2. station keeping
TRANSCRIPT
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TAMU PemexOffshore Drilling
Lesson 2
Station Keeping
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Lesson 2 - Station Keeping
Environmental Forces
Mooring
Anchors
Mooring Lines
Dynamic Positioning
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StationKeeping
The ability of a vessel to maintain
position for drilling determines the useful
time that a vessel can effectivelyoperate.
Stated negatively, if the vessel cannot
stay close enough over the well to drill,
what good is the drilling equipment?
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Station Keeping - contd
Station keeping equipment influences the
vessel motions in the horizontal plane.
These motions are: surge, sway, and
yaw. Generally, surge and sway are themotions that are considered.
Yaw motion is decreased by the mooringsystem and is neglected in most mooring
calculations.
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Station Keeping
When investigating or designing a
mooring system, the following
criteria should be considered:
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Operational Stage
1. The vessel is close enough over thewell for drilling operations to be
carried out. This varies betweenoperators, but is usually 5% or 6% ofwater depth.
Later, other criteria, based on riserconsiderations, will be discussed.
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Non-operational but Connected2. The condition from the operational
stage up to 10% of water depth:
Drilling operations have been stopped,
but the riser is still connected to thewellhead and BOPs.
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Disconnected
3. The riser is disconnected from the
wellhead and the BOPs, and the
vessel can be headed into the seas:
Displacement > 10% of water depth
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Station Keeping - contd
Example
Water Depth
= 1,000 ft
Drilling: 50-60 ft
Connected:
100 ft max
1,000
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Environmental Forces Actingon the Drilling Vessel
(i) Wind Force
(ii) Current Force
(iii) Wave Force
These forces tend to displace the vessel
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The Station Keeping System
Must be designed to withstand the
environmental forces
Two types:
Mooring System (anchors)
Dynamic Positioning (thrusters)
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(i) Wind Force
The following equation is specified bythe American Bureau Shipping (ABS)
and is internationally accepted:
ACCV00338.0Fsh
2
AA
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Wind Force
Where:
yaw.andheelbothithwchangesareaThis.ftsurfaces,
exposedallofareaprojectedAessdimensionl
2,-3TablefromtcoefficienheightCessdimensionl
1,-3TablefromtcoefficienshapeC
knotsvelocity,windVlbforce,windF
2
h
S
A
A
ACCV00338.0F sh2
AA
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Table 3-1. Shape Coefficients
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Table 3-2. Height Coefficients
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(i) Wind Force - example
VA
= 50 (wind velocity, knots)
Ch = 1 (height coefficient)
Cs = 1 (shape coefficient)
A = 50 * 400 (projected target area, ft2)
Then FA = 0.00338 * 502 * 1 * 1 * 50 * 400
FA = 169,000 lbf = 169 kips
?
ACCV00338.0Fsh
2
AA
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(i) Wind Force - example
VA = 50 (wind velocity, knots)
1 knot = 1 nautical mile/hr
= 1.15078 statute mile/hr
1 nautical mile = 1/60 degree = 1 minute
= 6,076 ft
ACCV00338.0Fsh
2
AA
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Where:
AVCgF2cscc
4
2
c
2c
sc
ft
sec*lbft1g
ftarea,projectedAft/seclocity,current veV
1)-3(TabletcoefficienwindtheasSameess.dimensionlt,coefficienragdC lbforce,dragcurrentF
(ii) Current Force
lbf
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Fc = 1 * 1 * 22 * 30 * 400
Fc = 48,000 lbf = 48 kips
(ii) Current Force - example
Vc = 2 (current velocity, ft/sec)Cs = 1 (shape coefficient)
A = 30 * 400 (projected target area, ft2)
AVCgF2cscc
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(iii) Wave Forces - (a) Bow Forces:
L0.332Tfor
4
22
bowT
LBH273.0F
T = wave period, secL = vessel length, ft
B = vessel width, ft
H = significant wave height, ft
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Bow Forces - contd
L0.332Tfor
4
22bow
)TL664.0(
LBH273.0F
NOTE: Model test data should be used
when available
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(iii) Wave Forces - (b) Beam Forces
2DB0.642Tfor
4
22beam
T
LBH10.2F
NOTE: API now has Recommended
Practices with modified equations
Where D = vessel draft, ft
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Beam Forces - contd
2DB0.642Tfor
4
22
beam )TD2B28.1(
LBH10.2F
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Figure 3-1. The catenary as used for mooring
calculations.
Floating Drilling: Equipment andIts Use
The Mooring Line
T
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The Mooring Lines Resist theEnvironmental Forces
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The equations used for mooring calculations for
one single weight line are:
H
xwcoshw
Hy
tanHHTV
)T/H(cos
TcoswdTH
22
1
The Shape of the Mooring Line:
T
H
V
cosh z = (ez + e-z)/2
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sAxL
)tan(secln
w
H
H
VTln
w
Hx
tan
w
H
w
Vs
More equations used formooring calculations:
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Where:
ftline).waterabovefairleaderoutboardofheightinclude(shoulddepthwaterd
ftlength,linesuspendedslb/ftlength,unitperweightlinew
T.ofegiven valuanyforlinesuspendedtheoflengthover theconstant
isHlb.force,restoringhorizontalHdegreeshorizontalthe
respect towithlinetheofanglelbline,theoftensionT
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ftlength,linemooringtotalA ftanchor,theto
vesselthefromdistancehorizontalLftH,forcefor theaccounttoused
conditionboundaryonaltranslatiaH/wftseabed,the
toucheslinetheepoint wherthetovesselthefromdistancehorizontalx
ft,w/Hdordinatey
and:
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Station KeepingTable 3-4. Example of Single Line
Restoring Forces
Try to duplicatethis Table
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T H
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Figure 3-2. The effect of changing line weight--
single-line calculations.
Too Hard
Looks OK
Too Soft
S
ingleLineRe
storing
Force
,kips
Offset - Percent of Water Depth
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Figure 3-3. The effect of changing initial tension only--single-line calculations.
Effect of Initial TensionWater Depth - 500 ft Chain
- 2 in., 42.6 lb/ft Initial
Tension
( KIPS )
SingleLine
Restoring
Force,
kips
Offset - Percent of Water Depth
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Figure 3-4. The effect of changing water depth only;
single-line calculations.
Effect of Water Depth
Initial Tension - 30 KIPS
Water Depth
, ft
Wire Rope
3 in.18.6 lb/ft
S
ingleLineRestoring
Force
,kips
Offset - Percent of Water Depth
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Station Keeping
1. In shallow water up to about 500feet, a heavy line is needed,
particularly in rough weather areas.
2. Chain can be used (but may not beadvisable) to water depths of about1,200 feet.
3. Composite lines may be used to~ 5,000 feet.
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Station Keeping - contd
4. Beyond about 5,000 feet, usedynamic positioning
5. Calm water tension should bedetermined to hold the vesselwithin the operating offset underthe maximum environmentalconditions specified for operation.
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Station Keeping - contd
6. Once the riser is disconnected, the
vessel heading may be changed todecrease the environmental forceson the vessel.
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Station Keeping
Typical Mooring Patterns for Non-
Rectangular Semis
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Typical Mooring Patterns for Ship-
Like Vessels and Rectangular Semis
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Typical 8-line Mooring Pattern
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Table 3-5. Effects of Mooring Line Patterns
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Figure 3-8. Drag anchor nomenclature.
crown
Anchor shackle
fluke
stock
Crownpad eye
shank
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1. Be able to calculate the total
restoring force and tension in the
most loaded line vs. offset.
2. Be able to handle a minimum of
ten mooring lines.
3. Be able to handle composite line
data for wire rope and chain.
Mooring Program should...
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Setting Anchor with Workboat
Anchor before touching bottom
Drilling vessel winching-in cable
Pendant
Mooring LineFluke
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6 strands, 19 wires per strand
Strand Construction for Mooring Lines
( IWRC - Independent Wire Rope Core )
T bl 3 7 Wi R S ifi ti
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Table 3-7. Wire Rope Specifications
6 x 37 Bright
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Wire Rope Specifications
6 x 37 Bright
Diameter
in
1
2
3
3.5
Weight
lbs/ft
1.85
7.3916.6
22.7
Strength
tons
49.1
190414
555
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Fatigue Life of 3/4 Wire Rope
Load = 30% of breaking strength: Life = ~105 cycles
Load = 20% of breaking strength: Life > 4*106 cycles
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Figure 3-15.
Chain Nomenaclature.
Stud Link Chain
Stud keeps chain from collapsing
3 chain has breaking strength > 1,000 kips!
WireDia.
Pitch
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Chain Quality Inspection
Chain quality needs to be inspected
periodically, to avoid failure:
(i) Links with cracks should be cut out(ii) In chains with removable studs, worn
or deformed studs should be
replaced(iii) Check for excessive wear or
corrosion
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Table 3-10. Table for RenewingStud-Link Chain
For 3 chain, renewal dia. = 2 11/16
Fi 3 18 T i l i
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A chain is only as strong as its weakest link
Figure 3-18. Typical wire rope
connection to chain.
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Wire Rope location for barge
Wire Line
Tensiometers
MooringWinches
Outboard
Fairlead
Read tension while moving slowly
S i K i
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Station KeepingFigure 3-20. Drum
Capacity andminimum drum-to-
sheave spacing
Rd > 200 dwire
= 1.5o (smooth)
= 2.0o
(grooved)Rd
Figure 3 21 Deck machinery
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Figure 3-21. Deck machineryarrangements for ship-like vessels.
Chain mooring requires a wildcat & chain stopper.Tension is usually measured with a load cell.
ChainStopper
DualWildcat
Figure 3 22 Typical chain wildcat and
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Figure 3-22. Typical chain wildcat andfairlead locations on a semi.
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Dynamic Positioning
Dynamic positioning uses thrusters
instead of mooring lines
to keep the vessel above the wellhead.
Glomar Challenger used dynamic
positioning as early as 1968.
The Ocean Drilling Program (ODP)
uses dynamic positioning.
Ad t f D i P iti i
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Advantages of Dynamic Positioning
(i) Mobility - no anchors to set or retrieve
- Easy to point vessel into weather
- Easy to move out of way of icebergs
(ii) Can be used in water depths beyond
where conventional mooring is
practical
(iii) Does not need anchor boats
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Disadvantages of Dynamic Positioning
(i) High fuel cost
(ii) High capital cost (?)
(iii) Requires an accurate positioning
system to keep the vessel above the
wellhead.
Usually an acoustic system - triangulation
Simple position-referencing system
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Simple position-referencing system
WH1 = WH2= WH3WH1 = WH3WH2 > WH1 ,WH3
W
H1
H2
H3
Acoustic Position Referencing
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To understand the operating principlesof acoustic position referencing, assume
that:
1. The vessel is an equilateraltriangle.
2. The kelly bushing (KB) is inthe geometric center of the
vessel.
Acoustic Position Referencing
Acoustic Position Referencing
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3. The hydrophones are locatedat the points of the triangular
vessel.
4. The subsea beacon is in thecenter of the well.
5. No pitch, no roll, no yaw and
no heave are permitted.
Acoustic Position Referencing
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Diagram of controller operations.