Preventing Damage to Corrosion Protection During Pipelaying
by Jim Britton (1996) from Offshore Magazine
Corrosion control for offshore pipelines is typically achieved with a combination of pipe coatings supported wilh cathodic protection to cover coating holidays or defects. The nature of offshore pipelines dictates that the most cost effective and reliable method of providing the cathodic protection component is the use of zinc or aluminum alloy bracelet anodes. This method has served the pipeline industry well for many years.
Larger diameter pipelines require a stabilizing weight coating, usually concrete, installed over the thin film corrosion coating. This weight coating provides mechanical protection to the corrosion coating as well as allowing the bracelet anodes to be flush with or even below the outer profile of the pipe. Thus. damage from contact with the pipelay stinger or burial sled to the anodes or coating is minimized.
As the oil and gas industry moves into deeper water, the trend is toward smaller diameter heavy-wall pipe with no weight coating. The smaller the diameter, the greater the risk that an anode can be hooked up on the stinger or in the trenching plow. If an anode is snagged, there is a risk that it will slide along the pipeline and damage the coating or even gouge the pipeline.
Since most of the possible damage will occur on the pipeline1s journey over the stinger, the anodes have a better survival rate if they are tapered. In that way, they would better traverse the rollers and straighteners on the lay barge.
This anode alteration does not solve the problem entirely, but it does help. Several operators have reduced the risk of a snag by top mounting the anodes on the pipe. Such a position helps passage through the stinger, but would not necessarily help the pipe through a trenching plow or jetting sled.
An additional concern is that the pipe usually will rotate on its axis between the point of exit from the stinger until seabed touchdown. In several documented cases, this rotation has resulted in burial in the mud of all the anodes over a long section of line, leaving the upper half of the pipeline exposed to seawater. The anode's electrical resistance will be significantly increased by this mud bllrial and consequently the current delivered by the anode is reduced.