Читать книгу Underwater Inspection and Repair for Offshore Structures - Gerhard Ersdal - Страница 40

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.