This fall Marintek, in cooperation with Statoil, will kick off a joint industry project addressing issues of riser/soil interaction - The Catenary Riser/Soil Interaction Model for Global Riser Analysis (Carisma).
Steel and titanium catenary risers have been proposed as cost-effective alternatives to flexible risers both in deep waters and for high-pressure, high-temperature service. However, existing mathematical models for riser/soil contact are too simplified to capture the complexity of this interaction, and the JIP aims to develop an enhanced model for use in design, says Knut-Aril Farnes of the Statoil Research Centre in Trondheim.
The project will be executed by Marintek, the Norwegian Marine Technology Research Institute, which will develop the model based on a series of geotechnical tests.
The interaction model which is the basis for the JIP is derived from a pipeline model developed by Statoil and the Sintef Research Institute. This model has clear advantages compared with the standard Coloumb friction model currently in use in the industry, but being based on pipeline behavior, also has acknowledged limitations, Farnes says.
Whereas a pipeline moves only when it is being laid, a riser is always in motion. These movements can lead to deep penetration into the clay soil of the seabed. Risers have been observed to sink in more than six times their own diameter. However, the pipeline model is based on laboratory data with pipeline penetration limited to one third of the pipe diameter. Hence extension and verification of the model for greater penetration is necessary.
Valuable benefits should accrue from the Carisma model. It will, for example, make possible more realistic predictions of fatigue life in the touch-down area. Another feature is that it will also predict trench development. Based on long-term statistics of vessel motion, riser and soil data, it will predict the depth and shape of the trench which will be formed.
The Carisma project should also throw light on a series of unknowns associated with the behavior of a riser on the seabed. For example, some preliminary studies indicate that the suction forces from the soil may increase the fatigue damage significantly when the riser is lifted from the seafloor due to the motion of the production vessel in line with the riser, Farnes says.
The project will study these suction effects carefully to see if the catenary shape of the riser will produce any zipper effects or if the suction forces are reduced by shear strength degradation due to small vibration of the riser prior to the uplift. Other effects to be studied are resistance in bell-mouth type trenches, overfill of soil on top of the riser, and consolidation of the soil beneath the riser.
Because of Carisma's expected inherent complexity, an important delivery from the project will be a guideline to support the designer in performing riser analysis. Although the focus is on metallic catenary risers, the model will also be applicable to other riser configurations and pipes, such as flexibles, cables and umbilicals.
The project has a budget of NKr 4.4 million and is scheduled to be completed in 2000.
