A method for manufacturing a reinforced structure of composite material comprising at least one stiffening element attached to a concave surface of a shell element, the stiffening element comprising a web, a first fixation flange and a second fixation flange, wherein the method comprises the following steps: - forming a concave semi cured shell element of composite material, - forming at least one semi cured stiffening element by: - forming a first semi cured profile and a second semi cured profile of composite material, the first semi cured profile comprising a first profile web and a first profile flange and the second semi cured profile comprising a second profile web and a second profile flange, - forming a semi cured reinforcing element of composite material, which semi cured reinforcing element comprises a first side, a second side, each side having an outer edge and an inner edge, and at least one forming bulge on at least one of the respective sides (31; 33), which at least one forming bulge extends from the outer edge to the inner edge, wherein a thickness of the at least one forming bulge is largest at the outer edge, tapers towards the inner edge and is levelled out at the inner edge, - positioning the semi cured reinforcing element between the first profile web and the second profile web, - positioning the at least one semi cured stiffening element on the semi cured shell element - co-curing the semi cured shell element and the at least one semi cured stiffening element in a curing tool, wherein the first semi cured profile and the second semi cured profile, adheres to the semi cured reinforcing element, whereby the web of the semi cured stiffening element adopts the shape of the semi cured reinforcing element, and wherein the semi cured stiffening element adheres to the semi cured shell element, - removing the reinforced structure from the curing tool. - forming a concave semi cured shell element of composite material, - forming at least one semi cured stiffening element by: - forming a first semi cured profile and a second semi cured profile of composite material, the first semi cured profile comprising a first profile web and a first profile flange and the second semi cured profile comprising a second profile web and a second profile flange, - forming a semi cured reinforcing element of composite material, which semi cured reinforcing element comprises a first side, a second side, each side having an outer edge and an inner edge, and at least one forming bulge on at least one of the respective sides (31; 33), which at least one forming bulge extends from the outer edge to the inner edge, wherein a thickness of the at least one forming bulge is largest at the outer edge, tapers towards the inner edge and is levelled out at the inner edge, - positioning the semi cured reinforcing element between the first profile web and the second profile web, - forming a first semi cured profile and a second semi cured profile of composite material, the first semi cured profile comprising a first profile web and a first profile flange and the second semi cured profile comprising a second profile web and a second profile flange, - forming a semi cured reinforcing element of composite material, which semi cured reinforcing element comprises a first side, a second side, each side having an outer edge and an inner edge, and at least one forming bulge on at least one of the respective sides (31; 33), which at least one forming bulge extends from the outer edge to the inner edge, wherein a thickness of the at least one forming bulge is largest at the outer edge, tapers towards the inner edge and is levelled out at the inner edge, - positioning the semi cured reinforcing element between the first profile web and the second profile web, - positioning the at least one semi cured stiffening element on the semi cured shell element - co-curing the semi cured shell element and the at least one semi cured stiffening element in a curing tool, wherein the first semi cured profile and the second semi cured profile, adheres to the semi cured reinforcing element, whereby the web of the semi cured stiffening element adopts the shape of the semi cured reinforcing element, and wherein the semi cured stiffening element adheres to the semi cured shell element, - removing the reinforced structure from the curing tool.