Neptune Project: Production Riser System Design and Installation
Authors: Berner Paul , Baugus Victor , Gendron Kevin , Young Ron 
Published: October 2008

The Neptune Field Development is the first Spar-based floating production system using multiple production risers from seafloor wellheads back to surface trees. To meet an aggressivedevelopment schedule the design, manufacture, and installation of these risers relied on extensive system and component analyses, field-proven equipment or derivative designs, innovative teamwork, and familiar offshore procedures.

Introduction

The Neptune Field, jointly developed by Oryx Energy and CNG, is located in Viosca Knoll Block 826. The development utilizes a "Spar" floating production system capable of handling 25000 bid and 30MM cf/d. The Neptune Field is located approximately 90 miles south of Mobile, Alabama in 1930 feet of water, at the Eastern end of the geological area known as the Flex Trend (see Figure 1). The Neptune Spar has a multilevel deck supporting well control equipment, first stage separation, and processing for both Spar-based wellsand future satellite well installations. Production is exported to a nearby platform.

The Neptune Spar is a permanently moored installation and can accommodate up to sixteen production risers andsurface trees. Riser and tree installation and removal, as well as workover operations, are performed from the Spar. Operations requiring a seafloor BOP, such as subsequent drilling of new wells, are carried out from a drilling vessel. The first seven wells were drilled before the Spar was brought on location, enabling expeditious installation of risers and trees and thereby minimizing the development schedule.

Production Riser System Design

Many different companies contributed to the design, analysis, and installation of the Neptune riser system components. Oryx assembled a team of companies and individuals with expertise in various riser system areas. Rapid communication and coordination within the team relied heavily on e-mail sent across the Internet. Open discussions via the Internet allowed everyone to be a part of and contribute to the effort on a schedule that was convenient to their workload. In some cases, discussions between individuals on a particular topic generated helpful and timely input from another individual who would not have normally been requested, available, oreven included if a conventional meeting had been held.

To meet the Neptune Field's aggressive development schedule, the production riser system uses component configurationsbased on field-proven equipment or modifications to existing designs. A typical riser system, shown in Figure 2, provides a pressure-containing link between surface tree and seafloor wellhead system. The riser system is designed for complete installation and removal using the Spar-based workover rig. The riser system meets the defined load and operational requirements for the range of conditions and events anticipated during the life of the field. The risers are a permanent part of the field development and, once installed, need not be removed during the life of the field.

The Neptune Field produces from various highly fractured formations requiring as many as sixteen Spar-based wells to develop the field's reserves effectively. During the life of the field, well workover and re-completions are anticipated and surface wellheads and trees provide a simple and economic approach for such situations.