Failure analysis and modernization of high-pressure hydraulic press for drilling tubes testing

Highlights

• Multi-disciplinary research is conducted on a high-pressure hydraulic press.

• Potentially weak elements and their failure modes are determined.

• Analytical models of static and dynamic impacts are implemented.

• Proposed design modifications provide the integrity of the existing structure.

• Simultaneously, the required performance of tubes testing is provided.

Abstract

This research paper represents the results of abrupt failure investigation in the structure of industrial plant for hydrostatic pressure testing of tubes. The significant damages of testing machine elements are observed after the moving tube impact when the opposite tube end cap has been suddenly released because of the screw-thread defect. The water pressure inside the tube at the time of the incident was only 20 MPa while the maximal pressure of 125 MPa is required by tubes tests specification. Therefore, based on admitted assumptions, the detailed analysis is conducted of static and dynamic forces acting on elements of structure to avoid full machine destruction in case of the next incidents under higher pressures. Conditions are determined of the sharp cap end penetration into thrust plate and possible damage of upper protection casing from the water jet and tube pieces impacts under conditions of the crack opening in the tested tube. The two options are proposed of plant modernization to reinforce structure for safety work under pressure from 70 MPa to 125 MPa depending on tube size and testing conditions. The developed improvements of machine structure allow simultaneously increasing the productivity of testing plant up to 14–20 tubes per hour depending on their sizes.

Introduction

The hydrostatic pressure testing of tubes is an obligatory technological operation either after their production in metallurgical plants or field-tests before drilling operations on oil and gas sites. During the tests, the tube is closed by screw-threaded caps from both sides and filled with water by special pumps to the high pressure of up to 125 MPa. Tubes are kept at this pressure within about 5–10 s to check possible leaks and defects, then they release water and reinstall caps on the next tube. Such a sequential technological process is realized by the special mechanized hydrostatic presses, which may be damaged from the tube exploded due to a crack or a defect of screw-thread of caps in case if the machine and surrounding protective structure are improperly designed. The examples of the design of the existing hydraulic presses for testing tubes with a diameter of up to 1420 mm and a pressure of up to 70 MPa are shown in Fig. 1 and Fig. 2. In these designs for tubes of various lengths, the end thrusts are provided, adjustable by a screw electric drive, as well as locks fixing the tube along the length with a hydraulic drive. However, such variants of hydraulic presses do not provide high productivity for a wide assortment of tubes. The safety issues arising during operation of high-pressure machines in the restricted areas of workshops and satisfying controversial demands to productivity always need great efforts from the designers. Therefore, industrial customers install testing presses of their design. Such example of the investigated hydraulic press is represented in Fig. 3.

During one of the tube tests on this press, under the pressure of 20 MPa (even less than maximum value by design), one of the end caps has suddenly released due to the screw-thread defect and tube being moving by a water jet shocked into the protective structure of the machine causing deformations and penetration damages. Inspection of the structure after hitting the cap on the left thrust showed visually noticeable signs of deformation of the foundation bolts M36 (Fig. 4a) and the bolts M24 of the upper beam joining (Fig. 4b). The front plate with a thickness of 14 mm on the right thrust was punched by the impact of the mass composed of the tube with water and the right cap with a sharp end of the pressure line tip (Fig. 4c).

This incident forced to conduct a detailed analysis of strength capacity of the existing protective structure and to develop a modernization plan to restrain all possible static loads and dynamic impacts for safe operation under conditions of higher pressures 69–125 MPa. Multi-disciplinary research is required (fluid dynamics, mechanical shock and vibration, materials penetration) for the diverse impacts estimation on the structure and the multi-variant calculations for a wide variety of testing tubes depending on their specifications.

The content of this research paper is organized into several parts. Section 2 gives information on the steel grades of tubes, testing pressures and tubes fracture conditions. Section 3 contains methods and procedure of impacts calculations on different elements of a structure. Section 4 represents the calculated impacts on the structure from the water jet and damaged tube fragments. Finally, Section 5 gives recommendations on press modernization to satisfy the strength capacity limitations of structure and testing performance for all types of tested tubes.