Advance Pipeliner

For long, high-pressure offshore pipelines in the range of 200-800 km, the pressure drop from the upstream to the downstream end is in the range of 50-100 barg. The downstream end of the pipeline will normally not see pressure close to the design pressure. Codes normally require a pipeline to be designed with a uniform design pressure and overpressure protection at the upstream end. There is, therefore, a potential for cost reduction and capacity improvement if two, or several, sections of different design pressure could be used without having to implement sub sea pressure regulation and overpressure protection facilities at the point of transition between the different sections of design pressure. In determining the lower design pressure, the shutdown of the pipeline outlet facilities, at any point in time allowing for a practicable, achievable delay for closure of the upstream inlet valve has to be taken into account.
A fundamental requirement for this concept is that the pressure regulation and overpressure protection systems, which are implemented, will give the same level of safety as a pipeline with a uniform design pressure.

In the Norwegian Continental shelf (NCS), there are three pipelines in operation using this concept, and three are under development.

1. The Zeepipe IIA is a 300 km 40″ pipeline system that transfers dry natural gas from the Kollsnes terminal to the Sleipner riser platform (SLR).
2. Europipe II is a 660 km 42″ pipeline from the gas treatment plant at Kårstø to Dornum in Germany.
3. Åsgard transport is a 710 km 42″ rich gas pipeline from the Åsgard field at Haltenbanken to Kårstø treatment plant.

The pipelines under development are:

1. The 670 km 42″ Langeled from Nyhavna in Norway to Sleipner riser platform
2. Zeepipe IIA from Kollsnes to Sleipner (New upgrading)
3. Zeepipe IIB (40″) from Kollsnes to Draupner

Zeepipe IIA/IIB are planned upgraded in 2005-2006 to higher design pressures and utilization according to DNV-OS-F101.

References:

1. Designing Offshore Pipeline Systems Divided into Sections of Different Design Pressures, G Staurlang, et al., IPC04-0217, International Pipeline Conference, 2004.

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Deep water risers are usually so long that significant currents will excite a natural bending mode that is much higher than the fundamental bending mode. Since, deep water currents usually change in magnitude (and direction) with depth; it is possible that multiple modes of the riser can be excited into vortex induced vibration (VIV). This makes deep water riser VIV prediction much more complex than that for the short riser spans typically of fixed platforms in shallow water.
For a pinned-pinned beam with varying tension, stiffness, and mass, the nth natural frequency is given by solving the following equation:

Where, T(s) is the tension, EI(s) the bending stiffness, mt(s) the mass per unit length, and ?n the nth natural frequency of the structure:
For this configuration the nth mode shape of the riser is:

And the curvature along the riser us expressed by the following relationship.

The riser stress can be calculated using the abovementioned equation and general long beam theory of elasticity.
The shedding frequency of the fluid passing the riser is;

Where, St is the Strouhal number, which is function of the Reynolds number and roughness of the structure, and V denotes the current velocity.
All riser vibration modes with a natural frequency within a frequency band around fs are assumed to be excited. The frequency band for cross-flow and in-line is respectively.
The fatigue damage at location x of the riser then can be calculated using the following relationship:

The fatigue damage Dr(x) due to excitation frequency, ?r, is given by:

Where, Sr,rms(x) is the RMS stress due to the rth mode, m, and K are material fatigue constants.
Experiences in deepwater riser design show that often, a deepwater riser will fail to meet the fatigue design criteria due to VIV. So, generally designers may choose one of the following methods to overcome this problem:

1. Redesign the riser either by changing the mass, increasing the tension, or changing the whole riser design
2. Add a VIV suppression devices to reduce the vibration

REFERENCES

1. DNV-RP-F204, Riser Fatigue.
2. Subsea Pipelines and Risers, Elsevier Science Ltd, 2005.
3. SHEAR7 Program Theoretical Manual, Massachusetts University of Technology, 2005.