MECHANICAL ASPECTS OF SUBMARINE CABLE ARMOUR

Transcription

1 MECHANICAL ASPECTS OF SUBMARINE CABLE ARMOUR Ernesto Zaccone

2 ARMORING OF SUBMARINE CABLES Submarine cables must be robust Mechanical aspects are at least as important as the electrical o nes Must withstand the mechanical stresses due to: - Storage Handling Installation Recovery (if damage ever occurs) SWA: One layer of armor (used for shallow water applications) DWA: Two layers of armor (used for deep water applications) Round or flat wires are possible. Round wires are cheaper, flat wires provide better coverage and compactness For deep water installations, the cable must be reasonably torqu e balanced to avoid uncontrolled twisting as it is lowered to the sea bed. Galvanized steel round or flat wires are used for 3/C AC cables or HVDC cables. Hard copper round or flat wires are used for single core AC cabl es (too reduce cable losses) Resistant to corrosion

3 ARMOR TYPES OF SUBMARINE CABLES

4 DEEP AND EXTRA DEEP WATER HVDC CABLES Copper or Aluminum conductor Semiconducting paper tapes Insulation of paper tapes impregnated with viscous compound Semiconducting paper tapes Lead alloy sheath Polyethylene jacket Metallic tape reinforcement Synthetic tape or yarn bedding Double layer of steel armor flat wires Polypropylene yarn serving Typical Weight = 30 to 60 kg/m Typical Diameter = 110 to 140 mm

5 Sometime armour wires are required to be covered by a plastic sheath, either individually ( each wire ) or overall, in particular for offshore use when the platform is actively cathodically protected. Individually Individually PE covered PE covered wires 4 mm wires Rock Armour ; 7mm angle Rock 50deg Armour A specially resistance armour to abrasion and crushing is the so called rock type. The outer layer is applied with a short pitch ( typically angle of 45 to 60 deg ), made of big wires (e.g. 6-7 mm), over a thick PP yarn bedding (4-5 mm).

6 TORQUE MOMENT Unbalanced armor low tension high load on the conductor τ = T r sin α? = torque moment N/m, T = tension applied to the cable N r = pitch circle radius of the armo r m? = angle of application of armo r T T Torque balanced armor high tension high load on the armor r α = T r 1 sin 1 2 sin T = tension applied to the cable N r 1 = pitch circle radius of the 1 st armo r m r 2 = pitch circle radius of the 2 nd armo r m? 1 = angle of application of 1 st armo r? 1 = angle of application of 2 nd armo r T α 2 α r α 1 α 2 r 1 r 2

7 CABLE STORAGE Cables wound: Rotating platform Bending stresses (no torsion) Smaller inner diameter Applicable to all kind of cables Cables coiled: No rotating platform Bending stresses and torsion Larger inner diameter Applicable to a limited type of cables

8 OTHER MECHANICAL ASPECTS Resistance to tension strain: In case of single wire armor the maximum tension strain is supported by the conductor, as for the CIGRE recommendations the presence of flexible joints shall be taken into consideratio n when subjecting the cable to the mechanical qualification tests For the double armor the max strain is supported by the armor. The stresses on the cable components shall be evaluated with the dedicated software in order to guarantee the expected life. Stability of the cable: a high ratio Weight/Diameter in order to have a good stability against waves and currents.

9 STANDARD REQUIREMENTS OR RECOMMENDATIONS There are mainly functional r equirements for submarine cables that are strictly depending on the design of the armor and the system requirements. These are the recommended tests as for the CIGRE Electra 171 req uirements: 1. Coiling Test (if applicable) - to define the coiling minimum diameter 2. Tensile bending test - to simulate the laying on the vessel 3. Tensile test - to verify the elongation and rotation (for eng. info.) 4. External water pressure withstand test - to verify pressure collapse 5. Internal pressure withstand test (if applicable) 6. Watertightness of earthing connection to verify leakages under pressure 7. Sea - trial test if there is no prior experience

10 APPLICABLE STANDARDS There are very few standards requirements for submarine cables, the IECEA /NEMA WC 74 are indicating the diameters of the armor wires, the standard is applicable to 5-46KV SHIELDED POWER CABLE FOR USE IN THE TRANSMISSION AND DISTRIBUTION OF ELECTRIC ENERGY but no particular mechanical tests are specified. Note: Table extracted from IECEA /NEMA WC 74

11 CIGRE COILING TEST The coiling test shall be carried out only on cables which are c oiled during the manufacture, the laying, and the recovery. The cable shall be long enough to obtain at least 8 complete turns The cable length shall contain at least 2 factory joints The height of the jockey Hm shall not be higher that that used in practice Both the cable ends shall be blocked in order to prevent rotation. The coiling and recoiling cycle shall be repeated the same times expected during the manufacture and operations The coiling direction and minimum diameter Dm shall be specified by the manufacturer After the coiling test the cable shall be subjected to the tensile bending test and the electrical test

12 TENSILE & TENSILE BENDING TEST Tensile test for the measurement of axial and torsion characteristics for eng. inf. Tensile bending test for the simulation of the cable laying and recovery plus electrical test On the laboratory On the laying vessel

13 SUBMARINE CABLE INSTALLATION T d T = Tension of the cable - N w = weight of cable in water - N/m d = max. laying depth - m H = max. Allowable bottom tension N D = is dynamic tension (waves) - N H Water depth:0-500m T = 1.3*w*d+H Factor 1,3 takes care of the additional tension caused by the laying and recovery forces and dynamic forces during laying and recovery situations. Water depth:0>500m T = w*d+h+1.2*/d/ 1.2 is the safety factor of dynamic force

14 SEA - TRIAL OF 500 kv MI CABLE Lay of 6 km of cable including a repair joint and an earthing connection at maximum depth (1620 m) Stay for 6 days st - by with cable suspended at max.depth Recovery of all cable and un - load back to factory HV test at 720 kv Inspection of most significant parts of cable and accessories

15 SUMMARY OF REQUIREMENTS The cable must withstand, without deterioration, the severe bend ing under tension, twisting and coiling which may occur during the manufacture and installation programmes. The cable must also withstand, without significant deformation, the external water pressure at the deepest part of the route. The cable, and where appropriate the terminal equipment, must be designed to ensure that only a limited length of cable is affected by local damages (i.e. water ingress if the metal sheath is damaged when in service). The armor must be sufficiently robust to resist within certain l imits to external impact damages. For deep water installations, the cable must be reasonably torqu e balanced to avoid uncontrolled twisting as it is lowered to the sea bed. The weight of the cable in water must be sufficient to inhibit m ovement on the sea bed under the influence of tidal currents. Movement would cause abrasion and fatigue damage to the cable. The cable must be adequately protected from all corrosion hazard s. All cable components must have adequate flexural fatigue life.

16 NIGHTMARES Pictures from the ELECTRIC CABLES HANDBOOK D. McAllister