Truss Section - Most recent designs utilize a truss section attached to the base of the main hull to support the bottom main ballast tank at the base of the SPAR structure. The truss is typically 300-325 ft. (91-100 m) long. Construction is standard tubular steel members with periodic heave plates, which provide guides for the riser systems. The truss section is normally left uncoated and fitted with conventional sacrificial anodes. Design criteria and monitoring are the same as for a conventional offshore structure in equivalent water depth.
Main Hull Tanks - The main hull will contain a number of internal compartments. Some of these are void tanks that contain only air and are never flooded. Other tanks are variable ballast tanks which will see raw sea-water some of the time and will be largely dry for the majority of the life. Being a part of the hull, these compartments are fabricated from basic grades of carbon steeL It is not uncommon for these tanks, particularly the variable ballast tanks, to be heavily baffled and reinforced thus creating a large number of shielded compartments.
Void Tanks - These tanks are usually vented to atmosphere and thus cannot be fully deoxygenated. They are often susceptible to condensation on the inner walls. Access is possible through man-ways and periodic visual and non-destructive examination is always a part of the required in-service inspection program. Access to effect coating repairs during operation is possible but economically undesirable. Two basic approaches may be adopted; economics will usually determine the most desirable approach.
1. Coating as the main control possibly supplemented by vapor phase control as a backup method. Where allowed, coal tar epoxies fill the role well being reasonably surface preparation tolerant. Other two or three coat marine epoxy systems will provide equivalent protection. Vapor phase inhibitors should contain soluble inhibitors to inhibit any condensed water and control corrosion at coating defects.
Remember that these tanks will require some non-destructive testing, often to gauge hull thickness on outer walls, perform weld inspection on critical joints (particularly deck connections) that are only accessible from inside the tank. While advanced NDT techniques such as ACFM, Pulsed Eddy Current and some advanced Ultrasonic methods allow evaluation through in-situ coating; it may be worth leaving some controlled bare spots as back-up test locations.
2. Dehumidification can be a very effective method of void space corrosion control. It is simplicity itself; no water = no corrosion. Vapor phase control as a secondary method is a smart strategy should the dehumidification system not be 100% for any reason. This method offers the lowest cost in many cases; it may be necessary to include serial weight loss coupon arrays at strategic in-tank locations as a method to verify level of corrosion control attained over the life cycle. Coupon systems should be retrievable without tank entry; this allows regular retrievals, between entry inspections, to closely monitor corrosion rates.
Variable Ballast Tanks (Hard Tanks) - Coatings and sacrificial anode cathodic protection are required in most cases. It is important to locate anodes in all compartments and distribute them preferentially lower in the tank to be commensurate with expected life cycle surface wetting times. Maintenance current densities for bare steel in these environments should be 3 mA/sq. ft. (30 mA/sq M).
Particular attention should be paid to the coating quality in the upper areas of the tank likely to be in wet atmospheric service for much of the time, in these areas it is worthwhile considering the use of metallic based primers or even thermally sprayed aluminum (TSA) as a stand alone coating or as an epoxy primer.
In-service access to these tanks is extremely difficult if not impossible; it is therefore wise to include permanent reference electrodes on the lower surfaces of these tanks with signal cables to the topsides.
Permanent Ballast Tanks - On the design in question, there is a single large tank structure at the base of the floating unit. This is called the "soft tank" or permanent ballast tank. Again, construction is basic grades of carbon steel. The external surfaces of these tanks may are simply left bare and cathodically protected. The inside surfaces present a fairly unique problem.
In order to provide maximum negative buoyancy at this location on the structure, it is necessary to fill the tank not only with sea-water but also with additional dense material. The ballast of choice is magnetite (FE30.) in a granular form. There is mixed opinion on the long-term effects of this strategy, and little or no historical data from a field installation. Some FAQ's are:
1. Is there a risk of galvanic action between the magnetite and the steel?
2. Will the magnetite drain cathodic protection current meant for the underlying steel? Or will it shield the steel?
3. What will happen to sacrificial anodes if located under the magnetite? Are zinc anodes a better option than aluminum (certainly if increased weight is desirable)?
4. Is there any risk of microbial influenced corrosion (MIC) in these tanks under the magnetite, particularly the anaerobes?
Based on the little that we do know  the most likely answers are (respectively):
2. Current drain does not appear to be a risk, the question of shielding is unproven in a field situation, but is considered unlikely.
3. No long term data are available, short term it has been observed that black colored corrosion products appear on the anode surface, the long term effects of the anodes corrosion product on its performance are unknown. There is no long-term information to suggest that zinc or aluminum will have any specific advantage over the other.
4. Research is required on this subject.
Bearing the above in mind conservatism is strongly recommended. Use anodes both above and below the magnetite a maintenance current density of 3-4 m Al sq. ft. (30-40 m Al sq. m) is recommended; coat the tank particularly where the magnetite contacts the steel. Use a one shot biocide. Install reference electrodes as well as current density and anode current monitors both above and below the magnetite surface. When the full long term effects are known it may be prudent to recommend a less stringent strategy.
Production Riser Components
The riser systems on this type of structures are referred to as top-tensioned risers (TIR's). They are actually small structures within a structure, having their own buoyancy systems to support them. The only designed contact to the hull structure is through the topside flow-line connections which are above water, having said this there is a very high probability of fortuitous contact through mechanical interference with the hull. The riser systems are free to move completely independently of the hull. The extreme mechanical loading and complexity of these systems provide a whole new set of concerns regarding corrosion control. The main areas of interest are:
Air Buoyancy Cans - These are large tank like structures built around the outer surface of the riser conductor near the top of the riser. Thus they are always housed within the center well of the main hull structure. There are a number of proprietary designs that are used but they have some common corrosion areas irrespective of specific design.
Outer Can Surfaces - These see the same environment as the center well areas of the hull. However buoyancy is critical so weight loading must be minimized, anodes are not a good choice for this reason and reasons of mechanical interference with the risers guide frames. The stroke length on these risers can be as much as 40 ft. (12 m) or more. While mechanical interference is absorbed on wear strips on the outside of the can there is good possibility of coating damage during installation of the risers.
Thermal Sprayed Aluminum (TSA) provides a good solution in this area, normally non-activated aluminum grades are used and thinned epoxy sealers are applied to the coating, 10 mils (0.254 mm) is adequate for most design life requirements. While serving primarily as a barrier coating the TSA can also provide an adequate level of cathodic protection to small areas of exposed steel. It has the added advantage of high adhesion and low cohesion that allows it to smear if mechanically impacted, thus reducing exposed steel area. Be aware that the coating is conductive and will generally be at a potential that is 50 mV or more positive than the anodes in the hull center well, and will thus drain a small amount of cathodic protection from those anodes when contact between the structures exists. A design rule of thumb is to consider the TSA as a 90% efficient coating; another reference  says I m N sq. ft. (10 m N sq. m). These numbers equate to about the same amount of CP.
Inner Can Surfaces - Here there is more variation in environment and recommended corrosion control based upon the air can design. In normal operation the cans are mainly void, they may however have an open bottom that is sea-water exposed. Depending on the rate at which oxygen can be consumed and replenished it may be prudent to coat the inside surfaces, and provide a limited number of anodes at the base of the cans. Epoxy coatings are favored due largely to the difficulty of applying TSA to the inner surfaces. Depending on the can fabrication method, there may be some internal areas that are subject to damage from outside closing welds, take this into consideration. Normally the cans are filled with nitrogen in order to exclude the sea-water during installation, periodic re-filling is recommended to keep oxygen concentration to a minimum.
Main Riser Sections - The main riser sections are of pipe-in-pipe type construction with the outer pipe acting as a conductor. At the base of the riser that may be several thousand feet long there is a stress joint (there may also be a similar section near the point of exit at the base of the hull - referred to as the keel). For "dry tree" systems there will be a tieback connector that connects the riser to the marine wellhead. Riser sections are installed in the field and are mechanically coupled. The annulus between the riser and the outer pipe is usually flooded with sea-water. Mechanical spacer I centralizers are clamped to the inner riser pipe to control movement within the conductor pipe. In addition there may be external vortex shedding strake sections clamped to some areas of the risers and syntactic foam buoyancy modules clamped around others.
Outer Surface of Conductors - These areas are exposed to sea-water (except inside the air cans), and see all resistivity layers in the sea-water as they transit almost the entire water column. The use of conventional coatings with bracelet anodes has several drawbacks:
1. Possible shielding and coating damage under clamped strakes and buoyancy modules.
2. Possible resistive build up through mechanical joints.
3. High risk of mechanical damage during installation, coupled with the need to minimize outside diameter upsets that may snag during running operations.
4. Weight limitations.
For these reasons most systems use sealed TSA as the corrosion control. The advantages as previously stated also address the drawbacks listed above. An additional incentive is the competitive cost of applying TSA to standard tubular sections in large volume. Consider the following points when formulating an overall strategy:
• Don't forget to coat the riser couplers; they will receive some damage from makeup tooling. Bare or poorly coated couplers could drain the TSA unduly, particularly in the mid-water sections of the riser.
• If the riser has designed or fortuitous electromechanical contact with other structures at its extremities, ensure that adequate additional anode weight is provided to account for current drain to the riser, attenuation models can be used to predict the length of influence from each end and should be used when calculating amount and location of additional anode weight.
• It is also critical to ensure that the riser with TSA is not coupled to a large under-protected steel entity as this could irreparably damage the TSA coating and compromise the protection system.
Internal Surfaces - The outside of the actual riser pipe should be treated like the outside of the conductor (TSA coated). The inner wall of the conductor can be left bare if sea-water in the annulus is suitably inhibited. Centralizers are always non metallic to prevent wear but also ensure no metallic contact exists between inner and outer pipes. Take care when specifying fasteners for the centralizers, these will be electrically isolated from every thing but could be subject to crevice corrosion if improperly specified. A precise knowledge of the annular fluid chemistry is required before specifying, however avoid thin film non-metallic coatings on carbon steel and low end 300 series stainless steels 316 or lower.
Stress Joints - As the name suggests these joints are designed to take the major share of bending moment on the riser, they are therefore made from high strength materials with good stress characteristics and are located in the outer conductor pipe. Some titanium grades are particularly suitable from a mechanical standpoint and are therefore a common choice in many systems. The propensity of titanium to suffer hydriding under cathodic protection at certain levels provides a dilemma, one made more difficult by the lack of long term field data and the very high consequence of a failure. A number of articles have been published on these phenomena [3,4]. As previously stated, the high risk and limited experience should justify a conservative approach. Three basic methods are available and are often specified in combination, each is designed to control the surface potential on the titanium.
1. Isolate the titanium from the rest of the system. This can be accomplished using flange isolation materials or a specially constructed isolation joint at each end of the stress section.
2. Cover the titanium to stop cathodic protection, heavy elastomeric coatings have been used, they provide a tough yet flexible barrier. This can be difficult to apply however and other coatings may be suitable. Be sure to cover all the titanium, flanges can de difficult. If this is the only method used it is wise to develop a proven field repair procedure and an in-situ repair procedure in the event that the coating barrier is penetrated.
3. Ennoblement systems couple controlled areas of a noble material to the titanium to depress its potential. This method is the least desirable of the three since it will impose a drain on the CP systems and the term polarization data are very limited to say the least.
Whichever method is used, these areas should be subject to close investigation during in-service sub-sea inspections. Potential measurements on non essential titanium coupons tied to the stress joint that can be stabbed by an ROV interfaced CP probe, can provide good operational verification that the stress joint is isolated.
Tie-Back Connectors - The tie-back connector mayor may not be electrically isolated from the riser above, in either case it can be treated as an extension of the riser and receive the same corrosion treatments. Ensure that if TSA is used that the wellhead to which it's connected has adequate and compatible levels of cathodic protection.
In deep water it is common practice to pre-drill a number of wells then temporarily cap them until the production structure can be located on site. Some of these pre-drilled wells have nowhere to attach anodes for corrosion protection. In these cases it is prudent to install a pod of anode material (Figure. 4). which can be electrically tied back to one or more of these wells using ROV installed clamps (Figure. 5). These pods can produce currents in the tens of Amperes range. Pod installation is straightforward, and an added benefit, the current from the pods can be easily monitored using an ROV stab interface.