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additional reinforcement steel is used to control cracking. Anchorages for the post‐tensioning tendons are generally terminated in prestressing pockets in the structure, which are also vulnerable. Inspection of tendons is, however, very difficult using conventional inspection techniques. In many cases, the reinforcing steel is very congested and this condition can lead to poor compaction of concrete immediately adjacent to the anchorage. The recess is fully grouted after tensioning. These conditions expose the critical tendon anchors to the marine environment, causing corrosion of the anchor and additional spalling and delamination of concrete and grout in the anchorage zone. Regular visual inspection of the anchorages is recommended.

      Experience has shown that the anchorage zones are vulnerable to distress in the form of localized cracking and spalling of grouted anchorage pockets. Where evidence exists for this type of damage, ISO 19903 (ISO 2019b) recommends a more detailed visual inspection supplemented by impact sounding for delamination. The visual inspection should focus on corrosion staining, cracking and large accumulations of efflorescence deposits.

      Partial loss of prestress is generally recognised as leading to local concrete cracking resulting from redistribution of stress. This should be investigated upon discovery. In addition, design documents should be reviewed by the inspection team to establish the arrangement and distribution of cracking that could be expected to result from partial loss of prestress.

      Durability of concrete and the corrosion protection system are addressed in terms of inspection and focus on:

       Concrete: factors that may change with time and may need to be surveyed regularly, such as chloride profiles, chemical attacks, abrasion depth, freeze and thaw deterioration and sulphate attack in petroleum storage areas.

       Corrosion protection: Periodic examination with measurements should be carried out to verify that the cathodic protection system is functioning within its design parameters and to establish the extent of any material depletion of the anodes. Cathodic protection is also provided for the protection of the reinforcing steel, which is important for the structural integrity of the concrete. In this respect the level of adequate potential should be monitored. In general examination shall be concentrated in areas with high or cyclic stress utilization, which need to be monitored and checked against the design basis. Heavy unexpected usage of anodes should be investigated.

      Examination of any coatings and linings is normally performed by visual inspection to determine the need for repairs. A close visual examination will also disclose any areas where degradation of coatings has allowed corrosion to develop to a degree requiring repair or replacement of structural components.

It is noted in ISO 19903 (ISO 2019b) that several techniques have been developed for the detection of corrosion in the reinforcement in land‐based structures. These are mainly based on electro‐potential mapping, for which there is an ASTM standard (ASTM 2015). These techniques are useful for detecting potential corrosion in and above the splash zone but have limited application under water because of the low resistance of sea water. ISO 19903 (ISO 2019b) also notes that it has been established that under many circumstances underwater corrosion of the reinforcement does not lead to spalling or rust staining. The corrosion products are of a different form and can be washed away from cracks and hence leave no evidence on the surface of the concrete (see Section 3.5). However, when the reinforcement is adequately cathodically protected, any corrosion should be prevented. In cases where cathodic protection of the reinforcement is limited, the absence of spalling and rust staining at cracks in the concrete cover should not to be taken as evidence for no corrosion.

      2.5.3 Department of Energy Guidance Notes

      The “Survey” section of the fourth edition Guidance Notes (Department of Energy 1990) focussed mainly on steel structures with regard to the requirements for re‐certification. At the time of preparing the fourth edition Guidance Notes, concrete structures were relatively new and a comment was made that experience to date had shown little requirement for maintenance and repair. However, specific mention was made of the lower elements of a GBS‐type structure, which were only likely to suffer significant damage if erosion or uneven settlement had taken place. It was noted that many units of this type have built‐in instruments to record the state of the foundations. With regard to corrosion of the steel reinforcement, it was noted that this is less likely in permanently submerged areas than in the splash zone. Hence, if the splash zone was found to be in good condition, then only a limited number of checks needed to be made at lower levels, except where there were sudden changes of section or high stress concentrations.

      The Background document to the Guidance Notes (HSE 1997) supported the limited problems to date but commented that this was probably a result of limited life, particularly for evidence of any corrosion of the steel reinforcement. Some problems with draw‐down had been found which were mentioned, due both to failure of the draw‐down system and accidental impact damage. Several platforms had suffered ship impact or impacts from dropped objects, with several cases where the damage had been minimal. Where damage had occurred, it had resulted in holes in cell roofs or a leg and in each case a repair had been undertaken. It was noted that the degree of marine growth had varied substantially between structures, some structures with almost no growth, others with extensive growth requiring cleaning.

      The “Repairs” concrete section of the Guidance Notes stated that the accepted materials for repair offshore were concrete, cement grouts and mortars and epoxy resins. The importance of the ability of the repair material to bond to concrete, reinforcement or pre‐stressing ducts needed to be considered. The Guidance Notes also noted that when selecting a repair material for protection of the reinforcement against corrosion, the properties of the material to be considered should include permeability to water, presence of chloride ions and resistivity. An important factor is the durability of the repair material in the marine environment.

      Repair of cracking in a concrete structure was addressed, with low viscosity epoxy resins being the usual material and the repair procedure should be established by trials.

      Repairs to steel reinforcement were considered where the reinforcement has been damaged or cut away. The new reinforcement should either be lapped on to the existing reinforcement with a full bond length or connected by a suitable coupler.

Repairs to prestress were also addressed making the point that it is very difficult to establish the effectiveness of any damaged prestress, as this is to a large extent dependent on the bond with the grout in the cable duct. The effectiveness of any repairs to tendons which have been broken would require re‐stressing of the new or repaired tendons, which is not easy in a repair situation.

      Repair of cracking in a concrete structure should utilise low viscosity epoxy resins, and the repair procedure should be established by trials.

      

      2.5.4 NORSOK N‐005—Concrete Structures

      NORSOK N‐005 (Standard Norge 2017b) has a special annexe covering inspection of concrete structures. This standard makes the point that concrete structures are normally damage tolerant because of their large dimensions and planning of inspections should take this into account. However, it also points out that ageing concrete structures in air may experience increasing amount of chloride ingress that may lead to corrosion of reinforcement in areas of poor‐quality concrete.

      Specific requirements for data collection and management relating to concrete structures include:

       operational criteria for caisson cells operated with differential hydrostatic pressure (drawdown) or normal hydrostatic pressure in cells;

       operational criteria for cells used for oil storage and/or water ballast;

       any reported repairs or discontinuities in the concrete from the fabrication phase. Specifically, repair of reported leakages in the concrete;

       description of mechanical systems for operating the ballast water, oil filling and draw down systems in the storage cells; and

       settlement

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