Pipe in Pipe

The main feature of pipe-in-pipe and pipeline bundle systems is that the pipeline is comprised of concentric inner and outer pipes. The inner pipe or pipes within sleeve pipes carry the production fluids and are insulated, whilst the outer pipe or carrier pipe provides mechanical protection.

 The first known pipe-in-pipe system was installed in 1973 by Pertamina Offshore Indonesia.This pipeline was 8 miles long extending from shore to a single point mooring facility. The outer and inner diameters of this pipeline were 40" and 36" respectively.Up till now nearly 36 pipeline bundles have been installed by controlled depth tow method (CDTM). The first one was installed at the Murchison field in 1980. The longest pipeline bundle is the one being designed, constructed and installed in Norwegian Sector by Rockwater. This bundle is 14 km long with 46" carrier pipe and three production lines.

There are several conditions under which pipe-in-pipe systems (including bundles in this definition) may be considered for a particular flowline application over a conventional or flexible pipeline.

a) Insulation- HP/HT reservoir conditions

HP/HT flowlines require thermal insulation to prevent cool down of the wellstream fluid to avoid wax and hydrate deposition. There are many thermal coatings available that can be applied to conventional steel pipe but they tend not to be particularly robust mechanically and have not been proven at the temperatures now being encountered in HP/HT field, typically 150°C and above. A similar problem exists for flexibles in this respect. An alternative is to place the flowlines(s) inside another larger pipe, often called a carrier or outer sleeve pipe. The annulus between them can then be used to contain the insulating material whether it be granular, foam, gel or inert gas.

b) Multiplicity of flowlines

The bundle concept (pipes-in-pipes) is a well established one and a number of advantages can be achieved by grouping individual flowlines together to form a bundle. For specific projects the complete bundle may be transported to site and installed with a considerable cost saving relative to other methods. The extra steel required for the carrier pipe and spacers can be justified by a combination of the following cost advantages.

 A carrier pipe can contain more than one flowline. Common applications have also contained control lines, hydraulic hoses, power cables, glycol lines etc. Insulation of the bundle by the use of gel, foam or inert gas is usually cheaper than individual flowline insulation.

In most cases there is no trenching or burial requirement due to the carrier pipe’s large diameter. Since there are multiple lines within the carrier, seabed congestion within the filed is also minimized.

 Bundle installation is commonly carried out through use of the Control Depth Tow Method (CDTM). The main limitation to the CDTM is the permissible length of bundle that can be installed, currently around 7.8 km. This is due to a combination of construction site and inshore launch area size.

 c) Trenching and Rock-dumping

Traditionally, flowlines less than 16-inch in diameter are trenched and/or buried. When contained within a sleeve pipe, which could be anything from 18-inch to 24-inch in diameter for single pipe-in-pipe systems and much larger for bundles, a reasoned argument for nontrenching can be made demonstrating that the line will not pose a risk to human life or the environment, nor will it become a hazard to other users of the sea. The cost associated withneeding to trench, backfill and rock dump is often greater than that of the installation cost of the pipeline. By not trenching, buckling of the pipeline will only occur in the lateral direction across the seabed and there are methods to control such an event, e.g. mid-line spools or laying in a ‘snaking’ configuration. Upheaval buckling through the seabed, which is the more severe situation, can only be controlled through sufficient over burden being placed on the linein the form of rock dumping. These issues are addressed later.

In terms of impact from trawl boards or fishing gear, the external pipe acts as the first line of defense and although it may be breached, the integrity of the flowline will not compromised.For certain applications, pipe-in-pipe systems offer significant cost saving over conventional pipelines, particularly when the need to trench, backfill and rockdump can be eliminated with additional mechanical and structural benefits as well.

Sumber :

Lee,J.E,2007, Introduction to Offshore Pipelines and Risers.

Palmer,C.A, 2008, Subsea Pipeline Engineering, Usa : Penwell Corporation

Mousselli,A.H,1981, Offshore Pipeline Design, Analysis, and Methods, Usa : Penwell Corporation

Guo,B.,2005, Offshore Pipeline, UK: Elsevier

Bai,Y.,2005, Subsea Pipelines and Risers, UK: Elsevier