Baseline Integrity Management |
January 19, 2012 |
By Todd Porter
I’ve had a number of discussions over the course of 2011 related to determining the accuracy of pipeline position. The basic question of “How accurate is my pipe centerline?” persists. My background is deep rooted in navigation and positioning, and all the survey systems and methods that wrap around the technology to provide accurate position information for a feature, structure or events. In this article, I’ll provide perspective on the pipeline positioning spectrum: the technologies, methods and value to the ultimate goal of safe and reliable delivery of hydrocarbon energy through pipeline integrity management (PIM), as well as the processes for assessing and mitigating pipeline risks in order to reduce both the likelihood and consequences of incidents. As industry professionals, we understand PIM as a systematic, comprehensive process designed to provide information to effectively allocate resources for appropriate prevention, detection and mitigation activities. Simply stated, this involves extensive spatial (where), geometric (what), condition (when), repair (how) and hopefully not the “who” when an incident takes place. Any valid baseline includes accurate position of the pipeline.
Let’s first define accuracy as the sum of precision + reliability; repeatable and correct to some standard. Absolute accuracy is with respect to a regional or global datum, such as the North American Datum (NAD83). This is the survey standard reference, which closely approximates the World Geodetic System (WGS84) to which the GPS system is related. This is what our latitude, longitude and elevation, or State Plane Coordinates, are based on. Relative accuracy, however, is based on a local reference: a mile post, girth weld, road crossing or any feature to which we can pull a survey tape from to locate a defect or area of interest. Relative accuracy will be discussed later when I review inline inspection mapping methods. Now you’re ready to challenge your staff or contract surveyors with this practical knowledge.
With the evolution and innovation of GPS technology, we tend to teeter between consumer and commercial systems, i.e., which combination of cellular phones or ruggedized GPS units will best serve and survive field conditions, make economic sense and will be adopted (and standardized) by demanding user groups across the organization. There are many grades of GPS receivers and modes of operation, including GPS-enabled cell phones. To improve accuracy, there are differential survey methods and real-time correction services, integrated pipe locating technology and inline inspection mapping systems. This provides opportunity for accuracy and consistency across the pipeline asset base to continuously collect field data, tag it with coordinates and improve the accuracy and resolution of the pipeline and features. The point is to include some metadata about the methods used in collection of the data — process, accuracy, etc. — to enable conflict resolution at some future date when the question arises: Which data is correct or more accurate?
As most of us have experience with legacy systems (i.e. not recent or new pipeline construction), a primary challenge is relating aboveground position and condition data to the buried pipeline that typically is linearly referenced (stationing, odometer). I’ll spend the balance of my word space discussing inline inspection mapping systems (ILI-MS), which offer a means to accurately correlate underground features to where the backhoe needs to dig, where a root cause analysis becomes more certain and where engineering design includes monitoring.
The essential electromechanical elements of an ILI-MS are gyro-accelerometer cluster for measuring orientation and movement changes, a synchronized clock and odometers. To cut the story short, complex post-processing is employed to compute high resolution position and curvature for the pipeline using cluster, odometer and aboveground marker (AGM) and feature (valves, etc.) position data. The PIM engineer needs to understand the benefits and limitations of this technology in the operations and technology dimensions.
Operationally, there is no better means to integrate position with ILI condition assessments when the tool is run in tandem. ILI vendors are providing reliable and cost effective combination tools (MFL, UT, caliper), thus reducing the operator to the risk and expense of multiple tool runs. Technically, ILI-MS is a relative survey tool. Its accuracy is pegged to the last known position on the pipeline. For example, many vendors quote 1:2,000 positional accuracy. This equates to an error of 1 ft for every 2,000 ft from the closest known point such as an AGM or valve, which have accurate GPS positions. It’s important to note that there are many factors that affect the ILI-MS accuracy: cluster grade (tactical or navigation), AGM spacing and tool detection, tool velocity profile, degree of horizontal bends and odometer reliability. These all contribute to the final outcome.
We have the opportunity to improve pipeline position accuracy on a continual basis. How much accuracy do we really need — a foot, meter, half the right-of-way width? The answers lie in resolving congested space conflicts, matching and aligning data, enabling root cause analysis with precise 3D pipeline geometries and increasing the overall confidence in the PIM system. Cost-benefit should consider all internal stakeholders, and whichever means are used, be sure to include the metadata describing how it was derived.
Todd Porter is vice president at New Century Software Inc. and is a member of the North American Oil & Gas Pipelines Editorial Advisory Board.