NEWSLETTER

The fitness-for-purpose evaluation of offshore structures requires the use of sub-sea inspection methods, both to detect and size defects, which in the past have often been fatigue cracks. Various inspection techniques are used during operation. A number of different techniques have been developed for diver use and more recently for deployment from remote operating vehicles (ROVs). A significant advantage of the use of ROVs is the elimination of risks to personnel. General visual inspection (GVI) and close visual inspection (CVI) methods are used initially. Where more detailed information on joint integrity is required, entailing the detection of surface-breaking cracks (e.g. from fatigue), the main methods used offshore are magnetic particle inspection (MPI), eddy current detection (ECD), alternating current field measurement (ACFM) and flooded member detection (FMD), though FMD is the only technique routinely used from an ROV. GVI and FMD have accounted for the detection of the highest number of significant failures, usually at the end of the fatigue life.

In recent years, increased reliance has been placed by the offshore industry on the use of FMD techniques as part of the overall jacket inspection strategy as a cost-effective means of verifying structural integrity. It is applied typically to bracing systems in jacket structures covering several types of weld, including closure welds, repair patches / windows, appertunances, etc.. The FMD technique is suitable for the detection of through thickness cracks (or other damage leading to water penetration) by the detection of flooding levels in tubular members. The procedure is fast and can be performed by ROV, thus providing a good tool for rapid screening of platform members for gross damage. There are two currently available technologies, based on either ultrasonics or radiography (using gamma sources).

As the method relies on the detection of the flooding of a member once a through-thickness crack has developed, it is necessary that the residual strength of the cracked member is sufficient to withstand the maximum applied load until the next inspection and that the inspection interval is not overestimated. These requirements limit the use of FMD to redundant members. It follows that an inspection plan employing the use of FMD needs to be supported by an appropriate level of structural analysis.

The use of FMD has many recognised advantages. However, its use as an inspection tool has several limitations:

• internal corrosion or other debris inside the member can lead to false readings and partial flooding of members can be difficult to detect;
• there is some uncertainty in the time required for a member to become flooded once a crack has penetrated the wall of the member and this will depend on the water depth, the applied loads, residual stresses and the 'tightness' of the crack;
• once flooding is determined, other methods are usually needed to detect the location of the damage, such as CVI or MPI;
• finally, once a through thickness crack has been found, the remaining fatigue life is likely to be very low.

Furthermore, in developing an inspection strategy, it should be recognised that FMD is not effective for various types and locations of fatigue cracking, namely

• a joint connected to a member already flooded (intentionally or as a result of damage);
• a joint where cracking occurs on the chord side, particularly for leg joints;
• a joint with poor access for NDE type inspection, e.g. in the splash zone.

It should also be recognised that information on the reliability of the method and the implications on safety is somewhat limited, particularly due to the high variability in defect shape, joint geometry and loading which can cause the interval between first leakage and complete failure to be highly variable. In practice, a baseline inspection following platform installation should identify the presence of flooding in particular members, as well as the degree of access for different inspection methods.

FMD is particularly effective for certain situations which are difficult to control at the design and fabrication stages. This generally applies to single-sided welds with the potential for significant root defects to occur. Geometries include joint stub to member, access windows, circumferential butt joints and possibly point to point construction (i.e. single-sided welded nodal joints).

Overall, this approach indicates that with careful design to eliminate features where FMD is not applicable its use in service can be an effective method of risk control. For structures where the use of FMD has not been considered during design it is important that this technique should be used with caution. It is considered that the increased dependence on FMD is only compatible with various requirements, including the ability to respond in a timely manner to the detection of gross damage. Other necessary compatibilities in the system are improved fabrication quality, a rigorous consideration of the implications of through-thickness cracking, high reliability of detection in both operator and technique and the need for special inspections following both accidental and design environmental events.

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