ON-BOTTOM STABILITY CALCULATIONS FOR FIBRE OPTIC SUBMARINE CABLES - - PowerPoint PPT Presentation

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ON-BOTTOM STABILITY CALCULATIONS FOR FIBRE OPTIC SUBMARINE CABLES Inge Vintermyr

Nexans Norway AS

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Presenter Profile

Inge Vintermyr graduated from the Norwegian Institute of Technology in 1989 with a Ph.D in Materials Science. He joined Nexans Norway AS (previous Alcatel Kabel Norge AS) the same year. He has Kabel Norge AS) the same year. He has been working with research, development and engineering of energy and telecom cables since 1989. In 2000 he was appointed Technical Manager for the communications cable division. Inge Vintermyr Technical Mgr Email: Inge.Vintermyr@Nexans.com Tel: (+47) 22 88 62 29 Mobile Tel: (+47) 95 23 57 43

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OUTLINE

• INTRODUCTION/BACKGROUND
• THEORY AND MODELS
• STANDARDS AND SOFTWARE PROGRAMS
• STANDARDS AND SOFTWARE PROGRAMS
• CALCULATIONS AND RESULTS
• SUMMARY AND CONCLUSIONS

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INCREASING DEMAND FOR FOR FO IN OFFSHORE INDUSTRY

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ON-BOTTOM STABILITY

• FO cables are installed along with other

cables, umbilicals and pipelines.

• Commonly specified that all installed items

shall be stable. shall be stable.

• Typically referred to DNV-RP-F109 ”On

Bottom Stability Design of Submarine Pipelines”.

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FO CABLES MUST OPERATE WITH OTHER SUBSEA INSTALLATIONS

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FO CABLES, POWER CABLES AND UMBILICALS

ROC -Cables URC -Cables ROC -Cables Power/Umb

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CABLE CHARACTERISTICS

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SEABED STABILITY THEORY

• Driving forces:
• Drag and inertia forces from flowing water, waves and current
• Resisting forces:
• Interaction with soil, friction + passive resistance due to penetration

FL FI +FD FLOW (ws –FL)u+ FR(z) ws

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SEABED STABILITY THEORY

• Drag and lift forces are proportional to velocity squared

Drag: Lift: Intertia:

( ) ( )

2

sin 2 1

C w D w D

U t U C D F + ⋅ ⋅ = ω ρ

( ) ( )

2

sin 2 1

C w L w L

U t U C D F + ⋅ ⋅ = ω ρ   ⋅ ⋅ = dU

2

ρ π

Intertia:

      ⋅ ⋅ = dt dU C D F

w M w I 2

4 ρ π

FL FI +FD FLOW (ws –FL)u+ FR(z) ws

High Ws/D ratio is positive for stability

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SEABED STABILITY THEORY

• Interaction between cable and soil provides resistance

– Consists of pure coulomb friction and resistance due to penetration into the seabed

FL FI +FD FLOW (ws –FL)u+ FR(z) ws

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SEABED STABILITY BASIS

Operation – For permanent operational conditions and temporary phases with duration in excess of 12 months, a 100-year return period applies. – When detailed information about the joint probability of – When detailed information about the joint probability of waves and current is not available, this condition may be approximated by the most severe condition among the following two combinations: 1) The 100-year return condition for waves combined with the 10-year return condition for current. 2) The 10-year return condition for waves combined with the100-year return condition for current.

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SEABED STABILITY BASIS

Temporary Phase – For temporary phases with duration in excess of three days and less than12 months, a 10-year return period applies. – This condition may be approximated by the most – This condition may be approximated by the most severe condition among the following two combinations: 1) The 10-year return condition for waves combined with the 1-year return condition for current. 2) The 1-year return condition for waves combined with the10-year return condition for current.

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STABILITY - APPROACHES

• Allowing accumulated displacement

– A certain maximum displacement allowed – Will break out of its cavity during extreme sea state state

• No break-out (virtual stability)

– Allowing small displacement, normally less than 0.5XD. No accumulated displacements

• Absolute stability

– All loads less than the resistance forces, no lateral movement

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STANDARS AND SOFTWARE PROGRAMS

• Time Domaine Dynamic Analysis

– A three hour severe storm is typically simulated – Results are a timeseries througout the storm:

• Lateral displacement
• Load effects (forces, stress, strain)
• Load effects (forces, stress, strain)

– Few companies use this method – Commercial software products:

• PONDUS (Marintek)
• AGA Level III (PRCI, USA)

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STANDARDS AND SOFTWARE PROGRAMS

• DNV-RP-F109, 2007 ”On-Bottom Stability Design of Submarine

Pipelines” (Replaces DNV-RP-E305, 1988

• RP-F109 provides design curves that are based on a large set of

full dynamic analyses performed by using PONDUS

• Simplified approach compared with the time domain dynamic
• Simplified approach compared with the time domain dynamic

analysis

• Includes four sets of design curves for the following methods:

– Sandy seabed with displacement < 0.5 x OD – Sandy seabed with displacement < 10 x OD – Clayey seabed with displacment < 0.5 x OD – Clayey seabed with displacement < 10 x OD

• Absolute stability
• No displacement allowed

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INPUT PARAMETERS

D: Pipe outer diameter [mm] ws: Pipe submerged weight per unit length [kgf/m] Hs: Significant wave height [m] for extreme sea states ( 1, 10 and 100 years return period) Tp: Wave Peak period [s] for Hs Uc: Current speed [m/s] for extreme events (1, 10 and 100 years return period) (1, 10 and 100 years return period) d: Water Depth [m] su: Un-drained clays shear strength [kPa] γ’s: Submerged unit soil weight. For sand normally in the range 7000 (very loose) to 13 500 N/m (very dense) d50: Mean sand grain size [mm] The oceanographic data (Hs, Tp, Uc) have to be derived by statistical methods from long term measurement of both wave and current Seabed data(su , γ’s , d50 ) are established by taking soil samples at different locations in the area where the pipe is going to be installed

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TYPICAL FIELD DATA

Area d (m) Hs (m) Tp (m) Uc (m/s) Su (kPa) d50 (mm) North Sea 290 16 18.5 0.41 10

• North Sea

290 16 18.5 0.41

• 0.5

Brazil 1300 7.16 14.8 0.58 10

• Brazil

1300 7.16 14.8 0.58

• 0.5

Gulf of Mexico 1913 14.2 14.9 0.1 10

• Gulf of

Mexico 1913 14.2 14.9 0.1

• 0.5

Persian Gulf 51 6.1 11.2 1.3 10

• Persian

Gulf 51 6.1 11.2 1.3

• 0.5

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CALCULATIONS – NORTH SEA

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CALCULATIONS – GULF OF MEXICO

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CALCULATIONS – PESIAN GULF

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CALCULATIONS – W/D RATIO

Area Min Sub W/OD -Sand Abs 0.5xOD 10xOD Min Sub W/OD -Clay Abs 0.5xOD 10xOD North Sea 50.0

• 31.8

109

• Brazil

17.2

• 69.7
• Gulf of

Mexico 17.2

• 17.2
• Persian

Gulf 600

• lift

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SUMMARY & CONCLUSIONS

• On-bottom stability calculations depending on oceanographic data

(Hs, Tp, Uc),derived by statistical methods from long term measurement of both wave and current – and seabed data(su , γ’s , d50 ), established by taking soil samples at different locations in the area where the “pipe” is going to be installed

• DNV-RP-F109 calculations have clearly shown that the submerged
• DNV-RP-F109 calculations have clearly shown that the submerged

weight to outer diameter ratio (Sub.w/OD) is the governing parameter of a cables’s on-bottom stability at certain weather conditions.

• The small diameter FO cables are less stable (as per DNV-RP-

F109 ) than power cables/umbilicals and pipelines.

• Self burial effects could possibly be more dominant for small cables

and thus make them more stable than calculated by the DNV rules?

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SUMMARY & CONCLUSIONS

• How to obtain a stable cable?

– Adding armour weight (Increase Sumberged weight/OD ratio) – Partly or fully bury the cable – Trencing – Trencing – Restrain the cable at certain points along the route by using piles, grout bags or rock dumping

• Will it always be required to obtain stability?

– The cable will normally be well protected for movements/abrasions/bendings – The bottom topograhy may protect cable from movements, proper route selcection, etc – So answer can be ”No” for some installations

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Thank you for your attention

2010

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