Post welding residual stress in P91 alloy steel

    Machines. Technologies. Materials., Vol. 15 (2021), Issue 2, pg(s) 73-78

    The chromium-molybdenum steel pipe P91 is widely used in the power plants construction, because they can withstand higher temperatures and operating pressures, in order to increase operating efficiency and output. This material has a complex microstructure which is extremely vulnerable to convert into Austenite form near the temperature for heat treatment. During the welding procedure should be very carefully, because otherwise the residual stress installed post welding will reach high values. A series of experimental studies was carried out, which has shown the development in the magnitude of transverse and longitudinal residual stresses and the values in both directions were determined. For but welded joints the residual stresses in the three direction were measured and the results are reported here. For measuring was used the hole drilling method through “measuring balls” or electrical strain gauge. Except the experiments the finite element method was used to determinate the value and the distribution of the residual stresses. The experimental and FEM results were compared between them and no discrepancy was found.


    Analysis of sinking of GRP pipe string DN 1600

    Machines. Technologies. Materials., Vol. 15 (2021), Issue 1, pg(s) 15-21

    The aim of this research it was the installation of a discharge sea water pipeline DN 1600 of GRP (glass-reinforced plastic) material in Vlore Bay. The analysis of the sinking of the pipe string has been carried out by using CAESAR II software and considering different load cases. The calculation was carried out for static condition (sustained load) and transient condition (occasional load). In consideration were taken the allowable stress due to bending for pipe and allowable stress for joint subjected to freezing. From the calculation it was evident that the most critical effect on the GRP pipe has done by the position of Concentrated Force due to the presence of sledge (approx.. 2 tons), not disassembled from the string after moving in the position from the shore. In these cases, where the central crane has been maintained fixed or moved downward, the maximum bending stress on the pipe was 12.03 MPa. According to the verification carried out for pipe wall in the transient condition (occasional load) the stability was good and for the joint, was shown, in the worst scenario, a bending stress 158% of the allowable one. In this case it is possible to have the crack of the resin used for the freezing of the joint but not the failure of the system joint + pipe that can withstand without, failure, a stress 3 times greater than the allowable stress for pipe according to EN1796.


    Mechanical and hydraulical stability of the offshore composit structure

    Mathematical Modeling, Vol. 4 (2020), Issue 1, pg(s) 22-25

    The aim of this research it was the installation of an intake sea water GRP (glass-reinforced plastic) structure in Vlore Bay. A detailed study about the extreme wave condition offshore was carried out by the SWAN (Simulating Waves Nearshore) model. Then the mechanical structure stability calculation was performed with AxisVM8, a finite element modeler & solver. Conservative hypothesis were taken, such as to use the max static pressure value as per energy balance calculation on the sea waves. The program Fluid Flow was used to model the pipe line end with pipe chimney connections. The scope of the calculation was primarily to balance fluid flows in each chimney in order to match the range of 0, 2 – 0, 25 m/s velocity at each chimney entrance. The result of the mechanical and hydraulically stability verification for the sea water intake structure take in consideration it was positive and this structure was installed successfully.