Floaters

Floaters are the drilling vessels that keep floating during the entire course of drilling and other operations

DRILLSHIP Drill ships are suitable for water depths 20m-3000 AnchorMoored type 8, 9 or 10-point mooring.

SEMI-SUBMERSIBLES -Floater type rigs in which the drilling deck is mounted on columns which are supported by submerged pontoons or hulls

Drill Ship

SEMI-SUBMERSIBLES

Anchor

FloatersAnchorMoored type 8, 9 or 10-point mooring, Self Propelled. Suitable for water depth up to 1500m.

Dynamically Positioned type Station keeping is done by DP system though anchors are also normally provided

Vessel motionsMotions restricted to horizontal plane areSurge- Translation fore and aft (X- axis).Sway- Translation port and star board (Y- axis). Yaw- Rotation about the Z- axis

.Motions that operate in vertical plane are-Roll- Rotation about the X-axisPitch- Rotation about the Y-axis. Heave- Translation up and down (Z-axis)

TURRET MOORING

Dynamic Positioning The process involving the action of thrusters which, commanded by a controller and opposing the environmental forces maintains a ship or any other floating vessel in the vicinity of a reference point and stabilize its heading.

DP System

Disadvantages of Dynamic PositioningHigher CapexHigh running costHigher noise level

How floater drilling is different.FLOATER Vessel keeps floating, is subjected to various movements like Roll, Pitch, Heave etc. 30 casing is lowered in a drilled 36 hole with seawater with no returns to the rig.26 hole is also drilled (generally) with seawater with no returns to the rig.JACK-UP RIG Once the vessel is jacked-up it stands firmly like a fixed platform.30 casing is piled in to the seabed.

26 hole is drilled with seawater and returns to rig / with mud returns to the rig.

How floater drilling is differentEvery casing string lowered in the well is terminated at the seabed (except liner casing).BOP stack (generally 18 bore) is lowered and installed at the seabed after 20 casing .It has more functions and requires special handlingEvery casing string lowered in the well is brought up to the surface (except liner casing).

The BOP stack (generally 13 5/8) has to be removed for the installation of each section of well head.

How floater drilling is different.BOP control system is much more complex for remote operation and redundancy.Drill String Motion Compensator is required

BOP control system is same as that is used on land rigs.

Not required

How floater drilling is differentSubsequent operations right up to abandoning of the well are carried through the stack .A string of risers about 20 bore is used requiring a special tensioning systemDimensionally this stack is much smaller in size and has less no. of rams, annulars and k/c line valves.A single riser pipe is used.

Sequence of drilling vessel moves to the new location running of anchors & pre-tensioning OR dynamic positioning 36 hole is drilled & 30 casing is run and cemented with no returns of circulating fluid . A permanent guide frame is secured with 30 casing head while running casing to guide the BOP stack

Sequence of drilling26 hole is drilled with a pin connector if required with diverter in case of shallow gas or riser less20 casing is run with 18 well head on top with a running tool and drill pipes and cemented 18 BOP stack is now run and latched on to the 18 well head. subsequent operations are all carried through the BOP stack

Advantages of Dynamic Positioning Faster rig positioningNot affected by water depth nor the type of seabed Independence from logistic resources The performance of Dynamic Positioning is generally superior to that of Mooring Systems. Re entry is fast and easyOffset is1 to 3 % of water depth

Components of a Dynamic Positioning SystemPosition Reference System Control and monitoring unit Thrusters

Sub sea Well-heads And Casingssub sea wellhead supports the BOP supports the suspended casings seals off the annulus between the casing strings

Difference from Jack upsfirst and second casing strings are cemented with returns to the seabed Casing is run with the last joint made-up on a casing hanger followed by drill pipescementing plugs are located at the wellhead and released remotely Casing seals are run and set remotely

Difference from Jack ups30 casing is run with PGBIf required 40 along with TGB is run in soft sea bed locationsSince all casings are run up to sea bed only casing tally must be very closely checked

Sub sea cementing system Cementing plugs are fitted in to the casing hanger joint Running tool fits into the casing hanger and attached to the drill pipe Bottom plug is launched by releasing a ball from the cementing head at surfaceTop plug is launched by releasing a dart from the cementing head at surface

Blowout Preventera) The main BOP stack &b) The LMRP (Lower Marine Riser Package).

A typical BOP stack consists of:-1) 3 pipe rams,2) 1 shear-blind ram,3) 1 annular BOP,4) Hydraulically operated BOP/Well-head connector 5) A mandrel on the top of lower annular BOP.

Blowout PreventerA typical LMRP consists of:-1) One or two annulars,2) Kill/choke coflexip hoses,3) One ball/flex joint,4) One riser adapter on top of ball/flex joint for connecting to risers,5) Subsea control pods for BOP 6) A riser connector.

Subsea BOP

BOP Control Systemtwo basic types of control systems Hydraulic Electrohydraulic

Acoustic Control System for emergency

Two pods are used for redundancy

Telescopic jointsTo accommodate higher heave telescopic joint having a stroke length of around 55 ft is normally used in deepwater.Based on riser string weight requirement riser tensioner requirement are calculated and provided on the rig for carrying out deep water drilling.

Major components of the control system

Hydraulic power unitHose bundlesPodsRemote control panels

Telescopic Joint and Support Ring

Pod Hose Bundle- Hydraulic Lines Main power line at centre- Min 1 dia Pilot lines or 5/16 dia reinforced 5/16 more preferable as expansion more in line at high pressure

The Marine Riser SystemRiser stringSlip Joint (also known as Telescopic Joint)Upper Ball JointLower Ball JointDiverter SystemJumper Lines

DRILL STRING MOTION COMPENSATOR

Crown Block type Traveling Block type.

Compensator system consists of Two cylinders & pistonsTwo HP hosesOne control hoseTwo Isolating valvesUpper Yoke ( connects Travelling block to cylinders)Lower Yoke ( connects Hook block to the pistons )Compensator locking arrangementGuide rails in the derrick

Operation of CompensatorThe basic principal of operation of a compensator is to support a desired constant load or to maintain a constant tension. This is done by pressurizing the oil/air below the pistons on which hook block is supported. The pistons along with the hook block remain stationary with respect to the seabed whereas the rig goes up & down

Operation of CompensatorDuring drilling, the pressure below pistons is kept so as to support string load minus WOB During casing landing, the pressure corresponds to little less than the weight of the string between the rig floor to well head During BOP landing, the pressure is according to the weight selected to be kept on the Blocks

SOME USEFUL POINTS FOR FLOATER During operations, especially, concerning wellhead & BOP, always account for "tide correction".Emergency Hang-off Tool should be kept ready, stacked in the derrickIf compensator is working, clear margin should be left for kelly top when it reaches kelly bushing

Drill pipes should not be kept on slips for extended periods

BOP should not left hanging with telescopic joint fully open under load of BOP for too long

When BOP stack is at surface, always disconnect LMRP & check for proper operation of "Riser Connector

DEEP WATER Station Keeping Riser and Riser Tensionerthicker wall riser of 5/8 thickness in place of earlier Floats are attached to the riser Booster line 4.5 x o.5 wall, 3000 psi Additional mud pump 12 P 160Additional Hydraulic Supply Line Auto Fill Up MechanismRemotely Operated Vehicle (ROV)

Flex Joint18-3/4 - 10 degree flex joint is normally used in deepwater which is more stronger and reliable in place of ball joint used mostly in shallow water. It is placed just above LMRP to allow for movement of risers above due to ship movement.

Booster Line & Pump As riser OD is more, cutting removal is a problem in deeper depths. For effective cuttings removal from riser, one additional steel line called booster line 4.5 X 0.5 wall, 3000 psi working pressure is provided on risers with provision for inlet just above BOP.One additional mud pump provided for this purpose. A booster termination spool also provided just above BOP in the form of a short riser to facilitate connectivity of mud booster line with a hydraulic valve.

Hydraulic supply line of BOP controlIn deepwater, flexible hydraulic supply line of BOP control is not very reliable, as it is prone to bursting. To avoid this problem one more steel line of 3 OD X 0.5 wall is provided on riser string for supplying hydraulic fluid for BOP operation.

RiserMostly 21 & 18-5/8 OD risers are used in deepwater. Due to heavier long riser string having axial rating of the order of 2 million pounds, thicker wall riser of 5/8 thickness is used in place of 1/2. Even with 2 million pounds rating string, weight of riser string is excessive.

Buoyant Riser

Due to higher currents in deeper water and increased weight of guideline wires, guideline system is not effective. Instead guideline less system is used using beacons on TGB, PGB & BOP. BOP also has funnel at the bottom for proper entry over wellhead. This system is very effective especially in areas where frequent riser disconnects are required due to storm conditions.

Multiplex System (MUX)Normally, pilot hydraulic signals are sent through a hose bundle for operating various stack functions. This system is effective up to 1000m water depths. Beyond this, advanced systems are used for better reliability and quicker response. Multiplex system (MUX) is one such system used in deepwater in which pilot signals are transmitted through optical fiber cable. This system reduces response time for operating BOP considerably as the signals from the surface travels at the speed of light. This is of paramount importance in deepwater operations

ROV-Intervention Model

Deepwater Field DevelopmentFPSOTLPSPAR

RECENT TECHNOLOGICAL DEVELOPMENT Taut leg mooring & Preset moorings Dual Density Mud SystemArtificial Buoyant Seabed (ATLANTIS) TechnologyLightweight riser Dual Derrick RigsSlender well technologyDeep Water Drilling With Surface BOP Stack