Corr Cont Meth for DW Float Prod Equip

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Corrosion Control Methods for Deep Water Floating Production Equipment

James N. Britton & Richard E. Baxter

Deepwater Corrosion Services Inc.

ABSTRACT

The rapid growth in the deep-water production sector worldwide has spawned a wholenew generation of floating production equipment. This paper will deal with some of the

unique corrosion challenges presented by these production systems and how corrosion is being managed using innovative coatings, cathodic protection designs and corrosion

resistant materials. The paper will also address some unique monitoring and inspectionissues that arise on these structures.

Key Words: Deepwater, Offshore Corrosion, Floating Production Systems, Moorings.

INTRODUCTION

The new generation of floating offshore production systems such as Floating

Production, Storage and Offloading facilities (FPSO), SPAR (originally designated aSingle Point Articulated Riser, now generically applied cylindrical floating structures),

Tensioned Leg Platforms (TLP), and Deep Draft Caisson Vessels (DDCV) present a

number of unusual corrosion control challenges, being as they are hybrids between shipsand conventional production platforms. These challenges arise from several sources

including:

• Use of high strength materials.

• Need to control weight and reduce corrosion allowance.

• Inclusion of many confined spaces and tanks.

• Lack of operational history.

• High number of dynamic components and non-welded connections.

• Challenges for inspection and monitoring.

Corrosion engineers should therefore err on the side of caution when specifying corrosion

control systems, and often must use laboratory simulation and parallel operational history

from other areas of the industry. The operator must also be aware of regulated in-service

inspection requirements and allow appropriate monitoring and inspection aids to bespecified during construction. Close co-operation between structural engineers (naval

architects), subsea engineers and corrosion / materials engineers is absolutely critical.



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Recent design experience, highlighting solutions and some conflicts of opinion are

presented.

SPAR / DDCV STRUCTURES

The Deep Draft Caisson Vessel (Fig. 1.) [5] and SPAR type structures have presented deep water operators with a cost effective production facility for prospects with

a moderate number of subsea wells, and are being widely used in the Gulf of Mexico. A

typical Truss SPAR design is shown Fig.2 [5]. These structures present the followingmain components, each of which has unique problems. These structures contain the

majority of the problems encountered on other types of floating production systems, so an

in-depth analysis of a corrosion control approach should serve well for many otherfloating production cases.

HULL COMPONENTS

Main Hull

The hull is typically a large cylindrical steel tube with a variety of internalcompartments that provide buoyancy to support the topside drilling and production

equipment with a central annulus (center well) through which all production is

accomplished. Hulls are typically 90–120 ft. (30–40 meters) diameter with a center welldiameter of 30-50 ft. (10-16 meters). Overall length of the hull section can be 600 ft.

(183 meters) or more in length. This construction presents two distinct seawater

immersed corrosion exposures.

Outer Hull – Normally basic grades of carbon steel exposed to natural seawater,these areas are normally addressed using conventional marine epoxy coatings through the

splash zone and topsides and conventional sacrificial anode cathodic protection to bare

steel in the immersed region. Design criteria are the same as for conventional platforms.Monitoring of CP system performance is simply accomplished using surface deployed or

remotely operated vehicle (ROV) interfaced monitoring equipment. Be aware that some

areas of the hull that are the outer wall of internal tanks may require some type of wall

thickness monitoring. This can be accomplished with an ROV, however the inspectionlocations should be clearly marked and treated to prevent excessive marine growth

accumulation.

Center Well – Similar basic carbon steel but exposed to quiescent seawater.

Exclusion of normal sunlight reduces marine growth accumulation and densely packed

riser systems make post installation access difficult to impossible, and can causetemperatures to riser a little above ambient seawater. The net result is a slightly lower

current requirement for cathodic protection of the steel on the wall of the center well.

Data from early fixed monitoring systems indicates a current density requirement of 6-9mA/sq.ft. (60-90 mA/sq.M) initially and 1-3 mA/sq.ft (10-30 mA/sq.M) finally. A good

design number for weight calculation would be 3 mA/sq.ft (30 mA/sq.M). Again

coatings are typically used only in splash zone and emerged areas, sacrificial anodes must



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be flush mounted to avoid obstructions during riser installation, this fact must be taken

into account when computing anode resistance. Be aware that additional anode weightmust be provided to offset current losses to the riser systems, (this will be discussed

further).

There are also a series of guide frame structures within the center well; these guidesrestrict lateral movement of the riser systems during vessel relocation about its

operational footprint and to offset normal hydrodynamic forces. It is important to locate

anodes on these guide frames, as they could be shielded from anodes on the inside wall.Bear in mind that it is these anodes that will provide a large proportion of current drained

to the risers through fortuitous contact.

The limited post installation access to this area makes it a good candidate for fixed

reference electrodes to monitor CP system performance, basic dual element electrodes

Fig. 3. can be conveniently hard wired to the surface.

Truss Section

Most recent designs utilize a truss section attached to the base of the main hull tosupport the bottom main ballast tank at the base of the SPAR structure. The truss is

typically 300 – 325 ft. (91 – 100 meters) long. Construction is standard tubular steel

members with periodic heave plates, which provide guides for the riser systems. Thetruss 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 ballasttanks which will see raw seawater 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 ofshielded compartments.

Void Tanks – These tanks are usually vented to atmosphere and thus cannot be fully de-oxygenated. They are often susceptible to condensation on the inner walls. Access is

possible through manways and periodic visual and non-destructive examination is always

a part of the required in-service inspection program. Access to effect coating repairsduring 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



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systems will provide equivalent protection. Vapor phase inhibitors should contain

soluble inhibitors to inhibit any condensed water and control corrosion at coatingdefects.

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 beworth leaving some controlled bare spots as back-up test locations.

2. Dehumidification can be a very effective method of void space corrosion control;it’s 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 benecessary 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 protectionare 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 theseenvironments 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 sprayedaluminum (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 withsignal cables to the topsides.

Permanent Ballast Tank(s)

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 seawater but also with additional dense material.



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The ballast of choice is magnetite (FE3O4) in a granular form. There is mixed opinion on

the long-term effects of this strategy, and little or no historical data from a fieldinstallation. 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 underlyingsteel? 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 [1] the most likely answers are:

1. No.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 theanodes 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 bothabove and below the magnetite a maintenance current density of 3-4 mA/sq.ft. (30-40

mA/sq.M) is recommended; coat the tank particularly where the magnetite contacts thesteel. 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(TTR’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 flowline connections which are above water, having said this there isa 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 ofconcerns 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



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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 guideframes. The stroke length on these risers can be as much as 40 feet (12 meters) 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 adhesionand 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 potentialthat 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 thestructures exists. A design rule of thumb is to consider the TSA as a 90% efficient

coating; another reference [2] says 1 mA/sq.ft (10 mA/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 mainlyvoid, they may however have an open bottom that is seawater exposed. Depending on

the rate at which oxygen can be consumed and replenished it may be prudent to coat theinside 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 aresubject to damage from outside closing welds, take this into consideration. Normally the

cans are filled with nitrogen in order to exclude the seawater 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 pipeacting 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 tiebackconnector 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 seawater. Mechanical spacer / centralizers are clamped to theinner 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.



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Outer Surface of Conductors – These areas are exposed to seawater (except inside the aircans), and see all resistivity layers in the seawater as they transit almost the entire water

column. The use of conventional coatings with bracelet anodes has several drawbacks:

•

Possible shielding and coating damage under clamped strakes and buoyancymodules.

• Possible resistive build up through mechanical joints.

• High risk of mechanical damage during installation, coupled with the need tominimize outside diameter upsets that may snag during running operations.

• Weight limitations.

For these reasons most systems use sealed TSA as the corrosion control. The advantagesas 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 themid-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 forcurrent 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 compromisethe protection system.

Internal Surfaces – The outside of the actual riser pipe should be treated like the outsideof the conductor (TSA coated). The inner wall of the conductor can be left bare if

seawater 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 isolatedfrom 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 stainlesssteels 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



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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 articleshave 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 ateach end of the stress section.

2. Cover the titanium to stop cathodic protection, heavy elastomeric coatingshave 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 methodused 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 thetitanium 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 long-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 couponstied 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.

Tieback Connectors – The tie-back connector may or may not be electrically isolated

from the riser above, in either case it can be treated as an extension of the riser andreceive 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.

WELLHEAD COMPONENTS

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 Fig. 4. which can be electrically tied back to

one or more of these wells using ROV installed clamps Fig 5. These pods can producecurrents in the tens of amperes range. An additional benefit is that current to each

structure can be easily monitored using an ROV stab interface, and are straightforward to

install. This method will begin to find acceptance for many types of subsea structuregiven the favorable economics. ]



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MOORING COMPONENTS

Lateral mooring systems are required to enable the hull structure to be moved

around its operational footprint. These multi-leg systems comprise several key

components that need to be addressed. Obviously the moorings are subject to very high

loading and a corrosion failure could be very costly.

Upper level Components

At the surface the moorings comprise a long section of chain, guided by a fairlead

structure attached to the hull.

Fairleads – These are basically a large sheave on a hinged support through which the

mooring chain makes the turn toward the seabed, they are normally located near the

bottom end of the main hull structure. Cathodic protection is a suitable method forcorrosion control, but the following precautions are required:

1. Ensure that a flexible continuity jumper cable is provided between the fairlead

structure and the hull.

2. Ensure that current losses to the chain are calculated when sizing anodes.

Upper Chains – Chains are very difficult to protect from corrosion due to their

construction and operational requirements. This is one area where a generous corrosion

allowance is normally provided. The problem is that they will draw C.P. current fromstructures to which they are electrically connected; the degree of connectivity is difficult

to predict or quantify. A rule of thumb is to assume that approximately 200 Feet (60Meters) of chain is likely to pick up C.P. in each direction away from a probable contact

point. It would be difficult to locate sufficient anode material on the cheek plates of the

fairlead to satisfy this drain, hence the importance of ensuring electrical continuity between the fairlead and the hull.

Catenary Mooring Lines

This is the long section of the mooring through the water column to the seabed

and is usually a large diameter spiral strand wire rope for most of its length. Large

connectors are used to couple the rope to the upper chain section and the lower chainsection.

Wire Ropes - There are a number of commercially available rope designs that adequatelyaddress corrosion control, usually rope strands are coated with either zinc or a zinc –

aluminum alloy, the bundle is often treated with a blocking compound to restrict the

water path to the inner strands and the whole bundle is sheathed with a polyethylene jacket. This scheme has proven to be quite acceptable in most cases.



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Connectors – The connectors are normally provided with anodes directly attached to the

body. Some designs provide electrical isolation of the connector, this is favorable, particularly when connected to a chain, and again it is difficult to provide enough anode

material to cover losses along the chain. The bearing surfaces of these connectors are

usually sheathed or lined with a hard corrosion resistant alloy which is cathodic to the

connector body material, thus it is important to maintain protected potentials on theseareas.

Seabed Components

The lower sections of the moorings usually comprise of a “ground chain” section,which is connected by shackle to either an anchor or an anchor pile. While corrosion

rates are low in this region some operators require protection. Given the difficulty of

attaching sufficient anodes to the pile or anchor, the anode pod concept previouslydescribed can be a viable alternative.

EXPORT RISER COMPONENTS

Export risers and flow lines from remote wells are either steel catenary design or

may utilize flexible sections. Various methods have been used successfully utilizing

either conventional Fusion Bond Epoxy (FBE) type coatings or TSA. Anodes can belocated on the riser catenary section but some operators prefer to use anodes located at

the touch down area and on the hull to provide protection from the ends.

SUMMARY & CONCLUSIONS

When formulating corrosion control strategies for these systems the following

points are important:

• Ensure system compatibility between all components, and make sure there is an

overview material compatibility review performed.

• Consider in-service inspection requirements when designing systems.

• Remember to account for CP losses to TSA coated components and other un- protected entities.

• Use conservative designs throughout.

REFERENCES

[1] In-House Deepwater Corrosion Services Study – Unpublished.[2] DNV RP B 401 (1993) Section 6.3.16

[3] Jeih Ing & Paul Chung “A Study of Hydriding of Titanium in Seawater

Under Cathodic Polarization” CORROSION 86, Paper #259[4] Ronald W. Schultz “Guidelines for Successful Integration of Titanium

Alloy Components into Subsea Production Systems” CORROSION 2001,

Paper # 003

[5] Operating Company Web Sites



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Figure 1. Conventional Spar – Deep Draft Caisson Vessel



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Figure 2. Truss Spar Structure Figure3. Standard Permanent

Reference Electrode (Dual Element)



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Figure 4. Seabed Anode Pod – Current MonitoringStabs on Top of Frame

Figure 5. ROV Installed Tieback Clamps

Corr Cont Meth for DW Float Prod Equip

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