Drilling Riser[1]

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TAMU - PemexOffshore Drilling

Lesson 5A

The Drilling Riser

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Lesson 5A - The Drilling Riser

Riser Components Riser Tensioning Fatigue Kill/Choke Lines Inspection & Maintenance Reentry

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Riser system for a floating drilling rig

BOP

RISER JOINTSKILL AND CHOICE LINES

SLIP JOINT OUTER BARREL

UPPER BALL JOINT

SLIP JOINT INNER BARREL

RISER TENSION

LOWER BALL JOINT

Mudline

Marine riser = drilling riser, get returns to surface, well control, communications link

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Integral Marine Riser JointsChoke and kill lines are integral with the marine riser , flanged connections; clamp, etc

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Integral Marine Riser Connector

Flanged connection - 6 bolts

NOTE: Choke and Kill Lines

O-ring type seals- inspect when running

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Integral Marine Riser Connector

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Marine Riser

Flexible Joint

Flexible joint, binding from high tensile forces, inject lubricant, up to 1,500,000 lbf!

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Upper Section Marine Riser System.

MARINE RISER DIVERTER ASSEMBLY

RISER TENSIONING

LINESTELESCOPIC JOINT

KILL AND CHOKE LINES

FLEXIBLE JOINT

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Control valve

Vent line

Mud returns

L.P. Annular

A diverter system.Re-directs flow from rig floor to blooey line (10”+), downwind, do not shut in, erosion -108

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Figure 6-6.

Subsea BOP Stack

Vertical steel loops used for kill / choke line transition around the ball joint.

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The Drilling Riser

Schematic diagram of riser with imposed forces

MEAN WATER LEVEL

RISER ELEMENT

Optimum riser tension to minimize damage to riser and wear-and-tear on tensioners, sag

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Str

ess

in r

iser

, ki

ps

Applied tension in riser, kips

Maximum stress

Minimum stress

Tension in riser must be not too low and not too high. Set at 118 kips - will fluctuate 100-136 kips ~ 15%

Mean tension

Insufficient Tension

(too much sag)

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Combined Effect of Mean Stress and Alternating Stress

Consider: Max Stress of 40 ksi

Alternating Stress: 10 or 20 ksi

Mean Stress: ?

Life: ?

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Mean stress, ksi

Modified Goodman diagram

Alt

ern

atin

g s

tres

s, k

si

30 10 (20-40 ksi) => 106; 20 20 (0-40 ksi) => 105 cycles. If Mean stress is high, alt. low

Yield strength

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Vessel offset, % of water depth

Max

. ris

er s

tres

s, k

si

Need adequate tension: Tensioned to 225 kips for 6% offset: at 3%, 11 ksi; at 10%, 30 ksi.

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The effects of losing one-sixth tensioning capacity on the riser system of previous slide

Max

. ris

er s

tres

s, k

si(196 * 5/6 = 163)

Need adequate tension: Tensioning to 296 kips looks the most advantageous.

(225 * 5/6 = 188)

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Estimating Pressure Dropin Choke Lines

It is important to be able to determine if the pressure drop in the choke line will be a problem (excessive).

Most drilling fluids are Non-Newtonian.

The Bingham Plastic or Power-Law models may be used. More about this later.

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Riser Considerations

Riser Metallurgy is very important. Correct heat treatment is

essential. 80,000 psi min. yield strength and

good toughness is preferred. Preheating, welding & normalizing

after welding is critical for riser integrity & long life.

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Riser Considerations – cont’d

No Field Welding on Riser!

Fatigue of riser cannot be measured prior to some indication of failure.

Routine inspection required.

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Riser Considerations – cont’d

Fatigue is an embrittlement of the metal. It often starts in the vicinity of welds or other places of high stress concentration.

Fatigue is caused by cyclic loading.

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Riser Inspection

Visual Inspection of the riser should occur every time the riser is run.

Check all the sealsCheck all the sealing areas

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Riser Inspection – cont’d

A complete inspection should be made annually.

Dyes: Will detect cracks. To use dyes, paint must be removed.

Magnetic Particle Inspection: Sand blast areas around welds prior to magnetic particle inspection.

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Riser Inspection – cont’d

Ultra-Sonic Inspection: May detect cracks below the surface. This test is run inside pipe. Paint removal is not necessary.

X-Ray Inspection: Is for cracks inside the metal. It may miss surface cracks.

No one technique will find all the cracks.

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Riser Instrumentation

Heave Gauge: Pointer attached to guide line moving in front of graduated board.

Riser Angle Indicator (at ball joint):

2/1y

2x

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2/12y

2x

tantantan

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Riser Instrumentation – cont’d

Accurate Positioning System:

For detecting and monitoring vessel position.

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Ball Joint

A Ball Joint Angle > 4 degrees is an indication that something is wrong!

Vessel has excessive offset

Riser tension is inadequate

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Ball Joint – cont’d

Must decrease ball joint angle before operations are resumed.

Remedial Action:Decrease OffsetIncrease Riser Tension

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Vessel to Seafloor Guidance System

Guidelines are used for guiding equipment from the vessel to the seafloor (except in deepwater)

Selection and care of guidelines is critical

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Vessel to Seafloor Guidance System – cont’d

Guidelines should not be tensioned beyond 1/3 of breaking capacity

But…inadequate tension is the most common cause of failure in guidelines

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Vessel to Seafloor Guidance System - cont’d

Tension should be maximum when landing the BOP stack, or when landing the riser onto the stack.

When the guidelines are not being used to run equipment, tension may be slacked off to ~ twice the weight of the line in seawater.

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Table 6-1. Recommendations for

Conventionally Used Guidelines

As water depth increases, larger diameter guidelines must be used.

Higher tensioning is required.

Don’t forget to limit tension to < 1/3 of breaking strength.