Materials for Seawater Pipeline Systems
Introduction
A range of materials is available for the construction of piping systems conveying seawater to machinery and plant installations. The purpose of the present paper is to review the technical and economic advantages and disadvantages of the different types of materials for the various applications concerned.
Applications
The main applications to be considered are seawater intakes and distribution systems for:
• Sea-going and coastal vessels of all types
• Offshore oil and gas platform installations
• Desalination plants producing fresh water from seawater
• Coastal petroleum and petrochemical processing plants
• Coastal electricity generating stations.
Material Requirements
The factors that are relevant in choosing a material for such applications are:
• Resistance to corrosion by seawater over a wide range of operating conditions
• Resistance to corrosion by the external environment
• Resistance to marine biofouling
• Permissible water velocities
• The physical and mechanical properties of the material
• Ability to cut, machine, bend and perform other fabricating operations
• Availability of suitable jointing techniques and of NDT methods to confirm the quality and serviceability of joints
• Availability of comprehensive ranges of components to enable complete systems to be assembled, including compatible pumps, valves, heat exchangers, etc
• Existence of adequate and reliable supplies of pipes and components and free availability of raw materials for their fabrication
• Initial cost of pipe and components and costs of fabricating and installing systems
• Life expectancy and the value of scrap when the system is dismantled
• Demonstrable reliability based on adequate service experience
• Ability to withstand hazards during construction and service, eg. mechanical damage, fire.
In considering behaviour in seawater, account has to be taken of many factors including:
• Rate of general and/or localised corrosion under steady state flow conditions
• Possibility of crevice corrosion and of deposit attack or pitting, particularly under stagnant or slowly moving conditions
• Resistance to stress corrosion cracking
• Effect of variations in composition of seawater including salinity, oxygen content, suspended material, pollutants, etc.
• Effect of chlorination of seawater, if practised 4
• Velocity limitations
• Effect of variation of temperature, possible spheres of operation being anywhere from arctic to tropical regions. In some applications hot brine has to be handled.
• Possible galvanic effects between different materials.
It must also be borne in mind that in the marine environment external corrosion of piping systems can be a hazard, e.g. occurrence of crevice corrosion due to ingress of chloride beneath sheathings, laggings, brackets, etc. There have been many recorded cases of piping systems failing prematurely from the outside.
Materials
The main types of material to be considered for seawater piping systems are:
• Copper alloys, particularly the copper-nickel series
• Bare carbon steel
• Galvanised steel
• Carbon steel internally coated or lined (e.g. with paint, bitumen, rubber, cement, mortar)
• Stainless steels
• Plastics or reinforced plastics
• Titanium.
A summary of the relevant properties of these materials is given in Table 1. To make a complete economic assessment of the various competitive materials taking into account all the factors enumerated above is a matter of extreme complexity, verging on the impossible. Adequate data on service conditions may not be available and even if the initial conditions can be specified fairly precisely, they may subsequently change in an unpredictable manner. Estimates of the probability of satisfactory behaviour for the various materials will have to be made. First costs must be balanced against subsequent costs of maintenance, repair and replacement and loss of revenue due to outage. The calculations need to include assumptions about variations in material costs, labour costs, interest rates, inflation, taxation policies, product prices and so on. In some situations the costs of breakdown are much higher than in others. In offshore installations, loss of production could quickly nullify any savings made in the first cost of the installation and the carrying out of repairs could be a matter of considerable difficulty and expense. Because of these complexities heavy reliance must be placed on actual or related prior experience. A change from an established material will not be considered unless there is a great enough economic incentive and a sufficient body of evidence of the reliability of the new material. Acceptable evidence could take the form of long and satisfactory service in related applications. A good example is the widespread change from carbon steel to 90/10 coppernickel alloy for seawater pipelines on offshore platforms, based on the proven satisfactory service behaviour of copper-nickel pipelines in ships and other marine installations
Introduction
A range of materials is available for the construction of piping systems conveying seawater to machinery and plant installations. The purpose of the present paper is to review the technical and economic advantages and disadvantages of the different types of materials for the various applications concerned.
Applications
The main applications to be considered are seawater intakes and distribution systems for:
• Sea-going and coastal vessels of all types
• Offshore oil and gas platform installations
• Desalination plants producing fresh water from seawater
• Coastal petroleum and petrochemical processing plants
• Coastal electricity generating stations.
Material Requirements
The factors that are relevant in choosing a material for such applications are:
• Resistance to corrosion by seawater over a wide range of operating conditions
• Resistance to corrosion by the external environment
• Resistance to marine biofouling
• Permissible water velocities
• The physical and mechanical properties of the material
• Ability to cut, machine, bend and perform other fabricating operations
• Availability of suitable jointing techniques and of NDT methods to confirm the quality and serviceability of joints
• Availability of comprehensive ranges of components to enable complete systems to be assembled, including compatible pumps, valves, heat exchangers, etc
• Existence of adequate and reliable supplies of pipes and components and free availability of raw materials for their fabrication
• Initial cost of pipe and components and costs of fabricating and installing systems
• Life expectancy and the value of scrap when the system is dismantled
• Demonstrable reliability based on adequate service experience
• Ability to withstand hazards during construction and service, eg. mechanical damage, fire.
In considering behaviour in seawater, account has to be taken of many factors including:
• Rate of general and/or localised corrosion under steady state flow conditions
• Possibility of crevice corrosion and of deposit attack or pitting, particularly under stagnant or slowly moving conditions
• Resistance to stress corrosion cracking
• Effect of variations in composition of seawater including salinity, oxygen content, suspended material, pollutants, etc.
• Effect of chlorination of seawater, if practised 4
• Velocity limitations
• Effect of variation of temperature, possible spheres of operation being anywhere from arctic to tropical regions. In some applications hot brine has to be handled.
• Possible galvanic effects between different materials.
It must also be borne in mind that in the marine environment external corrosion of piping systems can be a hazard, e.g. occurrence of crevice corrosion due to ingress of chloride beneath sheathings, laggings, brackets, etc. There have been many recorded cases of piping systems failing prematurely from the outside.
Materials
The main types of material to be considered for seawater piping systems are:
• Copper alloys, particularly the copper-nickel series
• Bare carbon steel
• Galvanised steel
• Carbon steel internally coated or lined (e.g. with paint, bitumen, rubber, cement, mortar)
• Stainless steels
• Plastics or reinforced plastics
• Titanium.
A summary of the relevant properties of these materials is given in Table 1. To make a complete economic assessment of the various competitive materials taking into account all the factors enumerated above is a matter of extreme complexity, verging on the impossible. Adequate data on service conditions may not be available and even if the initial conditions can be specified fairly precisely, they may subsequently change in an unpredictable manner. Estimates of the probability of satisfactory behaviour for the various materials will have to be made. First costs must be balanced against subsequent costs of maintenance, repair and replacement and loss of revenue due to outage. The calculations need to include assumptions about variations in material costs, labour costs, interest rates, inflation, taxation policies, product prices and so on. In some situations the costs of breakdown are much higher than in others. In offshore installations, loss of production could quickly nullify any savings made in the first cost of the installation and the carrying out of repairs could be a matter of considerable difficulty and expense. Because of these complexities heavy reliance must be placed on actual or related prior experience. A change from an established material will not be considered unless there is a great enough economic incentive and a sufficient body of evidence of the reliability of the new material. Acceptable evidence could take the form of long and satisfactory service in related applications. A good example is the widespread change from carbon steel to 90/10 coppernickel alloy for seawater pipelines on offshore platforms, based on the proven satisfactory service behaviour of copper-nickel pipelines in ships and other marine installations
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