For repair and hot-tapping of pipelines in the field, often so-called repair sleeves or split-tees are used. These consist of two semi-circular pipe segments, joined by two longitudinal welds and attached to the pipe by two circumferential fillet welds. Whereas the Time of Flight Diffraction (ToFD) method is successfully applied for inspection of the longitudinal welds, NDE possibilities for the circumferential fillet welds are limited. Radiography is not an option, a.o. because of the thickness variations. The weld's complex, varying geometry prevents a reliable volumetric ultrasonic inspection. N.V. Nederlandse Gasunie and Röntgen Technische Dienst bv in The Netherlands are conducting a modeling study in order to optimize parameters for inspection by ultrasonic phased arrays and ToFD. The ultimate goal is to achieve a level of NDE reliability for these welds that is comparable to that of circumferential butt welds. The presentation will highlight some results and provide an outlook on the implications of split-tee fillet welding and NDE. Phase 1 computer modeling of ultrasonic testing of circumferential fillet welds of split-tee and repair sleeves on pipelines has shown that, using manual UT, discrimination between relevant fusion defects and welding-related small, non-relevant imperfections is far from reliable. Large defects can give signals similar to those of small imperfections. This result was confirmed by a Round Robin test on four welds, using five operators. Phase 1 modelling study showed that this performance can significantly be improved by making better use of reflected and diffracted signals generated by defects and geometry. Although the current manual UT procedure gives room for improvement, significant improvements can be expected from the use of mechanized techniques using phased array and TOFD. This will be done in phase 2.