Spar Riser Alternatives for 10,000 ft Water Depth
Authors: Gupta Himanshu , Finn Lyle , Halkyard John 
Published: October 2008

This paper presents a discussion on design of Spar top tensioned risers for 10,000 ft water depth. A parametric study on Spar configurations, riser configurations and tensioning systems is reported. Performance of these concepts for such a deep water depth is compared. Assessment of various riser solutions for 10,000 ft water depth will provide guidelines for selecting the best riser solution. Effect of riser system on design of the Spar is also highlighted through the parametric studies. Results of the study show the difficulties involved in design of riser system for such water depth. Several alternatives are suggested to overcome the design problems. Response of the various Spar/Riser configurations is presented and recommendations are made for most suitable tensioning alternatives. As the offshore production moves to deeper water depths, riser design challenges increase. Applicability of present designs have to be reevaluated for the new design parameters. Present paper provides guidance for riser designs for 10,000 ft water depth.

Introduction

The offshore production is slowly moving towards deeper water depths. Recent drilling activity in 10,000 ft water depth has increased interest in evaluating options for production for this water depth. In past few years, the Spar concept has become a popular option for the development of deep-water fields. Three Spars of the classical configuration have been installed in the Gulf of Mexico (Ref. 4,6,7), three Spars of the truss configuration are being fabricated (Ref. 1), and a number of additional Spars are in advanced planning stage. Spar offers several advantages over other floater options that have helped to fuel this increase in popularity. Spar is a moored floater that is rather insensitive to water depth. This offers the possibility of reusing the vessel at another site in the future. The center of buoyancy of the Spar is designed to be above the center of gravity. Thus the vessel remains stable in all environmental conditions. The topside deck and facility are very similar to those used on conventional fixed platforms and can perform typical drilling, workover, and producing operations. The motions of a Spar are significantly less than a typical semisubmersible, allowing more drilling and operational up time. Several different riser systems have been developed for Spar platforms. Both top tensioned and steel catenary risers are viable with the Spar concept.

The design of risers and mooring lines is the primary challenge in extending the applicability of Spars to 10,000 ft water depth. This paper presents results of a parametric study on different Spar/riser configurations aimed at evaluating performance of various tensioning system options. This study mainly focuses on Truss Spar designs. Several Truss Spar configurations for different topsides operating weights and number of risers were studied to examine the performance of different riser and tensioning system options.

Description of Parametric study

Objective of this study was to evaluate the behavior of different Spar/Riser configurations. Primary parameters that are important to the sizing of Spar platforms are deck payload, number of top tensioned risers to be installed and environment, in addition to the water depth.