Developing new methodology for subsea cable analysis

AgileTek Engineering

AgileTek has developed a methodology that takes into account the local stiffness of all cable components and includes a new approach to their non-linear slip/stick behaviour.

The Problem

Power cable projects require a significant number of analysis studies at various phases of development.

These typically include:

• Installation analysis
• Cable fatigue analysis
• Storm response and clashing analysis
• Protection system design and verification
• VIV screening

To obtain solid results, these studies all rely on accurate cable data, specifically bending, axial and torsional stiffness. The most effective way of obtaining these values is to perform testing on the power cable. This testing can be complex and costly to perform and worse, such analysis is often required before a cable sample can even be produced, leading to ever more over-conservative assumptions. Due to cost and time constraints, this has resulted in manufacturers providing values which are merely based on simple analytical formulations and whose basis is not transparent.

These formulations grossly over-simplify the mechanical behaviour of power cable, introducing technical risk and leading to unnecessary project expenditure and delays.

The Solution

AgileTek has developed a methodology which takes into account the local stiffness of all cable components and includes a new approach to their non-linear slip/stick behaviour. In this approach slip regions develop which, after a critical curvature is reached, grow from the cable neutral axis, hence some wires on a cross-section are in the post-critical slip condition while others are not, dependent on the cable curvature. This leads to a transitional region in the moment-curvature relation between the low slip and full slip, residual stiffness regions.

Interaction between armour wires and the surrounding material is based on test data with zero applied tension. The non-linear moment-curvature response for two example cables is presented below along with the linear bend stiffness values from the supplied cable datasheets.

This provides the user with a realistic non-linear relationship between the bending moment and the curvature rather than simply a linear estimation.

The Results 

By using the non-linear curve rather than the linear curve, AgileTek have been able to calculate power cable fatigue lives of at least an order of magnitude larger than the conventional method. This methodology has been reviewed by DNV and validated through physical bend testing of a cable to an industry-recognised standard at an industry-leading cable manufacturer facility.

In addition, the use of non-linear stiffness curves in analysis models allows the hysteresis exhibited by cables in bending to be properly implemented. This can reduce unrealistic results in power cable simulations under the vessel and environmental loading.