Howard Wang

The Effect of Reeling on the Fatigue of Welded Risers

ExxonMobil Upstream Research Company (URC) conducted a pilot testing program to systematically assess the effect of reeling on the fatigue performance of welded risers. Given the criticality of deepwater risers, a robust reeling simulation method was developed. The method properly addresses the fundamental weld sampling issue arising from the randomness of the plane in which risers are reeled. To address that issue the nominal reeling strain is accumulated around the entire circumference of girth weld, thereby capturing the weak fatigue bending plane. The reeling simulation is implemented by using multi-plane bending. The number and orientation of bending planes needed to simulate a total cumulative reeling strain are based on (1) the strain experienced during the process being simulated (2) weld fabrication and inspection records and (3) residual stresses left after reeling calculated via finite element analyses. Thirty production-quality welds were made on 323mm OD × 20.5-mm WT (12.75-in OD × 0.81-in WT) seamless X60 pipe, yielding a total of 14 full-scale specimens. Two different welding procedures and two different cumulative nominal strain levels were used. The effect of reeling on fatigue was assessed by comparing the fatigue lives obtained with companion reeled and non-reeled full-scale specimens. Results indicate that, although there was a distinct deleterious effect of reeling on fatigue life, the performance may still be adequate for most riser applications when high quality welds are used and qualified.© 2010 ASME

Numerical Analysis of Experimental Data of Subsea Jumper Vortex Induced Vibrations

Vortex Induced Vibrations (VIV) may cause fatigue damage to subsea jumpers that are exposed to bottom currents. ExxonMobil Upstream Research Company (URC) has been conducting research on VIV of subsea jumpers since 2011. Model tests conducted on an “M” shaped jumper in 2012 showed that VIV can occur in subsea jumpers over a wide range of bottom current speeds and directions [1]. Presently, there is no well-established industry practice for assessing subsea jumper VIV and determining the need for its suppression. In this paper, we present two different methods for characterizing jumper VIV response based on the model test data described in [1]. Specifically, these methods consist of spectral analysis of local response and modal scalar analysis of global response. These methods are used to analyze measured response over a wide range of towing speeds and towing directions. A brief summary of the findings are provided and some general conclusions are drawn.Copyright

VIV Response of a Subsea Jumper in Uniform Current

Subsea jumpers are susceptible to in-line and/or cross-flow vortex induced vibration (VIV) fatigue damage due to sea bottom currents. However, there is no proven industry standard design analysis methodology currently available specifically for assessing subsea jumper VIV response.In 2012, ExxonMobil conducted a jumper VIV model test to assess the validity of potential jumper VIV prediction approaches. A towing test rig was used to expose a small scale jumper model to flow conditions simulating uniform bottom currents. The jumper model was instrumented to acquire acceleration, bending strain and end connection load data. Several accelerometers and strain gauges were installed to enable reconstruction of static and dynamic deformations and bending deflections along the jumper model. Towing tests at different orientations and tow speeds were performed on both a bare pipe model and a straked pipe model. The data were analyzed to examine the frequencies and amplitudes of the jumper vibration. The data from these experiments provide a benchmark for validating jumper VIV prediction approaches.In this paper, the model test program is presented including model testing philosophy, jumper design and fabrication, and high level model test results.Copyright

SCR Application for Turret Moored FPSOs in West Africa

Turret moored Floating Production Storage and Offloading (FPSO) systems with flexible risers have been successfully used in many deepwater developments in West Africa. However, Steel Catenary Risers (SCR) have not been used on a turret moored FPSO because of concerns with the riser fatigue and compression loading due to the large FPSO motions at the turret location. ExxonMobil has initiated a development program to establish the feasibility of SCRs on turret moored FPSOs. The goal of this work effort is enable the use of SCRs on West African turret-moored FPSOs and thereby expand the riser concept options to bring potential economic advantage for future prospects. Additionally, the SCRs may be needed to comply with local content requirements. The development program consists of identifying the limitations of a conventional SCR design, establishing feasible SCR concepts that meet the strength and fatigue requirements, and pre-qualification of a high fatigue performance welding technology. The program focuses on alternate SCR concepts including SCRs with locally weighted and buoyant sections. This paper presents the SCR concepts and the associated analysis results which demonstrate the elimination of compressive loads in the touch down area and the achievement of adequate weld fatigue performance throughout the riser. This paper further presents the results of the inconel weld pre-qualification program including fatigue tests on large and small diameter pipe to demonstrate enhanced fatigue performance.Copyright