Design Optimisation of Deepwater Hybrid Riser Base Jumpers
Authors: Shanks John , Hahn Guillermo , Xia Jianjun 
Published: May 2011

The design of rigid jumpers at the base of deepwater dynamic Free Standing Hybrid Risers (FSHRs) represents a complex engineering challenge. Design issues involve strength under extreme static loads including relative end displacements, strength and fatigue under forced dynamics due to riser vortex induced vibration (VIV), direct VIV due to seabed bottom currents, internal flow induced vibration, cyclic thermal loads, and installation. This paper considers design optimisation of the Cascade and Chinook Riser Base Jumpers (RBJs) in the Gulf of Mexico for strength and fatigue under static and VIV loads since these conditions are generally dimensioning. A distinguishing feature in the engineering of such jumpers is a balanced approach to reconcile requirements for strength with those for fatigue because these tend to drive the design in opposite directions. In particular, strength considerations dictate a low stiffness solution to avoid high stress under jumper end motions, while the avoidance of dynamic coupling with the riser, in order to reduce fatigue damage, generally requires a high stiffness design. These conflicting requirements were satisfied by parameterising a base case design geometry and performing an automated design optimisation to minimise the fatigue-producing dynamic stress range (object function) under defined dynamic end loads while subject to specified constraints arising from geometric and strength requirements. The resulting design geometry increased fatigue lives by approximately two orders of magnitude compared to an early configuration resulting from strength design only. Following the design optimisation, other less critical design conditions were checked and found to be satisfactory.

Introduction

Figure 1 illustrates the FSHR system employed on the Petrobras Cascade and Chinook Field Development in 2500m of water. The main vertical steel riser pipe is anchored at the bottom to a suction anchor foundation structure with a flexible Roto-latch connection and maintained in tension via a Buoyancy Can (BC) located at 200m depth. A flexible pipe jumper provides tieback to the disconnectable Floating Production Storage and Offloading (FPSO) vessel turret system while at the seabed a Riser Base Jumper connects the riser to the associated Pipe-Line End Termination (PLET) as indicated in Figure 2. Further details of the FSHRs can be found in related papers, References 1 and 2.

Key design considerations for the RBJ structures include:Strength under extreme static loads including relative end displacements,Strength and fatigue under forced dynamics arising from riser VIV,Direct VIV due to bottom current conditions,Internal flow induced vibration,Cyclic thermal loads andFabrication, load-out, transportation and installation.

When considered alongside the large number of possible design parameters associated with a typical RBJ structure, these numerous and varied design conditions have proven a significant challenge for several recent Projects involving deepwater FSHR systems. This paper addresses this challenge by presenting an automated design optimisation procedure for strength and fatigue of the Cascade and Chinook RBJ structures under static strength and dynamic riser VIV loads since these conditions are generally dimensioning.

An alternative design solution for isolating the RBJ from riser VIV motion is the use of a rigid connection between vertical riser and foundation structure. However this approach introduces different challenges including more complex strength and fatigue design of the lower riser (possibly leading to impractical Taper Stress Joint requirement), increased foundation loading, increased risk of suction anchor pull-out and consequent need for heavier, more fatigue resistant, foundation structure.