Rig Selection.pdf

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You should be able to:

• List types of rigs

• List selection criteria for various rig types

• State site preparation requirement prior to mobilizing

a rig onto a location

• Rig sizing

Rig Selection

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Rig selection involves effort of many groups in the up

stream sector.

A typical scenario of actions performed leading to rig

selection are as follows

:

 Geologist develops a prospect and define the

desired well location(s)

 Surveys or spots exact location of well on land or

coordinate of well location offshore

 Land acquisition for land based operation/location

of well/site preparation

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 Water depth, seabed soil condition, near

seabed seismic results

 Drilling engineer selects the rig

 Drilling rig owner (Contractor) defines the rig

sizing requirements, rig weight, loads to be

handled, drilling fluid volumes, rig power

requirements depending upon type of well

i.e Exploratory well and development well

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Drilling Rigs can be divided

into two main groups;

 Marine Rigs used for drilling

on water

 Land Rigs used for drilling on

land

 Cable Tool Rigs no longer in

operation are used for drilling

shallow wells on land

CABLE TOOL RIGS

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Drilling rigs used offshore are generally termed

marine rigs

Marine rigs are further grouped into:

Bottom supported rigs. Rigs rest on sea floor or on

pads built on the sea floor

Floating rigs where drilling operations are conducted

while the rig is in floating position

Drilling rig mounted on barge. Typically used for

drilling in 8-10 feet of water depth and self contained

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Land Rig Swamp Barge 10 ~ 30 ft Tender Assisted 30 ~ 400 ft Semi-submersable 7500 ft max Jack-up 450 ft max Drillship 10000 ft max

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Rig Selection

Rig Selection Process ~ Land and Shallow Waters

Land

Rig

Swamp

Barge

Jack-up

Tender

Assisted

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Rig Selection Process ~ Deep Water

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Major Rig selection criteria are as follows;

 Water Depth rating

 Rig capacity, bulk capacity, liquid and mud mixing

capacity

 Derrick, sub structure, drilling envelope

 Physical rig size and weight

 Stability in rough water

 Duration of drilling program

 Type of drilling i.e Exploration or Development

 Availability and cost

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Water Depth rating

 Primary consideration for Rig selection

 Selection based on Bottom Supported Units and

Floating Rigs

 Size of rig with respect to Drilling Equipment Set

(DES)

C

ommonly is never an issue of selection. Marine Rigs

are over-specified to meet a wide range of depth rating

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Bottom Supported Units consists of

Jack Up rigs

Limited to 400 feet water depth. Typical water depth from minimum 25 feet to maximum 300ft

Most widely used marine rig for both stand alone drilling of exploratory wells and multi-wells development drilling from jackets

 Common used independent legs cantilever jack-ups which can cover 9 – 15 wells on a jacket or small platform depends upon the drilling envelop. The derrick and substructure is skidded out on cantilever.

 Individual legs penetrate into below sea bed

 Popular designs are Baker Marine 300C, Marathon Le Tourneau 116C and Friede Goldman L780 mod II

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Bottom Supported Units consists of (continued) Jack Up rigs

 Mat type Jack up – the drilling hull is supported by legs from large mat/pontoon that rest on sea floor. It’s used for very soft sea floor soil condition

 Slot type jack up rig where drilling of wells done through the slot in the hull of the rig

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• Bottom Supported Unit

consist of Jack Up rigs

• Jack up rigs are self

contained and most are

for drilling depths of

25,000 feet

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Marine Rig Sizing and

Selection

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Rig Selection

Jack-up Risks

Punch Through Effect of RPD

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Bottom Supported Units consist of

Platform rigs

 Long term development drilling

projects from sufficiently large

platform (> 15 wells)

 Self contained, complete rig with

facilities are installed on the

platform

 Cost effective and limited only by

water depth limitation of the

platform

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Bottom Supported Units consists of

Platform rigs

 Variation is a Tender Assisted Platform Rig where

Drilling Equipment Set (DES) is positioned on the

platform. Prime movers, living quarters, rig pumps,

mud tanks on a floating tender anchored along side

 Water depth limited by the anchoring capacity of the

tender

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Floating rigs (Floaters)

• Water depth capability slowly increased to 7000 feet • Floating rigs do not rest on the sea floor

• Not restricted by rigs leg length

• Drillship and semi-submersible rigs • Different operating characteristics • Drillships are usually self propelled

• Semi submersible have lower hull. Below sea level and ballasted to maximize rig stability. More stable than drillship and some are self propelled

• Lower variable deck loading than drillship

• Specially designed for petroleum operation hence more costly

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Semi Submersible

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Marine Rig Sizing and Selection

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Floating rigs

1. Deepwater capability using dynamic positioning system

(DPS). Anchoring systems are not required

2. DPS maintains rig position by thruster and acoustic

beacons.

3. Power is provided by;

• AC (alternating current) generator with silicon controlled

rectified (SCR) to provide DC to the drilling rig. [AC generator SCR DC motor Rig component]

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Other Considerations for Offshore Location Are

 Check for subsea pipelines, marine cables,

telephone lines, shipwreck

 Usually sea bed features e.g slumping, steep

inclines, unusual debris at sea floor

 Very soft sea bed soil condition, low

anchor-holding capability

 Shallow gas

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If problems cannot be resolved, alternate rig site should be selected site survey studies proposed for Jack up leg investigation are as follows;

 Side scan sonar for sea bed features, debris, boulder and pipeline  High resolution shallow seismic for shallow gas. Correlation with

soil bore data

 Soil bore cores analysis to deepest expected penetration and for platform installation

 Penetrometer usually 3 feet in length to estimate undrained strength of sea bed clays/formation for Jack up leg penetration analysis

 Echo sounder for water depth determination

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Some common foundation problems are;

 Punch through during preload

 Inadequate leg length

 Scouring due to strong seabed currents and soft soil

 Seafloor instability

 Unable to extract legs

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 Most offshore rigs are rated /sized to drill well depth

of 25000 ft. One rig can be used to drill various type

of well and well depths

 Functionally, Offshore rigs becomes over specified

and rig sizing is not an issue

 Rig sizing are more pertinent to land drilling.

Specifications are tailored to suit drilling well depths

and well condition

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Land Rigs are further categorized depending upon;

 Conventional (unitised) rigs

 Trailer mounted rigs

 Helicopter transportable rigs (heli-rigs)

 Desert rigs

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 Generally uses telescopic mast

 Restricted to light work and medium depth drilling to 10,000 feet

 Generally mounted on a truck or large trailer. Available in drive-in or back in unit

 Low rig down, move and rig up time increase efficiency and lower cost

 Generally used, for land workover and well servicing jobs

 Usually limited in mast capacity (350 kips), limited rig equipment capacities

 Some rigs have “doubles” masts.

 Limited height of rig floor require cellar to

accommodate height of higher rated BOP stack

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 Largest land rigs are available with derricks or big jack

knife mast

 Rated for drilling 10,000 to 35,000 feet well depths

 Rig components are torn down and moved individually

on trucks due to size

 Rig mounted on a sub-structure to allow use of tall, high

pressure rated BOP stacks, large pipe stand-back

capacity

 Most rigs have 142 feet derrick or mast and able to pull

(3) joint stands of drill string

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After well site located, rig site preparation depends

upon;

 Onshore : Marsh,terrain/topography

 Offshore

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Onshore Rig site Preparation

 Well location usually vertical on sub-surface target location  Land survey “staking the well”

 Access road, land acquisition and land compensation, permits  Soil survey to check marshy or soft soil to take load of the rig  Require an area of 350 feet x 420 feet area to be cleared

 Water source for drilling water well

 Sometimes major civil engineering work is required

 Barge rig for marshy location require dredging channel to bring barge in

 Filling up or small platform to take rig

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 Main power requirements of a drilling rig are

the drawworks, rotating system, rig mud

pumps and power for rig ancilliaries

 Modern rigs are designed to meet minimum

rig power requirements to run drawworks.

Both pumps running in parallel and the

topdrive/rotary table in operations. These are

driven by DC motors

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 Rig ancilliaries consist of centrifugal mud pumps to run mud treatment equipment, rig lighting, air compressor motors, BOP accumulator and etc. Usually they require AC current on land rigs and these power requirement are met by AC generators from the utility house

 Modern rigs offshore has prime movers driving AC

generators where the power is transmitted to the drilling equipment DC generators via a AC-SCR system (SCR). Alternating current silicon controlled rectifier system. Older land rigs have mechanical engine compounds or have DC generator driving the DC motors called a DC-DC system

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Mechanical Drive System

 Commonly used for trailer-mounted rigs of medium

depth drilling range

 Two and three prime movers (diesel engines) are

compounded by chain, gears and belts to drive

drawworks and pumps

 Torque converters at the engine output are used to

reduce shock loading on engines. Provides torque

multiplication and constant

power output

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A TYPICAL AC-SCR-DC SYSTEM

As shown in schematically below :

Prime Mover (s) AC generator SCR system

Rig Ancilliaries (AC motors)

Drawwork Drive DC motor

Rig Pump Drive DC motor

Top Drive DC motor

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Total Rig power required at the Prime Mover can be presented by ;

HP _rig = HP _H+ 2 x HP _P + HP _RT

where

HP _rig = Total rig power required at the SCR power outlet

HP _H = Power required by the hoisting system at the input of the drawworks HP _P = Power required by each pump at the input

HP _RT = Power required by the rotary table or top drive system input

Assuming the SCR and electrical transmission system efficiency at 0.90 (range 0.85 - 0.90)

HP _RT

HP _engine = _{_}______ _{= 1.11 HP}

RT where = efficiency of the prime mover

0.90

Total HP required = HP _engine x 1/

API standard 7B-11C defines diesel engine performance variation resulting from harsh environment

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 Hoisting system provides the means for the vertical movement of the pipe in the well

 It consists of the drawworks, crown and travelling

blocks, wireline and ancilliary equipment such as hooks, bails and elevators

 The horse power required at the Travelling Blocks can be computed by

HP_TB= L x V 33,000 _B

Where L = heaviest hook load (lbs) V = hoisting rate (ft/min)

normally assumed 93ft/min

B=friction factor of the block

and tackle system

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As with all mechanical system, the block/tackle system is not friction less i.e _B< 1.0

Friction factor _B= (0.98)n _{where n = number of sheave pulse}

The following table indicates friction _B for various pull system ;

No of lines _B 6 0.886 8 0.850 10 0.817 12 0.785

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Drawworks reels in wireline as the pipe is lifted and thus is made up of drum to spool the wireline, shafts and chain driving the drum

A typical drawworks consists of four shafts and five chains and efficiency is given by

D = (0.98)n where n = number of chains and shafts

Therefore _D= (0.98)4+5 _{= 0.834}

Therefore HP _H horsepower required at the input of drawworks

HP _H= HP _TB = ____L x V_____

D 33,000 x B x D

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The hook load L is normally taken as the heaviest casing load in mud. Usually 9-5/8” casing represents the heaviest string.

Therefore ;

L = buoyant unit weight of casing in mud x length of casing

Mud Buoyancy factor is calculated by

B _F= 1 - _m/ 65.50 where _m = mud weight in ppg

L = B _Fx W _C x H where B _F= mud buoyancy factor

W_C= unit weight of casing Ibs/ft

H = total length of casing run (feet)

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 Rotating system impacts rotating action to the drillstring and bit  Rotary system consists of the kelly, rotary table and drive bushing  Top Drive system consists of the top drive (DC motor and gear

box), drill string and bit

 Rotating horse power requirements depend on speed of rotation, hole friction, angle, depth straightness. Basically it can be given by

HP _RT = T x N where HP _RT = rotating system HP (BHP)

5250 T = rotary torque required (ft-lbs) N = rotary speed (rpm)

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Power Requirements for Top Drive System

• For modern day rigs drilling complex wells with top drive system. The horse power required can be calculated based on extreme condition

HP _RT = T x N where T = rotary torque. Assume maximum

5250 torque rating of a 5-1/2” drillpipe = 35,000 x 120 with 35,000 ft-lbs

5250 N = RPM. Assume at 120 rpm = 800 BHP

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• The mud pumps is the heart of a rig circulating system

• Mud pumps are designed for pressure output, flowrate and horsepower requirements

• Power required by rig pump can be calculated by HHP (Hydraulic Horse Power) = P x Q

1714

Where HHP = pump output at fluid end in BHP P = total pressure drop in the system (psi) Q = pumping rate (GPM)

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 Derrick or a mast provides the vertical height necessary for the hoisting system to raise and lower the pipe

 API standards 4A provides specs for derrick and API standard 4D provides specs for mast type structure

 Derrick or Mast must be able to handle /support all loads, including drilling load and weight of pipe set in the derrick

 Derrick must be able to withstand wind loads acting horizontally on the pipe racked in it

 Selection for a derrick based on whether rig usage i.e drilling activities or workover and servicing

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 Drilling requires pipe to be handled in stands (3 joint

of connected pipe is about 90 feet in length)

 Height of derrick is roughly ascertained by [pipe

length 90 feet + 25 feet for travelling block, hook, and

bails + 3 feet stick up above rotary table + 5 feet for

buffer below crown block] These will total at about

123 feet

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The schematic below shows API 4A Derrick size classification

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Table 1 provides the General Dimension of Derrick size

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 Modern day Derrick or mast for drilling

activities require additional height to

accommodate Top Drive System

 Most widely used Derrick size is API 19

which provides 146 feet height and 30 feet

square base with pipe racking capacity of

160 stands of 5” drill pipe

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 Derrick or Mast for workover or well servicing activities

handle tubings which are limber and tend to bend due to

its own weight. Pipe are handled in

 Double (2 joints of connected pipe about 60 feet in

length)

 Single (1 joint of pipe about 30 feet in length)

 More time for pulling and running in pipes

 Normal heights are 90 feet (for handling pipe in singles)

and 102 feet (for handling pipes in doubles)

 Suitable for uneven terrains

 Small rig site preparation required

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 Sub structure provides the height for the blowout preventer stack required

 Sub structure similar to the derrick must be able to support all loads on rotary, weight of pipe set back racked in the derrick

 Provides the derrick floor space for pipe set back and people to work safely

 On offshore rigs, especially for drilling from platform must have sufficient longitudinal and traverse width to allow the drilling unit to skid from one well to another over entire drilling envelope of wells

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 Derrick load is defined by the heaviest hook load that can be handled with proper safety factor

 Effective derrick load can be evaluated by

F _DE = 4L (N + 4) 4N

 The heaviest hook load L is usually taken as the heaviest casing load in mud as given in earlier section

 Wind load is created by wind acting horizontally on the pipe set back in the derrick and is calculated by

Where

F _DE = Effective Derrick load, lbs

N = number of lines string up

over the block

L = Heaviest hook load, lbs

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Wind load is created by wind acting horizontally on the pipe set back in the derrick and is calculated by

Lw = 0.004 V₂ A

Please refer to Table for wind load areas

Where L w = wind load, lbs V = wind velocity, mph A = area of pipe set back