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Growing Undersea Infrastructure Redefines Quality Requirements

Since the early times of extruded high voltage subsea XLPE insulated cables, it has been mandatory to develop reliable in-house solutions for jointing either XLPE or lead extrusion lengths. The manufacturing lengths are connected by so called Factory Vulcanized Joints (FVJ). The factory joints make it possible to proceed with the application of subsequent cable layers in processes not sensitive to restarts after a break.

The development of XLPE subsea cables has been promising during the last 25 years, including progress in material quality, compound handling systems, extrusion technologies and quality control during manufacturing. This progress has led to an increase in the number of installations of high (HV) and extra high voltage (EHV) subsea XLPE cables. In addition, the installation lengths at all voltage levels have also increased during the same time, which makes it clearly important to develop a reliable concept for in-house jointing of subsea cables.

Maillefer technology

The technologies attributed to cable manufacturing are available on the open market. While FVJ has been an in-house competence for decades, we have recently developed FVJ specifically for HV AC subsea cables up to 245 kV. The development includes a vulcanization modeling software that optimizes the insulation curing process for joints. This technology is part of the Maillefer Consultation portfolio. Our FVJ know-how provides you with all the needed technology for manufacturing factory joints.

FVJ is an integral part of the whole subsea cable system. For reliability reasons, FVJ is required to have equal performance as the XLPE cable and a low statistical scatter in dielectric strength. Furthermore, an elaborated selection of equipment, consumables, tools, processing parameters, temperature and cross-linking simulations, quality control, and know-how throughout the manufacturing steps are all necessary for a mature technology.

Manufacturing FVJ is similar to the concept of manufacturing a short length of XLPE cable, having equal electrical, thermal and mechanical properties. The principal of FVJ is shown in Figure 1.

Factory Vulcanized Joint for HV AC subsea cables
Figure 1. Factory vulcanized joint for HV AC subsea cables.

Quality control

The manufacturing of a Factory Vulcanized Joint along with continuous quality control is divided in five main steps:

  • Welding of conductor
  • Inner semi-conductive layer restoration and vulcanization
  • Insulation restoration and vulcanization
  • Outer semi-conductive layer restoration and vulcanization
  • Final quality control, e.g. local Partial Discharge (PD) and/or X-ray detection.

For HV and EHV subsea cables, additional steps are commonly found, like the restoration of lead and polymeric sheaths. The temperature distribution during manufacturing must also be simulated in a modeling software, verified and fine-tuned. An example of temperature distribution from a simulation is shown in Figure 2.

Temperature distribution in a factory vulcanized joint
Figure 2. Example of temperature distribution in a factory vulcanized joint.

Quality control assurance during FVJ manufacturing includes:

  • Welding (inclusions, voids), X-ray
  • XLPE material cleanliness and handling
  • Processing parameters (temperature and pressure vs. time)
  • Dimensions, thicknesses and eccentricity.

The design and manufacturing techniques (handcrafting) and quality control of Factory Vulcanized Joints are also essential. By working together with us, you can reach the tight technical requirements within a realistic time frame without sacrificing FVJ quality.

Timo Mäkelä, Process Specialist, MV&HV Cable Solutions

Wed July 26 2017