With subsea projects continuing to increase in size and complexity, often being executed in deepwater and harsh environments, new technologies and capabilities must be developed to meet the fresh operating challenges.

A solution presented at the Offshore Technology Conference 2013 by Subsea 7 outlined details of a new riser concept for field developments in ultra-deep water and deep water. Many well-proven riser concepts have been used in different deepwater environments, in particular steel or flexible risers in catenary or lazy-wave shape and single or bundle hybrid riser towers. Nevertheless, for some applications new concepts are deemed more attractive, such as the buoy supporting riser for ultra-deep water offshore Brazil.

But other ideas of riser concepts emerge from the review and comparison of the pros and cons of existing riser systems, trying to take the better aspects of each one.

New catenary concept

Subsea 7’s tethered catenary riser is an adaptation of the buoy supporting riser but with an alternative tether arrangement and easier installation method. It also can be seen as a single hybrid riser with a number of steel catenary risers (SCRs) attached to the buoyancy tank.

Jean-Luc Legras of Subsea 7 said the tethered catenary riser concept consists of a number of SCRs supported by a subsurface buoy, which is tethered down to the seabed by means of a single pipe tendon and anchored by means of a suction pile. Flexible jumpers are used to make the connection between the floating production unit (FPU) and the buoy. Umbilicals run without interruption from the FPU to the subsea end while being supported by the buoy.

The system has all the advantages of decoupled riser arrangements: Flexible jumpers effectively absorb platform motions, and for that reason the rigid risers and tendon have very small dynamic excitation. The system can be installed before the FPU’s arrival on site, which improves the time before first oil. Analyses have shown that, with adequate geometry of the buoy, the latter is sufficiently stable to induce acceptable tilt and twist when different arrangements of SCRs and flexible jumpers are installed and under accidental scenarios during the in-place life.

The riser system is best designed for between four to eight risers in addition to a number of umbilicals; thus it is convenient for one or two conventional drilling centers.

Results of the basic engineering work on the tethered catenary riser indicate that it is possible to have a robust design using qualified materials and technology, according to Legras. The components used in the tethered catenary riser are all field-proven as they are commonly used in existing riser systems.

As a result of installation studies, a method very similar to the one commonly used by Subea 7 for single hybrid risers has been selected for the buoy and tether system. Installation of rigid risers, jumpers, and umbilicals is done by Subea 7 for the buoy supporting risers. This method is well adapted for installation by reel lay or J Lay, allowing the company a choice of vessels including the new Subsea 7 flagship vessel Seven Borealis, which is able to perform heavy lift and pipelaying duties.

Harsh environment alternative

A further riser solution is Subsea 7’s uncoupled riser system catenary offset buoyant riser assembly (COBRA), targeted at harsh ultra-deepwater environments.

In a paper at OTC 2013 by Daniel Karunakaran of Subsea 7 and Rolf Baarholm of Statoil, SCRs and hybrid riser towers were said to have been an attractive choice for recent deepwater field developments.

The design of SCRs for harsh environments or from large motion host platforms remains a significant challenge. The key issues for the design of SCRs in harsh environments are the fatigue near the hangoff and at the touchdown point. Hybrid riser towers have their own challenges and need special bottom assemblies with heavy foundation and complicated spools.

The COBRA system consists of a catenary riser section with a long, slender buoyancy module on top, which is tethered down to the seabed. The top of the catenary riser section is connected to the host platform by a flexible jumper.

This concept combines the advantages of SCRs and the hybrid riser tower, eliminating the fatigue challenges at the touchdown point of the SCR and avoiding the use of complicated bottom assembly and spools of hybrid riser tower. Since the platform motions are uncoupled in this riser system, the fatigue in the SCR part is very small.

COBRA is an efficient riser arrangement for host platforms with large motions, for example FPSO vessels or semisubmersibles. The flexible jumpers in this riser system effectively absorb the platform motions, and consequently the SCR section has almost no dynamic motions, which improves both strength and fatigue performance. The riser system has been developed for water depths ranging from 750 m to 3,000 m (2,461 ft to 9,843 ft) in harsh northern Norwegian environments.

“The results clearly indicate that it is possible to have a robust design of COBRA risers from large-motion host platforms in harsh environments using presently qualified material and technology. The first-order wave fatigue response of the steel riser section is negligible, and the fatigue is purely controlled by VIV [vortex induced vibration] and can be mitigated by the use of VIV strakes. The preliminary work also showed that this riser system can easily be installed in harsh environments. The riser components used in this riser system are all field-proven as they are used in other riser systems,” the paper’s authors note.

The concept also is applicable in less demanding environments such as Brazil. Furthermore, due to reduced dynamics in the SCR part of the risers, cost-effective corrosion-resistant alloy materials like mechanically lined pipes can be used in the SCR section, thereby reducing the costs considerably.

A material question

John Mair, Subsea 7

The oil and gas industry has widely adopted the use of composites in a variety of applications over the past 20 years.

The increasing demands on materials to meet future technical requirements for risers, flowlines, spools, and components such as stress joints are leading the industry to take a closer look at composites for these applications. Technical specifications to address the requirements associated with pressure, temperature, corrosion, and fatigue are stretching the suitability of established metallic and nonbonded flexible options.

Bonded composite thermoplastic pipe is a high-end composite solution that directly addresses many of these challenges while delivering substantial reductions in vessel payload.

Subsea 7 has undertaken a number of studies with two companies that specialize in bonded composite technology, Airborne and Magma, to evaluate the possibilities for this emerging technology and understand the opportunities and challenges in its design and installation.

A phased approach also is being adopted toward the introduction and potential applications of this technology by a number of operators in order to gain a detailed understanding for its deployment in such demanding emerging applications as deepwater risers. High sour environments and high-temperature applications that could potentially see a significant reduction in top-tension buoyancy requirements also are issues under consideration.

The riser designs of both Airborne and Magma use bonded composite thermoplastic-based technology. One area of consistent interest is for deepwater spools. Spool fatigue is one example where fluctuating flow regimes and riser movement may be better accommodated by the properties of composite products in terms of flexibility and fatigue resistance.

A composite spool can more readily accommodate dynamic loads, flex to absorb movement in the riser and flowline, and offer opportunities to streamline the deployment and connection approach. Subsea 7 is currently leading studies to assess these potential benefits.