聚乙烯管材专用料(PE-XRT70)应力松弛和蠕变行为

摘要

海洋油田的注水管道一般采用多层复合的结构设计,用于密封和防护的管道层多为高分子材料.注水海管设计寿命在25 a以上,且内压密封层直接承受管道的内压和温度工况,经长时间的压缩应力作用会产生蠕变,同时,其特定部位受到恒定的压缩应变经历较长时间也会发生应力松弛.因此,研究这类高分子材料在长期应力应变下的蠕变和应力松弛行为规律对于评估注水软管内压密封层材料长期服役行为具有重要意义.针对某海上油田开发项目聚乙烯(PE-XRT70)材料被用于油田海管工程注水复合软管道内压密封层的特定工况(75℃、16.5 MPa),通过对聚乙烯试样进行75、85、95℃、恒定应变10％和20％的应力松弛以及75℃、16.5 MPa恒定应力的蠕变实验,利用聚合物材料的时温等效性以及松弛时间分布计算出该材料在特殊条件下的平移因子αT.建立了Maxwell多元件并联模型,对应力松弛及蠕变实验数据通过1 stopt进行5参数拟合,得到预测材料实际工况温度下的应力松弛及蠕变行为的数学公式.根据聚合物材料的时温等效性以及松弛时间分布计算出PE-XRT70管材压缩试样在特殊条件下的平移因子αT.结果表明,在应变量为10％时,85℃平移到75℃的平移因子是10.97,95℃平移到75℃的平移因子是11.20.在应变量为20％时,85℃平移到75℃的平移因子是14.76,95℃平移到75℃的平移因子是15.77.广义Maxwell模型及模拟曲线能很好地预测该材料的应力松弛和蠕变行为,相关系数R2的数值在0.975以上,通过该数学模型可获得该材料在75℃下两种恒定应变中的平衡应力σ∞及75℃、16.5 MPa恒定应力下的平衡应变.应力松弛实验和数学模拟表明,75℃温度下,10％和20％的恒定应变的应力松弛过程的平衡应力σ∞分别为3.983和7.505 MPa,且两种条件下应力松弛在一周后基本达到平衡应力.蠕变试验数据和数学模拟表明,75℃、16.5 MPa压力下,PE-XRT70压缩试样的蠕变过程的平衡应变为31.479％,且在24h之后的蠕变基本达到平衡.%Polymer materials are used to seal and protect the pipeline layer in the multi layer c6mposite st ructure 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 factoroTwas calculated by the time-temperature equivalence and relaxation time distribution of polymer materials.Firstly,the shift factor αT of 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 mathenatical 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 coefficient R2 was 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 stress σ∞ 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.