Offshore Anodes

Offshore anodes are similar in shape to tank anodes. They are, however, much larger and weigh about 0.5 t. They are predominantly manufactured from aluminum alloys. On the basis of strength, most of them are cast onto pipes or profile iron as supports on which lateral protruding shackles are welded. The cross-section is usually trapezoidal (see Fig. 6-15). 

For pipelines, so-called bracelets are used (see Section 16.6). They consist of groups of plate anodes of more or less large breadth, partly curved, which are secured near to one another on an iron band support like links of a bracelet and laid around pipelines. With the same objective, half-shells are used which are fixed in pairs under stress around the pipe and then welded. 

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Reding, J. T., and Boyce, T. D. (1974). Cathodic protection performance of offshore anodes at a simulated depth of 650 feet. Mater. Performance, 13(9), 37-40. 

The low cost, light weight, and exceUent electrical conductivity of graphite anodes have made this impressed current protection system valuable for cathodic protection of pipelines, storage vessels, process equipment, and also for weU casings both on- and offshore. 

Oxide, hydroxide and basic salts of aluminum are less soluble at pH values of about 7 than those of zinc [17], which explains the easy passivatability. Galvanic anodes of aluminum alloys are primarily employed in the area of offshore technology. The anodes work in relatively pure seawater flowing with a high velocity so that by using suitable alloys, passivation phenomena are rare. Their low weight is particularly favorable in view of a service time of 20 to 30 years. 

Protection with impressed current, with galvanic anodes, and a combination of both processes is used for marine structures and offshore pipelines. Their properties, as well as their advantages and disadvantages, are given in Table 16-1. The protective measures must be optimized for every structure. In the impressed current protection of offshore platforms, for example, the difficulties of maintenance and repair will be of major importance, whereas in harbor installations these problems can be... 

Chemical analyses should be provided for all anodes used in the offshore and harbor area, together with results for current content in A h kg and current output in amperes [2,3]. The geometric shape and the number of anodes required is determined by these parameters. Expensive calculations for design based on grounding resistances are made only in exceptional cases because in practice there are too many uncertainties and the number and mass of the anodes have to be quoted with a corresponding safety factor. 

Sacrificial anode systems operate without external power source. The anodes are reactive metals such as magnesium and zinc or aluminum alloys. The energy for the process is derived from the anode material. Careful design is required to match the output and lifetime of the anodes with the polarization and life-expectancy requirements of the plant. Sacrificial anode CP is used for offshore platforms, sub-sea pipelines and the inside of ballast tanks on tanker ships. 

Fig. 9.23 Welded areas of an offshore platform structural tubular displaying premature corrosion due to (a) lack of surface grinding (resulting in poor paint adhesion and performance) and (b) the fact that the weld metal is anodic relative to the parent material of the tubular...

Technical Note, Fixed Offshore Installations, TNA 702, Fabrication and Installation of Sacrificial Anodes, Det. Norske Veritas (1981)... 

Metallurgical and Inspection Requirements for Cast Sacrificial Anodes for Offshore Applications, RP-0387-87, NACE, Houston (1987)... 

Zinc anodes do not find application at temperatures in excess of 50°C. Zn-Al-Cd alloys suffer intergranular decohesion, and high purity zinc will passivate. Zinc anodes are not predominant in onshore or offshore applications, but they find considerable use under both conditions. 

This calculation should yield a practicable number of anodes, i.e. 10 or 10000 anodes are both clearly unacceptable for the protection of an offshore oil production platform. N X M must be equal to, or greater than, the total weight of anode material, W, required. 

It is essential to ensure adequate current distribution such that all of the exposed structure remains protected particularly important, for example, for the nodes of an offshore steel structure. Similarly, over-protection should be avoided. Thus, sacrificial anodes need to be distributed to ensure that the protection potential over the whole structure is achieved. 

Fig. 10.20 Typical anode shapes and fixing methods, (t ) Offshore stand-off anode (ft)Sland-off anode - types of bowed core (c) Stand-off anode, clamp fixings (d) Typical tank fixing for shipping (e) Bracelet anode assembly...

These anodes, like platinised Ti may be supplied in different forms e.g. rod, tube, mesh, wire, etc. They may be used for the cathodic protection of offshore structures, heat exchangers, or even pipelines as they can be installed in the soil surrounded by carbonaceous backfill, and are comparable in cost to platinised titanium. ... 

Sacrificial anodes Small land based schemes and for avoidance of interaction problems. Marine structures, e.g. offshore platforms High soil/water resistivities and small driving e.m.f. may require a large number of anodes Reasonably uniform Cannot be applied in high-resistivity environments... 

Fig. 10.29 Protection of offshore marine oil-drilling rig. (o) With external zinc (or aluminium) anodes and (b) with impressed current using platinised titanium or platinised niobium. (Compare the large number of anodes used in (a) with the small number used in (b))...

For potential surveys on offshore platforms it is necessary to locate numerous reference electrodes at all levels on the structure. The hard-wire connections from these electrodes together with, for example, similar connections from specially monitored sacrificial anodes are best terminated and displayed at the surface on mimic display monitoring panels. 

Galvanic anode systems are generally used in well-coated electrically isolated structures, offshore structures, ship hulls, hot-spot pipeline protection, heat exchanger water boxes and other environments of resistivity below 10000 Q cm. 

Cathodic protection has many applications, e.g. in refineries, power stations, gas, water, and oil utilities on marine structures, e.g. jetties, piers, locks, offshore platforms, pipelines, ships hulls, etc. and on land structures, e.g. buried pipeline, storage tanks, cables, etc. For each use, the cathodic protection system requires careful design, either impressed current, sacrificial anodes, or a combination of both may be chosen. There may also be other protection systems, e.g. paint, the nature of which will affect the design parameters and must be taken into consideration. 

Sacrificial anodes are most commonly employed to protect the hulls of ships and smaller boats, for offshore oil rigs and underground... 

Direct current is arranged to flow ont from the impressed anodes into the surrounding electrolyte, which is the sea water for offshore structures or the damp ground for onshore structures. The current returns through the structure itself and then back to the negative terminal of the impressed current source. The direction of current as described prevents the loss of metal from the structure into the electrolyte. This is opposite in direction to the natural current present due to corrosion action. 

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