Versatility in riser systems with Subsea 7

Online Editor

Selecting the optimum riser system at an early stage in field development planning is the key to success

Commenting on his firm’s many years of product development, Yann Brouard, Subsea 7 riser technology manager, says, “We have demonstrated that selecting the optimum riser system at an early stage in field development planning is the key to unlocking economic hydrocarbon production.”

After designing, constructing and installing a wide range of riser systems, Subsea 7 is refining its established riser technology portfolio to meet the challenges of the future. The principal drivers in this programme are developing advanced capabilities to lower the installed cost, enhance fatigue performance, reduce loads on the host and improve corrosion resistance properties.“We are also looking to the near future when lightweight non-metallic pipeline materials will transform riser performance,” says Brouard.

The company’s riser portfolio consists of both coupled systems (for direct connection to the floating host facility) and hybrid decoupled systems with flexible jumper connections that are suitable for deepwater applications with large hang-off loads or strong dynamic motions.

The use of decoupled risers has diminished in recent years for cost reasons, although they are still deployed in specific cases where the field development is based on using refurbished FPSOs with limited payload.

The company is currently focusing on several key technology innovations. One is a riser (GJR) that will allow free-hanging rigid riser configurations that are more cost-effective than high-performance steel lazy wave risers (SLWRs) and require no buoyancy. Tethered catenary riser (TCR) is another innovation. This sees the consolidation of several field-proven decoupled riser designs into a new cost-efficient TCR assembly suitable for fabrication and installation on a global basis. Finally, advanced riser materials is another area of innovation. This involves the successful introduction of high-strength steel and non-metallic materials into riser systems, in particular corrosion-resistant alloy (CRA), polymer-lined and thermoplastic composite pipe (TCP).

The gimbal joint riser system

Launched in 2019, the Gimbal Joint Riser (GJR) is an ingeniously simple concept that is based on the insertion of a multiple hinged joint within the suspended catenary that absorbs the dynamic movements coming from the floating host. The mechanical joint acts as an exo-skeleton, providing added strength and maintaining flexibility in an area that is subject to continuous repetitive, fatigue-inducing motions.

The GJR comprises multiple tubular elements with a number of articulated rigid sections that transfer tensile, compressive and torsion loads and permit degrees of pivoting motion. These armoured elements decouple the dynamic excitation transferred through the riser from vessel motion and protect a pliant inner riser pipe that supports high curvatures.

This mid-riser solution addresses the problem of fatigue at the touchdown point (TDP), which is conventionally overcome through local material thickening with possible constriction of the internal diameter. “Platform dynamics are the main driver in the industry quest to develop free-hanging riser configurations which require no expensive distributed buoyancy modules,” explains senior technology manager, Ivan Cruz, of the GJR development team.

“In deepwater regions, operators are converting tankers for redeployment as spread-moored FPSOs, and these vessels can experience hang-off upward and downward velocities of 5-6 metres per second in harsh weathers. Such conditions make standard steel catenary risers unfeasible, and traditionally require the deployment of expensive buoyancy modules in an SLWR configuration.”

Already two years under development in collaboration with leading operators, the GJR is now heading for full-scale prototype testing to technology readiness level (TRL) 4.

As well as removing the costs of added buoyancy modules, the GJR delivers considerable cost savings by reducing the length of the riser lines compared with SLWR solutions. This cost benefit increases with multi-riser applications lined with expensive Alloy 625. “A common configuration is to have all the risers in Brazilian FPSOs on one side of the floating facility,” adds Cruz. “Deploying GJRs could also allow operators to physically locate an FPSO nearer the TDP, potentially saving hundreds of metres of flowline sections in addition to the length reductions in the suspended catenaries.”

The tethered catenary riser system (TCR)

This new market-ready concept pulls together up to six riser lines into a buoyant midwater arch, which prevents clashing. The riser lines can be accessed individually and, as with other decoupled risers, the buoyant assembly reduces the combined weight imposed on the floating host by the risers.

The TCR has a lower installed cost than other hybrid riser designs while still delivering excellent dynamic and fatigue performance. By grouping multiple risers within a single assembly, the TCR concept benefits from economies of scale over a conventional single hybrid riser design.

The TCR design allows for reductions in riser components, including mechanical joints, bottom spools, pipeline end terminations (PLETs), monitoring systems and buoyancy tanks.

New riser pipeline materials

Using higher strength steel to decrease pipe thickness is an effective way to lower material and fabrication costs and significantly reduce installation costs. Subsea 7 has performed material and welding qualification for X80 installed by Reel-lay including CRA mechanically lined pipe. A recent study by the firm’s Pipeline Production Team identified a potential 20% reduction in the weight of SLWRs and an 11% reduction in buoyancy material costs by replacing X65 riser steel with higher-strength X80.

It is non-metallic materials, however, that could become serious game-changers in the next generation of riser design. Subsea 7’s Swagelining product is currently progressing the qualification of its first polymer-lined water injection SCR, which, with its inherent fatigue-resistance and lower weight, offers a highly cost-effective corrosion barrier for riser applications.

With its 50-year qualified design life, reduced weight and improved flow assurance, the Swagelining product has the potential for greater system simplicity, potentially avoiding the need for the complex integration of flexible risers with the rigid pipe flowline.

Looking further to the future, the industry is anticipating substantial benefits from the deployment of thermoplastic composite pipe (TCP) in riser configurations.

Subsea 7 has made a strategic investment in leading TCP manufacturer Strohm, which has already presented a detailed business case for free-hanging TCP catenary risers in deepwater developments. Yann Brouard believes that TCP represents the future of deepwater riser design.“TCP is beautifully simple, suffers no corrosion or fatigue issues and is thinner, lighter and more spoolable – that’s what makes it the key to the future,” he says.