The multiple-lines hybrid riser tower which was first applied in the late 1980's in the Gulf of Mexico, and more recently to deep-water FPSO projects at the Girassol field in Angola, is now starting to be applied more widely on other deep water projects. This paper presents the lessons learnt from the design of recent hybrid riser tower applied to meet the requirements of several different deep water field developments. The paper specifically addresses the various changes made to the Hybrid Riser Tower relating to the design, fabrication and installation. The performance aspects are discussed along with information covering the overall costs associated with the use of riser towers as opposed to other single riser line concept. The paper also concludes on the possible future evolution of the HRT to meet the future ultra deep-water developments.
The first generation of multiple-lines hybrid riser tower (HRT), called FSPR (Free Standing Production Riser), was designed as a cost effective re-usable riser system by Cameron Iron Works in 1983 and installed in 1988 by Placid on the Green Canyon field block 29 (see reference 1). The structure was then upgraded and reinstalled on the deeper Garden Bank field (2096 feet water depth) by Enserch in 1994 (see reference 2). This early concept of HRT used the drilling technology to assemble the riser bundle with adequate buoyancy from a drilling rig; the riser tower foot and spools were connected to a subsea base manifold and flexible jumpers at the top were connected to the rig. Refer to fig 1. The concept proved to be cost effective and well adapted to operate in the Gulf of Mexico environment.
In 1997 Total called for a design competition to define the subsea architecture of the Girassol field (1450m WD) discovered in Angola. Stolt Offshore proposed three riser towers of what can be considered the second generation of HRT after the first experience in the Gulf of Mexico. This technological choice was mainly motivated by the stringent thermal insulation requirement imposed by the field operations and the layout (see reference 3). This second-generation design of HRT was completely designed to accommodate the specific fabrication and installation requirements associated with the Girassol field and its geopolitical environment.
The AMG consortium consisting of Stolt Offshore (67%) and Saipem (33%) was awarded the contract for designing, fabricating and installing the Umbilical, Flowline and Riser system which included the three HRTs. The Girassol field, which was designed for a nominal production of 200,000 bbl/d has in practice been capable of significantly higher production rates. This success encouraged Stolt Offshore to further develop the concept and use the lessons learnt from the Girassol project to improve the HRT design and take it into the third generation of HRT refer to Fig 2. The Greater Plutonio field operated by BP in Angola will be equipped with an HRT of the third generation. Other Contractors and Operators also consider the HRT for their deep-water field development.
