The Dorada Field Production Risers
Authors: Wybro P.G. , Davies K.B. 
Published: October 2008

The Dorada Field floating production system, located offshore Spain in 310 ft of water in the Mediterranean Sea, utilizes three individual production risers. This paper outlines the riser analysis methodology, identifies several new aspects of production riser design analysis, describes some pertinent results, and discusses the riser instrumentation system developed to verify these results.

The riser analysis utilized a time-domain finite element program to predict survival state conditions and develop stress transfer functions. The fatigue analysis assessed both the familiar wave-frequency fatigue damage and damage due to wind-gust induced low-frequency vessel motion. A computer model of platform motions excited by steady and slowly varying environmental forces was developed for this analysis. The riser fatigue life was computed by a spectral method for expected environmental conditions. The importance of considering a succession of several "bad" years in designing a limited service life structure like a production riser was emphasized.

A riser instrumentation system was designed and installed to assist in production riser operations and to provide data for validating riser analysis models. Local riser stresses, flex-joint angle, vessel motions, and environmental factors are measured by the system. The results of a preliminary comparison between predicted and measured results are presented.

Introduction

The Dorada Field is located 12-miles offshore Spain in the Mediterranean Sea in approximately 310-ft of water. The field is being produced by a semisubmersible floating production system (FPS) positioned directly over three clustered subsea wells (Figure 1). ENIEPSA is the operator of the field for partners Getty Oil Company of Spain S .A., and Union Texas petroleum. The overall project design and operation is described in [1]. This paper is concerned only with the riser design analysis that was performed.

The Dorada riser design is unique in that it is the first functional installation of individual string production risers. Each riser is individually tensioned to prevent buckling and to allow direct vertical access to the wellbore. Four variations of the individual riser design were investigated to arrive at a riser concept which best satisfied the site specific design and operating criteria:Concentric string riser system using a taper joint end termination.Concentric string riser system using a flex joint end termination.Single string system with a taper joint end termination.Single string system with a flex joint end termination.

For reasons of structural design, and the prototype level technology associated with taper joint risers, the latter configuration was selected.

Discussions in this paper will be limited only to the final configuration and will include the following:Design criteria for the riser.Design analysis methodology, including some new considerations for production riser design.Results of the engineering calculations.Riser instrumentation program Preliminary results of field performance verification tests on the risers. The Dorada riser design study has shown the importance of accurate prediction and simulation of the moored vessel motion under storm conditions. Normal riser design practice considers only mean platform offset due to static conditions of winds, waves and currents combined with the platform wave frequency motion.