A pipeline element assembly, comprising at least two pipeline elements and a pipeline element coupling piece for coupling the at least two pipeline elements each of the at least two pipeline elements having a groove and a groove circumference at a groove bottom of the groove, wherein the pipeline element coupling piece comprises: two or more coupling units, wherein each of the two or more coupling units includes two engaging keys respectively, wherein each of the engaging keys has an engaging surface having a variable radius of curvature; at least one gasket disposed in a cavity defined between the two or more coupling units; and at least one fastening element configured for fastening the two or more coupling units together for pre-sleeving at the at least two pipeline elements while aligning the engaging keys with the grooves of the at least two pipeline elements, such that when the at least one fastening element is operated to fasten the two or more coupling units together, the engaging keys are engageable in the grooves of the at least two pipeline elements and each of the engaging surfaces is deformable to vary the variable radius of curvature so as to substantially match with a radius of curvature of each of the groove circumference of the groove bottom of each of the at least two pipeline elements; and at least one pivot connecting element, wherein the two or more coupling units include a first coupling unit and a second coupling unit and the at least one pivot connecting element is configured to pivotally connect the first and second coupling units (10A; 10B); wherein, each of the two coupling units comprises a housing, a pivot connecting end portion integrally extended from the housing and a fastening end portion integrally extended from the housing, wherein the pivot connecting end portions of the first and second coupling units (10A; 10B) are pivotally connected with each other by the at least one pivot connecting element, and the fastening end portions of the first and second coupling units (10A; 10B) are configured to be fastened by the at least one fastening element; and wherein, each of the engaging surfaces of the two coupling units has a proximate end point adjacent to the pivot connecting end portion thereof and a distal end point adjacent to the fastening end portion thereof, characterized in that the radius of curvature of each of the engaging surfaces gradually increases from the proximate end point to the distal end point. two or more coupling units, wherein each of the two or more coupling units includes two engaging keys respectively, wherein each of the engaging keys has an engaging surface having a variable radius of curvature; at least one gasket disposed in a cavity defined between the two or more coupling units; and at least one fastening element configured for fastening the two or more coupling units together for pre-sleeving at the at least two pipeline elements while aligning the engaging keys with the grooves of the at least two pipeline elements, such that when the at least one fastening element is operated to fasten the two or more coupling units together, the engaging keys are engageable in the grooves of the at least two pipeline elements and each of the engaging surfaces is deformable to vary the variable radius of curvature so as to substantially match with a radius of curvature of each of the groove circumference of the groove bottom of each of the at least two pipeline elements; and at least one pivot connecting element, wherein the two or more coupling units include a first coupling unit and a second coupling unit and the at least one pivot connecting element is configured to pivotally connect the first and second coupling units (10A; 10B); wherein, each of the two coupling units comprises a housing, a pivot connecting end portion integrally extended from the housing and a fastening end portion integrally extended from the housing, wherein the pivot connecting end portions of the first and second coupling units (10A; 10B) are pivotally connected with each other by the at least one pivot connecting element, and the fastening end portions of the first and second coupling units (10A; 10B) are configured to be fastened by the at least one fastening element; and wherein, each of the engaging surfaces of the two coupling units has a proximate end point adjacent to the pivot connecting end portion thereof and a distal end point adjacent to the fastening end portion thereof, characterized in that the radius of curvature of each of the engaging surfaces gradually increases from the proximate end point to the distal end point.