Polymer materials are used to seal and protect the pipeline layer in the multi layer composite structure flexible pipe in the offshore oil field.The design life of those pipes are more than 25 years,and the long-term working temperature is high (75 ℃),the pressure is 16.5 MPa.After a long period of the compressive stress,the creep of materials can be produced,and at the same time,the specific parts of the compressive strain will be subjected to a constant compressive strain.Therefore,it is very important to study the creep and stress relaxation behavior of these polymers under long-term stress and strain to evaluate the long-term service behavior of the inner pressure sealing layer.The stress relaxation experiments of polyethylene(PE-XRT70),which was used as the specific operating pressure sealing layer of an oifield composite flexible pipe under sea water were detected carried out at the temperatures of 75 ℃,85 ℃ and 95 ℃ by 10% or 20% constant strain and creep experiments which were determined at 75 ℃ and 16.5 MPa constant stress.The shift factor αTwas calculated by the time-temperature equivalence and relaxation time distribution of polymer materials.Firstly,the shift factor αTof PE-XRT70 pipe was calculated by the time temperature equivalence and the relaxation time distribution of the polymer materials.Then,by multielement parallel Maxwell model,1 stopt used five parameters was applied to fit the stress relaxation and creep experimental data and forecast mathematical model of the material stress relaxation and creep behavior.The results showed that the Maxwell model and simulation curve fitted well,the balance stress model closed to the experimental values and the value of coefficient of correlation coefficientR2was above 0.975.The equilibrium strain of the two constant strain at 75 ℃ and the equilibrium strain at constant stress of 16.5 MPa at 75 ℃ were obtained.The simulation and experiment of stress relaxation experiments showed that at 75 ℃,the stress relaxation process of balance stressσ∞were 3.983 and 7.505 MPa under constant strain 10% and 20%,respectively.The creep test data and mathematical simulation showed that the equilibrium strain of the creep process of the PE-XRT70 was 31.479%,and the creep basically reached the equilibrium in 24 h under the pressure of 16.5 MPa.
