Subsea Production Riser
A subsea production riser consists of conductor pipes connected to offshore production platforms or FPSO and the flowlines/manifolds/wellheads on the seabed. It is also the primary devices of the floating production system to convey fluids to and from vessels. It is one of the most complex aspects of a deepwater production system.
There are essentially two types of subsea risers: rigid risers and flexible risers. A hybrid riser is achieved through a combination of these two types of risers.
There are mainly four types of risers:
- Steel catenary riser (SCR)
- Top tensioned riser (TTR)
- Flexible riser
- Hybrid riser
Steel catenary riser
A Steel catenary riser (SCR) is a common method of attaching an offshore pipeline to a deepwater floating oil production platform. The pipeline is extended into a flexible pipeline up to a catenary on the production platform, a free-hanging riser with no intermediate buoys or floating devices. . Relative rotational movements between risers and platform can use a flex joint, stress joint, and pull tube to offset the movement.
They were first used by Shell on the Auger tension leg platform in 1994 which was moored in 2860ft of water.. Two 12-in lines were used for oil and gas export. Recently they have been used in large numbers as production risers in Bonga FPSO.
The SCR is a cost-effective alternative for oil/gas export and water injection lines in deepwater fields where the large diameter flexible risers present technical and economical challenges. The SCR is self-compensated for the heave movement(the riser is lifted off or lowered to the seabed). It needs a ball joint to allow rotation of the risers induced by waves, currents, and vessel motion at the upper end connection.
The SCR is sensitive to waves and current due to the normally low level of effective tension on the riser. The fatigue damage induced by vortex-induced vibrations (VIVs) can be fatal to the riser. Use of VIV suppression devices such as helical stakes and fairing can help reduce the vibration to a reasonable level.
Top tensioned riser
Top tensioned risers(TTRs) connect the sub-sea well bore to the floating production vessel, enabling the installation of Christmas trees on the dry floating deck. TTR pipes are normally in contact with high-pressure production fluids and are subjected to dynamic loads. They rely on a top tensioner in excess of their apparent weigiht for stability. The hydro-pneumatic tensioners allow the riser to move axially or stroke relative to the platform. These risers typically appear in a group arranged in a rectangular or circular fashion and are commonly used on a tension leg platform (TLP) or spar dry tree production platforms.
Depending on the workover requirements, Top Tensioned Risers can be single or dual cased. The risers are installed directly from the drilling derrick on the platform by making up a series of riser joints equipped with threaded connectors. The connectors are available in the following configurations:
- Weld on (forged connectors to be welded on plain end pipe)
- Threaded and coupled (fatigue-enhanced premium connections)
- Integral premium connections.
Flexible risers are multiple-layer composite pipes formed by an inner flexible metal structure surrounded by polymer layers and spiral wound steel ligaments, also known as armor wires. Flexible risers have been a successful solution for deep and shallow water riser and flowline systems worldwide. Flexible risers are the result of an extraordinary development program that was based on flexible pipes, which were found out to be suitable for offshore application.
Hybrid riser's principle feature is that it accommodates relative motion between a floating structure and a rigid metal riser, by connecting them with flexible jumbers.