The present invention concerns a device in connection with a riser
for the transport of oil and/or gas, possibly including water, from a well head,
transport pipe or similar on the sea bed to a platform.
Based on exploration activities in the North Sea in recent years,
it seems today as if the era of large oil and gas finds is over. On the other hand,
there have been many finds of a smaller size. At current oil/gas prices, they are
not really commercially attractive/profitable, but in the future they may be interesting
if oil prices increase and/or the technical solutions for extraction of oil become
Many platforms which are already installed and in use have reserve
risers for future developments. A common feature of these risers is that they were
meant for large new fields. Their dimensions are therefore often much larger than
what can be used for production from marginal fields with modest production and
The situation is, therefore, that many risers which are held in reserve
will never be used. In theory, it is conceivable for a transport pipe with a small
diameter from a marginal production field to be connected directly to an existing
reserve riser of a larger diameter. However, this is not possible as the authorities
and the regulations require, among other things, that the riser and the production
pipe can be reamed (pigged), which can be difficult or impossible to do at the
transition from a large to a small pipe.
The alternative, therefore, is to lay a new riser which meets the
statutory requirements for strength and installation. A new riser is, however,
very expensive and may, as indicated above, contribute to making the development
of small oil and gas fields less attractive to the developers. In the worst case
scenario, development may be stopped for lack of profitability.
The present invention represents a solution which makes it possible
to use existing reserve risers with the pressure class and strength with which
such risers were installed and which meet the requirements of the existing regulations.
The solution makes it possible to perform pigging and is also less expensive to
install and use compared with the installation of new risers.
In accordance with the present invention, the solution is characterised
in that, in order to use a riser for the transport of oil from a small oil field
with low oil/gas production where a transport pipe with a small diameter is used,
a flexible pipe with generally the same internal diameter as the transport pipe
is arranged inside the riser. At its upper and lower ends, the riser is provided
with a coupling, which forms a seal between the transport pipe and the flexible
pipe and also forms a seal at the ends of the riser so that it is closed.
The present invention will be described in further detail in the following
using examples and with reference to the attached drawings, where:
- Fig. 1
- shows a simple sketch of a riser.
- Fig. 2
- shows a similar simple sketch modified in accordance with the present invention.
- Figs. 3-5
- show enlargements of three different solutions for connecting a production
pipe to an existing riser in accordance with the present invention.
- Fig. 6
- shows an enlargement of an alternative embodiment with distance rings between
an internal flexible pipe and the existing riser.
As stated, Fig. 1 shows a simple sketch of a riser 1. When new platforms
are installed at sea in connection with the development of new oil fields, the
platforms (not shown) are provided with additional (reserve) risers 1 for use in
connection with subsequent field developments in addition to risers which are intended
for immediate use. Such additional risers 1 are often provided with a blinding
in the form of a plug 2 or similar at the end, for example fixed via a flange connection
The platforms are provided with the additional risers because it is
not expensive to lay the pipes while the platform is being constructed but very
expensive once the platform has been launched and positioned in the oil/gas field.
The pipes are classified as risers and meet the given requirements for pressure,
temperature and corrosion resistance.
For large new oil fields, the production or transport pipe on the
sea bed will have a diameter equivalent to that of the necessary risers on the
platform so that the transport pipe can be connected directly to the riser. For
small fields, where transport pipes with a smaller diameter are used, such direct
connection is often not possible, as stated above. However, the present invention
represents a solution which makes it possible to use existing large, available
pipes. The solution is based on inserting a flexible pipe 4, with generally the
same internal diameter as the transport pipe 5 from a near or remote well head
underwater production plant (not shown), inside the riser 1. The transport pipe
5 is connected to the flexible pipe 4 via a coupling 6 at the lower end of the
riser. An equivalent coupling 7 can be used between the flexible pipe 4 and the
process pipe system 8 on the platform.
The coupling 6, 7 also forms a seal at the ends of the riser so that
it is closed. This represents an important feature of the present invention, as
the internal, flexible pipe is expediently perforated or permeable in some other
way so that the oil/gas penetrates out into and equalises the pressure against
the space (annulus) between the flexible pipe 4 and the riser 1. This results in
the maintenance of a constant pressure, P1 = P2 (see Fig. 5), between the annulus
and the internal, flexible pipe 4. This, in turn, results in several advantages:
- The original reserve riser 1 is used to the full in relation to its classification
with regard to pressure, temperature and material. Consequently, it is possible
use a much less expensive, non-classified, flexible internal pipe 4 of various
material types and designs.
- Installation of the internal flexible pipe 4 is very simple and inexpensive
compared with the installation of a new riser with the correct dimensions.
- It is possible to perform pigging on the flexible pipe.
Overall, therefore, the present invention represents a much simpler
and less expensive solution than having to install a new riser with a small diameter
within the existing qualification requirements for an existing platform.
Figures 3, 4 and 5 show alternative couplings between the transport
pipe 5, riser 1 and flexible internal pipe 4. It is expedient to use an intermediate
pipe coupling 9 between the transport pipe 5 and the flexible pipe 4. It can advantageously
be connected to the respective pipes by welding. The pipe coupling 9 is fixed tightly
in an end piece 15, which, in turn, may be connected to the riser 1 by means of
a weld connection 10 as shown in Fig. 5, a sleeve clamp connection 11 as shown
in Fig. 4 or a flange connection as shown in Fig. 3.
In connection with installation of the flexible pipe 4 in the riser
1 and during regular production of oil/gas for the production system, water may
accumulate in the annulus between these pipes. This may expediently be sucked into
the flexible pipe via a venturi nozzle 13, as shown in Fig. 5, to reduce potential
Moreover, regarding the installation of the flexible pipe 4, it is
important that it is held in place inside the riser, particularly in connection
with pigging the pipe.
This may expediently be done using distance rings 16 arranged at intervals.
With regard to the expression "flexible pipe" 4, as defined in the
claims, this concerns not only pipes which can be bent without suffering plastic
deformation, but also pipes which are divided into sections and which are articulated.
The pipe must be able to be pushed or drawn through a riser and adapt to the bends
or curves of the riser. Furthermore, the riser need not be a reserve riser, but
may equally well be a riser which has previously been used for production from
a large field with high production.