Coiled tubing was developed in 1970s and is one of the most important pieces of well intervention equipment in the oil and gas industry. There are several different types of it available in the industry market with several different designs of a coiled tubing unit; however, the components of a coiled tubing unit are very similar. The main differences are performance capabilities and hydraulic power control systems. This article will give an overview of the essential components of a coiled tubing unit. (Read about the history of coiled tubing here – Introduction to Coiled Tubing (CT) in Oil and Gas)
Figure 1 shows the mounted truck coiled tubing unit, which is normally used for the land operation and Figure 2 is the coiled tubing unit used for operating in an offshore environment.
The main components of a coiled tubing unit are;
- Power Pack
- Control Cabin
- Injector Head
- Pressure Control System
- Coiled Tubing Reel
Power Pack Unit
A power pack unit provides hydraulic power to control and operate a coiled tubing unit and pressure control equipment. Generally, the power pack units have an independent electric or diesel power source which is designed to operate in Zone 1 and Zone 2 areas.
Coiled Tubing Control Unit
The control unit contains all of the necessary controls for operating a coiled tubing unit from this location. Typically, a control unit is located behind a coiled tubing reel. However, there may be a situation where the location does not permit rigging this up, so a control unit may be relocated to another place where operators will also feel safe when operating the unit. Figure 4 demonstrates the controls inside the unit and it is clearly seen that there are many instrumentations for the unit control and gauges. The level of control and instrumentation are dependent on the type of operation being performed. For example, a normal well intervention coiled tubing until will have less instrumentation than a highly sophisticated coiled tubing drilling unit.
An injector head is one of the critical parts in a coiled tubing unit and it consists of several hydraulic systems that allow a coiled tubing unit to operate with a high degree of operational variability. Its function is to supply pulling and pushing capacity required for running and retrieving a coiled turning in and out of a well. When working a live well, it is important to ensure that chain gripper blocks have adequate gripping power to overcome well head pressure force and the injector hydraulic pressure must initiate sufficient force to push a coiled tubing into a well against well bore pressure.
Pressure Control Equipment
In order to safely operate in a live well, coiled tubing units must have pressure control systems in place for well control. Three classification of pressure control equipment for a coiled tubing unit are as follows;
A primary barrier is an initial barrier for pressure containment which is normally closed.
Stripper/packer is a primary barrier in a coiled tubing unit and the stripper is designed to provide a pressure seal around a coiled tubing unit when it is being run into or pulled out of a live well. The sealing mechanism is activated by hydraulic pressure controlled by an operator.
Tandem stripper assemblies, which are used together with a fixed stripper, provide a backup for a stripper when the primary stripper fails or wears out during a coiled tubing operation.
A secondary barrier is a mechanical normally open system and it is used when the primary barrier fails or becomes impaired.
A tertiary barrier is a mechanical normally open system and it will be used when a primary and a secondary barrier fails and the well integrity is badly impaired.
Coiled Tubing Blowout Preventer
A coiled tubing blowout preventer provides secondary and tertiary barriers to back up the primary barrier system. The BOP is used to secure the coiled tubing and isolate the wellbore pressure during normal and emergency situations. The typical arrangement of a coiled tubing BOP consists of blind rams, shear rams, slip rams and pipe rams and the diagram for the BOP is shown in Figure 8.
From top to bottom, the BOP has the following rams:
Blind rams are designed to close and seal off a wellbore with no coiled tubing or any tools across the BOP.
Shear rams are designed to cut the coiled tubing in an emergency situation, but they don’t seal rams.
Slip rams are designed to close around the coiled tubing and hold the tubing string whether the well forces are acting up or down, but they don’t seal rams.
Pipe rams are designed to close around the coiled tubing and seal around an annulus area.
Coiled Tubing Reel
The primary function of a coiled tubing reel is to store a coiled tubing. Typically, the coiled tubing reel is hydraulically driven, whether by a chain or a direct drive. A reel driven system is designed to keep the coiled tubing in a dynamic tension between the gooseneck and the reel, while the coiled tubing is un-spooled and re-spooled.
Capacity of coiled tubing is dependent on several factors, such as the size of coiled tubing, weight and length of coiled tubing, footprint availability, etc. It is imperative to select coiled tubing which is suitable for the expected operation. Over capacity coiled tubing can lead to excessive cost and space issues for some offshore locations. However, insufficient capacity of a coiled tubing unit can result in an unsuccessful job.
Jr. Adam T. Bourgoyne, 1986. Applied Drilling Engineering (Spe Textbook Series, Vol 2). Edition. Society of Petroleum Engineers.
J.J. Azar, 2007. Drilling Engineering. Edition. PennWell Corp.
George E. King. 2009. Coiled Tubing Introduction. [ONLINE] Available at: http://gekengineering.com/Downloads/Free_Downloads/Coiled_Tubing_Surface_Equipment.pdf. [Accessed 14 August 2016].
Worl Oil , 2005. Coiled Tubing Handbook. 1st ed. Texas: World Oil.