flexible foamed elastomer 2 to 82
Cellular glass -129 to 149
Glass fiber 4 to 190 or 454 (depending on type)
Mineral wool 60 to 649 or 982 (depending on type)
Calcium silicate 60 to 649
Perlite silicate 60 to 593
2.3.5 Fireproofing
The contribution of insulation systems used on offshore facilities or other types of
applications to a fire hazard is a very important property to be considered especially
36
where fuels, liquids or other flammable materials are involved in the operational
activities. Offshore facilities are a good example of this case and are always exposed to a
potential fire. Exploration and production activities involve the use, handling and
processing of flammable products such as diesel, condensates or natural gas for power
generation, or oil and gas that is produced from the offshore reservoirs.
Any part of the offshore structure and equipment including their contents may
contribute to fire hazard by sustaining combustion or producing smoke [20]. Usually
insulation systems can be divided into two groups, those that have the ability to withstand
fire exposure or those that have the ability to develop smoke or spread flame [19].
Generally, insulation materials are tested for smoke developed, flame spread, and
fuel contributed. The materials are compared to red oak flooring rated at hundred and
asbestos cement board rated at zero. The accepted value for flame spread is 25 and 50
for smoke developed and fuel contributed. However these values may vary from one
application to another [20].
2.3.6 Sound Attenuation
This property is considered in some applications where sound transmission may
be a problem. Usually in this case, an extra thickness of insulation or special jackets is
used to reduce the sound to an acceptable level [20].
2.3.7 Chemical Neutrality
Insulation materials should not contribute to the deterioration of metal, mainly if
water and moisture diffuse into the insulation. The material should be chemically neutral
or alkaline to prevent corrosion. Figure 2.2 shows the corrosion rate of iron versus pH
levels of aerated water. The red line represents the rates of corrosion under insulation
37
systems. Some insulation materials contain substances that are leached out when they are
wet that may decrease the pH of water and create a very corrosive medium for the
insulated pipe or equipment. Therefore this characteristic should be considered
principally for offshore application where a risk of water intrusion is present.
Figure 2.2: Effect of pH on corrosion rate of iron in aerated water [26]
2.3.8 Other Properties
Additional properties and factors may be considered in the selection of the proper
insulation system. The available form of the insulation material is one of them. Some
insulation materials can fulfill the thermal and other requirements for a particular
application, but they may not be available in a compatible form. The most common
forms of insulation materials are: rigid boards and blocks, flexible sheets and blankets,
pre-formed shapes such as curved segments and halve pipes [19]. Figure 2.3 and 2.4
38
show an example of a typical rigid block insulation used on vessels and a pre-formed
pipe insulation system.
Figure 2.3: Typical vessel insulation using rigid blocks [27]
39
Figure 2.4: Typical pre-formed pipe insulation multilayer construction [27]
Another factor that may dictate the selection of the insulation system is the
capacity of the insulation to be removable and reusable. Some equipment such as valves
and flanges require frequent maintenance and if they are insulated, the insulation material
could lose its insulation capacity if the product is not capable of withstanding the removal
and reinstallation action on a regular basis [21]. Figure 2.5 shows an example of a
removable and reusable insulation system on a valve.
40
Figure 2.5: Removable and reusable insulation system [28]
2.4 Insulation Materials
Nowadays there are a variety of insulation materials available for any type of
application. Some products have been in the market for a long period of time, while
others such as the new type of areogels are relatively new. In the following section, a
general description of the primary insulation materials is presented. In the Appendices
section there are tables available that provide a list of the most common insulation
materials and their properties.
2.4.1 Calcium Silicate
Calcium silicate is a rigid insulation produced from silica and lime and reinforced
with organic and inorganic fibers. This insulation product is known for its excellent
compressive strength property and durability. The recommended service temperature
41
varies from 35C to 815C depending on the manufacturer [18]. However, because it can
absorb nearly 400 % of its weight when immersed in water and in humid conditions 20 to
25% by weight water; most manufacturers recommend a lower temperature limit of about
150C for outdoor applications [27].
This type of material when wetted has a pH between 9 and 10. Some coatings that
are applied on the surface of metals before the insulation such as inorganic zinc may be
affected with high pH solutions [27].
2.4.2 Expanded Perlite
This product is made from perlite mineral that during its manufacturing process is
expanded and combined with sodium silicates as binders. It has a maximum
recommended service temperature of 593 C. At higher temperature values, it starts to
shrink very fast [25]. Its physical structure is based on small air cells surrounded by
vitrified product. This insulation material resists moisture penetration due to the addition
of water resistance additives, is non-combustible, and comes in sheets and rigid pre -
formed shapes [18].
Expanded perlite starts losing its water resistance property at temperatures around
315C, because some additives burn out and water absorption increases [27].
2.4.3 Glass and Mineral Fibers
Fibrous mineral and glass products are available in a variety of forms such as
rigid and semi-rigid boards, flexible blankets or semicircular sections for pipe insulation.
They are produced from the molten state of rocks, slag or glass that is converted into a
fibrous form with the combination of organic heat resistant binders [27].
42
Fiberglass is the most popular insulation material, having a bulk density that
ranges from 24 to 96 kg/m3 depending upon the manufacturer, has a poor compressive
strength property, a thermal conduction (k) value between 0.22 to 0.26 W / m x C and
a thermal resistance (R) value between 3.8 to 4.5 m x C / W . Service temperatures
range from 1.5C to 422C. The binder systems employed during the manufacturing
process are the important factor that dictates the highest temperature at which it can be
used [20]. Some binders get damaged in the presence of water combined with high
temperatures where the resulting solution could act as a triggering factor for a corrosion
process [27].
Fibrous insulations have the capacity to absorb water and moisture due to their
porous structure. Therefore, weather barriers such as metal jackets are used to prevent the
ingress of water and moisture into the insulation.
2.4.4 Cellular Glass
Cellular glass insulation is composed of pure sealed glass cells. This product
comes in rigid forms such as boards and pre-formed pipe coverings. It is completely
inorganic and has an average compressive resistance value of 690 kPa [19].
This product does not absorb any quantity of moisture or water; has good structural
strength, but is brittle to some extent. It is also resistant to common acids and corrosive
environments and has excellent fire resistant properties [18]. However the thermal
conductivity value is higher compared to other insulation materials, but because of its
special features, this type of insulation material is highly recommended for offshore
applications [19].
43
2.4.5 Polyurethane and Polyisocyanurate Foams
These two types of insulation materials are available in rigid forms and are
commonly used in industrial applications. They have an excellent value of thermal
conductivity that ranges from 0.020 W / m x C to 0.042 W / m x C, but they have poor
fire resistance characteristics, especially the polyurethane foams. Polyisocyanurate
insulations were created to improve the fire resistant properties but they still have not
reached the 25/50 fire hazard classification (25/50 FHC) [19].
Polyurethane and Polyisocyanurate foams do not absorb water as long as their cell
structure is not affected. The recommended service temperatures range from -73C to
149C for Polyisocyanurate foams and from -73C to 82C for polyurethane foams with a
compressive resistance value of 17 kPa at 5 % of deformation [25].
These materials, as well as other insulations contain substances such as chlorides,
fluorides, silicates and sodium ions that when wet, leach out of the insulation and may
produce a low pH solution that accelerates the corrosion process of any insulated metallic
equipment. The pH value could range from 1.7 to 10, but when the value is below 6.0, the
corrosion rate of metals usually increases and special concerns should be given [27].
2.4.6 Elastomeric Foams
Elastomeric foam insulations are a mixture of foamed resins and elastomers that
produce a flexible closed cell material. They are manufactured in a variety of forms
including pre-formed shapes and sheets. The maximum recommended temperature is
around 105C depending upon the manufacturer. This product is commonly used for cold
service systems and does not require vapor barrier protection. The principal disadvantage
of this type of insulation is its smoke generation capacity. [18].
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2.4.7Aerogels
Aerogel insulations were first manufactured in the year 1931, but due to the
complicated manufacturing process, their large scale commercial application was not
possible. They are produced from a polymerization reaction where polysilicic acid creates
a firm structure that during the drying process, the processing water is removed and
replaced with air that is hold in its structural matrix [21].
During the last few years, new technologies have made possible the improvement
of the production process by reducing the drying time and the manufacture of flexible and
thin blankets. The new aerogel product has smaller pores in its structure that reduce the
free diffusion of gas molecules through the insulation and thereby improves its thermal
performance. The product offers the lowest thermal conductivity and does not absorb
moisture due to its hydrophobic property [21].
2.5 Protective Coverings and Finishes
The proper performance of insulation materials depends upon their protection
from mechanical and chemical damage and also from water and moisture ingress. A
variety of jacketing systems and finish materials are produced and applied in conjunction
with insulation materials to ensure the long term performance of the whole insulation
system [18]. In the appendix section, detailed tables are presented with more
characteristics of protective material and accessories.
The following section presents a general description of the additional accessory
materials that are used with the insulation systems.
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2.5.1 Adhesives
For some applications, adhesives materials such as adhesive tapes are used to
secure insulation materials to equipment surfaces. The principal problem that has been
experienced with the use of some adhesives on austenitic stainless steel is that they have
caused stress corrosion cracking. The main reason is that some adhesives are
manufactured with chlorides and other components that when wet are leached out and
produce corrosive solutions that attack the metal surface [27].
2.5.2 Cements
Cements are used to bond insulation materials into the desired shape. Asphaltic
based cements are used for cold systems. Special concern must be given to some cement
materials which contain chlorinated polymers that are intended to be used for insulating
austenitic stainless steels, because they may promote the initiation of corrosion processes
if those polymers are leached out when water ingress into the insulation [27].
2.5.3 Coatings and Mastics
Coatings and mastics are applied over insulation materials to retard the diffusion
of water vapor into the insulation. If they are used without jacketing systems in outdoor
applications, they must be capable of resisting ultraviolet radiation and fire exposure.
Therefore frequent inspection is necessary to maintain the integrity of the insulation
system [18].
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2.5.4 Sealants and Caulks
Sealants and caulks are designed to seal jacket systems, joints and protrusions. A
common cause of water ingress into the insulation is the failure of sealant and caulking
systems [26]. Figure 2.6 shows an example of a typical insulation system with caulking
compound near a pressure gauge attached to the pipe.
Figure 2.6: Typical insulation system where caulking compounds are used [27]
Because caulking and sealant systems are very susceptible to fail due to
mechanical abuse and other factors, frequent monitoring programs are necessary to keep
insulation systems in good condition and prevent the ease of water intrusion [27].
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2.5.5 Jacketing Systems
Jacketing systems, also known as weather or vapor barriers, represent the first line
of defense and protection of insulation systems against mechanical abuse, corrosive
atmospheres, water intrusion and fire exposure. Special consideration should be given to
jacketing materials that are used for mechanical protection of insulation materials with
low compressive strength, because they are very susceptible to physical damage,
allowing water ingress [18].
Jacketing materials that are frequently used include fiberglass reinforced plastic,
stainless steel, aluminum, galvanized steel, tape systems and reinforced fabrics [27]. The
condition of the insulated equipment and the insulation material itself will depend upon
the capacity of jacketing systems to maintain their technical integrity over the planned
lifecycle of the equipment [20]. Figure 2.7 shows an example of a vapor barrier applied
over an insulation material
Figure 2.7: Rubberized asphalt vapor barrier membrane on an ammonia system [24]
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2.5.5.1 Aluminum Jackets
Aluminum jackets come in different thicknesses and in corrugated or smooth
shapes. Because they are less costly than stainless steel jacketing, their use is more
common. Aluminum jackets are usually secured with screws, straps or with a patented
seam in a Z or S pattern. Figure 2.8 shows an insulated pipe with aluminum jacket
secured with screws [20]
Figure 2.8: Aluminum jackets secured with screws [29]
Usually a variety of coatings and vapor barriers are applied to aluminum jackets,
especially if the insulation may have some substance that can cause corrosive attack on
the aluminum. For application where the insulated equipment suffers frequent expansions
and contractions, corrugated aluminum jackets are used in order to absorb the physical
changes of the equipment [20].
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2.5.5.2 Stainless Steel Jackets
Stainless steel jackets come in corrugated or flat shapes. The most frequently
available alloys are types 302, 304 and 316. They are available in a variety of thicknesses
and are secured in the same way as aluminum jackets. Since they are more expensive
than aluminum jackets, their use is restricted to special applications such as insulation
systems that are required to be fire resistant [27].
This type of material is susceptible to stress corrosion cracking in contact with
leachable chloride ions presented in insulation materials. Therefore stainless steel
jacketing is usually supplied with a inner coating film to prevent the rapid deterioration of
the metal. In order to prevent the occurrence of galvanic corrosion, stainless steel bands
are used to secure this type of jacket [27].
2.5.5.3 Plastic Jackets
Plastic jackets are available in a variety of materials, such as polyvinyl chlorides
(PVC) and polyvinyl fluorides (PVF). These thermoplastic materials are not often used
for outdoor applications because of their poor resistance to mechanical abuse and
ultraviolet radiation, low melting point and corrosion by different chemicals. These
materials are commonly used for indoor applications [27].
2.5.5.4 All Service Jackets
The all service jacket (ASJ) or all purpose jacket consist of three layers of
different materials that form the complete jacket system. The most common material that
serves as the base of the system is kraft paper that has been coated. A fiberglass cloth is
placed over the kraft paper in order to provide strength to the system. Finally a layer of
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aluminum foil or metalized film is added over the fiberglass cloth. A special adhesive is
used to bond permanently the three materials and provide the desired strength and water
vapor resistant properties [20].
2.6 Insulation Failure Mechanism
The most common failure mechanism of all insulation systems is the one related
to water ingress into insulations. If water in the liquid, solid or vapor state is present in
the insulation, it will cause serious effects on the thermal properties of the insulation
system; it may affect the physical structure of the insulation material and also it may
cause deterioration of the insulated equipment due to corrosion [17].
The hydroscopic properties of insulation materials are very important in the
prevention of water diffusion into the insulation, but in reality there is no ideal insulation
system currently available that will protect against water ingress during its designed
operating life.
Mechanical abuse such as personnel walking on insulated equipment can be
considered as the primary cause of water ingress into insulation systems. Mastics,
sealants, weather and vapor barriers are the critical components of insulation systems that
are more vulnerable to mechanical abuse since they are used and designed to protect and
seal the insulation. As the time passes, ultraviolet radiations, water and chemicals used
for cleaning purposes may also promote the damage and failure of vapor and weather
barriers. Therefore periodic inspections must be performed in order to maintain insulation
systems in a good and dry condition [27].
Sometimes jacketing systems are not properly installed and finished, leaving a
gap between joints and allowing water to bypass the insulation. Figure 2.9 shows a
typical example of a jacket material installed without proper finish [2].
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Figure 2.9: Improper finishing of jacketing system [2]
Unsealed insulation end sections are another example of improper installation of
insulation systems where weather barriers may