Page 42: of Marine Technology Magazine (October 2011)

42MTROctober 2011Trenching First, the cable is laid between two points. At some point after the lay, a trench is then created by a self-propelled trencher that allows the cable to fall into a trough in the seabed. The seafloor material is fluidized by powerful water jets or mechanically cut with a chain or wheelequipped with hardened bits. This is commonly referred to as PLIB ? Post Lay Inspection and Burial. General Plough vs. Trencher Pros and Cons (relative to OWF Inter-Array Cable) Plough Pros Can be completed in one process Generally faster rates than trenching Generally good burial Can penetrate harder sea beds More economical over longer distances Instant cable burial and protection Plough Cons Must be completed properly in one pass Difficult to monitor product cable tension Can cause cable damaging tension spikes Difficult to maneuver and position near foundations More difficult to steer as it is towed Requires tow cable catenary management Requires umbilical management Risk of mechanical impact/abrasion/stress on product cableHigher risk in the vicinity of adjacent cables Plough ?grade-in? and ?grade-out? at each end of span or a recovery leaves exposed cable requiring BLIB Can?t ?cut & grip? damaged cable Requires bigger ship and more crew Trencher Pros Self-propelled control Maneuverable to optimal cable position Can work adjacent to vessel and foundation Multiple pass option to improve burial More deploy/recover operation flexibility Mobilization layout flexibility ? stern or side launch Generally single man launch capable Wider operating weather window Can remain working for long periods Can perform cable/foundation inspections Can ?cut and grip? damaged cable Can conduct cable and scour surveys Can assist with messenger lines in the cable ?pull-in? process Scheduling flexibility ? lay then bury Trencher Cons Generally slower than a plough Less effective in harder sea beds Post-lay burial ? two-part process Generally requires returning after the lay to then trench Requires more power from the ship for cutting or jettingGeneral Mechanical Cutting vs. Jetting Pros and Cons Trenchers are self-propelled, and use either a mechanical cutting or water jetting technology for penetrating theseabed. Since the pros and cons regarding maneuverabili- ty are similar, we will look further at the pros and cons of the soil cutting technology when water jetting is com-pared to mechanical cutting. Mechanical Cutting Pros Can cut harder sea beds Control of cutting rate Completed in one pass Mechanical Cutting Cons Cable is at greater risk of mechanical damage Extreme caution needed around cable Bigger machines 75T+ Cable needs to be ?loaded? into a protective cable highway on the vehicle Must be done in one pass Difficult to bury cable with slack, bends or loops Generally slow to very slow rate of progress Generally more expensive to buy/lease a system Generally more expensive to operate and maintain Jetting Pros Risk to cable minimized Control of cutting force/rate Can run multiple passes to improve burial Jetting arms deploy over cable to capture Can bury cable with slack, bends and loops Adjustable nozzles and arm configurations to accommodate varying seabed conditions Generally less expensive to buy/lease system Generally less expensive to operate and maintain Generally smaller in size using smaller ships MTR#8 (34-49):MTR Layouts 10/11/2011 8:52 AM Page 42