Subsea cables - a concept for long runs

PUBLISHED: 15 Jun 12

The need for renewable power generation has increased the demand for submarine power cables. And the growing demand seems to continue in the foreseeable future due to several new offshore wind projects and additional grid interconnections.

Interconnections from offshore wind farms to the mainland grid often require long cable lengths. Joining shorter lengths is expensive, time consuming and represents the most critical element in the cable installation. Ideally, utilities seek to employ a continuous uninterrupted section of cable which covers the full distance. However, production or deployment constraints make this requirement difficult if not impossible to realize.

The cable manufacturer's goal is to produce the longest quality cable length without interruption, and as a result, limit the number of cable joints required over a given cable length. Specific techniques are required to ensure the highest insulation quality and cleanliness in the XLPE insulation process. Maillefer has placed much effort in this area, as highlighted in the following list.

  • Insulation cleanliness has to be guaranteed when conveying raw material from the material box into the extruder. For that purpose, Maillefer has developed the XLPE insulation material handling concept. It includes the Polymer Discharge Isolator PDI 50, which easily helps maintain a clean room atmosphere for the most critical connection between the material box and the material conveying line.
  • With the adaption of the pellet scanning system the use of screens can be avoided and thus increase a continuous insulation run remarkably without stops. The system can detect and sort out contaminants down to 50 µm before reaching the extruder.
  • After the material is conveyed safely into the extruder, the melt quality in the extrusion process must be secured. Maillefer has developed a new screw design, which enables runs for at least two weeks without pre-scorch. The new screw design is available for 150 to 200 mm XLPE insulation extruders.
  • With an extrusion process modeling software, the extruder process parameters can be optimized for long runs. Melt temperature, which is the most critical parameter in XLPE insulation, is controlled online with an ultrasonic non-contact measurement device. Measurements are continuously taken from within the extruder’s crosshead adaptor. Melt temperature is adjusted according to the required production length.
  • Extrusion melt cleanliness can be controlled on-line with the Cleanliness Scanning System CSS2. Melt is scanned after extrusion just before the crosshead. Cleanliness is controllable by bleeding the melt at production startup as well as continuously during the production run.
  • For controlling the melt temperature in the crosshead Maillefer has developed its own reliable and accurate Integrated Water Tempering. IWT is a fully integrated part of the line control system. Water cooling of the extrusion screws and the crosshead flow channels are controlled as accurately as the independent zones of the insulation extruder.

With such measures taken on an optimized Maillefer line, one can expect runs reaching up to two weeks or more, depending on the specific manufacturing conditions.

Maillefer Extrusion Oy, Finland
Tel: + 358 9 88 665 600

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