D. B. JOHNSON, D.D. BALDWIN, J.R. LONG
Over the HNBR was wound a layer of glass hoops and a layer of glass helix which compact the rubber and provide extra abrasion protection for the HNBR. After the overwrap was completed, the mandrel/wound assembly was placed back into a rotating cure dolly, and oven cured. After cool-down, the mandrel was removed and the part was inspected, before proceeding to acceptance testing.
Acceptance Testing. As part of the fabrication process, each CPR joint was subjected to a hydrostatic leak test of 1000 psig to verify liner and composite integrity. Next, the joint was weighed, and a final dimensional inspection was performed. Finally, the data logbook with material certifications was reviewed and approved, and the joint was crated for shipment.
Testing Program Objectives
To confirm the advantages of and secure operational staff and certifying authorities confidence in CPR's, a comprehensive testing program was required to fulfill the following main objectives:
1. Establish performance data for basic materials (fiber, resin, adhesives, liner, metals), composite constituents, and the metal-to-composite interface for use in design and analysis of the CPR joint.
2. Identify performance limitations and establish the expected failure envelope (failure modes and associated failure criteria) for the full-scale diameter CPR with the proposed metal-to-composite interface under the different loading scenarios (short term rupture, static fatigue and cyclic fatigue).
3. Confirm the approach and the strength factors used for designing the CPR.
4. Validate that the manufactured CPR meets the performance requirements for application on a TLP in the Gulf of Mexico.
5. Generate data which, in conjunction with the analysis effort, could be used to design CPR's of different sizes and for different operating conditions. This will allow for site- specific qualification of a CPR with minimal testing.
The testing program focused on testing the CPR joint in axial tension, internal and external pressure, and a combination of these loads. The bending capability of the CPR was not assessed since bending is not required by the functional specification and performance criteria proposed for the CPR in this project. These effects are encountered in the bottom-most tapered stress joint and the upper section of the riser, generally above water. The remainder (approximately 90%) of the riser experiences tensile loads in combination with pressure loads. The focus of the current development is the central section of the riser. Recommendations are that the bottom-most tapered stress joint and the upper section of the riser from the platform to approximately 180 feet below the water line remain steel, as in current practice, due to fire safety and in-service damage considerations. Therefore, the testing did not include assessment of fire resistance or in-service damage, although handling and transportation damage were assessed.
Testing Procedures
The testing program was organized into two parts:
1. Tests to verify the proposed design and manufacturing process and to establish the static failure envelope of the CPR.
2. Tests to characterize the final design and establish performance limits.
Ultimate Pressure Test with End Load Effect. The objective of these tests was to evaluate the capacity of the CPR specimens under combined pressure and axial loads. End caps were used to close the specimen ends, and the pressure end load was reacted by these end caps back into the specimens. The specimens were placed on wood supports on the ground, with the ends free and unrestrained. A pressure transducer was placed in the port of one of the end caps and the pump was connected to the port of the other end cap. Strain gauges were mounted on each specimen at the center and over the trap. Displacement transducers were installed to measure the length change over the entire sample length, and over each MCI. The specimen was pressurized to a low initial pressure (approximately 1000 psi) and the strain gauges and displacement transducer functionality was checked. Pressure was applied continuously to burst, with a time to reach failure of 2-3 minutes. Fig. 7 shows a typical pressure vs. time plot.
Ultimate Pressure to Burst without End Load Effect. These specimens were tested in similar manner to the above except that the effect of pressure end load was removed from the sample. This was achieved by pressurizing the specimen in a load frame and using the frame to apply a compression load at the ends of the specimens equal to the end load generated by the pressure. This load was increased as the internal pressure was increased until pipe burst occurs.
Ultimate Tension, These specimens were tested in axial tension in a 4000 kip test frame, with no applied bending, or external or internal pressure. Specimens were instrumented with strain gauges and displacement transducers similar to the pressure tests. The specimens were loaded to a low initial load (approximately 100 kip) and the gauges and displacement transducer functionality checked. Tensile load was then applied continuously to failure, with a time to reach failure of 2-4 minutes. Fig. 8 shows a typical tensile load vs. time plot.
