Cathodic Protection Design in Deep Water: Be Safe, Not Sorry!
by Jim Britton (1999)
Offshore structures in deep water are now quite common. However, a deep water development project still requires significant capital investment on the part of the operator. Corrosion failure is not acceptable. This paper presents a common sense approach to cathodic protection design on deep water projects. Some practical tips for avoiding pitfalls are presented as well as an analysis of the role of ROV's in maintenance and monitoring of these assets.
The worldwide development of deep water oil and gas prospects has resulted in the emergence of many alternative schemes and structures designed to produce oil and gas more cost effectively. Early developments used large fixed jacket structures such as the Shell Cognac and Bullwinkle platforms, which are located in water depths of 1000 feet and 1350 feet respectively in the Gulf of Mexico. As drilling moved to even greater depths, these fixed structures were no longer cost effective, and new structure designs emerged: Tension Leg Platforms (TLP's), Floating Production Systems (FPS's), SPAR designs. All of these structures have process facilities located on the surface, above subsea wellheads. The completion of remote subsea wells connected by flow lines and control umbilicals to a surface production facility is now the most common design, being a very cost effective method for developing smaller reservoirs. These remote wells are often several miles from the production facility, and it is not uncommon for several subsea wells to be connected to a single subsea manifold structure, which is in turn connected by pipeline to a production or storage facility on the surface. Cathodic protection (CP) for these wells, manifolds, flow lines, jumpers and umbilicals is the main focus of this paper.
Understanding Deep Water - There are three major factors impacting cathodic protection design in deep water. It is important to understand these factors and how they work together to drive CP designs in this environment:
1. Water Temperature - As depth increases, water temperature decreases, and temperature has a direct affect on water resistivity. By affecting the solubility of nutrient salts, lower temperatures change the composition and morphology of calcareous deposits formed at the cathode.
2. Water Resistivity - The increase of water resistivity raises the anode to seawater resistance, and this increase in resistance decreases current output from fixed anodes. Water resistivity is therefore a major factor in sizing anodes (rather than using stock sizes) to meet the desired weigh- to-current ratios.
3. Calcareous Deposits - These deposits are formed on the cathode surface as a result of electrochemical reactions associated with cathodic protection. This phenomenon is the major contributing factor to why cathodic protection systems work in seawater. The calcareous deposit, acting as a barrier coating, dramatically reduces the current density required for cathodic protection to occur. Calcareous deposits form much slower in cold water, and in general are less dense than deposits formed in warmer waters. Less dense deposits require a higher current density to maintain required cathodic polarization.
In summary, designing deep-water cathodic protection systems requires the following basic design criteria modification:
1. Use higher initial (polarization) and maintenance current density values than would be used in shallow warm water.
2. Use higher seawater resistivity values when computing anode resistance/current output.
3. Use coatings to reduce cathodic protection current requirements.
Cathodic Protection Design (basic considerations)
Design Conservatively - If "conservative" is defined as "being within sensible limits," then we must design deep water cathodic protection systems very conservatively. The cost associated with deep-water oil and gas projects is measured in tens, or even hundreds of millions of dollars. Costs associated with in-situ repairs quickly escalate when there is a problem. For this reason, corrosion failures are not an option. Cathodic protection systems for these pieces of equipment have relative costs in the tens of thousands of dollars. Cathodic protection is relatively cheap; use it wisely. Try to get it right the first time.
Use Coatings - As previously stated, cathodic protection design in cold water requires more cathodic protection current per unit area, and in cold water a standard anode geometry will make less current available. It quickly becomes obvious that covering up some surface area with coatings makes a CP design much more manageable. Coatings also provide additional benefits such as increased in-situ visibility, and corrosion protection during onshore fabrication and storage.
Coating Efficiency - What is the appropriate coating breakdown-factor to use? This question has been debated among corrosion engineers for decades. Published guidelines vary in their advice on this subject. One prominent code considers some coatings completely worthless after 20 years . The same guideline later provides this statement: "Operator's experience of a specific paint coating system may justify the use of less conservative coating breakdown factors than specified in this document." Clearly conservative common sense must rule. For pipelines, it is common practice to assign a coating breakdown factor of 5% over the design life. For coatings on seawater immersed surfaces near the bottom, we would recommend increasing this numbers to 10-15% initial damage and 20-25% over the design life. This will require more anodes, but remember: be safe, not sorry.
Quality Control - It is critical that sacrificial anodes work as designed. Anodes which fail to activate, or which perform with significantly reduced galvanic efficiency could mean that an anode retrofit will be required prematurely. Deep water anode retrofit projects are not impossible, but they are usually very expensive. Anode performance can be guaranteed if the following quality control guidelines are observed:
1. Write a clearly defined specification with special emphasis on electrochemical potential and efficiency testing. Any serious deficiencies in chemical composition will be exposed during these tests. A helpful document has recently been revised by a NACE T7-L technical committee .
2. Use an anode alloy with a proven track record. The Al-In-Zn-Si anode alloys are preferable because their behavior in mud environments is more predictable, and because there are less environmental concerns than with Mercury (Hg) or Tin (Sn) activated materials. Aluminum (Al-In-Zn-Si) anodes are more efficient and lighter than zinc anodes and have the highest available open circuit potential.
3. Ensure that the anode specifications require that the testing be performed at the minimum anticipated service temperature.
4. Specify an anode chemistry that will work in cold water and cold saline mud. A suggested modification of an ambient temperature chemical composition versus one intended for cold water is presented in Table 1. This will result in fewer rejects at the testing phase and will reduce the risk of anodes not activating when installed.
5. Ex-pect what you In-spect. Use qualified third party inspectors to assure anode quality.