Direct Steerable Pipe Thrusting

Pipelines of various types, sizes, and purposes are routinely installed below the ground surface in every community around the world. The most common method of pipeline installation is open cut, also known as trenching. However, open-cut installation results in obvious and sometimes significant unwanted ground disturbance. Direct Steerable Pipe Thrusting (DSPT) was developed to fill a need for a pipeline installation in ground conditions challenging to other trenchless methods. The history of DSPT begins with two entirely different underground installation methods: horizontal directional drilling and microtunneling. DSPT technology is more akin to microtunneling technology. However, not all the principles of microtunneling are valid for DSPT. History has shown that new trenchless technologies, adaptations, and applications require new approaches to design and construction.

The direct steerable pipe thrusting (DSPT) method presents a set of new options for the pipeline installation market by combining the benefits of widely accepted trenchless technologies to form a new methodology. This chapter provides a summary of the DSPT process; the DSPT microtunnel boring machine, and other equipment technology, as well as some general design parameters. The DSPT method incorporates the sequential process of a typical horizontal directional drilling installation including site preparation, prewelding (long) sections of thrust pipes, integrity testing and certification of joints within these long sections, application of joint coating, tunneling, and pipe installation. A full spread of DSPT equipment is characterized by several aboveground and belowground components. The chapter provides a comprehensive overview of the key generic elements.

Selecting the best route for a direct steerable pipe thrusting (DSPT) installation depends on a variety of factors. These considerations include avoiding known buried objects and working within existing or available ROW or temporary workspace, permit restrictions, access to workspace, geological conditions, and existing topography. Prior to determining the feasibility of a DSPT project, a preliminary geotechnical investigation should be performed to evaluate the ground conditions. During the planning phase, published geology and historic explorations should be reviewed prior to conducting a detailed geotechnical assessment. Worksites are configured using different sizes and shapes based on the length of the crossing, available length for pipe sections, contractor preference, and size and type of thrusting equipment. A variety of factors need to be taken into consideration to develop a cost estimate for a pipe installation using DSPT.

This chapter addresses site investigations that are important for planning, constructability/feasibility evaluation, detailed design, construction, long-term operability, and resiliency of direct steerable pipe thrusting projects. These site investigations include geotechnical assessment and characterization, utility and other structure surveys (both aerial and subsurface), topographic and bathymetric surveys, environmental conditions, and seismic conditions. Environmental conditions can be naturally occurring or the result of human activity. Naturally occurring conditions include hydrocarbons and asbestos, as well as active or dormant landslides, active erosional features, and corrosive soils. Seismic conditions including ground shaking and fault offset, and related ground responses should be considered when selecting entry and exit locations, pipeline alignments, and design requirements for the entry and exit pits. The seismic considerations include protection of personnel and minimization of damage from fault offset, ground shaking, landslides, liquefaction, lateral spreading, and tsunamis.

The design process for a direct steerable pipe thrusting (DSPT) installation draws from both horizontal directional drilling and microtunneling. This chapter discusses the following topics as they relate to the engineering design of DSPT installations: geometric design, thrust force estimate, hydraulic fracture evaluation, overcut and settlement, slurry and lubricating fluid considerations, launch and reception area considerations, and acceptable geometric tolerances. The geometric design for a DSPT installation typically consists of five basic elements: launch tangent, launch curve, bottom tangent, reception curve, and reception tangent. Engineers should use sound engineering judgment when evaluating methods to estimate the total thrust forces. The MTBM can be launched from the ground surface or from a shallow shaft and pit. If launched from the ground surface, consideration should be given to the use of a conductor casing to provide fluid containment, support of adverse ground, and a controlled launch for the MTBM.

This chapter covers the following topics as they relate to the direct steerable pipe thrusting method of trenchless steel pipe installation: installation related axial, bending, hoop, torsional, and combined stresses; installation related pipe buckling evaluation and considerations; allowable thrust force calculation; operational stresses; and pipe coating considerations. As the steel pipe is thrust through the ground, stresses are induced in the pipe that must be assessed to avoid overstressing the pipe and yielding of the steel. If using the conventional Euler buckling theory as described, the relatively long unbraced lengths of thrust pipe typically predict buckling at axial loads that are much lower than are necessary to install the pipe. The design engineer should factor the thrust force estimate in accordance with their knowledge of the geotechnical conditions at the site, capacity of the pipe thruster, their experience, and other considerations.

This chapter focuses on the contract documents for direct steerable pipe thrusting (DSPT) projects. Contract forms, delivery methods, and potential legal issues, such as differing site condition clauses and alternative dispute resolution, are also briefly addressed. The contract form dictates who is responsible for the project design and how the contractor will be paid for the work. In general, the most significant risks associated with trenchless projects, including DSPT projects, are delays, damages, or losses that result from unanticipated, poorly understood, or misinterpreted subsurface conditions. All geotechnical data collected during various phases of the project should be compiled, provided to bidders, and become part of the contract documents. This is typically accomplished using a Geotechnical Data Report.

This chapter summarizes the direct steerable pipe thrusting (DSPT) construction process from preconstruction activities through as-built documentation. Topics discussed as they relate to the construction phase of DSPT installations are safety, preconstruction activities and submittals, staging and layout, launch preparations, machine operations, contingency measures, and construction documentation and as-built submittals. As with all construction projects, DSPT construction requires a site-specific construction safety plan. Because each project is unique and site conditions vary, a safety plan should be developed for each phase of the DSPT construction. The preconstruction survey for DSPT consists of verifying available workspace and laying out or staking the major design components. The purpose of the launch seal is to keep the lubrication fluid in the overcut annulus throughout the DSPT process.

This back matter contains a glossary.