Groove sizing beneath the waves - an in-depth probe

TWI Bulletin, July/August 1993

After obtaining a degree in Mechanical Engineering, followed by a PhD for work on the stress analysis of discontinuous pipe bends, Graham joined TWI in 1975. He worked in the Fatigue Department on a wide range of projects, including many associated with the fatigue design of offshore structures. In 1985 he joined the Edison Welding Institute as Manager of Engineering and was responsible for setting up the Engineering Department and establishing mechanical testing facilities in the laboratories in Columbus, Ohio. After returning to Abington, he was appointed Manager of the Marketing and Membership Group. This group covers membership development, member services and marketing and information.

Unless you have ever tried to measure the depth of a groove you would be entitled to wonder why an instrument such as the Linear Angular Measurement (LAM) gauge is necessary. It is actually quite a difficult task. If you can accomplish this for a simple groove, try it on an angled groove on a curved surface, add 50 feet of murky water and a layer of marine growth, and the advantages of the LAM gauge become more and more apparent. But why do we need to take such measurements at all? Graham Wylde looks at the LAM gauge and some of its potential uses to discover why it is becoming an important asset in underwater inspection.

A common observation during inspection is a linear indication at the weld toe. These indications may be caused by a number of factors. They may be a feature of the weld geometry, incipient corrosion, or more seriously, they may be the first signs of cracking. This cracking may have originated during fabrication, but had not been detected during previous inspections, or possibly is the first indication of fatigue or other service related cracking.

The consequences of cracks in offshore structures can be very serious and thus, once a linear indication is detected at a weld toe, remedial action will generally be necessary. Weld repairs offshore can be extremely expensive and thus the first stage, when an indication of the type described above is detected, is to attempt to establish its true nature and extent.

One relatively quick method to establish the extent of the problem is to use a pencil or burr grinder to remove a shallow depth of material from the weld toe along the length of the indication. Once this has been carried out to a depth of a few millimetres, the joint can be re-inspected and one of two things will have happened:

• the indication will have been removed - end of problem;
• the indication will still be seen at the bottom of the groove.

Provided that the cracked members are relatively thick and the width of the indication is sufficiently small to suggest that the crack is unlikely to extend through the full thickness, a viable alternative to any form of weld repair may be remedial grinding. Tests ^[1,2] have shown that under some circumstances a weld with a relatively deep groove at the weld toe may still possess a significant fatigue strength and may thus be fit for service.

Tests on fillet welded plate specimens with groove depths of 20, 33 and 50% of the plate thickness at the weld toe, have indicated that the fatigue strength of the joints was surprisingly good. Indeed, a specimen with a full width groove equal to 20% of the plate thickness had a similar fatigue strength to an as-welded joint. ^[1] Tests on welded tubular T joints under out-of-plane bending, ^[2] showed that joints containing groove repairs, ranging from 25 to 69% of the chord wall thickness, all gave fatigue lives to through-thickness cracking in excess of the mean for as-welded joints.

The reason for this apparently strange observation is that as-welded joints have an inherently low fatigue strength compared with plain material. This is primarily because welded joints contain imperfections at the weld toes in the form of sharp slag intrusions. ^[3] These intrusions form sites for fatigue crack initiation, which can occur at a much lower applied stress range than for plain material. By grinding and thus removing these sharp discontinuities, a significant improvement in fatigue strength can be achieved. As grinding depth increases, this improvement has to be set against the increased stress across the reduced section and the stress concentration at the base of the groove. At a groove depth of approximately 20% plate thickness for a flat plate with a fillet welded attachment, these two factors are approximately equal.

Of course, the situation offshore is more critical than in the laboratory, but the principle of the groove repair as a viable repair method remains, and is used offshore. However, in such instances an assessment of the groove depth, joint geometry and applied loadings must be made by a competent engineer.

Groove depth measurement

In groove repair, the need to measure groove depth to a reasonable accuracy plays a vital part in ensuring the subsequent integrity of the structure. Before the development of the LAM gauge, various other instruments were used. TWI has produced the weld gauge for many years ( Fig.1). This is perfectly satisfactory for measuring very shallow groove depths, such as joint undercut, but was never intended for grooves of any significant depth. It was a diver inspector who, out of frustration with the lack of a suitable instrument for measuring grinding depth in complex tubular joints offshore, developed the LAM gauge.

The LAM gauge ( Fig.2) comprises a telescopic depth probe mounted through an angular pivot. The instrument is held in place using pivoting magnetic clamps. This is vital when used underwater as it leaves the diver with both hands free.

The gauge is extremely simple to use. For grooves in flat plate, which are normal to the surface, the angular pivot is first locked to the 90° position. The gauge is placed on the plate surface with the depth probe resting on the plate surface adjacent to the edge of the groove. The scale is zeroed using a simple adjustment. The gauge is moved, allowing the depth gauge to slide to the base of the groove. It is locked, removed and the groove depth read directly from the scale.

For angled grooves, the telescopic probe is first used to assess the angle of the groove, which is read from the angular scale. Once this is established the scale is locked and the telescopic probe zeroed on the edge of the groove as before. Once again, it can be lowered to the bottom of the groove to determine groove depth.

In this instance, however, it is important to appreciate that a cosine correction must be applied to the groove depth to determine the remaining ligament size. This is illustrated in Fig.3.

The operation is equally straightforward on curved surfaces such as those of tubular members in offshore structures. In this case, in addition to the cosine correction, a further correction for the plate curvature is necessary. Corrected depths are tabulated for ease of use and can be applied to the actual measurements at a later stage.

In addition to remedial grinding depth, the LAM gauge has many other applications. Figure 4 shows how it can be used for measuring excess weld metal.

Another interesting application in relation to structural integrity is its use for measuring corrosion pitting depths and other surface damage - dents or grooves. For estimating the remaining ligament after extensive local corrosion, an additional straight edge may be used as a bridge, as illustrated ( Fig.5).

Benefits of the LAM gauge

The LAM gauge has many different uses and thus many potential benefits. However, it was developed specifically for measuring groove depth in complex tubular joints underwater and for this job it is unique.

The LAM gauge is the first instrument to enable such measurements to be taken with sufficient accuracy to allow engineers to determine the remaining ligament (the depth of sound material beneath the groove) with confidence. This measurement is vital when assessing the fitness-for-purpose of groove repairs.

Although the LAM gauge has only been available for one full North Sea diving season, it is already seen as a vital tool of the diver inspector. Divers have found it easy to use and the only reliable method for obtaining consistently accurate measurements of groove depth.

It is likely that the gauge will find uses in many other industry sectors, including chemical plant, ship repair and nuclear plant inspection.

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