Bigger and more sophisticated

In the last four or five years the capacity of trailing suction hopper dredgers has increased tremendously, but dredger builders and operators in the Netherlands say that size is not all important, and highlight the need for highly manoeuvrable, accurately controlled vessels. They also highlight the growing demand for sophisticated control systems or dynamic positioning and tracking (DP/DT) and dredge track presentation systems (DTPS), which enable the crew to “see” the seabed they are excavating in real time, using sonar. Although the performance of a dredger is still largely defined in terms of its capacity to dredge, load and transport material, along with installed power and cost, other factors come into effect in the design of a modern trailing suction hopper dredger. These include its hullform (which must be hydrodynamically efficient in order to minimise transit time), its maximum draught (which must be limited, to enable it to work in confined waters), and its ability to dredge accurately within prescribed tracks and to a prescribed depth. Moreover, say the principal dredger builders, increasingly owners are not just specifying ships with a certain installed power, capacity and speed, but are also specifying the accuracy with which the dredger must be capable of working, and its ability to maintain track. Thus, they claim, in due course, far from remaining unsophisticated workhorses, the most efficient dredgers will be fitted with particularly accurate, fully integrated navigational equipment, and with manoeuvring gear such as azimuthing thrusters and even podded propulsors. The future of dredger design lies, they believe, in very manoeuvrable all-electric ships with diesel electric propulsion, azimuthing thrusters, and flexible power management equipment, all controlled by software which mimics the actions of experienced operators. Larger vessels on the wane? In its most recent financial statements, the Netherlands most successful dredger builder, IHC Holland, said that demand for dredgers “remained steady”, but highlighted the fact that there had been no further orders for “jumbo” hopper dredgers since it last reported on its results. “There have, however, been a number of smaller but complicated dredgers to compensate for this,” said IHC in its statement, thus drawing attention to one of the most obvious trends in the market for dredgers at present. “There are also a number of promising projects in the market for complicated custom built ships,” noted IHC, although “pressure on prices in the Group?s market segments is less intense from the Far East? it is increasing from within Europe, where Spanish yards have won a number of orders”. This was, said IHC, mainly due to what it called “the availability of favourable tax lease structures”. Ironically, Dutch yards found themselves unable to build what is currently the world?s largest trailing suction hopper dredger, Jan de Nul?s Vasco da Gama, because capacity was at full stretch building many somewhat smaller vessels. Intended primarily for use on large-scale land reclamation jobs, the Vasco da Gama was built in Germany, by a joint venture of Thyssen Nordseewerke and dredging equipment specialists LMG. Reclamation projects In the past decade, tremendous changes have taken place in the dredging market. Traditionally a highly cyclical business, and one where demand has gone through regular peaks and troughs, the dredging market in the past 6-7 years has enjoyed a period of steady growth, fuelled in large part by the growing number of large-scale land reclamation projects around the world. Several contractors now operate vessels in the 18,000-23,000m3 range, but the capacity of the Vasco da Gama is such that on a single voyage it can transport 2-3 times the load of most competing vessels. Another key feature of the latest generation of jumbo trailing suction hopper dredgers exemplified by the Vasco da Gama is the ability to work at great depths. As demand for clean sand for reclamation work has grown, so dredging contractors have had to work further and further offshore to obtain it. For this reason, in addition to her very large capacity hopper, Vasco da Gama is also capable of working at depths that exceed those of any other dredger. The largest Dutch-built vessels remain Royal Boskalis Westminster?s Queen of the Netherlands and sister vessel WD Fairway. Not far behind are vessels such as Rotterdam, the 21,500m3 trailing suction hopper dredger IHC built for Anglo Dutch Dredging Company Ltd (ADDC), the launch of which took place on December 16, 2000. At 190m in length, this trailing suction hopper dredger is the second largest in the world, and is almost 190m long. As Dutch dredging contractor HAM noted recently, the trend towards the acquisition of larger and larger trailers was driven mainly by the enormous land reclamation projects in the Far East, and the availability of dredge sand for these reclamation projects at the lowest possible price. “Stiff competition keeps us focused on the cubic metre price of sand,” said HAM recently. HAM?s latest trailer, the HAM 318, was launched on May 25, also having been built by IHC Holland NV, in co-operation with Van der Giessen – de Noord. The sheer size of the ship – she is 170m long with a breadth of 32m – meant that IHC had to sub-contract the hull to Van der Giessen de Noord. She was due to start work in October, on land reclamation jobs in Singapore and Hong Kong. With a capacity of 23,700m3 the HAM 318 is approximately the same size as the Queen of the Netherlands and WD Fairway. In keeping with the design of most modern trailers, it is equipped with two suction pipes, each with a diameter of 1,200mm and can dredge at depths of up to 110m. With a total installed power of 28,500kW, it is capable of making slightly more than 17.3kt. At the beginning of last year, Dredging International?s latest acquisition, the 17,000m3 Nile River made its operational debut, carrying out demanding assignments in the Mediterranean and Persian Gulf. An enhanced version of the trend-setting Pearl River – the first of the new generation of jumbo trailers – the Nile River was also built by IHC Holland and went to work on a channel deepening scheme off the French coast near Monaco. Like the Pearl River, the Nile River combines a jumbo-sized hopper capacity of 17,000m3 with a draught of just 10.6m, thus enabling her to work close to shore and in confined fairways. Going offshore Land reclamation jobs apart, one of the major tasks assigned to larger trailing suction hopper dredgers in recent years has been work offshore, dredging and backfilling trenches for pipelines. Doing so requires a very high level of accuracy on the part of such large vessels, but DP/DT systems also play a key role for pump ashore operations and many other tasks, and enhanced positioning tends to increase dredging efficiency. Perhaps the first vessel to fully embody the trend towards the use of DP/DT systems was the 13,700m3 Lange Wapper. As representatives of IHC Systems explained at the recent 16th World Dredging Congress in Malaysia, the Lange Wapper was the first vessel with a fully integrated bridge in which all dredge instrumentation and automation systems were integrated with the ships DP/DT, ECDIS and dredge track presentation system. The same high level of integration between dredging control and automation and the ship?s navigation equipment is also a feature of the Nile River. As representatives of IHC Systems explained at the conference, the integrated DP/DT and dredge track presentation system help to ensure that the unsought movements of the draghead – the device which actually excavates material from the seabed – off track are minimised. “The force on the suction pipes can change in seconds from zero to very high values, which can force the draghead in unwanted directions,” explained IHC Systems George Mallee. “Also, the draghead has a natural tendency to follow tracks that have already been made in the seabed, and can switch into an adjacent track very quickly. Without a DP/DT and DPTS, it can take several minutes to steer the vessel back onto the correct track, by which time it may be several metres past a ridge, thus missing some of the seabed. “If the ship?s navigator has a good idea of the seabed profile, which can be gained from the DTPS, he can predict whether a draghead will shift position on the next ridge or mound. Having an up-to-date bottom profile of the type provided by the DTPS enables him to anticipate movement of the draghead, and if it does occur, the DP/DT enables him to recover the required track more quickly”. Vision of the future Sophisticated as Lange Wapper and Nile River are, the IHC group companies have moved beyond such designs in their thinking, and have developed alternative designs. Among these are an enhanced Lange Wapper design with azimuthing thrusters, and an all electric ship with still more advanced DP/DT performance, the latter design benefiting from a relatively short engine room, positioned as far aft as possible and making use of three diesel gensets for power generation. With a shorter engine room, located further aft, the arrangement of the hopper and pump rooms can be enhanced, and the size of the hopper can be increased. Having the pump room further forward makes for a more flexible suction tube installation, and enables the ship to carry longer suction pipes. It can thus dredge at greater depths, and having the suction pipes further forward improves DP accuracy. With the pump room forward of the hopper, the ship?s centre of buoyancy is shifted aft, thus reducing the differences in trim between the fully loaded and empty vessel, providing a finer entry of the hull, reducing wave resistance and pitching, increasing speed and reducing wash. Although more costly than a conventional dredger, the greater efficiency of such a design results in much more efficient dredger, and a quicker return on investment.