Lightweight influence mine sweeping system for the future Norwegian - - PDF document

UDT 2020 UDT Extended Abstract Presentation

Lightweight influence mine sweeping system for the future Norwegian unmanned MMCM concept

• R. Fardal1 and M. Nakjem2

1Principal scientist, FFI, Horten, Norway 2Project manager, FFI, Horten, Norway

Contact author: Rune Fardal, rune.fardal@ffi.no

Abstract — The Royal Norwegian Navy (RNoN) Alta class mine sweepers are close to the end of their lifetime and Norway has an overall plan to replace the capability in the timeframe of 2025 – 2030. An increased focus on reduced cost, flexible scalable solutions and a desire to remove personnel from the minefield, have resulted in a future mine sweeping concept based on unmanned surface vehicles (USV). The Norwegian Defence Research Establishment (FFI), the RNoN, the Norwegian Defence Material Agency (NDMA) and the industry have started to investigate in future concepts with use of a mothership, positioned outside the minefield, and forward deployed autonomous unmanned maritime mine countermeasures (MMCM) systems. Several tests have been performed and this paper will give an

• verview of the concept, technology and testing of a USV-based magnetic, electric and acoustic light weight influence

mine sweeping system.

1 Introduction

Effective MMCM requires a combination of mine sweeping, mine hunting and clearance diving. Hunting, sweeping and clearance diving are complementary techniques and it is absolutely necessary to master all three in order to handle all mine threats and operate effectively in all areas with varying environmental conditions. Extensive bottom profiling and mapping shows that more than 50% of Norwegian areas relevant for mine clearance

• perations are unsuitable for mine hunting and a robust

mine sweeping capability is necessary. The RNoN Alta class mine sweepers were built for a lifetime of 25 years in the period 1994 to 1997. Towards the end and past the designed lifetime, a significant increase in the maintenance cost is expected to sustain operational availability. Assessment about the future of Norwegian MMCM is therefore needed and FFI have several projects related to technological risk reduction for the future mine sweeping concept.

2 Current Norwegian MCM

The current Norwegian MMCM forces consist of two Oksøy class mine hunters, two Alta class mine sweepers and mine divers. The Oksøy class is equipped with hull mounted sonar and a remotely operated vehicle for mine

• disposal. In addition, the Navy has a contract with

Kongsberg for delivery in 2020 of four new Hugin autonomous underwater vehicles (AUV). Two of them

• perated from the Oksøy class and two containerized

systems operated from ship of opportunity. The Hugin AUV is equipped with Synthetic Aperture Sonar and

• ptical camera with strobe lights. The Alta class has the

ELMA cable sweep and AGATE acoustic system for magnetic and acoustic influence sweeping. The sweepers are also capable of performing mechanical sweeping. Both the Oksøy and Alta class have identical hull mounted mine hunting sonars and Kongsberg’s Minesniper One- Shot Mine Disposal Weapon. This enables the Alta class to perform complete mine sweeping and mine hunting

• perations.

The Mine Diver Command (MDK) supplement the Naval Mine Warfare Service with an additional capability for clearing mines from the MCM vessels and in very shallow water and surf zone where the vessels cannot operate. The MDK is also equipped with small Remus100 AUVs.

3 Future Norwegian MCM

In 2015 the Norwegian Ministry of Defence (MoD) issued the order for start-up of the conceptual phase for the planned replacement of the current Norwegian MMCM

• capability. The order stated the conceptual phase to focus
• n the possibilities of using unmanned systems in the

future capability to get the “man out of the minefield” when possible. This statement was based on the successful experiences from operational testing of the Hugin AUV, giving the Navy great confidence in the use of unmanned systems for MMCM operations. The conceptual phase is finalized and it states that the future MMCM capability will be based on use of unmanned systems. Further, the conceptual phase recommend that the preferred future MMCM solution should consist of unmanned systems operating from a manned ship located outside the mined area. This is also in line with the new long-term plan for the Norwegian Armed

• Forces. Table 1 shows the timeline for the transition to the

next generation MMCM capability.

UDT 2020 UDT Extended Abstract Presentation

Table 1. Timeline for the Norwegian MMCM capability procurement program. Phase Year Conceptual phase 2015-2019 Definition Phase 2020-2021 Project Phase 2021-2028 Initial operational capability 2025 Full operational capability 2028

3.1 Concept for Future Norwegian MMCM Capability The Norwegian Navy future MMCM concept will be based on use of USV, AUV and clearance divers as shown in Figure 1. The unmanned MCM components are transported on- and operated from a manned ship placed

• utside the mined or assumed mined area. AUVs will be

used for detection, localization and identification of the mines, while an operator controlled mine disposal weapon (MDW) will be used for classification and neutralization. The USV will have a multi role function including towing

• f the influence sweep and control platform for the MDW.

The USV can also be used for AUV transportation and deployment, and as a communication relay station between the operator and the AUV.

• Fig. 1. Future Norwegian MMCM concept (figure: NDMA).

USV based influence mine sweeping has a key role in the future concept and the following sections of this paper will focus on how the mine sweeping operations and the related technological development at FFI.

4 Norwegian Influence Mine Sweep

The current Norwegian Alta class mine sweepers have ELMA cable sweep and AGATE acoustic system for magnetic and acoustic influence sweeping. The influence sweeps are used in Mine Setting Mode (MSM) which means that they have sufficient energies to satisfy the triggering logic of mines. They are also used in Target Simulation Mode (TSM) where the idea is to emulate the ship to protect and then the mine logic set for the specific ship will trigger. ELMA and AGATE can emulate ship signatures for large civilian merchants. The Alta class is also capable of performing mechanical sweeping. 4.1 Concept for Future Norwegian Unmanned Mine sweeping capability The Norwegian unmanned influence mine sweeping concept will use USVs to tow magnetic, acoustic and electric sweep sources in various configurations. The USVs will be transported by a dedicated supporting vessel

• r by other military transportation (air/sea/land), and they

will be operated from the supporting vessel or a mobile land station. The sources will be towed behind the USVs in order to get the required field distribution and prevent the USV from damage after a mine explosion. The sweep sources will most likely be launched from the manned ship outside the mined area and then transferred as a “clip-on” sweep to the USV before the USV leaves the mother ship rather than having the sweep systems integrated onboard the USV. This clip-on solution will remove the complexity of unmanned launch and recovery of the sweep from the USV, and will also allow for a smaller USV size. The USV with the lightweight influence sweep deployed will transit into the operation area and start the mine sweeping

• peration in a predefined pattern. When the operation is

completed or when the USV needs refueling the USV will transit back to the supporting vessel. FFI has two 35-feet boats with 600 hp and water jets for use as test and development platforms as shown in figure

• 2. The boats are equipped with relevant sensors and used

for autonomy development related to obstacle detection and avoidance, track planning and GPS-independent

• navigation. The boats are also used for all USV-based

development and testing at FFI.

• Fig. 2. FFIs test USV “Odin” (picture: FFI).

The following influence sweep sources and con- figurations will be described in this section of the paper:

• Acoustic sweep
• Open electrode magnetic cable sweep
• Closed loop magnetic and electric cable sweep

4.1.1 Acoustic Sweep A traditional acoustic sweep source can be used to mimic the acoustic ship signature, but it has to be a light weight

UDT 2020 UDT Extended Abstract Presentation source in order to be used from the USV. The acoustic source is attached to the magnetic sweep cable and the source must be operational over a large speed interval in

• rder to be efficient in different scenarios. This can be

solved by using a cable powered source rather than a system with acoustic output dependent on the speed through water. FFI has made a “dummy source” of a torpedo-shaped acoustic source for testing purposes. The source is shown in figure 3. It is comparable in size and weight to sources

• ffered by industry. The source has been adapted with a

float to keep it surfaced. The float is designed to create a downforce and submerge the source a few meters when towed at sweep speed. In spring 2019 FFI successfully performed trials with an acoustic source towed by USV.

• Fig. 3. Dummy acoustic source with float (photo: FFI).

4.1.2 Open Electrode Magnetic Cable Sweep The open electrode cable sweep consists of a straight cable towed behind the USV. The cable has two or more electrodes and the electric current passing through the cable and seawater produces the magnetic sweep

• signature. This is the preferred setup for a MSM operation.

The straight tail configuration can be towed by a single USV, is easy to maneuver and flexible regarding operating

• speed. The effectiveness of the open electrode cable sweep

does however depend on the mine threat. If the mines are equipped with advanced sensors and signal processing, the

• pen electrode sweep may have reduced effectiveness

since advanced mines are more capable of discriminating between the sweep and ship signatures. FFI successfully performed trials with straight tail magnetic sweep towed by a USV in summer 2019. 4.1.3 Closed Loop Magnetic and Electric Cable Sweep In order to obtain the required risk reduction a closed loop configuration is preferred in some cases. Advanced mines may call for a more ship-like electromagnetic sweep signature than an open electrode configuration can produce and if the mine threat is unknown, it may be necessary to perform TSM operations. In such cases, a closed loop configuration will be more efficient than an

• pen

electrode configuration. The closed loop configuration consists of an electric cable, but in this configuration no electrodes are used and the electric current does not flow through the water. Instead, the electric current flows in a closed loop circuit and this produces a more ship-like magnetic field than the open electrode configuration. The closed loop influence sweep is towed behind two USVs in formation in order to obtain the required loop-

• size. This limits the maneuverability and operational speed

compared to the single-USV straight tail configuration. The use of two USVs also requires more time for launch and recovery, refueling and maintenance. A visualization

• f the closed loop setup is shown in Figure 4.
• Fig. 4. Closed loop influence sweep (figure: NDMA).

During the fall 2019 FFI and the Norwegian navy successfully performed trials with the closed loop magnetic sweep and two acoustic dummy sources towed by two USVs. A picture of this is shown in figure 5. The USVs operated autonomously in pair. The sweep is slightly buoyant and surfaces when not towed. When it is towed, it submerges to a few meters. This is to avoid sudden changes in tension when the cross-cables are pulled through surface waves. The tow force on each USV was approximately 600 kg at 6 kn and 850 kg at 8 kn.

• Fig. 5. Closed loop influence sweep testing from USV. Floats

indicate the forward and aft cross-cables (picture: FFI).

UDT 2020 UDT Extended Abstract Presentation The use of a closed loop configuration also allows for an independent electric sweep signature. By placing a separate circuit with smaller electrodes on the magnetic cable, a ship-like Underwater Electric Potential (UEP) signature can be produced when electric current is passing from the electrodes into the seawater. This gives an efficient capability to also sweep mines with UEP sensors. 4.2 Launch and recovery The complete electromagnetic and acoustic sweep can be launched from land or ship. The sweep is then attached to the USV(s) and towed into the area of operation. This “clip-on” solution is used in order to avoid a complex unmanned launch and recovery system (LARS) from the

• USV. The USV size can also be reduced when the entire

sweep is not transported onboard the USV. The procedure for LARS of straight tail and closed loop configuration is similar, but closed loop is more complex thus making the launch and recovery phases more advanced and time consuming. The procedure for closed loop LARS from land and ship is described in the following sections. 4.2.1 Closed loop sweep LARS from land The standard procedure for launch and recovery of the closed loop sweep during testing at FFI is from land. The sweep cables are placed on drums on the pier and the USVs pull out each of the two straight cable sections as shown in figure 6. A small manned boat is then used to connect the acoustic dummy sources, and the forward and aft cross- cable sections. The entire procedure takes about 30

• minutes. The procedure is reversed when the sweep cables

are recovered.

• Fig. 6. Closed loop sweep launched from land (picture: FFI).

4.2.2 Closed loop sweep LARS from ship When launched from a ship, the magnetic sweep cables and acoustic source are deployed in a given sequence. When the entire sweep is launched, the cables are transferred to the USVs positioned on each side of the vessel as shown in figure 7. The USVs then releases from the davits and transits to the area of operation.

• Fig. 7. Closed loop sweep launched from ship (figure: NDMA).

The sweep has not yet been launched from a ship and transferred to USVs at sea, but a complete closed loop sweep was in 2018 launched from two manned harbor vessels positioned side-by-side. The vessels then separated to a given sweep width and successfully towed the sweep in 8 knots at sea state 2-3.

5 Conclusions

FFI, NDMA and industry have since 2015 developed a technology demonstrator for a USV-based lightweight influence mine sweep with the capability to sweep mines aimed at large civilian merchant ships. The sweep can be configured either as a straight tail sweep towed by a single USV, or as a closed loop sweep towed by two USVs working in formation. The complete influence sweep has successfully been towed by the FFI test USVs. FFI and industry will now start a close cooperation and develop a more advanced prototype that can be used as a part of the future Norwegian MMCM capability.

Author/Speaker Biography

Rune Fardal is a principal scientist at FFI. He has a background from hydro acoustics and has worked with influence mine sweeping and underwater ship signatures for the last 17 years. Fardal is currently involved in the definition phase of the Norwegian procurement program for future maritime mine countermeasures capability.