IBP1070_09 METHODOLOGY FOR DEFINITION OF BENDING RADIUS AND PULLBACK FORCE IN HDD OPERATIONS Danilo Machado L. da Silva1, Marcos V. Rodrigues2, Asle Venås3 4 Antonio Roberto de Medeiros Copyright 2009, Brazilian Petroleum, Gas and Biofuels Institute - IBP This Technical Paper was prepared for presentation at the Rio Pipeline Conference and Exposition 2009, held between September, 22-24, 2009, in Rio de Janeiro. This Technical Paper was selected for presentation by the Technical Committee of the event according to the information contained in the abstract submitted by the author(s). The contents of the Technical Paper, as presented, were not reviewed by IBP. The organizers are not supposed to translate or correct the submitted papers. The material as it is presented, does not necessarily represent Brazilian Petroleum, Gas and Biofuels Institute’ opinion, or that of its Members or Representatives. Authors consent to the publication of this Technical Paper in the Rio Pipeline Conference Proceedings. Abstract Bending is a primary loading experienced by pipelines during installation and operation. Significant bending in the presence of tension is experienced during installation by the S-lay method, as the pipe conforms to the curvature of the stinger and beyond in the overbend region. Bending in the presence of external pressure is experienced in the sagbend of all major installation methods (e.g., reeling, J-lay, S-lay) as well as in free-spans on the sea floor. Bending is also experienced by pipelines during installation by horizontal directional drilling. HDD procedures are increasingly being utilized around the world not only for crossings of rivers and other obstacles but also for shore approach of offshore pipelines. During installation the pipeline experience a combination of tensile, bending, and compressive stresses. The magnitude of these stresses is a function of the approach angle, bending radius, pipe diameter, length of the borehole, and the soil properties at the site. The objective of this paper is to present an overview of some aspects related to bending of the product pipe during HDD operations, which is closely related to the borehole path as the pipeline conforms to the curvature of the hole. An overview of the aspects related to tensile forces is also presented. The combined effect of bending and tensile forces during the pullback operation is discussed. 1. Introduction Originally used in the 1970s, directional crossings are a combination of conventional road boring and directional drilling of oil wells. Horizontal directional drilling (HDD) is an alternative construction method in the trenchless industry and it has experienced rapid growth in the construction industry over the past few decades. The horizontal- directional-drilling process represents a significant improvement over traditional open cut method for installing pipelines beneath obstructions, such as rivers, highways, railroads, islands and others. HDD is also increasingly being utilized for shore approach of offshore pipelines mainly because it has less environmental impact in certain cases vs. alternative methods. Installation of a pipeline by HDD is generally accomplished in three stages. The first stage involves drilling a small-diameter pilot hole along a designed directional path. The second stage consists of enlarging (reaming) the pilot hole to a diameter that will support the pipeline and the third stage consists of pulling the pipeline back into the enlarged hole. Despite its growth and popularity, a number of issues related to HDD installations remain poorly understood. These issues are even more significant in shore approach HDD installations. Shore crossing installation using HDD are much more complex and challenging than typical surface to surface installations. The increased challenges and complexity arise from an inability to readily access the exit location, less geotechnical information associated with ocean floor sediments, elevation differences between entry and exit locations, complexity of coordinating diving operations, tidal and storm influences, bore instability and drilling fluid management, product pipe installation strategies, and buoyancy control. An extensive theoretical research is needed in order to develop rational analysis procedures for the combined behavior of soil, product pipe and drilling fluids during HDD installations method. Such procedures are necessary to ______________________________ 1 D.Sc, Engineer – Det Norske Veritas, DNV – Rio de Janeiro, Brazil 2 D.Sc, Senior Engineer – Det Norske Veritas, DNV – Rio de Janeiro, Brazil 3 Segment Director Offshore Pipelines – Det Norske Veritas, DNV – Oslo, Norway 4 HDD Coordinator – Subsea 7 – Brazil Rio Pipeline Conference and Exposition 2009 establish rational design guidelines for HDD. In general, current design methodology relies on the experience and judgment of contractors, manufacturers and engineers. 2. Drill-Path Design As in any other installation method, in HDD operations the pipeline is under combined tension and bending. The pipe conforms to the curvature of the hole, which makes the magnitude of the stress a function of the HDD path design. This means that it is a function of the approach angle, bending radius, product pipe diameter, length of the borehole, and the soil properties at the site. The combined tension and bending ovalizes the pipe cross section reducing its resistance to external pressure. In case of a pipe in contact with a curved surface, the applied tension is reacted by a distributed force acting on the surface of the pipe. This contact force tends to increase the ovalization induced by bending (Kyriakides, 2007). The designed drill path should meet all the location and depth control points while keeping the drill length as short as possible. Drill paths are made up of a series of straight lines and curves, which are typically sag bends, over bends, or side bends depending on their axial plane. It is not uncommon for HDD drill paths to have compound bends even though they are generally avoided if possible. Combined small radius in vertical and horizontal plans, dog legs, and average 3D curvature deviated of the predicted profile is a common problem to avoid while drilling the pilot hole. The location and configuration of a drilled profile are defined by its entry and exit points, entry and exit angles, radius of curvature, and points of curvature and tangency. When designing the drill path it is desired to keep the number of bends to the minimum required. This reduces pullback loads and extends drill-rod life. The best bore path starts with a straight tangent section at the prescribed entry angle to gain the depth required for steering control and the depth of cover. At the required depth the drill head is steered upward with a curve, then transitions to a horizontal segment, and again turns upward with another curve before transitioning to another straight tangent section at the desire exit angle. Therefore, the key parameter in HDD path design is the bending radius. In a few cases HDD profiles can be defined with only two curvatures and two straight sections. This paper presents the most common used equations for definition of the bending radius for HDD. These equations are derived from established practice rather than from theoretical analysis. Of course, the bending radius has a significant influence over the pullback operation, in a way that small radius and dog legs increase the pullback forces. An overview of aspects related to the definition of the bending radius and the pullback forces in HDD operations is presented. Several relevant aspects for the success of HDD operations, mainly related to bending radius and pullback procedures, are presented and discussed. 3. Bending Radius An important parameter in the design of a crossing by means of horizontal directional drilling is the bending radius or radius of curvature. The bending radius is determined by the bending characteristic of the product pipe, increasing with the diameter. It is usual in designing HDD paths to consider a bending radius equal to 1000 times the nominal diameter of the pipe to be installed. Another general “rule-of-thumb” for the bending radius is 100ft/1in diameter for steel line pipe, which is equivalent to 1200 times the nominal diameter of the pipe. These connections between pipe diameters and bending radius are derived from established practice for steel pipe rather than from theoretical analysis. Typically, the minimum radius determined using a stress-limiting criterion would be substantially less than 1000 times the nominal diameter. For this reason, bending stress limits rarely govern geometric path design but are applied with other stress-limiting criteria, in determining the minimum allowable bending radius. The following equations, from the Strength of Materials, can be used to determine the minimum bending radius for a steel pipe. M σbend · I 1 M E·I E· D σbend = rpipe → M = ; = → rbend = ; σbend = (1) I rpipe rbend E·I M 2·rbend Where, σ is the bending stress, M is the bending moment, I is the moment of inertia, E is the modules of elasticity, rpipe is radius of the pipe, rbend is the bending radius and D is the pipe outside diameter. The following equation is commonly used for calculating the allowable bending radius for steel pipe (Willoughby, 2005). 3·E· rpipe rmin = (2) 2·Sa 2 Rio Pipeline Conference and Exposition 2009 Where rmin is the recommended smallest radius of curvature that can be used without overstressing a straight pipe,