DNV develops a new concept to deal with seabed pipeline expansion

12 March 2013

DNV has developed a new concept—SliPIPE—to deal with the challenges of transporting oil and gas from subsea high-pressure and high-temperature reservoirs through pipelines.

A pipeline laid on or buried in the seabed responds to high pressure and high temperature by expanding, resulting in axial displacement (also known as end expansion), lateral buckling, upheaval buckling, or a combination of these. Such pipeline movements can cause failures and are critical to the integrity of a pipeline.

When a pipeline is subject to high pressure and high temperature, its ends expand longitudinally and exert large forces and bending moments onto adjacent tied-in structures connected to it. The tied-in structures must be designed to withstand these expansions and forces. Dumping rocks along the pipeline or a giant spool installed at the pipeline end have traditionally been costly alternatives.

SliPIPE works to reduce the end force expansion exerted at the tie-in by absorbing the end expansion through sliding within itself and simultaneously reducing or eliminating the effective axial compressive force in the pipeline.

The concept consists of an outer pipe connected alongside a pressure chamber and an inner pipe that can slide inside them. Seals are placed at the contacts between the pressure chamber and the inner pipe. The inner pipe slides in or out of the outer pipes in response to an axial stress that can either be more or less than a certain value. This value is pre-determined in the design and causes an axial tension in the pipe wall to develop, which opposes the effective axial compressive force component arising from the inner fluid pressure.

SliPIPE is a new concept developed to deal with the end expansion of a rigid pipeline subject to high-pressure and/or high-temperature ( HPHT). Click to enlarge.

The axial tensile pipe wall force is produced by letting fluid pressure in. Between the outer pipe/pressure chamber and the inner pipe of the SliPIPE concept are two main seals, a partition wall seal, an environmental seal and a scraper seal. The seals are made of materials that allow them to function at high temperatures and pressures.

A SliPIPE concept used for absorbing end expansion may be pre-installed on a PLET (pipe line end terminal) which is then transported and installed offshore on the end of the pipeline, lowered onto the seabed and connected to a manifold or riser via a short tie-in spool. A misalignment flange may be included. Alternatively, a direct tie-in (without a PLET and short tie-in spool) is also feasible with the use of a suitable installation guide.

Key advantages with the SliPIPE concept are:

• avoiding the fabrication and complicated installation associated with giant spools;

• minimizing costly post-installation subsea intervention work; and

• space-efficiency and suitability for areas congested with many subsea facilities.

At this stage SliPIPE is conceptual and will require refinement, engineering and qualification before it can be realized in an actual project.

—,DNV’s pipeline director Asle Venås