Submarine power cables are commonly buried below the seabed to protect them from external hazards and to ensure long-term operational reliability. In morphologically active areas, however, trenching is complicated by sand waves and megaripples. These bedforms introduce local slopes and crest regions where the cable must adapt to rapidly changing seabed geometry. In practice, this can lead to insufficient burial and the formation of standing bights, where a section of cable remains locally elevated instead of settling into the trench.

This thesis investigates the mechanical response of submarine power cables during trenching over sand-wave-dominated seabeds, with particular focus on the conditions that may lead to standing-bight formation. A back-analysis of DEME project data is first used to identify practical trends in burial performance. The field data show that rougher seabeds, steeper local gradients, and cable properties such as submerged weight and bending stiffness influence the achieved depth of lowering. However, the available operational data do not allow the individual mechanisms to be isolated with certainty.

To study the cable mechanics separately, a simplified static model is developed. The cable is represented as a free-hanging, inextensible slender rod subjected to submerged self-weight and internal force components. The governing non-linear elastica equation is solved numerically using a shooting-based procedure. For each cable type and prescribed uphill geometry, admissible equilibrium configurations are identified and evaluated using end tension, horizontal distance, exit angle, and overall cable shape.

The results show that standing-bight susceptibility cannot be assessed from residual tension alone. The same end tension can correspond to different cable geometries, depending on the horizontal and vertical internal force components. For a given uphill geometry, a practical tension window exists. If the tension is too low, the cable sags excessively and reaches the prescribed height too early, creating a standing-bight-prone geometry. If the tension is too high, the cable remains too straight and may require too much horizontal distance to reach the target depth, leading to loss of depth of lowering. The favourable window depends strongly on cable properties, prescribed height, and available trench length.

Within the scope of this thesis, standing-bight formation is therefore interpreted as a loss of geometric mechanical compatibility between cable properties, residual tension, and seabed geometry, rather than as the result of a single dominant parameter. The results provided indicate mechanical tension windows that can support trenching assessment and motivate further dynamic and soil-interaction modelling. ... Read more