Synchronized Towing of Flexible Riser in Shallow Water and Varying Current

Abstract Towing of offshore pipelines can be heard during the mid-nineties, and it has been continuously improved with the advent of 2nd and third generation vessels. The operation poses a great challenge due to the dynamics and the instantaneous tension which the pipelines need to face. There is a high risk of flow line / pipeline getting damaged with potential increase in tension loads. It could also happen that the pipeline can get buckled with unexpected environmental loads. There can be a situation wherein the flow line gets damaged due to difference in tow speeds. In such situations the pipeline most likely touches the seabed for a shallow water depth. If a pipeline is in high tension in which greater than the allowable axial load, then there is likely pipeline can get damaged. If the pipeline is a flexible type, then there is a high chance for the outer sheath to get damaged. It is one of a most delicate operation which requires a focused interaction from different parties, viz. the Tow master, captain of the forward and trailing vessel and the monitoring vessel which have to move in a synchronized speed along with the Tow speed. There are in all three (3) major phases for a typical Towing Operation: a) Initiation and setup for Towing; b) start of Towing operation; and c) Tie-in or relocating the Pipeline / Riser to a new location. The seabed route selection and the bathymetry features are decided and firmed upfront. It should be as much as possible select the shortest route with minimum changes in the Vessel azimuth and bearing direction. The bathymetry should be free of any elevated obstruction, subsea anchor, mooring buoys and any mooring anchor chain from the nearby floating facility. This paper will be discussing flexible dynamic riser with near seabed tow at shallow water depth and high current. The flexible dynamic riser is an existing production line, which was required to be detached from a floating facility and then to be towed away to a safe distance. The facilities were temporarily shut down and during that time flexible riser was planned to be towed to a safe location and wet stored. The flexible riser was then again towed back to the same field after changes were made in the infrastructure facilities. The additional challenges faced were on manipulating the tensions around the bend stiffener and maintaining allowable Minimum Bend Radius (MBR) near the bend stiffener portion. It discusses about the sequence of operation, the initial planning preparation work, the simulated engineering activity, the offshore marine spread and its associated activities, and the monitoring systems in place.