UNDERWATER INSTALLATION AND METHOD FOR BUILDING OF AN UNDERWATER INSTALLATION

INSTALLATION SOUS-MARINE ET SON PROCEDE DE CONSTRUCTION

English Abstract

An underwater installation for use in offshore drilling of and production
from,
respectively, oil and/or gas wells at deep and moderate deep water comprises
an
intermediate station in the form of a buoyancy based hollow body (16) tension
strut
anchored at the seabed (10) and located at a depth substantially closer to the
surface
of the sea than the seabed (10). At least one hydrocarbon conveying pipeline
extends
between the buoyancy body (16) and seabed depth, for the transfer of
hydrocarbons from
the reservoir to said buoyancy body (16). At least one casing liner string
(18a-18h)
constitutes the tension leg/strut anchoring of the buoyancy body (16). The
invention
also relates to a method for the building of an underwater installation.

French Abstract

Installation sous-marine à utiliser dans le forage offshore et l'exploitation, respectivement, de puits de pétrole en eau profonde et moyennement profonde. Ladite installation comporte une station intermédiaire se présentant sous la forme de jambes de force d'un corps creux flottant (16), ancrées dans le fond marin (10) et situées à une profondeur sensiblement plus proche de la surface de la mer que du fond (10). Au moins un pipeline de transport d'hydrocarbures s'étend entre le corps flottant (16) et le fond marin pour le transfert d'hydrocarbures depuis le réservoir jusqu'audit corps flottant (16). Au moins un train de chemises du tubage (18a-18h) constitue l'ancrage formant jambe de force du corps flottant (16). L'invention concerne aussi un procédé de construction d'une installation sous-marine.

Claims

5
The embodiments of the invention in which an exclusive
property or privilege is claimed are defined as follows:
1. An underwater installation for offshore
drilling of a hydrocarbon well or wells in the seabed for
the production of hydrocarbons from a reservoir below the
seabed, the well or wells having an exposed casing or
casings on the seabed, said installation comprising
an intermediate station in the form of a buoyancy
based hollow body, said body being totally submerged
below the surface of the water and above the seabed at a
depth at which the influence from waves is small, said
hollow body being positioned over the well or wells; a
tension leg anchoring means for anchoring the hollow body
to the seabed over the well or wells, said anchoring
means comprising at least one hydrocarbon conveying
pipeline formed of a casing extension pipe string
extending between, and being coupled to, the exposed
casing or casings of the well or wells on the seabed and
the hollow body for forming the tension leg anchoring
means for the hollow body and for transferring
hydrocarbons from the reservoir in the seabed to the
hollow body, said tensioning leg anchoring means forming
the sole means for anchoring said submerged hollow body
to the seabed; and means for supplying hydrocarbons from
the hollow body to the surface of the water.
2. An underwater installation according to claim 1
further including a frame on the seabed for the exposed
casing or casings of the well or wells.

6
3. A method for constructing an underwater
installation for offshore drilling of a hydrocarbon well
or wells for the production of hydrocarbons from a
reservoir below the seabed, the installation having a
totally submerged, intermediate station in the form of a
buoyance based hollow body with an upwardly extending
production hose for said hollow body for the transfer of
said hydrocarbons to a surface of the water, said method
comprising the steps of:
a) using G seabed frame, drilling one or more
wells in the seabed to leave an exposed casing at the
well or wells and thereafter temporarily plugging the
well or wells;
b) submerging the hollow body beneath the surface
of the water and above the seabed to a depth at which the
influence from waves is small;
c) positioning the hollow body over the well or
wells
d) retaining the body over the well or wells by
means of guys attached to the frame on the seabed and the
hollow body;
e) forming a hydrocarbon conveying casing pipe
string extension from the exposed casing of one or more
wells to the hollow body
f) removing the guys to tension the casing pipe
string extension so that the casing becomes a tension leg
forming the sole means for anchoring the submerged hollow
body to the seabed; and
g) removing the plugging of the well or wells to
supply hydrocarbons to the casing pipe string extension.

7
4. A method as set forth in claim 3, including the
steps of providing a fastener in the form of threads on
the exposed end or ends of the casing or casings and
connecting said end portion to an adjacent end of a pipe
comprising the casing pipe string extension, the opposite
end of said pipe being anchored to the hollow body.

Description

CA 02248683 1998-09-10
WO 97/34074 PCT/N097/00068
1
Underwater installation and method for building of an
underwater installation.
The invention relates to an underwater installation for use
in offshore recovery of oil and gas, particularly at large
depths of the ocean.
Oil drilling from floating vessels is a well established
technique which can be carried out even at large depths of
the ocean. Conventionally, production of oil and gas has
taken place by means of fixed installation resting on the
seabed. It is difficult to build fixed installations at large
depths. Therefore, technique has been developed wherein
wellhead and valves belonging thereto are placed on the
seabed, and where risers carry hydrocarbons to a vessel at
the surface.
Seadbed based equipment is to a high degree remote controlled
and adapted to the use of a remote control vehicle (a RCV or
a ROV) for maintenance, etc. This prior art technique can be
used at moderate depths. Using known technique, large depths
are difficult to access, and a finished installation will be
very expensive.
From Norwegian patent application No. 924962, it is
previously known to dispose wellheads on a submerged buoyancy
body, from where conductor pipes extend downwardly
to wells on the seabed. From the wellhead, hydrocarbons

CA 02248683 1998-09-10
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2
are conducted upwardly to a vessel as previously known. Thus,
the buoyancy body serves as an artificial seabed, wherein
well completion and production are carried out using prior
art technique. If the artificial seabed has a sufficient
buoyancy, it may in itelf carry a common fixed oil
installation.
According to this technique, production wells are drilled in
two phases. By means of a floating vessel, a well is drilled
to a part of the planned length, e.g. until a 13 3/8 inches
casing is set, whereafter the well is plugged and left.
Thereafter, neighbour wells are drilled in the same manner.
The last set casings are, at the upper ends thereof, provided
with fasteners in order to be extendable upwardly, e.g. in
the form of internal or external threads, to be screwed
together with another pipe.
A submerged buoyancy body is anchored above the well area and
conductor pipes extend from the buoyancy body and downwardly
to the wells, where the conductor pipes are attached to the
last set casings. The buoyancy body is positioned at a depth
so deep that the wave influence becomes insignificant, the
body being attached to the seabed by means of tension struts,
such as known from floating tension leg platforms.
On the top of the conductor pipe, within the buoyancy body, a
blowout valve is mounted as previously known, risers
extending upwardly to a drilling vessel. Drilling of the
wells may, thus, continue by means of prior art technique,
but now from a substantially less depth than the first phase
of the drilling, e.g. one hundred and fifty metres. Second
drilling phase which is introduced by drilling out the plug
set in the first phase may, thus, be carried out by means of
simpler equipment than during the first phase.
Finished drilled wells are completed and put in production
as previously known.

CA 02248683 2004-09-08
3
Use of a submerged buoyancy body forming an artificial
seabed makes it possible to recover oil and gas from
substantial depths of the ocean. However, the state of
the art, such as represented by said NO 924962, falls
unnecessarily expensive, substantially due to a very
expensive anchoring.
The present invention is directed towards the provision
of a reasonable anchoring of submerged buoyancy bodies of
the kind serving as bases for wellheads to wells at
larger depths of the ocean.
In accordance with one aspect of the present invention,
there is provided an underwater installation for offshore
drilling of a hydrocarbon well or wells in the seabed for
the production of hydrocarbons from a reservoir below the
seabed, the well or wells having an exposed casing or
casings on the seabed, the installation comprising an
intermediate station in the form of a buoyancy based
hollow body, the body being totally submerged below the
surface of the water and above the seabed at a depth at
which the influence from waves is small, the hollow body
being positioned over the well or wells; a tension leg
anchoring means for anchoring the hollow body to the
seabed over the well or wells, the anchoring means
comprising at least one hydrocarbon conveying pipeline
formed of a casing extension pipe string extending
between, and being coupled to, the exposed casing or
casings of the well or wells on the seabed and the hollow
body for forming a tension leg anchoring means for the
hollow body and for transferring hydrocarbons from the
reservoir in the seabed to the hollow body, the
tensioning leg anchoring means forming.the sole means for
anchoring the submerged hollow body to the seabed; and

CA 02248683 2004-09-08
3a
means for supplying hydrocarbons from the hollow body to
the surface of the water.
In accordance with another aspect of the present
invention, there is provided a method for constructing an
underwater installation for offshore drilling of a
hydrocarbon well or wells for the production of
hydrocarbons from a reservoir below the seabed, the
installation having a totally submerged, intermediate
station in the form of a buoyance based hollow body with
an upwardly extending production hose for the hollow body
for the transfer of the hydrocarbons to,a surface of the
water, the method comprising the steps of: a) using a
seabed frame, drilling one or more wells in the seabed to
leave an exposed casing at the well or wells and
thereafter temporarily plugging the well or wells; b)
submerging the hollow body beneath the surface of the
water and above the seabed to a depth at which the
influence from waves is small; c) positioning the hollow
body over the well or wells; d) retaining the body over
the well or wells by means of guys attached to the frame
on the seabed and the hollow body; e) forming a
hydrocarbon conveying casing pipe string extension from
the exposed casing of cne or more wells to the hollow
body; f) removing the guys to tension the casing pipe
string extension so that the casing becomes a tension leg
forming the sole means for anchoring the submerged hollow
body to the seabed; and g) removing the plugging of the
well or wells to supply hydrocarbons to the casing pipe
string extension.

CA 02248683 2002-O1-31
3b
A characteristic features of the invention consist in that
the buoyance body is anchored to the seabed by means of
conductor pipes extending between one of the casings of the
well and the buoyance body.
An example of a diagrammatically shown embodiment of the
invention is shown in a perspective view when an underwater
installation occupies a production phase.
On the seabed 10, a subsea frame 12 has been installed in an
introduction phase, forming an anchor for lines 14 for
submerging and lateral positioning of a buoyancy based hollow
body 16.
The underwater installation according to the invention is
suitable for use at large depths of the ocean, e.g. 400 -
4000 metres, and the submerged buoyancy body 16 may e.g. be
placed at a depth of e.g. 150 metres, where the influence
from the waves is very small, causing a moderate variation in
the load on tension struts for the anchoring of the buoyancy
body 16 on the seabed.
After e.g. eight wells are predrilled to 13 3/8 inches,
casing liner strings 18a, 18b, 18c, 18d, 18e, 18f, 18g, 18h

CA 02248683 1998-09-10
WO 97/34074 PCT/N097/00068
4 -
are lowered from a floating platform (not shown) to be
screwed to the upper threaded end of last set casings, during
which the liner strings 18a - 18h pass through vertically
aligned passages, not shown, in the buoyancy body 16. In the
production phase, the buoyancy body 16 is closed uppermost,
except for a central opening for a production hose 20. After
having been screwed to said upper end of last set casings,
these casing liner strings 18a - 18h, the number of which
corresponds to the number of wells, are attached to the
buoyancy body 16 with the upper end thereof. During this
attachment, the temporary anchor lines 14 may be tightened
for, thereafter, to be slackened, transferring the tension
strain to "the tension struts" 18a - 18h according to the
invention which, thus, are tensioned and tightened.
The temporary anchor lines 14 may be removed when the
underwater installation is ready for production.
As mentioned, one or more casing-liner strings 18a - 18h have
a double function, namely as a casing and a tension strut. In
the embodiment shown, the wells upon completion are
considered as being continuous from the reservoir up to the
buoyancy based, tension leg/strut anchored, submerged hollow
body which is positioned at a depth of another order than the
seabed. The invention represents large simplifications in
relation to known and conventional technique, and enables the
utilization of already established technology such as
operations associated to drilling at very large depths of the
ocean as well as socalled floating production.

Representative Drawing