ABSTRACT

An oil field X in South China Sea was considering using TLP, which was the first time in China. The interference of TLP drilling and production risers may result in fatigue damage and even diffusion. Based on local conditions, an optimized finite element method was used to analyse the riser minimum clearances. The sensitivity analysis were further conducted, parameters including water depth, top tension and platform offset. The results showed that the most dangerous condition is the combination that the upstream is production riser and the downstream is drilling riser. The sensitivity analysis revealed that water depth has little effect on interference, and the minimum clearance is more sensitive to top tension and platform offset. Larger top tension tends to reduce the vibration amplitude and plays a positive role in avoiding the interference.

INTRODUCTION

X oilfield in the South China Sea plans to use TLP (Tension Leg Platform) for the first time. TLP is an important platform for deep-water oil and gas development. The advantages of TLP include good athletic performance, strong resistance to adverse environment and good economic performance. Drilling riser and production riser are important auxiliary equipment of TLP. When waves and currents flow through drilling risers or production risers, different degrees of fluid-induced vibration will occur in the pipe body due to the periodically changing fluid force.

The shielding effect between the risers can change the local velocity of flow, which will lead to a large relative displacement between risers, resulting in collision between risers (Huse, 1996; Leira, 2002). The collision may create fatigue damage and even fluid diffusion, causing ocean pollution. Therefore, it is of significance to study the interference collision between risers to ensure drilling and production safety under TLP development mode.

On the interference responses of multiple riser system, Scholars have studied and analyzed for decades. Huera-Huarte and Bearman (2011) and Huera-Huarte and Gharib (2011) conducted vortex- and wake-induced vibrations of a tandem arrangement of two identical flexible circular cylinders with far and near wake interference respectively. It was found that the downstream cylinder experiences wake-induced vibration (WIV) in the cases of larger gap spacing at high reduced velocity. Xu et al. (2018) also carried out an experimental investigation on multi-mode flow-induced vibrations of two identical flexible cylinders in a tandem arrangement. Results show that the downstream cylinder, subjected in the wake of the upstream one, is different from the upstream cylinder. Besides, some model tests of two cylinders in tandem were also conducted by Armin et al. (2018), Sanaati and Kato (2014), Huang and Herfjord (2013), Assi et al. (2010) and so on. Additionally, computational fluid dynamics (CFD) is another way to investigate the problem (Wang et al. 2017; Zhao et al. 2016).

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