The Dorada Field Production Risers
Authors: Wybro P.G. , Davies K.B. 
Published: December 2007

This paper outlines the riser analysis methodology for the Dorada field floating production system and identifies several new aspects of production riser design analysis. The analysis considers both the familiar wave-frequency fatigue damage and damage caused by wind-gust-induced low-frequency vessel motion. Pertinent analytical and experimental results are presented and the riser instrumentation system developed to produce those results are described.

Introduction

The Dorada field is located 12 miles (19.3 km) offshore Spain in the Mediterranean Sea in approximately 310 ft (94.5 m) of water. The field is being produced by a semisubmersible floating production system (FPS) positioned directly over three clustered subsea wells (Fig. 1). positioned directly over three clustered subsea wells (Fig. 1). ENIEPSA is the operator of the field for partners Getty Oil Co. of Spain S.A. and Union Texas Petroleum. The overall project design and operation is described in Ref. 1. This paper is concerned only with the riser design analysis that was performed.

The Dorada riser design is unique because it is the first functional installation of individual string production risers. Each riser is tensioned individually 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 that 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, andsingle-string system with a flex-joint end termination.

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

Discussions in this paper are limited to the final configuration and include (1) design criteria for the riser, (2) Copyright 1982 Society of Petroleum Engineers of AIME design analysis methodology, including some new considerations for production-riser design, (3) results of the engineering calculations, (4) riser instrumentation program, and (5) preliminary results of field performance program, and (5) 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. Riser design practice traditionally has considered only mean platform offset resulting from static conditions of winds, platform offset resulting from static conditions of winds, waves, and currents combined with the platform wavefrequency motion. In the present design, the low-frequency platform motions induced by wind gusts and platform motions induced by wind gusts and wave slow-drift forces are shown to have a significant effect on maximum riser stresses and fatigue life. This is because the selected design wind gust spectrum of Davenport had a peak energy at frequencies close to surge-sway platform resonance, and the lower riser stress was strongly dependent on vessel offset.

This design study also has shown the importance of considering year-to-year variation in wind and wave climate at the site for proper fatigue design of production risers. This is not usually a concern for fixed platforms because of the longer design life.

The riser top tension is an important design factor for the riser. An optimal tension for the riser design can be found from stress and fatigue considerations. This optimal value does not change substantially for the range of annual wind and wave climates that could be experienced.

A riser instrumentation project was undertaken to provide the field operators with an on-board monitoring provide the field operators with an on-board monitoring capability and to verify the design analysis work. Preliminary results of this program generally substantiate Preliminary results of this program generally substantiate the analysis work described.

Design Criteria

The overall riser design strategy was to produce a riser configuration that satisfied operational requirements and minimized production interruption.

JPT

P. 2947