Section 1 Corrosion protection

1.1.1 The requirements cover the corrosion protection of offshore units of the general types defined in Pt 1, Ch 2, 2 Definitions, character of classification and class notations see also Pt 3, Ch 1 General Requirements for Offshore Units. Requirements are also given for riser systems, see Pt 8, Ch 1, 2 Riser systems.

1.1.2 All structural steel work is to be suitably protected against loss of integrity due to the effects of corrosion. In general, suitable protective systems may include coatings, metallic claddings, cathodic protection, corrosion allowances or other approved methods. Combinations of methods may be used when agreed by Lloyd’s Register (LR). Consideration should be paid to the design life and the maintainability of the surfaces in the design of the protected systems.

1.1.3 The basic Rule scantlings of the external submerged steel structure of units which are derived from Pt 4 STEEL UNIT STRUCTURES assume that a cathodic protection system will be effective and in use continually. Unless agreed otherwise with LR no corrosion allowance will be included in the approved scantlings, see Pt 3, Ch 1, 5 Corrosion control.

1.2.1 The type of protection of the steelwork is to be suitable for the structural location of the unit and for this purpose the steel structure is to be considered in terms of zones.

1.2.2  Submerged zone. That part of the external structure below the maximum design operating draught.

1.2.3  Boot topping zone. That part of the external structure between the maximum design operating draught and the light design operating draught. For column-stabilised units, see Pt 8, Ch 1, 1.2 Zone definitions 1.2.6.

1.2.4  Splash zone. That part of the external structure above the boot topping zone subject to wet and dry conditions.

1.2.5  Atmospheric zone. That part of the external structure above the splash zone.

1.3.1 The minimum requirements for corrosion protection of the external steelwork of offshore units is given in Pt 8, Ch 1, 1.2 Zone definitions 1.2.6.

1.3.3 The structural steelwork in the atmospheric zone is to be protected by suitable coatings.

1.3.4 The structural steelwork in the submerged zone is to be protected by an approved means of cathodic protection using sacrificial anodes or an impressed current system, except where noted otherwise in Pt 8, Ch 1, 1.2 Zone definitions 1.2.6. High resistance coatings may be required or used in conjunction with a cathodic protection system but they will not be accepted in lieu except where noted in Pt 8, Ch 1, 1.2 Zone definitions 1.2.6. An alternative means of protection such as increased scantlings may be considered in special areas. Where In-Water Survey notation OIWS is to be assigned corrosion, protection in submerged zone shall be provided by high resistance coatings supplemented by cathodic protection; this is considered to be the optimal means of protection for all submerged components.

1.4.1 Ballast tanks shall be protected from corrosion by a combination of anti-corrosion coatings and cathodic protection.

1.4.2 At the time of new construction, all salt-water ballast tanks shall have an efficient protective coating, epoxy or equivalent, applied in accordance with the manufacturer’s recommendations. The durability of the coatings could affect the frequency of survey of the tanks and light coloured coatings would assist in improving the effectiveness of subsequent surveys. It is therefore recommended that this be taken into account by those agreeing the specification for the coatings and their application.

1.4.3 Storage tanks and other compartments require corrosion protection where the storage product may be corrosive. Particular attention should be paid to the likelihood of water in the bottom of hydrocarbon storage tanks and the effects of bacterial induced corrosion. Suitable protective measures may include coatings, corrosion inhibitors together with biocides.

1.4.4 In deep draught caisson units and other units with combined oil storage and ballast tanks which remain full during the service life of the unit, special consideration will be given to the requirement for internal corrosion protection of the tanks. In general, the minimum Rule scantlings of tanks as required byPt 4, Ch 6, 7 Bulkheads are to be suitably increased based on a study of likely degradation rates and service life of unit.

1.5.1 Where bimetallic connections are made in the structure, suitable measures are to be incorporated to preclude galvanic corrosion. Details are to be submitted for approval on the structural plans required in Pt 4, Ch 1, 4 Information required The combination of painting the less noble material and leaving the more noble material uncoated for an immersed bimetallic couple is not permitted. In submerged zones cathodic protection is considered to be a suitable mitigation measure as it will eliminate the potential differences across the bimetallic connection.

1.6.1 Chain cables and wire ropes for positional mooring systems are to be protected from corrosion and the requirements ofPt 3, Ch 10 Positional Mooring Systems are to be complied with. Current drain from the mooring system is to be considered in cathodic protection design as required in Pt 8, Ch 4, 1 External steel protection.