In-service experience

01 Sep 1999

Lauritzen Kosan Tankers is pleased with the in-service performance of two T-type LPG carriers, delivered by Hyundai in September and October last year.

The pair of 5,750m3 capacity LPG carriers, Tessa Kosan and Tenna Kosen, are semi-pressurised and fully refrigerated vessels, with two independent holds. Designed for worldwide trading, they started their operational life carrying isoprene monomer for Mitsui from Japan to Europe. Lauritzen reported that the vessels’ relatively high service speed of 16 knots succeeded in shaving 100 hrs off the usual time taken to cover the 11,040 naut miles, from 789 to 690 hrs.

The nitrogen generator fitted to these T-type ships has speeded up the process of changing the cargo system atmosphere, a necessary procedure when switching between incompatible products such as LPG and propylene or isoprene monomer and LPG, says Lauritzen.

There is no longer need for the vessel to deviate to a port at which it can take nitrogen from shore, and the time required for inerting and gassing up is now accomplished during the ballast voyage, says Flemming, technical director, Lauritzen Kosan Tankers. On many occasions, this has resulted in considerable time being saved at the berth.

A first for plates

Two 78,500m3 LPG carriers under construction at the Gydnia Shipyard, Poland for delivery to Bergesen in June and December next year will be the first LPG carrier newbuildings to be fitted with reliquifaction units incorporating plate heat exchangers instead of shell and tube condensers.

Alfa Laval describes the units as having Alfal Laval’s AlfaRex gasket-free, all-welded plate heat exchangers fitted with titanium plates, which are impervious to seawater corrosion, says the company. The plates are laser welded together one by one in alternate grooves to form a plate rack. Four of these customised skid mounted units are to be supplied by Hamworthy KSE.

Plate heat exchangers were chosen by Bergesen following the success of a trial unit which was fitted to its 12,000m3 LPG/NH3 carrier Havsol in 1996, following problems experienced with corrosion in its shell and tube condensers.

Kvaerner Masa-Yards’ new generation of LNG carrier design comes in two versions, both incorporating higher capacity, stretched cargo tanks. These are achieved by the addition of a collar between the top and bottom halves of the cylindrical tanks.

A 1.35m deep collar would add 5,000m3 of cargo capacity to a four-tank, 135,000m3 capacity vessel. Alternatively, a collar of around 2.63m in depth would provide an extra 5 per cent capacity, giving a total of 141,750m3.

The hull dimensions remain the same as for a standard four-tank vessel as this has enough residual ballast capacity to compensate for the higher tanks.

One of the two versions of the design has the accommodation block sited in the bow. The advantages of this include reduced windage, a lower bridge, and better visibility, says KM-Y.

Taking the BOG out of the propulsion equation means that the vessels be equipped with diesel electric propulsion. In this case five medium-speed diesels are specified, powering twin 14.5MW azipod drive units.

A European Commission funded project to develop a medium-speed dual fuel gas diesel engine for LNG tankers is to report its findings in late September. The project partners are Wartsila Diesel, Chantiers de l’Atlantique, Lloyds Register, and Shell International Trading and Shipping Co.

Wartsila has developed two medium-speed dual-fuel engine (type 46, and 64) burning varying ratios of gas and fuel oil. One has gas injected into the engine at high pressure (300 bar). The second has the gas injected into the air inlet at standard pressure, ignited by a pilot oil flame.

Chantiers de l’Atlantique has studied the feasibility of a diesel electric propulsion plant, and to draw up a financial comparison with steam turbine propulsion over 20 years of use.

Shell International Trading and Shipping has provided Chantiers de l’Atlantique with input on the start of the art in current projects, helped to identify solutions that are currently considered feasible, and give input on how owners assess cost issues.

Lloyds Register has examined the diesel electric propulsion plant for compliance with class rules and regulations.

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