Chapter 14 Sea Bed Boundary Effects - Dredging Engineering · November 2005 Outline Chapter 14 -...
Transcript of Chapter 14 Sea Bed Boundary Effects - Dredging Engineering · November 2005 Outline Chapter 14 -...
![Page 1: Chapter 14 Sea Bed Boundary Effects - Dredging Engineering · November 2005 Outline Chapter 14 - Boundary layer under Currents and Waves - Bed Material Stability - Sediment Transport](https://reader031.fdocuments.in/reader031/viewer/2022020315/5ae851537f8b9acc268ff342/html5/thumbnails/1.jpg)
November 2005
Chapter 14
Sea Bed Boundary Effects
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November 2005
Outline Chapter 14
- Boundary layer under Currents and Waves- Bed Material Stability- Sediment Transport- Sea Bed Changes- Laboratory Modeling- Vertical Pile in Current- Small Objects on the Seabed- Pipelines
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November 2005
• Objective of this chapter :
- Obtain insight of the flow in the vicinity of the sea bed- Forces on man made objects and its consequences
Introduction
• Erosion and deposition of seabed material around an offshore platform pilehave significant effect
• Pipelines can be buried or lacks of foundation support
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November 2005
• Convention on co-ordinate system (widely used in Offshore Engineering) :
Origin Z-axis in Still water level+ Z is upward.
Plan View
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November 2005
Boundary Layer Under Currents & Waves
• From basic fluid mechanics concerning Boundary Layers :
- Result of Velocity Differences between ambient flow and object (sea bed = object !)- Needs time or Equivalent distance to develop- Surface roughness plays a significant role on their development
• Types of currents
- Tidal currents -> entire depth of the sea.- Wind wave current -> surface water motion- Oceanographic current -> current less than 1 km deep
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November 2005
• Prandtl - von Karman velocity distribution :
• Tidal current : caused by gravitational effects of the Sun & Moon
- Driving force uniformly over the depth- Despite this, distribution is NOT uniform over the depth, due to friction force
over the seabed
Linear ‘Patch’ with slope dV/dz
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November 2005
Bed Shear Stress : Currents Alone
Newton’s friction model :
• Tidal variation takes a long time -> Current can be assumed to be Constantfor a few minutes to 1 hour.
-> Enough time to develop a‘ Well developed’ boundary layer.
-> Shear stress constant in that time span
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November 2005
Bed Shear Stress : Currents Alone , continued
From River Engineering comes derivation based on …
1- The Flow’s driving force : loss of potential energy (decrease of elevation)2- Resistance on the flow coming from friction with river (or sea) bed
… which yields :
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November 2005
Bed Shear Stress : Currents Alone , continued
Empirical CHEZY (18th century) formula
Depth averaged velocity in river :Eliminate ( i =1 )for Offshore case
Combination (with previous formula) yields :
…no (direct) height variable..!
Estimation for Chezy coefficient :
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November 2005
Bed Shear Stress : Currents Alone , continued
Closer look to the formulae :
Newton -> -> Proportional to V
Chezy -> -> Proportional to V^2
Generally in practice more faith to Chezy !
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November 2005
Boundary Layer Under Waves :
Water motion due to Wind caused Waves (so called Wind Waves)Near the sea bed is NOT to be neglected, even in deeper waters.
Three necessary conditions for a boundary layer by waves :
1- Water motion relative to sea bed2- Sufficient time/distance to develop
• otherwise : very thin layer develops above seabed and the flow above it and is ‘ignorant’ to the seabed.
3- Sea bed Roughness
e.g. Storm wave : - Height = 30 m.- Period = 20 sec.
-> Water velocity amplitude at 300m depth: 0.23 m/s. !
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November 2005
Boundary Layer Under Waves, … continued
• Assumed : Patched (linear) velocity profile also useful :
• Linear characteristic gradient characterized by Zt .
• Boundary layer retards the flow
-> Characteristic velocity for Shear Stress determination less than predicted in wave-theory !
Elevation of tangency
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November 2005
Shear Stress Under Waves
• Characteristic velocity of wave ut can be utilized for Shear Stress determination
• Remind: ut is periodic function
-> Averaged Resultant of Shear Stress over n waves will result in zero !!
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November 2005
Shear Stress Under Waves Plus Current
• Boundary layer caused by Waves OR Current are 2 separate mechanisms
• Direction of current often NOT equal to wave propagation
-> Superposition needs to be performed
• PLAN VIEW of Velocity components on the sea bed, in Vector notation :
P . Ub goes back and forth
Vt = Constant Vector velocity ofcurrent. (here normalized to 1 m/s)
μ = angle current – wave(here ~ 60 degr.)
Vr = resultant velocity( here : max = 1.3 m/s
min = 0.90 m/s)
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November 2005
Shear Stress Under Waves Plus Current, … continued
• Projection of Wave velocity components in the current (X,Y ) co-ordinate system :
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November 2005
Shear Stress Under Waves Plus Current, … continued
• The resultant velocity (Vr ) in the current (X,Y ) co-ordinate system :
• Resulting bed Shear Stress magnitudeat any moment is proportional to Vr^2 :
Chezy ->
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November 2005
Time Averaged Shear Stress Magnitude :
• Shear Stress magnitude averaged over a wave period :
NOT dependent of this angle !!
• In combined situation, Wave increases the average bed shear stress, while wave only does not contribute to this !
• Average bed shear stress magnitude plays a crucial role in Sediment Transport !
Current + wave
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November 2005
Time Averaged Shear Stress Components :
• … from earlier derivation…-> the X and Y component of the square of the resultant velocity (Vr ) :
Assumed : Vt >ux
• Time averaged shear stress in the X-direction :
-> for μ ≠π/2 , it increases !
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November 2005
Time Averaged Shear Stress Components , ….continued
• Time averaged shear stress in the Y-direction :
-> only ZERO if μ = π or 0 !.
• If μ ≠ π or 0 :
-> >0
-> means that the resultant bed shear stress is NOT parallel to the current direction
-> there is a resultant force acting on the water flow and perpendicular to the original current direction !!
-> this force tends to DIVERT the current, so that μ does approach π-> current forced to turn and be parallel with the wave crest !!
• In offshore : the shift in current direction is smaller due to the smaller wave influence near the (deep) sea bed.
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November 2005
Bed Material StabilityA discussion about the forces on and stability of cohesionless grains of sea bedmaterial.
Force Balance
Horizontal Force equilibrium of cohesionless grain on the sea bed.
• Horizontal Drag Force (FD) due to tiny wake downstream with low pressure.
• FD is resisted by horizontal inter-granular friction force. This friction force isdependent on vertical intergranular normal force
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November 2005
Force Balance, ... continued
Vertical Force equilibrium of cohesionless grain on the sea bed.
• Vertical intergranular Normal force is a summation of :1- Net submerged weight (= weight minus buoyant force)
of the grain2- Vertical resultant of hydrodyn pressure force distribution
around the grain, Lift force (FL).
Net submerged weight
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November 2005
Force Balance, ... continued
Total Force Picture
• Complete force balance cumbersome to carry out
-> A ‘Global’ approach is highly preferable.....
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November 2005
Shields Shear Stress Approach
This Global approach ralates the time average bed shear stress (τcw )to a stability parameter for the soil grains.
Shields grain stability curve :
Dim
.less
bed
shea
rst
ress
.
Reynolds number
Band of uncertainty
• Area of uncertainty due to e.g. particle interlock• Assumed that the(very small) slope of the bed doesn’t affect the grain stability.
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November 2005
Sediment Transport Process
A discussion about sediment transport mechanisms
• Unstable material in Shields movement area is a necessary but not asufficient condition to be transported !
• Requirements for Sediment Transport are :
1- Particles must be loosened from the seabed -> Shields (stability) criterion !2- Presence of sufficient resultant current !
CASES :
1- Weak Current : -> Could be too weak to transport sediment.2- Waves alone : -> No NET sediment mass transport, only back and forth.3- Strong Waves : -> Strong enough to ‘Stir Up’ the sea bed material,
small resultant current (e.g. Case 1) is enough forsediment transport.
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November 2005
Time and Distance Scales
Assumptions regarding sediment transport :
RIVERS : Flow conditions don’t change rapidly
-> accelerations could be neglected-> conditions remained essentially constant along a streamline-> Case of disturbance :
For typical river depth of 5 m, approx. 500 meter downstream from disturbance the steady statecondition is restored.
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November 2005
Time and Distance Scales, …. continued
OFFSHORE :
-> Case of disturbance :Due to depth, eg. 50m., distance to regain sediment transportequilibrium is approx. 5 km.:
• Trajectory to regain equilibrium is significantly larger than the structures’dimensions.
• Near an offshore structure : No completely stable sediment transport due toit’s local disturbance.
• Offshore engineering -> continually confronted with transient situation of sediment transport
Nevertheless :
-> Start with stable/steady state situation for convenience of explanation-> Transient follows
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November 2005
Mechanisms
Three methods to transport bed material in a RIVER :
1. Solution2. Suspension3. Saltation (moving along the bed.)
Solution : - transport on molecular level- Not important at all for our cases
Suspension : - relative fine particles- move along with the water, at any elevation- make the water turbid or ‘hard to see through’- occasionally important for OE applications
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November 2005
Mechanisms, ...continued
Saltation (bed load transport)
- ‘Never really gets off the ground’- Particles bounce and roll along the bed- With a speed less than adjacent flow in sea bed boundary layer
Focus will be set on the suspension and saltation methods.
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November 2005
Suspended Transport of sea bed material
Mechanism which keeps bed material in suspension :
Downwards : particles fall back towards sea bed with fall velocityUpwards : moved back upward as a result of turbulent diffusion, and
water exchanged upward has a higher sediment concentration thanwater swapped downward at the same time.
concentration
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November 2005
Suspended Transport of sea bed material, .... continued
Given the facts :
- Free exchange of material between flow and sea bed- No suspend material is lost at sea surface
-> differential equation for an equilibrium situation :
a measure of the scale of distribution in the flow
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November 2005
Suspended Transport of sea bed material, .... continued
.... making an assumption for the distribution of εs(z)
-> after mathematical manipulations, solution is :
Must be known
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November 2005
Suspended Transport of sea bed material, .... continued
When c(z) is known, the total rate of suspended material transport is :
Sediment transport over a time average (over a wave period) is much morerelevant than an instantaneous value,
-> U(z,t) can be replaced by its time averaged value
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November 2005
Bed Load Transport
....., it “Never gets off the ground”.
• Stays near the bed in a thin layer below the suspended sediment transport
• For convenience :
- from earlier: at elevation ht the bed linear velocity profilechanges into a logarithmic Prandtl-Karman profile.
- arbitrary assumption : Suspension sediment transport above this levelBed Load Transport (Sb) occurs below this level
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November 2005
Bed Load Transport, ... continued
• A pragmatic assumption :
- Sb occurs in layer of thickness ht, with a velocity Vt
- the equivalent concentration is then :
• Total steady state sediment transport :
S = Sb + Ss
Suspended transp.Bed transp.
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November 2005
Importance Bed vs Suspended Load
In offshore case :
• Sb; Bed load transport reacts very quickly to flow changes due to objectsof a typical offshore scale.
-> most important transport component
• Ss; Suspended transport does not : sediment concentration profile(the driving force) nearly affected by object.
-> seldom important
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November 2005
Sea Bed Changes
Sediment Transport Not Sufficient for Bed Changes
• Bed material stability (Shields) usually not sufficient to cause Morphologicalproblem (erosion/deposition).
-> grains replaced locally by others-> Dynamic Equilibrium
• To reveal Morphological changes :
- change in sediment transport along a streamline must be positive- between A and B more material carried away than supplied
-> dS/dX > 0 : -> leads to erosion
- if dS/dX < 0 :-> sedimentation/deposition
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November 2005
Sediment Transport Not Sufficient for Bed Changes, ... continued
- if dS/dX < 0 :-> sedimentation/deposition
• Effect of time dependency, e.g. tides :
-> introduces dS/dt which is the same in the entire vicinity-> does NOT cause erosion or deposition.
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November 2005
Bed Change Time Scale
• Typical objects that is of importance regarding morphology :
- pipeline- base of a jackup platform leg- base of a tower structure- communication cable- anchor
• OE morphological phenomena occurs within a distance of tens of meters
- usually less than 100m3 of material involved- occurs rather fast : e.g. During a single tide period !- a storm enhances the wave action and therefore the bed shear stress !
-> stirs additional bed material loose-> resulting current locally influenced by an object can easily
transport it
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November 2005
Laboratory Modeling
Physical modeling of local morphological changes
Theoretical Background and Scaling
• Offshore morphological problems are dominated by bed load transport
-> No necessity to model the suspended transport properly (neglect !)-> No necessity to model entire ocean depth : bed load occurs in a thin
layer• Laboratory model : some meters of near bed flow is reproduced
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November 2005
- Avoid blockage of flow : object ~10% of cross section- Current velocity : reproduce the lower part of the velocity profile- Waves :
- reproduce the velocities caused by waves at the sea bed- scale the wave length according to Froude
• Successful physical modeling : properly scale the bed shear stress wrt. the stability (Shields) of bed material
Theoretical Background and Scaling, ... continued
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November 2005
Case : Stability of stone berm to cover exposed subsea pipeline
- Test carried out with wave and current in the same direction
- Only current has caused rapid erosion- Waves superposed on the Current
-> did NOT cause erosion !!- Measuments revealed that the velocity profile
, so the shear stress, were greatest abovethe berm crest with current alone
- Superposition of wave has caused a reduction of thevelocity profile, due to interaction with environment andcurrent
A Modeling Experience
Bed Shear Stress plays a crucial role in modeling bed load sedimenttransport !!
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November 2005
Vertical Pile in Current
Physical Model : - Isolated vertical pile- Penetrates well into the seabed- Pile height is several diameter above sea bed
Two Dimensional Approach
Stagnation point WakeVelocity at cylinderside is 2x undisturbedVelocity.
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November 2005
Two Dimensional Approach, ... continued
Stagnation point WakeVelocity at cylinderside is 2x undisturbedVelocity.
One would expect :
- Fact that no velocity at stagnation point-> nothing would happen with
bed material on the leading partof the pile
- At both sides, where velocity is doubled-> shear stress increase-> dS/dX > 0 : erosion !
- In the wake, turbulence will enhance erosion on the lee side of the cylinder
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November 2005
Two Dimensional Approach, ... continued
FACT :
A significant erosion hole on the entire leading side of the cylinder
-> 2D model is not sufficient !
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November 2005
Three Dimensional Flow
• 3D flow pattern model includes the verticalvelocity profile caused by bed friction in theambient current
- Current assumed to be quasi constant such astidal current
- Waves neglected for convenience
• Velocity at point A higher than at B- stagnation point pressure (1/2 ρ v^2) at A
is higher and decreases downwards- dynamic press. gradient is steeper
than hydrostatic pressure (ρ g h) gradient
- residual quasi-static downwardpressure
-> downward flow !!
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November 2005
Three Dimensional Flow, ... continued
• Downward flow hits the sea bed and turnsupstream .....
- .... against the approaching flow with lowerkinetic energy(lower velocity).
- after a distance (~1 pile diameter) upstream, theflow bends up ....
- .... and sweeps back to the cylinder-> a VORTEX is formed
• Vortex grows in length on each side
- the flow around the pile sweeps (stretch)the vortex to the lee side
- viewed form above : a horse shoe pattern isformed
“Horse shoe vortex”
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November 2005
Three Dimensional Flow, ... continued
• Common features of vortices
- local- higher flow velocity than the flow in vicinity (ambient)- relatively thin boundary layer ....- .... and therefore relatively high velocity gradients and turbulence
-> high velocity gradientscause high shear stresses
-> erosion pit develops on the entire front and sides ofthe pile :
SCOUR HOLE !Local scour
Global scour
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November 2005
Three Dimensional Flow, ... continued
Depth of erosion pit is limited !
- As erosion pit gets deeper-> slopes at its sides gets steeper !-> upward slope more difficult to transport bed material-> at a certain moment ultimate depth reached
- Ultimate depth is in the orderof 1.5 x pile diemeter
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November 2005
Three Dimensional Flow, ... continued
The downstream side of the pile
- The vortices in the pile wake cause increased turbulence.- The story about the stagnation pressures can be
repeated here , but in the opposite direction.- A small secondary flow from the seabed and horse shoe
vortex will be drawn upward in to the wake- Increased turbulence enables sediment
to be carried in suspension !- At downstream : no new turbulence is
added -> vortices die out gradually.- Sediment falls out on a larger area
downstream
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November 2005
Drag Force Changes
Drag force on the cylinder influenced bythe velocity gradient caused by bed friction
Known equation :
The drag force of a vertical cylinder in a (2D) constant flow at a certainelevation :
Additional effects :
- at the sea bed- at the water surface
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November 2005
Drag Force Changes, ... continued
Secondary Flow Effect at the Sea Bed
Secondary downward flow at the upstream side of the pile ....
- supplies extra volume of water flow around the cylinder at bed elevation- flow around the cylinder actually greater than undisturbed flow.
The drag force formula is related to the undisturbedvelocity at a certain level, which is smaller thanactually present around the cylinder
-> at seabed elevation : Cd a few percent larger
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November 2005
Drag Force Changes, ... continued
Water Surface Effect
Pile cause a a kind of standing wave at water surface, ...
- this wave propagates in the upstream with the same velocity as the flow atsurface level
- dynamic pressure field is generated by this wave :- wave crest (higher pressure) located at the upstreamside of the pile
- wave trough (lower pressure) located at the downstreamside of the pile
-> cause of Net additional force componentin flow direction
-Drag force related to undisturbed near-surface flow
-> Cd a few percent Higher at this elevation
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November 2005
Drag Force Changes, ... continued
Free End Effect
A third possible effect is the ‘Free End’ effect of a vertical cylinder towed in a water tank, as has been done in the Hydromechanic Lab.
- The measured force is assumed to be uniformly distributed over thesubmerged length of the cylinder, after which the Cd can be determined.
- This assumption is NOT precisely correct :
- at the water surface : force will be disturbed (earlier explained)- at the free end : 3D flow pattern which reduces the force !
- Nevertheless : force disturbances are not significant to predict loads onoffshore structures.
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November 2005
Drag Force Changes, ... continued
“End Effects” is the collective name of these three effects.
Elimination of these effects for tests with currents only :
- Thin rigid plate located just below the water surface , the upper plate :
- should be larger than the wave length of the surfacedisturbance wave.
- the wave’s dynamic pressure will be absorbed by the plate, preventingdisturbing the pressure field lower down in the water.
- Thin rigid plate located at its free end , the lower plate :
- forces the flow to go around the cylinder to remain a 2D flow.
Measurement :
- sensors should be placed between the plates- if sensors located far enough from the ends -> no guiding plates needed
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November 2005
Small Objects on The Sea Bed
Types of small object on the sea bed :
- intentionally deployed : - anchors- subsea positioning beacons- military devices- ..... etc.
- not intentionally deployed- overboard cargo- ..... etc.
Preferences :- remain exposed on the sea bed, e.g. subsea beacon- self-burial desired, e.g. Communication cable,
Military device to provide detection
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November 2005
Burial Mechanisms :
- Object hits the sea bed hard enough to form a crater. Crater re-filled by‘conventional’ sediment transport -> small occurrance
- Object sinks into the the soil under its own weight, soil bearing failure. Bed material has to be weak and/or the object is heavy and specially shaped.
- Object got buried due to local erosion and deposition. Main focus !
- Object may be covered or exposed caused by large scale bed mobility,e.g. slow migration of sand banks.
- Sudden large scale bed movements, caused by high storm/earthquake,e.g. Tectonic plate movement damaging transatlantics communic. cables
- Slow build up of excess pore pressure in the soil (loosely packed sand).
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November 2005
Local Morphology
Morphology near the small object is much incommon to the pile case :
- downward secondary flow develops on theupstream side -> horse shoe vortex
- erosion pit will be formed upstream and besidethe object
-> bed material from under the object will fall into the pit-> object support is eroded on the upstream side-> object tumbles forward into its own erosion pit !
Velocity profile
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November 2005
Pore Pressure Build Up and Bed Instability
Surface (storm) wave action able to cause pressure changes on the sea bed surface,which in turn cause minute cyclic soil deformations,
-> loosely packed soil tend to consolidate (reduce volume)-> water will escape from the saturated soil during this process-> low permeability of fine soils and the over supply of pore water
cause the increase in pore pressure
Terzaghi’s rule :
-> Increase of pore pressure leads to decrease of Effective Stress
A limit case : Effective Stress not able to withstand applied load ......
-> condition for Quicksand !!
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November 2005
Pore Pressure Build Up and Bed Instability, ... continued
Floating on Quiscksand :
- object has less density than quicksand (1800 kg/m3)
Examples : - Pipeline filled with air/gas (1300 kg/m3) , float upward througha beach and become re-exposed.
- Electricity or communic. cables (4000 kg/m3) buried in sea bedalways sink.
Occurance of failure : soil’s shear strength (effective intergranular stress) is reduceduntil below the imposed stress level !. Quicksand may notyet fully developed at moment of failure.
Failure occurs intermittent : cyclic wave action stimulate cyclic variations inpore pressure.
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November 2005
Pore Pressure Build Up and Bed Instability, ... continued
Relatively large pressure cycles are needed to build up sufficient pore pressure-> essentially a shallow water phenomenon !!
Precautional measure against sinking of pipelines :
1- Consolidate the sand artificially by backfill the pipeline/cable trench.-> expensive method for pipelines.
2- Backfill the trench wit coarser material providingsufficient soil permeability to prevent pressure build up.
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November 2006
Illustrations regarding SCOUR Forming/Protection around a pile.
CourtesyMrs L. de VosResearcherGent UniversityBelgium.
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November 2006
Illustrations regarding SCOUR Forming/Protection around a pile, …. continued
CourtesyMrs L. de VosResearcherGent UniversityBelgium.
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November 2005
Pipelines
Discussion on the forces of exposed pipelines and sea bed morphology in theirvicinity.
The considered situation :
- pipeline is initially laying on the sea bed- minimum penetration on the sea bed- current flow approx. perpendicular pipeline route- sea bed consists of sand
Flow and forces
The entire approaching flow have to go over the pipeline. The sea bed (wall) prevent the flow to go under the pipeline.
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November 2005
Drag force
- The Cd in such situation is higher than for cylinder in unrestricted flow
- Cd used in conjunction with undisturbed current at pipeline center line elevation
- Drag force is resisted by soil friction, which on its turn is dependent on thevertical force balance.
-Stagnation area on the leading side near the sea bed :
-> very high quasi-static pressure
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November 2005
Lift force
- velocity on the upper side is higher than for isolated cylinder
- pressure on the top side is lower , bottom side is stagnant-> Lift force present -> Lift force conteracts the pipe weight
-Vertical equilibrium : -> Lift force reduces soil contact (friction) force-> pipe will slide before it lifts off the bottom
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November 2005
Lift force, ... continued
- Adding weight to prevent lifting :
- usually most economical by adding high-density concrete coating- for small pipe diameter : increase steel wall thickness
- Change in size and form can affect lift and drag -> optimization !
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November 2005
Morphology
- The approching flow has a velocity profile
- Collision with the pipe object will generate vortex on the upstream (luff) side
- Vortex is present along the exposed pipeline
- The very unsymmetrical cross-section generate on downstream (lee) sidea strong and large one-sided vortex
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November 2005
Self Burial Steps !!
- Upstream vortex-> causes the LUFF erosion
- Downstream vortex-> causes the LEE erosion
- When both trenches further develops -> sand under the pipe becomeunstable, falls in the trences and finally washed away.
Luff and Lee erosion
Tunnel erosion
- Remaining ridge of sand cannot support the pipe’s weight and resist the hydrodynamic pressure differential between upstream and downstream side
-> finally it fails, and the water flows under the pipe.
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November 2005
Tunnel erosion, ... continued
- Flow under the pipe is squeezed-> velocity gradient is locall very high-> bed shear stress is large under
the pipe-> TUNNEL erosion occurs
- The high velocity flow under the pipe reduces the original Lift force due to :- less water flows over the pipe now- pressure on the bottom side is reduced
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November 2005
Pipeline Sag
- At the start of tunnel erosion the pipeloses its support
- Shear forces convey the weight of the suspended pipe segment to adjacentand intact sea bed, and stimulating theirfailure as well
- The tunnel erosion extends along the pipe axis, and the pipe acts as a beam
- Eventually the pipe wil sag under its own weight into its own trenchformed by tunnel erosion
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November 2005
Pipeline Sag, ... continued
- In some cases :
- Pipeline axis (centerline) is at the originalsea bed level, while a narrow tunnel stillexists under te pipe
- If the pipe continues to sink, it blocksless of the original flow -> reduce driving force in the tunnel
- Streamlines under the pipe are getting longer-> increasing the frictional resistance
- At a certain point in this process :-> current in the tunnel become to weak to transport sediment-> tunnel will plugged with sand !!
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November 2005
Repeated Cycle
- If the pipeline is still high enough wrt tothe original sea bed, then the luff and leeerosion will start again
-> new cycle starts
- After each cycle the position of the pipeis lower wrt. to the original sea bed
- If the pipe is deep enough, then its disturbance will be small and local erosionstops
-> any remaining trenches will be re-filled with ambient sediment transport
- Final situation : Pipeline can be buried completely
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November 2005
Tunnel Erosion Stimulation
- Natural burial method is cheap- Goverment regulations co-operative
-> diam. < 406 mm, then must be buried within 1 year
- Smaller lines are weaker and easier to bedamaged
- Naturally buried pipe lines still vulnerable for dragged anchors- Pipelines are buried deeper (using artificial means) to cross a
shipping channel/route
Method for stimulation : Use a spoiler ->
- Blocks the natural flow over the pipe- Stimulate/increase the flow under the pipe- Makes the lift force negative ! -> pulls the pipe down- Drag larger by spoiler
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November 2005
Cover Layers
There are occassions where artificial means are desirable to cover/protectan object on the sea bed,
Examples :
- Cover an expose pipeline or back-fill its trench- Locally cover a pipeline to enable pipeline crossings- Protect a long power or communication cable- Providing intermittent supports for pipelines to cross a
deep valley in the sea bed without sagging too much
In all cases : 1- How to guarantee the stability of a cover or support ?2- How to install the necessary materials, especially in deep waters ?
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November 2005
Stability
- Shields criterion for stability not quite suited in this case
- Roughness of the dumped material often different than the natural sea bed
- The first part of the cover layer (luff side) have to remain stable under thevelocity profile associated to the original sea bed and resulting shear stress
- On the lee side, the original sea bed has to withstand the velocity profilewhich has adapted to the roughness of the cover material
- If cover material is rougher than sea bed, flow is more turbulent !-> Local erosion of sea bed material at the lee side can occur-> Cover material of the ‘trailing edge’ can fall into the erosion pit and lost.
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November 2005
Installation
- In shallow waters : push gravel or stone overboard from a ship- In deeper waters : a Fallpipe has to be used.
The fall pipe is a vertical pipe which extends from the workship to a few meterabove where the material is to be deposited.
The fallpipe can be made of :
- Conventional pipe sections- A series of loosely coupled funnels- A ‘loosely braided hose’ of chain links. This is a porous construction.
During utilization of the fall pipe :
- Ships forward speed and currents will generate drag force on the fallpipe- At the lower end, a remote controlled vehicle with thrusters is present
to place the pipe in the desired position.
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November 2005
Internal Fallpipe Hydraulics
Consider the following fallpipe configuration :
- Impermeable fallpipe- Top end above the sea surface- Still water in the pipe, until sea level
When dumping the gravel in the pipe :
- Hydrostatic pressure of surrounding sea watermatches the static pressure of mixture water+gravel !
- Gravel/stone increases the overall density of mixture-> its level is lower than surrounding sea water !
- Faster dumping of gravel into the pipe, gives higherconcentration of solids in the mixture
-> higher overall density of mixture-> level in the pipe will drop more !
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November 2005
Internal Fallpipe Hydraulics, .... continued
Constant dumping rate :
- Density of mixture also constant- ‘Water’ level in the pipe come to rest- Gravel/stone move downward with falling speed
through still water
Dumped gravel in pipe :- Falling speed
-> first falling through air : high falling speed-> then impact in water : lower falling speed
- Maintaining mass transport :-> concentration of mass (gravel) must be higher
where the speed is lower
- If dumping speed too high, gravel can bridge across the pipe and block !!
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November 2005
Internal Fallpipe Hydraulics, .... continued
Increasing the productivity can be done byletting water flow down the fallpipe along withthe gravel.
The low falling speed in the pipe is a limit forhigh productivity.
Letting water into the pipe :
- Make opening in pipe wall at certain elevations- Use porous pipe (chain links hose)- Simply lower the top end below sea level
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November 2005
Internal Fallpipe Hydraulics, .... continued
Now the particles (gravel/stone) sink through the moving water :
- Flow of water and particles are discharged at the bottom end
- Compared with the closed pipe, for the same rateof material supply :
- concentration of particles discharged is lower- discharge velocity is higher !
- Now higher production is possible without blockingthe pipe
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November 2005
Discharge Morphology
Too high discharge velocity :
- a vertical jet of mixture collide with the sea bed and spreads out
- this local spreading current generates too much shear stress forthe material just discharged -> swept away from the wanted location !
Grain size distribution :
- material consists of different fine and coarse particles- the coarse parts will fall faster than fines- discharged mixture is not uniform :
- first only coarse is discharged- at the tail more fine particles
- Fine particles must able to withstand the discharge jet- A non-uniform discharge mixture can be disastrous for e.g. insulation function
of hot oil pipes.