11. Side Shell and Deck Structures Flashcards
What are the purposes of side shells and deck plating?
In addition to forming a watertight skin around the hull, they also act as major strength providers to help resist longitudinal ben ding, vertical shearing forces and impact loads.
Side shell framing
Provides necessary strength to withstand deformation or damage to the hull in the most adverse conditions possible. Also defines the shape of the hull.
Can be longitudinal or transverse but always divide into:
Primary strength members : heavier, deeper frames that are more widely spaced giving substantial strength to the main structure of the vessel
Secondary strength members: lighter, more closely spaced frames that provide support to the immediate surrounding area.
Typical regions identified in construction regulations
- Forward region (0.3L from FP looking aft) pounding, panting, slamming
- Aft region (0.3L from AP looking forward)
- Amidships/ mid body (totalling 0.4L between aft and forward) Subjected to hogging and sagging
Plating terminology
Seams : longitudinal joints between plates
Butts: vertical or transverse joints
Strake: fore and aft line of plates with their butt joints welded together
Sheer strake : upper strake of shell plating at the main deck, connects with the deck plating
Bilge Strake: at the turn of the bilge extending outward to a point where the side rises vertically
Garboard strake: first strake of plates on each side of a keel, bar keel or keel plate
Strakes slide 15
Sheerstrake
Normally the uppermost strake of hull plating and is therefore the furthest away from the neutral axis of the hull girder form. Subjected to considerable shear and ben ding stresses. The joint between the sheerstrake and the deck stringer is usually a T-shaped arrangement having full penetration welds. This shape results in an exposed upper edge of plating with high degrees of stress that could easily result in cracking of the free edge
Bottom shell plating
Thicker than the side shell plating. The keel plate is the thickest strake due to the requirement for increased strength against longitudinal bending, dry dock stresses, corrosion, and the possibility of grounding. The garboard strake provides the transition from the keel plate thickness to that of the bottom shell plating.
Thickness calculation of bottom plates based on the lateral pressure and longitudinal buckling strength of the plate. Change of plate thickness done gradually to reduce stress concentration points.
Strakes of bottom plating have their greatest thickness over the 40%L of the midship where bending stresses are the highest.
Side plating
Plates extending from the bilge strake to the sheerstrake and considered the outermost structure on the hull.
Usually lower thickness than bilge and sheer strake.
Assists in structural strength of the vessel
Openings in side shell should have rounded corners to avoid concentrations of stresses.
Bilge strake: line of plating where transition occurs from bottom plating to side shell plating. Usually thicker due to added stress and internal corrosion problems when a margin plate is fitted.
Side shell plating and stresses
Chine: abrupt change from bottom plating to side shell plating. A smoother chine contributes to reductions in stresses.
Round bottom
Soft Chine deep V
Soft chine flat bottom
multiple ch ine
hard chine shallow V
hard chine deep V
Special plates
Fashion plate : extension of the side plating of a superstructure which tapers off into the sheerstrake. Stress concentration reduction
Shoe plate: connection of the flat plate keel to the stem at the collision bulkhead
Coffin plate: attach the flat plate keel to the sternframe
Boss plate: shield shaped plate that fits over the boss formed in the sternframe through which the tail shaft is fitted. Maintain hydrodynamic efficiency of the vessel
Oxter plate: plate having double curvature immediately below the transom floor and running into the fore side of the sternpost. Molded plate used to continue shell plates immediately above the propeller aperture of a ship. AKA Tuckplate
Stealer plate: a wider plate that replaces two narrow plates as the girth of the vessel reduces towards the forward and aft ends. Prevents the use of narrows plates with the extra welding necessary. Usually assigned the letter of the lower strake that it replaces.
Shell stresses
The midregion of the vessel generally has thicker plating than the remaining side shell to accomodate a higher stress regime. It is possible that areas of plating on the ship’s sides will be found to be thicker than the bottom plating like:
- sheer streake
- areas near a change of structure or high vertical shear stress (i.e. superstructures)
- panting region
- plating around hawse pipes
- bulbous bow plating
- plating around rudder horn
- plating around shaft brackets
- plating around sternframe
Longitudinal bending and fatigue
In cyclic stress environments, metal fatigue can develop and the appearance of cracks may occur. These cracks may then propagate vertically down the side of the hull. Special care must be taken to provide the area with crack arresting properties. The first line of defense would be proper thickness plating and proper construction techniques to handle the anticipated load. The upper edge of the sheer strake is dressed (rounded) to avoid irregularities. Any change in section are fitted with suitably well-rounded plates. Sheer strake is made of higher quality steel and often made notch resistance (higher manganese). Attachments and openings are restricted to certain areas and sizes.
Rounded sheer strake
Eliminates the upper free edge of plating. Plating is rolled to provide a smooth and suitable curve radius = 15x thickness provide a very gradual change in direction
Transverse side shell structures
Used in relatively short vessels, less prone to longitudinal ben ding.
Primary strength structure: deep, heavily constructed web frames fabricated from pieces of steel welded together
Secondary strength: ordinary frames made from angle or bulb bar.
Assume brackets (beam knees) are fitted at both the top and the bottom of the frame. Tween deck frames are only considered to have a bracket at the top. The fitting of brackets increases the strength of the frame by reducing the unsupported span.
The use of stringers (horizontal plates) is common. They run longitudinally to help support the side frames. They are bracketed to bulkheads and suitably attached to web frames.
In ice breakers: anti-tripping bracket would be installed to keep frame in upright condition at all times.
Spacing: rule of thumb ordinary frames every 600-800 mm, web frames every 4th or 5th frame space.
Longitudinal side shell framing
Ships over 120 m in length b/c more prone to longitudinal bending. Also when a significant amount of deck material is removed to provide large unrestricted cargo hatches (container ships).
Advantage: increase in longitudinal strength and large reduction in longitudinal bending. Could result in thinner deck and bottom plating.
Primary strength: side transverses (large web frames), depth = 2.5x slot for longitudinal.
Secondary strength: continuous side longitudinals, usually bulb bar and are passed through and welded to slots cut out of the transverses.
Spacing: rule of thumb 2.5 - 4.2 m between side transverses, 600-850 mm between longitudinals
Side shell openings
Any openings in plates reduce structural strength. Therefore, must have rounded corners to reduce stress concentrations. Could also be fitted with double plates (2nd plate fitted around opening and welded to surface) or insert plates (larger hole, plate welded inside have 1.5-2x thickness).
Large openings are often fitted with heavier inserts, edge stiffening, additional internal support
Examples: doors, discharges, ventes, suction, equipment ports, portholes/windows, anchor pockets, bitt inserts
Side plate appendages
Will require reinforcement of the attachment point and will consist in tying the appendage to a frame structure by deep welding or doubler plates.
Examples: fairleads, propeller bars
Shell expansion drawing
3D shape of the hull plating in a 2D view. Only shows 1 side of the vessel as hull is symmetrical about centerline. Garboard strake is A. Individual plates are numbered aft moving forward but may change with classification society.
Include the position of frames, stringers, bulkheads, floors, decks, plating thickness, welding particulars any other unusual arrangements.
Deck plating arrangements are given in 2D form. Strakes are identified alphabetically starting port/starboard of center strake and moving towards ships sides.
Drawings slides 57-58
Deck plating
Resist longitudinal bending, designed to support various loads and weights from deck equipment, superstructure, other devices.
Mid-region of the ship fitted with thicker plating to better resist longitudinal bending. Areas of high stress (hatch openings, concentrated loads, change in structure) will have thicker plating.
Main strength deck = freeboard deck
Tween deck is generally thinner than the strength deck. Plating is cut to allow passage of side shell framing. Attachment to ship sides made watertight by chocks or using sealing plates around the frames.
Sheathed decks
Permanent (provides minor strength, can reduce plate thickness by 5-10%) or non-permanent (sacrificial deck covering but water cay lay between deck and sheathing leading to corrosion)
Usually wood planking (teak, pine, douglas fur), sometimes synthetics, done for aesthetic purposes. Wood can be thermal insulator
