Steel or titanium catenary type risers are suggested as cost effective alternatives to flexible risers even for medium water depth, but particularly for deeper water (>300 meters). Risers with simple tubular cross-sections offer several advantages over flexible risers built up from numerous layers of different materials.

Figure 1.
Riser/soil interaction for catenary risers.

Existing analysis tools for riser response have proven reliable for most types of risers. However, in the area where catenary risers are resting on the seafloor (refer Figure 1), existing models for riser/soil contact are too simplified to capture the complexity of the interaction. Some of the key findings of earlier work are that the fatigue life predictions at the touch down area (TDA) are sensitive to the FEM modelling, fatigue accumulation procedures and, in particular, the soil model.

Hence, an important step forward in improving riser analysis tools is to improve the riser/soil interaction model. This issue has been the key objective of the CARISIMA project.

Organization

The CARISIMA work has been organized as a Joint Industry Project (JIP) with MARINTEK and Statoil as executive partners, and with The Norwegian Geotechnical Institute (NGI) as sub-contractor for execution of the laboratory tests.
The project was kicked off during autumn 1999, and will be terminated within spring 2002. The overall budget is 5.0 MNOK in external funding, plus 1.5 MNOK as in-kind contribution. The work has been sponsored by BP, Conoco, Statoil, Norsk Hydro, ExxonMobil, Agip, Texaco, Petrobras and Halliburton.

Laboratory tests

The CARISIMA model development has been based on small-scale laboratory tests performed by NGI. The tests were performed in two phases, phase 1 (14 vertical and 17 horizontal tests) was completed during spring 2000, and phase 2 (10 vertical and 6 irregular tests) was completed during spring 2001.

The clay was taken from a NGI reference site at Onsøy, in the south-east part of Norway. A cement truck was used to transport the clay from Onsøy to Oslo. At NGI the clay was filled into 4 steel bins, and left to consolidate under dead weight for six weeks or more.

Originally, the NGI test rig was designed to be configurable depending on what type of tests to perform, ie, either vertical tests (suction) or horizontal tests (pullout). However, when designing the test series for phase 2, it was decided to modify the rig to allow for simultaneous motion of the pipe sample both vertically and horizontally. Hence, an additional actuator was installed, and the test rig ended up as illustrated in Figure 2.

Figure 2. The NGI test rig.

The vertical and horizontal loads (or displacements) was applied to the model by one horizontal and one vertical actuator. Both actuators were equipped with low friction seals. The actuator system is servo-hydraulic (servo valves on actuators controlling oil flow from constant speed and pressure pump) and closed-loop (operation based on measured performance feedback). As the actuators move, the pipe sample will also move. Stroke range for the horizontal actuator is ±500 mm, for the vertical actuator ±80 mm. Controlling the actuators is done through a computer-based operating system, consisting of a control unit and a PC program.

The main parameters logged during execution of the tests (both vertically and horizontally) were load, displacement and acceleration.

Model development

The raw test data as acquired during the tests went through massive post-processing procedures before generally ending up as dimensionless functions of typically force and displacement (penetration), as illustrated in Figure 3, where all results from the vertical tests in phase 1 are shown.

For the vertical tests, one of the important observations made was that the mobilised soil resistance was very dependent of the lift-off velocity, as illustrated in Figure 4. Hence, this effect has been incorporated into the numerical model.

Figure 3.
Results from the vertical tests.

Figure 4
Effect of lift-off velocity on suction.

Verification and validation

The developed numerical model has been verified by back-calculation of the small-scale laboratory tests at NGI.

Furthermore, as part of an information/data exchange agreement between CARISIMA and the STRIDE JIP (www.stridejip.co.uk), the CARISIMA project gained access to all results from the full-scale Watchet Harbour riser/soil interaction tests. The tests were carried out in spring 2000 in a tidal harbour in West England using a 110 m pipe of 16.83 cm diameter. An actuator was used to introduce vertical or horizontal motions similar to those experienced by a Gulf of Mexico catenary riser (the test site setup is illustrated in Figure 5).

Figure 5 The test rig setup at Watchet Harbour.

The CARISIMA riser/soil interaction model, as implemented in MARINTEK’s RIFLEX software, was validated by back-calculation of a selection of the test series performed at Watchet Harbour.

Deliveries

The main result from the CARISIMA JIP is a mathematical model for riser/soil interaction in clay. The model has been designed for implementation into existing computer codes for global riser analysis and is able to predict both vertical (suction) and horizontal soil resistance. It has sufficient accuracy to give reliable predictions of fatigue life and extreme stresses in the touch down area.

Furthermore, due to the inherent complexity of the model, another important delivery from the project is a guideline to support the designer in applying the model when performing riser analyses.

MARINTEK contact:

(Article in MARINTEK Review No 1-2002)

Published January 31, 2005