The distance between where the cable leaves its seabed covering and reaches the turbine entry point typically covers about 12-15metres. This cable section is highly dynamic, being subject to sea currents and drag-related water flow disturbances around the foundation and rock-scour protection. So, cable protection is usually provided on 20-25 metres of cable.
Most common cable entry methods include external Ior J-tubes attached to the monopile foundation or a jacket or tripod leg, or through a hole in the monopile.
"Mechanical cable damage can occur during installation and in the operating phase, and has proven to have a noticeable affect on overall projects costs," says Ton Tuk, managing director of CP/NL Engineering. "Statistics suggest that following cable installation, the condition of one in forty cables is already problematic and may not reach its projected 25-year lifetime. In practice, we see that certain decisions made in the CAPEX phase insufficiently anticipate cable damages and the financial consequences during OPEX. The retrofit of cable protection is thereby of crucial importance."
Remedying operations are by definition costly due to turbine shutdown and sometimes even switching off a complete string with multiple turbines, he points out. Cable replacement costs amount to between EUR3-6 million per cable, excluding additional turbine downtime-related losses, sometimes affecting whole arrays.
The articulated pipes comprise many individual spherical cast iron elements interlinked by a fast hooking-in method that eliminates bolt connections common for competing third-party offerings. "This feature enables fast assembly in a confined deck space," says Tuk. "This reduces the required size and cost of the cable laying vessel."
It also speeds up removal of the articulated pipe for cable repair and reassembly, or for reuse following decommissioning, he points out.
The system requires only 20 bolts for the connecting piece at the cable entry to the turbine, and a saddle clamp which goes around cable and keeps it central inside the pipe at the point where it enters the seabed. The components are made in cast iron instead of polyurethane to eliminate any risk of cable overheating. According to Tuk, it can also be installed and removed without diver support.
A picture of an articulated pipe shows bright yellow and black coated elements, which was painted at the client's request. The articulated pipe does not require painting explains Tuk. "We know an example where uncoated articulated pipes were retrieved after 90 years, and without visible signs of corrosion damage."
He also refers to the 1992 National Association of Corrosion Engineers study where a spherical cast iron sample is exposed to a seawater flow test. Initially it experiences a high rate of corrosion, which is measured in mass loss. This peaks after about three days and then oxidation slows. By 17-21 days a steady state occurs and virtually no more oxidation occurs afterwards. This chemical process offers a natural protection of the cast iron.
CP/NL has developed and tested several prototypes of articulated pipe systems, and aims to continue to innovate to stay ahead of the competition. "We do not limit ourselves to a single innovation path," says Tuk. "We can afford such flexibility because production is fully outsourced to manufacturing partners close to our key markets. Last year, we manufactured 95% of our hardware demand in the UK with our UK foundry partner."
Last November it became the first cable protection supplier to receive DNV GL certification for monopile direct cable entry, I-tubes and J-tubes. Tuk adds that these demands exceed the less stringent certification of the Joint Industry Project. It has recently been contracted to deliver over 100 cable systems for the German Nordsee Ost project.
Tuk believes that the industry can benefit from better cooperation between all projects partners to improve procurement strategies. "Many decision makers have never set foot on an installation vessel, which creates the attitude of focusing on known products and paper references. As a result they continue choosing these products even if they are more expensive or have known problems. A project's full cycle lifecycle should be considered, and this would have a positive impact on CAPEX, OPEX and in driving down the cost of energy."