Rigs are classified according to type (bottom supported, floating), environmental capacity (harsh, moderate), water depth, and specification (standard, premium).
MODUs are
classified as bottom-supported or floating rigs. In bottom supported units, the
rig is in contact with the seafloor during drilling, while a floating rig
floats over the site while it drills, held in position by anchors or equipped
with thrusters using dynamic positioning.
Bottom
supported units are used for shallow-water drilling and include barges,
submersibles and jack ups Floaters are used for deep water drilling and include
semisubmersibles and drill ships Jack ups, drill ships and semisubmersibles
comprise the majority of the offshore fleet.
Rigs are
classified as harsh or moderate environment units. Harsh environments are
characterized by frequent and severe storms as occur during winter in the
Northern Hemisphere (North Sea, Norwegian Sea, North Pacific, Eastern Canada).
In the Gulf of Mexico and much of Asia,
moderate environmental conditions predominate for most of the year, but
tropical storms may cause severe weather events. In Brazil, Australia, West
Africa and the Persian Gulf, severe weather is rare.
In order to
work efficiently in a region, a rig must be capable of operating during average
1 year storm conditions and surviving 100-year storm conditions Due to tropical
storms, the 100-year storm conditions in the North Sea are similar to
conditions in the Gulf of Mexico and Asia; however, 1 year storm conditions are
far more severe in the North Sea As a result, harsh and moderate environment rigs differ in maximum
operating conditions but do not differ in maximum survival conditions.
Harsh
environment units have a number of design modifications to decrease weather
related downtime, including increased variable load to reduce the need for
resupply, increased airgap to increase wave clearance, and changes in the
geometry and spacing of the legs and columns to decrease wind and wave loads.
Harsh environment rigs are larger, heavier and more expensive to construct and
operate than moderate units.
Drilling
operation is mainly of two types, namely, “exploratory” and “development,”
depending on the purpose and type of wells. Exploratory drilling is usually
carried out in the virgin area or in the field whose lithology or subsurface
characteristics are unknown.
On the other
hand, development drilling is undertaken in a producing field with known
reservoir characteristics to enhance production capacity.
Based on
capacity, depth of drilling, and size of production hole, rigs are categorized
as “shallow” or “deep drilling” rig.
Similarly,
depending on the type of operation performed, on‐land rigs are termed as
“drilling” or “workover” rigs.
Old wells
occasionally require maintenance and recompletion jobs, especially in matured
fields that include revival of sick wells, water shutoff jobs, well deepening,
sidetracking, fishing, servicing, and so on, that necessitate working over the
wells.
These jobs are generally done with
smaller‐sized, cheaper rig with lesser footprint called “workover rig.”
A deep drilling
rig consists of large‐sized modules and high‐capacity equipment like draw work,
engine, mud pump, mud system, traveling block, crown block, swivel, rotary
table, and huge and tall tower called “derrick,” apart from varied downhole
tubular such as drill pipe, collar, stabilizer, jar, and so on.
A shallow or development drilling rig has a
smaller‐sized module as compared to a deep drilling rig. Workover rigs, on the
other hand, are generally truck mounted and have much lesser equipment.
The on‐land
drilling rigs are of various types, which may be broadly termed as mobile rig,
desert rig on wheels, and rig with high floor masts and substructure, all of
which are elaborated as follows:
1. Mobile rig:
This type of rig is mounted on wheeled carrier and has telescopic mast, which
can be driven to the desired location along with equipment, engines, and other
accessories.
It is used for drilling shallow wells. Rig
move time ranges between 16 and 24 h within a distance of 4–5 km.
2. Desert rig
on wheels: There are various combinations of rig types depending on capacity,
need, varying (heavy) loads on wheels, partially or fully disassembled units,
and so forth for which rig move time would also vary.
Desert rig on
wheels that moves with mast in erect condition takes around 1–2 days for rig
move within the same area. Desert rig that moves with mast down condition takes
around 3 – 4 days.
3. Rigs with
high floor mast and substructure: These are usually higher‐capacity rigs used
for drilling deeper wells.
The rig
components are dismantled and transported to the new location by heavy‐duty
trucks and trailers.
Rig move time
varies from 1 to 2 weeks depending on the design and capacity of the rig.
The current study specifies rig move
activities of 1500/2000 HP desert electrical rig on wheels with fully
disassembled units (drilling depth
capacity—16,000/20,000 ft).
The decision to
use a platform rig should be made prior to the design of the platform. This
decision would be influenced by size of field, depth of production, radius of
drainage of platform, volume of production, availability of rigs and water
depth.
The platform
rig would consist of the basic rig components with the living accommodations
being part of the platform.
This would require additional space and
strength to handle the loads imposed. The day cost of this type rig would be
less than others but could be offset by the additional expense of the platform.
The rig would be, perhaps, a land rig that
would be loaded on a barge and towed to the platform.
It would then
be hoisted on to the platform and positioned over the well template for
drilling.
Jack up rig
The decisions
to use a jack up rig would be influenced by the same actors, but generally a
lesser number of wells and shallow water would be strongly in favor of the jack
up.
The platform
design should allow sufficient room to position the jack up around the platform
without damaging it.
This would require the operator to look at a
particular rig and sign a contract for this rig in order to assure the
compatibility of the two.
The rig would
be spotted near the platform with tugs and work boats. The rig anchor system
would be put out and the tugs would assist in spotting the rig over the
platform.
The rig anchor
pattern should ensure good holding power and not interfere with the normal
operations of the platform such as pipelines, crew and work boats, etc.
Anchor piles are sometimes installed on the
platform to reduce the anchor-handling problem.
The increased
depth of operational waters has been met with the semisubmersibles.
The development of subsea equipment has been
such that the continued use of the semis is insured.
The semi (or
floater, as it is sometimes referred to) is towed to the location buoy and
spotted.
The tugs will
run the anchor pattern that has been previously selected for minimum
interference with operations and weather.
The semisubmersible will then be positioned
over the proper location and maintain its position automatically within preset
limits.
Some are
self-propelled or have thrusters to assist in towing. The self-propelled speed
is generally low, and some government regulations require tugs.
The
semisubmersibles have very good motion characteristics that permit drilling
operations to continue in waves of 35 to 40 ft high.
Wind and
current forces tend to push the unit off location.
This calls for a heavy mooring system that
uses chains (instead of wire rope) and 45,000 lb anchors.
This system requires good handling.
In the United
States, the Coast Guard has been assigned the responsibility for developing
measures to ensure the safety of life at sea.
The American
Bureau of Shipping (ABS) determines the maximum safe draft to which a drilling
unit may be loaded and requires that a visible marking be placed on the vessel
at this draft.
This maximum
draft, or load line, is indicated by the Plimsoll Mark.
Samuel Plimsoll was a member of the British
Parliament and was largely responsible for the passage of the British Merchant
Shipping Act of 1876.
This act called for the placing of a mark at
the maximum safe draft in order that all persons on board the vessel could see
that it was not overloaded.
Drillship is a
ship with the center part hollowed out for drilling activity. This moonpool
area is positioned by the ships power system and maintained within
predetermined limits.
Drill ships are generally used for the remote
wildcat type operations, or in very deep water, or for scientific exploration.
The water depth is limited only by its mooring system.
Wind, wave, and
current action tends to force the unit off its, location. A mooring system of
up to 10 anchors and mooring lines could be used to offset forces in any
direction.
The deployment
and recovery of this system is difficult and hazardous. It cannot be
accomplished safely in waves of more than 8 to 10 ft.
Drill ships are
the least expensive floating units to build by conventional mooring. The
addition of dynamic positioning increases construction cost substantially, but
no other alternative is presently available for ultradeep water.
The disadvantage to the drills hip is its
susceptibility to wave action. In 20 to 25 ft waves, the vessel may
heave 8 to 10 ft.
Drilling operations are usually suspended when
heave reaches 5 to 7 ft, so in rough-water areas much of the time is spent
waiting on the weather.
The submersible
rig is similar to the jack up rig in that they both rest on the bottom. The
submersible has hulls on which it floats while being towed to the location.
Upon reaching the location, the hulls are
flooded and sunk to rest on the bottom. The drilling deck is supported by
columns from the hull and is well above the water level.
Submersibles
have a fixed operating water depth like the jack ups and both are limited to
drilling in relatively shallow waters. The submersible and jack up rigs provide
a very stable platform for operations.
Upon reaching
its location, the submersible is submerged by flooding one end of the hull to a
reasonable tilt angle.
The other end
is then flooded, and the vessel is more or less rocked to bottom. The drilling
deck must be at such an elevation that the waves can pass safely underneath.
On leaving the
location, the submerging process is reversed. (Some units submerge at a nearly
level condition.) Upon reaching the seafloor, the unit is leveled and all tanks
are flooded to bring the unit to the desired bearing load on the bottom.
Care must be
taken to ballast or DE ballast the unit evenly about the center line.
Too much weight
on one side can bring the unit to the surface heeling to one side. Planning can
eliminate this problem.
A careful
operator knows the weight of the unit and how much ballast must be removed to
cause it to float freely off bottom.
As the unit starts to break suction from the
seafloor, observant rig personnel can detect the first movement of the unit as
it starts to move with the waves.
The DE ballasting operation should be stopped;
the force of the sea gradually causes water to permeate the soil beneath the
hull and allows the unit to float. The unit is then raised to the surface.
If you want
to learn more about Drilling Rigs you could do so in my book,
economic
study of oil and gas well drilling.
which is published on amazon, check it out at the link below
تعليقات
إرسال تعليق