Development of a Failure Detection System for Flexible Risers
Figure 2: FLEXASSURE System Operating Diagram
information to protect the asset from further damage or potential loss of the riser system.
Sub-system 4 – Detailed Data Analysis and Reporting
The data recorded on the vessel are stored in a database for each riser. These data can be sent onshore for detailed post-processing.
Laboratory Test Description
Sequences of tests were carried out in the Submarine Technology Laboratory (LTS) at the Federal University of Rio de Janeiro (UFRJ). The objectives of the tests were to determine the effectiveness of several sensor packages to consistently detect the successive failure of armour wires. The test set-up and sensors used are described below.
The flexible riser test piece is a typical 6-inch flexible riser approximately 4.3m in length, terminated with a connector at each end, as shown in Figure 3. The internal diameter of the flexible pipe is 5.9 inches.
Figure 3: Flexible Riser Tension Test Apparatus
Connector A
Tension
Connector B
The flexible riser contains a total of 144 armour wires, of which 73 make up the outer layer of wires and 71 the inner layer. The pitch of the wires is 30º, and each layer is wound in the opposite direction to the other, as shown in Figure 4. The thickness of each armour wire is approximately 2.6mm.
Flexible riser Fully fixed end Simple support Exposed wires
The test rig consists of a steel structure with two load frame assemblies equipped with hydraulic actuators to tension the flexible pipe. Tension in the test piece is carefully controlled by a computerised data acquisition and closed-loop control system.
Figure 4: Flexible Riser Armour Wires
External coating Connector A
Tension
Connector B 30º
During the flexible riser failure test, the riser pipe is installed in a horizontal position and a section of riser coating close to the fixed end is removed to expose the armour wires (see Figure 5A). Connector A is simply supported at the point where the tension is applied, and the opposite end of the riser is fully fixed.
Flexible riser Fully fixed end Simple support Exposed wires
After the tensioning system has been calibrated and the monitoring devices installed, the following test methodology is applied for the laboratory test:
Outer armour wire layer
Figure 5: Onshore Test Sequence
AB
Inner armour wire layer
• The nominal tension load applied is 80 metric tons. This represents normal operating conditions and is approximately 30% of the breaking strength.
• Both static and cyclic tension loads are applied from 40 to 80 metric tons. For some cases the maximum tension load is increased to 100 metric tons.
• The outer and inner armour wires are cut by hand using a handheld rotary tool fitted with a cutting wheel. The wires are cut to a depth of approximately 2.0mm (see Figure 5B).
• The riser is placed under increasing tensile load until the wire fails. • As soon as the wire fails, all tensile loadings are suspended (see Figures 5C and 5D).
C D
• A square cut-out is made in the external coating 70mm from the region where the wires are cut. Tensions are measured in the wires in this region to verify whether the external coating has any effect on the dissipation of wire tension after breakage.
This procedure is repeated for a total of 30 wires, 27 located at the external armour wire layer and three located at the inner armour wire
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EXPLORATION & PRODUCTION – VOLUME 8 ISSUE 1
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