A field worthy method of accurately measuring localized coiled tubing (CT) wall thickness using ultrasonic (UT) techniques has been developed. This paper discusses the development of this new method, the system required to implement the method, and presents data from testing of the system.
Introduction
A failure of CT, either in a well or while being bent at surface, can have a large safety, environmental and/or economic impact. Significant improvements have been made in the industry, which has reduced the number of CT failures. These include improvements in:The materials, manufacturing process and quality control of the pipe before it is sent to the fieldThe modeling of the fatigue damage in the pipe, allowing it to be removed from service before it reaches the end of its useful lifeHandling and treatment of the CT to mitigate corrosion and mechanical damageTraining of personnel in the proper use and maintenance of the pipeThe understanding of failure statistics through failure tracking systems that require each failure to be analyzed
However, the demands being placed upon the CT are increasing. Fractruting and acid stimulation through the CT erode or corrode the steel. Certain well environments such as chrome well tubulars cause external abrasion. CT is being used in "high-pressure" applications, with the definition of high-pressure constantly increasing. These increased demands require a better means of monitoring the integrity of the CT.
Several CT inspection systems have been developed to try to meet this requirement. Some systems measure only diameter and ovality. Other systems seek to find cracks and pits, and give an average wall thickness. These systems have found applicability in some niches of the industry, but none of them are completely satisfactory for one or more of the following reasons:They are too difficult to calibrate and operate on a continuous basis in the field environmentThey do not meet the zone requirements to be used in some areasThe output they give is un-interpretable and/or requires an expert in pipe inspection to interpretThey are cost is prohibitive, especially in low cost marketsThey measure a few parameters (such as diameter and ovality) which are of interest in certain cases (such as high-pressure), but fail to measure other parameters.
Due to these limitations, more research and development is being done in CT pipe inspection. The UT system discussed in this paper does not purport to solve all of the CT inspection issues, but it is a significant step forward in CT inspection technology.
Pipe Inspection Basics
Many systems have been developed to inspect pipe. Different systems measure different parameters, some of which include diameter, ovality, and wall thickness. They also look for flaws in the pipe such as cracks and pits. Some versions of these systems have been adapted for inspecting CT. These systems can be categorized in three major types:Nuclear - some type of radiation such as gamma rays are passed through the pipe and the reflections are measuredElectromagnetic - magnetic flux is passed through the pipe and the flux flow or flux leakage is measuredUltrasonic (UT) - ultrasonic sound is passed through the pipe and the reflections are measured.
Nuclear measurements require a nuclear source, which makes them impractical for many applications.
Systems using electromagnetic measurements have been used for diameter and ovality measurement, and for finding anomalies, flaws and defects in the pipe, such as cracks, but often such systems have not proven accurate for measuring the wall thickness in one localized area of the pipe. An electromagnetic inspection system for CT is discussed in reference 1.
UT inspection systems have been used for measuring the localized wall thickness of a tubular. They are also capable of measuring diameter, ovality and finding anomalies. An exemplary UT system for inspecting CT is shown in reference 2.
