Deepwater pipeline Mechanical design

In deepwater, the current and wave effects are limited, causing little dynamic loading. Also, water depth is not a limiting criteria as per DNV (refer Sec.5 A201 of DNV-OS-F101 : 2013) in the design of the submarine pipelines. Therefore, it is not understood why there are very few pipelines installed in deeper waters i.e. more than 2500 m depth. It shall be recognized that the increasing water depth is not the only constraint in designing the deepwater pipelines. The complexity of an offshore pipeline is typically expressed in terms of the water depth and pipe diameter i.e.

Selection of pipe diameter for the given depth becomes very critical, since on it lies the commercial viability of the project i.e. higher the pipe diameter more is the throughput. Therefore, this article focuses upon large diameter pipelines in ultra-deepwater. This paper broadly elucidates the nuances involved in mechanical design (or wall thickness selection) of deepwater pipelines such as requirement of thick-walled line pipes to sustain collapse due to external over-pressure and tensile stresses generated due to installation forces etc.

This article expects the reader to be acquainted with the offshore pipeline design code DNV-OS-F101.

For ultra-deepwater pipelines, the investment involved is considerable and the ability to transport significantly more gas at limited additional cost improves the commercial performance of the project. This can be achieved in following two ways:

1. Increasing design pressure of the pipeline
2. Increasing diameter of the pipeline

Increasing design pressure: Thick walled pipes are required for meeting the external collapse criteria. Thus the increased thickness of pipes adds to the project cost without any substantial benefits towards reducing the transportation cost of gas through the pipeline.

However, this give-away scenario can be altered if the pipeline internal pressure is increased to a value at which internal pressure becomes the governing criteria i.e. pressure containment (bursting) becomes the dominant criteria in wall thickness design and not collapse criteria. Then the increase in steel thickness can contribute in reducing the transportation cost of the gas and in turn improves the economic viability of the project. Therefore, the deepwater pipelines are generally high pressure (HP) pipeline, which leads to selection of thick walled pipes in the shallow waters too. HP pipelines require sensitive Pipeline Safety System and Pipeline Control System to safe guard the pipeline from extreme variation of high internal pressure. It will be wise if the internal pressure is increased to such an extent that the pipeline experiences a net hoop’s stress under operation. Therefore, the deepwater pipelines are generally high pressure (HP) pipeline, which leads to selection of thick walled pipes in the shallow waters too. HP pipelines also require sensitive Pipeline Safety System and Pipeline Control System.

Increasing diameter: Increase in diameter has significant benefits for the project economics, enabling more gas to be transported over longer distances. During Gas Flow Verification Analysis for a long distance deepwater pipeline, a typical relationship inferred between inlet pressure and outside diameter for different throughputs showed that inside diameter increase from 20” to 28” allows more than twice the volume of gas to be transported. While the friction loss increases exponentially for smaller diameters, it also increases with the higher velocities required to transport the same volume through a smaller pipe.

While this figure only relates to a typical pipeline length, the same considerations apply for shorter distance pipelines, justifying the desire to implement larger diameter pipelines for deep water application.

Core to the capability to develop large diameter pipelines in ultra-deepwater is the wall thickness design in combination with the manufacturability of the steel plates and line pipes to the required specification. The deepwater pipelines are antagonized by the tremendous external pressure which it has to resist at high water depth for safe operations. The wall thickness design for ultra-deepwater pipeline is mainly focused upon pipe wall buckling criteria. Pipe wall buckling criteria for external overpressure includes system collapse and propagation buckling.

System Collapse The thick walled large diameter pipes fabrication is feasible by two pipe manufacturing processes i.e.:

• JCOE, and
• UOE and

However, DNV imposes 15% reduction in compressive strength (i.e. fabrication factor, α_fab = 0.85, refer Sec.5, Table 5-5 of DNV-OS-F101: 2013) for UOE manufacturing process under the inference that the UOE manufacturing process introduces cold deformations giving different strength in tension and compression. For the development of one of the deepwater pipeline project, DNV was engaged to find out ways for improving the collapse strength of the UOE pipes. Critical findings of the study are:

• Modest heating during external corrosion protection coating application on UOE pipes can enhance the compressive strength of the pipes and thus higher fabrication factor i.e. α_fab = 1.0 may be utilized. However, the same shall be documented during the pipe manufacturing by performing compressive tests and/or ring collapse tests.

• Enhance production testing and tighter manufacturing tolerances for ovality and wall thickness measurement may be employed.

These findings of DNV are now being widely applied in wall thickness design of ultra-deepwater pipelines, such as Middle East to India Deepwater Pipeline (MEIDP), South Stream etc.

Achieving the desired material parameters for the wall thickness required for deepwater pipelines using standard calculation methods is on the edge of what can be produced. A small reduction in wall thickness can result in a major improvement in manufacturability, and thereby drive the actual feasibility of the project for a specific throughput and OD combination.

During the mechanical design of Middle East to India Deepwater Pipeline (MEIDP), which is planned to be laid at a water depth of 3450 meter (approx.), wall thickness of the pipe required for meeting the system collapse criteria was 44.6 mm considering the fabrication factor, α_fab = 0.85, as specified by DNV for UOE pipes, for DNV SAWL485 FDU (API 5L Gr. X-70 equivalent) line pipes, which is at the limit of manufacturing capabilities of most of the pipe mills with current configurations worldwide. After applying the recommendations from DNV regarding the enhancement of fabrication factor from 0.85 to 1.0, the wall thickness of the pipes selected for the ultra-deep section of MEIDP was reduced to 40.5mm.

Propagation Buckling For deepwater pipelines in water depths greater than 1000m, it is not economical to protect the pipeline from propagation buckling by further increasing the pipe wall thickness. Therefore, provision of buckle arrestor at fixed intervals to meet propagation buckling criteria shall be contemplated.

Other factors The supplementary requirements which are used to further reduce the wall thickness of the pipes selected for ultra-deepwater installation is “high utilization” (U) (refer I 500, Sec. 7 of DNV-OS-F101 : 2013) and “dimensions” (D) (refer I 400, Sec. 7 of DNV-OS-F101 : 2013). The supplementary requirements, high utilization (U) is applied to raise the material strength factor, αU from 0.96 to 1.00 (refer Table 5-4, Sec. 5 of DNV-OS-F101 : 2013) and supplementary requirement, dimensions (D) is applied to reduce the fabrication tolerance to ± 1.0 mm (refer Table 7-26, Sec. 5 of DNV-OS-F101 : 2013). Therefore, the pipes suitable for ultra-deepwater application require a full scale testing program for building the confidence of the designer, manufacturer as well as the owner. These aspects will be dealt further in the line pipe manufacturing section of this article.

Deep water pipelines are subjected to very high external pressures which makes the collapse strength of the pipe a major consideration. The line pipe manufacturing process involves a cold expansion operation that reduces the pipe’s transverse compressive yield strength, which is attributed to the Bauschinger effect. This reduction in compressive yield strength increases the wall thickness requirement for the external collapse criterion.

The line pipes for the deepwater pipeline projects are generally coated by externally three (3) layer polypropylene for corrosion protection. During, the coating process, the pipes are heated to approximately 5 - 6 minutes. This mild heat treatment or “Heat Soaking” can recover the compressive strength of the line pipe material lost during forming process (DNV Technical Report, Collapse of thick walled pipeline for ultra-deep water pipeline, Report no. 2007-1867). To prove this on a consistent basis, design is suggested to adopted a collapse ring test as regular production test for testing 50 mm wide rings, cut from the line pipes.

In addition to the DNV requirements, manufacturing of line pipe for deepwater application shall meet following other requirements:

• Tensile testing shall be conducted during various stages of pipe fabrication i.e. plate, after J-ing/ U-ing at the base of ‘J’/ ‘U’, after longitudinal welding, after cold expansion and after coating application. This multi-stage tensile testing helps in closer monitoring of the nuances in the tensile strength of the pipe during various stages.

• Compression testing of the pipes, as per the recommendations of the study conducted by DNV shall be included as a part of regular production to monitor the compressive strength of the pipe material. The compressive test may be conducted in accordance with ASTM E9.

• Residual stress measurement test, elevated temperature test and tests to ensure zero centerline segregation in the line pipes shall also be included as part of regular production testing.

Manufacturing of line pipes for ultra-deepwater application is most challenging as it involves pipe fabrication within tighter dimensional tolerances such as tolerance for ovality within 0.5% and tolerance for wall thickness within +/-1.0 mm. One more thing, which makes manufacturing of line pipes more critical, is that these tolerances are applied on thick walled pipes of upto 40.5 mm thick.