Tensile Bending Test Services according to Cigré Electra 171
Pro Subsea offers Complete Tensile Bending Test according to Cigré Electra 171.
The tensile bending test according to Electra 171 demonstrates the cable’s ability to withstand tensional forces in combination with bending over the laying wheel during installation. The test is to be performed on a piece of cable before the electric tests are performed on the same piece of cable. A flexible joint can be qualified when included in the test cable. The test cable is laid halfway around a large sheave, which represents the laying wheel of the cable-laying vessel. A pulling force is applied to the cable ends while they are moved three times back and forth around the wheel. The wheel diameter is often 6 or 10 m representing typical laying wheel diameters of existing cable ships
Test arrangement for tensional bending test according Electra No. 171.
The test pulling force is given in Electra 171 depending on the cable weight in water and the water depth. For a depth over 500 m, also the dynamic forces caused by wave movements are included. Experience has shown that actual pulling forces on the laying wheel can considerably exceed the forces given in Electra 171, depending on the weather situation during laying.
For many submarine transmission projects the laying depth is below 100 m, which leads to rather moderate test forces according to the Cigre recommendation. Sometimes the required pulling force is so small that the heavy cable is not even straight but has considerable sag during the test. Tests with the Cigré-recommended forces for laying depth under too m often have no practical relevance. Purchaser and manufacturer should consider an agreement to omit the test to save costs.
The tensional bending test confirms the design values of tensional strength and side-wall pressure (SWP), i.e. the uniform lateral force onto the cable side per unit length.
Other mechanical stresses might be experienced during installation, such as localized side impact by caterpillar pads or wheeled cable engines, or stresses caused by trenching machinery. Also the transport of cable over curved roller tracks or cable gantries is not covered by the Cigré test recommendations, unless the rollers in the gantry are very close together and the roller diameter is sufficiently large. No type tests have been devised yet for this type of impacts. As installation conditions and gear vary largely between projects, it would not be possible to construct type tests covering all type of impacts. Instead the manufacturer, purchaser and installer should agree on the necessity and character of additional confirming tests.