When NAM/Shell UK decided to invest in a ‘Walk-to-Work’ (WTW) offshore vessel for use with unmanned gas platforms, they requested Royal Niestern Sander (RNS) to make the preliminary design. The challenge was to design and build a stable platform for the transfer of personnel and equipment, to efficiently support the gas production platforms in the weather and sea states of the southern part of the North Sea.
With the experience gained over years in the adapting and mobilisation and demobilisation of Platform Supply Vessels (PSVs) into Maintenance Support Vessels (MSVs), RNS was ideally placed to provide the solution to that challenge. However, there was a single large problem to overcome first and that was NAM did not have a donor vessel suitable to convert. Furthermore the mobilisation and demobilisation of an existing vessel would be a lot of hard and expensive work. Against this setting and the early discussions came thoughts of a brand new vessel. The aim now was to find a partner/ designer/charterer combination willing to provide a custom WTW vessel for a ten year charter agreement.
The solution to the NAM challenge
RNS was a willing partner as they saw a future in such vessels, because in many respects the vessel’s design faced the same set of criteria as those used in the wind farm industry.
The first assignment for Niestern Sander was to conduct a design study, based on a list of the functional demands of the vessel.
With this ‘list’ and the design study tucked under their arm, NAM/Shell UK commenced a tender procedure to assemble a list of candidates interested in the design and build of a vessel for them to charter on a ten-year contract. The successful candidate Wagenborg, in combination with RNS, was awarded a contract in July of 2013.
So it was, that on 9 February 2015 at 15:00 hours in the afternoon, the champagne flowed as Lydia Schotman-Bulterman, the wife of Gerald Schotman, managing director of NAM, named the vessel Kroonborg. Based in Delfzijl and intended for use in the NAM/ Shell UK offshore gas fields in the southern part of the North Sea, Kroonborg now fulfils the owner’s requirements with style.
As stated above, Kroonborg is a ‘Walk-to-Work’ vessel, designed for the provisioning and maintenance of NAM/Shell UK’s unmanned gas platforms. To satisfy the NAM requirements, it must be capable of operating round the clock 300 days a year in up to 2.5 metre significant wave heights in Dynamic Positioning (DP) mode, with a crew of 20 persons and 40 maintenance staff of technicians and engineers. The vessel is primarily to provide a constantly level platform and so, in a break with tradition, it was designed around the motion-compensated gangway and crane. The real challenge here was to dampen the movements and their accelerations in the prevailing sea states.
The honourable task of developing the hull form and lines, which were tested at the Maritime Research Institute of the Netherlands (MARIN), was given to Conoship of Groningen. For this newly developed shape, Groot Ship Design of Leek in the Netherlands conducted the engineering design and produced all the drawings of the ship’s construction.
With the naval architect’s aspects taken care of, careful attention had to be paid to the design and selection of the propulsion components to satisfy the requirements of the DP2 system. The additional requirement for a very high level of comfort in the accommodation dictated low sound and vibration levels. To achieve this RNS invested in sophisticated techniques in their propulsion and DP components.
Initially the vessel was to be BV (Bureau Veritas) classed according to the LHNS (Liquid Hazard Noxious Substances) code for liquid chemicals below deck. However, the list with functional demands also required the ability to carry small amounts of specific chemicals, which were not defined in this LHNS code. For that reason, RNS upgraded the tank lay-out and systems to meet the requirements of the IBC (International Code for the Construction and Equipment of Ships Carrying Dangerous Chemicals in Bulk) notation.
In addition, the vessel features a dedicated Cold Start Up (CSU) unit: a high pressure pump unit consisting of two hydraulically driven pumps and large heaters for injection of anti-freeze liquid into ‘sleeping’ existing wells. To accommodate these facilities, a separate dedicated pump room is provided below deck on portside aft. To allow storage and handling of the bunker hoses for liquid cargo and CSU, reels and mounted to portside on main deck. During bunkering and CSU, these bunkering hoses can be handled with the deck crane.
The areas below decks are primarily used for cargo tanks and machinery spaces. As the expected cargo includes dangerous liquid and gaseous chemicals (and to follow BV Rules), the cargo tanks are arranged in a special Outokumpu supplied Duplex steel configuration and Kroonborg is fitted with intrinsically safe pumps made and supplied by PerGjerdrum.
Above main deck Kroonborg is designed to carry deck cargo, such as pipe, containers, palletised coiled materials, casing segments and other heavy items on her large 500 m2 cargo area. This cargo decking has a load rating of five tonnes per square metre. With the accommodation located as far forward as possible, the optimum use is made of the unobstructed ‘deck length’.
For loading and unloading duties Kroonborg is equipped with a Lagendijk semi-custom built crane, with a lift capacity of five ton at 30 metre outreach. The crane sits on a custom designed motion compensated platform. Made by Bargemaster, the BM-T40 hydraulic motion stabilised platform encompasses all three ships freedoms of movement, the whole unit having the smallest footprint ever for a crane of this size.
An Ampelmann self-stabilising access system provides the ‘walk to work’ facility. Like the Bargemaster/Lagendijk combination, this gangway system is also motion compensated and equipped with ‘long-stroke’ hydraulic cylinders. This system ensures, that it can meet the 2.5 metre wave height operating requirement, which is very important for the personnel making the 22 metre ‘walk.’ Furthermore in the interest of minimising sound and vibration levels, it was decided to water-cool the hydraulic power pack. To facilitate this, the power pack was purchased from Ampelmann and then customised before being installed on the vessel.
The vessel is fitted with a full suite of anchoring and mooring equipment, appropriate to the size and purpose of the vessel. In addition the vessel features two five-tons tugger winches on the cargo deck for cargo handling. The straight and vertical bow, designed to minimise the effect of slamming, in combination with the forward superstructure provides a covered foredeck to protect the crew during mooring operations.
Also on the cargo deck, at the forward end on the port side, adjacent to the superstructure, is the SOLAS/IMO required Fast Rescue Craft (FRC), whilst on the starboard side is the Daughter Craft with a capacity of 15 persons. With a planing hull and water jet propulsion the FRC is capable of 40 knots. Both vessels are deployed and recovered A-frame davits; vessels and davits are delivered by Palfinger/Ned-Deck. Immediately behind the boats, eight inflatable life rafts are provided, four on either side, in racks. They provide the capacity required by the authorities.
As stated above, considerable thought went into the design and selection of equipment to permit the vessel to perform its duties efficiently and operate under the DP system. As a result a diesel-electric power option was inevitable. Power is generated by four diesel alternator sets, each of 1,700 ekW. The prime mover diesel engines are Caterpillar 3512C HD, Tier II units, delivered by Pon Power. They are located, together with the alternators, in the engine room under the superstructure. The alternators produce 690 Volts AC at 60 Hz and by means of the power management system can be run in any combination. This ensures that the load at any time is adequately provided for, whilst at the same time fuel usage is minimised and operational hours are balanced across the units optimising the maintenance requirements.
In accordance with SOLAS and Class requirements, an emergency generator is located on the top deck. This set is powered by an air-cooled Caterpillar C9 coupled to an alternator delivering 269 ekW. The entire electrical system was designed and installed by Eekels Technology including the switchboards, power management system, engine room alarm and monitoring and pumping system.
The propulsion line comprises two Voith Schneider Propellers (VSP) located in the stern with two Voith Inline Thrusters (VIT) in the bow. The aft VSPs are 28R5 ECS/234-2 units, driven by their own dedicated electric motors rated at 1,850 kW each. These Voith Schneiders are ideal instruments for use with DP applications, not only are they able to thrust in a full 360 degrees to a very high level of accuracy, but they can also respond to a change of demand within a split second. These levels of response and precision put them in a class of their own when compared to conventional azimuthing thrusters, which can take up to 30 seconds to make large change of thrust angle. In addition, in this instance, Voith Schneider recommended the motors to be variable speed by means of variable frequency converters. With the variable speed and variable pitch potential provided by the Voith Schneider’s own combinatory control programme, the thrust management is almost limitless and ensures low fuel consumption. Furthermore the VPSs are placed under an athwartships angle (following the ‘dead rise’ bottom angle of the hull), thus also providing a force that compensates for the rolling of the vessel. In ‘Roll Compensation’ mode, the VSPs anticipate the rolling motion of the vessel and reduce this motion by means of a response in thrust variation, thus contributing to the comfort on board.
At the forward end of the vessel, the bow thrusters exhibit a similar level of innovation. These two VITs are rim-driven propellers and do not require an electric motor hub in the centre of the thruster or complicated shaft drive and gearing. Instead the stator and rotor are located around the outside of the tunnel with a permanent magnet rim rotor, that directly drives the propeller. The thrusters are fitted with six bladed units, making for a quiet and smooth delivery of thrust. Even with the bow thrusters at full power the sounds levels in the accommodation right above them (the day/ mess room) remain well below 45 dB. This is important for NAM and Wagenborg, as it is important that the engineers and technicians are not troubled with seasickness and are well rested, as any such inconvenience may prevent them from working efficiently.
Wolfard & Wessels Werktuigbouw BV of Hoogezand conducted the design and integration of the machine spaces and their equipment. They also provided the entire engine room and cargo piping systems prefabricated, complete with the machinery in a turnkey installation package. In addition Kroonborg is equipped with an Oceanguard ballast water management system, supplied by Reikon. This system kills microbes, bacteria and viruses in ballast water, using an electro-chemical excitation system. The choice for specifically this system was because of the small footprint of the unit, making it ideal for use in restricted spaces.
The accommodation is at the forward end of the vessel to permit the optimum cargo space usage. On the same level as the tank tops and forward of the switchboard room are the gymnasium and laundry. Above them, at the same level as the cargo deck, are the galley, provision store, changing rooms and the mess room. Moving up to the next level A-deck, there are 17 cabins for the ship’s staff and the hospital. One deck higher, B-deck, holds 16 cabins and one office for the maintenance technicians. Continuing the climb up through the superstructures leads to C-deck, where the cabins for the ship’s officers, captain, chief engineer, second engineer, two dynamic positioning officers (DPO) and an offshore installation manager (OIM), are. The DPOs are available for round the clock duties, whilst the OIM has his own dedicated office as master of the offshore platform. Next level up is D-deck, providing direct access to the Ampelmann gangway, which is the ‘gateway to Walk to Work’. As such it is the assembly and transfer area, comprising of the changing room, waiting room and a smoking room.
The accommodation throughout reflects the ‘high level’ of decor and finish specified by the owners and charterers, even as far as the art and paintings onboard, which depict views of the Groningen area. A further example of this commitment to comfort, is the fact that the chef has worked at the prestigious Amstel Hotel in Amsterdam.
A central access island characterises the bridge deck, with the duties split between the forward section solely for transit and the aft section for DP operations. The bridge outfit is provided by Eekels, this includes the main transit console, the two DP consoles, GMDSS and overhead consoles. Populating the various consoles are the Alphatron telephone and talk back system and the navigation equipment supplied by SAM Electronics. These include the two radars, gyrocompass, DGPS, AIS, Navtexand complete suite of radios. Kongsberg provided the accurate DGPS, CyScan and RADius position reference systems.
The solution revisited
This description of the Kroonborg started by detailing the challenge(s) and the eventual arrival at the solution. RNS has built a vessel around the functional aspects that were defined by all major stakeholders. The tour through the vessel has described an extremely well thought out state-of-the-art vessel: a comfortable level work platform. Kroonborg with its distinctive axe bow design may well turn out to be the ‘ideal solution’ in more markets than just that of ‘fossil fuels’.
Tom Oomkens and Andrew Rudgley