Abstract

The deepwater drilling riser is an important structure which is used for connecting the offshore drilling platform and the subsea wellhead. During service, the riser is subjected to severe loads which will result in complex mechanical behavior. However, the marine environment of deepwater drilling engineering is harsh, complex and changeable, especially in extreme situations such as typhoon. In order to protect the wellhead and the drilling platform, disconnection operation between the riser and the wellhead must be carried out. Thus, the riser is in the state of suspended. In addition, emergency evacuation will be implemented when the severe weather occurs. Generally, the hard hang-off mode is usually selected as an operation mode of the riser evacuating from severe weather. Therefore, it is of great significance for the safety control of deepwater drilling to study the mechanical behavior of riser in hard hang-off mode. In this paper, the mechanical model of a hard hang-off riser is regarded as a cantilever beam fixed at the top end and free at the bottom end. The static mechanical response of the suspended riser under different evacuating speed, evacuating directions and different suspended length are studied. Then, the feasibility of the allowable speed and the maximum allowable suspended length when the riser evacuates from typhoon are figured out. Finally, the conclusions are drawn based on the analysis. This study has reference value for safety control of the deepwater drilling riser during emergency evacuation.

INTRODUCTION

Deepwater drilling riser is the most important device connecting the floating platform and the subsea wellhead. In some complex marine environment, such as typhoon, disconnection operation between the lower marine riser package (LMRP) and the subsea blowout preventer (BOP) should be carried out. Under this circumstance, the riser is suspended below the floating platform. Therefore, it is necessary to analyze the mechanical behavior to ensure the safety of the riser during evacuating from severe weather.

At present, a large number of scholars have analyzed the mechanical behavior of the deepwater drilling riser. Burke (Burke, 1974) has proposed the static and dynamic analysis models for marine riser. Simmonds (Simmonds, 1980) has addressed the riser vibration characteristics using the finite difference method through transforming the partial differential equation into a set of nonlinear ordinary differential equations. Trim (Trim, 1991) has derived the equation of the riser after emergency disconnection. Shi et al. (Shi et al., 2004) have analyzed the dynamic characteristics of the riser in deepwater drilling using three-dimensional dynamic model. Wang et al. (Wang et al., 2014) have studied the longitudinal and transverse mechanical behavior of deepwater drilling riser during installation, and figured out the safe window of riser during installation. Ambrose et al. (Ambrose et al., 2001) have studied the soft hang-off mode of ultra-deepwater drilling riser and introduced the advantages, limiting factors and operating procedures of the soft hang-off mode, which pointed out that the soft hang-off mode could greatly reduce the axial tension fluctuation of the riser and widen the operating window of the drilling ship. Brekke et al. (Brekke et al., 2004) have studied the mechanical behavior of the riser under installation and hang-off operation, which pointed out that the main factors restricting the installation operation are ocean current and the heave movement of the drilling platform. Besides, the soft hang-off operation is mainly limited by the deflection angle of the upper flexible joint and the stroke of the telescopic joint while the hard hang-off operation is mainly limited by the fluctuation of excessive tension force. Zhang et al. (Zhang et al., 2010) have analyzed the longitudinal vibration behavior of the riser through regarding the riser as a homogeneous beam. Ju et al. (Ju et al., 2011) have provided a method to determine the top tension of the riser based on the residual tension at the bottom of the riser system and the maximum hook load of the riser system. Sun et al. (Sun et al., 2009) have studied the axial dynamic characteristics of ultra-deepwater riser under hard and soft hang-off mode and discussed the management strategy of the emergency evacuation.

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