In typical insulated Pipe-in-pipe (PIP) systems, both extremities of the outer jacket pipe are swaged down close to the inner pipe and are welded directly to the outer surface of the inner pipe. This full penetration weld (hereafter called swaged weld) works as a structure connection between the inner and outer pipes and seals the annulus between the two pipes. The integrity of this swaged weld ensures the structure integrity of whole PIP as well as its thermal properties.

The non standard geometry of this swaged weld presents a real challenge for its Non Destructive Examination (NDE). Despite of its long existence and its importance, the inspection of this swaged weld has always been performed by Manual Ultrasonic Testing (MUT) or Semi Automated Ultrasonic Testing.

This paper presents an Automated Ultrasonic Testing (AUT) system recently developed to inspect this type of weld more efficiently and more completely. The principle of this system and its Det Norske Veritas (DNV) qualified performance in term of defect detection capability and sizing accuracy will be described.

To enhance the integrity of the inspected welds, fitness-for-purpose acceptance criteria have to be developed for this AUT system. Due to the range in weld geometry, the analytic approach conventionally employed for girth welds is no longer applicable. A Finite Element Analysis (FEA) based on Engineering Critical Assessment (ECA) approach has to be applied and this approach will be presented.

In a recent West African SURF project, this AUT system coupled with fitness-for-purpose acceptance criteria has been applied with success.

1. Introduction

Developments of the high temperature high pressure (HTHP) field bring on stage more strict requirements on thermal insulation of the pipelines. Conventional wet thermal insulation such as 5-layer polypropylene foam (5LPP foam) or 5-layer syntactic polypropylene (5L syntactic PP) turns out to be limited both in terms of the insulation efficiency (U value over 1.5W/ m2.K or 0,264BTU/hr.ft2.°F) and the maximum service temperature (approximately 130°C or 166°F). The Pipe-in-pipe (PIP) system presents a good alternative where conventional wet thermal insulation is not applicable. A typical PIP system can provide U value down to 0.5W/ m2.K or 0.09BTU/hr.ft2.°F, which extends the cooling down time of the pipelines and reduces the risk of hydrate formation and waxing during shutdowns and start ups.

PIP systems were introduced to the fields inWest Africa more than a decade ago. The first typical PIP system was installed in Tchibeli field; one PIP production line, with a length of 25km, was installed in 1999. More recently such system was installed in several other fields such as Bonga and Kizomba Satellite.

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