DNV GL has developed a new concept to deal with HPHT pipeline expansion design. Chia Chor Yew explains.
Transporting oil and gas through pipelines from high pressure and high temperature (HPHT) reservoirs continues to be a major challenge as the industry pushes into new energy frontiers. This poses greater issues for pipeline systems and their integrity.
Depending on whether the pipeline is fully restrained or unrestrained, HPHT pipelines laid on or buried in the seabed can experience a wide variety of reliability and safety issues. Frictional resistance from the soil can result in a combination of axial displacement, lateral buckling, upheaval buckling, and pipeline walking, to an extent that varies with seabed soil resistance. These pipeline movements can cause failures in the midline, or at the tie-ins connected to the pipeline end, and thus are critical to pipeline integrity.
Image Caption: SliPIPE concept A telescopic joint integrated with a pressure chamber
DNV GL, working with a team of engineers from Singapore, Oslo, Perth and Groningen, developed SliPIPE to control the expansion at the end of a pipeline operating under HPHT conditions.
During development, the team considered comments from the offshore pipeline industry, academia, personnel from two major installation contractors, and a seal company.
At this stage, SliPIPE is conceptual and will require refinement, engineering and qualification before it can be realized in an actual project.
SliPIPE consists of an outer pipe connected to a pressure chamber. An inner pipe 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.
The axial stress value is pre-determined in the SliPIPE 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.
The axial tensile pipe-wall force is produced by letting fluid pressure in, through holes in the inner pipe, to one side of the pressure chamber, separated from the other side of the pressure chamber by an annular partition wall. As the pressure in that side of the chamber freely builds up, it pushes against the partition wall and the pressurized end of the chamber in opposite directions to one another until an equilibrium is reached.
This in turn develops a tensile force in the pipe wall, which can be scaled to a desired value by pre-sizing the crosssectional area of the pressure chamber.
Between the outer pipe/pressure chamber and the inner pipe of the SliPIPE are two main seals, a partition wall seal, an environmental seal, and a scraper seal.
Each main seal consists of a pair of chevron seals (made of thermoplastic) and T-seals (made of elastomer) with backup rings, capable of preventing a single failure from causing the loss of both barriers. Other equivalent double barrier seals may be used.
Around the rim of the annular partition, which moves within the pressure chamber, is a set of double T-seals. Each T-seal is reinforced with backup rings on either side and these provide efficient resistance to extrusion of the seals.
The seals are made of materials that allow them to function at high temperatures up to 150°C and pressures of 100-400 bar. Environmental seals and scraper seals remove marine growth and other contamination on the surface of the inner pipe before it makes contact with the main seal.
Before use, all seals must first be qualified for HPHT conditions and to ensure the long-term reliability of the seals to function under the frequent two-directional sliding of the surfaces that come into contact with them.
Traditionally, a giant tie-in spool would normally be required to absorb large pipeline end expansion to a level low enough for economic design of the tie-in. Alternatively, or additionally, expensive post-lay subsea intervention work that limits expansi.on would be deployed.
SliPIPE could minimize financial implications by avoiding the spool material procurement costs, handling costs and offshore installation time associated with giant spools. Preliminary cost estimates indicate that direct tie-in of a pipeline with SliPIPE at the ends can lead to potential savings in CAPEX for installing the pipeline of up to US$5.2 million.
This represents an approximate 50% reduction in CAPEX for a conventional tie-in compared to using the giant spool method, or 10% compared to a typical installation of 10km pipeline using conventional lateral buckling design and tie-in methods.
Several practical issues that will influence how SliPIPE operates have been studied, and ways to overcome these are being looked into.
As a concept, SliPIPE is suitable for installing tie-ins between a submerged rigid pipeline and a subsea well, subsea structure, or riser, typically from 10.75- to 24in (273-610mm) in diameter.
This may be pre-installed on a pipeline end termination (PLET), 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. This is designed to minimize end expansion, external forces, and bending movements acting on it. Alternatively, a direct tie-in (without a PLET and short tie-in spool) is also feasible, with the use of a suitable subsea installation guide.
In the direct tie-in method, SliPIPEs have to be locked to restrict any uncontrolled movement and the lock released before tie-in. SliPIPE must be designed to have at least the same capacity as the adjacent linepipe, which has already been designed to resist the maximum tensile forces and bending movements.
As a relatively simple yet effective alternative to traditional giant tie-in spools and expensive post-installation subsea intervention, analysis shows SliPIPE could potentially offer cost savings in material and offshore installation.
In constructing this concept, DNV GL has taken into account comments from industry and academia to address and overcome challenges around this issue, and is determined to further develop the SliPIPE technology through to commercialization and deployment. OE
Chia Chor Yew is head of department, subsea, structures and pipelines, DNV GL Singapore. He has 31 years’ experience in structural, civil and pipeline engineering, mainly related to the offshore and marine industry. He has a BSc and MSc in civil engineering from the National University of Singapore.
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