Due to the great volume of the oceans, literally anything that is placed in it and is not anchored in some way will gradually drift, no matter the size or weight. Of course the heavier the object, the slower the drift will be. Concerning an HC, with a full HC weighing about MT and being placed in waters which on average have ocean current speeds of m/s ( ft/s) it is calculated that an HC will drift about cm ( in) per year.
At that drift rate an HC is expect to take years to move 2.5 cm (1 in) or only cm ( in) in 1000 years.
This is actually the projected drift for any size of an HC, because HC's are projected to expand in proportion to the population increase, therefore a minimum sized HC will the same weight to floating unit ratio as a full HC. In other words, it will have the same weight density and floating units in the water as a full HC.
The main reason why this drift is slow low is because of the combination of the great weight of an HC and the fact that an HC will be built on floating units rather than having a full hull base. Therefore ocean currents will have a 'path of least resistence' to flow through and thus mostly will flow around a floating unit. This is also like a ship propelled by sails, pulling down most of its sails. There are therefore less sails to catch the wind and therefore less forward force that can be captured. With a full HC being planned to have about floating units this represents only of the space that a full hull would have taken, thus reducing the potential drift power of the passing ocean current by .
In addition to this, to help streamline the flow of water around a floating unit, during construction, each floating unit will be placed diagonally to the location's prevailing water current direction. So instead of having a perpendicular front face to full push upon, the flowing current will have a front at an angle of 45o. (See image below). Simple calculations show that this streamlining will result in about a reduction in the ocean currents power. Also, to insure that this favorable diagonal direction is maintained, an simple mechanical system which enables each floating units to be turned may also be implemented. It would be a sort of a turntable palced between the top of a floating unit and the HC's main deck.
In order to be prepared for the unlikely event that drift of an HC should increase significantly, several measures can also be easily used. An increase in drift force upon an HC can easily be countered by having a citywide waterjet propulsion system, with, at the most, an output jet nozzle at each floating unit, which would be periodically activated, for a short period of time, to return or keep an HC at its original (or preferred) 'geographic' location. By being a waterjet system, very little material will be needed to build the system. It could also be built in into the city's water intake system.
Another already present citywide system than can be used in counter-drifting is the HC's possible marine turbine current (MCT) energy system. A sufficient amount of the propellers of these Marine Turbines can easily made to spin at a necessary 'reverse' force to reposition an HC. The feasibility of this measure, along with the waterjet system, can easily be seen in the fact that (1) engines, light enough to be aboard a floating vessel, are used to propel large ships through waters at many times the speed of oceans currents; (2) rope(s) can be used to keep a boats and ships acosted to a dock; (3) an anchor can hold a large ship in place in the middle of the ocean. Incidently, mooring an HC can technically also be done, but having millions of ropes thousands of meters|feet long moored to the bottom of the ocean would physically be a tremendous task and would have a significant, if not detrimental, enviromental impact. May 28, 2008