Beyond the Code - Subsea Cable Stability

Transcription

1 Beyond the Code - Subsea Cable Stability All-Energy Conference, Aberdeen 2012 Chas Spradbery

2 Outline Company Overview Why this presentation could save you Code limitations What actually happens Going Beyond the code Conclusions 1

3 Engineering Market Leading Positions Structures & topsides 40 years of unrivalled offshore expertise We have worked on 75% of GOM deepwater facilities 30% of deepwater facilities worldwide We have engineered the WORLD S DEEPEST TLP - Magnolia 4,744 SPAR Perdido 7,800 Production platform Independence Hub Semi 7,920 Subsea solutions LARGEST independent SUBSEA technology group of its kind in the world Engineered more than 500,000km of cables & pipelines Engineered, managed or appraised OVER HALF of all subsea development worldwide We have DESIGNED and PROJECT MANAGED subsea developments in water depths up to 3,000 METRES Experienced working in HIGH ENERGY areas large waves, currents, cyclones, etc Involvement in access of 800 RENEWABLE ENERGY projects to date Assessed OVER 60,000 MW of renewable energy development internationally 2

4 Background Cable stability only needed prior to protection Conservative code requirements may result in: Reduced schedule flexibility between install & protect Additional cable weight (armour/fillers) 3

5 Required SG for Generalised Stability (10D limit) Keulegan-Carpenter Number Cable Stability Just Like Pipelines? DNV-RP-F109 Stability Design for Pipelines Required SG very dependent on pipe/cable diameter Driven by Keulegan-Carpenter Number ( 1/D) Drives SG >> 1.5 for D < 0.4m Is this REAL? 5 SG_ K Pipe / Cable Diameter (m) 4 4

6 Boundary Layer Assumed profile is conservative Boundary layer will depend on flow conditions and actual seabed very site specific 5 5

7 Stable Cables on an Unstable Seabed? Test Case: 50mm Dia. 5mins ramp up Fine sand Observe: Storm build-up Scour Cable movement Cable lowering Accretion Burial Movie courtesy UWA from Mini-O-Tube Facility 6 6

8 Field Observation - Partial Burial is Typical 7 7

9 STABLEpipe JIP: UWA O-tube Project STABLEpipe JIP: Chevron, Woodside Sponsors UWA and JP Kenny Participants UWA Large-O-Tube Project: Closed-loop recirculating wave and current flume 200mm Dia. Model pipe (50mm &100mm also employed) Storm build-up Scour Cable/Pipe movement Cable/Pipe lowering Accretion Burial 3yr Research Programme Photos courtesy UWA from Large-O-Tube Facility 8 8

10 Model: Scour Mechanisms Flow Water particles Flow separation High pressure Low pressure A typical scour process (Cheng et al.2009) 9 9

11 COMPLEXITY Cable-Soil-Fluid Interaction Model We need a model which can bridge the gap between: The (too simple) hysteresis friction models and The (too expensive) continuum FEA / CFD models Separately model soil profile and pipe/soil interaction & response? ACCURACY 10 10

12 What is the CSF Model? Cable-Soil Interaction Vertical Reaction Force Soil Resistance Soil deformation due to cable movement Cable-Fluid Interaction Cable hydrodynamics Soil-Fluid Interaction Suspended sediment transport Fluid Cable Soil Cable on Sea Floor 11 11

13 CSF Model: Sediment Transport Bedload Transport: Rolling, sliding and hopping of sediment grains Suspended Sediment Transport: Sediment advected by turbulent eddies of the fluid Change in soil node elevation: Sediment advected into and out of the element Change in mobile sediment between timesteps 12 12

14 Model Outputs What can we achieve with this model? -Better understanding of the Cable seabed interaction -Prediction of self burial -Potential for reduction in weight -Faster simulation times resulting in reduction of costs to operator -Simplified certification if cable is to be left exposed for long periods

15 Conclusions Reduced stability requirements leads to: reduced cable weight, reduced cost increased flexibility in schedule reduced risk. Lack of public domain knowledge Models/experience is available to save money on single projects or groups of projects

16 Thank you