Technical paper

1. Does the platform have sufficient spare current capacity to power the system?

2. Is there a place to install the transformer rectifier(s)?

3. What is the distribution of pipelines on and around the structure, is there a risk of interference?

4. Does the structure's geometry lend itself to impressed current cathodic protection? Extremely intricate structures or structures that use very high strength steels may obviate the use of ICCP or make installation unpractical.

5. Are there any safety concerns? Sport divers, fishermen, endangered wildlife etc. could all be adversely affected by, or could adversely affect the system.

6. Are trained personnel available to operate and maintain the system?

System components

In order to compare the performance of various types of systems, let us first look at the system components and address some of the pro's and con's of various materials and methodologies. Let us discern what's good, bad and just downright ugly.

Anode material - Over the years the following anode materials have been used in offshore impressed current systems. When selecting the most appropriate material the various properties can be compared in Table 2.

Table 2

Comparison of Impressed Current Anode Performance

Scrap steel - Used in early systems because it is cheap and readily available. That's about all it has going for it. Unpredictable corrosion patterns, high consumption rate and low operating current density make this a poor selection.

High silicon (14%) Cast Iron - Usually alloyed with 3-5% chromium for seawater application, the anode has good predictable performance in sea water and mud but the low operating current density, bulkiness and relatively fragile nature restrict its use to low current systems.

Graphite - Usually impregnated with either wax or phenolic resin, the material has similar properties to silicon iron, except that it is somewhat less dense. Large cylindrical graphite anodes in the order of 12 in - 18 in diameter and 72 in - 84 in long have enjoyed success in the Cook Inlet when deployed as remote sea bed anodes. Other applications are again restricted to those with low current requirement.

Lead silver - Lead anodes alloyed with either 2% Silver or 1% Silver and1I% Antimony were very popular in the late 1960's and into the early 70's. They offered the advantage of reasonably high operating current density coupled with a fairly low consumption rate. The anode material was particularly popular for ship systems and for close fitted di-electrically shielded anodes (Figure 1). Concerns with the use of lead, coupled with technological improvements in other areas have rendered the material virtually non-existent today.

Magnetite (Fe₃O₄) - Cast magnetite anodes were pioneered largely in Scandinavia in the 1970's [1]. They had good current density and low consumption but suffered from being extremely brittle and difficult to connect to, electrically. These anodes are not commonly in use today.

Platinized titanium - Platinized anodes offer high current densities coupled with extremely low wear rates. [2] [3]. Titanium is a good substrate material being relatively readily available and reasonably priced. One major drawback however was the applied voltage limitations [4]. The rutile (TiO₂) film was susceptible to breakdown at applied DC voltage in excess of 12 Volts. This could result in rapid substrate corrosion. Because the most available form of the material was coated small diameter rods, this 12 Volt limitation restricted the use of the product in high current applications offshore, but the material was used extensively in inshore applications.

Platinized niobium - The use of Columbium (Niobium) as a substrate material offered performance advantages over Titanium in that the operating current density was much higher and the voltage restriction was, for all practical purposes, eliminated. Platinized niobium anodes could happily operate at surface voltages in excess of 50 Volts [5]. The Platinum was applied to the substrate in several ways; cladding was a popular method [6]. Other systems used a small platinum wire spiraled around and stitch welded to the substrate. The most popular anode material for offshore use was an evenly deposited coating [5]. The material did exhibit one weakness that resulted in a failure: the coated material was subject to accelerated corrosion rates if excessive AC ripple was imposed on the DC waveform [7]. Once this phenomenon was understood it was possible to modify the power supplies to filter the rectified supplies. The material was used in a number of North Sea retrofits [8] and on at least one new system [9].

Mixed metal oxide - As the name would suggest, these anodes are a mixture of metal oxides deposited onto a titanium substrate. Developed in Italy for the chlor-alkali industry, they became popular for cathodic protection in the late 1970's. The material offeres advantages over both types of platinized anodes, not the least of which was price. The anodes can operate at high current density and are available in a variety of shapes. Most modem offshore retrofit systems utilize this anode material. [10] [11].

Anode shape / geometry - Offshore systems will normally be required to operate at high current density. Anode shapes which concentrate current discharge will generally not be as reliable for long term use as those which promote even surface current density distribution. Of course the point at which the current is introduced into the anode will also be a factor. There are only two basic configurations, cylindrical shapes (rods, tubes or wires) and flat shapes (plates, expanded meshes). Generally speaking the cylindrical shapes more evenly distribute current, especially if they are center connected and if "end effect" is suppressed. This is accomplished by placing a flat dielectric plane normal to the end of the cylinder (Figure 2). The plates and meshes tend to suffer more from edge and end effects that can lead to premature coating failure on some types of anodes, when operating at a high current density.

Cable to anode connection - This is a critical area in the design; the point at which the DC current is transferred from the supply cable to the anode element. Any degradation that exposes a copper conductor to the seawater or mud environment will lead to rapid failure of the anode system. Common errors here are trying to use "onshore" splicing and joining techniques, which do not work for deep submerged application, and also inappropriate material selection for encapsulations.

Anode support structure - Different types of systems have various specific support requirements, however the following general guidelines should help a designer avoid any major pitfalls. Ensure that the support structure is capable of withstanding offshore deployment and installation. If it is not practical to tie the structure into the platform (remote sleds for example), ensure that the structure will not corrode and fail. Either provide independent cathodic protection to the structure or fabricate it using corrosion resistant materials. To help ensure that the structure will not be subject to interference, place it in the high local field gradient around the anodes. Try to avoid asymmetrical designs or placement of the support elements directly between the anode and the protected structure. If the anode support structure is not rigid, ensure that there are no areas that will fatigue or abrade, to avoid possible fretting failure. Ensure that the support structure does not entrap evolved chlorine gas. This could result in localized pH values below 1, which could have adverse consequences on materials in the area.

Dielectric shields - Some system designs that are close-fitted to the platform require dielectric shielding to promote current spread and to avoid localized over-protection of certain parts of the structure. Be careful to esure that shields are of sufficient size to achieve the desired result. The material should be chlorine and seawater resistant; it must also have the required dielectric properties. Care should be taken to ensure that the shield does not create a crevice and that the area behind the shield will be completely sealed or else will receive adequate cathodic protection. Platform applications will require that the shield be monolithic, therefore it must be assured that the shield is adequately supported and will not be subject to undue flexing, which could cause cracking. "Painted on" type shields will not normally be used for retrofit applications.

Positive anode cable - The main supply cable to the anode or anode array is probably the most common location for an impressed current cathodic protection system failure. Designers have a tendency to try to cut new-comers here, because the cable represents such a large portion of the hardware cost. The importance of correct cable selection cannot be overemphasized. When designing a cable construction the following points have to be considered:

Conductor - Select a stranded copper conductor of sufficient size to accommodate the high currents that will not impart too much IR loss. Consider providing a completely redundant cable for designs which have cable routed across the seabed. 19-strand construction is generally preferred for cables that lack a conduit, because a heavy armor will be required. Standard multi-conductor cables are often viable; the conductors can simply be made common.

Insulation - A 600 Volt grade insulation is sufficient. However care should be taken to ensure that the material will be compatible with the proposed jointing / splicing method. PVC, Polyurethane (PU) or Neoprene (PCP) are normally good candidates.

Bedding - The bedding material and configuration must compress over and protect the primary insulation. It must also fill the cross sectional voids in the construction. Keep in mind that it forms the "bed" for the armor layers.

Armor - It is strongly recommended that all exposed cables are armored using hard steel wires. These constructions have consistently provided the best long term mechanical reliability offshore. For cable layed directly on the seabed, double wire armoring is recommended. The armor wires must be contra-helical. This improves wire handling and provides good conductor support for the long vertical runs that are to be expected in these applications. Basket-weave-type armors are of little use. Most standard designs use galvanized improved plow steel (GIPS) for armor wire construction.

Jacket - The jacket is the first line of defense for cable integrity. Materials should be chemically inert in seawater, chlorinated water and UV stable. The jacket must have excellent abrasion and impact resistance and should not propagate notches or cuts. High density cross-linked polymers are generally favored; cross linked high density polyethylene is a good selection in most cases.

Cable joining - Some system designs require that a number of anodes be connected to a single header cable. This requires more joints that will probably be subsea. A lot of the same rules apply as for anode to cable joining. In some cases a better option may be to use oil filled, pressure compensated enclosures to join the cables. If correctly designed, these systems can be made virtually fool proof. An example is shown in Figure 3.

Cable support / routing - In those cases where a system failur occurs as a reult of cable failure, virtually all can be attributed to poor support or cable routing. The most common failures occur because of inadequate subsea support, whcih leads to either fatigue or fretting/abrasion failure. This is a particular risk in the splash zone, where the wave forces are amplified and the number of wave cycles in a 10-year period could well exceed 15 million. No cable movement should be permitted in this area.