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       GVI to detect obvious or extensive damage such as impact damage, wide cracks, settlements and tilting. Prior cleaning of the inspection item is not needed for this type of inspection.

       CVI to detect less extensive damage. CVI requires direct access to the inspected area and prior cleaning of the item to be inspected is normally needed.

       Non‐destructive testing involves close inspection by electrical, electrochemical or other methods to detect hidden damage such as delamination. Prior cleaning of the inspection item is normally required.

       Destructive testing such as core drilling which is used to detect hidden damage or to assess the mechanical strength or parameters influencing concrete durability.

       detecting any leakage;

       biological activity;

       temperature, composition of sea water and pH values in connection with oil storage;

       detecting any corrosion of reinforcement; and

       concrete cracking.

       Splash zone; it can experience damage from impact of supply vessels and is also sensitive to corrosion of the reinforcement. It can also deteriorate from ice formation with ensuing spalling in surface cavities where concrete has been poorly compacted. Ice abrasion and freeze‐thaw cycling can also lead to early deterioration even with high quality concrete. This deterioration can lead to subsequent loss of cover over the reinforcement steel leading to potential steel corrosion. ISO 19903 (ISO 2019b) recommends that repairs to these damaged surfaces should be made as soon as possible to prevent further deterioration and structural overload.

       Construction joints in the concrete structure; these represent potential structural discontinuities where water leakage and corrosion of reinforcement are possible negative effects. As a minimum, the monitoring programme should identify construction joints located in high stress areas and monitor the performance with respect to evidence of leakage, corrosion staining or local spalling at joint faces (which indicates relative movement at the joint). In addition, evidence of poorly placed and compacted concrete, such as aggregate pockets and delaminations and joint cracking or separation, are areas for investigation.

       Embedment plates; these may create a path for galvanic corrosion to the underlying steel reinforcement. The main concerns are corrosion and spalling around the plates. Galvanic corrosion is a serious possibility where dissimilar metals are in a marine environment and could lead to deterioration of the reinforcing steel, which is in contact with the embedment plates.

       Repair areas and areas of inferior construction; ISO 19903 (ISO 2019b) states that concern is associated with areas that provide a permeable path through which a flow of seawater can take place as the continuous flow of saline and oxygenated water can cause corrosion of the reinforcement. In addition, attention is recommended to be given to the surface and the perimeter of patched areas for evidence of shrinkage cracking and loss of bond to the parent concrete surface.

       Penetrations; these are areas of discontinuity and are therefore prone to water ingress and spalling at the steel to concrete interface. Any penetrations added to the structure during the operational phase are particularly susceptible to leakage resulting from difficulties in achieving high‐quality concrete in the immediate vicinity of the added penetration. ISO 19903 (ISO 2019b) states that all penetrations in the splash and submerged zones require frequent inspections (but not specified).

       Steel transition ring to concrete interface; this interface is the main load transfer point between the concrete towers and steel topsides. ISO 19903 (ISO 2019b) recommends that the steel transition ring should preferably be examined annually. The examination should include the load transfer mechanism (flexible joints, rubber bearings, bolts and cover) and the associated ring beam. In addition, the concrete interface should be inspected for evidence of overstress and corrosion of embedded reinforcing steel. Corrosion‐potential surveys can be used to detect ongoing corrosion that is not visible by visual inspection alone.

       Debris; it can cause structural damage through impact, abrasion or by accelerating the depletion of cathodic protection systems. It can also limit the ability to inspect various parts of the structure (e.g. tops of storage cells). Drill cuttings can build up on the cell tops and/or against the side of the structure and should be assessed for lateral pressures exerted by the cuttings, and whether they cause an obstruction to inspection. As a result of this assessment the removal of drill cuttings needs to be assessed accordingly.

       Scour; this is the loss of foundation‐supporting soil material and can be induced by current acceleration round the base of the structure or by “pumping” effects caused by wave‐induced dynamic rocking motion. It can lead to partial loss of base support and unfavourable redistribution of actions. GVI is the recommended method.

       Drawdown (differential hydrostatic pressure); a level of drawdown in many concrete structures is necessary for structural integrity. Structural damage or equipment failure can lead to ingress of water and loss of hydrostatic differential pressure. Special inspection may be required if loss of drawdown is detected.

       Temperature of oil sent to storage; the storage cells are designed for a maximum temperature differential. In cases where differential temperatures have exceeded these design limits, special inspections may be required.

       Sulphate‐reducing bacteria (SRB); these can occur in anaerobic conditions where organic material is present (such as hydrocarbons). The bacteria produce H2S (hydrogen sulphide) as their natural waste which could cause a lowering of pH value of the cement paste in the concrete. Favourable conditions for SRB growth can be present in unaerated water in, for example, the water‐filled portion of shafts and cells. An acidic environment can cause concrete softening and corrosion of reinforcement. However, it is recognised that an inspection of a concrete surface likely to be affected by SRB activity is difficult to undertake. Some guidance can be obtained by adequate monitoring of SRB activity and pH levels (HSE 1990).

       Post‐tensioning; the tendons are usually contained within ducts which are grouted. Post‐tensioning anchorage zones are commonly areas of complex

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