W4- O&G Multiphase Flow & Assurance

Question 1

Flow assurance

Answer 1

How fluids interact in different operations

Question 2

Reservoir types

Answer 2

NG, gas condensate, crude oil, heavy crude

Question 3

Fluid characterisation

Answer 3

Governs material and equipment specs, affecting P drop, heat transfer, pipe corrosion, deposits

Question 4

Thermodynamic properties

Answer 4

Bubble point
Dew point
2 phase region
Cricondenbar: max pressure above no gas will form
Cricondentherm: max temp above which no liquid will form
Critical point: liquid and gas phase properties equal

Question 5

Fluid flow & effects of gravity

Answer 5

Gravity affect flow of liquids more than gases
Inclination affects multiphase flow
Gravity alters local liquid hold-up (from velocity changes)

Liquid accumulation occurs at low pipeline points due to gravity and inclination
Liquid velocity decreases in upward flow, but downward flow causes liquid acceleration
Gravitational effects less critical at high velocities due to inertia

Question 6

Liquid hold-up

Answer 6

Ratio of liquid to total volume

Question 7

Gas hold-up

Answer 7

Ratio of gas to total volume

Question 8

What will be the liquid hold up for equal flow rates of liquid/gas

Answer 8

HL > 50% as gas will slip through due to greater velocity (lower viscosity, no internal friction

Question 9

Pipeline angle effect on liquid hold up

Answer 9

Upward flow have higher liquid hold-up as gravity slows liquid down and densities differ vastly

Question 10

Superficial velocity

Answer 10

velocity of fluid if occupied by full cross sectional area:
U= Q/A
HG + HL = 1

Question 11

No-slip (homogeneous) assumption

Answer 11

Gas and liquid travel at same velocity (faster gas phase doesn’t slip past liquid)

No-slip hold up: Q/Qtot

Question 12

Vertical flow regimes

Answer 12

Increase gas flow:
Bubbly: distribution of bubbles throughout liquid

Slug: gas flow rate increased where bubbles coalesce to form slugs

Churn: churning of irregularly shaped portions of gas & liquid

Annular: liquid flowing at wall and gas in core, with liquid droplets (entrained droplets do not coalesce)

Question 13

Horizontal flow regimes

Answer 13

Increase gas flow:
Bubbly: bubbles confined at top of pipe, liquid below

Plug: bubbles coalesce to form larger bubbles

Stratified: gas plugs join to form a continuous gas layer on top

Wavy: waves on surface of liquid from interfacial shear stress

Slug: waves grow until crest at top pf pipe and gas breaks through, with liquid distributed over wall

Annular: liquid flowing at wall and gas in core, with liquid droplets (entrained droplets do not coalesce)

Spray/mist: majority of liquid dispersed as droplets in gas core, liquid film very thin

Question 14

Horzonal flow effects of pressure, inclanation, viscocity

Answer 14

Pressure: increased pressure, less slug formations

Upward inclanation- slug formation amplified as gas flow easily up liquid

High viscosity- slug formation area amplified

Question 15

Vertical flow regime effect of pressure

Answer 15

Increased pressure, less slug formation area

Question 16

Total pressure drop over a pipeline

Answer 16

Delta PT = Delta Pf + Pel + Pacc

Single phase Delta Pf = f(L/d)[(p*mu^2)/2]

Question 17

What force dominates for different flow rates at high pressure losses?

Answer 17

Low flow: Gravity dominated

High flow: friction dominated

Question 18

Elevation pressure loss, Delta Pel

Answer 18

Delta Pel = psgL*sin(a)

For slip: ps = pLHL + pGHG
L = segment length
a = segment angle v horiztonal

Question 19

Acceleration pressure loss, Delta Pacc

Answer 19

Pacc = (1/pL){(m/A)^2]{[(1-xe)^2/(1-a)] +[(xe^2pL)/(aPg)]}

Question 20

Frictional pressure loss, Delta Pf (no slip)

Answer 20

Delta Pf = fms(L/d)[(Pns*Vm^2)/2]

pms = /\LpL + /\GpG

Question 21

Frictional pressure loss due to fittings

Answer 21

Delta Pf = K[(pV*m^2)/2]

Question 22

Frictional pressure loss, Delta Pf (slip)

Answer 22

Delta Pf = ftp(L/d)[(pns*Vm^2)/2]

ftp = mtp*fns

Question 23

Multiphase flow models/correlations categories

Answer 23

1) No slip + no flow regime considered (LM & Homogeneous flow model)

2) Slip + no flow regime considered (two phase friction factor and H-B liquid hold-up correlation)

3) Slip + flow regime considered (computational)

Question 24

Slugging and types of slugging

Answer 24

Unstable multiphase flow of intermittent L+G surges

Hydrodynamic & terrain-induced

Question 25

Hydrodynamic slugging

Answer 25

Induced from flow regime
L+G interactions induce instabilities (liquid waves) spreading to entire pupe by merging and growth

Question 26

Terrain-induced slugging

Answer 26

Formed from inclination (hold-up changes) and elevation pressure drop.

Liquid waves form slugs and merge.

At low points, liquid hold up increases so area for gas to flow decreases meaning instability to liquid

Question 27

Sever riser slugging

Answer 27

Special case of terrain-induced slugging at low points of inclined and vertical riser section.

1) Riser fills with liquid as gas pressure too low to overcome liquid hydrostatic pressure developing
2) Slug growth and gradually blocks riser- obstacle causing upstream gas pressure build up until riser full and begins to empty
3) Riser backpressure (due to gas) increases due to blockage and empties - gas pushes liquid out riser more rapidly as hydrostatic head decreases
4) Force balance shift causes large liquid slugs up riser volume, then large gas surges. Gas blowdown

Question 28

Pipeline operation slugling

Answer 28

Transition phenomena in operations- during flow or pressure changes (start-up/shut down) liquid hold up changes and creates slugs

Question 29

Pigging

Answer 29

Where pig removes wall deposits against corrosion and pressure drop

Interference with waves causes slugs

Question 30

Slugging mitigation

Answer 30

Early routine decisions
Line minimisation
Riser base life gas injection
Control systems

Question 31

Slug catchers

Answer 31

Downstream of pipelines to absorb impact of large liquid surges associated with slugging

Large volume to allow sufficient residence time to settle and withstand large hydraulic forces (thick walls & long, parallel pipes)

Question 32

Pipeline insulation methods

Answer 32

Pipe in pipe (PIP) insulation: insulation in outer pipe

Multilayer insulation: layers of different materials

Bundle flowline insulation: several fluid pipes in a larger pipe

Pipeline burial: partial/full burial in seabed

Question 33

What can reduce insulation of pipes?

Answer 33

Deep-water hydrostatic pressure allows water to seep into it, reducing effectiveness

Question 34

Heat losses between reservoir/pipeline/facility temps

Answer 34

Weak points- manifolds, joints, valves

Riser heat losses

Manifold heat losses

Pipeline heat losses

Question 35

Active heating and stabilisation of pipeline temp methods

Answer 35

Indirect heating of the pipeline

Inductive heating:

Direct electrical heating: