El-Chayeb, Abdul Rahman; (1999) On the Mechanics of Electrical Cables in Subsea Control Umbilicals. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis is concerned with the mechanics of electrical cables in subsea umbilicals. Subsea umbilicals are used in the offshore hydrocarbon industry to control and operate remote seabed equipment. They are characterised by an armoured and sheathed bundle of hydraulic hoses and electrical cables. Experience to date has shown that the electrical cables in subsea umbilicals are prone to mechanical failure. One of their most common modes of failure is the formation of kinks in the insulated copper conductors leading to rapid fatigue damage. The key physical phenomenon involved in this buckling type of failure is the non-linear material properties of the copper conductors which results in the build up of compressive forces under strain controlled cyclic loading. Two main analytical models are developed in this thesis to investigate the mechanical loads in the electrical cables in subsea umbilicals. One model is concerned with the structural response of the umbilical for axi-symmetric loads and the other is concerned with the structural response for flexural loads. These models use an energy formulation and are based on the differential geometry of deformed helices. The models incorporate an original sub-structuring approach to take into account the compliance of the core. The models also include new features to take into account the non-linear material properties of the conductors, finite friction coefficients and a methodology to predict the equivalent material properties of the composite core. The above analytical models are verified with full-scale tests and with other experimental data published in the public domain. A good level of agreement is achieved giving confidence in the application of this work to the kinking and fatigue analysis of the electrical cables in subsea umbilicals. The kinking analysis uses the analogy of the insulated copper conductors to beams on elastic foundations to predict the critical buckling stresses. The critical stresses calculated from this model are expressed as a function of the global load limits. The fatigue analysis is carried out using the as-built material properties of the conductors which are determined experimentally. The thesis concludes by proposing a design methodology that can be used to optimise the mechanical performance of electrical cables in subsea umbilicals.

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