Semisubmersible and method of its operation.

Semisubmersible and method of its operation

The invention relates to a semisubmersible arranged for offshore operation, comprising a main deck that optionally supports a crane, wherein at subsea level one or more pontoons are provided to give buoyancy to the semisubmersible.

Such a semisubmersible is well known in the art and may be used for different kinds of operation in relatively shallow water of less than 20 m depth, as opposed to jack-islands that are operated at sea with water depths of more than 30 m. Jackislands differ further from semisubmersibles in that they are standing on the bottom of the sea rather than floating on the sea as do the semisubmersibles. The level at which the semisubmersibles float in the sea is controlled by regulating the amount of ballast water in the pontoons.

The instant invention is aimed at resolving the problem that attaches to semisubmersibles in that they are relatively sensitive to waves and tides of the sea.

For this purpose the semisubmersible of the invention is having the features of other more of claims 1-4.

In accordance with the invention the main deck is provided with one or more legs that are arranged to stand on the bottom of the sea. This makes possible that a stable positioning of the main deck with reference to sea level can be attained, whilst avoiding the drawbacks of the known jack-islands.

Beneficially the legs are movable in a direction that is perpendicular to the main deck, and further it is desirable that the legs are provided with hydraulic means for moving them with respect to the main deck.

A suitable embodiment is having the feature that the legs extend through the pontoons.

With the known jack-islands the entire load which consists of the mass of the main deck, the mass of the legs, and the mass of the payload must be supported by the legs which are relatively long, i.e. 40 m or more. The total mass may be as much as 6000 tons. With the semisubmersible of the invention the legs are of considerable less length, notably approximately 10 m, whereas -in a further aspect of the invention- the load that is to be carried by the legs can also be kept low.

In order to meet the objective of keeping the load of the legs of the submersible of the invention at a relatively low level, the invention is also embodied in a method for operating this semisubmersible, which method is characterized in that the legs of the semisubmersible are extended until they stand on the bottom of the sea, and an amount of ballast water is introduced into the pontoons sufficient to stabilize the semisubmersible standing on the bottom of the sea. The amount of ballast can then be restricted to for instance 200 tons, which is only supplemented by the weight of the payload totalling to an amount of for instance 400 tons.

The invention will hereinafter be elucidated with reference to the drawing showing schematically a semisubmersible in accordance with the invention.

In the drawing: -figure -figure shows shows a view of the semisubmersible;

a side view of the semisubmersible with empty pontoons;

-figure ing the pontoons

-figure shows the filled with shows the semisubmersible in a side view havballast water; and semisubmersibles of the invention with its legs standing at the bottom of the sea.

Wherever in the figures the same reference numerals are applied, these numerals refer to the same parts.

With reference first to figure 1 and figure 2, the semisubmersible of the invention is shown having a main deck 1 that optionally supports a crane 3. The semisubmersible is having at subsea level one or more pontoons 6, and the main deck 1 of the semisubmersible is further provided with one or more legs 5 that are movable in a direction that is perpendicular to the main deck 1, so as to arrange that these legs 5 can stand on the bottom of the sea. Suitably the legs 5 are provided with hydraulic means for moving them with respect to the main deck. The way how this can be implemented is known to the person skilled in the art, hence a further discussion with reference to the drawing can be dispensed with.

From figure 1 and figure 2 it is further clear that the legs 5 extend through the pontoons 6.

Figure 3 shows the case in which the pontoons 6 of the semisubmersible are at least partly filled with ballast water, therewith lowering the level at which the semisubmersible is floating in the sea.

Figure 4 in turn embodies the situation that the legs 5 of the semisubmersible are extended until they stand on the bottom of the sea, wherein an amount of ballast water is introduced into the pontoons 6 sufficient to stabilize the semisubmersible while it is standing on the bottom of the sea. As mentioned above the amount of ballast water for realizing this stable positioning can be restricted to an amount of for instance 200 tons .