ABSTRACT

For a deepwater vertical riser cluster, interference between the individual risers in strong ocean currents is a key design concern. The riser lateral deflections are likely to be large and the risers are prone to wake-induced clashing with possible detrimental effects. In this paper, a stability analysis methodology is developed to address the wake-induced instability of a pair of vertical risers with one located in the wake of another. A comprehensive parametric investigation is carried out to examine the effects of various parameters varying in wide ranges. In particular, effort is made to investigate the different instability mechanisms of a marine riser situated in the near or far wake field of the upstream riser. It is found that the fluid-elastic instability of the downstream riser can be caused by either a Hopf or a stationary bifurcation depending upon its relative position to the upstream riser.

INTRODUCTION

Marine production riser systems can be broadly divided into two groups, i.e. top tensioned vertical risers as used in TLPs and SPARs and flexible/steel catenary risers as used in conjunction with spread moored tanker-shaped floating production units. For the top tensioned vertical riser system, the vortex-induced vibration was and still is a primary design concern which has been under extensive investigation since the early days of offshore oil and gas exploration (Pantazopoulos, 1994; Sarpkaya, 1979; Vandiver, 1998). As the industry moves into ever deeper waters, an additional problem of the riser system emerges, i.e. the interference between individual risers in the riser cluster in strong ocean currents as the riser lateral deflections are likely to be large and the risers are prone to wake-induced clashing. The central thrust of his theoretical work on risers clearance was based upon this wake shielding drag model to predict the minimum gap between two vertical risers under various currents.

This content is only available via PDF.
You can access this article if you purchase or spend a download.