Dynamic Compression Buckling in Flexible Riser
Authors: Ribeiro E.J.B. , Roveri F.E. , Mourelle M.M. 
Published: October 2008

This paper focuses on the global analysis of a free hanging cateruuy flexible riser connected to a turret moored FPSO (floating production storage and oftloading) vessel. By global analysis is meant that riser response in terms of minimum radius of curvature and minimum effective tension at touch down point region and maximum effective tension at top will be of primary interest. Loads taken into account are weight, buoyancy, current, vessel static offset and vessel first order motions. The influence of current, vessel static offset, riser structural damping and riser departure angle with regard to vertical at top on riser global response are investigated.

Introduction

A free hanging cateruuy flexible riser connected to a FPSO may be subjected to negative effective tension and consequently to high curvatures and this has been a concern for the oil industryl. The magnitude of the resulting axial tension depends on various parameters, such as vessel static offset (including second order motions), current intensity and direction, riser departure angle with regard to vertical at top, hydrodynamic damping and vessel first order motions. However, it is likely that the most important of these excitations is the vertical motion (displacement and acceleration) imposed by the vessel at the wave frequencies. In a VLCC (very large crude carrier) being converted to a FPSO to be installed at 780 meters water depth at Campos Basin, offshore Brazil, the distance between vessel center of gravity and the internal turret axis located at bow can be as much as 140 meters. The combined effect of heave and pitch of the vessel during extreme environmental events can produce turret vertical motions in the order of 10 meters for displacement and 2 mJs2 for acceleration (single amplitude). These prescribed motions may cause the riser to undergo compression and exceed the allowable limit for curvature.

Turret vertical motions

Environmental action and vessel draft.

Although the turret provides a weathervane capability to the FPSO, the angle of the wave propagation direction with regard to the vessel longitudinal axis at equilibrium is one aspect which deserves a more thorough investigation in order to develop the load cases for the riser analysis. For this particular FPSO, directional stability is attained passively and no thruster assistance is required. If current, wave and wind are collinear vessel equilibrium is reached at head seas. However, if the environmental actions are not collinear, vessel equilibrium may be such that the wave heading is different from head seas, for instance quartering seas. The consequence of this fact is that turret motions are amplified as the wave incidence departs from 180 degrees (head seas), see Fig. 1. For the environmental conditions at Campos Basin it is assumed that at equilibrium the angle of wave direction with regard to thevessel longitudinal axis lies in the 135-225 degrees range, where 180 degrees corresponds to head seas. The influence of vessel draft on turret vertical motions can also be seen in Fig. 1, where the higher turret vertical displacements are associated to the 21.6 meters vessel draft.