2. station keeping

7/31/2019 2. Station Keeping

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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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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



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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



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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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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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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.




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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.



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Diagram of controller operations.

2. station keeping

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