Hybrid Ultradeepwater-Riser Configuration Saves Costs, Pushes Depths
Authors: Carpenter Chris 
Published: April 2020

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 29389, “Single Independent Riser: A Cost-Efficient Ultradeepwater Riser,” by François Lirola, Eric Revault, and Jean-François Lunven, Saipem, prepared for the 2019 Offshore Technology Conference, Houston, 6–9 May. The paper has not been peer reviewed. Copyright 2019 Offshore Technology Conference. Reproduced by permission.

The single independent riser (SIR) is a hybrid riser configuration optimized for ultradeepwater field development. The SIR is composed of a flexible jumper and a rigid part vertically tensioned by means of distributed or continuous buoyancy. This configuration features improved dynamic behavior under fatigue and extreme environmental conditions for the rigid riser section thanks to its compliant shape. The design is versatile and can be staggered easily to comply with design constraints or congested layouts.

Introduction

The SIR is adapted to both turret-moored and spread-moored vessels as well as most other types. The configuration is also suitable for a wide spectrum of environments, from the mild conditions of west Africa to those in the Gulf of Mexico or the North Sea. The configuration can be designed for production risers, whether single-pipe or pipe-in-pipe, and injection and export risers. The strength of this configuration lies in its compliant, fatigue-resistant behavior. Its steep-wave configuration facilitates the mitigation of potential interference; the relative position of the sag and hog of each neighboring riser can be adjusted while reducing the load at the hang-off location.

Overview

The SIR features a steep-wave shape. The upper part is a flexible jumper, and the lower part is a rigid, steel-pipe string maintained in a near-vertical position either by means of continuous buoyancy or distributed buoyancy modules. These methods are used instead of a conventional air can in order to provide the up-thrust necessary to keep the riser vertical. The use of continuous or distributed buoyancy implies that each section compensates for its own weight, meaning that the concept is applicable to any water depth for which buoyancy materials are qualified.

The transition between the rigid and the flexible parts is achieved with a sub-sea connector. An overview of the SIR configuration is given in Fig. 1.

The SIR features the following technical advantages:

The design relies on field-proven technology such as rotolatches to connect the rigid section to its anchor or standard shallow-water flexible-jumper technology to perform the transition between the rigid section and the floater. Though the SIR can be deployed in any water depth, the cost of the flexible jumper limits the suitability of the SIR for water depths below 1000 m, which should be considered the minimum applicable water depth.

The SIR is compatible with any installation method.

The fabrication process of the SIR can be reversed fully, meaning the system can be retrieved for relocation or maintenance purposes.

The SIR features a small dynamic response, thus leading to less-stringent fatigue and welding requirements.