Project 6300 RNoN Skjold-class corvette - Design
What is revolutionary about RNoN Skjold is the particular combination of technologies integrated into a truly unique vessel. Skjold has the qualities that enable it to be an effective shield in the most varied and extreme situations, inshore and in open seas – peacetime as well as wartime.
RNoN Skjold is truly a pioneering vessel that successfully incorporates stealth technology and a Surface Effect Ship platform. It is the first craft where radar-absorbing properties have been built into the actual load-carrying structure of the ship. The Skjold Class LCC is the only vessel in which the superstructure, hull and fittings are produced largely of radar-absorbing materials. In addition to enhancing stealth characteristics, this yields a considerable weight advantage.
RNoN Skjold has shown remarkable speed capability of over 60 kn, even in high seas. It has operated in wave heights of more than 25 feet (8m) without difficulty and remains as the fastest combat ship in the world.
Using a catamaran (twin-hull) design, Skjold is a rigid-sidewall surface effect ship (SES) that can sustain 50-plus knots and can operate in waters as shallow as 3 feet. With its light weight and two lift fans for SES operation, Skjold can lift about 70 percent of her weight out of the water, yet keep some "keel" in the water via her two hulls. The ship's high-speed capability could not be achieved with a traditional hull or catamaran design and without the lift fans. Having a minimum portion of the hull in the water reduces "skin drag:" friction created by the portion of a hull beneath the water's surface as a ship moves forward (or backward). Reduced skin drag thus allows for higher speeds. The ship's main propulsion comes from two Rolls Royce-Allison gas turbines, each producing 8,160 shaft horsepower. Two diesels power the lift fans, and Skjold also has two auxiliary diesels and two waterjets.
Composite materials used in the ship's construction are the key to the Skjold prototype project and to the very concept of this class of littoral combat ship. It is by far the strongest and lightest material around. Skjold will float high in the water because she is so much lighter than water. There are different kinds of foam in different places on the ship because we want it to be as light as possible but still strong enough. And there's different laminate outside, with varying thicknesses. All reinforcing components and all the hull plates--everything--are made of composite material.
Composite materials used in the are mainly glass-fiber material, with some carbon fiber. It's the way the fibers are laid down and the way they are pressed together that give the material its strength. And, while there are some radar-absorbing areas, most of the ship is radar-reflecting. It's difficult to absorb a radar signal; it's much easier to reflect it. Only in special situations does a ship want to absorb it. The ship does have infrared (IR) and radio-energy shielding molded into its composite structure to reduce its radar and IR signature.
The composites are fire-retardant, meaning they will not catch fire, and they also are 'isolating.' Normally, when there is a shipboard fire, it is attacked from all directions. The crew would try to seal it off, and cool it down so it doesn't spread because - with steel plates - heat will go right through [the plates], and the next compartment will catch fire. That will not happen here. It will not help to start cooling this side if there's a fire on the other side because the heat will not spread through the composite material. Adding fuel under pressure and oxygen, for a prolonged period of time, eventually will generate enough heat to melt it down, and then ther also could be toxic fumes.
A SES is in principle based on a catamaran hull where lift fans blow air into an air cushion between the hulls. Rubber finger type seals in the bow and a bag seal in the stern close the air-leakage. An air pressure equivalent to abt. 0.5m water column is maintained and controlled by a Ride Control System. The RNoN has acquired a substantial experience with naval SES. The unique features of the SES concept as verified by the Oksøy class SES MCMVs are
- improved shock resistance due to low wetted surface
- improved seakeeping
- reduced magnetic signature elevated position of main magnetic components
- improved manuvering capability due to twin hull and waterjets
- large deck spaces
In addition the FPB utilise the extreme low resistance offered by the SES. The wetted surface of the side hulls is significantly reduced compared to catamarans. The wave resistance caused by the air-cushion is heavily speed dependent and vanishes at high speed, resulting in low high speed resistance compared to any displacement or semi-displacement vessels.
An important advantage is the low draft of the SES when operating on-cushion. A draft of less than one meter offers significant advantages in shallow waters, i.e. reduced likelihood of grounding and less vulnerability due to less exposed area for impact of floating objects. The twin hulls have important advantages regarding survivability and redundancy. All important systems are redundant, and the vessels can operate with one engine room set out of work. Machine Control Room where all ship tecnical equipment can be controlled. All engines may also be controlled from the bridge.
A combination of global and local FEM calculations have been carried out extensively, considering the anisotropy of the sandwich structure where needed. The theoretical laminate properties are based on the Tsai-Wu failure criterion, which have been verified by extensive laminate testing performed by the yard. The hull materials are selected following detailed assessment of mechanical properties and producibility. Special attention to waterjet interface with adjacent structure and mast were paid. Stiffness requirements to secure sufficient stiffness to the structures which are supporting the gun and EO/K-band radar director required extensive analyses.
All structures are built with FRP sandwich using uni-axial glassfibre and carbon laminates with vinylester or polyester resin. PVC core material is used in main structural elements below main deck and PMI core material is used elsewhere and for the complete superstructure.
The advantages with such a sandwich structures are multiple:
- high strength to weight ratio
- excellent shock resistance
- embedded stealth capability
- structural design without secondary stiffeners
- simplifies construction
- simplifies outfitting
A large quantity of the internal structural elements, internal and external decks, beams and the complete superstructure with the carbon fibre mast were fabricated applying the closed SCRIMP process, which is a vacuum assisted resin injection method used in order to reduce the styrene emission, to reduce the structural weight and improve the laminate properties. Heated walking zones on the upper deck to avoid ice build up is installed.
The extensive use of carbon is used in structures with high stiffness requirements, as beams, the mast and the supporting structure for the gun and EO/radar director. For the first time radar absorbing materials are included in the loadbearing structure (structural RAM) of a vessel over large areas. This contributes to significant weight reduction compared to the conventional method of cladding this material on the outside of the load bearing structure. The ship is designed with a low number of reflective panel orientations and none right angled corners. This is the reason for the faceted external shape of the vessel.
Much effort is applied to minimise secondary structure and outfitting penetrating the flush exterior of the vessel to minimise the contribution to the radar cross section. All exterior doors and hatches are made flush and have exactly the same radar reflecting/absorbing characteristics as the adjacent panels elimination the reduction of the signature properties. Necessary deck outfitting as mooring pullers are either covered with radar absorbing covers or made de-mountable (rails). Window screens are made radar reflective and are installed flush without any visible coaming.
The Skjold class is designed with low IR and optical signature. A painting system with high IR absorption properties and a colour pattern to reduce optical signature measured for typical Norwegian coastal waters contributes to a balanced low level of signatures. By turning the two waterjet nozzles independently KNM Skjold is capable of moving perfect sideways without side- or bow propellers
State-of-the-art weapons and technology aboard Skjold were supplemented with up-to-date training based on the developing concepts of littoral warfighting, something with which the Norwegian navy is quite familiar. The ship's main offensive weapons are surface-to-surface missiles, designed specifically for littoral warfare and, as such, having very high-resolution imaging and passive infrared seekers so they can discriminate targets in an archipelago area, along with a highly accurate navigation system.
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