FRANCE: Hyperstatic riser system lessens buoyancy, wall thickness needs

May 1, 2001
Stress distributed between main, peripheral tubes

Institut Francais du Petrole

Drilling in ultra-deepwater and in very harsh operational and oceanographic environments requires the use of risers with the following characteristics:

  • Large mass and weight in water (particularly when full of heavy mud) leading to very high top tension demands (>1,000 tons)
  • High axial strength to resist the tension and the pressure end-effect of the peripheral lines, which can contribute an additional 400 tons
  • Suppression of buoyancy modules at the riser bottom to improve axial behavior while in hung-off mode, and to decrease the natural period
  • Reduced weight, where possible, of associated buoyancy modules and peripheral lines, which do not improve the riser's axial resistance
  • High dynamic loads, which can lead to fatigue of the main tube and the connectors.

IFP's new Hyperstatic Integration System (HIS) will generate improvements in all these areas. The HIS' peripheral lines are integrated so that they contribute to the riser's axial resistance. The lines play a structural role in the riser, instead of representing a dead weight.

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Under the presently accepted system of peripheral line integration, the lines are fixed to the connector at one extremity of each riser joint, but are free to slide at the other extremity. Hence, they do not participate in the sharing of axial loads. This system is simple, but it can lead to significant movement between the riser main tube and peripheral lines (1-in.).

Another problem is that the tubes are subjected to independent and differing loads due to pressure, tension, temperature and bending effects. To allow such motions, the seal box geometry must be relatively complex. Further-more, the peripheral lines are subjected effectively to compression when under pressure, which can cause buckling. Clamps then have to be applied, which increases the riser's mass.

Circumventing difficulties

The HIS has been designed to circumvent all these difficulties. With this new system, the peripheral lines are fixed to the man riser tube at both extremities of each riser joint. Consequently, the lines participate in resistance to axial riser loads. The stress distribution between the main tube and the peripheral lines is governed by the laws of hyperstatics.

IFP has built a model to evaluate stress distribution between the peripheral lines and the main tube and to validate theoretical computation. This model consists of a small-scale (1:4) riser joint. It has a main tube and two peripheral lines. The integration system can be modified easily to simulate either a standard integration (with sliding end) or a hyperstatic integration system by changing the gap of a stop ring.

Three clamps have been added to the model. They have been designed either to maintain the peripheral lines in a straight position or to permit buckling (with a radial gap). Each tube can be pressurized independently and the whole structure can be tensioned using a hydraulic jack. The peripheral lines can also be heated by means of special belts to simulate temperature effects.

Preliminary development of the hyperstatic riser system has led to the following initial conclusions:

  • Wall thickness of the main pipe (and hence the riser mass) can be reduced significantly. This reduction leads to a large decrease in the amount of buoyancy required
  • The peripheral lines are maintained in a positive effective tension in all configurations. This implies that clamps are no longer required to prevent buckling
  • No sliding problems occur in the peripheral line end fittings as the lines are linked to the connector. The design is therefore simplified
  • There is a possibility of using hybrid tubes to decrease the mass of the peripheral lines. In this case, distribution of stresses between the tubes (main pipe and peripheral lines) would be improved because the peripheral lines are less thick
  • Composite hoop windings could also be applied beneficially to the lower part of the riser main tube, leading to a reduction in wall thickness. With the HIS, this thickness would be governed by the internal pressure, instead of axial tension.

An accompanying table compares characteristics of a classical riser with those of a hyperstatic riser fitted with hybrid steel/composite kill and choke lines, and with a main tube hoop with carbon strips in a sub-2,000-meter water depth. The dry weight (mass) of a riser with hyperstatic integration is 780 tons less than that of a conventional steel riser. The required top tension is reduced by at least 136 tons, and the buoyancy material is reduced by 284 tons.

Such savings are of great importance for drillship design and riser management economy. The riser's relatively low mass and the fact that its resonant period of axial vibrations is well below the usual wave range leads to important improvements in the system's dynamic behavior, both in drilling and hang-off mode. This, in turn, enlarges the operating envelopes and improves the riser's fatigue life.

Comparison of a conventional riser system involving peripheral line integration with the Hyperstatic Integration System.
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