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Pipe-in-pipe System

Many of the newly emerging generation of high pressure high temperature (HP/HT) reservoirs in the North Sea are being exploited using pipe bundles and single pipe-in-pipe configurations as part of subsea tie-backs to existing platforms. Not only are reservoir conditions more harsh but there is a need to insulate the flowlines to prevent wax and hydrate formation as the product cools along the length of the pipeline.

With the use of pipe-in-pipe systems come additional design features that are not present in conventional pipeline design. Challenging engineering problems rang from structural design of spacers and internal bulkheads to the understanding of the structural behavior both globally and locally under a variety of loading regimes. Due to the increased number of components in a pipe-in-pipe system compared with conventional pipelines, the design process is therefore more iterative in nature as the interactions of the components may necessitate design alteration.
A pipe-in-pipe system is essentially made up of an insulated inner pipe and a protective outer pipe. The function of the inner pipe is to convey fluids and therefore is designed for internal pressure containment. The inner pipe is insulated with thermal insulation materials to achieve the required arrival temperature. The outer pipe protects the insulation material from external hydrostatic pressure and other mechanical damage. Concrete weight coating is not normally required due to high submerged weight and usually low ocean current speeds in deepwater areas.
For the exploitation of HP/HT reservoirs, pipe-in-pipe system can provide the necessary thermal insulation and integrity for transporting hydrocarbon at high temperature (above 120 ^C) and high pressure (in excess of 10000 psi). Pipe-in-pipe system comprises a rigid steel flowline inside a rigid sleeve pipe. The two pipes are kept apart by some form of spacer at the ends of each joint, and by bulkheads at the ends of the pipeline. The various proprietary systems in the market differ in the details of the spacers and bulkhead arrangements. The air gap between the inner and outer pipes provides the means of achieving the high thermal insulation. This air gap accommodates the insulation, which typically consists of either granular material poured into the inter-pipe annulus, or of a blanket form, which is wrapped around the inner pipe. In either case, the insulation material needs to be kept dry in order to maintain its insulation properties.

Why Pipe-in-pipe Systems

 

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 with needing 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 line in 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.

Configuration

 

Various configurations of pipe-in-pipe can be used. The followings should be considered when determining the configuration.

• Gap thickness between the internal and external pipes:
• This should be optimized to maintain the heating;
• Thermal stability;
• Overall feasibility.

Source:
Bay, Yong and Qiang Bai. 2005. Subsea Pipelines and Risers. Oxford: Elsevier.