A hybrid that can accommodate a number of different situations, flexible risers can withstand both vertical and horizontal movement, making them ideal for use with floating facilities. This flexible pipe was originally used to connect production equipment aboard a floating facility to production and export risers, but now it is found as a primary riser solution as well. There are a number of configurations for flexible risers, including the steep S and lazy S that utilize anchored buoyancy modules, as well as the steep wave and lazy wave that incorporates buoyancy modules.
Flexible risers are being usurped by metallic systems in some of the newer deepwater developments. Shell chose steel catenary risers for its three latest TLPs in the Gulf of Mexico, and Petrobras has followed suit for gas export from the Marlim semi. In Norway, titanium is making its mark in the riser for the Heidrun TLP.
The downside of metallic systems is the extra expense perceived for many applications. For this reason, flexible risers will remain popular for all water depths. However, there are serious design challenges to surmount as development heads deeper.
In a paper delivered at Deeptec '96, Aberdeen, Svein Are Lotveit of SeaFlex put collapse capacity and weight limitations at the top of his list. He also drew attention to related concerns for deepwater flexible pipe, namely:
- General demand for large bore risers
- High pressure, high temperature and aggressive fluids
- Complicated replacement/repair of deep water risers, which needs to be avoided.
SeaFlex, a Norwegian engineering consultancy, is also part of the technical support team evaluating Coflexip and Wellstream pipes at Shell's facilities in The Hague. The project, known as the Deep Water JIP, is led by four major oil companies.
Coflexip and Wellstream are two of the dominant producers of non-bonded flexible risers (bonded pipes are currently not in the picture for deepwater risers). Another emerging heavyweight is NKT (Furukawa). Lotveit went on to outline this trio's current R & D work.
The market leader for deepwater, large diameter and high pressure applications is Coflexip, which favors a five-layered construction for its pipe. The innermost section is a carcass, providing collapse resistance. This is normally cold formed from stainless steel (316L), although Coflexip is working on alternate, T-shaped carcasses made from aluminum or ferritic stainless steel.
A key thrust of its development has been the middle layer hoop spirals. Main aim has been to increase the pressure rating and to improve service life. Traditionally, the hoop spiral has a Z-shaped cross-section (Zeta). However, a new hoop spiral wire geometry has been developed, called T-wire, which has been used in several test pipes but has so far not been installed offshore.
Advantages of the T-wire geometry are said to be:
- Larger and stronger hoop spirals can be made
- Less susceptible to initiation of fatigue cracks than the Zeta. In accelerated tests, it has been shown to improve service life of Coflexip risers.
The T-wire is currently being tested by the Deep Water JIP. To date, thicknesses up to 14mm have been examined, and wires up to 16 or even 18mm may also shortly be tested. This will increase substantially the pressure rating of large-bore Coflexip risers. A further increase in capacity is achieved through use of a secondary hoop spiral outside the T-wire.
T-wire's main impact will be in extreme pressures and dynamic risers. However, long length production of Coflexip risers with T-shaped wires will likely involve substantial investments, meaning that the cost of these risers will be relatively high.
Tension armors in the company's risers are normally made of rectangular steel wires. To increase the depth range of flexible pipes, Coflexip has qualified tension armors of reduced weight composite material. One composite riser has been installed offshore Brazil and is still in operation.
However, the cost of composite-reinforced systems is higher than a steel-reinforced pipe. They are only justifiable, therefore, at great depths where traditional pipes are too heavy. Maximum depth for a steel-reinforced flexible riser in a free-hanging configuration is typically 1,000 meters. For greater water depths, riser weight is critical and there composite may become attractive again, according to Lotveit.
During 1994-95, a number of failures occurred in risers in high temperature service (above 80C). Both Coflexip and Wellstream responded by re-evaluating their designs, making significant changes. This work is still some way from completion.
Coflexip had considered 20 different designs before selecting a prototype for testing. A 20 meter test pipe and a second sample with the old end fittings were both manufactured and tested by cycling the temperature from ambient to 130C. The new end fitting included a steel sleeve inserted under the main fluid barrier in order to control accurately the diameter and circularity of the barrier close to the main pressure seal.
While the failure rate of the old design in tests confirmed misgivings, the new sample did not perform perfectly either. Coflexip then designed and built five new pipes with different end fittings, incorporating different methods of PVDF layer termination. Since June 1995 when the first set of tests were completed, the second series of pipes have proven these end fittings to be stable.
One customer was sufficiently convinced to accept the first dynamic jumper with a new end fitting design, for short-term service (two to five years) in the Far East. This has been in service since last October. Following further tests, Coflexip is now predicting 20 years of service life.
Sources : http://www.rigzone.com/training/insight.asp?insight_id=308&c_id=17http://www.offshore-mag.com/articles/print/volume-56/issue-4/news/general-interest/deepwater-risers-steel-catenary-flexible-risers-battle-for-technical-supremacy.html
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