Design of Offshore Pipelines on Erodible Seabed

Transcription

1 Design of Offshore Pipelines on Erodible Seabed Winthrop Professor Liang Cheng School of Civil, Environmental and Mining Engineering The University of Western Australia

2 OFFSHORE PIPELINES Key links between production wells and storage and processing units (mainly onshore) Functions: transport of oil and gas products, power and control fluids Costs of pipelines are generally high Consequences of failures are high Costs associated stoppage of production and repairs Environmental and social impacts

3 TYPES OF OFFSHORE PIPELINES Rigid pipe Small: 3 to 14 ; Medium: 16 to 28 ; Large: 30 to 56 Flexible pipe HP to around 14 ID LP to around 20 ID Materials: carbon steel, corrosion-resisted Alloy

4 Cost per km*inch (US$) TYPICAL PIPELINE COSTS Gas Export Pipeline Cost UKCS Eng Mat Inst 120, ,000 80,000 60,000 2nd Trunkline 40,000 20, ,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 Pipeline Length*Diameter (km*inch)

5 COSTS FOR PIPELINES Stabilisation 30% Management & Engineering Installation 10% (excluding Stabilisation) 30% Materials & Fabrication 30%

6 PIPELINE ON-BOTTOM STABILITY Trenching or rock berm in shallow waters ( approximately < 30 m) Primary stabilization: self weight + concrete coating in medium to deep waters ( > 100 m) Primary + Secondary stabilizations: in medium water depth between 30m to 100 m

7 SECONDARY STABILIZATION METHODS Rock Berms Gravity Anchors Piles,

8 PIPELINE DESIGN CHALLENGES Corrosion pipeline integrity Anti-corrosion coating and metal cladding Flow assurance steady productions Liquid plugging, hydrates, wax, On-bottom stability present topic Resist extreme hydrodynamic forces Very expensive

9 PIPELINE STABILITY DESIGN Design requirement: pipeline be stable under extreme environmental conditions Absolute stability methods Pipeline movements not allowed Often conservative, leading to high costs Dynamic stability methods Pipeline movements are allowed Considers seabed resistance changes Structure integrity needs to be checked

10 FLAWS IN CURRENT DESIGN METHODS Calculations of wave forces are too conservative Velocity reduction in wave boundary layers in force calculations are not considered Sediment transport processes ignored Use static seabed profiles Reality: seabed profiles around the pipe significantly modified before storm peaks arrive Hydrodynamic loads Soil resistance Use of a slice of pipeline is not acceptable

11 Effects of Sediment Transport Movement of sandy seabed sediments modifies seabed profiles around pipelines Local scour occurs if wave orbital velocity exceeds a critical value Local scour can cause pipeline self-burial into the seabed, improving pipeline stability Hydrodynamic forces decreases Soil resistance increases Dalian University of Technology

12 LOCAL SCOUR BELOW PIPELINES

13 SELF-BURIAL MECHANISMS A B A A B B A A B B A A B B A A A B B School of Civil and Resource Engineering B

14 FIELD EXPERIENCES Significant self-burial on sandy seabed 80%-100% burial for 60%-80% of the lengths of existing small diameter pipelines School of Civil and Resource Engineering

15 Field Measurement Results Measurement using high resolution laser surface profilers Pipeline is partially buried in the seabed 1 year after installations This occurs to entire pipeline Dalian University of Technology

16 WAY FORWARD Conduct more research Quantify the effect of wave boundary layers on hydrodynamic forces on pipelines Develop robust and reliable methodology that can take into account the effect of sediment transport on pipeline stability Joint Industry Project: StablePIPE JIP Aims: develop new design guidelines Sponsored by two industry partners Involved University and engineering

17 Research Aims and Methods Develop a new design method to consider the effect of pipeline self-burial on the stability of pipelines Method: Experimental investigation Quantify the effects of sediment transport and wave boundary layers on pipeline stability Simulate dynamic response of pipelines under extreme environmental conditions Dalian University of Technology

18 OUTCOMES A new research facility is established A large scale recirculating flume Able to generate prototype random storm velocity time series up to 2.8m/s with a period of 13s. Simulate flow/pipeline/seabed interactions near prototype conditions to overcome scaling difficulties Completed a number of research projects Developed a new pipeline stability design guideline.

19 NEW FACILITY Functions Conduct pipeline stability testing at large scales to reduce scaling effects; 1:1 scale for small diameter pipelines (<8 inch) 1:5 scale for large diameter pipelines (<40 inch) Simulate flow conditions induced by cyclonic storms Oscillatory flow up to 2.8 m/s with a peak period of 13 seconds Steady currents up to 3.0 m/s Combined random storm time series.

20 O-TUBE CONCEPT Flow is generated by controlling the rotation of impeller One direction only steady currents Two directions of equal speeds oscillatory flows Two directions of different speeds oscillatory + steady Rotations from any spectra random storms

21 PIPE CONTROL SYSTEMS Model pipe Mounting actuators Data acquisition system Active feedback control operations

22 MODEL PIPE D=0.2 m and L Internal DAQ system, communicating via Ethernet Up to 1 MHz, 8 channels per box Internally pressurised to protect DAQ system.

23 RANDOM WAVES + CURRENTS

24 Movie -1: DURING TEST

25 MOVIE-2: PIPE BREAKOUT

26 NUMERICAL MODELS SCOUR-2D & SCOUR-3D Model scour below pipelines and subsea structures Hydrodynamic forces on pipelines and risers Vortex-induced vibrations WAVEFLUME-3D Wave-structure interactions Application examples

27 2D SCOUR - PIPELINES

28 PIPELINE SELF-BURIAL

29 3D SCOUR BELOW PIPELINES

30 SCOUR AROUND A TRUNCATED PILE

31 COUPLING OF VIV AND SCOUR

32 RESEARCH IMPACTS

33

34 New design code: with DNV

35 Validation using field data

36 Validation using field data

37 Validation using field data

38 SUMMARY A new pipeline stability design methodology has been developed and validated against both laboratory tests and field data The new design method is being applied to a number of real engineering projects More research efforts are needed to improve the design method.