An improved design method for CRLP maximum allowable operating pressure

Compared to the traditional line pipe, the composite reinforced line pipes (CRLP) have the advantages of high pressure bearing capacity and light weight. However, researches on pressure design of CRLP are still insufficient. The design ideas of maximum allowable operating pressure in CSA Z662-2015 Oil and gas pipelines systems and GB/T 35185-2017 Composite reinforced line pipe for the petroleum and natural gas industry are different and the design results vary greatly. For this problem, analysis of CRLP under pressure was carried out based on the combined thin-wall cylinder model, the nominal and the true allowable stress of the material under the maximum allowable operating pressure was distinguished, and the explicit expression of the true allowable stress was derived and taken as the evaluation indicator of the design safety margin. The analysis results show that design formulas in the existing code and standard cannot ensure the true allowable stress of the line pipe and composite material is a constant. In both of the two design methods, the true allowable stress of various constituent materials varies with CRLP of different specification in the process of pressure test and pre-stressing. Based on the above analysis and the bearing capacity of CRLP under the ultimate state, an improved pressure design method was put forward, which preferably guarantees the true allowable stress of the composite material as a constant and it can be determined directly by the design factor. In the calculation of the maximum allowable operating pressure, the effect of the loading process was considered. This new method has a clear physical meaning, and the design examples show that the new method has a more reasonable safety margin compared to existed standard methods, providing useful references for CRLP design.