SMD Hydrovision unveils AMP500 ploughVessel & ROV News // July 9, 2004
The Advanced Multipass Plough, or AMP 500, represents the culmination of over 20 years of development by SMDH in subsea pipeline burial.
The plough, built for CTC Marine of Darlington, UK, and its host vessel - the Maersk Assister - will, claims the company, be the most advanced and capable pipeline trenching system in the world.
The AMP500 boasts a number of innovations which will extend the capability of the 350 tonne bollard pull plough to an effective 500 tonnes.
In simple terms the AMP500 comprises a chassis which carries a share (the part which digs the trench) split in two halves; pipe grabs which can load up a 52" OD pipe through the open share; support rollers on which the pipe rests during trenching; skidswhich support the front of the plough and control the depth of cut; and a towing arrangement which also steers the plough. The latter which is a patented system used on all SMDH ploughs, is very important since it enables the plough to steer accuratelyalong the pipeline independently of the course of the towing vessel.
The AMP500 can create up to a 2m deep 120° stable vee trench in a single pass. It can also take a second cut, or "multipass", to create up to a 2.5m deep trench. The 180 tonne machine is fitted with a buoyancy system which means it can cope with soil strengths as low as 4 to 6 kPa. This is the type of soil the average person could stand on without sinking.
Ploughing's principle advantages over other pipeline burial techniques such as jetting or mechanical cutters is its speed and its ability to cope with a wide range of soils from very stiff clays to sands.
However, certain fine, dense sands with low permeability can reduce ploughing rates to a snail's pace below 50 m/hr. This can be very costly if the sand is over any significant distance. With existing systems the only way to improve progress is to shallow out which may mean the burial being "out of spec". This is also potentially very costly if remedial work such as rock dumping is required. The AMP 500 is fitted with several innovations to overcome this problem.
The energy which is imparted to the plough through towing is divided into useful energy which fails the soil, and energy which is wasted overcoming friction between the plough and the seabed. The latter is significantly larger in sand which is highly frictional. The AMP 500 employs a new "Hi-Pull" tow geometry which increases the proportion of useful energy and hence the trenching performance of the plough.
The plough also has a more visible innovation for reducing its friction and improving performance in sands - the "Active Share Drive". This comprises a driven caterpillar type track which supports the plough at the rear, eliminating a large proportion of friction and providing additional energy for trenching.The final way the plough can improve its trenching performance in sand is through jetting with the "Tri-Jet System". This injects high pressure water through an arrangement of jets on the share. Their purpose is to assist in overcoming the suctions that are created in shearing low permeability sands, and thus improve the trenching speed. The system has been carefully designed to ensure the bulk of the sand is not fluidised which could destabilise the excavated soil and cause premature backfill.
The combination of the Hi-Pull tow geometry, the Active Share Drive and Tri-Jet System gives the AMP500 an effective maximum bollard pull of 500te. This is a quantum leap over earlier generations of plough systems which are generally 250 to 300 tonnes maximum pull.
To complete the system SMDH will also deliver a Backfill Plough, fitted with a special compactor to improve the quality of the backfill, and a 200te Constant Tension Motion Compensator which will enable the plough to be landed safely over the pipe in high sea states. The combination of these with AMP500 will enable it to set a new standard in pipeline trenching.