Double bottom Floors

Vertical transverse plate floors are provided both where the bottom is transversely and longitudinally framed. At the ends of bottom tank spaces and under the main bulkheads, watertight or oiltight by closing any holes in the plate floor and welding collars around any members which pass through the floors. Elsewhere ‘solid plate floors’ are fitted to strengthen the bottom transversely and support the inner bottom. These run transversely from the continuous centre girder to the bilge, and manholes provided. Lightening holes are cut in each solid plate floor. Also small air and drain holes may be drilled at the top and bottom respectively of the solid plate floors in the tank spaces. The spacing of the solid plate floors varies according to the loads supported and local stresses experienced. At intermediate frame spaces between the solid plate floors, ‘bracket floors’ are fitted. The bracket floor consists simply of short transverse plate brackets fitted in way of the centre girder and tank sides (see Figures 10 and 11).

The IMO International Gas Carrier Code, became mandatory in 1986.

The requirements of this code are incorporated in the rules for ships carrying liquefied gases published by Lloyd’s and other classification societies.

The code covers damage limitations to cargo tanks and ship survival in the event of collision or grounding, ship arrangements for safety, cargo containment and handling, materials of construction, environmental controls, fire protection, use of cargo as fuel etc. Of particular interest in the context of ship construction is the section of cargo containment which defines the basic cargo container types and indicates if a secondary barrier is required, i.e. a lining outside the cargo containment which protects the ships hull structure from the embrittling effect of the low temperature should cargo leak from the primary tank structure. The cargo containment types are described next.

Keel Plating

At the centre line of the bottom structure is located the keel, which is often said to form  the backbone of the ship. This contributes substantially to the longitudinal strength and effectively distributes local loading caused when docking the ship. The commonest form of keel is that known as the ‘flat plate’ keel, and this is fitted in the majority of ocean-going and other vessels (see Figure 8 above ). A form of keel found on smaller vessels is the bar keel . The bar keel may be fitted in trawlers, tugs etc., and is also found in smaller ferries.

Where grounding is possible this type of keel is suitable with its massive scantlings, but there is always a problem of the increased draft with no additional cargo capacity. If a double bottom is fitted the keel is almost inevitably of the flat plate type, bar keels often being associated with open floors, where the plate keel may also be fitted.
Duct keels are provided in the double bottoms of some vessels. These run from the forward engine room bulkhead to the collision bulkhead and are utilised to carry the double bottom piping. The piping is then accessible when cargo is loaded, an entrance to the duct being provided at the forward end of the engine room. No duct is required aft of the engine room as the piping may be carried in the shaft tunnel.

CONTROL OF LEAKAGES in hydraulic system

Leakage. Any hydraulic system will have a certain amount of leakage. Any leakage will reduce efficiency and cause power loss. Some leakage is built in (planned), some is not. Leakage may be internal, external, or both.

a. Internal. This type of leakage (nonpositive) must be built into hydraulic components to lubricate valve spools, shafts, pistons, bearings, pumping mechanisms, and other moving parts. In some hydraulic valves and pump and motor compensator controls, leakage paths are built in to provide precise control and to avoid hunting (oscillation) of spools and pistons. Oil is not lost in internal leakage; it returns to a reservoir through return lines or specially provided drain passages.

Too much internal leakage will slow down actuators. The power loss is accompanied by the heat generated at a leakage path. In some instances, excess leakage in a valve could cause a cylinder to drift or even creep when a valve is supposedly in neutral. In the case of flow or pressure-control valves, leakage can often reduce effective control or even cause control to be lost.

Normal wear increases internal leakage, which provides larger flow paths for the leaking oil. An oil that is low in viscosity leaks more readily than a heavy oil. Therefore an oil's viscosity and viscosity index are important considerations in providing or preventing internal leakage. Internal leakage also increases with pressure, just as higher pressure causes a greater flow through an orifice. Operating above the recommended pressures adds the danger of excessive internal leakage and heat generation to other possible harmful effects.

A blown or ruptured internal seal can open a large enough leakage path to divert all of a pump's delivery. When this happens, everything except the oil flow and heat generation at a leakage point can stop.

b. External. External leakage can be hazardous, expensive, and unsightly. Faulty installation and poor maintenance are the prime causes of external leakage. Joints may leak because they were not put together properly or because shock and vibration in the lines shook them loose. Adding supports to the lines prevents this. If assembled and installed correctly, components seldom leak. However, failure to connect drain lines, excessive pressures, or contamination can cause seals to blow or be damaged, resulting in external leakage from the components.

c. Prevention. Proper installation, control of operating conditions, and proper maintenance help prevent leakage.

(1) Installation. Installing piping and tubing according to a manufacturer's recommendations will promote long life of external seals. Vibration or stresses that result from improper installation can shake loose connections and create puddles. Avoid pinching, cocking, or incorrectly installing seals when assembling the units. Use any special tools that the manufacturer recommends for installing the seals.

(2) Operating Conditions. To ensure correct seal life, you must control the operating conditions of the equipment. A shaft seal or piston-rod seal exposed to moisture, salt, dirt, or any other abrasive contaminant will have a shortened life span. Also, operators should always try to keep their loads within the recommended limits to prevent leakage caused by excessive pressures.

(3) Maintenance. Regular filter and oil changes, using a high-quality hydraulic oil, add to seal life. Using inferior oil could cause wear on a seal and interfere with desirable oil properties. Proper maintenance prevents impurity deposits and circulating ingredients that could wear on a dynamic seal

           

Elaborate Description about PANTING & POUNDING

Pounding

When a ship meets heavy weather and commences heaving and pitching, the rise of the fore end of the ship occasionally synchronizes with the trough of a wave. The fore end then emerges from the water and re-enters with a tremendous slamming effect, known as pounding. While this does not occur with great regularity, it may nevertheless cause damage to the bottom of the ship forward. The shell plating must be stiffened to prevent buckling. Pounding also occurs aft in way of the cruiser stern but the effects are not nearly as great.

Panting           

As the waves pass along the ship they cause fluctuations in water pressure which tend to create an in-and-out movement the shell plating. The effect of this is found to be greatest at the ends of the ship, particularly at the fore end, where the shell is relatively flat. Such movements are termed panting and if unrestricted, could eventually lead to fatigue of the material and must therefore be prevented.

The structure at the ends of the ship is stiffened to prevent any undue movement of the shell.

Arrangement to resist Panting & Pounding:

Arrangement to resist panting:

The structure of the ship is strengthened to resist the effect of panting from 15% of the ships length from forward to the stem and aft of the after peak bulkhead.

In the forepeak:

o   Side stringers are fitted to the shell at intervals of 2M below the lowest deck.

o   Channel beams are fitted at alternate frames in line with the stringers and connected to the frames by brackets. Intermediate frames are bracketed to the stringer. The free edge of the bulkhead stringer may be stiffened by one of the beam.

o   The tank top is not carried into the peak, but solid floors are fitted in each frame. Floor are thicker & flanged on free edge.

o   The solid frames are 610mm a part and being so well supported.

o   The deck beams are supported by vertical angle pillars on alternate frames, which are connected to the panting beams (channel) and lapped onto the solid floors.

o    A partial wash plate is usually fitted to reduce the movement of the water in the tank.

o   Inter-coastal plates are fitted for two or three frame spaces in line with the centre girder.

o   Between the collision bulkhead and 15% length from the forward, the main frames together with their attachment to the margin, are increased in strength by 20%. In addition, the spacing of the frames from the collision bulkhead to 20% of the length from forward must be 700mm.

o   Shell plating thickness is increased by 5% to 15% depending upon ships length.

o   The collision bulkhead is stiffened by vertical bulb plates spaced about 600mm apart inside the peak.

o   The structure in the aft peak is similar in principal to that in the fore peak. The stringers and beams may be fitted 2.5m apart.

Arrangement to resist pounding:

The structure of the ship is strengthened to resist the effect of pounding (slamming forces) from collision bulkhead to 25% of the ships length from forward.

o   The flat bottom shell plating adjacent to the keel on each side of the ship is increased in thickness between 15% - 25% depending upon the length.

¥      The frame spacing in this region is 700 mm compare with 750mm – 900mm amidships.

¥      Longitudinal girders are fitted 2.2m apart, extending vertically from shell to the tank top.

¥      Intermediate half height girders are fitted to the shell reducing the unsupported width to 1.1M.

¥      Solid floors are fitted at every frame space and are attached to the bottom shell by continuous welding.

o   In longitudinally framed ship:

¥      The spacing of the longitudinal is reduced to 700mm and they are continued as far as forward as to the collision bulkhead.

¥      Transverse floor may be fitted at alternate frames.

¥      Full height side girders may be fitted 2.1m a part. 

Fore End Structure: