Connection assembly (1; 200) for connecting a branch pipe to a cylindrical structure (2; 102; 202), said cylindrical structure (2; 102; 202) comprising a circumferential wall (3; 103; 203) having an inner surface (4; 104; 204) and an outer surface (5; 105; 205), said wall provided with an opening (10; 110; 210) for connecting the branch pipe to the cylindrical structure, said connection assembly (1; 100; 200) comprising: a first part (20; 220), comprising a first tubular section (21; 221), and a first flange (22; 222) having an outer diameter larger than a diameter of said opening (10; 110; 210), and a second part (40; 240), comprising a second tubular section (41; 241) having a diameter smaller than the diameter of said opening (10; 110; 210), and a second flange (42; 242) having a diameter larger than said diameter of said opening (10; 110; 210), wherein said first and second parts are adapted for clamping a portion of said circumferential wall of said cylindrical structure between said first flange and said second flange, wherein said first and second tubular sections (21,41; 221,241) are provided with first locking means (30; 230) and second locking means (50; 250) on said facing surfaces (21a,41b; 221a,241b) of said first and second tubular sections (21,41; 221,241) respectively, characterized in that said first tubular section (21; 221) is adapted for fitting inside said second tubular section (41; 241) or vice versa, with circumferential surfaces (21a, 41b; 221a, 241b) of said first and second tubular section (21, 41; 221, 241) facing each other, said first and second locking means (30,50; 230,250) are adapted for engaging each other for allowing linear movement of said first flange (22; 222) toward said second flange (42; 242) while said flanges remain substantially rotationally fixed to each other, and for blocking movement of said first flange (22; 222) away from said second flange (42; 242). a first part (20; 220), comprising a first tubular section (21; 221), and a first flange (22; 222) having an outer diameter larger than a diameter of said opening (10; 110; 210), and a second part (40; 240), comprising a second tubular section (41; 241) having a diameter smaller than the diameter of said opening (10; 110; 210), and a second flange (42; 242) having a diameter larger than said diameter of said opening (10; 110; 210), wherein said first and second parts are adapted for clamping a portion of said circumferential wall of said cylindrical structure between said first flange and said second flange, wherein said first and second tubular sections (21,41; 221,241) are provided with first locking means (30; 230) and second locking means (50; 250) on said facing surfaces (21a,41b; 221a,241b) of said first and second tubular sections (21,41; 221,241) respectively, characterized in that said first tubular section (21; 221) is adapted for fitting inside said second tubular section (41; 241) or vice versa, with circumferential surfaces (21a, 41b; 221a, 241b) of said first and second tubular section (21, 41; 221, 241) facing each other, said first and second locking means (30,50; 230,250) are adapted for engaging each other for allowing linear movement of said first flange (22; 222) toward said second flange (42; 242) while said flanges remain substantially rotationally fixed to each other, and for blocking movement of said first flange (22; 222) away from said second flange (42; 242).