Multibore Dynamic Risers
Authors: Pettenati-Auziere C. , Dridi H. , Birch A. , Fjell K. 
Published: October 2008

ABSTRACT
A range of flexible dynamic multibore riser structures have been developed which are able to significantly increase the capacity of flexible riser systems by reducing the number of individual lines required for a typical sub-sea development.
Multibore riser structures represent a major breakthrough in flexible riser technology further extending the application limits of floating production systems.
This paper presents an overview of the multibore development programme from the market forces driving the initiative, through the conceptual design, to the manufacture and testing of full size prototypes.

INTRODUCTION
Floating production systems based on flexible risers have progressed rapidly over the last ten years from the clement environments of Brazil, the Mediterranean and the Far East to extensive multi-well developments in the North Sea such as Sun Oil's Balmoral development.
The trend toward increasing numbers of sub-sea wells is likely to continue.
Field developments with 20 sub-sea wells are now becoming typical. Such developments require flexible riser systems able to accommodate at least 60 individual lines (assuming no sub-sea manifolding) with a production line, gas lift line and control umbilical for each well. However, this requirement currently exceeds the maximum capacity of flexible riser systems based on existing techniques. A typical example would be a Floating Production System in 150m water depth which is able to accommodate about 40 individual lines; assuming two riser arrays, with five sub-sea riser supports in each array, and four individual lines on each sub-sea riser support. There is clearly a necessity to increase the capacity of flexible riser systems to accommodate the needs of future field developments. The use of larger sub-sea riser supports and the reduction of the number of individual lines are two methods aimed at increasing the capacity of flexible riser systems. The use of large sub-sea riser supports have been the subject of various investigations within the industry. Fixed structures or towers, which eliminate the clearances necessary between adjacent subsea buoys and permit more efficient utilization of the available space, are typical examples. However, larger sub-sea riser supports alone are not a complete solution as the number of individual lines to be handled remains unchanged. This, inevitably, results in lengthy and cumbersome installation methods with attendant increases in risk and cost.
Therefore, effort has concentrated on the development of multibore riser structures, the subject of this paper, which increase the capacity of flexible riser systems by bundling the various individual lines into a smaller number of more handleable packages simultaneously easing installation, reducing congestion and minimizing the potential for interference.
The multibore development programme, the subject of this paper, commenced with the definition of a design philosophy together with an analytical prediction model and culminated in the manufacture and testing of full size prototypes to validate the performance predictions.

DESIGN PHILOSOPHY
Multibore structures compr1s1ng of relatively large diameter individual lines for use in dynamic flexible riser systems are a new concept. The design must take into account the hydrodynamic characteristics of the multibore structure in the flexible riser configuration as well as the mechanical strength and the dynamic service life.