The rock manual - Dredging Engineering Research Laboratory

New Possibilities for Ripper Dredging of Rock 11BERNARD MALHERBEgraduated in 1976 with a MSc inGeology at the University of Leuven(Belgium). In 1980 he completed hisstudies as a Master of EngineeringScience in Geological Engineering atthe École Nationale Supérieure deGéologie (France). He worked in thedredging industry for more than20 years – for a contractor jointventure and as an engineeringconsultant – before joining theJan De Nul Group in 2004. Since thenhe has been employed as AreaManager on many offshore projects.Presently he is Engineering Managerfor project development.Figure 4. Artist’srendering of theVasco Da Gamawhich has amaximum dredgingdepth of 135 m.PETER DE POOTERgraduated in 1990 with a MSc in CivilEngineering at the University of Ghent(Belgium). He worked in theengineering and construction industryfor more than 10 years before joiningthe Jan De Nul Group in 2003. Sincethen he has been employed asEngineering Manager on the offshoreproject Sakhalin II in Sakhalin (Russia)and as Project Manager on severaloffshore projects. Presently he isworking as Project Manager of theManifa Field Causeway and IslandConstruction Project in Saudi Arabia.Consequently, the Vasco da Gama wasselected and fitted with a specially modifieddraghead equipped with an adaptablenumber of ripper-teeth (Figure 4).During this work, this heavy and solid rockripperingdraghead would prove its uniqueefficiency and flexibility. The extreme installedpropulsion power of the Vasco da Gama(29,400 kW of a total installed 37,000 kW)ascertained a continuous and efficientoperation and production, despite the factthat each dredging track commencedinshore with a ground-speed of 0 knots,because the ship is in fact going backwards.Moreover, the vessel was equippedwith an onboard multibeam echo-sounderin its central moon-pool for continuousonline monitoring the dredged profile byFigure 5. Ripper draghead with 1 m long ripper teeth.Figure 6. Drawing of draghead with ripper teeth on seabed.the client and project team on boardwithout having to wait for survey results.The ultimate generation of rock-ripperingdragheads, developed by Jan De Nul forthe Vasco da Gama weighs 60 tonnes, is8 m wide and is fitted with 7 ripping-teeth(1 metre long) on the draghead’s heel and20 pick points on the draghead’s visor(Figures 5 and 6). For security reasons incase of excessive ripping forces, a securityflangesystem with breaking bolts wasinstalled at the draghead/suction-tubeflange.By the time the ship arrived on site inSeptember 2006, new geotechnicalinvestigations had been done and the coresindicated heterogeneous coral reef rocksover the full thickness of the reef:Unconfined Compressive Strength (UCS)values of up to 12 MPa of the coral rock