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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 the

motions that are considered.

Yaw motion is decreased by the mooringsystem but 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 the

well for drilling operations to be

carried out. This varies betweenoperators, but is usually 5% or 6% of

water depth. Later, other criteria,

based on riser considerations, will bediscussed.

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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 the

wellhead 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.

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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 Acting

on 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

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(i) Wind Force

The following equation is specified by

the American Bureau Shipping (ABS)

and is internationally accepted:

ACCVFshAA ***003388.0

2

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Wind Force

Where:

yaw.andheelbothithwchangesareaThis.ftsurfaces,

exposedallofareaprojectedAessdimensionl

2,-3TablefromtcoefficienheightCessdimensionl

1,-3TablefromtcoefficienshapeC

knotsvelocity,windVlbforce,windF

2

h

S

A

A

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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)

ACCVFshAA***003388.0

2

Then FA = 0.00338 * 502 * 1 * 1 * 50 * 400

FA = 169,000 lbf = 169 kips

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(i) Wind Force - example

VA

= 50 (wind velocity, knots)

1 knot = 1 nautical mile/hr

= 1.15078 statute mile/hr

ACCVFshAA***003388.0

2

1 nautical mile = 1/60 degree = 1 minute

= 6,076 ft

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Where:

AVCgF 2cscc

4

2

c

2

c

s

c

ft

sec*lbft1g

ftarea,projectedA

ft/seclocity,current veV1)-3(Table

tcoefficienwindtheasSameess.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) Bow Forces:

L0.332Tfor

4

22

bowT

LBH273.0F

T = wave period, sec

L = vessel length, ft

H = significant wave height, ft

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Where:

ftdraft,vesselD

ftlength,beamvesselB

ftlength,vesselL

ftheight,t wavesignificanH

lbforce,waveF

secperiod,waveT

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Bow Forces:

L0.332Tfor

4

22

bow)TL664.0(

LBH273.0F

NOTE: Model test data should be used

when available

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Beam Forces:

2DB0.642Tfor

4

22

beamT

LBH10.2F

NOTE: API now has Recommended

Practices with modified equations

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Beam Forces:

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 and

Its UseThe Mooring Line

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The Mooring Lines Resist the

Environmental Forces

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Station Keeping

1. In shallow water up to about 500

feet, a heavy line is needed,

particularly in rough weather areas.

2. Chain can be used (but may not be

advisable) to water depths of about

1,200 feet.

3. Composite lines may be used to

~ 5,000 feet.

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Station Keeping

4. Beyond about 5,000 feet, use

dynamic positioning

5. Calm water tension should be

determined to hold the vessel

within the operating offset under

the maximum environmental

conditions specified for operation.

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Station Keeping, Continued

6. Once the riser is disconnected, the

vessel heading may be changed todecrease the environmental forces

on 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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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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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.

ODP uses dynamic positioning.

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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

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Fig. 3-23. Simple position-referencing system

WH1 = WH2= WH3WH1 = WH3WH2 > WH1 ,WH3

W

H1

H2

H3

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To understand the operating principles

of acoustic position referencing, assume

that:

1. The vessel is an equilateral

triangle.

2. The kelly bushing (KB) is inthe geometric center of the

vessel.

Acoustic Position Referencing

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3. The hydrophones are located

at the points of the triangular

vessel.

4. The subsea beacon is in the

center 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.