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November 2005
Chapter 14
Sea Bed Boundary Effects
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
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
November 2005
• Convention on co-ordinate system (widely used in Offshore Engineering) :
Origin Z-axis in Still water level+ Z is upward.
Plan View
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
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
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
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 :
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 :
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 !
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. !
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
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 !!
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)
November 2005
Shear Stress Under Waves Plus Current, … continued
• Projection of Wave velocity components in the current (X,Y ) co-ordinate system :
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 ->
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
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 !
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.
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
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
November 2005
Force Balance, ... continued
Total Force Picture
• Complete force balance cumbersome to carry out
-> A ‘Global’ approach is highly preferable.....
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.
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.
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.
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
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
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.
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
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
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
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
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
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.
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
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
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.
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
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
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
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 !!
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.
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
November 2005
Two Dimensional Approach, ... continued
FACT :
A significant erosion hole on the entire leading side of the cylinder
-> 2D model is not sufficient !
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 !!
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”
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
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
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
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
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
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
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.
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
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
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).
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
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 !!
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.
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.
November 2006
Illustrations regarding SCOUR Forming/Protection around a pile.
CourtesyMrs L. de VosResearcherGent UniversityBelgium.
November 2006
Illustrations regarding SCOUR Forming/Protection around a pile, …. continued
CourtesyMrs L. de VosResearcherGent UniversityBelgium.
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.
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
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
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 !
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
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.
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
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
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 !!
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
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
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 ?
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.
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.
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 !
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 !!
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
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
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.
