Q? What is Active Nautical Depth?

A. Active Nautical Depth is an alternative to capital and maintenance dredging to keep harbours accessible to ships. It is an “anti-siltation”, as opposed to a dredging, method. The former relates to suspensions, whereas the latter involves digging up bed deposits. Active Nautical Depth involves fluidising the bed, keeping it that way and then permitting vessels to sail through overlying water, together with the carefully controlled near-bed “conditioned” fluid mud layer. It is an in-situ method requring neither a dredging nor a disposal licence. The method is applicable to pure mud as well as to sandy mud mixtures. In the latter case, sand is separated and disposed of. The method is inapplicable to sands, which are inert and less problematic. Deliberately sailing through natural fluid mud layers has been practised since 1974. The refinement of fluidising firm mud deposits as an alternative to removing them has been implemented since 1990. Both practices have been ratified for world-wide use by PIANC – the body regulating the international port and shipping industry.

Q? Emden (the prototype port for Active Nautical Depth) is a special case?

A. Laboratory (rheological) tests on 210 mud samples from ports all round the world, including Emden, show a common behaviour . It is reasonable to imply that every one of these could be managed by Active Nautical Depth. In practice, the procedure has been applied to muds in 10 or more ports, all of which behaved in the same was as that in Emden. The method is being adopted now by ports in a range of countries as a routine management procedure.

Q? Fluidisation might apply to locks, docks and port entrances but not in tidal rivers.

A. The rheological properties of muds and the vessels used to fluidise them are identical irrespective of location. Active Nautical Depth has been successfully applied to port entrances, fairways and at vessel anchorages. Long term observation of a fluid mud cloud in an entrance shows that its volume “waxes” and “wanes” with time without it either flowing away completely or building up and overwhelming the entrance. Conditioned material in a channel axis has been observed to migrate up- and down-estuary with the tide. Thus, rather than forming a silt trap, as expected using conventional methods, the estuary behaves as if the harbour were no longer present. This has a number of major coastal engineering and environmental benefits. In contrast, a trailer suction hopper, cutter suction or water injection dredger uses a fast speed and high power dredge pump, leading to significant plumes of disturbed material in the water body. Overspilling a hopper to raise its solids content, together with dumping a hopper load back into a water body, also induces large scale spreading of released mud.Consequently, Active Nautical Depth is quite different from conventional mud dredging.

Q? Fluidisation associated with Active Nautical Depth will degrade surrounding water quality by raising the biochemical oxygen demand. (Implied anaerobic organic material disturbed with the mud uses up dissolved oxygen once mixed into oxygenated overlying water).

A. The opposite is the case. As explained in 3 (above), the gentleness of the method ensures that fluidised mud is kept segregated from the overlying water body. Furthermore, passage of the propeller of a vessel close to a natural settled or fluid mud bed entrains large quantities of anaerobic mud, leaving a turbid wake behind the ship. This wake induces a biochemical oxygen demand. Equivalent passage over or through oxygenated fluid mud engineered by Active Nautical Depth leads to beneficial entrainment of aerobic mud. The concept known as “Environmental Windows”, whereby conventional dredging, with its unavoidable turbid clouds, is forbidden when water temperatures fall below 10°C, is irrelevant as this modern procedure is not accompanied by widespread entrainment. With conventional dredging such action is undertaken to protect the organisms in the water body or on the bed.

Q? Fluidisation, the essential process in Active Nautical Depth, destroys bed fauna and flora.

A. The opposite is the case: Natural fluid mud deposits are always barren. Their lack of physical strength combined with anaerobic nature render them unable to be colonised by burrowing or surface-dwelling organisms. Similarly, conventional dredging of consolidated mud deposits results in removal and death of all of its associated fauna and flora (usually material is disposed of in water bodies – temperature, salinity, burial risks – where organisms can’t survive). In contrast, Active Nautical Depth oxygenates the engineered fluid mud cloud rendering it less hostile to organisms.

Q? If no silt is removed the mud in the entrance or in the system generally must build up.

A. The opposite is the case. Periodic removal of mud from a sheltered entrance creates a demand to fill it again in an attempt to regain an equilibrium situation. Replacing the settled mud formerly dredged with the engineered fluid mud cloud causes the entrance to behave as if it is already silted up, thus cancelling the demand. Conventional large scale dredging and disposal leads to undesirable whole-system erosion. It is not the duty of a port authority to maintain a fine sediment equilibrium in a coastal system. Instead it is to handle ships for trade.

Q? Active Nautical Depth operations are not profitable for dredging companies.

A. The opposite is the case. The purpose of the operations is to “create and guarantee navigability”. Actual operations are highly profitable for the contractor. Similarly, slow-running of all onboard systems minimises fuel consumption and plant wear. Operations are undertaken within a commercial niche in which traditional dredging operations are forbidden. Hence these activities complement rather than compete with traditional market practice. Once navigability is assured the vessel goes to standby until such time as material changes demand further operations. This is still paid work. One of the two inventors of this technology has been a long term President of CEDA (Central Dredging Association).

Q? Active Nautical Depth is not accepted by shippers.

A. The opposite is the case. Navigating through a fluid mud cloud was first appraised and ratified for world-wide use by PIANC, the body regulating the international port and shipping industry in 1983. In 1997 a more detailed evaluation was carried out by PIANC, IAPH (International Association of Ports & Harbours – a sister organisation to PIANC), International Association of Harbour Pilots and International Association of Lighthouse Keepers. This endorsed and supplemented the earlier study. In 2008 PIANC published Report 102 entitled “Minimising Harbour Siltation” in which all generic methods to manage muddy sediment in the optimum manner were properly specified and quantified. The effects of fluid mud on all aspects of vessel behaviour have been evaluated over a prolonged peirod and properly documented.

Q? What sequence of procedures is needed at a new site?

A. As the first step, disturbed (ie. grab) samples of the mud at the site in question are obtained and subjected to tests in a laboratory rheometer. This quantifies the strength, mode of failure, and flow attributes of the material. Experience has shown the behaviour will fall within a typical swarm or field of data points. In the second step, a temporary conversion of a standard trailer suction hopper dredger is made. The main alteration is providing a second (down) pipe to complement the dredge (raising) pipe. Such a vessel is inappropriate in a number of respects but these inexpensive alterations permit the vessel to fluidise the beds in question as an inexpensive initial step. Such vessels have too much power, unsuitable pumps etc. but can create a crudely engineered fluid mud cloud. The third step is to apply an instrument to quantify navigability and the nautical depth. Later this instrument will be the primary tool used regularly in bathymetric surveys proving the available depth above non-navigable bed. These days the navigable depth is specified by a dynamic resistance parameter, generally the shear strength and viscosity, rather than a static materials property like density. The nautical depth is the point of minimal viscosity on the shear curve. The fourth step is to provide access to a properly designed and built Conditioning Vessel to be available on standby to guarantee safe ship passage 24 hours a day, 365 days a year as monitored and confirmed by the nautical depth surveys.

Q? Does sailing through the fluid mud cloud have any detrimental effects on ship behaviour or cooling water flow to engines?

A. The effect of sailing with a negative keel clearance in natural and engineered fluid mud clouds is well established both from physical tests and mathematical model studies as well as from prolonged fullscale experience. The lengths of fluid mud clouds in fairways, turning circles or dock entrances is generally short. Influences include squat, trim, draught, power, turning ability (rudder angles), speed, etc. (see above). Mud entering the cooling water system of a vessel will be pumped at high speed and be Newtonian or near-Newtonian in its flow properties. The ingested muddy suspension does not pass through the engine but, instead, through a heat exchanger or radiator around the engine. Many vessels sail regularly into fluid mud ports and there has never been any example of engine or cooling water system damage caused by such prolonged operation.

Q? Does fluidising the muddy bed destabilise harbour works such as piled jetties, stone, concrete or steel quays or wharfs, etc?

A. Active Nautical Depth to create a fluid mud cloud offers more support for such structures than ambient water itself, but less support than a rigid muddy bed with an elevated shear strength. Consequently, neither the close proximity of such harbour works nor, indeed, channel slopes themselves should be excavated without due regard for the geotechnical properties of channel or bank materials. Usually the depth excavated below an equilibrium or regime elevation is modest compared to the depth of foundations themselves. Active Nautical Depth is generally an alternative to whole bed removal occasioned by maintenance dredging. Capital deepening, either by dredging or Active Nautical Depth, needs to take account of geotechnical properties of surrounding media.

Q? How often do Active Nautical Depth operations need to be undertaken?

A. Were fluidising operations to be undertaken in the absence of the other necessary sediment changes, the fluid mud would immediately begin “de-watering” and consolidating. This would be equivalent to the well-established “hindered-settling” condition long recognised and well appreciated in laboratory settling columns. Natural, anaerobic fluid muds behave in this manner. After as little as 3 weeks further mixing would be necessary. Active Nautical Depth involves the element of oxygenating the medium in order to activate aerobic bacteria, subsequent to which the material is placed back in situ. Prolonged experience at the prototype port, Emden, mirro-red by Bremerhaven, Botlek, and 2e Petroleum Haven in Rotterdam, is that the more times fluid mud is “conditioned” the longer it can remain stable without further intervention. In other words, it has a “memory”. Re-mixing episodes every 4 weeks progressively extend until in Emden it has now reached 1 year (with regular monitoring). Over the 20 year period any sand has been removed, anaerobic bacteria have been disabled (so no methane, ammonia or hydrogen sulphide gases are produced), and the noxious contaminant, tri-butyl tin, has been broken down and destroyed.

Q? Can Active Nautical Depth be applied to systems with no or negligible through flow – such as impounded dock systems?

A. These, so called, auto-flushing systems for muddy materials have been experimented with for at least 160 years. Using simple systems – a settling system (via the water body), a collection system (gravity flow down an inclined slope), and a discharge system (a bed-mounted submersible pump), it is now possible to collect and discharge fine material back into a coastal water body at the same rate at which it enters. This is much more desirable than by resorting to occasional large scale dredging and disposal episodes.