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Semi-Submersible

So-called "pocket" submarines used either for warfare or peaceful purposes in underwater exploration or various underwater interventions are well known. These submersibles cannot be used as instruments of leisure, since they call for considerable training of the users and also for a top physical condition. A semi-submersible is less affected by wave loadings than a normal ship, which is advantageous while performing offshore operations. The advantages of semi-submersible vessels are well-known in the art.

Fighting trim is a term used in the navy for the immediate readiness of a warship for battle, or for the proper trim to commence an action. To come to fighting trim meant to adjust the sails or yards with reference to the direction of the wind and the course of the ship, or to distribute the load of a ship so she floats on an even keel. By the end of the 19th Century, some vessels were designed to be submerged to the fighting trim by means of a system of valves opening into the water-tight compartments of the double bottom. These are worked from the berth-deck. The vessel can be submerged to the fighting draft in five minutes. When Jess Willard fought Jack Dempsey he had been living an easy life for some time. He had not taken the coming battle very seriously and was not in fighting trim. The result was that he lost.

The process of trimming covers two functions. Firstly, adjustment of Fore and Aft trim provide a tank at each end of the submarine, with a pipe connecting them, and a pump in the middle. These are known as Trim Tanks, and water can be transferred from one end to the other without pumping any out or flooding any in.

The second function of trimming is the adjustment of bodily weight to achieve neutral buoyancy, and secondly the adjustment of fore and aft weight so that the submarine remains level. Adjustment of bodily weight is done to compensate for changes in buoyancy and also to make the submarine as nearly as possible neutrally buoyant before going to sea. It is carried out by admitting water to tanks near the centre of the submarine, or by pumping water from these tanks to sea. There are known as compensating tanks. Getting water into the tanks is easily done by opening a hull valve and allowing the water to flood in through a pipe and valves to the tanks. Pumping it out again requires a powerful pump, but the same system and valves can be used.

Fighting Trim refers to a warship sailing condition that could be set by some warships of the late 19th and early 20th centuries. These ships had the ability to alter their freeboard. i.e. the portion of the ship above the waterline. Water-holding tanks designed specifically for the purpose were intentionally flooded, to lower the silhouette and the target area of the ship.

The words `ship-shape` indicate that a vessel has a `normal` hull shape, in the sense that at an intersection plane where the hull intersects the water-line, the cross-sectional area of the hull, when viewed from above, is substantially the same as the surface area of the hull below the water-line. A ship-shape vessel advantageously has a relatively large allowable ratio of dead-weight to displacement at operational draft, which allows it to carry a substantial deckload and a substantial amount of equipment on deck. Ship-shape vessels may have a relatively cumbersome dynamic behaviour in certain conditions. This is due to the fact that when viewed from above, the waterline area of the vessel is relatively large, which makes the ship-shape vessel sensitive to wave forces, resulting in relatively large roll, pitch and heave motions of the vessel.

Semi-submersibles have replaced ship-shape vessels for a wide variety of offshore operations in recent years. The word `semi-submersible` indicates that the vessel has a variable draft, which draft is controllable. The displacement of a vessel is the actual weight of the vessel and its contents. The displacement gives an indication of the size of the vessel. The deadweight is the maximum weight that a ship can safely carry when fully loaded. It includes the cargo, fuel, water, crew and stores. Fixed equipment, such as heavy cranes, a drilling rig and a pipelay tower are formally part of the ship and not of the deadweight.

An advantage of semi-submersible vessels over a normal ship is that a limited sensitivity to waves and good seakeeping characteristics can be obtained by providing ballasted, watertight, lower hulls below the water surface and wave action. The operating deck is situated above the sea level and kept well away from the waves.

Semisubmersible marine structures

Semisubmersible marine structures are well known in the oil and gas industries. Such structures are typically only moveable by towing. These semisubmersibles have a relatively low transit draft that allows them to be floated to a stationing location, where they can add ballast, usually by taking on seawater, to assume a relatively deep draft or semisubmerged condition for operation. Semi-submersible platforms have the principal characteristic of remaining in a substantially stable position, presenting small movements when they suffer the action environmental forces such as the wind, waves and currents.

Flotation of semisubmersibles is usually accomplished with pontoons on which an upper deck is supported by columns. The pontoons provide a relatively large waterplane area, as is desirable for transit, but when submerged for stationing, the columns connecting the pontoons to the upper deck present a lower waterplane area for operation. The low waterplane area is desirable to reduce motion characteristics from waves, especially during swell seas and storms. The upper deck from which rig activities are conducted must be maintained above the water plane at all times.

In recent years, the drilling operations have been conducted at increasingly greater distance from the shoreline, placing the offshore production or drilling facilities in often severe weather conditions. In such environments it is particularly important to have a stable floating facility for supporting the mineral exploration and production operations, as well as providing living accommodations to the crew and storage for the necessary equipment. In deep waters, over 7500 feet, it becomes particularly advantageous to deploy floating semi-submersible vessels, as opposed to fixed bottom anchored structures.

Designs of semi-submersible vessels utilize buoyant pontoons, or lower hulls which support a plurality of vertically extending columns, the upper portions of which carry a working platform. Some of the semi-submersible vessels can have a single caisson, or column, usually denoted as a buoy while others utilize three or more columns extended upwardly from buoyant pontoons.

In many such structures, vertical or diagonal braces are used between the columns, the braces contributing to the water plane area of the vessel. The braces are usually constructed with smaller diameters than that of the columns and are therefore more vulnerable to the environmental and mechanical damage. If the connecting braces are damaged, the entire structure becomes jeopardized.

One example of a single-brace structure is a two-pontoon, four-column structure, with a pair of columns being mounted on a respective underwater hull, or pontoon. One transverse horizontal stay is mounted between each pair of the columns at the ends of the underwater hulls. The object of the design is to simplify the construction and to reduce the resistance to water flow. However, the minimal number of braces may be less beneficial where spreading forces acting on the four columns are relatively high and torque imposed on the columns by the pontoon lateral bending tends to twist a column structure in the direction of the prevailing wind and wave forces.

There also exist numerous designs of semi-submersible vessels using diagonal braces in addition to horizontal stays. These tend reinforce the support structure of the platforms and resist destructive forces of the ocean waves. One of the disadvantages of the diagonal braces is increase in water plane area of the vessel, which adversely affects the weight, wave resistance and overall cost of the vessel.

Another consideration that is taken into account when designing semi-submersible vessels is resistance of the vessels to heave and roll motion induced by waves. The vessels must have sufficient stability to withstand wave motions to allow the mineral exploration and production operations to be carried out in safety. This consideration becomes particularly important in harsh environments where strong winds and waves are prevalent throughout the year.




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Page last modified: 26-09-2019 10:21:38 ZULU