First South East European

Regional CIGRÉ Conference

Portoroz, Slovenia, 7—8 June 2016

SEERC

Deep Water Power Cable Systems?

Indeed!

M. JEROENSE, O. HANSSON, A. TYRBERG, E. REBILLARD,

K. CRONHOLM, J. LINDHE

2-06

Ocean and sea depths

• The average ocean depth is nearly 3,700 meters

• Maximum of 10,994 m (some state 11,034 m) Mariana trench close to Japan and Philippines

• Mediterranean: averagedepth of 1500 m and deepest 5267 m (Calypso Deep in Ionian Sea)

Transatlantic Telecom cable

• 1858 first message sent

• That early already – so what’s the difficulty?

• 550 kg/km (!) ≈ 0.5 kg/m

• Power cable ≈ 50 kg/m

• Tension ca. 100 x larger!

Laying a cable

• Catenary length

• Tension

𝐿0 = 𝑑 ∙ √ 1 + 2 ∙𝐻

𝑤 ∙ 𝑑

𝑇𝐸 = 𝑤 ∙ 𝑑 + 𝐻 + 𝐷

weight depth

Calculating the dynamic tension

• Using special purposesoftware

• Or using the equationsin TB623

• 𝐷 = 𝐷𝐼2 + 𝐷𝐷

2

• DI stands for inertiaforce

• DD stands for drag force

Requirements and challenges

• Tension, side-wall pressure and squeeze forces

• Hydrostatic pressure

• Longitudinal water penetration

• Recovery forceSidewall pressureSqueeze force

0 500 1000 1500 2000 2500 3000

Wei

ght

Power [MW]

Cable design rules

• Aluminium vs Copper• For same power: lower

weight. But lowermaximum power

• Insulation thickness• Increased electric stress

• Lower weight

Larger weight Larger tension Smaller depth

aluminum

copper

10

15

20

25

30

30

35

40

45

50

55

60

20 25 30

Ave

rage

Ele

ctri

c fi

eld

Str

ess

[kV

/mm

]

Wei

ght

[kg

/m]

Insulation thickness [mm]

Cable design rules

• More armour makes cable more stiff (EA)• Less strain on conductor

(ε)

• But larger weight• Increases the tension

• Gain?

𝑇𝐸 = 𝑤 ∙ 𝑑 + 𝐻 + 𝐷

Extruded and MI DC cableU = 525 kV

P = 1200 MW (pair)

A = 1000 mm2 Aluminium

W = 49 kg/m

EA = 705 MN

U = 525 kV

P = 1200 MW (pair)

A = 1400 mm2 Aluminium

W = 52 kg/m

EA = 697 MN

The Comparison

Top tension Conductor strain

0

100

200

300

400

500

600

700

800

900

1000

0 500 1000 1500 2000

Top

te

nsi

on

[kN

]

Water depth [m]

XLPE 1000mm2 Al

MI 1400mm2 Al

0,00

0,02

0,04

0,06

0,08

0,10

0,12

0,14

0 500 1000 1500 2000

Co

nd

uct

or

stra

in [

%]

Water depth [m]

XLPE 1000mm2 Al

MI 1400mm2 Al

Quite similar top tension and conductor strain

The Comparison

Extruded DC MI DC

0,0

100,0

200,0

300,0

400,0

500,0

600,0

700,0

0 500 1000 1500 2000

Forc

e p

er

un

it le

ngt

h [

kN/m

]

Water depth [m]

Required squeezeforce - 2-track[kN/m]

Required squeezeforce - 4-track[kN/m]

Side wall force - 5m radius [kN/m]

0,0

100,0

200,0

300,0

400,0

500,0

600,0

700,0

0 500 1000 1500 2000

Forc

e p

er

un

it le

ngt

h [

kN/m

]

Water depth [m]

Required squeezeforce - 2-track[kN/m]

Required squeezeforce - 4-track[kN/m]

Side wall force - 5m radius [kN/m]

Quite similar squeeze and side wall forces

Tensile test on conductor joints

• Conductor joints for deep sea applications arewelded

• Heat Affected Zone

• Some annealingreducing yield stress and tensile strength

• The test result show thatMediterranean depthscan be reached

Squeeze test

• Cable tensioned..

• ..and squeezed

• With optical fibre– no significantattenuation

• Electrical test

• Tested at > 500 kN/m

Conductor water penetration test

• Tests up to at least 200 Bar have beenperformed

• Large cross sections

• With good results

• Mediterranean depthscan be reached

Conclusion

• XLPE and MI cables areboth suitable for deepwater installation• Similar top tension,

conductor strains, squeeze and side wallforces

• Vertical Laying Systems suitable for extreme depths• Avoid Side Wall Pressure• Lower recovery force