Abstract

Petrobras has around 17.3 billions barrels of oil reserves being half of them under more than 1000 meters water depth. In addition, 60% of offshore exploratory concessions are also deeper than 1000 meters water and half of these concessions at more than 2000 meters. Therefore, the future Brazilian oil discoveries should be located offshore in ultra-deep waters. Beyond 2000 meters water depth flexible riser technology is so far not proved. Therefore, the new challenge is to develop an ultra-deep water hybrid riser technology in order to incorporate these reserves in the Company's assets.

This paper shows a preliminary technical feasibility study considering a hybrid riser configuration for a typical deep water field in Campos Basin, Offshore Brazil. The water depth is 1800 meters and the riser is attached to a turret at bow FPSO (Floating Production Storage & Offloading) vessel. The results indicate an attractive alternative design concept that fulfil some typical difficulties related to ultra-deep waters such as high vertical loads on the production floating unit.

The analysis methodology uses the Finite Element Method through commercial computer codes packages MSC/Patran and Abaqus. The hydrodynamic loads are computed through programming using PCL (Patran Command Language).

The strength of the pipe under bending, tension and external pressure is considered through the standard DNV OS-F101 [1]. This standard is proposed specifically for the analysis of submarine pipelines.

Introduction

The steel hybrid riser concept [2,3,4,5] is composed by a SCJ (Steel Catenary Jumper) connected to a SSVR (Self-Standing Vertical Riser) through a goose neck spool piece, as shown in Figure 1. The connection of the two rigid risers is provided by a typical flex-joint. The SSVR requires a subsurface buoy at the top to keep its vertical stability. In addition, the SCJ is attached at the base of a bow turret of a typical FPSO built on the hull of a former VLCC (Very Large Crude Carrier). At the seabed the connection is modeled as a hinge, however, a taper joint or a flexjoint could be considered based on a further local analysis not included in this paper.

One of the main advantages of such configuration is that it eliminates entirely riser soil interaction, therefore, any TDP (Touch Down Point) design constraints. Another important advantage is that the concept is made completely of steel pipes, which gives more flexibility in terms of construction and procurement.

The proposed configuration in this work was defined through the analysis of the results of a preliminary study of several simulations.

The System Specification is described below:

  • WD (Water depth) = 1800 meters

  • Pipe Outer Diameter = 12.75 in

  • Pipe wall thickness = 1 in

  • Pipe material: API 5L X65

  • SCJ length = 1700 meters

  • SSVR length = 800 meters

  • Buoy upthrust = 1470 kN

  • Flex joint connecting SCJ top and FPSO

  • Flex joint connecting SCJ and SSVR

  • Operational offset: 10% of water depth

  • Accidental offset: 15% of water depth

  • VLCC ship with turret system

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