The Analysis of HVDC Submarine Power Cable Bundle Installation

Abstract

The transition to renewable energy has positioned offshore wind power as a cornerstone of sustainable electricity generation. As offshore wind farms expand further from shore, High-Voltage Direct Current (HVDC) transmission systems have become essential for efficient power delivery. These systems consist of multiple cables, preferably installed as bundled units for various reasons, such as cost savings and minimising environmental impact. This thesis investigates the mechanical properties and the static and dynamic behaviour of HVDC submarine power cable bundles during the normal lay operation, a critical installation phase.

The study focuses on a 2 GW 525 kV HVDC transmission system comprising two single-core HVDC cables, a metallic return cable, and a fibre optic cable. Bundled installation is examined due to its potential to reduce installation costs, enhance spatial efficiency, and minimise environmental impact. However, the mechanical complexities of installing bundles with varying cable properties necessitate an in-depth exploration of their behaviour under operational conditions.

A multi-faceted approach was adopted, beginning with a comprehensive characterisation of the mechanical properties of an HVDC submarine power cable bundle. This characterisation included an axial, bending, and torsional stiffness analysis, evaluated under full-stick and full-slip conditions. Analytical and computational models were developed using diverse theories and advanced modelling techniques in OrcaFlex. Particular attention was given to the role of bundling straps in influencing intercable interactions and ensuring overall bundle stability during installation. The analysis revealed that cable bundles predominantly operate in the full-slip regime, where the aggregate stiffness of individual cables primarily defines bundle stiffness.

The installation process was studied from both static and dynamic perspectives, focusing on the behaviour of the bundle during normal lay operations. Three distinct cable (bundle) tensioning strategies
were evaluated, each offering unique insights into the resulting mechanical properties, responses to environmental loads, workabilities, and required strap tensions. The dynamic simulations incorporated
realistic environmental conditions, such as wave-induced motions and seabed interactions, to assess the robustness and efficiency of each strategy. The analysis identified the individual tension control
strategy as the most effective approach. By independently managing the tension in each cable within the bundle, this method offers superior control over compression loads, bending radii, and twist, ensuring the bundle’s structural integrity during installation.

Future research should focus on developing theoretical and computational models to improve the accuracy of predictions regarding the twisting behaviour of cable bundles. These models should comprehensively account for environmental conditions, internal loads, and stick-slip interactions to provide precise estimations and must also be validated and verified. Furthermore, the integration method for the physically sensitive fibre optic cable requires refinement to ensure its structural integrity during installation.