An Accurate and Efficient Fitness-For-Service Assessment Method of Pipes with Defects under Surface Load

With continued urbanization in China, the construction of urban gas pipelines is increasing, and the safety of gas pipelines are also increasingly affected by urban development and the increased scope of buildings and roads. Pipes with defects are more likely to fail under the surface loads. In this study, uniaxial tensile tests of high-density polyethylene (HDPE) pipes were carried out to obtain the real material parameters of pipe. A pipeline-soil interaction finite element model of HDPE pipeline with defects under surface load was established. The failure mechanism of the urban gas pipeline was studied and the influence of parameters such as internal pressure, defect position, defect depth on the mechanical behavior, and failure of pipelines were analyzed. A failure criterion for HDPE pipes with defects under surface load was proposed based on the limit-state curves obtained under different working conditions. Furthermore, an accurate and efficient fitness-for-service assessment procedure of pipes with defects under surface load was proposed. The results showed that maximum Mises stress of the pipeline gradually increased with increasing surface load and the position of maximum stress changed from the top and bottom of the pipe to the defect position and both sides of the pipe. Finally, when Mises stress of the HDPE pipe exceeds the yield limit, failure will occur. Internal pressure, defect location, and defect depth were found to influence the failure process and critical surface load of the pipeline. Safety evaluation curves of the gas pipeline with defects under surface load were obtained by calculating the critical failure load of the pipeline under various working conditions. Finally, a nonlinear fitting method was used to derive a formula for calculating the critical surface load under different defect parameters. The proposed method provides a useful reference for urban gas pipeline safety management.

Pressure Effects on the Lifetime of Gas High Density Polyethylene Pipes

The purpose of this study was to propose low gas pressure effects on lifetime of natural gas high density polyethylene (HDPE) pipes by thermal-oxidative aging (TOA). The new method to assess the lifetime of HDPE natural gas pipes is based on gas pressure testing. An approach to monitor oxidative induction time (OIT) has been used to predict lifetime. Natural gas HDPE pipes were used to evaluate the effects of low gas pres-sures on oxidative induction time. In order to emphasize the pressure effects, relatively low temperatures at 45, 55, 65 and 75 °C were utilized for the exposure. The low-pressure conditions were created using air at levels of 0, 0.1, 0.2, 0.3 and 0.4 MPa. The property of high density polyethylene pipes was effectively moni-tored using the low pressure oxidative induction time (OIT) test. The results show that the aging reaction rate of high density polyethylene pipes increased exponentially with temperature and gas pressure according to the Arrhenius equation. Analytical models were developed to predict the aging reaction rate and lifetime of natural gas HDPE pipes.

Application of Offshore HDPE Pipes Route Design in North Maluku Indonesia

The lack of fresh water for the inhabitants of Maitara island is a very urgent problem to be solved. Two main factors at least must be taken into account to deliberate the right of way of subsea High-density polyethylene (HDPE) pipes, namely the hydrodynamic conditions and of a block analysis. This paper presents the study to justify the best route of subsea HDPE pipes based on hydrodynamic model analysis and concrete block strategy. The method used to analyze the best route includes 2 aspects. Firstly, the investigation method consisting of a bathymetric survey conducted by a single beam echosounder, 15 days tidal observations and seabed sediment sampling. Secondly, the hydrodynamic modelling analysis using Mike 21 FMHD and concrete block analysis, all these studies have been completed in August 2018. In the morphological behaviour analysis, three alternative routes are considered for the subsea HDPE pipes from Tidore Island to Maitara Island. The outcome of the analysis shows that the second track line option has the smallest impact by the hydrodynamic conditions, with a current speed of less than 0,5m/sec and a significant wave height of fewer than 1.2 meters. Furthermore, the uniformity of the lithology along the route is the other reason to select the second route. Finally, the concrete block analysis generated a minimum dimension of 75cm x 60cm x 30cm, and a free span of 3 meters is safe to absorb the uplift and drag forces acting on the pipe.