Tenaris and ExxonMobil conducted a qualification program focused on Heavy Wall Seamless Line Pipe (HW SMLS LP) and its girth welds, to experimentally demonstrate and validate the material potential to be used as riser or fatigue sensitive flowline for the development of challenging deep and ultra-deep offshore fields. The qualification program was divided in three phases. Phase I [1] included: 1) HW SMLS LP, X65QS, 273 mm OD, 46 mm WT plain pipe qualification; and 2) development and qualification of a WPS (STT ® process for root pass and GMAW process for hot, fill & cap passes, narrow groove J-bevel type, 1G welding torch position) and full scale fatigue testing qualification. Phase II [2] was focused on evaluating the root girth weld fatigue endurance, taking into account the fatigue behavior observed in Phase I (fatigue crack initiation mainly occurred from the outer surface). The fatigue strength of the weld root surface is needed as the weld root surface is in direct contact with corrosive fluids and is the most sensitive area for sour service applications. Thus, to obtain the root girth weld fatigue endurance, full scale fatigue specimens were manufactured with their caps fully removed (by grinding and flapping) to generate weld cap toe free of stress raisers/imperfections. In addition, pipe outer surface was grinded to mitigate the fatigue crack nucleation from outer pipe surface. With this special preparation, the fatigue strength of weld root surface was able to be assessed. This paper presents Phase III qualification outcomes. The objective was to develop and qualify a WPS representative of J-Lay installation, meaning 2G welding torch position (CMT ® for the root pass and pGMAW process for hot, fill and cap passes, Narrow groove J3°-bevel type). The qualification included full scale fatigue assessment as well. The LP material was from the same batch used in Phase I & II (HW SMLS, X65QS, 273 mm OD, 46 mm WT). The WPS was successfully developed and qualified. The full scale resonant fatigue testing included three specimens with cap smoothed similarly to Phase I, and three specimens with full cap removal plus outer pipe surface grinding similar to Phase II. The fatigue results surpassed the expectations (Class D, S-N curve). Most of the specimen’s fatigue lifetime is above the class B, S-N curve classification (reference for plain material). Obtaining a smooth root weld overfill was key to delaying the fatigue crack nucleation from the weld root toe.

As the offshore oil & gas industry moves into deeper waters, more and more offshore projects, specifically the subsea, riser and flowline designs, rely on heavy wall line pipe materials. These pipe materials must be capable of operating in stringent working conditions such as high pressures, high temperatures, large deformations, fatigue loading, sour environments, etc. Within this context, ExxonMobil Development Company and Tenaris have jointly conducted a detailed technical assessment of the mechanical and fatigue performance of the newly developed heavy wall X65 line pipe (LP) developed by Tenaris. The main goal of such project is to evaluate and demonstrate via full-scale testing the fatigue performance of this new generation of heavy wall line pipe materials and the associated girth weld also recently developed by Tenaris. Although conclusive fatigue performance results at ID are not included in the present paper, the comprehensive test/qualification plan also includes a fatigue performance comparison at ID and OD. The present work clearly demonstrates weldability of this Heavy Wall X65 (273 mm OD × 46 mm WT) line pipe. A narrow-groove bevel welding procedure utilizing the STT ® process for the root pass and single torch GMAW process for hot-pass, fill, and cap has been successfully developed. Four full scale fatigue tests were also successfully conducted using a resonant fatigue test machine. The presented fatigue results help demonstrate a realistic level of fatigue performance achievable with this thick wall LP/girth weld technology which will help facilitate assessment of riser/flowline design feasibility in ultra-deep water applications and/or high pressures fields.

The needs for oil and gas exploration in deep water (DW) and ultra-deep water (UDW) severe environments involve critical requirements of heavy wall materials. Offshore DW and UDW impose demanding service conditions of sour environment, mechanical properties, fatigue performance, gas service, high pressure and wide temperature ranges not only for heavy wall seamless line pipe materials but also for the girth weld performance. Thus, the development of heavy wall materials for severe applications is essential for DW and UDW, where complex material requirements are sought. Additionally the girth welding of heavy wall materials has imposed particularities typical of large wall thickness materials’ welding. The latter requires the development of particular solutions for pre-production and GMAW narrow groove offshore welding procedures. The present work presents the development of two welding processes of a heavy wall seamless pipe material: 273.1 mm OD × 46 mm WT, X65 steel grade. Pre-production welding involves STT®+SAW using a dual slope V-bevel, filler material for root processing was an AWS ER80S-G, while welding deposition for fill and cap passes was made using twin-wire technique, with two different electrodes (ENi1K and EG AWS designations), in combination with a neutral flux. On the other hand, narrow groove welding procedure considered a J-bevel, 3° angle, applying STT®+GMAW; filler material for GMAW was as well an ER80S-G AWS designation. Both welding procedures are aimed to deliver adequate mechanical properties to meet sour-service requirements (<250HV10), weld metal overmatching (120 MPa minimum) and toughness (CVN 45J AVE /38J IND ) at low temperature. Mechanical characterization included hardness Vickers measurements using a 10 kgf load, tensile tests in all-weld metal and transverse impact fracture Charpy V-notch tests and CTOD tests.

Offshore industry has evolved to meet numerous challenges, e.g. deep water, high currents, high pressure and high temperature (HPHT), and sour reservoirs, facing deepwater exploration. The trend in flowline specifications for deepwater offshore fields is a consequence of complex oil-gas field conditions, such as HPHT and developments in design criteria (i.e. limit state design), welding and laying technologies. The technological evolution exhibits a trend towards an increasing wall thickness (WT) to provide sufficient resistance for the very high operating pressures. Furthermore, the pipelay operations, especially when linepipes are installed by means of the reel laying method, cause repeated plastic bending and straightening deformation cycles. These cyclic loads affect final material stress-strain properties. Reeling is currently applied to an increasing range of pipe geometries, being the present limit given by pipes with 16 ″ outer diameter (OD) and 30 mm wall thickness (WT). Other pipeline installation techniques, for example, J-lay, S-lay and steep S-lay also introduce plastic strain. All previous factors mentioned before and adding one more variable when exploring and producing in regions alike to the Artic where low temperatures implied several material challenges calls for high performance seamless pipes tailored to the specific application required by the oil and gas industry. In this paper, a description is given of the results of latest fundamental studies on high-strength heavy-wall steel materials manufactured by Q&T processing. This work is part of an on-going development program on high performance heavy wall seamless pipes for special applications such as HPHT, low temperature design criteria, sour requirements and studying the material under the strain based design criteria involving metallurgical modeling, laboratory tests, industrial trials and advanced metallographic examinations. The most recent findings and overall conclusions are reported hereafter, these results have been exploited by Tenaris to manufacture a limited production seamless pipes in a wall thickness range from 40 mm to 48 mm in steel grade X65 Sour Service.