Photo courtesy of Wärtsilä Corporation Pump test tower
Dehumidification
Elastic deformation – Deformations that, after the removal of the external load disappear. The material thus gets back to its previous form and shape.
Plastic deformation – Deformations that, after the removal of the external load are permanent.
Dehumidification – Reducing the humidity. Humidity and condensation can cause damage to cargoes and also to the holds or tanks containing them. Dehumidifying of the areas prevents these problems, and the two most widely used systems employ either refrigeration or desiccant.
In the refrigeration systems the air is cooled, and the amount of water vapour it can hold is reduced, with the excess condensing on the chiller surface. The condensed water has to be either collected into a container or pumped away.
In the desiccant method, a medium (silica gel, for instance) is used to absorb the moisture from air before it is blown into the hold to replace the humid air. Heat is then used to dry the desiccant. The released water vapour is vented outside the controlled atmosphere.
Dehumidifier – A substance or a machine used to reduce humidity in holds or cargo tanks.
Calcium chloride and silica gel are water-absorbing chemicals (desiccants) commonly in use.
Delamination – Peeling from undercoat or substrate.
Demersal species – Fish which live at the bottom of the sea, for instance a sole, a hake and a halibut and those which live near the bottom like a cod, a haddock.
Dendamix Marine – A spray-on mineral-fibre insulation material. Using Dendamix Marine as noise or thermal insulation is said to give substantial savings to shipyards, since the adhesive used to spray the fibre directly onto shotblasted and primed steel gives a permanent bond. It precludes any necessity for advanced paint coatings. For A60 steel bulkheads, the material is applied at 45mm thickness over plate and 18mm over stiffeners, at a density of 112kg/m3.
DeNOx – Process of removing nitrogen oxides (NOx) from the exhaust of lean-burn engines, especially by using selective catalytic reduction (SCR).
Density – Mass per volume unit. Expressed in kg/m3 at a specified temperature, normally at 15°C.
Deposit attack – An attack at the edge of a local deposit formed on metal surface in the presence of an electrolyte.
Depth –
Depth moulded – Moulded depth of a ship is measured at the middle of length L, from the top of keel to the uppermost continuous deck at side.
Extreme depth – The depth of the ship from the upper deck to the underside of the keel.
Depth of fusion – The distance that fusion penetrates the base metal or previous bead from the surface melted during welding.
Derating – The operation of a diesel engine at normal maximum cylinder pressure for its continuous sea service rating, but at a lower mean effective pressure and shaft speed.
Derelict – Deserted goods or any other commodity, especially a vessel abandoned at sea.
Derrick
-1. A crane consisting of a boom and a mast with a whip and a tackle connected to a deck winch. Derricks may be arranged for fixed outreach working or slewing derricks may be fitted. Most older ships use winches together with derricks for cargo handling.
2. A framework forming a tower over the drilling slot. It is built onto the deck of a drilling rig.
Desalination – The removal of any chemical salts from seawater to produce distilled water. The equipment used may be described as a distiller, an evaporator or a freshwater generator.
Desiccant – A substance which can absorb moisture, e.g. anhydrous calcium chloride. It is often used as a drying agent.
Design depth of the underwater unit – The depth in meters (feet) of water equivalent to the maximum pressure which the underwater unit is designed for and approved to operate, measured to the lowest part of the unit.
Design pressure of a piping system – The pressure which each piping system element is designed for.
Design internal pressure of the hyperbaric chamber – The maximum pressure which the hyperbaric chamber is designed for and approved to operate.
Design mission time – The maximum effective recharging interval for life support, compressed air and electrical systems which the underwater vehicle or hyperbaric chamber is designed for and approved to perform the intended function under normal operating conditions.
Design temperature of a piping system – The maximum temperature which each piping component is designed to operate at.
Despatch days – The days saved by loading or discharging quicker than stated in the charter party. The charterer may seek compensation if a provision exists to this effect.
Destination – Port that a vessel is bound for.
Desulphurization – The removal of sulphur and sulphur compounds from gases or petroleum liquids.
Detection – The determination of the location of survivors or survival craft, (SOLAS, Chapter III). See also Lifesaving.
Devil’s claw – A stretching screw with two heavy hooks or claws. It is used to secure an anchor in a hawse pipe.
Dew point – The temperature below which water vapour in the air condenses.
DF-electric concept, also dual-fuel/diesel-eletric propulsion – The propulsion plant developed by Wärtsilä for LNG carriers. It is an electric propulsion with main generators driven by dual-fuel engines that can run on either boil-off gas or MDO. The main generators feed the ship electrical system and, through a variable-speed drive system, the propulsion motors. See also LNG tanker GAZ DE FRANCE ENERGY.
Diaphragm –
1. A circular plate made up of two halves. It fits between the rotor wheels of a steam turbine and has nozzles in its upper half.
2. A flat or corrugated thin metal plate used in pressure measuring instruments.
3. A piece of flexible synthetic rubber material which forms part of a pressure-tight chamber for a pneumatically operated valve actuator.
Dielectric – A solid, liquid or a gas in which electric field can be maintained with little or no external electrical energy supplied. It is therefore called an insulator.
Diesel Combined Cycle (DCC) – Technology utilizing both the shaft output and thermal output of a diesel engine. The thermal output is used to drive a steam turbine, for example.
Diesel-electric propulsion – Electric propulsion with diesels used as prime movers.
Diesel-electric drive takes less space than the equivalent direct-drive two-stroke engine allowing the aft section to be slimmer and giving better flow over the propeller. Not only is the diesel-electric drive train lighter than a two-stroke engine, but also its weight can be distributed more evenly. Also there is no need for auxiliary generators.
Diesel-electric propulsion
Diesel engine
Diesel-electric (DE) propulsion emerged early in the 1900s but for most of the century was confined to specialist niches.
A much wider application is now enjoyed thanks to the developments in AC drive technology, the central power station concept for propulsion and ship services, and increasing interest in low emissions and propulsion plant redundancy.
More and more vessels are fitted with electric propulsion plant that have succeeded in cruise ships, icebreakers, ferries, shuttle tankers, chemical carriers, and research vessels. The recent Gaz de France LNG tanker project marks another ship type reference for this transmission system.
For offshore vessels it may be selected for flexibility. Where the vessel has to be efficient under numerous operating conditions, it may be that propulsion is only one of several requirements, such as drill ships where the main propulsion system is used in transit, otherwise the power is split between dynamic positioning (DP) and deck power consumption for equipment. Icebreakers are a special case. Here DE propulsion in various forms is a favourite one since it meets the requirements for maximum output at very low speeds. For vessels such as FPSO, the choice of diesel-electric drive is almost automatic, since about 80% of time is spent on production at low power. The remaining time is split between transit, offloading, standby and production at variable loads.
Diesel engine – A type of internal combustion engine which ignites fuel by injecting it into hot, high-pressure air in a combustion chamber. It has neither carburettor nor ignition system. The fuel is injected in the form of a very fine spray, by means of a nozzle, into the combustion chamber. There it is ignited by the heat of compressed air which the chamber has been charged with. The diesel engine operates within a fixed sequence of events, which may be achieved either in four or two strokes. The low-speed (i.e. 70 to 120 rev/min) two-stroke diesel is used for main propulsion units, since it can be directly coupled to the propeller and shafting. The medium speed four-stroke engine (250 – 1200 rev/min) is used for the auxiliaries such as alternators and also for main propulsion with a gearbox.
A four-stroke diesel engine resembles a gasoline engine as it works on the four-stroke cycle, that is: admission, compression, power and exhaust. When the piston gets down on the air admission stroke, the lower pressure in the cylinder allows a charge of air into the cylinder through the inlet valve which opens just before top dead centre.
Once the piston has passed the bottom dead centre and is beginning to ascend, the inlet valve closes and the upward movement of the piston compresses the air charge in the cylinder causing a quick rise of temperature. Before the second stroke is over, the charge of fuel oil is gradually injected into the cylinder by an injector.
The burning of the air-fuel charge makes the gases expand. They push the piston downwards and create the power stroke. Before the piston reached the bottom dead centre, the exhaust valve opens and, as the piston goes up again, the burnt gases are forced out through the exhaust valve. Just before top dead centre the inlet valve opens and the cycle begins again.
High-speed diesel engine – Trunk piston type engine having a rated speed of 1400 rpm or above.
Medium-speed diesel engine – Trunk piston type engine with speed range from 400 rpm to 1200 rpm.
Low-speed diesel engine – Crosshead type engine with rated speed of less than 400 rpm.
Wärtsilä RT-flex 82T Wärtsilä RT-flex 35