Einar Magnus Bjelland and Jon-Steinar Andreassen look at the scope of work for the 768 fibre riser cable, including the arrangements for the pull-in operation
A riser cable containing 768 fibres, developed by Nexans Norway, is claimed to be a world record with regards to number of fibres. Because of the large number of fibres required it was necessary to develop a system optimised for installation efficiency and which minimised the time needed for optical measurements and tests during and after installation.
The total scope of work for the 768 fibre riser cable included arrangements for the pull-in operation, the topside cable hang off arrangement, the assembled riser cable, and an arrangement for distribution of fibres into four 'dead end' cables for future connection.
To meet these requirements the design incorporates four individual steel armoured cable cores helically stranded together. Each of the four cable cores contains 192 fibres making a total of 768 fibres (Fig. 1). The armoured cable cores will be connected to the various subsea cable segments in the future. The cable cores are robust and act as individual cables on the sea bed. Nexans fibre in a laser welded steel tube filled with a water impregnation preventing compound is a Nexans proven technology and a basic building block in all our sub sea cables.
The transition between the complete cable pulled in to the J-tube and the individual cables supporting sub-sea cable segments was made without any fibre splices. A special mechanical component was developed to protect and secure the transition, where the assembled riser cable is split into four. The cable ends lying on the sea bed are prepared with half-joints ready to be connected to the seabed segments.
A pre-mounted cable hang-off was used for the pull-in operation. The hang-off including the mechanical termination of the stranded steel wire armoured cables could then be prepared in the Nexans facility. Critical cable handling and installation time during the offshore pull-in sequence was avoided. In addition, to the pre-mounted hang-off, eight meters of flexible steel segments was put together over the cable cores to protect them during the pull-in sequence. These segments are flexible to manage the J-tube bend and in the same time transfer the load down to the hang-off terminated to the riser cable during the pull-in sequence. Having eight metres of prepared cable cores inside the pull-in arrangement makes the installation more efficient and safe. The cables can be routed directly to their junction boxes with minimum cable handling.
The riser cable system, including pull-in and hang-off arrangements as well as the transition splitter arrangement was supplied on one drum.
The above describes a static cable solution for J-tube pull-in. For dynamic fiber optic riser cables, the basic cable technology is still applicable however the scenario in terms of cable loads and exposures for cable service period requires different and more complicated design criteria. For dynamic risers, comprehensive site specific engineering and analysis is necessary. Such analysis involves meteorological data and data on platform movements (RAO) for verifying the compatibility between cable structural properties, cable configuration, subsea components and topside interface. Fatigue is a key issue for a dynamic cable, and results from dynamic analysis are used for establishing fatigue criteria end determination of parameters for equivalent fatigue life test, a test which is necessary to include in the qualification test program.
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Einar Magnus Bjelland and Jon-Steinar Andreassen are with Nexans Norway AS, Etterstad, Oslo, Norway. www.nexans.no
