HEAVY DUTY PLOUGH PERFORMANCE IN VERY SOFT COHESIVE SEDIMENTS N° MS05 HEAVY DUTY PLOUGH PERFORMANCE IN VERY SOFT COHESIVE SEDIMENTS Geoff Holland & Sarah Dzinbal ALCATEL-LUCENT STUDY AIMS HD ploughs are now commonly used in the industry: heavier more tow force and up to 3m burial capability. Still somewhat limited data for burial predictions. Alcatel-Lucent has commissioned a series of plough studies in the last few years, to add empirical data for BAS predictions. STUDIES COMMISSIONED PRESUMED BOUNDING CONDITIONS FOR HD PLOUGH BEHAVIOUR * Presented at SubOptic 2010 130T pull 2.3m Share HD PLOUGH PERFORMANCE IN VERY SOFT CLAY: STUDY RESULTS & CONCLUSIONS Cable Route 1, Baltic Sea HD Plough, 1.5m share, just <30T in air, widened skids to minimise sinkage. More variable seabed, areas of very stiff glacial clay, which were removed from the study. The very soft clay had shear strength 2-11kPa. Average tow speed 0.56m/s (2km/hr) Water depth range generally 100-200m Minimal tow wire effect seen, due to the shallow water depths. No correction was made for the tow wire (max 0.5T). Average tow tensions were 15-16T. Higher tensions than Routes 2 or 3, due to accumulation of heavy, stiff glacial clay on the plough. Cable Route 2, Mediterranean Sea  HD Plough, 2.3m share, 30T in air, unmodified. Most representative of a typical Alcatel-Lucent installation. Seabed was primarily very soft clay, with shear strength 2-10kPa. Average tow speed 0.39m/s (1.4km/hr) Water depth range 33-1255m. A significant tow wire effect was seen, in the deepest water depths. An equation was generated to correct for this, and tow tensions were normalised. Average tow tensions were 12T. Maximum tow tension trendline indicates deeper and deeper burial only occurs in soft and softer sediment. Cable Route 3, Mediterranean Sea  Slightly lighter HD Plough, 2.3m share, 25T in air, unmodified. Seabed was primarily very soft clay, with shear strength 2-11kPa. Average tow speed 0.2m/s (0.72km/hr) Water depth range 111-1245m. Tow tensions were already corrected for the tow wire (wire out/unit weight), but an additional minor correction was able to be made from an equation, and tow tensions were normalised. Average tow tensions were 12-14T. The range of average tow tensions was only 2T across the whole burial spectrum, similar to the other two routes. Since the results from the three separate analyses were similar (average 12-16T tensions & similar seabed conditions, 2-12kPa), the data from all three routes could then be analysed together as one data set. Average, minimum and maximum normalised tow tensions were calculated for every cm of burial. The relatively large difference between maximum and minimum tow tensions are probably a result of localised areas of firmer and softer sediments. The study shows that tow tensions, irrespective of share length or plough modifications are 12-14T on average. Increasing burial depth has very little effect on tow tensions — tow wire has more effect. Final Analysis Data From All Three Routes The slight plough configuration differences provided a good opportunity. to cross correlate plough behaviour. Separate analyses were conducted first, as described in the section above. CONCLUSIONS Minimum bearing capacity is 5kPa for HD ploughs. However, under dynamic conditions, plough sinkage and overburial still occur in clays with strengths >12kPa. Average tow tensions in very soft clay (5-12kPa) are 12-14T, and increasing the burial depth has little effect on tension, but… The tow wire itself has more of an effect on tow tensions as water depths increase. An equation has been generated to correct BAS Predictions for the tow wire effect as water depth increases. It can be approximated as: TT Correction (T) = 1.85% x Water Depth Tow Speed does not significantly affect burial depth or tow tensions in very soft clays, but it is believed to minimise sinkage to a degree. Burial Assessment Predictions: Correcting for the Effect of the Tow Wire Very soft clay = low tow tensions. The tow wire has a greater effect than the sediment as water depth increases—as much as 28T at 1500m water depth. Route 2 provided the best data set with regard to a typical Alcatel-Lucent.installation—2.3m-share, unmodified 30T HD Plough. The table above can be used to correct for the tow wire effect, as can the general equation: TT = (0.0189 x WD) + 11.797 This equation can now be used for all seabed types, not just very soft clay.