How to Build the Next Generation of Greener Hopper Dredges
For decades, the leadership and technological supremacy in the field of trailing suction hopper dredge design and construction has resided in Western Europe, particularly the Netherlands. While there have been significant changes in the maritime, shipbuilding and dredging industries, much of the current fleet of U.S. hopper dredges includes obsolete design features that would not be found on a contemporary European dredge. These out-of-date features economically and environmentally penalize the operators for the life of the vessel and restrict the efficiency of the fleet for the ships’ entire lives.
This graphic shows a medium speed diesel electric arrangement with shaft generator power take-off (PTO).
The Europeans build several dredges per year. This repetitive dredge construction has facilitated the designers and builders to hone and tightly integrate their standard design features and construction details. These improvements can be seen in several papers published in the proceedings of the dredging conferences. This experience was not easily transferable to U.S. shipyards and U.S. dredge construction because, until recently, the technology was held close.
The focus of this article is to conceptualize the design aspects of a new, modern, highly efficient, environmentally and economically sustainable hopper dredge for the U.S. fleet, and provide some editorial analysis about the future of the U.S. fleet.
There are some major philosophical differences in the procedures used to manage the respective European and U.S. fleets. The standard economic life of a European hopper dredge is 18 years (according to CIRIA’s A guide to cost standards for dredging equipment 2009). At the end of that time the dredge is fully depreciated and is usually sold. This is not the case in the U.S. Although the U.S. private hopper dredge fleet has built several new ships since 1980, they tend not to replace or renew their dredges with the same frequency of the Europeans. Modernizing, rehabilitating or repowering a dredge is often done to extend the useful life of a dredge, but rarely will a modernization or rehabilitation project change the fundamental characteristics of the ship (length, beam, draft, cargo capacity, dredging efficiency, hull efficiency, etc.) enough to radically affect the overall economic analysis of the dredge, except to reduce a little fuel use, lengthen the life, and extend the amortization time.
HOPPER DREDGE DESIGN FEATURES
The following design features and success criteria of a new hopper dredge are listed irrespective of mission (size and application) of the new dredge. The collective result of these features and criteria are to facilitate achieving the mission at the lowest cost per cubic yard of dredged material. For a given size of hopper dredge, the dredge with the most production and the least cost will be the most efficient, and would theoretically underbid the competition. The relationships between these features are often conflicting and the subject of trade-off analyses during design. The factors affecting each of those two criteria are listed below:
• Largest Hopper
• Most Maneuverable
• Highest Density Excavation
• Most Efficient Hopper Loading
• Fastest Loaded Speed to Disposal Site
• Fastest Unloading
• Fastest Speed Returning Empty
• Least First Cost
• Least Maintenance Cost
• Most Efficient Hull
• Most Efficient Propulsion System
• Most Efficient Pumping System
• Crew Size Matched to Automation
The hull of the hopper dredge is the most important feature as it supports and sustains the ship. The following features are briefly discussed below:
• Minimum Lightship Weight (weight of the ship without cargo)
• Vessel Trim
• Hull Resistance (EHP)
• Hopper Shape
The lightship weight should be as low as possible, consistent with the life expectancy of the dredge. Minimizing lightship weight maximizes deadweight (cargo carrying capacity) for the same loaded displacement. Minimizing lightship weight minimizes construction cost. Lightship weight should approach 32 percent of the displacement for a modern efficient dredge hull. It can however increase maintenance costs as there is less steel available to be sacrificed to corrosion/erosion over the life of the ship.
A modern hopper dredge should carry the maximum cargo for a given displacement. Hopper dredges typically have very high block coefficients (ratio of the underwater volume to a block of the same Length x Beam x Draft) providing the maximum buoyancy force and to provide adequate volume for a large hopper. To maximize the cargo carrying capacity, the dredge design must efficiently arrange machinery to achieve the largest hopper volume, while balancing the trim of the vessel so that the full hopper volume may be utilized before maximum draft is attained. Adjusting the trim for optimal loading is a weight balancing act that involves the longitudinal location of the cargo, the type and location of machinery, the pump room location, the deckhouse size and location, and other factors.
Effective horsepower (that required to push the hull through the water) should be as low as possible for a particular displacement, thereby reducing machinery weight and fuel usage. The horsepower required by the hull is also affected by the trim at different loading conditions; the resistance of the various appendages: skegs, bow thruster openings, hopper door recesses, sea chests, specialized bow designs; block coefficient; and the chosen hull shape. Hulls with high block coefficients typically require higher effective horsepower than more sleek hulls. Hopper shape is critical for efficient structural arrangement (least weight/cost), longitudinal strength, hopper loading rate, and hopper discharge rate. The loading rate is a function of the hopper dimensions, shape and location of overflow(s) to reduce overflow losses. The unloading rate is affected by the size, number, type and location of the hopper doors. The doors dictate the arrangement and amount of sloping plate surfaces in the hopper, and the structural arrangement. If equipped with pumpout, the discharge rate is a function of the hopper collection system and jetting system arrangement.
The least weight and lowest cost machinery concept is often direct-drive medium-speed diesel via reduction gear and controllable pitch propellers, including a large shaft generator driven off of the reduction gear and the jetting and dredge pumps driven off of the front end of the main engines. This plant concentrates all power production into the large efficient main engines. The shaft generator is key because in the event of a main engine failure, the shaft generator can be converted to a motor and driven via the auxiliary generators to get home at reduced speed. Though this is the lightest solution, it does present some disadvantages, like a lack of redundancy and weight balance/trim issues, since the deckhouse location tends to be forward to balance weight of all major machinery aft. A fixed pitch propeller is usually more efficient than CPP (controllable pitch propeller), however CPP is necessary while dredging to drive the pumps from the same engine at the most efficient pump speed. CPP is not required for diesel electric dredges.
Diesel-electric offers some excellent advantages:
• Diesel-electric offers flexibility in arranging machinery weight, maximizes cargo volume and trim balance, and speeds up construction.
• With a commonly used arrangement of multiple prime movers and a power management system (PMS), the number of generators online can be matched automatically to the power requirement. This achieves the optimum brake specific fuel consumption (BSFC) for any given load.
• A PMS can quickly shed non-vital loads in an emergency, keeping the vital machinery powered and online.
• Hybrid technologies, such as batteries, can be incorporated easily. Battery hybrid systems allow peak torques above the cumulative power of the prime movers, as well as smooth transitions between the number of prime movers online.
• Cold ironing with shorepower is an environmentally responsible option in port that becomes more economical with diesel-electric power.
• A newer development–DC power grid with variable speed generators is now a viable alternative.
DC Power Grid provides several uncommon advantages made available by technology developed for the utility industry’s low-loss DC transmission of electric power. New developments in the interrupting capability of DC circuits during an electrical fault condition promise to bring those advantages to the marine industry. Brushless DC generators and motors developed for electric automobiles reduce the higher cost of DC rotating equipment, which was also a previous limiting factor to DC power.
The advantages of a DC Power Grid include:
• A DC bus eliminates the need to synchronize generators.
• A DC system minimizes losses and problems due to inductance and harmonics which remain isolated in a DC system but circulate in an AC bus.
• A DC bus eliminates the rectifier stage in a typical diesel electric drive system, simplifying the system and reducing losses and heat.
• In a DC grid, generator engines can operate at the most efficient RPM for the load, which can represent a significant fuel savings over isochronous operation of AC generator engines.
Dynamic Positioning (DP) should be incorporated if pumpout is required. DP allows the dredge to automatically maintain station during retrieval and connection of the floating pumpout hose and during pumpout operations, without the need for deploying anchors or manual station-keeping by watch-standers. DP can easily be extended to Dynamic Tracking (DT), which allows the captain to automatically dredge along a prescribed route. These systems are commonly available and typically used on offshore supply boats in the U.S. and on dredges overseas. Multiple manufacturers and numerous vendors make for competitive pricing in a commercial off-the-shelf (COTS) procurement environment.
Z-drive propulsion and podded propulsion each have their own advantages and share a few. Z-drives and pods (and Voith cycloids) are the epitome of maneuverability. They each enhance dynamic positioning and the Z-drives can often be removed from the ship for repair without dry-docking, which is a significant maintenance cost savings. Z-drives and pods each put all the torque where you want it rather than divert the stream as a rudder would, adding efficiency to weather-compensating angleorders, especially where a bow drive is available. For maneuvering, Z-drives and pods can mean avoiding tug escort fees and assist fees at docking and passing under bridges. Voith drives are too expensive and unforgiving for most dredging applications.
New, lighter-weight materials for machinery can be used to minimize machinery weight and in turn, lightship weight, contributing to maximum cargo capacity. Without sacrificing safety, materials such as fiberglass reinforced plastic (FRP) or cross-linked polyethylene (PEX) pipe can be applied in the dredging systems as permitted by rules and class. The distinction between “industrial systems” and “vessel systems” on U.S. dredges, as well as the developed set of U.S. Army Corps of Engineers safety rules, in addition to the USCG and class rules, creates a complex regulatory environment for innovation but innovation is possible. For a dredge owner willing, the market disruption of increased efficiency could bring a greater market share of yards moved.
Environmental sustainability must be considered in the very beginning and throughout the design process. New environmental developments are being introduced continuously. New anti-fouling hull coating systems prevent the formation of marine growth keeping hull resistance low.
Another transfer technology, like the DC Power Grid, is the use of water hydraulic systems for the marine industry. Using freshwater as the fluid medium has been proven extremely effective on the Corps of Engineers Dredge Murden’s split-hull open/close system (Bossert & Olson, 2013). This technology is especially applicable to hopper door operators, dredging/jetting valves, overflow weir level-adjusting, draghead visors and vacuum-relief valves. All these systems are directly over or submerged in the sea and, if traditional oil is used, can result in hundreds of gallons of oil leaked from a failed flex hose, resulting in downtime and fines from regulatory bodies. With water hydraulics, the purchasing, storage, handling and disposal costs of the hydraulic fluid (water) with a food-grade glycol additive in winter, is almost negligible compared to other environmentally acceptable fluids, which cost between $30 and $50 per gallon.
The latest EPA tier ratings for diesel engines require reduced emissions. Combining Tier III or IV engines with a power management system, new hull coatings and the most efficient hull form, results in the minimum fuel consumption and minimum exhaust emissions for a given horsepower requirement.
Automation may be the single most powerful tool in improving dredge efficiency. Part of this is related to monitoring. While the best engine room watch stander only has two eyes, automation systems can monitor thousands of data points simultaneously. By reducing crewing requirements, automation systems enhance engine room safety and can reduce equipment maintenance costs. For the engine room, the level of automation should be matched to the crew size.
Many more dredges have been designed and built with unmanned engine rooms than have used that status to reduce crew size. ACCU status is awarded by the American Bureau of Shipping (ABS) and considered by the U.S. Coast Guard in determining crew size. It is considered most when cited in the application for a certificate of inspection and especially when called out in requesting a reduced crew size. This should be done early on to allow for modification of remote stations to suit the local captain of the port of registry. There is some latitude beyond the minimum standard for remote operation and reporting. Remote video of the engine room is one option. Software tools employed by engine-specific applications integrated to ship-specific motion-triggered visioning and actuation applications offer many possibilities. For instance, dredge pump torque can be dynamically displayed above the chief’s bunk (lucky chief.)
Some of the newest commercial dredges are capable of one-person bridge operations. The mate on duty is both pilot and dredging officer. This requires an extensive level of automation. The highest attainable efficiency gains are realized on dredges that use sophisticated algorithms to continuously adjust, improve and learn from the operation and interrelationship of the propulsion speed, draghead visor position, and dredge pump speed (Mourik, 2014). Efficiency gains as great as 26 percent can be achieved over non-automated dredge control. Automated engine room and automated bridge are very much separate issues and features though often carried out by the same systems integrator. Heave compensation is integrated with differential GPS (DGPS) and detailed hydrographic survey data, often aided in real-time by survey quality SONAR depth measurement and bottom profiling and contractor-maintained laser or radio frequency aids to navigation and temporary radar buoys. Self-optimizing algorithms are the “secret sauce” of dredge controls, along with proportional-integral-derivative (PID) loop controls and sand slurry specificgravity prediction methods to optimize dredge pump and booster efficiency, and throughput. It is all about maintaining the highest specific gravity without plugging the pipe. PLCs (programmable logic controllers) can adjust in milliseconds and not ever need to reboot. They can hot-swap like RAID V blade drives in a data center. 24/7.
The service life of the electronic platform for the engineering, navigation and dredging plant must be considered in the economic analysis of the dredge. The software and hardware associated with monitoring, alarm, control and automation typically has a life of five to 10 years, at which time it is replaced, so the electronic platform needs replacing several times over the life of the hull.
LNG should be considered as a fuel for any new dredge. Full Tier IV compliance is achievable without aftertreatment – no urea formaldehyde injection needed for NOX emissions, no sulfur and no particulates. LNG is plentiful and produced in the U.S. LNG may or may not be the most economical fuel at the lowest possible oil prices we have seen recently, but LNG remains inexpensive as oil prices rise. Natural gas will remain inexpensive in the U.S. for many years because so many gas wells have been drilled and fracked. Even with increased export, this over-supply will keep LNG prices low for the foreseeable future. The known reserves of natural gas are enormous and production costs low. As with many efficiency improvements, passing on of fuel cost increases to the end user has reduced incentives for owners to spend money to conserve.
The latest commercial hopper dredges include the following dredging features:
• One large diameter dragarm; typically, maintenance dredging requires two dragarms due to severe restrictions in their ability to maneuver in harbor entrance channels;
• A bow discharge connection for direct pumpout and rainbowing, if required by the mission. Rainbowing is the process of pumping out the material in the hopper via a nozzle fitted near the bow coupling. The airborne material arcs over a containment barrier to build islands;
• Jet-assisted, adjustable visor, excavating dragheads;
• Shallow loaded draft to get as close as possible to the pumpout location and shortest pipeline to the shore;
• Large hopper doors to unload dredged material quickly and efficiently and leave the least material in the hopper following disposal;
• Dredging automation to include automated deployment and retrieval of dragarms, and automatic loading and unloading.
Recently, there has been a mission change at the Corps of Engineers to include increased beach nourishment and wetlands creation verses traditional dredged material placement via bottom doors. This requires higher pressure dredge pumps, more installed power to drive those pumps, and additional booster stations to pump through miles of pipe to the shore. These booster stations are a good place to use cleaner technologies like natural gas. Booster stations also present good opportunities for pipeline automation and automated slurry control to maximize pumpout efficiency, pressure, velocity and specific gravity. This is best done by computer and not by radio and hand-throttle. Systems are available with high-security wireless, marine-level redundancy and no-boot platforms.
Jet-assisted excavating dragheads have been standard equipment for some time. They use high pressure water jets to cut and fluidize the bottom sediment and use abrasion-resistant teeth, coupled with the towing force of the dredge’s propulsion to scoop the loosened material and force that material up into the dragarm. Jet-assisted excavating dragheads greatly increase the density of the mixture entering the dredge. This creates greater efficiencies but also greater opportunities for failure, making automated control and monitoring even more important. The best dredger can’t adjust something in 24 milliseconds.
CONCLUSION: EDITORIAL ANALYSIS
A hopper dredge is not simply a cargo ship hull with dredging equipment inserted. A hopper dredge is a dredging system, from stem to stern, with every component integrated to do one thing as efficiently as possible. Every pound of piping, wire, machinery or steel that is not optimized for this one purpose, contributes to inefficiency, with the cumulative effect of commercial failure of the dredge.
Efficiency is everything. Efficiency represents the amount of fuel used and the amount of fuel wasted. Efficiency represents the underlying bid factor and the economic viability of the vessel. Efficiency on a larger scale represents the viability of the project and the contractor. Personal injuries are inefficient. Environmental impacts are inefficient. Unnecessary after treatments are inefficient. Environmental fines contribute to inefficiency. Over-crewed vessels contribute to inefficiency. Antiquated practices and legacy equipment contribute to inefficacy.
Achieving the same future as the Western European dredging community is possible – improved dredging efficiency through optimization, automation and environmentally responsible energy management. These systems lead to more cubic yards of material moved for less money. Though the new Jones Act dredges being built are a major step forward in modernizing the U.S. fleet, the opportunity was missed to really break from the pack and take advantage of some readily available and proven technologies used successfully elsewhere.
The skeptical viewpoint could be represented as: U.S. dredging is a closed market, the market-driven forces propelling innovation are neutered and the U.S. dredging fleet remains backward with regard to most European dredging technology, particularly from the Netherlands. A more optimistic view of the U.S. dredge market could be characterized as: U.S. dredging is an open-bid, dollars-per-yard, competitive business, and there is every reason to believe the U.S. dredging industry will improve its efficiencies and replicate, duplicate or better its European colleagues. Although the industry has a way to go, American dredgers must make their best effort to make the more optimistic projection come true – achieve Dutch efficiencies with the U.S. dredging fleet.
Vinton Bossert retired in February 2015 from the U.S. Army Corps of Engineers Marine Design Center, as the senior marine engineering specialist. He now operates Bossert Dredge Consulting, LLC. He can be contacted at email@example.com. Stephen Wright is manager of electrical systems and manger of shipyard and commissioning teams at SanSail Group. Ethan Wiseman is vice president of engineering at SanSail Group.Edit Module