Completion and Workover (C/WO) Risers

Two different types of risers are used for installation and intervention in and on a well: completion risers and workover risers. A completion riser is generally used to run the tubing hanger and tubing through the drilling riser and BOP into the wellbore. The completion riser may also be used to run the subsea tree. The completion riser is exposed to external loading such as curvature of the drilling riser, especially at the upper and lower joints. Aworkover riser is typically used in place of a drilling riser to reenter the well through the subsea tree, and may also be used to install the subsea tree. The workover riser is exposed to ocean environmental loads such as hydrodynamic loads from waves and currents in addition to vessel motions. The C/WO riser can be a common system with items added or removed to suit the task being performed. Either type of risers provides ommunication between the wellbore and the surface equipment. Both resist external loads and pressure loads and accommodate wireline tools for necessary operations. Riser connectors are one of the most important riser components. As drilling depths have increased, riser connectors have evolved to address issues concerning high internal and external pressures, increasing applied bending moment and tension loads, and extreme operating

conditions such as sweet and sour services. For connector design, the material selection and fabrication of bolts are critical issues. The spider is a device with retractable jaws or dogs used to hold and support the riser on the uppermost connector support shoulder during running of the riser. The spider usually sits in the rotary table on the drill floor. The gimbal is installed between the spider and the rotary table. It is used to reduce shock and to evenly distribute loads caused by a rig’s roll/pitch motions, on the spider as well as the riser sections. A slick joint, also known as a telescope joint, consists of two concentric pipes that telescope together. It is a special riser joint designed to prevent damage to the riser and control umbilicals where they pass through the rotary table. Furthermore, it protects the riser from damage due to rig heave. Riser joints are the main members that make up the riser. The joints consist of a tubular midsection with riser connectors in the ends. Riser joints are typically provided in 9.14- to 15.24-m (30- to 50-ft) lengths. For the sake of operating efficiency, riser joints may be 75 ft in length. Shorter
File:Stack-Up Model for a C WO Riser.png
Stack-Up Model for a C WO Riser
File:Key Components in a Drilling Riser System (Courtesy of World Oil(1)).png
Key Components in a Drilling Riser System (Courtesy of World Oil(1))
File:Riser Connector (Courtesy of World Oil (1)).png
Riser Connector (Courtesy of World Oil (1))
joints, called pup joints, may also be provided to ensure proper space-out while running the subsea tree, tubing hanger, or during workover operations.

Depending on configuration and design, a drilling riser system also consists of the following components:

1. The BOP adapter joint is a specialized C/WO riser joint used when the C/WO riser is deployed inside a drilling riser and subsea BOP to install and retrieve a subsea tubing hanger.

2. The lower workover riser package (LWRP) is the lowermost equipment package in the riser string when configured for subsea tree installation/ workover. It includes any equipment between the riser stress joint and the subsea tree. The LWRP permits well control and ensures a safe operating status while performing coiled tubing/wireline and well servicing operations.

3. An emergency disconnect package (EDP) is an equipment package that typically forms part of the LWRP and provides a disconnection point between the riser and subsea equipment. The EDP is used when the riser must be disconnected from the well. It is typically used in case of a rig drift-off or other emergency that could move the rig from the well location.

4. The stress joint is the lowermost riser joint in the riser string when the riser is configured for workover. The joint is a specialized riser joint esigned with a tapered cross section, in order to control curvature and reduce local bending stresses.

5. The tension joint is a special riser joint, which provides means for tensioning the C/WO riser with the floating vessel’s tensioning system when in open-sea workover mode. The tension joint is often integrated in the lower slick joint.

6. The surface tree adapter joint is a crossover joint from the standard riser joint connector to the connection at the bottom of the surface tree.

7. The surface tree provides flow control of the production and/or annulus bores during both tubing hanger installation and subsea tree installation/ workover operations.

Diverter and Motion-Compensating Equipment

A diverter is similar to a low-pressure BOP. When either gas or other fluids from shallow gas zones enter the hole under pressure, the diverter is closed around the drill pipe or kelly and the flow is diverted away from the rig. All floating drilling units have motion-compensating equipmen installed to compensate for the heave of the rig. Compensators function as the flexible link between the force of the ocean and the rig. The quipment consists of the drill string compensator, riser
File:Motion-Compensating Equipment (Courtesy of World Oil (1)).png
Motion-Compensating Equipment (Courtesy of World Oil (1))
File:Complete Riser Joint (Courtesy of Offshore Magazine, May 2001 (3)).png
Complete Riser Joint (Courtesy of Offshore Magazine, May 2001 (3))
tensioners, and guideline and podline tensioners. The drilling string compensator, located between the traveling block and swivel and kelly, permits the driller to maintain constant weight on the bit as the rig heaves. Riser tensioners are connected to the outer barrel of the slip joint with wire ropes. These tensioners support the riser, and the mud within it, with a constant tension as the rig heaves. The guideline and podline maintain constant tension on guideline wire ropes, and wire ropes that support the BOP control podlines as the rig heaves.

Choke and Kill Lines and Drill String

Choke and kill lines are attached to the outside of the main riser pipe They are used to control high-pressure events. Both lines are usually rated for 15 ksi. High pressure is circulated out of the wellbore through the choke and killed lines by pumping heavier mud into the hole. Once the pressure is normal, the BOP is opened and drilling resumes. If the pressure cannot be controlled with the heavier mud, cement is pumped down the kill line and the well is killed. The drill string permits the circulation of drilling fluid or liquid mud. Some functions of this mud are to:

  • Cool the bit and lubricate the drill string
  • Keep the hole free of cuttings by forced circulation to the top.
  • Prevent wall cave-ins or intrusions of the formations through which it passes.
  • Provide a hydrostatic head to contain pressures that may be present.


[1] World Oil, Composite Catalog of Oilfield Equipment & Services, forty fifth ed., Gulf Publishing Company, Houston, 2002/03.

[2] International Standards Organization, Petroleum and Natural Gas Industries – Design and Operation of Subsea Production Systems – Part 7: ompletion/Workover/Riser System, ISO 13628-7, 2005.

[3] T. Clausen, R. D’Souza, Dynamic Risers Key Component for Deepwater Drilling, Floating Production, Offshore Magazine, vol. 61, (2001), May.

[4] P.R. Geiger, C.V. Norton, Offshore Vessels, Their Unique and Applications for the Systems Designer, Marine Technology, vol. 32 (No. 1) (1995) 43–76.

[5] American Petroleum Institute, Recommended Practice for Design, Selection, Operation and Maintenance of Marine Drilling Riser Systems, API-RP- 16Q (1993).

[6] American Petroleum Institute, Design of Risers for Floating Production Systems (FPSs) and Tension-Leg Platform (TLPs), API-RP- 2RD (1998).

[7] H. Matlock, Correlations for Design of Laterally Load Piles in Soft Clay, OTC, 2312, Offshore Technology Conference, Houston, Texas, 1975.

[8] K. Vandiver, L. Lee, User Guide for Shear7 Version 4.1, Massachusetts Institute of Technology, Cambridge, 2001, March 25.

[9] J.F. Archard, Contact and Rubbing of Flat Surfaces, Journal of Applied Physics, vol. 24 (No. 8) (1953) 981.