Fullscreen HTML5Med Res (??MB)HTML4

Making old ships go faster Photo: Blohm + Voss GmbH A new marine propulsion system using an ABB motor and generator has boosted the speed of three rebuilt US container ships from 18 to 21 knots. Instead of “power take off” (PTO), it makes use of the “power take in” (PTI) concept to transmit 4000kW to the ships’ main shafts. Vessels (ACV). Ten years later, speed has become the dominating factor for these vessels, with large container capacities given only a second priority. The most obvious way to increase the speed of the ships was to reduce their length and to equip each of them with a new forebody and afterbody plus a new propulsion plant. However, that idea was rejected as being eco­nomically inviable. Three US container ships were recently upgraded at the Hamburg shipyard of Blohm + Voss GmbH by shortening the vessels and installing a new marine propulsion system. The reasons for the modifications to the three vessels were straightforward. Operators of container ships have had to adjust to a vastly changed economic situation in recent years. The need today is for faster, smaller container vessels with a capacity of 2500 to 6000 TEU (equivalent unit for 20-foot containers) and able to travel at a speed SL-31 – a brand new concept 72  Silicon Chip of at least 21 knots. Recognising this trend, the SeaLand Division of CSX Corpo­ ration in the USA began looking for ways to modernise its large and relatively slow container ships. These were built in Korea in the early 1980s for United States Lines (USL). The vessels, which when new were among the most economical container ships in service, originally had a storage capacity of approximately 3900 TEU and a speed of 18 knots. USL operated the ships as Atlantic Class The shipyard and shipowner eventually agreed on a complete­ ly new concept. The project name that was chosen was SL-31 (SL stands for Sea-Land, 3 for 3000 TEU and 1 for 21 knots). It proposed a reduction in the length of the ACV container ships by three hatch groups (bringing their length down from 279 metres to 248 metres), a more streamlined forebody and a higher power rating for the propeller. Extensive calculations and tests were carried out by the shipyard at the HSVA marine test insti­ tute in Hamburg to make sure that a speed of 21 knots would actually be achieved. An increase in the drive power rating would be necessary, as would modifications to the shape of the fore­body. The changes that had to be made to the body of the ship called for precision work. For example, during the removal of the midbody, a flamecut with a length of 330m had to be made in one operation and with an accuracy that would ensure that no re­machining of the storage structures would be necessary after the forebody and afterbody had been floated back together again. In addition, the electrical power connections between the two halves of the ship, involving about 350 cables and large numbers of pipes, had to be separated. After the midbody had been cut out and temporary bulkheads had been fitted, the forebody and the midbody were floated and pulled out of the dock by tugs. After this, the forebody was moved to within about 300mm of the afterbody. The dock was then floated again and the forebody pushed up against the afterbody, aligned, tacked and welded in place. The most critical part of this operation was the manoeuvring and alignment of the two halves. Very high precision was necessary, as a deviation of just a few millimetres from the original longitudinal axis would translate into a loss of speed. Optical measuring equipment was used to ensure a perfect fit. It is worth remembering that the Photo: Blohm + Voss GmbH The “Sea-Land Pride” in Dock 10 after the forebody had been cut away and with the midbody being prepared for removal. The photo on the facing page shows the ship after conversion. Next to it, on the left, is the “Sea-Land Value”. parts being manoeuvred weighed several thousand tonnes and that they had to be moved by tugs to precise positions in the dock. This part of the moderni­sation alone was a considerable achievement on the part of the shipyard. Increasing the drive power A new approach was also necessary for the upgrade of the propulsion systems. The installed machines, Sulzer 7 RLB 90 engines, were rated at 20,590kW (100%) and 18,530kW (90%). In order to run the ships at 21 knots without modifying the ves- sels, it would have been necessary to increase the engine power to about 30,000kW. By streamlining the forebody through hydrodynamic improve­ ments, an initial power saving of 3700kW could be achieved. Also the reduction in length by three hatch groups reduced the ships’ frictional resistance, allowing a further saving of 1500kW (or 5200kW in total). This meant that, to achieve the required speed of 21 knots, an additional 38004000kW would have to be fed into the propeller shaft system. To raise the drive power rating to the a b Fig.1: ACV container ship conversion based on the SL-31 concept: (a) forebody cut away and midbody removed; (b) short­ened ship with new forebody. November 1997  73 Photo: Blohm + Voss GmbH This photo shows the new, more streamlined forebody being fitted to the “SeaLand Pride”. 2 n = 102 min-1 New propeller 1 3 4 5 6 7 8 Fig.2: design of the new marine propulsion system with booster motor: (1) Sulzer diesel engine, 20,588kW; (2) Controllable-pitch propeller; (3) Gearing; (4) Booster motor, 4000kW; (5) Switch­board, 6.6kV; (6) Wartsila diesel engine, 4860kW; (7) Generator, 4374kW; (8) To bow thruster, 1800kW. 2940 7700 2860 1 ø 620 2 Fig.3: shaft arrangement for the booster motor and tunnel gear­ing: (1) Booster motor; (2) Tunnel gearing. 74  Silicon Chip required level, Blohm + Voss GmbH developed a new, unconventional concept that “reverses” the standard shaft generator system commonly in use. Previously, electrical power has been fed into the onboard power system from the main machine by means of a gear system with an attached generator. The new drive makes use of the “power take in” instead of “power take off” concept (Fig.2). In this method, 4000kW is transmitted via a 6.6kV electric motor to the main drive shaft by means of tunnel gearing which is flanged via a Vulcan coupling to the flywheel of the main machine (see Fig.3). The electric motor is fed with 4860kW (100%) or 4374kW (90%) from an additional Wartsila-Diesel generator set with a 6MVA alternator. The high-voltage switchgear and diesel-generator set are installed in a new engine room on the main deck. Many new, innovative control features were required to link the slow-speed main machine to the electric motor via the tunnel gearing. The power is transmitted to the water by a new controllable pitch propeller with a diameter of 7.1 metres. This propeller can absorb up to 24,400kW which is also the maximum power transmitted to the shaft. Although the new propeller is 0.5m smaller in diameter than the original unit, its special shape enables it to produce 20% more power. Using the machine data as a basis, the propeller power was calculated to be 19,160kW. Tests with a draught of 10m and a speed of 21 knots showed the power demand to be 18,639kW, giving a safety margin of 521kW. After converting this extra power into speed, the maximum predicted speed possible is 21.2 knots. Booster drive system As with many seemingly simple solutions, it was the small details that caused the main problems. A slow-speed diesel engine with oscillating torque had never before been combined with a constant-torque electric motor on a propeller shaft. To protect the electric motor and gearing system from the vibrations caused by the main machine, tunnel gearing was chosen. This transmits the electric motor power via a multi-disc clutch to the gear system and then via a Vulcan coupling direct to the flywheel of the main machine and the propeller shaft. The energy flow in the shaft is shown in Fig.4. To enable the two different systems to be used together, new automatic controls had to be developed for the drive system. These had to be completely reliable in every operating mode. This problem alone presented a major challenge, especially in view of the limited time that was available for the development work. For this particular application, a new digital control system was installed. The new main-machine/booster system was rigorously tested by the US Coast Guard (USCG) and the American Bureau of Shipping (ABS) with the help of Failure Mode Effective Analysis (FMEA). This involved a run-through of all possible service profiles, both in the dock and at sea, to ensure the safety and reliability of the booster system. The new electrical auxiliary system for the booster in­stallation receives its power from the booster diesel-generator set via a 6600/480V, 500kVA transformer. Photo: Blohm + Voss GmbH The new booster generator is used to feed an additional 4000kW (before losses) to the main drive shaft. 87 kW Electrical loss Self-supporting system 400 kW Booster drives bow thruster Since the booster diesel-generator is not required for docking man­oeuvres or when the ships are in port, it can also be used to drive the newly installed ABB bow thruster. Thus, the booster diesel-generator has two tasks in that it supplies: (1). additional energy for the main drive (PTI); and (2). drive power for the bow thruster. The diesel-generator set supplies power to a 6.6kV substa­tion with load feeders to the booster motor, the bow thruster and an auxiliary transformer. For this project, the Marine, Oil and Gas Industry Division of ABB Indust­ rie­technik AG supplied the electrical booster plant, the electrical equipment for the bow thruster and all of the cabling for the electrical systems. During the conversion, it was necessary, among other things, to shorten all of the cables to the forebody. This involved cutting a 40m-long section out of approximately 350 cables and then reconnecting the cables using heatshrink joints. This work was carried out in close collabora­tion with the shipyard and the suppliers of the other systems to ensure full compliance with ABS and USCG regulations. 80 kW Electrical loss Booster motor 4000 kW 159 kW Gear loss 210 kW Shaft loss Alternator Wärtsilä6000 kVA Diesel engine 12R 32 4860 kW Main engine Sulzer 7 RLB 90 20,588 kW Gearing 3822 kW 24,410 kW 20,588 kW Sea margin 3184 kW Fig.4: energy flow in the propeller shaft. Sea trials with the first ship to be completed, the Sea-Land Pride (formerly Galveston Bay), were carried out in the summer of 1994 and underscored the success of the project. The vessel, which had been running with a speed of 18 knots, achieved 19 knots without the booster system and almost 22 knots with it. In the same year, its two sister ships, Sea-Land Value and Ra­leigh Bay, were also handed over to the customer after successful SC conversions. Acknowledgement: this article has been adapted from an article that appeared in the March 1997 issue of ABB Review, published by Asea Brown Boveri Ltd. November 1997  75