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As-Built Mapping of Critical Crossings: Project And Operational Risk Management

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Pipeline infrastructure is crucial for the safe and reliable delivery of energy, municipal, telecom and other services. There is significant congestion and conflicts, resulting in increased operational risk, especially at waterway, transportation, and utility crossings. This article will focus on projects in the energy pipeline space, some of which are complex, and others that have adopted this new standard in mapping critical trenchless crossings installed using HDD methods. There is immediate and long-term value of accurate 3D data to manage risk, enhance damage prevention, confirm clearances and achieve permit compliance at these critical crossings. 

Asset Mapping

For HDD installs, it’s important from the outset to make a distinction between “tracking” and “mapping.” The drilling industry employs tried and true methods for steering and tracking the drill head to follow design profiles. Each of these methods ultimately have trade-offs in accuracy, interference, setup, cost and required operator experience. There is no substitute for choosing the best combination of these technologies and experience, and the HDD industry continues to deliver within design tolerances. Therefore, the value of this discussion is complementary to prudent geotechnical design, HDD monitoring, steering and quality assurance. This value can be quantified with these gains:

  1. Accurate 3D mapping of HDD installed pipe, the final in-service location (not a pilot hole).
  2. Reduced risk from future threats.
  3. A complete project “close-out” package for “As-Builting.”
  4. Mapping continuity and GIS ready data.

This article will focus on a complex HDD crossing of the Houston Ship Channel, and typical corridor and DOT crossings that have become the new standard in conflict avoidance and As-Builting.

Accuracy

As an industry, we speak of tolerances as “the amount of variation in a product that will still allow it to function as intended.” In this discussion, we will talk about accuracy, which is the sum of precision (repeatability) plus reliability (bias from truth). For example, we can measure a distance 100X with a standard deviation of +/- 0.01 m. However, our measurement device has a bias of 0.5 m. Therefore, in order to increase accuracy, we must remove the systematic biases (through calibration, procedures) and select a consistent (low random error) device. To complete the “accuracy” discussion, we need to define relative and absolute accuracies; where relative is “internal” to itself and absolute is with respect to some “external” datum or reference point. This is where the results are reported in a defined datum and coordinate system for overlay into GIS and CAD systems for reporting and further analytics.

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Achievable relative accuracy of the pipe centerline (or top of pipe) is 1:3,000 and with RTK-GPS control points (+/- 0.01 m) at the entry (launch) and exit (trap) points of the installed pipe will yield the same absolute accuracy in the final XYZ coordinates. This approach exceeds all other methods of both ILI tools and above-ground methods.

Example calculation: pipe segment = 3,000 ft, 20-in. diameter; therefore 3D Accuracy = 3,000 ft / 2 * 1/3,000 = 6 in.

The largest error occurs at the midpoint (furthest from the control points). In this case, the error estimate is well within the pipe diameter “envelope.” Accuracy = Precision + Reliability, where precision is achieved for towed tools through repeated measurement of the segment through forward-backward, transit-backward-forward runs. Free swimming tools employ additional along-line control points to reduce errors.

Reliability is achieved through factory calibration (annual), forward-backward averaging and/or additional along-line reference points. This averaging eliminates most systematic errors (biases) that occur through heading sensitivity of the gyros and velocity excursions due to pull force and pipe curvature. In addition to high accuracy GPS for establishing absolute coordinate reference and scaling, velocity and along-track distance is measured with redundant odometer wheels. Velocity updates are a critical element of the inertial navigation processing.

The towed tool (p. 25) offers simplicity and operational efficiency, due to the open-ended pipe segment. The tool run occurs immediately after a gauge plate is run post-hydro test of the pulled through pipe, with or without tie-in fabrication. The complete mapping takes less than two hours with minimal disruption to construction operations. For free swimming tools (shown above) used in segments over 5,000 ft, launch/trap barrels are required to push the tool with air (head pressure required) or water. This requires expert operations support that may span days.

Case Examples and Drivers

Major TXDOT Adjustments — These adjustments are driven by transportation corridor expansions, which in some states are significant infrastructure buildouts. Generally, these projects largely reimbursable to the asset owner and increased value awareness of the state DOTs.

IH-69 Bridge Span — requirement for 3D mapping of a deep products pipeline to insure safe offset from planned bridge span piles. This called for better than one-foot accuracy with confidence to avoid conflict.

FM 1960 Adjustment — relocation (adjustment) of natural gas pipeline under secondary highway, in congested corridor, and requiring high accuracy position for any future conflict or to defend future damage claims.

U.S. Army Corps of Engineers (USACE) 408 Permit

Houston Ship Channel — a hybrid project consisting of 7,500 ft HDD, 4,300 ft Bottom Lay and 2,300 ft HDD, total of about 14,100 ft. RTK-GPS control points were established at launch, bottom-lay girth welds and trap locations. Tool speed control was achieved with adequate head-tail pressures and CFM volume to move low friction tool. Combining post-construction bathymetry produced depth of cover in addition to the accurate as-built.

towed mapping tool

Key Considerations

  • HDD drill logs are not a final as-built. However, these mapping methods are deterministic. Confirmation of the final pipeline position is provided and compared to design.
  • Utility conflicts and avoidance requirements are addressed.
  • Compliance for USACE 408 Jurisdiction permits.
  • Risk Management — benefit of critical segment 3D position and curvature (a by-product of inertial mapping).
  • Confirmation in the field and uploaded into GIS and company workflows. “Map the Gap,” these critical segments typically go unmapped, but can now be tied into open cut girth-weld surveys with consistent accuracy.
  • Subsequent ILI tool runs may be years away, only to deliver less accurate position results.
  • For congested corridors and crossings (especially USACE jurisdiction or state DOT), this level of accuracy is critical for avoiding damage from construction and expansion projects.
  • This process is easily included in any construction project, with minimal impact to schedule and cost.

Risk Reduction

The HDD-3D Mapping contributes to addressing these threats:

  1. Third-party damage — such as from highway or rail expansion, line crossings or utility conflicts and other excavation projects.
  2. Internal corrosion — the value of sag/overbend for flow modeling, liquid holdup.
  3. Natural forces — having an accurate “baseline” from which subsequent ILI can overlay (subsidence, free span) and bending strain assessed.
  4. Construction — validation of design prior to in-service.

Experience Counts

Confidence in asset location accuracy is essential in all aspects of risk management, achieving operational excellence and permit and regulatory compliance. This includes Subsurface Utility Engineering (SUE) practices for locating utilities in corridors and facilities, using in-pipe systems, GPR, EM technologies for 3D imaging and direct examination for confirmation and discrimination.

Todd Porter, MSc, MBA, PE is the founder and CEO of PORTER 4D LLC (www.porter4d.com) and leads the infrastructure industry in providing technology, expertise and demonstrated value with high accuracy mapping of underground pipelines and conduits.

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