A flowline is a pipeline for flow of fluids and hydrocarbons which is used for connecting a wellhead situated on the seabed to a sea surface installation. The flowline is generally comprised of two main parts. The riser extends from the surface installation to the seabed and touching the seabed at a location called the touch down zone. The shape of the riser is typically a catenary line. A flowline extends along the seabed from the touch down zone to the wellhead. A flow line may also consist of a pipeline extending from one well to another well.
Usually, several wells and their respective flowlines are connected via respective risers to the same surface installation. Environmental conditions (wind, hydrodynamic loads, wave and current) exert forces upon the flowlines and the risers and may cause the flowlines to move. Due to the number of flowlines and subsea devices, the movement of one flowline can cause considerable damage to other subsea equipment. Connections, for example, at the end termination of the flowline and the well head are not designed to support excessive forces. To reduce or avoid any damage to the flowline and the subsea equipment, it is recommended to anchor the flowline on the seabed at specific location(s).
Another example of an action which anchoring of the flowline improves is buckling of the flowline. Due to variations of the pressure and the temperature of hydrocarbons transmitted through a flowline during its lifetime, the flowline may extend itself in the longitudinal direction due to the heat of the hydrocarbon and shrink at the cooling due to absence of the hydrocarbon. This phenomenon is longitudinal buckling. In consequence, forces are exerted on the end termination at the well head, and movement of the pipe in the touch down zone may cause damage because of friction with the seabed. In order to limit the longitudinal buckling, it has been proposed in U.S. patent application Ser. No. 11/995,296 corresponding to PCT/FR/06101676 to affix the end of the flowline opposite to the end connected to the wellhead.
Lateral buckling, may also occur along the flowline. As illustrated in publication WO2007/010102, a wave shape may be imposed on the flowline in order to control lateral buckling of a flowline. The wave is formed by anchoring the line at regular intervals along the flowline.
In a known process for anchoring a flowline, the flowline is anchored generally during its laying operation. The location of the anchoring point is predetermined before the laying operation. A device for anchoring a flowline generally comprises a foundation on which the flowline is to be attached. The foundation is of a known type, installed in known manner by one of suction applied to the foundation at the seabed, a driven pile or a gravity base for the foundation. The device further comprises a clamp mounted around the flowline at predetermined lengths along the line. Further, some means connect the flowline to the pile, for example, a cable or a chain. The clamp around the flowline must resist pulling forces which are quite strong in at least one of an axial and a longitudinal direction. Thus, anchoring the flowline should not rely on gripping force or friction between the pipe and the clamp. In order to resist forces exerted on the flowline, the clamp is preferably engaged with a collar or other fixture that is attached and, usually in a reeled lay method, butt welded between the pipe sections. The collar has a recess or other fixture on which the clamp is mounted.
Typically, the collar is butt-welded to a pipe, and typically to ends of sections of the pipe at predetermined lengths along the flowline. In a stove piping method (a J-lay method), individual pipe sections are assembled on the vessel and each sections is attached to the preceding section as the pipe is laid. This method requires periodically stopping the pipelaying in order to weld an additional pipe section to the flowline. This requirement makes it is easy to butt weld a collar between the ends of two pipe sections without adding additional steps. However, applying a collar to a pipe is quite different in the reeled lay method, because the sections of pipes had been welded before the full flowline length is placed on the vessel (in a factory called a “spoolbase”). The pipe lay is then of the reeled flowline and is carried out without stopping and in a continuous manner. The pipeline is unwound from a reel. To add a collar to that unreeling flowline would require stopping the pipe laying, cutting the flowline, and welding or forging a collar to the line. The time required to lay a flowline would be considerably increased.
One can imagine pre-installing a collar at selected predetermined lengths along the flowline prior to winding it on a reel. However, it is not possible to do this without affecting the reelability of the flowline. One of the parameters to be considered for the reelability is the thickness of the outer diameter of the pipe. The variations caused by the thickness induced “rigid” portions at the collars would affect the reelability and may cause ovality of the flowline (e.g. from sharper bends at a collar), which is well known to initiate buckling phenomena.
Taking this problem into account, a subsea device that is intended to be connected to the flowline, such as a collar, a buckle arrestor, etc. are typically mounted on the outer surface of the pipe after the unreeling operation and therefore are attached to the flowline on the vessel.
In order to save time during a pipe laying operation, a new buckle arrestor design has been developed. Instead of mounting a collar before the laying, a “reelable buckle arrestor” has been proposed. U.S. Pat. No. 4,364,692 refers to the reeled pipe method and the problem that arises with a known buckle arrestor. A known buckle arrestor for rigid pipe is a shoulder welded onto the outer surface of the rigid pipe. It is explained that this kind of buckle arrestor is not adapted for continuous reeled pipelaying because the buckle arrestor can not be installed before reeling without affecting the reelability of the pipe. Known buckle arrestors (shoulder) are replaced by a rod wound around the pipe. Such prior art clearly demonstrates that a person skilled in the art would never have welded a collar having a shoulder onto a rigid pipe to be laid by the reeled lay method.