PET 306 Lecture Note 2 (Prepared by Dr. N.U.

Okereke)

1.1 Drilling Overview (Highlight)

Background:

Land drilling for oil has been done for well over a century, it was only since the Second World
War that marine drilling really came into existence as an industry in its own right, and many of the
practices now used offshore evolved only in the last twenty five years (Cranfield Subsea Note,
2012).
The first ‘offshore’ wells, however, were a shallow well drilled over the water from a pier at Santa
Barbara in southern California in 1897, and a well drilled in 1911 in Caddo Lake, Louisiana, where
a steam-powered rotary rig was erected on a wooden, bottom-supported platform.

In the 1930s techniques were introduced for drilling in the swamps of Louisiana, USA, in Lake
Erie in Canada, and in the large-scale developments then taking place in Lake Maracaibo, in
Venezuela. The first floating rig was a simple barge used in 1933 to drill in the bayous of southern
Louisiana. All subsequent ‘marine’ drilling, which at first confined to swamps and lakes, was
carried out using ordinary land drilling equipment mounted on simple, flat-topped or flat-bottomed
barges.
As the US oil industry rapidly expanded following the end of the Second World War, so it set its
sights on drilling in the seas off the US Gulf states, necessitating the development of special craft
capable of supporting drilling equipment in a sometimes hostile environment. The first
seagoing ‘mobile offshore drilling unit’ of any kind was a submersible platform that drilled in
a water depth of only 20 feet in 1948, while the first ‘floater’ was a converted US Army wartime
transport barge that had a rig fitted on an overside cantilever. Many other ‘submersibles’ were
built for use in the Gulf area, these having decks supported by tubular, bottle-shaped columns fitted
above pontoons which could be ballasted like those of a modern semi-submersible. These
platforms were, in fact, the direct descendants of the modern ‘semi’, although some of them bore
little resemblance to today’s heavy-duty, harsh environment units (Cranfield Subsea Note, 2010).
The subsequent development after submersibles came in the form of self-elevated barges that had
tall legs on which the drilling platform could be jacked up to sit well clear of the water, the feet of
the legs resting in or on the sea bed. These craft, of which the first was built in 1954, evolved from
floating docks that were used by the US army during the War. The first crude ‘jack-up’ units were
used initially in the shallower parts of the Gulf of Mexico and the Arabian Gulf, but the designs
grew steadily more sophisticated with the increasing demands of the industry for oil from deeper
water, eventually producing units with legs 300 feet high. However, this was still not high enough.

After the early makeshift barges and submersibles, converted ships were used to support
drilling rigs, but as exploration moved out into deeper water, so fixed platforms were developed
that could be towed into position and sunk so that they rested squarely on the drilling location. At
first these, like jack-up units, were only used in relatively calm and shallow waters near the coast,
but larger and stronger platforms were later built for the deeper and less tranquil waters further
offshore (Cranfield Subsea Note, 2010).

In 1955 the first well was drilled by a vessel on which a rotary rig was mounted and by 1957 a
further milestone was reached when a well was drilled by a drill ship in 100 feet of water. The
equipment used on these occasions was the most advanced then available, but it would look
primitive alongside the sophisticated offshore drilling units operating today. The ships and barges
then used could only operate in a maximum depth of 600 feet, while submersibles were restricted
to 90 feet. By 1960 about 70 barges and converted ships were being used for offshore drilling,
much of this work going on off California.

The 1960s saw the introduction of the first semi-submersibles, which could either sit on the bottom
like a submersible or float like a drill ship. About 30 of these units were built during the decade,
of many different types. The number of jack-ups quadrupled at the same time, and the new designs
started to utilise the canted legs that are now a common feature of many units.

Purpose-built ocean-going drillships with large storage capacities for fuel, drilling fluids and other
supplies were also developed for use in remote areas far from supply bases. They were built with
their own propulsion so that they could move themselves between locations and dispense with the
need for tugs. In the 1960s drillships were first fitted with one of the most important developments
in deep-water drilling technology: dynamic positioning (DP) systems. These systems enabled
drillships, and later, semi-submersibles, to maintain position with the aid of computer controlled
thrusters that responded to the commands of positioning monitoring inputs and dispensed with the
need for anchors. However, the systems were, and still are, expensive and the vessels using them
relatively few.

The upward surge in the price of crude oil and the availability of offshore concessions and
exploration licenses from many countries prompted a frantic spate of rig-building in the 1970s,
when an average of 30 drilling units were built each year – more than at any time before or since.
This brought the world’s offshore rig fleet up to over 500 units, and with this increase came many
improvements in equipment and operating techniques, including the use of surface motion
compensators.

Having reached a level of 750 units of which many are surplus to requirements, the 1980s saw a
fall in the number of drilling units built, long with a fall in the price of oil. Offshore technology
has, however, kept moving ahead. The discovery of very large deepwater fields off West Africa
led to the come-back of Drillships, with Dynamic Positioning capabilities. Jack-ups with legs 600
feet long are now able to drill 450 feet of water, while dynamically-positioned semi-submersibles
can now operate in 10,000 feet depths. Offshore drilling is going on in nearly every maritime area
of the world, and seems likely to continue and expand as oil prices again rises. Even in the Arctic
wastes drilling is carried out from man-made islands – the lastest type of semi-submersible – whose
huge caissons are floated into position, ballasted and sunk, and fortified against the powerful
natural forces within the ice-cap. Sea-bed cores have been recovered by a drill ship from a depth
of 23,000 feet and experimental well drilling has been conducted in water 13,000 feet, although
no commercial oil drilling has yet been carried out in such depths. However, the technology is
available to exploit oil reserves in the deep oceans well away from continental shelves; it only
requires a stronger demand for oil at a commercially viable price to stimulate the drawing board
ideas into action (Cranfield Subsea Note, 2010).

Key steps involved in drilling are as outlined below (SITP Training Manual, 2008);

1.1.1 Drilling Steps/Stages

Once the drilling vessel is positioned over the location and a seabed survey carried out, a
Temporary Guidebase (TGB) is lowered onto the seabed. If drilling is conducted through an
existing Template structure no TGB will be required as a dedicated slot with guide funnel will
already be in place.

A 36” hole is then drilled, or jetted, to a predetermined depth using the TGB as a guide structure.
A Permanent Guidebase (PGB) with four Guideposts (for connecting guidewires to) is attached
to the top of a string of 30” casing and the whole unit run and landed on the TGB, or template slot,
on the sea floor. The casing is centralised then cemented in place. Guidewires are now connected
to the posts.

A 26" hole is now drilled, with mud returns to the seafloor or to the rig via a Marine Riser with
attached Diverter. The marine riser is a large diameter pipe which is attached to the rig with a
Telescopic Joint and held rigid with a Tensioning system.

The hole is drilled with the Drillstring Motion Compensator engaged. This Derrick mounted
system compensates for vessel heave and keeps the drill pipe stationary, maintaining a constant
weight on bit. Once the hole has been conditioned and flowchecked the marine riser is unlatched
from the PGB and retrieved. The 20" casing is now made up and run. The casing is topped out
with a high pressure Wellhead Housing which lands and seats in the PGB. Wellhead housings are
available in many sues. styles and pressure ratings (5, 10 and 15,000 psi systems), depending upon
the required casing string design and whether the well is intended to be a Subsea Completion or
a Tie Back. The housing has a mandrel profile machined into the outer diameter of the upper
housing as well as several internal profiles which are used as receptacles for landing and setting
the smaller casing string hangers. During drilling operations the casing hanger landing profiles are
protected by a Wear Bushing.

Once the housing has been landed and then locked into the PGB an overpull test is carried out
using the drill pipe running string. The 20” casing is now cemented in place and the running tools
retrieved. This now leaves the wellhead housing with its mandrel profile (commonly called a H4
connector profile) ready to receive the next item of drilling equipment. namely the hydraulically
operated Blow Out Preventer or B.O.P Stack.

The B.O.P stack is run down the four guidewires on the marine riser and is hydraulically locked
onto the wellhead mandrel profile via the wellhead connector. The stack is then subjected to a
comprehensive series of operating and pressure integrity tests to ensure its well control abilities
before each hole section is drilled. All subsequent drilling and completion operations take place
through the stack. B.O.P stacks are available in 5,000psi, 10,000 psi or 15,000 psi working
pressures.

The 17 l/2” hole section is now drilled with all mud returns being put through the mud processing
equipment and then recirculated. Prior to running the 13 3/8 casing the wellhead housing wear
bushing is removed and the internal surfaces of the housing jetted clean. The casing is now run
and the hanger set in its respective profile inside the housing. Following casing cementation the
running tool is rotated to energise the hanger Packoff Assembly. This locks the hanger in the
housing and also forms a seal against the hanger and housing. Once the casing has been
successfully pressure tested the wear bushing is replaced.

Subsequent smaller casing strings can be run. locked and sealed inside the wellhead housing in the
manner described above. It is common for the production liner to be hung off near the bottom of
the previous casino string with a Liner Hanger and not in the wellhead housing. Typical North
Sea Casing design is as shown in
Table 1 below.
Table 1: Typical North Sea Casing Design

Hole Size 36" 26" 17 1/2" 12 1/4" 81/2'

Casing Size 30" 20" 13 3/8" 9 5/8" 7”

Depth Set 150 – 500 – 2000 – 5000 – 6000 –

Suspension In PGB Wellhead Casing Casing Casing or

1.1.2 TYPES OF DRILLING RIG

The Main types of Drilling Rig are :-

In addition to these MODUs, there are the wells drilled from offshore platforms.

These will be detailed in further sections.

FIELD EXAMPLES
In this Introduction several examples of Drilled Fields are given. These include :-

Britannia Field – North Sea

Girassol Field - West Africa

Deep Gulf of Mexico

Drilling Key Personnel:

Key personnel involved in drilling are highlighted in the diagram below: