File Name: external corrosion and corrosion control of buried water mains .zip
Download this paper. This basic methodology works equally well in deep water as it does in shallow. There are however some very important differences in the way corrosion control systems operate in deep water versus shallow water.
Download this paper. This basic methodology works equally well in deep water as it does in shallow. There are however some very important differences in the way corrosion control systems operate in deep water versus shallow water. There are also other obvious differences in the way the pipelines are built and installed that have an effect on the corrosion control strategy, it is these differences which will be addressed.
To understand how corrosion mechanisms vary in deep water, it is necessary to appreciate how the deepwater environment differs from the shallow water areas. There is a common misconception that steel does not corrode in very deep water. It is true that the corrosion rate is less in deep water, but it is far from being zero.
Calcareous deposits are the result of the cathodic protection polarization process. These deposits are critical in the cathodic protection process as they act like crude coating systems to reduce the current required from the anodes, thus it is possible to provide cathodic protection for very long life with relatively small anodes. At low temperature the deposits form much more slowly, and when formed, are generally less dense than deposits formed under equivalent conditions in shallow water.
This slowed formation requires higher levels of cathodic protection current to sustain anti corrosion protection maintenance current density , and thus, more anodes are needed. This means that the amount of cathodic protection current which could be expected from a conventional anode will be much lower in deep water than in shallow water. So we have a dilemma, more current is required, less is available so we need more anodes, or anodes of a different shape.
Coatings operated by simply providing an oxygen diffusion barrier between the steel and the seawater. From the previous paragraph, the importance of coatings in deep water can be appreciated. The majority of new pipelines are protected with fusion bonded epoxy coating FBE. These pipeline coatings perform equally well in deep or shallow water but are subject to mechanical damage during the pipe lay operation.
When formulating a corrosion control design, it will be necessary to assign a coating breakdown factor, this should be done wisely. None of the design guidelines gives a good recommendation for a number to use, and the most appropriate number will vary depending on the type of coating and the pipeline installation method.
This is a pessimistic number, pipeline survey results on old lines have typically shown that the coatings hold up very well, and the supplemental cathodic protection seems to enhance the long term integrity of the coatings by eliminating corrosion undercutting. This coating offers the benefits of low cost with excellent corrosion protection over a wide range of temperature and conditions.
While TS coatings do provide a measure of cathodic protection, they still require supplemental anodes but the quantity can be significantly reduced. These coatings are still under development for broad-based pipeline applications but expect to see continued progress.
The material was cheap, easy to cast because of its relatively low melting point, it was also easy to alloy. The material is also able to operate in either sea water or seabed sediments. Some operators still favor zinc. In the mid seventies, the use of indium activated aluminum anodes became the norm. The aluminum anodes were lighter, operated more efficiently and were also able to work in seabed sediments. The use of aluminum on platforms was also on the riser mainly due to weight saving, and thus the high demand for aluminum anodes made the price very competitive.
The use of aluminum, activated with indium, carried into the deep water market again because of price and weight. The adjusted chemistry being widely used in the Gulf of Mexico is shown below Table 1.
When procuring anodes is is critical to ensure that the anodes are electrochemically tested at the anticipated service temperature, not ambient seawater. It is also important to monitor the chemical composition of the anodes carefully. It takes a lot less current to get the job done if the pipe is buried, however, the lower conductivity of the seabed sediments versus the seawater mean that the anodes cannot deliver as much current.
In addition, ensure that the design will still be correct if an anode is detached from the pipe, this means that the spacing should be closer than normal. In most conditions, anodes on deepwater pipelines should not exceed feet spacing.
Anodes may be detached through mechanical contact with the pipe lay equipment or coatings may be damaged. On projects employing the reel-lay method, it is not uncommon to attach both halves of the anode bracelet on the top half of the pipe Figure 1 , this minimizes the potential for damage from the stinger. The use of cast in place polyurethane tapers is not uncommon for pipe lay situations where the anodes can be pre-installed, the tapers provide a smooth transition and also anchor the anode grounding wire in place.
Taken from the corrosion control perspective, we always recommend isolation of pipelines under the following circumstances:. Remember that it is much easier to electrically short a pipeline than to electrically isolate it when in service. Having the pipeline isolated also give far more options for troubleshooting and monitoring over the life of the equipment. Some smart pigs are able to detect external corrosion pitting. This is usually too late, as the goal of monitoring is to stop corrosion, not to find it once it has already happened!
Starting with good specification writing and vendor qualification, as well as in plant inspection of coatings and cathodic protection materials. A critical phase is the installation, inspection on the pipe lay vessel is the last chance to catch problems before they become major problems.
The use of stinger mounted sensors  to detect anode detachment of coating damage should be considered, these electrochemical sensors give instant indication of a problem which can usually be remedied just as quickly, thus expensive retrofits can be avoided . The most important inspection is the post lay, usually performed by an ROV to verify that here are no spans or other anomalies, this is the perfect opportunity to verify the operational status of the cathodic protection and coating system.
If this survey proves that all the anodes are attached, activated and adequately protecting the pipe, and that the coating system has not sustained serious damage, then it will not normally be necessary to re-survey the line for over 10 years. The decision to use high or low resolution will depend upon the level of information required.
High-resolution systems will give not only accurate potential profiles of the line but also current density, this is particularly important if a remaining life estimate is required. A typical high-resolution survey plot is shown in Figure 2.
An ROV equipped with high-resolution survey equipment is shown in Figure 3. Low-resolution systems give only potential profiles. These are usually sufficient for post lay inspections. Note that any survey of a pipeline cathodic protection system which utilizes a trailing ground wire, will be erroneous, and could give dangerously optimistic or pessimistic results.
New equipment has recently become available for cathodic protection monitoring on deep water equipment, a self contained 10, ft rated CP measuring system Figure 3 , requires no ROV interface.
This system is useful for checking a few isolated points on a pipeline with minimum operational hassle. Updated model of this probe - Deep C Meter. Expect to see continued improvement and development in the area of thermally sprayed aluminum and zinc coatings, and a greater use of ROV integrated inspection systems. Offshore Magazine - April - J. Winters and A. Sign up for our Corrosion Newsletter using the form below. You can unsubscribe at any time. If multiple papers are published in that period, they will be sent in the same email.
Read more. The Deep Water Environment To understand how corrosion mechanisms vary in deep water, it is necessary to appreciate how the deepwater environment differs from the shallow water areas.
Table 1. Figure 1. Taken from the corrosion control perspective, we always recommend isolation of pipelines under the following circumstances: 1. The pipeline is connected to a bare steel structure i. The pipeline is connected to any structure of foreign ownership. A new pipeline is tied into an old pipeline. The pipeline is to be cathodically protected with impressed current.
The pipeline is operating at elevated temperature. The pipeline is made of a corrosion resistant alloy CRA. Figure 2. Three Electrode Survey Plot. Figure 3. Figure 4. Figure 4a. References 1. Newsletter Subscription First Name :. Last Name :. Email :. Company :.
Related Content. Bracelet Anodes Sacrificial anode bracelets for pipelines Read more.
Corrosion and cathodic protection CP are electrochemical phenomena. Understanding corrosion and CP requires a basic knowledge of chemistry and electrochemistry. Electrochemistry is a branch of chemistry dealing with chemical changes that accompany the passage of an electric current, or a process in which a chemical reaction produces an electric current. Examples of structures where cathodic protection is employed include underground tanks and pipelines; aboveground storage tanks; water tank interiors; ship hulls; ballast tanks; docks; sheet piling; land and water foundation piles; bridge substructures; tube sheets; oil heater treaters; and reinforcing steel in concrete. NACE International is committed to providing CP solutions that reduce costs, maximize asset availability, keep personnel safe, and preserve the environment. Our CP solutions include CP education and certification programs, hundreds of books, ebooks, and CP-focused publications, as well as CP standards that drive the industry. Our resources are designed to serve a wide range of CP industry areas.
Specifically, a Corrosion Control Master Plan has been developed which incorporates use of systematic risk based approach to corrosion control whereby facilities.
The sacrificial metal then corrodes instead of the protected metal. For structures such as long pipelines , where passive galvanic cathodic protection is not adequate, an external DC electrical power source is used to provide sufficient current. Cathodic protection systems protect a wide range of metallic structures in various environments. Common applications are: steel water or fuel pipelines and steel storage tanks such as home water heaters ; steel pier piles ; ship and boat hulls; offshore oil platforms and onshore oil well casings; offshore wind farm foundations and metal reinforcement bars in concrete buildings and structures. Another common application is in galvanized steel , in which a sacrificial coating of zinc on steel parts protects them from rust.
Norsworthy, Richard. As important as cathodic protection is to external corrosion control, it may not be the most important issue facing those concerned with preventing external corrosion. The NACE International SP document clearly states the intent of the document is to provide guidance for effective control of external corrosion. Many seem to ignore the intent of this document and concern themselves only with cathodic protection and related criteria. We must consider the other parts of the document and how to effectively control external corrosion with all possible methods with an understanding of how these methods work together to provide expected performance. SP This standard practice presents procedures and practices for achieving effective control of external corrosion on buried or submerged metallic piping systems.
Not a MyNAP member yet? Register for a free account to start saving and receiving special member only perks. The offshore pipeline industry, since its first ventures into the Gulf of Mexico and the waters off California more than 40 years ago, has steadily improved its operating practices, with new materials, more robust designs, and more efficient techniques for construction, operation, and maintenance. Today it operates with confidence in waters as deep as 1, feet, with near-term plans for depths of 3, feet Salpukas, Technology is being developed for pipelines in much deeper waters, up to perhaps 6, feet.
Buried Water Mains Solving Corrosion Problems at Water Main Breaks. 8. Cathodic Protection Performance Verification. Topics to be.Tongueudeep 23.03.2021 at 10:35
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These have an outer coating of highly insulating material to prevent the ingress of water that will also act as a barrier to cathodic protection currents. If possible an.