Our Drive - Part IV: Special Vessels

The continuous advancement of alternative propulsion technologies enables even special purpose vessels to operate in an environmentally friendly manner without operational downtime.

Unlike cargo and passenger ships, special purpose vessels are difficult to categorize. They are characterized by a wide variety of functions, dimensions, and propulsion concepts, all of which must be precisely tailored to their specific operational purpose. Compared to other ship types, the functional diversity relative to vessel size is significantly higher, which often poses challenges in terms of available space.

Each special purpose vessel is designed for clearly defined tasks, allowing us to tailor all design parameters precisely during the design process. During the conceptual phase, we apply a kind of filtering process that enables us to develop solutions for all customer requirements.

The motion and operational profile defines the framework for the design

The area of operation and the distances to be covered determine many key parameters, such as vessel dimensions, speed, and range. The operational area also plays a decisive role in selecting the energy carrier. In environmentally protected areas, emission limits are already in force, in some cases requiring complete local zero-emission operation.

The availability of different energy carriers—such as diesel, methanol, ammonia, and hydrogen—or sufficient shore power connections for battery charging can further restrict the choice of propulsion concept.

However, many special purpose vessels offer a planning advantage in that they operate within a limited area and have defined berths in specific ports. This allows any required shore-side infrastructure to be planned reliably and implemented at a single, clearly defined location.

The primary function of the vessel gives rise to a wide range of design-specific characteristics

We have been involved in the design of special purpose vessels since 1994 and have addressed a wide range of operational applications. Unlike other projects, the conceptual design process focuses not on the vessel itself but on its functions—the vessel effectively grows around these requirements. In combination with size limitations imposed by the operational profile, this can become a significant challenge.

Especially for special purpose vessels with multiple functions, available space is the most critical resource. The required technical equipment competes directly with the propulsion system for installation space. Alternative energy systems require considerably more space than conventional diesel engines and fuel tanks.

In addition, the technical equipment of a special purpose vessel often represents a substantial share of the total construction costs. For shipowners or authorities, this can mean that the higher initial investment required for more environmentally friendly propulsion systems exceeds the available budget.

Commercial utilization often has a significant influence on the available budget

Large shipping companies often operate fleets of special purpose vessels dedicated to maintaining their revenue-generating ships (e.g., cruise vessels). High reliability in daily operation is essential, as the failure of a special purpose vessel can directly impact the availability of the commercial fleet. Consequently, substantial budgets are allocated to ensure uninterrupted operation. Alternative propulsion systems are also increasingly favored, as these vessels are visible to end customers during operation and help promote an environmentally responsible image.

Companies performing specialized construction and maintenance work use their special purpose vessels as profit-generating assets. Any downtime of these core working platforms directly results in lost revenue. As a result, a significant portion of the budget is invested in robust and proven technology, with alternative propulsion solutions being adopted only to a limited extent.

Public authorities operate special purpose vessels that are not commercially profitable. These vessels are publicly funded and rarely generate direct revenue. Budgets are tightly defined and typically offer little flexibility. At the same time, authorities are keen to meet governmental environmental targets and to assume a pioneering role.

An environmentally friendly platform for diverse operational concepts

In 2021, we developed a research vessel for waterway monitoring, equipped with state-of-the-art onboard laboratory facilities for water quality analysis. Water pumps located in the lower deck transport samples directly to the laboratory on the main deck, which is equipped with multiple computer workstations, laboratory benches, and analytical instruments.

In addition, the vessel serves as an information platform for expert groups, committees, and school classes. The operator is a regional authority, classifying the vessel as non-commercial.

Particular emphasis was placed on an environmentally friendly propulsion system capable of being operated with increasing sustainability as future technologies mature. The vessel’s motion and operational profile is non-stationary and spans several river systems across Germany. To ensure flexibility for varying route lengths, a diesel-hybrid system based on a DC intermediate circuit was selected. This electrical architecture allows the future integration of additional energy sources. Accordingly, sufficient spare capacity was provided in the electrical cabinets during the design phase.

The installed battery capacity is designed to absorb load peaks on longer journeys (peak shaving). In the local operating area, fully battery-electric operation is possible, enabling near-silent sampling in sensitive biotopes. The vessel’s shallow draft ensures good accessibility to riverbanks and shoreline areas.

The generators are designed to recharge the batteries while simultaneously supporting vessel operation, thereby avoiding downtime and ensuring the engines operate within an optimal load range.

To accommodate potential future changes in the operational profile, the vessel is equipped with a sufficiently large battery compartment allowing for capacity expansion. A future change in fuel type has also been considered: the open deck with integrated container fittings allows the installation of containerized hydrogen systems. In the long term, the diesel generators can be replaced by hydrogen fuel cells.

The vessel’s modular concept can be adapted to a wide range of operational scenarios. For example, instead of a large laboratory, additional crew cabins could be installed.

Universal emergency response for saving human lives

In another project, we were commissioned to design an emergency firefighting and rescue vessel (HLB) for a professional fire brigade in the Ruhr region. The vessel is currently under construction. With its onboard equipment, it functions as a floating work platform capable of supporting a wide range of technical emergency operations.

For optimal situational awareness, the wheelhouse is located aft, ensuring that both the bridge crew and engineers have an unobstructed view of the operational area at all times. Among its many tasks, firefighting is paramount—both via fixed water monitors onboard and as support for shore-based operations, with the vessel acting as a pumping station.

Another key function is the recovery of objects and persons from the water using divers, supported by a telescopic crane. The vessel is also equipped with a pressurized citadel to prevent toxic or explosive gases from entering the interior. Special attention was paid to the rescue of persons from the exterior, who can be transferred into the citadel through an airlock.

The HLB differs significantly from other special purpose vessels in that its crew members are not professional mariners. It is operated by a professional fire brigade whose primary operations take place on land. An intuitive and user-friendly control system with a high degree of automation, dynamic positioning, and joystick control significantly reduces crew workload during high-stress operations.

Further design considerations arise from the motion profile: the vessel is expected to operate only 200–500 hours per year. With few exceptions, all voyages take place between two locks, and the vessel returns to its home berth after each deployment.

As a result, the fuel tank capacity was deliberately limited to less than 1,000 liters, which is sufficient for typical missions lasting only a few hours. For extended operations, fuel can be replenished via the fire brigade’s internal logistics by land or water.

With a maximum speed of 35 km/h, the HLB operates at a moderate speed for a firefighting vessel. Higher speeds were excluded due to the confined operating area between locks and the risk of severe damage from excessive wake in canals. The moderate engine power requirement allowed for reductions in weight and cost.

After extensive concept evaluation, a conventional diesel-mechanical propulsion system was selected. One key reason was the low annual operating hours. Both financially and in terms of emissions, the investment in a hybrid system would be so high that amortization over the vessel’s lifetime would not be achievable. In addition, the limited space resulting from the vessel’s compact dimensions and extensive functional requirements would have made integration of an alternative propulsion system extremely challenging.

As an emergency response asset, the HLB is not intended to be commercially viable. While it may indirectly support commercial shipping through obstacle removal or emergency assistance, its primary focus is on lifesaving, hazard prevention, and disaster response.

Waterway conservation in a nature reserve with low local emissions

In this project, we designed a small tugboat with a length of less than 20 meters for operations in tidal flats. The vessel combines multiple functions within a compact hull. Its primary tasks include coastal protection work and debris recovery. Due to operation in the Wadden Sea, a shallow draft of less than one meter is required, and the tug must be capable of drying out.

Despite its small size, the vessel is capable of towing barges in the tidal flats and maneuvering them in harbors. Adequate thrust is provided by a comparatively large propeller. In addition to its primary tasks, the tug is used for icebreaking near bridges and hydraulic structures. The operator is a regional authority, and the vessel is rarely used for commercial purposes.

A key requirement for the propulsion concept was minimal emissions. The suitability of various systems—fully battery-electric propulsion, fuel cells, and internal combustion engines using alternative fuels—was evaluated based on a realistic operational profile.

The vessel must be capable of operating at maximum power for more than two consecutive days, with a maximum speed of 8 knots. Due to the vessel’s small size, neither batteries nor fuel cells are feasible. The limited buoyancy reserves associated with the required shallow draft also preclude the use of a hybrid system. Consequently, only an internal combustion engine using an alternative fuel is a viable option.

Because the operating area is a nature reserve, ammonia was excluded as a potential fuel. The vessel is intended to operate on methanol.

At the time of design, methanol engines in the required low power range were not available. Therefore, the vessel will initially be equipped with an IMO Tier III-compliant diesel engine running on HVO fuel and will be prepared for future conversion to methanol. Fuel tanks and spaces were designed in accordance with current regulations, such that conversion to the alternative fuel will require only the replacement of the main engine and a few minor auxiliary components.

The end of the diesel era?

Technological progress already enables designs that rely exclusively on alternative energy carriers or batteries. In practice, however, high initial investment costs and the limited operational experience with new systems often present significant barriers for shipowners and authorities. Diesel-hybrid solutions currently represent the state of the art.

Conventional diesel-mechanical systems have become significantly more complex due to new regulations on exhaust aftertreatment and emission limits. When used on special purpose vessels that frequently transition between long-distance operation and maneuvering, system durability can be adversely affected. SCR catalysts and exhaust gas recirculation systems are particularly sensitive to frequent load changes and the associated fluctuations in exhaust temperature and pressure. Variable load profiles, especially at low engine speeds, can lead to soot formation and clogging in exhaust aftertreatment systems.

Hybrid propulsion systems with buffer batteries can smooth these load variations more effectively and minimize system wear.

We expect future special purpose vessels to be designed from the outset for alternative energy carriers. Achieving this will require pioneering efforts by shipowners, authorities, and shipyards to gain operational experience with these technologies. Prototypes already exist for most applications. Bringing these solutions to a level suitable for widespread adoption will require entrepreneurial courage and a willingness to embrace change.