A method of installing an In-Line Structure (ILS) to a fluid-filled pipeline extending from a reel, over an aligner, and through a lay-tower during offshore reeling, the pipeline having an oblique part from the reel to the aligner, and a vertical part from the aligner through the lay-tower, including at least the steps of: (a) draining fluid in the fluid-filled pipeline from the vertical part of the pipeline to create a drained portion of the vertical part of the pipeline up to and around the aligner; (b) cutting the pipeline at or near the draining of step (a) to create upper and lower open ends of the pipeline; (c) installing the In-Line Structure to at least the upper open end of the pipeline; (d) locating a vent hose through a vent port in the In-Line Structure; (e) adding fluid into the drained portion of the pipeline and venting air from the drained portion of the pipeline through the vent hose to wholly or substantially fill the drained portion of the pipeline with the fluid.