In Depth: Texelstroom

The largest of the Dutch Wadden Sea Islands, Texel, is connected by a modern ferry service from Den Helder. The service has been operated since 1907 by ship owner Texels Eigen Stoomboot Onderneming (TESO), which is a company owned almost entirely by the inhabitants of the island Texel.

The fleet exists out of two vessels. On average, every 12½ years, the company builds a new vessel, which takes care of the service for the first fifteen years of its life and then continues as a back-up vessel for the next ten years. In 2012, the process was started to design and build the replacement vessel for the current ship, Schulpengat.

With a population of 13,600, the island of Texel is a popular tourist destination, offering unspoiled nature at just a short and convenient ferry ride from the mainland. Tourists make up the majority of the passengers on TESO’s ferry service. In addition to the regular ferry connection throughout the day, the ferry service is on call at night for emergency sailings, as there is no hospital on the island. This happens about three times a week, and it’s an important aspect in the design of the vessel. The main considerations in the design are that it should well represent the splendid nature of the island and that it should be possible to continue the service in all weather conditions, with less than 0.1 per cent downtime per year. Finally, the car capacity had to be increased on the new ferry, as a long-term forecast shows that the annual numbers of cars can increase by 400,000 in the coming 25 years.

Clean slate

At the start of the project in 2012, TESO invited six Dutch ship design companies – including the designers of their previous vessels – to submit proposals in a tender process. They chose the relatively young and fast-growing company C-Job based on a combination of proven expertise, offered price, shown commitment to this special project and the ability to come up with out-of-the-box solutions. Not having designed a similar ferry before, the company started out with a clean slate, which was exactly what TESO wanted. While the design of the ferry is revolutionary, the way how it came about is nothing less.

Profound design process leads to radical innovation

Before actually designing the vessel, a big survey was held among TESO’s personnel, its stakeholders and passengers to generate ideas for improvement of the vessel. The survey resulted in over 600 ideas which were subjected to a feasibility study, and no less than 400 of these ideas made it into the final design. To evaluate whether an idea or solution would be applied, a rigorous vetting system was applied, in which each idea was scored on reliability, safety, ecological benefit, economic payback period and user-friendliness.

T-shaped section

One of the contenders in the initial tender process was Vripack (Sneek), who have successfully applied their yacht design skills to a number of commercial ships over the recent years. They didn’t win the tender for the technical design, but nevertheless were invited by TESO to do the interior and exterior styling.

The design of Texelstroom is very much a custom design, specific for its trading route and mooring facilities. The operational profile is characterized by a short and frequent crossing of about 20 minutes every half hour at a service speed of 10.5 knots. This leaves a very short time of less than ten minutes for loading and unloading of the vehicle decks. The mooring facilities, with a double-tier linkspan at either side, are owned by the government and were not to be changed. Basjan Faber, design director at C-Job: “By using a T-shaped cross section, we managed to add an extra lane on each side of the upper car deck without increasing the space needed at the berth.“

Slamming

One thing which was determined very early on is that the Texelstroom had to be a double-ended ferry like her predecessors, to keep manoeuvring times to a minimum. While this has clear operational advantages, it also presents a challenge, as the flow over the hull is very different at the bow than at the stern, even though they are shaped identically. One issue from the present vessel Dokter Wagemaker which required specific attention was the occurrence of some slamming in stormy weather. It was assumed that this was due to bow-flare slamming, derived from bowquartering waves impacting on the overhang over the water.

The initial idea was to break that sudden pressure surge with protrusions above the waterline, as can be seen under the cross-structure of some catamarans. Model tests at the model basin in Vienna however showed a different source of the slamming.
When the vessel is sailing, a wave trough is created near the forward thruster. Even though the ship doesn’t pitch, slams occur when this wave trough is temporarily below the chine in the hull, and the space under the bottom suddenly fills up with a short-crested wave. A solution was found in placing the chine deeper under the waterline.

CFD simulations

For this project C-Job contracted Van Oossanen Fluid Dynamics (Wageningen) for a CFD study early on, to verify if the thrusters could generate sufficient thrust sideways – taking into account the interaction between them – to manoeuvre away from the mooring piers in a wind of Beaufort force 11. It was a combination of a CFD wind study – with a strong side wind blowing over the superstructure – and a CFD study of the hydrodynamics in play under the waterline. Based on this study, the optimal orientation of the thrusters was advised. The choice for thrusters, rather than conventional shafts and rudders is both due to the manoeuvrability and the possibility to replace a thruster with the spare thruster in only 24 hours in case of a problem.

Peak shaving

The operating profile of Texelstroom consists of moored unloading and loading for about ten minutes, manoeuvring and acceleration for about three minutes (which takes about 5,500 kW of power for two minutes), a crossing of 14 minutes at 10.5 knots (which requires about 1,700 kW), and finally again three minutes for decelerating and manoeuvring. As on TESO’s previous ferries, the propulsion is electric – in this case gas-diesel- electric – mainly for redundancy and reliability. The designers adopted almost a similar approach as a DP-2 vessel: with any single failure, the vessel can still carry out its normal service without compromising speed
or comfort.

Because there is such a short burst of high power demand, it was decided to install a battery bank for peak shaving. This battery bank is charged during low-load periods of the generator sets. Its use avoids the chronically inefficient low-loaded running of a standby generator or the very frequent starting up and shutting down of one of the generators. Reports of the first crossings show that this system works well. In fact, the battery capacity is such that the vessel could make a back-and-forth crossing purely on the batteries which increases the safety of operating a ferry service. During normal operations, the battery pack is only drained by 20-25%, which leads to an expected lifetime of the batteries of up to 7,5 years.

Solar

Because this area has the most sunshine hours of the Netherlands, the Texelstroom also features 700 square metres of solar panels on its upper deck, which on sunny days can supply up to 75 per cent of the hotel load (up to 150 kW). The use of solar panels on ships is often debated, as they rarely have an ideal orientation, which significantly reduces their efficiency. As the double-ended ferry Texelstroom always has the same heading, the panels could be mounted so they face towards the South all the time, making it in this case a valid option, with an expected payback period of only six years.

CNG-electric 

The combination of cleaner emissions, no visible smoke, and less variable fuel pricing than marine diesel placed Liquid Natural Gas (LNG) high on the wish list from very early on. There are however still some complications with LNG, the biggest of them related to provisioning. For that reason TESO took a closer look at alternative solutions and contacted PitPoint Clean Fuels (Nieuwegein), which resulted in the choice for Compressed Natural Gas. Almost every house in the Netherlands is heated with natural gas, which is distributed in a piping network at a pressure of three bar. A compressor station was built by PitPoint close to the berth on Texel for the refuelling of the vessel during the night. Here the natural gas is compressed to a pressure of 200 bar and piped on board to two pressure tanks, which are stored in two UAC 30-foot containers on the upper deck. The connection to refuel the vessel with CNG is automatically uncoupled, ensuring no dangerous situation can occur when the vessel has to depart quickly for an ambulance service.

Dual fuel 

To optimise the efficiency of the gas filling station a seven-kilometre long high pressure pipe was installed all the way from Oudeschild, where the supply line comes from the mainland. Another issue with CNG is that it is not of the same quality as LNG. It contains some additives and has a lower methane content and energy density than LNG. The supplier of the main generators, Anglo Belgian Corporation (ABC), modified their settings to be able to run on CNG, rather than LNG. Two of the four main engines feature dual-fuel engines (CNG/diesel), while the other two are regular diesel engines. The dual fuel engines will be responsible for at least 80 per cent of total energy production. They use a small quantity of diesel to achieve ignition of the gas in the combustion chamber. As a fall back option Texelstroom can also run entirely on marine diesel oil and has sufficient tank capacity to sail for two weeks. In CNG-electric mode the ship complies already voluntarily with Tier III requirements, which are not yet mandatory. A hood was created above each main generator to capture any escaped gas and ventilate it to the atmosphere. When sailing in normal weather conditions, only one of the four generators will be running, in combination with the battery bank for peak loads.

Efficiency

A lot of smart solutions were adopted to reduce the fuel consumption and to allow for quick loading and unloading of the car- and passenger decks. At the port of Den Helder, the mooring pier is equipped with an automated mooring system, based on vacuum suction, from Cavotec , which eliminates the need for thruster power in port. To reduce the wind resistance, the corners of the vessel were rounded, and the headwind during sailing is used for much of the car deck ventilation. The ventilation fans are frequency-controlled and their speed is adjusted automatically based on the measured CO2 content in the air on the car deck. Double glass is used on all accommodation windows and ample insulation provided to reduce the heating load. A large tank of 90 cubic metre stores thermal water which recycles the waste heat from the main generators. This water is used for the central heating system on board, also during night time. Taking into account the slight increase in displacement (and therefore resistance), the payback period for this solution is still very good at about three years.

Vripack

The car decks were designed for rapid loading and unloading, which is helped by the absence of curbs and the ample provision of natural light through windows in the side. The interior designers of Vripack created a beautiful interior, which reflects in colour and style the nature of the island of Texel and the Wadden Sea area. There is also a marked difference between the side of the vessel which always faces the North Sea and the other side, which always faces the Wadden Sea. The exterior profile is inspired by a local bird, the little stern, in-flight. Vripack carried out a numeric simulation to optimise the flow of passengers. One consequence of this was to create an extra pair of staircases, to ensure that passengers making their way to the car decks are not held up by foot passengers queuing up to disembark. The buffet is of an open-shop type, so people can take the food and beverages they like and head straight for the cash register, rather than queuing up in a single line. This is a service improvement, especially because passengers stay in the saloon less than 15 minutes.

La Naval

Texelstroom will start sailing with passengers this autumn, and all parties involved are rightfully proud, including class society Lloyd’s Register, which was challenged by this project ‘off the beaten track’ with so much innovation. A thorough design and development process was followed by a relatively short building period. The ship was built at shipyard La Naval in Spain, which rather than a pure cost consideration was chosen for its track record as a builder of complex one-off vessels and its capacity in engineering and building complex vessels. What is remarkable is that TESO is a non-profit organisation that strives to deliver a high quality service to all passengers while being able to keep the crossing very affordable. A foot passenger pays only € 2.5 for a return trip while a car (including passengers) pays € 25 to € 37 for a return trip, depending of the time. All this on one of the most advanced and comfortable ferries in the world.

Without a doubt, Texelstroom will be a serious contender for the KNVTS Ship of the Year Award. The island of Texel will certainly receive a spike in visitors with a background in the maritime industry, just for the ferry ride.

Bruno Bouckaert

This article was previously published in Maritime Holland edition #5 – 2016.

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