Design Curves for Deepwater Compliant Risers
Authors: O&apos;Brien P. , O&apos;Sullivan E. , Frazer I. , Light M. , Brennan G. 
Published: October 2008

This paper describes a design methodology which uses design curves to evaluate the global dynamic response of compliant risers attached to floating production units in deepwater. The objective of this methodology is to use the "Riser Design Curves" as an easy-to-use preliminary screening tool for quantitatively evaluating the technical feasibility of potential riser configuration options. In this paper the basis for the development of these design curves is outlined and the procedure for their use is presented. Design curves are developed and verified for the specific application of flexible risers to a deepwater harsh environment West of Shetland Offshore UK and useful conclusions are drawn from the results of this work.

Introduction

As the search for oil and gas reserves extends into deepwater, so the requirements for offshore facilities will be similarly extended. Floating production units based on either ships or semi-submersible vessels can provide attractive solutions for these conditions. However, the compliant risers used to connect these floating production units to the subsea wells are a critical component within the system and require close assessment. A riser pipe cross-section will initially be designed to resist thermal / external pressure loads and collapse propagation using materials compatible with the internal/external environment. In the case of flexible pipe, the pipe construction which includes the number and size of component layers is designed by the supplier. In this context, the purpose of the "Riser Design Curves" is to assess the capability of the pipe cross-section against global riser response criteria such as axial tension, minimum bending radius, top angle response etc. This paper describes how such design curves can be developed and used for this purpose. Once the curves have been developed for a generic location with characteristic environmental/water depth properties, they can be used by an operator planning to develop a number of fields in the area. In this respect, the important uses of the riser design curves are:To determine how the overall field concept affects the feasibility of the various riser options under consideration. Key issues include the type of floater and the stationkeeping capability of its moorings; the subsea layout including the number and location of wells / well manifolds; and pipeline sizes.To continuously investigate various "what if' scenarios regarding the riser system as the field concept develops through the feasibility and conceptual engineering stages of a project.To provide basic riser configuration parameters such as section lengths, buoyancy etc., which in tum can be used to estimate relative costs for the feasible solutions developed. For this paper, the design curves have been developed for a typical West of Shetland harsh environment in approximately l000m water depth.

Development of Design Curves

The objective of developing the design curves it to relate important riser response parameters to fundamental properties of the riser cross section. These riser design curves then enable the riser configuration and cross section design processes to occur both interactively and simultaneously thereby reducing engineering cost and achieving an optimized solution.