The flow assurance aspects of all subsea projects have a major contribution tothe pipe design, field layout, choice of lifting equipment (subsea-pump or airlift), power requirement and system operability. The context of deepwatermining pushes the design theories beyond the existing application cases due tothe significantly larger particle size combined with small diameter riser andjumper including wave shape to accommodate vessel motions and excursionrequirements. In order to correctly assess pressure drop and erosion rate closeto real flow conditions, TECHNIP and GIW have built a large scale experimentalbench operated at the same flow condition as forecasted for the deepseaproject. This large scale test is using an innovative method to allow thereproduction of realistic erosion rate in the pipe by preventing the solidparticle to be eroded when looping through the pump.
The current paper summarizes the findings and results from this large scaleexperimental set-up, testing concentration from 10% to 45%, velocities from2.5m/s to 5.5m/s in an 8" flexible pipe with equivalent rocks particles.
As described in (Espinasse, 2010), Technip is supporting an internal R&Dprogram that should allow the understanding of critical parameters essential tothe design and operability of a subsea mining system. Within this R&Dprogram, an extensive study of the abrasion and erosion mechanism inside theflowline is needed to:
• Understand the inner pipe wear mechanism function of flow conditions
• Define the proper flowline pipe material providing the best compromisebetween wear resistance and pipe cost.
• Define a procedure to evaluate the lifetime duration of the flowline pipeduring operations to schedule inspection and maintenance.
To capture and understand the abrasion during subsea mining operations, Techniphas setted-up a full scale test with the help of GIW. In addition of tacklingflowline wear issues, this test is used to validate at large scale thehydraulic modeling exposed in (Parenteau, 2010) and (Parenteau, 2011).
STATE OF THE ART
The particularity of subsea mining is to transport large and dense particle inrather small diameter pipe compared to what the industry of slurry transport isused to. Subsea Mining Partcicle size disctribution can range from 1 mm to60mm. Crushing experience conducted in (B. Waquet, 2011) indicated that atleast 50% will exceed 25mm and more than 25% of the solid will exceed 50mm[Figure 1]. The particle densities range from 2500 kg/m3 up to 4000 kg/m3. Pipediameter will range between 8" to 10", and evolving into wavy shapes.
A recent LWD density log in an exploration well showed excessive abrasive metal loss on the density measurement stabilizer. Towards the end of the drilling run it was noticed that the bottom quadrant density correction (delta rho) was slowly moving from values normalized on zero to a more positive number of about 0.15 g/cm3. Measurements of the density stabilizer diameters performed after the logging run showed the diameter had been reduced by abrasion by approximately 0.2 inch along the entire length of the stabilizer. Therefore, the compensated density measurement was logically questioned.
A post-job calibration showed a significant difference from the pre-job calibration, as expected. What was unexpected was that the compensated density computed from the pre- and post-job calibrations compared favorably at the end of the well, but not at the beginning of the well. This implies that the density correction algorithms derived during characterization will compensate for metal loss but not for metal gain. Monte Carlo N-Particle (MCNP) modeling is used to review this finding and investigate a method to define the amount of metal loss that can be tolerated before compensated density measurement inaccuracies exceed specifications.
In order to compute an accurate photoelectric effect (PEF) and caliper that are derived from the individual short and long detector densities, the pre- and post-job calibrations need to be utilized for processing the data. A new methodology of blending the pre- and post-job calibrations as a function of metal loss was developed to accurately reprocess the density count rate data over the entire drilled interval. The final compensated density measurement from this reprocessing compared favorably to the original compensated density measurement (with only the pre drilling calibration in effect). This blending process resulted in valid single detector and compensated density data over the entire interval confirmed by independent measurements.