Improvement of a Subsea Hot Stab Operation

Abstract

Subsea pipelines that are used for the transportation of oil or gas are installed by pipe-laying vessels. When a pipeline is laid to the seabed, both ends of the pipeline are sealed with an end-piece. This end-piece fulfills an important role when the pipeline is being commissioned. In order to be accepted, the pipeline has to undergo Flooding, Gauging and Testing. For the hydro-test, water is pumped into the pipeline from the supply vessel through a flexible hose. The connection between the flexible hose and the end-piece is made with a so-called hot stab. To align and to connect male and female sections of the hot stab, a Remotely Operated Vehicle is used. This alignment, and stabbing process is proven to be challenging and sometimes resulting in damaged equipment and project delays. This thesis comprises a conceptual design and motion analysis for an improved concept of the subsea hot stab operation specifically developed for large diameter flexible hoses and heavy stabs.

The approach for this thesis starts with a research to find out limitations of the current equipment used. As a result, it has been concluded that a high stiffness of the flexible hose is the main cause of time consuming alignment and related problems, experienced by the Remotely Operated Vehicle operator. Various concepts have been generated, combined and rated using a Multi Criteria Analysis. The solution that scored the highest rank has been analyzed in order to predict the dynamic behavior of the whole system. System parameters have been optimized to reduce the undesired motions and to minimize forces that are required for alignment of a stab by the Remotely Operated Vehicle.

Combining the obtained knowledge from the research, the concept study and the analytical model, the following is concluded: it is beneficial to use a vertical orientation of the hot stab, instead of a commonly used horizontal orientation, in combination with buoyancy modules to support the flexible hose in an ‘S’ shape orientation as it reduces the loads during a subsea hot stab operation. This solution is adaptable for large range of water depths and does not exceed the maximum load capacity for the manipulators of the Remotely Operated Vehicle thereby preventing damaged equipment and project delays.