A ten-month test of large diameter, high pressure flexible pipe was conducted in the Norwegian Sector of the North Sea in 1982-83. The test configuration consisted of a 135 meter length of flexible pipe suspended in a catenary between a subsurface buoy and a surface buoy designed to respond in ten meter seas similar to the response of a large tanker in survival conditions. Test objectives, design, installation and instrumentation are described. General conclusions on the suitability of large diameter, high pressure flexible pipe for near surface, severe environment applications are presented.
Flexible pipe offers the compliance needed to link floating surface platforms with rigid subsea structures and accommodate the relative motions between the end points. Mobil's Deepwater Production Riser, described in detail in an OTC 83 paper, is one application (Fig. 1): the IFP-ELF-TOTAL system, mentioned in another OTC 1983 paper, is another (Fig. 2).
The objective was to provide technical data on the fatigue characteristics of large diameter, high pressure flexible pipe for application in production risers. Specific objectives were to:
Conduct an endurance test on a ten inch, 4500 psi working pressure flexible pipe under severe conditions;
Determine the loadings and motions to which the flexible pipe and its end connections are subjected;
Compare and analyze the components of the flexible pipe before and after the endurance test; and
Estimate the extent of degradation of the flexible pipe.
Configuration of the test installation is shown in Figure 3.
Components of the installation included:
A moored surface buoy.
A 135 meter length of nominal ten-inch internal diameter flexible pipe.
A subsurface bouy.
A deadweight to anchor the subsurface bouy.
Instrumentation at selected points along the length of the flexible pipe and on the surface buoy.
A telemetry system to transmit data from the instrumentation to a data recording location.
Data recording equipment on a nearby platform.
Environmental data from platform instrumentation were also collected at the recording center.
The site for the test was the Frigg Field in the Norwegian Sector of the North Sea. North Sea environmental conditions provided a severe test of the flexible pipe. The Frigg Field facilities offered a convenient location for routine observation of the test installation and for accommodation of data recording equipment, and provided access to existing environmental recording devices. A flat bottom topography and 106 meter water depth were suited to dimensions of the test configuration.
The flexible pipe (Figure 4) was nominal ten-inch (240mm) inside diameter, 2 inch (56mm) wall thickness, of layered steel and thermoplastic Rilsan R (trademark) construction. Its operating pressure was 4500 psi and its burst pressure was 11, 000 psi. Pressure was to be maintained at 4500 psi (about 310 bars) for the duration of the test. The 135 meter long flexible pipe terminated in a pulling head arrangement at both ends.