Monitoring Pipeline Cathodic Protection Systems

Fig. 3-1 Computer-aided data storage system for monitoring the cathodic protection of a long-distance pipeline.

A large percentage of mains (57%) and services (46%) are metallic systems (steel/cast iron/copper), and corrosion is a major problem. For distribution pipe, external corrosion is the primary problem and internal corrosion has also been observed in some instances. The methods of corrosion monitoring on cathodically protected piping are similar to the methods used in the case of transmission pipelines... 

The coupons have been used to monitor cathodic protection systems in Europe since 1960 [70]. Most of these coupons were installed since 1975. Coupons are used in the form of steel electrodes of a strictly determined shape and surface area, protected by cathodic protection together with the structure. The schematic of the coupon installation on a pipeline is shown in Fig. 15.10. 

The successful commissioning and effective use of a cathodic protection system is usually monitored by measuring the spatial distribution of the cathode potential on the structure. This may be achieved via regular ground stations in the case of a buried pipeline (Fig. 10.30) or a portable, heavy-duty reference electrodc/digital voltmeter probe (Fig. 10.31) in the case of offshore rigs and platforms. 

Monitoring cathodic protection systems and corrosion damage to pipelines (and other structures) under their influence is a highly specialized subject. This material is therefore presented separately from the general corrosion monitoring chapter, although readers should be able to identify some overlap in basic concepts. 

A large percentage (57%) of mains and services (46%) is metal (steel, cast iron or copper), and corrosion is a major issue. For distribution pipe, external corrosion is of primary importance, although internal corrosion has been noted in some cases. The methods of monitoring corrosion on cathodically protected pipe are similar to those in the transmission pipeline sector, including pipe-to-soil potential and coating surveys. One difference is that in distribution systems, leak detection is an acceptable method of monitoring for these pipelines without CP (nearly 15% of the steel mains). 

The effectiveness of cathodic protection can be accomphshed by monitoring the pipeline potential using the close interval potential survey (CIPS) method or by using direct current voltage gradient (DCVG) or IR coupon techniques. Also, physical and electrochemical methods can be used to estimate the corrosion rates of a cathodicaUy protected system. 

Underground pipelines are essential requirement of any developed country. Transportation of crude oil, gas, and many hydrocarbons are carried out through underground CCPs, as they are one of the safest modes of transportation, causing minimum disturbance to mankind. Since the material of construction of pipelines is steel, which is prone to severe corrosion, both from inside and outside, safety of pipelines depends upon a foolproof system of corrosion protection. All three major techniques of corrosion protection, namely, coatings, inhibitors, and cathodic protection, are used to protect pipelines. Protection of pipelines alone is not enough it is also important to monitor... 

Figure 9.14 Cathodic protection with an impressed current, (a) Leadjsilver anode for the protection of a ship s hull. Note that the anode is insulated from the hull and surrounded by a non-conducting shield to improve the potential distribution over the hull The anodes will be placed at intervals around the hull (b) Anode cans for the protection of an underground pipeline (some distance away). The anode cans contain a Fe/Si rod surrounded by coke breeze (to increase anode area), (c) Monitor and controlled power supply for a ship s corrosion protection system. Photographs supplied by Corrintec UK Ltd.

Recognize corrosion problems in materials used at the site and make monitoring a normal part of the operation. Sour oil and gas operations are often conducted under high pressure and corrosive conditions. Therefore, in addition to temperature and pressure considerations, system designs for the wellhead, downhole equipment, and pipeUnes must have features to minimize the effects of corrosion and prevent an accidental release of H2S. Corrosion-inhibiting fluids can be used to prevent internal corrosion and cathodic protection can be used to prevent external corrosion. Also, during extended periods of shut-in and injection into pipelines, inhibitor applications may be beneficial. 

In the case of important underground (main pipelines, tanks) and underwater (drilling rigs) structures, monitoring systems of cathodic protection installations are being more widely used. The main reason for introducing these systems is to enable... 

In the discussion of cathodic protection monitoring, two important distinct areas can be identified. The first domain lies in monitoring the condition and performance of the CP system hardware. Monitoring of rectifier output, pipe-to-soil potential and current measurements at buried sacrificial anodes, inspection of bonds, fuses, insulators, test posts, and permanent reference electrodes are relevant to this area. The second domain concerns the condition of the pipeline (or buried structure) itself and largely deals with surveys along the length of the pipeline to assess its condition and to identify high corrosion-risk areas. 

Corrosion coupons. Corrosion coupons connected to cathodically protected structures are finding increasing application for performance monitoring of the CP system. Essentially these coupons, installed uncoated, represent a defect simulation on the pipeline under controlled conditions. These coupons can be connected to the pipeline via a test post outlet, facilitating a number of measurements such as potential and current flow. 

Wherever possible, there should be a continuous secondary barrier for the entire pipeline system, in the form of jacket pipes, collecting basins, and channels. In some cases, this is not possible because it would impede cathodic corrosion protection or because differential expansion of the pipe and outer jacket threaten the systan. The secondary barrier can then only consist of intensive organizational and operative backup measures shorter inspection periods, shorter distances between shutoff points to limit possible losses, more elaborate provisions for leak detection, continual ronote monitoring of the pipeline systan, both internally and externally, equipment for tackling anergency events and catastrophes, contingency plans, and so forth. 

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