Bulk carrier and oil tanker ship construction

Types of ships discussed: bulk carriers & oil tanker

Here I discuss about two types of ships which are bulk carrier and oil tanker. The pictorial explanation gives general difference between both the vessels.

Bulk carriers

The name itself says that they carry products in bulk quantities. Ex: Edible commodities, raw materials and finished steel products. Bulk carriers can be with or without cargo gear. Those without will use the shore gear for cargo operations. The construction will be usually single or double hull with DB’s and TST’s. & Side hopper tanks. During rough weathers the ballast is taken into the cargo hold so that CG comes down there by minimizing the stresses. Cargo grabs can enter easily due to wide opening of cargo hold. The excessive cutting area reduces deck strength, but suitably compensated with the hatch coaming structure. Pontoons will cover the cargo hold & will differ in size, shape and operation depends on manufacturer. Pillars with deck girders supported by brackets and beams will take the weight of pontoons. The tanks tops are strong enough to bear stress caused by bulldozers or bob cats when they operate to collect cargo in cargo holds.

Tankers

Tankers are designed to carry liquid cargos in bulk quantities depending on their size. The entire cargo operation will be through pipelines. For carrying cargo operations they have to call berths fitted with chicksen or hoses. Smaller tankers usually used to supply bunkers and lube oils to the bigger vessels at anchorages. The vessels that carry gas also called as tankers. In shipping world tankers constitutes 35% tonnage presently. Tankers vary in size. From 500 GT to the world’s ultra large crude carrier MT.Hellespont Alhambra with dwt of 441,585. This type of ultra large tankers usually carry crude from the point of production to the refineries. The size and number of the cargo tanks will vary from ship to ship depending on the size and construction of the vessel. The divisions of the tanks are made possible by both longitudinal and transverse bulkheads with strengthened frames and corrugations to give extra strength to the bulkheads. These bulkheads in turn reduce racking stresses and prevent fire and flooding to progress to the other part of the vessel.

Panting and pounding arrangements as expressed below

Panting : The force exerted on ships side plating due to the vessels rising and falling due to waves is called panting. The bow part experience more pressure as compared to stern. Panting beams, stringers and deep floors will make the vessel to overcome this problem.

Pounding : During excessive pitching the fore foot bottom plating will slam the water with high force which is called as pounding. It could be because of excessive speed, much stern trim or too little ballast. By increasing the bottom plate thickness, longitudinal framings, transverse frames with side girders and more side frames helps the vessel to withstand these stresses.

Regional stresses affecting bulk carriers

Care to be taken always while loading and continuous monitoring for hogging and sagging. Sagging comes in the areas where more weight and hogging comes in the area of less weight. Uneven distribution of weight could lead the ship breaking into two. However even after loading to even keel still the vessel experiences this problem due to the waves. During rough weather extra care to be taken as the high raising waves provides extra buoyancy.

Transverse Stress: These are caused due to force exerted from cargo or structural weights or motion in sea way which in turn affects the ships cross sectional area.

Racking: Rolling causes ship’s deck to move laterally when compared to bottom thereby causing the side shells to move vertically relative to each other

At any given time the stresses on board should be within the limits of harbor and sea going conditions as given in loadicator.

References

1. College Hand out, Moodle.stc.uk
2. Classroom Running Notes
3. Merchant ship construction by D.A.Tailor 3rd edition 1992
4. Merchant ship construction by P.J.Pursey 7th edition Glasgow 1983
5. Safe Tanker Operations by Capt.K.S.D.Mistree, A Marinez Publication