Production risers

Large integrated drilling and production facilities employ Dedicated Drilling Risers which have been part of the industries practice for decades. In 1984 Conoco Hutton facility in the North Sea was the first facility to use the top-tension riser system. In the Gulf of Mexico there are several facilities using the dedicated drilling risers. The drilling risers’ work as an extension from the wellbore to the drilling rig which is located on the floating production system. The typical riser configuration considered consists of the subsea wellhead connector assembly, tapered stress joint, drilling riser, tensioning joint, riser landing joint, hydro-pneumatic tensioning system and a surface blow-out preventer. Risers systems have to sustain surface floater induced wave motions and direct environmental in addition to functional loadings.

Top tension risers

Top tension risers are suitable for only floater applications with limited lateral excursions. These should not be used for semi-submersibles or on ship shaped floaters. These risers are supported by hydraulic heave compensator system or individual buoyancy tanks as the vertical risers must always have tension throughout their length.

Flexible risers

Flexible pipe with multi-layered structures and riser configurations

In shallow conditions the unbound flexible pipes are the original technology for the floating production system and are the main riser system used in production. With additional buoyancy modules which are used to form loops which uncouple the riser bottom section from the floating unit motions. The result of the configurations allows risers to be more suitable to the different field conditions by allowing different flexibility such as the “Lazy Wave”, “Steep Wave”, “Lazy-S”, “Steep-S”, “Pliant Wave”, etc. Flexible riser systems are appropriate for shallow water and cyclonic conditions and are used in a number of fields located in West Africa, Gulf of Mexico, Asia Pacific regions, Brazil and in the North Sea. The recent development of composite materials that resist corrosive fluids, combined with high temperature and high pressure limits, the flexible risers are now qualified for 3,000m water depth.  

Steel catenary risers

“As floating production systems moved into deeper water in the 1990’s, steel catenary risers became feasible as the longer length of pipe cure provides adequate compliance relative to the magnitude of vessel motion.” Shells Auger TLP in the Gulf of Mexico was one of the first applicants of Steel Catenary Risers. The deepest depth of Steel Catenary Risers is installed in the Perdido Spar at depths of 2470m. All types of steel risers are prone to Vortex Induced Vibrations fatigue issues, particularly in deep waters where long lengths of risers are exposed to current (mulit-directional) loadings. Such risers are often fitted with anti- VIV devices.

Hybrid risers

The development of the “Hybrid Riser” was brought about by the need of to compliment the conventional top tensioned risers, flexible or catenary risers. Hybrid Riser incorporated steel and flexible pipe technologies this allows the flexible pipe to absorb most of the dynamic motions of the riser while the Steel Catenary Riser or vertical leg is connected to a sub-surface buoy for support. These Hybrid Risers systems have been developed mainly for the reduction of the decoupling effect between the floating production unit and the risers.

Hybrid risers allow for the reduction of the:

1. Riser loads transmitted onto the floating production unit
2. Minimizing riser fatigue issues
3. Installation planning risk decoupling as the hybrid risers can be pre-installed prior to the floating production unit site arrival. 

The hybrid systems would bring about a more cost effective, technical solutions, and higher productivity providing access to deeper field developments. “The most common used Hybrid Risers are the Free Standing Hybrid Riser with a single leg configuration or several bundled in a Hybrid Tower Riser”. 

Free standing flexible riser (FSFR)

Free standing flexible riser

The Free Standing Flexible Riser was developed in order to optimize the riser installation tension and insitu dynamic behaviors. There are some basic concepts to these FSHR.

Three basic concepts are found in these FSHR:

1. A vertical leg of the riser system for seabed to the subsurface buoy is a flexible pipe for a low bending radius of the pipe for the double-catenary installation
2. The traditional slender buoyancy tank is replaced by a flat buoy
3. The flat buoy minimizes the Vortex Induced Motions and Rotations.

The FSFR buoy can be fabricated offshore, lifted into the sea and towed to the site at sea surface where it is ballasted to its final sub-surface location. In most cases a lower capacity flexible pipe installation adding to lower cost for the project.

Multi-Lines free standing riser

Multi-Lines free standing riser

The Multi-Line Free Standing Riser system is currently developed to provide the functional requirements as a combined assembly in a single standing riser system. The functional requirements are a production line combined service line for round-trip rigging, a water ejection line to maintain reservoir pressure, and a gas-lift at the production riser base for produced fluid. The Multi-Line Free Standing Riser features many of the following components: a single buoyancy tank, a stem pipe-in-pip riser fitted with top and lower assemblies, a Top Riser Assembly allowing for the connection of several flexible jumper in double-catenary shape, a Lower Riser Assembly performing the connection of the riser foundation, two lateral risers for water injection and service, a foundation system, and three Riser Based Jumpers for production, service, and water injection.

Deep steep riser

The deep steep riser system is meeting the challenge of the oil and gas industry by reaching the 4000m ultra deep water developments. For this water depth a flexible riser system is being considered for its technical feasibility and costs. This innovative system will consist of a single leg tensioned riser, which can be fully flexible or a hybrid riser solutions. The deep steep riser would have a flexible jumper at its upper sections to decouple from the FPU and at the riser lower section would be flexible or rigid steel to reach the seabed foundation. Distributed buoyancy modules would be added at the “wave” and at the “hog” sections which would be similar to shallow water wave systems.

Noteworthy paper in OnePetro

Gouveia, J., Sriskandarajah, T., Karunakaran, D., Manso, D., Chiodo, M., Zhou, D., … Escudero, C. 2015. Steel Catenary Risers (SCRs): From Design to Installation of the First Reeled CRA Lined Pipes. Part I - Risers Design. Offshore Technology Conference. http://dx.doi.org/10.4043/25839-MS

Jung, D., Kim, H., & Moon, D. (2010, January 1). Dynamics of Large Diameter Riser. International Society of Offshore and Polar Engineers. https://www.onepetro.org/conference-paper/ISOPE-I-10-042

Ng, D. J. T., Teng, Y. J., Magee, A. R., Ahmad Zukni, N. B., Aramanadka, S. B., Abdul Malik, A. M., … Abdul Ghani, M. P. 2014. Riser VIV Suppression Device Test. Offshore Technology Conference. http://dx.doi.org/10.4043/24874-MS

Whitfield, S. (2015, April 1). Current Developments in the World of Risers. Society of Petroleum Engineers. http://dx.doi.org/10.2118/0415-0034-OGF

External links

Drilling riser

Oil Platform

See also

Offshore and subsea facilities

Compliant and floating systems

MODU riser and mooring systems

Flow assurance for offshore and subsea facilities

MODU equipment and capabilities

PEH:Fluid Mechanics for Drilling