FRANCE: Hybrid lines: lightening loads to increase drilling riser performance

Institut Francais du Petrole is working on technology to reduce riser mass by replacing the present peripheral lines of drilling risers (kill and choke lines, booster lines, etc.) with lighter lines.

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By C. Gaillard, J. Guesnon, E. Laval
Institut Francais du Petrole

Institut Francais du Petrole is working on technology to reduce riser mass by replacing the present peripheral lines of drilling risers (kill and choke lines, booster lines, etc.) with lighter lines. This is achieved by using hybrid tubes composed of a steel core hoop-wound with a carbon/polyamide thermoplastic strip. End fittings are welded onto the steel core prior to hoop winding.

Kill and choke lines have to withstand high internal service pressures, up to 15,000 psi according to the blowout preventer, as well as axial loads depending on the integration and testing procedures. Conventional 4.5-in. inside diameter (ID) lines would normally require a 1.5-in. or more wall thickness, whereas the use of carbon polyamide strips, hoop-wound over the steel core, allow the steel wall thickness to be reduced to 0.5-in. This translates into a weight reduction of the peripheral lines of approximately 50%.

During fabrication of the hybrid lines, the tube rotates around its axis and an arm, moving parallel to the tube, applies a well-controlled tension onto each strip, thus providing a pre-stress in the steel core. Whenever such a hybrid line is pressure loaded, the steel core mainly resists the internal pressure until the hoop stresses reach the yield strength of the steel, which occurs beyond the test pressure of 22,500 psi. Then, any further increase in the internal pressure is mostly supported by the composite hoop winding. Burst of the hybrid tube will not occur until the fibers of the strip break.

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Prototype of a hybrid line ready for field testing.
Click here to enlarge image

Thorough studies have been carried out at IFP to optimize the number of carbon layers and the tension to be applied to each layer to meet particular specifications for peripheral lines of drilling risers.

Composites Aquitaine built a dozen prototypes based upon this design. These were extensively tested at IFP with the supervision of an independent certifying authority. The various tests have been chosen as representative of the whole working life of the peripheral lines of drilling risers such as simple burst, burst with temperature, long-term pressure, cyclic pressure, cyclic temperature, and external pressure tests.

Particular attention was paid to the transition sections between the hybrid tubes and the end fittings. Tests were carried out to check the designed geometry (cone angle, local reinforcement). Results of the qualification tests have proven the good behavior of hybrid tubes, even after fatigue. A hydrostatic test under 10 ksi external pressure has also been performed without collapsing the tube. Additional tests have been carried out, such as impact tests to determine the burst resistance of hybrid tubes after shock, or thermal insulation tests to assess their insulation power.

The next step of the development phase has consisted in building three lines, each 70-ft long, to be fitted into existing riser joints, and field tested under actual operating conditions. The lines have been shipped to Angola where the test is being carried out aboard a drillship operating in the Gulf of Guinea. Following this campaign, the lines will be brought back to France for inspection and testing.

Benefits of hybrid lines

Advantages of hybrid lines are presented below through an example. Considering a 7,500-ft long, 21-in. riser for the Gulf of Mexico environment, with two, 4-in. ID by 0.953-in. kill and choke lines rated to 15,000 psi working pressure; and a 6-in. ID by 0.5-in., 5,000-psi working pressure booster line; the mass of such a riser is approximately 4,800 kips. The maximum top tension is about 1,700 kips (with 16-ppg mud density and a residual tension at bottom of 220 kips).

As mentioned before, the main advantage of hybrid lines is the reduction of the riser mass. These tubes are approximately 50% lighter than equivalent, all-steel tubes (22.8 lb/ft versus 50.5 lb/ft for kill and choke and 18.5 lb/ft versus 34.75 lb/ft for booster), which leads to a mass reduction of 1,000 kips, including the reduction of required syntactic foam. The top tension is thus decreased by 100 kips, the outside diameter (OD) of buoyancy modules by 5-in. In this case, the overall OD is 48-in. vs. 53-in. Moreover, because of the decrease in tension, the wall thickness of the main pipe can be reduced by 1/16-in., thus improving the dynamic behavior and increasing the fatigue life of the riser.

Hybrid lines also can be an effective solution for upgrading drilling risers. Existing 10,000-psi lines can be replaced with hybrid, 15,000-psi tubes without increasing weight and top tension. Similarly, existing 3-in. ID lines can be replaced with hybrid 4-in. or 4.5-in. ID lines.

These advantages can be translated into economic terms. Direct savings, due to the reduction in OD of the buoyancy modules and the reduction of the wall thickness of the main pipe, can be readily quantified. Induced savings on the rig, such as the reduction in dry weight, storage space, and tensioning capacity, must be assessed on a case-by-case basis.

Based on actual projects, the ballpark cost of a drilling riser equipped with hybrid lines, taking into account only the direct savings, is approximately 5 to 8 % higher than the cost of a drilling riser with conventional steel peripheral lines. However, when the induced savings are also taken into account, the drilling riser with hybrid lines leads to an equivalent reduction of 10 to 15% of the total cost of the riser.

Due to its technical and economic benefits, as well as a significant improvement of drilling riser performance, this innovative technology is well suited to ultra-deep drilling operations.

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