Design and installation of high voltage cables at sea

Abstract

Underwater telecommunication cables have been connecting continents since the 1860s. These days large electric power cables are connecting countries to ensure optimization of energy use. When the production of energy from renewable resources, like wind and solar, are low, energy from other sources may be imported and vice versa. At the moment a cable is planned between Norway (for hydropower) and Scotland (for wind power). Furthermore, large electric power cables are needed offshore to bring electricity from offshore wind turbines to shore and cables are required to bring onshore generated electricity to offshore oil and gas platforms to reduce pollution. Over time, the process of laying cables at sea has developed into a state-of-the-art operation. Now these operations are becoming more technologically advanced and it is possible to lay large diameter electric cables over large distances. A particular challenge occurs in case an unplanned splicing/ jointing will be necessary. In this paper, we explore the design criteria for such cables and the procedures and challenges of installation and splicing. Furthermore, the effects of how dynamic motions of the vessel and sea influence the situation in deep water are explored. We have analysed the effects of waves on vessel motion, and how this may affect the cable during a jointing operation of two cable ends at different water depths. The effects of current forces on the cable are also analysed and how the cable reacts to both current and wave forces. This analysis method can assist in determining the weather criteria for a jointing operation to prevent excessive bending, compression or fatigue damage in the cable. One finding in the analysis is that there are different requirements for laying the cable and the jointing operations. The suitable sea states for jointing are more limited than for laying. When the vessel and the cable are standing still, all bending occurs at the same place in the cable, resulting in increased risk of fatigue damage, hence it is necessary with a calmer sea state for this kind of operation. The examples referred to in the paper are based on realistic assumptions; a summary of these assumptions is included. Furthermore, a HAZID, carried out for cable installation, shows that there are several risks and hazardous events that may occur during the installation operation in connection to the cables integrity. Identifying and handling these risks early may reduce both their probability of occurring and the related consequences.