APPENDIX H POTENTIAL AREAS FOR FUTURE DEVELOPMENTS

In order to identify areas for potential future developments in POD/POS it is important to highlight the place of NDT in the overall process of arriving at safe, welded structures. The elements to arrive at safe structures can be put in the following three categories (Table H1):

- design and design codes - welding and inspection - defect assessment H.1 Design and design codes

For the purpose of this discussion it is assumed that the design is carried out using a set of well-recognised design codes and that the occurrence of design errors can be disregarded. As stated in Section 6.5 the design codes are acceptable only when good workmanship, including the use of good materials, can be assumed. In that case compliance with the code is a sufficient condition for an acceptable structure.

The design code itself is based on historic developments for which any major failure has been checked and, if necessary, incorporated in subsequent revisions of the code.

Hence the only item which requires further attention is the definition of good workmanship in terms of welding and non-destructive inspection which are addressed in the next sections.

H.2 Welding

Welding is a well established method of construction. The aim should be, as identified in Table H1, to optimise welding to reduce defects. This is achieved through the detailed description of the welding procedures and a QA plan to ensure that the procedures are complied with in practice. In addition, much attention is paid to welder qualification.

Secondly, once the code for fabrication has been defined then also defect acceptance from NDT is part of the fabrication. Here an economic argument comes in: if the number of defects are too high then the manufacturer has an economic interest in improving welding because the repair of welding defects is a costly process which also has a bearing on the scheduling of the manufacturing.

It is important not only from an economic point of view but also for structural safety to have an indication of the unacceptable defects left in-place. If the CRR (correct rejection rate) is of the order of 60% then the number of repairs per metre of welding provides a good, first order estimate of the number of rejectable defects left in place as well. Therefore:

Item 1: More information should be collected on the number of repairs per metre of welding.

The number of repairs, or rejectable defects left in place, should have a bearing on the defect assessment.

It will depend on the type of structure and on the adoption of a manual or automatic welding process.

H.3 Inspection

The main objective of this report is on POD/POS of inspection. Historically the main methods for the detection of buried defects are RT and UT. Much effort is put in optimising inspection methods by procedures and to train inspectors and insist on inspector qualification. Much work both in the USA and Europe are ongoing in this area.

In the report a CRR of 60% was quoted as a suitable first approximation for the detection rate of rejectable defects. The question should be asked how the size of rejectable defects are determined. It is based on historic evidence: the detection rate of the rejectable defects should be sufficiently large so that the majority of these defects can be found. Often inspection is directed through economic arguments: i.e.

what is the cheapest, accepted inspection method for a certain application. Therefore:

Item 2: More information on the economics of inspection should be gathered and analysed.

The POD can be improved by using two independent methods. In that way the CRR can be improved from say 60% to over 80%, which is a major step. This leads to the following question:

Item 3: Analysis should be carried out to determine the economic advantage in increasing the correct rejection ratio (CRR) from 60% to 80%.

In other words, if a CRR of 60% leads to a structure which is fit-for-purpose then the increase to a POD of 80% is an unnecessary expenditure.

An other item with regards to POD, which should be further addressed, is related to the high variability in POD for MPI. Both in UCL and ICON, where underwater MPI was used, the POD even for small defects was high whereas for TIP and Nordtest the POD for MPI, using land-based methods, showed large variations. Therefore:

Item 4: More fundamental work is required in the area of MPI to explain the large difference in POD between onshore and offshore practices.

If inspection is considered as a QA tool then the fabricated structure, after inspection and repair, is a sufficient condition to ensure that the structure is fit-for-purpose. In other words: in that case NDT ensures good workmanship.

Finally, it is not uncommon to use RT to check for defects and supplement it by UT for the sizing and categorising (reject/accept) of the defect. Particularly the developments of TOFD are worth mentioning:

it is the application of geo-science applied to welded structures. It provides an independent method with excellent potential for automation (as for example shown on pipeline inspection) and currently particularly suitable for defect sizing in simple geometries. Therefore:

Item 5: The development of TOFD for the sizing of defects in complex geometries should be stimulated.

This is an ongoing activity for example at NIL.

H.4 Missed rejectable defects

For further assessment it is essential to determine a characteristic defect or the maximum associated defect. Historically it is based on expert opinion and it is often the primary variable once the defect assessment procedure (e.g. PD 6493) for a given structure has been adopted.

Item 6: It is necessary to develop a rational basis for the defect size for defect assessment.

It is understood that this is one of the objectives of SINTAP.

H.5 Defect assessment

The beauty of defect assessment is that for known defects a criticality evaluation can be carried out and repairs can be avoided on a rational basis. However, the methodology is known to be conservative.

As recently demonstrated on cracked tubular joints, there appears to be a simple alternative to defect assessment, namely to consider the net effective area only. This approach seems justifiable for modern ductile material with proper, modern welding practices. Therefore there is a need for a more rational basis for defect assessment of real structures:

Item 7: There should be more full scale tests to support and give direction to defect assessment.

CTOD is often the basis for defect assessment. However, in the old days with poorer materials and welding practices fully acceptable structures were designed and built based on adequate Charpy values of the material and the welding. This provides a vast database and should be used as well. Hence:

Item 8: Historic data on older structures can also be used to calibrate defect assessment procedures.

H.6 Closing remarks

The topic addressed under items 7-8 of full scale testing and re-assessment of older structures falls outside the scope of the present study. However, it seems to be the only rational basis to ensure that a higher performance in inspection is cost effective and fit-for-purpose.

The full scale testing of specimens with known defects has been applied before; for example, in Ref. 22, tubular joints with fatigue cracks were tested to destruction. It has been demonstrated in these tests that for good quality steel the detrimental effect of defects can be calculated by considering the net effective area only. Hence the effect of small defects on the ultimate capacity of tubular joints is small.

Secondly, in the NIL project it was mentioned that it is very well possible to weld structures with pre-determined welding defects. Also JRC-Petten is able to fabricate surface defects of known shape through spark-erosion. Ref. 23 addresses this topic of full scale testing of pipeline structures and the consequences of given Charpy and CTOD values. A similar, more general approach is proposed in Ref. 24.

DESIGN CODES Design Codes

1. Design assumes good workmanship

2. Compliance with the code is a sufficient condition for an acceptable structure

WELDING welding

assume defect ignore defect

assessment How reliable is defect assessment?

ISBN 0-7176-2297-5