Your client is correct. TOFD has a near surface dead zone necessitating a two sided scan to provide full coverage. Check out

http://www.mpa-lifetech.de/TOFD/

Go to TOFDPROOF Public Documents and check out 'Recommendations For Applying TOFD'   

Steve Jones
Materials & Corrosion Engineer
http://www.pdo.co.om/pdo/

For explanation of TOFD dead zones go to

http://images.google.com/imgres?imgurl=http://www.olympusndt.com/data/Image/148-fig-1.gif&;imgrefurl=http://www.olympusndt.com/ndt-application/87-id.209715220.html&h=288&w=409&sz=21&hl=en&start=3&tbnid=Id0yYe0rrmpT_M:&tbnh=88&tbnw=125&;prev=/images%3Fq%3DTOFD%2Bdead%2Bzone%26gbv%3D2%26svnum%3D10%26hl%3Den%26sa%3DG


luismarques

First, the fundamental of the TOFD examination method is that it relies on detecting very weak tip-diffracted waves from discontinuities. However a sub-surface "lateral wave" travels directly from the transmitter probe to the receiver probe. Depth of penetration of this lateral wave depends on test set-up parameters but is usually considered to reach between 2 and 5 mm deep. Any small tip-diffracted signals originating from this zone may be swamped by the much higher amplitude lateral wave signal. For the ID the reflected back-wall signal is also of high amplitude and may also obscure weak tip-diffracted signals from discontinuities.

The strenght of the TOFD technique is that tip-diffraction is non-orientation dependent, so the PoD for weld defects is much higher. In addition, through-thickness defect height-sizing has a proven higher accuracy than conventional pulse-echo (PE) technique. The lower sizing accuracy of the PE technique is hat has hampered the introduction of Fitness-For-Purpose acceptance criteria replacing the traditional workmanship-based approach as typified in API 1104 and ASME VIII where height of a defect is not taken into consideration.


ASME Code Case 2235-9 Use of Ultrasonic Examination in Lieu of Radiography Section I; Section VIII, Divisions 1 and 2; and Section XII states:

"The ultrasonic examination shall be performed in accordance with a written procedure conforming to the requirements of Section V, Article 4.1 The procedure shall have been demonstrated to perform acceptably on a qualification block(s). The qualification block(s) shall be prepared by welding or the hot isostatic process (HIP) and shall contain a minimum of three flaws, oriented to simulate flaws parallel to the production weld’s fusion line as follows:

(1) one surface flaw on the side of the block representing
the vessel OD surface
(2) one surface flaw on the side of the block representing
the vessel ID surface
(3) one subsurface flaw"

If, after optimisation of the TOFD set-up parameters, the examination is shown to be incapable of detecting, positioning  and accurately sizing (height and length) from the OD alone there are other means of rectifying this problem than resorting to an ID scan. Pulse-echo probe pairs checking the near-surface and root welds areas can be incorporated in to the test procedure. An expensive business as the procedure qualification should be reiterated.

Surely you can refuse the internal TOFD scan on ergonomics alone? Maintaining probe contact on a concave surface can be difficult. For external circumferential scans the TOFD probe jig can easily be guided around the circumference maintaining a constant distance from the weld. Inserting the necessary guide strips and the TOFD jig inside the vessel and obtaining a good scan will be difficult and mitigates against an internal scan.

My advice would be to consult with TOFD specialists and let them determine how best to demonstrate and deliver full inspectability of the welds to your client.

P.S. On Sept 2006 at a conference in Berlin a European pre-standard CEN/TS 14751 TOFD on Weld Seams was presentd. TS denotes Technical Specification which in turn means that the "state-of-the-art is not yet stable enough." A corresponding acceptance levels document is "in progress".

Nigel Armstrong
Karachaganak Petroleum
Kazakhstan