Reduced J tube pull force

A method and apparatus are provided for reducing the force required to move a pipe through a J-tube riser attached to an offshore platform. The J-tube riser is provided with at least one movable section and the movable section is oscillated as the pipe is moved through the J-tube riser, thereby reducing the force required to move the pipe.

BACKGROUND OF THE INVENTION 
This invention pertains to the use of J-tubes on offshore platforms for 
installing pipelines extending upwardly from the seafloor. 
In offshore petroleum operations, a platform frequently serves as an 
operational base and a production terminal for oil and gas wells. A 
problem common to all such offsore platforms is providing a connection 
from an above water facility to a pipeline which lies on the seafloor. A 
typical method of providing the connection is the use of a J-tube riser 
guide member which is a continuous tubular structure having the shape of a 
J and through which a riser pipeline is pulled from the seafloor to the 
above-water facility. This J-tube runs down along the length of the 
platform, bends near the seafloor and thus runs generally parallel to the 
seafloor a short distance. The lower end of the J-tube frequently includes 
a flared portion referred to as a bell mouth. The upwardly pulled 
pipeline, i.e. the riser, provides a fluid flow path from the above-water 
facility to the pipeline on the seafloor. 
At least two basic methods are known for installing the pipeline riser in 
the J-tube once the J-tube is positioned. In the so-called J-tube method, 
the pipeline riser is inserted in the seafloor end of the tube and pulled 
through it with a cable or other means. In the so-called reverse J-tube 
method, a section of the pipeline riser is inserted in the deck end of the 
J-tube and is pulled or pushed down through it as additional sections of 
conduit are joined to the pipeline riser at the deck. Both of these 
methods place tension on the riser as it is forced through the J-tube. 
This tension may cause displacements of the pipeline riser and J-tube 
which produce stresses that exceed the limits of the materials and cause 
damage to the pipeline riser or J-tube. 
Unfortunately, the J-tube is generally useful for riser pipeline diameters 
only up to about 12 inches. For riser pipelines having a diameter greater 
than 12 inches, the high pulling force required to install the pipeline 
riser is a major deterrent to further use of the J-tube. The J-tube riser 
has the advantage of a controlled and simple installation process which 
does not require elaborate pipeline manipulating equipment, unlike other 
known installation processes. 
Accordingly, it is a purposes of the present invention to provide a J-tube 
which has all the advantages of the conventional J-tube and which can be 
utilized with pipeline diameters greater than 12 inches without placing 
undue stresses upon the pipeline or the J-tube. Other purposes, advantages 
and features of the invention will be apparent to one skilled in the art 
upon review of the following disclosure. 
This application is relevant to the following copending applications: 
application Ser. No. 868,485 filed May 30, 1986, application Ser. No. 
876,153 filed June 19, 1986 and application Ser. No. 876,152 filed June 
19, 1986. 
Applicant is not aware of any prior art references which, in his judgment 
as one skilled in the pipeline art, would anticipate or render obvious the 
novel J-tube of the instant invention; however, for the purposes of 
developing the background of the invention and establishing the state of 
the requisite art, the following are set forth: U.S. Pat. Nos. 4,098,091 
and 4,523,877. 
SUMMARY OF THE INVENTION 
The present invention provides a J-tube through which a pipeline can be 
moved by a reduced force. 
Accordingly, the present invention provides a method and apparatus for 
reducing the force required to move a pipe through a J-tube attached to an 
offshore platform, comprising providing the J-tube with at least one 
movable section and oscillating the movable section as the pipe is moved 
through the J-tube. The movable section may be either hinged to the 
J-tube, or joined to the J-tube by telescoping joint having a loose-enough 
fit to permit movement of the movable section. Preferably, the movable 
section is oscillated with means such as an hydraulic jack.

DESCRIPTION OF PREFERRED EMBODIMENTS 
In accordance with the present invention a solution is provided to a 
significant problem which exists on offshore platforms when a pipe is 
pulled via a J-tube to the top of the platform for a final tie-in. 
Friction and pull forces exerted are tremendous. Accordingly, the present 
invention is directed to the need which arises to reduce such forces, thus 
enabling the pulling of larger diameter pipes. Primarily, the present 
invention is directed to the modification of the existing conventional 
J-tube. A section of the J-tube riser may be hinged at one or several 
locations giving it a type of knee action movement that is externally 
actuated with a hydraulic jack or counterweight means of the like. As the 
pipe in one embodiment is fed into the J-tube at the bottom and pulled via 
a cable from the top of the platform, the knee action movement is actuated 
and the J-tube acts as a bender, nibbler, and come-along, all in one. The 
movable sections of the J-tube apply lateral bending forces to the riser 
pipe being pulled into the J-tube which significantly reduces longitudinal 
pull force. Thus, the invention realizes reduced pull force during 
installation and reduced residual stress in riser pipe and conduit during 
operational lifetime. 
Having thus generally described one embodiment of the invention, the 
following more particularly describes this and another embodiment of the 
invention in connection with the accompanying drawings. In FIG. 1 a knee 
action J-tube is provided to accomplish the benefits of the invention. An 
offshore platform 1 is provided with a J-tube 4 fixed to the platform. 
Within the J-tube 4 is a riser pipe 3 having an attached pull cable at the 
upper end of which is applied a pull force 5. A movable section 6 is 
connected at the lower end of the J-tube by means of a hinge 7, although 
other attached means can be provided which will make the lower section 
movable. A hydraulic jack 8 is employed to give the movable section 6 a 
cyclic vertical motion 9, as the pipeline 10 passes into the movable 
section 6. 
In FIG. 2 of the drawings another embodiment of the invention is disclosed 
which involves a double action. Thus, an offshore platform 11 is provided 
with a vertical section 12 of J-tube fixed to the platform, a telescoping 
joint 13, typically in three places, which telescopes with a movable 
section 14 which in turn telescopes with a movable section 15, which 
finally connects to a section 16 which is also preferably fixed to the 
platform. One or more of the sections is connected to some means such as a 
hydraulic jack 17, preferably actuated from above water which produces 
cyclic motions of sections 14 and 15. In this drawing, a pipeline would be 
passing from the seafloor into the J-tube riser but is not shown for 
purposes of clarity. 
The foregoing description of the invention is merely intended to be 
explanatory thereof. Various changes in the details of the described 
method and apparatus may be made within the scope of the appended claims 
without departing from the spirit of the invention. For example, 
combinations of knee action and double action may be employed in the same 
J-tube, and the pipe may be pulled or pushed, or both pulled and pushed, 
through the J-tube from the top and/or bottom.