Truss Riser Tower in Deep Water
Authors: Dailey J.E. , Healy B.E. , Zhang J. 
Published: October 2008

This paper discusses a hybrid riser tower termed a truss riser tower. It is best suited to applications where steel catenary risers may be less applicable and to offshore arenas that offer conventional land-based fabrication capabilities. For complete field development, a single truss riser tower can accommodate a remarkably large number of flowline and service line risers. The design builds on offshore construction principles proven through long experience. Results presented in the paper show that a single-piece, towed-out tower configuration can be fabricated on land, launched, towed to an offshore installation site, up-righted, and connected to a pre-installed foundation on the sea floor.

Introduction

The truss riser tower configuration was developed with reference to projects offshore Brazil and offshore West Africa. Preliminary subsea layouts for these two projects are shown in Figures 1 and 2. Both field developments contemplate an array of wells on the seabed that serve both production and injection purposes. On the seabed, these wells are clustered in templates that are connected by a network of flowlines and service lines.

Flow assurance is a very important consideration in the design of production flowlines. By keeping flowline lengths as short as possible, pressure and temperature drops in produced fluids can be minimized. Thus, maximum length of any given production flowline on the seabed may be limited by placing the network gathering point near the nominal center of a seabed well array. Service line spatial orientation is driven by injection well distribution on the seabed, which generally should follow the production well distribution.

In both subsea layouts, the flowline and service line networks nominally converge to single points where risers connect the lines to floating production vessels (FPSOs) at the sea surface. Figures 3 and 4 show a typical truss riser tower and FPSO arrangement at the nominal point of convergence. The truss riser tower can interface with a broad range of different FPSOs.

At the sea surface, flowlines and service lines enter the FPSO through an I-tube array (Figure 5). Manifolding and system design considerations organize the flowline and service line I-tubes into groupings associated with production/gas lift, water injection, gas injection and control functions. Depending on the FPSO configuration, it may be desirable to separate the production and injection groupings for both process and personnel safety reasons.

Inherently, line split-out requirements at the top and bottom of the riser tower must differ. At the bottom, lines must exit outwards in all azimuth directions to accommodate the global field development configuration. This notion is illustrated in Figure 6. At the top, lines must exit outwards in azimuth directions that are compatible with the floating production vessel. Further, split-outs at the top of the tower can be arranged in multiple levels to mitigate potential interference problems (Figure 7).

Within the riser tower, differences in split-out requirements at the top and bottom may be reconciled by crossover piping.