ABSTRACT

A dynamic riser system is a key component in offshore deepwater development. The long-term fatigue performance of a riser is a crucial design consideration in such systems. As water depth increases, the riser fatigue assessment becomes increasingly challenging. Overly conservative designs can be costly, whilst less conservative designs can result in the higher probability of catastrophic failure. In some cases, the target fatigue life cannot be achieved with only conventional riser design and analysis. Additional physical fatigue mitigation means are required and need to be developed.

The Weld Fatigue Enhancer (WFE), a physical device, was jointly developed by ExxonMobil, Balmoral and Stress Engineering Services, Inc. with the goal of improving riser fatigue performance. A patent for this device has been recently granted by US government (Wang, H et al, 2018). The qualification tests have shown that the WFE can reduce hotspot stress experienced at a girth weld by ∼40% or more. This effectively increases the riser fatigue life in the range of ∼5-10 times, which can make riser systems feasible in some challenging environments. It can also provide the possibility of using more cost-effective welds. In addition, the WFE can also be used for a variety of fatigue sensitive applications, such as pipeline and dynamic jumpers.

This paper introduces the weld fatigue enhancer, including the design, functionalities, and potential applications. It discusses the full-scale testing on the prototype model of the weld fatigue enhancer. Additionally, it briefly discusses the potential design improvements.

INTRODUCTION

Dynamic riser systems, as shown in Figure 1, are a key component in offshore deepwater development. The long-term fatigue performance of a riser is a very challenging technical issue and requires special design consideration. The fatigue performance becomes increasingly challenging with increasing water depth especially for rigid riser systems including steel catenary risers (SCR) and lazy wave steel catenary risers (LWSCR). Overly conservative design can be costly, whilst less conservative design can result in the possibility of catastrophic failure. In some cases, the target fatigue life cannot be achieved by means of normal riser design and analysis. Because of these limitations, additional physical fatigue mitigation means could be explored and developed to allow for the design of riser systems in challenging environments.

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