Riser Tower Installation
Authors: Alliot Vincent , Carré Olivier 
Published: October 2008

The paper describes the offshore operations and associated engineering - riser base and umbilical arch installation, surface tow, upending, docking, deballasting, etc - that led to the successful installation of the three riser towers in the deepwater environment of the Girassol field. The paper concludes on the feasibility of installing riser towers in different field environments.

Introduction

The riser towers, designed, fabricated and installed by the Alto Mar Girassol consortium (AMG) for the Girassol field, are a new and unique design. The riser tower concept was originally proposed in October 1997 as part of the design competition for the Girassol project. In July 1998, TotalFinaElf Angola and its partners, finally selected the riser tower solution for the Girassol UFL (umbilical flowline) system.

The engineering team had to design the riser towers primarily to meet stringent thermal insulation requirements but also to ensure that the structure would be installable with conventional installation spreads. This challenge was overcome by developing safe and efficient installation methods verified by proper analysis and testing.

The three riser towers were fabricated at the Angolan yard at Lobito and delivered on schedule to be towed to the Girassol field. At this stage of the project, those who had put so much efforts in the concept had to go through a very stressful period. Meanwhile the offshore construction team carried out the operation successfully three times in less than 25 days, finishing one day prior to the arrival of the floating production, storage and offloading (FPSO) unit on site.

This success and the experience gained from the Girassol project, firmly establishes the riser tower concept as an important solution for deep and ultra-deep field development projects. The flexibility of the concept makes adaptation to greater water depths and hasher meteocean conditions relatively straightforward.

Brief description of the riser tower

Fluid and piping system

The riser tower consists of following:Four production risers (8in)Four gas lift risers (3in)Two gas/water injection risers (8in)Two service line risers (2in)

Riser tower main components

The riser tower integrates the following components (fig a-1):A buoyancy tank (fig a-2) with a support balcony for the flexible risers and gooseneck spools to ensure flow continuity.A bundle (fig a-3) comprised of a 22in steel central core pipe with production, injection, gas lift and service risers placed inside sets of buoyancy/insulation foam elements.A taper joint and transition pieces between the buoyancy tank and the bundle.Two independent umbilical support arches (fig a-4) to allow installation of up to four umbilicals per tower.A riser foot structure (fig a-5) with a male Rotolatch connector, a temporary buoyancy tank and riser configuration designed to interface and ensure flow continuity with the static flowline system through flanged rigid spool pieces.A riser tower anchor (RTA) (fig a-6) embedded in the seafloor with a female receptacle for the Rotolatch connector and two sheave mechanisms used to pull down the tower on its anchor.