Читать книгу Underwater Inspection and Repair for Offshore Structures - Gerhard Ersdal - Страница 39

ISO 19903 (ISO 2019b) includes an inspection programme for concrete structures similar to that in, for example, ISO 19902 for steel structures, requiring an initial (baseline), periodic and special inspection to be performed. The initial inspection is required as soon as possible after installation to verify the original design and that all the major parts of the installed structure have no obvious defects. Following this initial inspection, inspection and condition monitoring of the structure shall be carried out regularly in accordance with the established programme. Special inspections should be conducted after direct exposure to an accidental or design environmental event (wave, earthquake, etc.). These inspections should encompass the critical areas of the structure. However, following an accidental event such as a boat collision and a dropped object, the inspection may, in certain circumstances, be limited to the local area of damage. In addition, the standard requires that measures should be taken to maintain the structural integrity appropriate to the circumstances in an event such as change of use, life extension, major modifications and when inspection has revealed damage or deterioration.

Similar to the standards already discussed for steel structures, assessment of the condition of the structure should be carried out following the inspection activities. This assessment should include a summary evaluation being prepared at the end of each programme for inspection. It is also stated that the data gathered from each periodic inspection should be compared to data gathered from previous inspections, with the purpose of establishing any data trends that can indicate time‐dependent deterioration processes.

The typical inspection methods used for a concrete structure include:

 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.

In the standard, structural monitoring methods are proposed to be used in areas with limited accessibility or for monitoring of, for example, action effects and corrosion development. The sensors needed for this monitoring, such as strain gauges, pressure sensors, accelerometers, corrosion probes should preferably have been fitted during construction.

The structure may also have been instrumented in order to record data relevant to pore pressure, earth pressure, settlements, subsidence, dynamic motions, strain, inclination and possibly temperature in oil storage. These monitoring systems are required by the standard to be tested and inspected regularly.

Required locations for inspection on a concrete structure include a survey of the atmospheric zone, the splash and the tidal zones and the important areas of immersed concrete. It is generally recognised that the splash zone is most vulnerable to corrosion. To determine the extent and frequency of inspection for different structural parts it is recommended that the exposure or vulnerability to damage is considered. ISO 19903 (ISO 2019b) states that the inspection of the internal parts of the structure (e.g. inside the legs) should focus on:

 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.

Special areas for inspection:

 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 stress patterns. Because of this, considerable 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.