A plastic envelope element for an underwater pipeline, configured for, in fitted condition, at least partly enveloping the pipeline, wherein the envelope element is modular for forming a tube around the pipeline with similar such envelope elements linked up in the circumferential direction of the pipeline in their fitted conditions, the envelope element comprising an inner side which in the fitted condition faces the pipeline and an outer side which in the fitted condition faces away from the pipeline, and further comprising at least one local spring portion (5, 6) manufactured integrally with the envelope element, which with respect to remaining portions of the envelope element located next to the spring portion functions as a local spring and is deformable under its spring force from a relaxed condition of the spring portion to a tensioned condition of the spring portion, and vice versa, wherein: - the relaxed condition occurs at least in a not yet fitted condition of the envelope element, and in the relaxed condition the spring portion projects at least partly on the inner side of the envelope element; and - the tensioned condition occurs at least in the fitted condition of the envelope element, and in the tensioned condition the spring portion does not project or projects to a lesser extent than in the relaxed condition on the inner side of the envelope element such that the spring portion in the fitted condition presses under its said spring force against the pipeline; characterized in that the spring portion (5, 6) is at least formed by a wall portion of the envelope element projecting in the relaxed condition on the inner side of the envelope element, which projecting wall portion is resiliently hingeable with respect to said remaining portions of the envelope element located next to the spring portion, while said projecting wall portion extends bridgingly between two mutually parallel and mutually spaced hinge edges (10, 11) of said projecting wall portion, and wherein the projecting wall portion is resiliently hingeable about both said hinge edges simultaneously with respect to said remaining portions of the envelope element located next to the spring portion, wherein at least a portion of other edges (14, 15), not being the two said hinge edges (10, 11), of the projecting wall portion does not adjoin said remaining portions of the envelope element located next to the spring portion, in a manner such that between the non-adjoining portion of these other edges (14, 15), on one side, and said remaining portions of the envelope element located next to the spring portion, on the other, in the relaxed condition at least one passage is formed in the envelope element. - the relaxed condition occurs at least in a not yet fitted condition of the envelope element, and in the relaxed condition the spring portion projects at least partly on the inner side of the envelope element; and - the tensioned condition occurs at least in the fitted condition of the envelope element, and in the tensioned condition the spring portion does not project or projects to a lesser extent than in the relaxed condition on the inner side of the envelope element such that the spring portion in the fitted condition presses under its said spring force against the pipeline;