Abstract

The objective of this paper is to allow operators/ contractors to envisage a new and effective design solution for Risers to be used for deep-water fields with High Pressure & High Temperature conditions (HP/HT).

The Top Sliding Bundle Riser (TSBR) is an alternate solution adapted to HP/HT conditions from Bundled Hybrid Risers (BHR) designs previously engineered by DORIS Engineering for benign environments.

The mechanical and structural particulars of the proposed concept are outlined. The hydrodynamic and structural analyses of the TSBR performed with an efficient in-house software package validate the design. The fabrication and installation modes specifically adapted to the new concept are then detailed, with the feedback of lessons learned on recent projects. Finally, the key benefits from the Concept are summarized and potential further improvements are drafted.

Introduction

Doris pioneered the design and realisation of Riser Towers in the end of the 70s with the Cosmos Project for the CEA (Commissariat à l'Energie Atomique) as shown on Figure 1.

The concept of Bundle Hybrid Risers was then derived in Gulf of Mexico (1988 & 1992) then by DORIS Engineering for deepwater offshore applications in West of Africa benign environment, and lead to the installation of the first 3 Riser Towers on the Girassol Field Development in Angola.

DORIS Engineering since then carried out a study for the French ministry of Industry on the applicability of hybrid riser towers to HP/HT environments. This study, funded by the Comité d'Etudes Pétrolières et Marines (CEPM or Comity for Marine and Petroleum Studies) was initiated in 2004 and will be further detailed this year with the support of FMC.

Figure 1 : The Cosmos Project (Available in full paper)

Study Objectives

The design drivers of hybrid riser towers in HP/HT conditions in benign environment are identified. The resulting design choices, made based on experience at this conceptual stage, are further described in this paper; they are being currently validated with more detailed analyses and support from various disciplines' specialists.

TSBR System Description
Design outline

The TSBR is a bundled riser with a flexible joint at the bottom and a vertically sliding pinned connection along the FPU riser porch, whose schematic is presented in Figure 2.

The tower is composed of six parts, from bottom to top:

  • The suction anchor,

  • The flexible joint,

  • The bottom assembly,

  • The rigid spools at the Tower foot to the Flowlines,

  • The Main Riser Section (MRS),

  • The Top Piece (TP), from the top of the MRS up to the top spools, allowing the guidance of the TSBR to the FPU side,

  • The spools from the MRS to FPU top flanges.

Figure 2: TSBR Schematic (Available in full paper)

The Anchor

A suction pile anchor, similar to Girassol holds the tower in place at the seabed.

The typical range for uplift force to resist for the suction pile is of 1 to 5 MN as described here below:

  • 5 MN max if all risers are empty,

  • typically 2 MN in normal operations,

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