GB2201220A

GB2201220A - Method for ballasting pipe line

Info

Publication number: GB2201220A
Application number: GB08802382A
Authority: GB
Inventors: Karel Karal
Original assignee: Norwegian Contractors AS
Current assignee: Norwegian Contractors AS
Priority date: 1987-02-03
Filing date: 1988-02-03
Publication date: 1988-08-24
Anticipated expiration: 2008-02-03
Also published as: NO870428D0; NO870428L; GB2201220B; NO172456C; GB8802382D0; NO172456B

Abstract

The position or level of a pipe body adapted to be towed in substantially horizontal position for subsequent submersion of the pipe line down to a position on the sea bottom relative to the bottom is controlled by supply/removal of a ballast medium to/from ballast chambers arranged in the pipe body, which chambers have longitudinal, preferably continuous form with an effective cross-section substantially less than the cross-section of said elongated pipe body and may have varying cross-section longitudinally. <IMAGE>

Description

A METHOD FOR BALLASTING A PIPE LINE, AND PIPE LINE FOR USE WITH THE METHOD.

The present invention relates to pipe bundles for installation on a sea bed produced on a shore based production site having a limited extent. The pipe bundles are adapted for towing along the surface with a possibi)ity for preliinay submersion of the pipe bundle down to the sea bed in order to avoid large loads arising from waves and streams. The pipe bundles are further adapted to be equipped with various connections in accordance with known methods. The invention opens possibility for production, transport and installation of extraordinary long pipe bundles and also provides a simple and safe transport method for such construction.

The invention relates more particularly to a method for ballasting and deballasting of elongated pipe bodies, and further various types of pipe elements to be used for carrying out the method.

A pipe bundle is as herein defined a nurber of pipes installed in a surrounding protection- and supporting pipe. Pipe bundles for installation on the sea bed are knows technology.

The length of conventional pipe bundles and their size represented by the number, diameter and wall thicknesses of the sepa#rate pipes are limited by the environmental conditions along the transport route and on the instalgation site and the selected transport method. The transport method also puts demands to the arrangement and the outfit of the pipe bundle.

Pipe bundles for offshore use are usually equipped with a series of chain lengths about 2 to 4 meters long and being suspended on the under side of the protection- and support pipe for instance spaced about 12 meters from each other. The chains are necessary in order to carry out trimming and weight adjustments of the pipe bundle on the sea bed at the production site prior to launching. The chains may also be helpful- for other purposes, such as to keep the pipe bundle at a definite distance from the sea bottom and in order to reduce the magnitude of required tension loads during towing.

The chains are not necessary if trimming is not required or can be carried out by means of buoyant bodies, and it simultaneously is possible to place the pipe bundle directly on to the sea bed.

Known solutions for pipe bundles are based on the following ballasting principles: 1. The pipe bundle is produced with a certain submerged weight in correlation with the transport and hydrodynamic conditions at the sea bottom. The pipe bundle is ballasted supplying water into gas-filled spaces inside the protection- and support pipe for the pipe bundle subsequent to that the same has been situated in desired location. This principle is used for the said first transport methodS as described in the following.

2. The pipe bundle is produced in ready ballasted state, transported on a vessel and positioned in the sea.

The following transport methods are known: a) Bottom towing where the pipe bundle due to buoyancy is suspended in chains which during towing slides on the sea bed; or the pipe bundles slide directly on the sea bed. The method brings demands to the nature of the sea bottom and the sea depth along the towing route. The length of the pipe bundle is limited as to weight and friction against the sea bottom and available towing power. Special evaluations are demanded when the object shall pass over -for instance lines or in vicinities with broken sea bottom.

b) Towing above the blotto - but-below the sea surface (control depth tow) where the pipe bundle is suspended at a certain depth between two towing vessels which control the actual vertical distance through tension control. Reduction of the tension below a certain value will lead to fracture of the pipe bundle. The length of the pipe bundle is limited by the weight of the pipe bundle, the depth conditions along the towing roue and available towing power.

c) Surface towing where the pipe bundle is attached to buoyant members supporting the submerged weight of the pipe bundle. Long pipe bundles may be transported in this fashion.

There exist two important limitations for use of this method, namely firstly, release of float members and transfer of the pipe bundle to the sea bottom and, secondly, the strength of the pipe bundle versus waves.

d) Transport on vessels, for instance coiled up on a large diameter drum. Very large pipe bundles of special design can be transported in this fashion. One of the most important limiting factors~ is that the number of pipe lines in a such bundle is relatively low and that the diameter of the pipe lines necessarily must he small.

The present invention removes the above mentioned limited tions which are represented by the conventional technology.

The construction in accordance with the present invention leads to a transport- and installation method which: - increases "'axi-wum length for bottom towing and "control depth" towing, - removes li.'-'itations in regard severe bottom conditions and the depth conditions along the entire towing route, a limitation whitey otherwit R present in connection with the bottom towing methnd, - opens possibility for transport in sea surface position without extra or supplementary float menlbers and in areas where it hi therto has not been possible to tow pipe lanes at the surface along great distances.

Not even the number and the size of: the pipe lines which are included in the bundle Is limiting factor.

The method for hallasting and if desired, a deballasting of an elongated pipe line body in accordance with the invention is of the kind wherein the pipe body is adapted for towing in substantially horizontal position for subsequent submersion down to the sea bottom, the position of the pipe body relative to the sea bottom being controlled by supply/removal of a ballast medium to/from arranged ballast chambers in the pipe body, and the method in accordance with the invention is characterized in that a ballast medium having suitable density and/or specific weight, is supplied with a certain pressure into said chambers, which chambers have an elongated, continuous shape with cross-sectional area substantially less than said elongated pipe body whereby whereby step-wise and section-wisp ballasting of the pipe body is provided in accordance with a pre-selected sequence.

pipe bundle or pipe line in accordance with the invention is generally characterized in that it includes one or more pipe members for ball asting purposes and./oz- transport media.

In a preferred emhodbnent the pipe or pipes for ballasting purposes is arranged as flexible or inflated hose member which can be attached along the outside of the support pipe andor be positioned inside ths support pipe.

The technical solution of the invention involves an inherent possibility for stepwise or sectional ballasting and/or deballasting of the pipe bundles. With the expression "ballasting" and "deballasting" is meant alterations in the buoyancy of the pipe bundle and the submerged weight of the same.

Ballasting or dehallasting is obtained by replacement of the contents in the pipe-shaped members. Suitabl; contents is usually a ass (nitrogene, air) and liquid (water, water with added ingredients~,.

pecial. pipes may either have rigid walls (steel. p ] as-tics) Or pyres which may be deflated by the outside pressure when wholly or partly out of function (hoses, for instance fire houses!. It is natural to position drainage enhancing rigidi fixation in deflatable pipes/hoses in order to prevent that ths entire cross-section thereof is being clamped together, a situation which may lead to uncontrolled and insufficient liquid drainage.

Stepwise ballasting and deballasting is obtained by positioning several ballasting means, the contents of which are shifted out in accordance with different time intervals.

Sectional ballasting may be obtained in that several hallasting means having varying diameter/cross-sectional area are arranged along the pipe bundle.

Typical pipe bundles are such used in connection with production of hydro carbones from offshore oil- and gas fields, For such pipe bundles it is usually sufficient with a two-step ballasting and a one-step deballasting with the following characteristic features: The first ballasting step consists in rendering the buoyant body with a weight which (1) is sufficient for transfer from the sea surface to the sea bottom, (2) is sufficient for "anchoring" the construction on the sea bed in sufficient degree- such that the pipe bundle can sustain stream forces without lateral skidding, and (3) permits manipulations with the pipe bundle along the sea bed, for for instance in order to carry out couplings, hauling or bend-to-connect operations.

This ballasting step is carried out in case of bad weather during towing and on installation site during positioning and possibly also during coupling operations. First deballasting step consists in conveying the pipe bundle from the sea bed up to sea surface when the towing operation shall proceed, respectively when the position of the pipe bundle shall be adjusted on the installation site. The second ballasting step consists in obtaining the final, permanent weight in installed condition.

With twostep or sectionwise ballasting is obtained varying buoyancy/submerged weight along the pipe bundle which can be desirable for towing- or coupling operations.

Typical pipe bundles for offshore use are equipped with ballast chains which are suspended from the pipe line. Thf invention makes use of such chains superfluous. Subjecl-to certain conditions it may, however, in supplement to the solution of the present-invention, be advantageous to use chains, for instance in case of rocky sea bottom in the submersion area. The invention makes it possible to reduce/ eliminate the number of ballasting chains.

The described ballasting arrangement offers the following advantageous production-, transport- and installation procedures The production can take place in a limited land area with shielded waters as storage medium for the ready made part of the pipe bundle. It is natural that the pipe bundle during all production steps has sufficient buoyancy, such that auxiliary external buoyant bodies are superfluous. The need for trimming is eliminated due to the larger weight/buoyancy tolerances which are offered by the towing method.

The pipe bundle is being towed in surface position. The pipe bundle may possibly be sectionally ballasted. If waves are developing during towing towards a limit defined by the strength of the pipe bundle, the towing proceeds to the nearest most submerged position which is selected for this purpose. In this location the pipe bundle is being ballasted (first step and submerged onto the s-ea bottom where it is kept until the wave conditions are improving). Thereafter the pipe line is deballasted by pressing water from the ballasting chambers by means of gas. Internally positioned inflatable pipes and hoses are particularly advantageous in such connections because the removal of water from the same can be carried out by the existing gas pressure which is surrounding such pipes and hoses.

Alternatively the towing can proceed as a bottom towing or as above bottom-be ] ow surface towing, or as a combination of these.

On the installation site the pipe bundle is transferred to the sea bottom. at the first ballasting step which permits manipulation of the pipe line for correct positnoning.

Coupling can be made in this phase in a fashion reducing the need for tensioning the pipe bundle under such operation. In case need for larger displacements, the pipe bundle may entirely or partly be deballasted, adjustments are carried out and the first deep ballasting step is carried out. When the final positioning is achieved, the second ballasting step is carried out, usually by water filling of the space between the separate pipe lines and the protection pipe.

The invention shall be described further with reference to the accompanying drawings which are showing some embodiments for pipe bodies in accordance with the invention and wherein: Figures la-c are showing cross-sections through a pipe bundle consisting of support- and protection pipe 1, and pipe lines 2,3 and 4. The Figures are showing two different steps in a phase wherein the pipe is filled with air as shown in Figures la and lb, and the final phase in step two shown in Figure lc wherein the protection pipe 1 is resting on the sea bottom in water filled state. The pipe line 2 is also used as ballasting means during the first step. This is being emptied in operational condition and serves as pipe for transport.

Figures 2a-c are in a similar manner as shown in Figures la-c, a cross-section through a pipe bundle~wherein a separate ballasting member 5 consists of a pipe with rigid walls, is provided. This can be filled with water also in the operation period. The pressure difference on the inside and outside, respectively, of the means 5 may be very small, a fact which implies that thin-walled- piping can be used.

Figures 3a-c are in a similar fashion showing a crosssection through an important embodiment wherein a special ballasting member 6 consisting of a flexible or inflatable hose is positioned inside the protection pipe 1. In the hose itself is further provided a drainage pipe or hose 7 which prevents formation of: air or gas pockets during the drainage of the hose at the ballasting. The hose is initially deflated by the gas pressure inside the support- and protection pipe 8. During the first step ballasting the hose is filled with water and gas and the pressure in the pipe is increased due to the Suprn íed volume of liquid. At the deballasting, the water in the hose is being pressed out by means of the gas pressure.

Figures 4a-c are showing cross-sections through a pipe bundle equipped with an external ballasting merger 9.

Figures 5a-c are showing a further embodiment of this version wherein two ballasting pipes 10 and 11 are being used, mounted on the under side of the support- and protection pipe 12. This embodiment possesses the advantage that during manipulation of the pipe bundle subsequent to the first ballasting the main pipe 12 is not In contact with the sea bottom which otherwise can create a risk for damage. On the other side this arrangement ray lead to a reduced floating stability during the towing operation.

Figures 6a-c are in a similar fashion showing a crosssection through a pipe bundle partly ballasted with chains 13 and equipped with a flexible or inflatable ballasting member consisting of a hose 14 and with a drainage pipe 15, similar to the one shown in Figure 3, but wherein the hose 14 is attached to#the outside of the protection pipe. The hose 14 can he arranged continuously along the pipe bundle or in interrupted sections and the hose as such can be divided into separate sections which can communicate or not, thereby making It possible to carry out sectional and controlled regulation of the ballasting and/or deballasting in accordance with the need both during the towing, submersion and later during operation when the pipe bundle is installed on the sea bottom.

Figure 7 is showing cross-section along a section of the pipe bundle and further showing a member for sectional ballasting. Said member consists of pipe lines 16 and 17, each of which is equipped with alternating narrow sections and enlarged sections. In a such solution the.reduction/expansion in the two pipes 16 and 17 are arranged out of phase with each other. In one operation the pipe bundle is ballasted with liquid in the pipe 17. The first step ballasting is carried out and fulfilled by water filling of the #pipe 16. The pipes 16 and 17 can extend continuously through the pipe line or only through certain sections of same or arranged in intervals. In the preferred embodinents the pipes 16 and 17 will run continuously, but ray be divided up in separate sections.

Figure 8 is showing a piston-like means 18 for controlling separation of liquid from gas. This can be equipped with a positioning r.#nItoring instrument making it possible to obtain full control over the ballasting procedure.

Claims

1. A method of ballasting and/or deballasting an elongate pipe body which is arranged to be towed in a substantially horizontal attitude and subsequently submersed down to a sea bed, the position of the pipe body relative to the sea bed being controlled by the supply or removal of a ballast medium to and from ballast spaces forming part of the pipe body, characterized in that the ballast medium is supplied to or removed from the spaces which are provided as chambers having an elongate, continuous form with cross-sectional areas substantially less than the cross-section of the elongate body, whereby one can obtain stepwise and ~sectional ballasting or deballasting of the pipe body in accordance with a stipulated pre-selected sequence.

2. A method in accordance with claim 1, characterized in that the pipe body is deballasted by pressing or squeezing the ballast medium out of the ballast chambers by means of gas pressure, whereby one obtains gradual stepwise sectional deballasting of the pipe body in accordance with a stipulated pre-selected sequence.

3. A method in accordance with claim 1 or claim 2, characterized in that the removal of the ballast medium is provided by means of the existing pressure in the medium surrounding the ballast chambers, which are so designed that the existing pressure reduces the cross-sectional area of the same to the selected minimum value which is necessary for sufficient drainage and removal of ballast medium from the ballast chambers.

4. A method of deballasting an elongate pipe body which is arranged to be towed in a substantially horizontal attitude and subsequently submersed down to a sea bed, the position of the pipe body relative to the sea bed being controlled by the supply or removal of a ballast medium to and from ballast spaces forming part of the pipe body, characterized in that the pipe body is deballasted by pressing or squeezing the ballast medium out of the ballast chambers by means of gas pressure, whereby one obtains gradual stepwise sectional deballasting of the pipe body in accordance with a stipulated pre-selected #sequence.

5. A method of ballasting and/or deballasting a pipe boy, substantially as described with reference to the accompanying drawings.

6. A pipe body for use with the method in accordance with any one of the preceding claims, characterized in that the elongate pipe body comprises an outer support pipe encompassing one or more internal pipes for ballasting and/or transport media.

7. A pipe body in accordance with claim 6, characterized in that one or more of the inside pipes are formed as a deflatable and inflatable hose member.

8. A pipe body in accordance with claim 6 or claim 7, characterized in that one or more of the inside pipes has/have expanded and/or reduced sections.

9. A pipe body in accordance with any one of claims 6 to 8, characterized in that one or more of the ballast chambers is/are realized as one or more deflatable and inflatable pipes attached along the outside of the support pipe.

10. A pipe body in accordance with any one of claims 6 to 9, characterized in that inside the or each pipe is positioned a drainage pipe.

11. A pipe body for carrying out the method according to any one of claims 1 to 5, characterized in that the pipe body is sectionally divided longitudinally by means of piston means or the like in order to facilitate controlled separation of liquid from gas phase.

12. A pipe body, substantially as described with reference to the accompanying drawings.