Deepwater Tieback SCR: Unique Design Challenges and Solutions
Authors: Song Ruxin , Stanton Paul 
Published: October 2008

A steel catenary riser (SCR) is often the preferred option for subsea tie-back to an existing deepwater platform due to its conceptual simplicity, ease of construction and installation, and simple interface with the flowline. Unlike new field development, which provides more room for the design of new SCR, the engineering design of a tie-back SCR has unique challenges and requires special solutions. This is mainly because initial prelimainry SCR design during design of the floating production system kacks project specific information for the tie-back. In addition, the early SCR design draws much less attention compared to the detail design. Based on deepwater tie-back SCR engineering experience, key tie-back SCR design challenges are summarized from the engineering execution perspective. These challenges include potential concept changes, platform stringent weight budget limits, existing riser porch strength limits, SCR top arrangement, thermal performance requirement, subsea layout, riser interference, SCR and flowline interface design, SCR fatigue design, and installation. They call for design optimizations and even innovative ideas. The engineering analysis and design to handle the challenges are discussed together with solutions. The lessons learned from recent tieback SCR engineering projects are outlined. These lessons should have significant relevance to future tie-back SCR projects.

To demonstrate the challenges and solutions, an example is given based on a recent deepwater Spar tie-back SCR project with solution to challensge design that makes the design reliable and robust.

Introduction

In recent years, exploration and production activities have increased dramatically in deepwater nearly doubling the water depth compared to the activities a couple of years ago. The offshore industry is building systems today for production from even deeper water, progressively using new technology and significantly extending existing technologies. The targeted water depths for oil and gas developments in areas such as Gulf of Mexico (GoM), West of Africa (WoA), Brazil and North Sea are increasing every year.

Regardless of the floater concept adopted for offshore field development, there is always a need of risers. Risers are some of the more complex aspects of deepwater developments. It became evident that riser system plays bigger and bigger role as part of the offshore infrastructures. There are different ways to group deepwater risers. The most widely used and field proven deepwater riser concepts include SCRs, hybrid risers with a single line or bundled multi-lines, flexible risers with different configurations, and top tension risers (TTR) as shown in Figure 1.

Figure 1: Illustration of Deepwater Riser Concepts Among the riser concepts, SCRs have been enjoying a widespread acceptability for deep and ultra-deepwater oil and gas production in recent years. Up to today, more than 100 deepwater and ultra-deepwater SCRs have been installed worldwide mainly in the GoM, WoA, and offshore Brazil. The impetus for developing SCR technology is the continuing push into deep and ultra deep water, and the growing belief that SCRs offer the best technical and commercial solution.