Wednesday, 13 August 2014

Deep Sea Risers

Technological advances have created economically viable solutions to the complications of well-control methods that are created by subsea blowout preventer systems. Further, wells are being drilled in waters deeper than before, and subsea technology has made it possible. Innovations have led to the invention of different types of Marine Riser Systems(Deep Sea Riser).

How Do Risers Work?

They transfer the materials from sea floor to production and drilling facilities atop the water's surface, as well as from the facility to the seafloor. Subsea risers are a type of pipeline developed for this type of vertical transportation. Whether serving as production or import/export vehicles, risers are the connection between the subsea field developments and production and drilling facilities.

Similar to pipelines or flowlines, risers transport produced hydrocarbons, as well as production materials, such as injection fluids, control fluids and gas lift. Usually insulated to withstand seafloor temperatures, riser can be either rigid or flexible.

The following video gives us a rough idea about Marine Riser System, later in the article the different types of risers are explained.

Types of Risers

There are a number of types of risers, including attached riser, pull tube risers, steel catenary risers, top-tensioned risers, risers towers and flexible riser configurations, as well as drilling risers.

Steel catenary risers

Build on catenary equation that has helped to create bridges around the world. Used for connecting the sea floors to the floating facilities above,as well as connect two floating platforms together. These are common on TLP's, spars & FPSO's, as well as fixed structures, compliant towers & gravity structures. While this curved riser can withstand some motion, excessive movement can cause problems.

Fig. 1: Steel catenary riser
(Image Courtesy: Google Images)

Attached risers

 They are developed on fixed platforms, compliant towers and concrete gravity structures. They are clamped to the side of the fixed facilities, connecting the seabed to the production facility above. Usually fabricated in sections, the riser section closest to the seafloor is joined with a flowline or export pipeline, and clamped to the side of the facility, until the top riser section is joined with the processing equipment atop the facility.

Top-tensioned risers

Used on TLP's and spars. These are completely vertical riser system that terminates directly below the facility. Although moored, these floating facilities are able to move laterally with the winds & waves. Because the rigid risers are also fixed to the sea floor, vertical displacement occurs between between the top of the riser and its connection point on the facility. There are two solutions for this issue. A motion compensator can be included in the top tensioning riser system that keeps constant tension on the riser by expanding and contacting with the movements of the facility. Also, buoyancy cans, can be deployed around the outside of the riser to keep it afloat. Then the top of the rigid vertical top-tensioned riser is connected to the facility by flexible pipe, which is better able to accommodate the movements of the facility. 

Fig. 2:  Top-tensioned riser
(Image Courtesy: Google Images)

Pull tube risers

These are pipelines threaded up the center of the facility. For pull tube riser, a pull tube with a diameter wider than the riser is preinstalled on the facility. Then, a wire is attached to the pipeline or flowline on the seafloor. The line is then pulled through the pull tube to the topsides, bringing the pipe along with it.

Riser Towers

Ideal for ultra-Deep water environments, this riser design incorporates a steel column tower that reaches almost to the surface of the water, and this tower is topped with a massive buoyancy tank. The risers are located inside the tower, spanning the distance from the seafloor to the top of the tower and the buoyancy tanks. The buoyancy of the tanks keeps the risers tensioned in place. Flexible risers are then converted to the vertical risers and ultimately to the facility above.

Fig. 3:  Riser Tower
(Image Courtesy: Google Images)

Flexible risers

A hybrid that can accommodate a number of different situations. It can withstand both vertical and horizontal movement, making them ideal for use with floating facilities. This flexible pipe was originally used to connect production equipment aboard a floating facility to production and export risers, but now it is found as a primary riser solution as well. There area number of configurations for flexible risers, including the steep S and lazy S that utilize anchored buoyancy modules, as well as steep wave and lazy wave that incorporates buoyancy modules.

Fig. 4:  Flexible riser(Image Courtesy: Google Images)

Drilling risers

These transfer mud to the surface during drilling activities. Connected to the subsea BOP(blowout preventer) stack at the bottom and the rig at the top, drilling risers temporarily connect the well bore to the surface to ensure drilling fluids to not leak into the water.LSD

Fig. 5:  Drilling riser
(Image Courtesy: Google Images)

Article By: Tanumoy Sinha

Recommended Readings: Riser TechnologyRigzone training