Continuous-String Light Workover Unit
Authors: Slator Damon T. , Hanson W.E. 
Published: December 2007

Abstract
A new high-speed workover method is described, wherein a continuous work string wound on a reel is injected into the oil or gas well, which may or may not be under pressure, to perform light workover operations or set tubing at depths to 15,000 ft. The first field-type operational system of this type was employed successfully by The California Co. in Southern Louisiana to remove sand bridges from the production tubing in an oil well at depths from 5,510 to 8,628 ft. Other similar workover jobs at greater depths have since been performed. The history, methods and economic analysis of these jobs are discussed.
Introduction
Operation PLUTO, the cryptographic name for pipe lines under the ocean, involved laying prefabricated pipelines across the English Channel to supply fuel to the Allied Forces after the World War II invasion of Normandy in June, 1944. The lines were prefabricated and coiled in cable-laying ships and on large floating drums. During a period of several months beginning on Aug. 14, 1944, a total of 23 lines were installed. Seventeen were approximately 30 miles in length and six were approximately 70 miles long. The pipe was laid as the ships and floating drums moved across the Channel, resulting in individual pipelines being installed in a matter of days, rather than months as required by conventional methods. Years later, R. V. Cross, an engineer for The California Co., charged with the responsibility for seeking new technique's to improve production methods and reduce costs, conceived the idea of applying the Operation PLUTO technique for laying pipelines in the Gulf of Mexico. Carrying the thought further, he conceived the idea of operating a continuous work string from a reel for performing light workover operations in oil and gas wells. Pilot model and prototype tests were done to prove the feasibility of the continuous-string method applied to oilfield workover operations. From these tests and evaluations, criteria and design parameters were established which formed the basis for design and construction of the first operational continuous-string light workover unit.*
Principles of Operation
The salient feature of the workover unit is, as the name implies, the continuous string. Tubing is welded into one continuous length, and is wound on a reel much as is a large diameter wire line. On the job site, the tubing, which is more or less rigid, is spooled off the reel to form a loop, the free end of which, with bottom-hole tools attached, is fed through and injected into the well by means of a traction device, the tubing injector, mounted on the wellhead. Fluid is circulated through the full length of the work string, and is returned through the annulus.
General Description of Rig
The unique components of the unit, illustrated in Fig. 1, include the powered reel unit, on which is wound the continuous work string; the tubing injector; and the hydraulic power and control system for operation and coordinated control of the reel unit and injector. Wellhead pressure control and safety equipment, and a conventional fluid circulating system complete the rig. A typical arrangement of rig components installed on a work barge is indicated schematically in Fig. 2, The relative arrangement of the reel and tubing injector set-up on a keyhole-type well structure, and the general configuration of loop in the work string are indicated in Fig. 3.The tubing injector and wellhead equipment are mounted on and attached directly to the top of the wellhead assembly so the weight of the equipment and the forces required to inject or extract the work string are carried directly into the wellhead.
JPT
P. 39ˆ