Top Tensioned Riser Systems

TTRs are used as conduits between dynamic floating production units (FPUs) and subsea systems on the seafloor, for dry tree production facilities such as spars and TLPs. TTRs are individual risers that rely on a top tension in excess of their apparent weight for stability. TTRs are commonly used on TLPs and spar dry tree production platforms. TTRs are generally designed to give direct access to the well, with the wellhead on the platform. This type of riser has to be capable of resisting the tubing pressure in case of a tubing leak or failure. The four types of TTRs are drilling risers, completion/workover risers, production/injection risers, and export risers. Tensioned Riser Applications Specifications:

Drilling

MODU drilling risers
Surface wellhead platform drilling risers
API RP16Q
API RP 2RD

Completion/workover

MODU completion/workover risers
Surface wellhead platform completion/workover risers
API RP 17G
API RP 2RD

Production/injection

Surface wellhead platform production risers
Subsea tie-backs
API RP 2RD
API RP 1111

Export

Surface wellhead export risers
API RP 2RD
API RP 1111
ASME B31.4
ASME B31.8
30 CFR
49 CFR

Top Tensioned Riser Configurations

TTRs consist of long, flexible circular cylinders used to link the seabed where the wellheads are located to a floating platform. TTRs are provided with tension at the top to maintain the angles at the top and bottom under environmental loading

File:Typical TTR System Configurations.png

Typical TTR System Configurations

conditions. TTRs often appear in a group arranged in a rectangular or circular array. The following major design considerations are important when designing a TTR to prevent failure:

Allowable floater motions;
Allowable stroke of tensioning system;
Maximum riser top tension;
Size of stress joints and flexjoints;
Keel joints;
Increasing length of riser joints;
Design criteria for the safety philosophy of liquid barriers, valve, and seals;
Current, interface with array, and VIV;
Impact between buoyancy cans and hull guides.

Top Tensioned Riser Components

The TTR configuration depends on the riser function and the number of barriers selected (single or dual).

Riser System

In general, the riser configuration comprises the following components:

The main body is made up of rigid segments known as joints. These joints may be made of steel, titanium, aluminum, or composites, although steel is predominantly used.
Successive joints are linked by connectors such as threaded, flanged, dogged, clip type, box, and pin connectors.
The riser is supported by a tensioning system, such as traditional hydraulic tensioners, air cans,RAMtensioners, tensioner decks, and counterweights.

Buoyancy System

File:TLP Tensioned Riser System.png

TLP Tensioned Riser System

For water depths exceeding 2000 ft, buoyancy systems are required to provide lift, which reduces the top tension requirements, prevents excessive stresses in the riser, and reduces the hook load during deployment/retrieval of the BOP. Both synthetic foam and air-can buoyancy systems have been used for deepwater riser systems, either individually or in combination.Buoyancy cans decouple the vertical riser movement from the vessel, and can be built by the fabrication yard. However, either a heavy lift vessel or a specially designed rig is required for offshore installation. It has large relative vertical motion in storm conditions and may generate lateral loads between the buoyancy cans and the spar center wall.

Design Phase Analysis

File:Spar Tensioned Riser System.png

Spar Tensioned Riser System

Before designing the TTR, several analyses need to be conducted to ensure that the riser design is up to specifications. To design a TTR system, these analyses must be performed:

Top tension factor analysis;
Pipe sizing analysis;
Tensioning system sizing analysis;
Stroke analysis;
Riser VIV fatigue analysis [27,28];
Interference analysis;
Strength analysis;
Fatigue analysis.

References

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[2] American Bureau of Shipping, Guide for Building and Classing: Subsea Pipeline Systems and Risers, ABS (March, 2001)

[3] T. McCardle, Subsea Systems and Field Development Considerations, SUT Subsea Awareness Course, 2008.

[4] American Petroleum Institute, Recommended Practice for Flexible Pipe, API RP 17B, (2002).

[5] American Petroleum Institute, Specification for Unbonded Flexible Pipe, API Specification 17J, (1999).

[6] American Petroleum Institute, Design of Risers for Floating Production Systems (FPSs) and Tension-Leg Platforms (TLPs), API RP 2RD, ( June, 1998).

[7] R. Burke, TTR Design and Analysis Methods, Deepwater Riser Engineering Course, Clarion Technical Conferences, (2004).

[8] World Oil, Composite Catalog of Oilfield Equipment & Services, Fourty Fifth ed., 2002, March.

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[12] L. Deserts, Hybrid Riser for Deepwater Offshore Africa, OTC 11875, Offshore Technology Conference, Houston, Texas (2000)

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[14] C.T. Gore, B.B. Mekha, Common Sense Requirements (CSRs) for Steel Catenary Risers (SCR), OTC 14153, Offshore Technology Conference, Houston, Texas, (2002).

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[17] R. Franciss, Vortex Induced Vibration Monitoring System in Steel Catenary Riser of P-18 Semi-Submersible Platform, OMAE2001/OFT-1164, Proceedings of OMAE’01 (2001).

[18] E.H. Phifer, F. Kopp, R.C. Swanson, D.W. Allen, C.G. Langner, Design and Installation of Auger Steel Catenary Risers, OTC 7620, Offshore Technology Conference, Houston, Texas, (1994).

[19] G. Chaudhury, J. Kennefick, Design, Testing, and Installation of Steel Catenary Risers, OTC 10980, Offshore Technology Conference, Houston, Texas, (1999).

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[21] K. Huang, X. Chen, C.T. Kwan, The Impact of Vortex-Induced Motions on Mooring System Design for Spar-Based Installations, OTC 15245, Offshore Technology Conference, Houston, Texas, (2003).

[22] M. Hogan, Flexjoints, ASME ETCE SCR Workshop, Houston, Texas, 2002, February.

[23] J. Buitrago, M.S. Weir, Experimental Fatigue Evaluation of Deepwater Risers in Mild Sour Service, Deep Offshore Technology Conference, Louisiana, New Orleans, (2002).

[24] International Standards Organization, Petroleum and Natural Gas Industries Design and Operation of Subsea Production Systems, Part 7: Completion/Workover Riser Systems, ISO 13628–7: 2005, (2005).

[25] R. Jordan, J. Otten, D. Trent, P. Cao, Matterhorn TLP Dry-Tree Production Risers, OTC 16608, Offshore Technology Conference, Houston, Texas, (2004). [26] A. Yu, T. Allen, M. Leung, An Alternative Dry Tree System for Deepwater Spar Applications, Deep Oil Technology Conference, New Orleans, (2004).

[27] Massachusetts Institute of Technology, User Guide for SHEAR7, Version 2.0, Department of Ocean Engineering, 1996.

[28] Massachusetts Institute of Technology, SHEAR7 Program Theoretical Manual, Department of Ocean Engineering, 1995.

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[30] J. Remery, R. Gallard, B. Balague, Design and Qualification Testing of a Flexible Riser for 10,000 psi and 6300 ft WD for the Gulf of Mexico, Deep Oil Technology Conference, Louisiana, New Orleans, (2004).

[31] B. Seymour, H. Zhang, C. Wibner, Integrated Riser and Mooring Design for the P-43 and P-48 FPSOs, OTC 15140, Offshore Technology Conference, Houston, Texas, (2003).

[32] P. Elman, R. Alvim, Development of a Failure Detection System for Flexible Risers, 18th International Offshore and Polar Engineering Conference, (2008).

[33] D.E. Thrall, R.L. Poklandnik, Garden Banks 388 Deepwater Production Riser Structural and Environmental Monitoring System, OTC 7751, Proc. of the 27th Offshore Technology Conference, Houston, Texas, (1995).

[34] L. Deserts, Hybrid Riser for Deepwater Offshore Africa, OTC 11875, Offshore Technology Conference, Houston, Texas, (2000).

[35] M. Wu, P. Jacob, J.F.S. Marcoux, V. Birch, The Dynamics of Flexible Jumpers Connecting A Turret Moored FPSO to A Hybrid Riser Tower, Proceedings of D.O.T XVlll Conference, (2006).

[36] J.K. Vandiver, L Li, User Guide for SHEAR7, Version 2.1 & 2.2, for Vortex-Induced Vibration Response Prediction of Beams or Cables with Slowly Varying Tension In Sheared or Uniform Flow, Massachusetts Institute of Technology, 1998.

[37] J.K. Vandiver, Research Challenges in the Vortex-Induced Vibration Prediction of Marine Risers, Offshore Technology Conference, Houston, Texas, (1998).

Top Tensioned Riser Systems

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