Extending the Life of Cathodic Protection Systems for Offshore Pipelines. Some Recently Applied New Technologies.
by Jim Britton and Dick Baxter (2008)
This paper discusses the practicalities of installing replacement cathodic protection to ageing offshore pipelines using diver assisted and diver-less systems. Current technology for condition assessment and life extension design criteria will be discussed in conjunction with new hardware technologies being utilized to improve reliability and reduce cost of life extension projects. The paper will also present some leading edge technology designed to reduce life cycle inspection costs which has been applied to pipeline systems in the USA Gulf of Mexico and Atlantic areas.
In essence we are really talking about technologies to facilitate installation of cathodic protection systems subsequent to installation of the steel asset. This is how virtually all onshore systems have always been deployed, first install the pipeline and then install the cathodic protection. Offshore, the trend toward pre-installation (bracelet anodes) was driven by installation methods and economics. When considering a life extension the drivers are the same but the challenges completely different.
Having made the decision that a life extension is required, the key questions when planning an offshore pipeline project of this type are:
“How far apart can I install the new anodes and how much time will I need to spend at each location?”
The answers to these two questions allow the cost of the project to be estimated, the installation is normally at least 75% of the project cost.
“Will I need to use divers or can it be done with an ROV (Remotely Operated Vehicle)?”
This question will normally determined by the local conditions around the subject pipeline, mainly the water depth and extent of burial. From the above we get to the key elements for the CP engineer to consider.
a. How to confidently maximize the spacing between retrofit current sources.
b. How to have the anode system provide the required CP current which will cover this optimized spacing.
c. Ensuring that the installation procedure and hardware design is fit for the subsea conditions and the resources available.
d. How to minimize time for installation of the anode array and connection to the pipeline.
Codes / Standards - The most widely used design guidelines (ISO 15589-2: 2004, DNV RP F103) do not specifically address pipeline retrofits. There is than an erroneous reference in the ISO document (section 5.2.5) which suggests “impressed-current systems may be preferred as a retrofit system on pipelines with galvanic anode failures, excessive anode consumption, operation beyond original design life or excessive coating deterioration."
This would lead one to believe that impressed current will “throw” further than galvanic anodes, and thus allow greater distances to be covered. This simply is not the case; the limiting factor is the pipeline linear resistance and resultant attenuation of potential as a function of current. This essentially limits the amount of current that can be drained from the pipeline at any single attachment point (without causing over-protection issues); this current limit does not consider the source of the current. The impressed system does allow better control of the drain point potential, but will not allow significantly more pipe to be protected.
For this reason, impressed current is typically only used if it is already in place, and only to protect the first couple of miles of pipe. Virtually all offshore pipeline life extensions have been completed using sacrificial anodes.
Our Design Philosophy - Neither of the referenced codes offers any real guidance as to how to design a retrofit cathodic protection system, what current density to use, how to take into account the effects of the existing anodes on the pipeline, etc. Both standards list coating breakdown factors for limited numbers of coating systems. The ISO standard being the more conservative of the two. In the absence of specific survey data, or instructions from the client, this is the number that we typically use (more info).
For this reason we have elected to take a conservative approach and design the pipeline life extension system assuming no contribution from the existing anodes. Indeed we assume that all the existing anode bracelets have depleted, leaving connected anode steel core structure as additional coating defects on the pipeline. The coating breakdown factors we apply are based on regional experience from direct assessments, pipeline surveys, and previous retrofit projects, this being in accordance with the code recommendations. Average current density is estimated as (maintenance or mean) level for calculation of potential attenuation.
Anode Array Design Criteria - Over the last several years we have developed a series of standard retrofit arrays that have been attached to pipelines, the three most commonly used are shown graphically [Figs. 1 - 3].