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General Amended April see note on front cover 1. General 1. This anode is also the current source for the CP system and will be consumed. In this RP, the cathode of the electrochemical cell i. The design is simple, the system is mechanically robust and no external current source is needed. In addition, inspection and maintenance during operation can largely be limited to periodic visual inspection of anode consumption and absence of visual corrosive degradation.
However, due to weight and drag forces caused by galvanic anodes, impressed current CP systems are sometimes chosen for permanently installed floating structures. It prevents localised forms of corrosion as well as uniform corrosion attack, and eliminates the possibility for galvanic corrosion when metallic materials with different electrochemical characteristics are combined.
However, CP may have certain detrimental effects, for example hydrogen related cracking of certain high-strength alloys and coating disbondment as described in 5. Still, CP is often fully effective in preventing any severe corrosion in a tidal zone and has a corrosion reducing effect on surfaces intermittently wetted by seawater.
Whilst the requirements and recommendations are general, this document contains advice on how amendments can be made to include project specific requirements. Mobile installations for oil and gas production like semi-submersibles, jack-ups and mono-hull vessels are not included in the scope of this document. However, to the discretion of the user, relevant parts of this RP may be used for galvanic anode CP of such structures as well.
Considerations related to safety and environmental hazards associated with galvanic anode manufacture and installation are also beyond its scope. CP of submarine pipelines is further excluded from the scope see 1. However, this RP is applicable for CP of components of a pipeline system installed on template manifolds, riser bases and other subsea structures when such components are electrically connected to major surfaces of structural C-steel.
Furthermore, some informative text in the old revision has been contained in such notes. All tables and figures associated with Sec. The document has further been revised to facilitate specification of Purchaser information to Contractor, and optional requirements associated with CP design, manufacture and installation of anodes see 1.
Additional comments on revisions in this issue are made in the Introduction last paragraph of Sections 6, 7, 8 and Annex B and C. References — Page 7 1. It may also be used as an attachment to an inquiry or purchase order specification for such work.
If Purchaser has chosen to refer to this RP in a purchase document, then Contractor shall consider all requirements in Sections of this document as mandatory, unless superseded by amendments and deviations in the specific contract.
Referring to this document in a purchase document, reference shall also be made to the activities for which DNV-RP-B shall apply, i. CP design in Sections 6 and 7, anode manufacture in Sec. Different parties issuing a contract i. For definition of contracting parties and associated terminology, see Sec. Tabulated data for CP design are compiled in Annex A.
Annex B and C contain recommended procedures for laboratory testing of anode materials for production quality control and for documentation of long-term electrochemical performance, respectively. References 2. The latest editions apply. Terminology and Definitions Amended April see note on front cover 2.
Terminology and Definitions 3. Party Owner or main contractor issuing inquiry or contract for CP design, anode manufacture or anode installation work, or nominated representative. Party to whom the work i. CP design, anode manufacture or anode installation has been contracted. B, ISO References within parentheses refer to the applicable paragraph. For items applicable to quality control and CP design parameters, reference to the applicable paragraph is made in the list of abbreviations 4.
Abbreviations and Symbols — Page 9 4. Abbreviations and Symbols 4. The content of this section is informative in nature and intended as guidelines for Owners and their contractors preparing for conceptual or detailed CP design. Nothing in this section shall be considered as mandatory if this RP has been referred to in a purchase document. It follows that cathodic protection is not an alternative to corrosion resistant alloys for components with very high dimensional tolerances, e.
In addition, variations in seawater pH and carbonate content are considered factors which affect the formation of calcareous layers associated with CP and thus the current needed to achieve and to maintain CP of bare metal surfaces.
In seabed sediments, the major parameters are: temperature, bacterial growth, salinity and sediment coarseness. It is not feasible to give an exact relation between the seawater environmental parameters indicated above and cathodic current demands to achieve and to maintain CP. It has been argued that a design protective potential of - 0.
However, in the design procedure advised in this RP, the protective potential is not a variable. Towards the end of the service life, the potential increases rapidly towards - 0. Such potentials will not apply for CP by galvanic anodes based on Al or Zn.
However, with surface preparation to achieve an optimum surface roughness, some coating systems e. For coating systems whose compatibility with galvanic anode CP is not well documented, Owner should consider carrying out qualification testing, including laboratory testing of resistance to cathodic disbondment.
Within the potential range for CP by aluminium or zinc based anodes i. The hydrogen atoms can either combine forming hydrogen molecules or become absorbed in the metal matrix. The same applies for welding or hot forming according to an appropriate procedure. For precipitation hardened austenitic stainless steels, the susceptibility is lower and a hardness of max. In the intermediate hardness range i.
The qualification of coatings for this purpose should include documentation of resistance to disbonding in service by environmental effects, including CP and any internal heating. However, laboratory testing has demonstrated susceptibility to HISC during extreme conditions of yielding. It is recommended that all welding is carried out according to a qualified procedure with HV as an absolute upper limit. With a qualified maximum hardness in the range to HV, design measures should be implemented to avoid local yielding and to apply a reliable coating system as a barrier to CP induced hydrogen absorption.
It is widely recognised that untempered martensite is especially prone to HISC. Welding of materials susceptible to martensite formation should be followed by post weld heat treatment PWHT to reduce heataffected zone HAZ hardness and residual stresses from welding.
The same recommendations for hardness limits and design measures as for ferritic steels 5. However, failures due to inadequate heat treatment have occurred and for critical applications, batch wise testing is recommended to verify a maximum hardness of HV. This is related to an increased ferrite content rather than hardness. Qualification of welding should therefore prove that the maximum ferrite content in the weld metal and the inner HAZ about 0.
Forgings are more prone to HISC than wrought materials due to the course microstructure allowing HISC to propagate preferentially in the ferrite phase. Cold bent pipes of small diameter uncoated and with mechanical connections, i.
Design precautions should include 1 measures to avoid local plastic yielding and 2 use of coating systems qualified for e. For high-strength titanium alloys, documentation is limited and special considerations including e. For more quantitative testing, uni-axially loaded tensile specimens with constant load , 4-point bend specimens with constant displacement , crack tip opening displacement CTOD and other testing configurations have been applied at controlled CP conditions.
Such testing is, however, beyond the scope of this document. The risk is moderate with Al and Zn-base galvanic anodes but at least one explosion during external welding on a water flooded platform leg containing such anodes has been related to this phenomenon.
Closed water flooded compartments will not normally require CP, see 6. The thickness is typically of the order of a tenth of a millimetre, but thicker deposits may occur. The calcareous layer reduces the current demand for maintenance of CP and is therefore beneficial.
A calcareous layer may, however, obstruct mating of subsea electrical and hydraulic couplers with small tolerances. This may be prevented by applying an insulating layer of a thin film coating e. An alternative measure is to electrically insulate the connectors from the CP system and use seawater resistant materials for all wetted parts.
High-alloyed stainless steels, nickel-chromium-molybdenum alloys, titanium and certain copper based alloys e. The generic type of anode material i. However, zinc based anodes have sometimes been considered more reliable i. Purchaser may require that the anode manufacturer shall document the electrochemical performance of their products by operational experience or by long term testing in natural seawater.
A recommended testing procedure is contained in Annex C. The anode type determines the anode resistance formula 6. The current output, Ia A , in relation to net anode mass, Ma kg , is high, as is the utilisation factor u.
Stand-off anodes are manufactured up to a net anode mass of several hundred kilograms. In surface waters, drag forces exerted by sea currents are significant. Bracelet anodes are used primarily for pipelines but have also found some use on platform legs in the upper zone, combining high current output to weight ratio with low drag. All flush mounted anodes should have a suitable coating system applied on the surface facing the protection object. This is to avoid build-up of anode corrosion products that could cause distortion and eventually fracture of anode fastening devices.
For stand-off type anodes, special precautions may be necessary during anode design and distribution of anodes to avoid impeding subsea operations 7. For weight-sensitive structures with a long design life, the combination of a coating and CP is likely to give the most cost-effective corrosion control.
For some systems with very long design lives, CP may be impractical unless combined with coatings. This includes deep water applications for which the formation of calcareous deposits may be slow see 6. It should further be considered for surfaces that are partly shielded from CP by geometrical effects. The CP design procedure in this document does not account for a voltage drop in the seawater remotely from anodes.
DNVGL-RP-B401 Cathodic protection design
DNV-RP-B401: Cathodic Protection Design