The Impacts of working in the Transition Zone

For companies specialising in dredging and offshore construction, there are many operational challenges that require specialist skills and equipment to overcome. 

Working within the transition zone is just one example. Constant exposure to waves and fluctuating tides accelerates wear on equipment and structures, leading to corrosion and marine growth.

Positioning in this area is difficult, and typically cannot be done with established subsea positioning methods, such as USBL. The MiniPods acoustic positioning system serves to fill this gap and is designed to be fully submerged below the surface, providing accurate positioning when above the surface for vehicles, divers and towed arrays.

What is the transition zone?

The transition zone in subsea exploration refers to the area between dry land and navigable offshore survey areas (where USBL or other positioning methods take over). It can be a challenging area for exploration and production departments to work within, and is one of the least explored areas.

What are the challenges for Dredging and Cable laying companies working in the transition zone?

Harsh environmental conditions

The harsh environmental conditions created by strong wave forces, structure-induced currents and seawater make dredging, jetting and trenching operations particularly difficult. Especially when trying to maintain accurate positioning when strong tides and currents can affect the topology of the seabed, making the laying of cables or subsea foundations particularly dangerous and difficult in this area.

Accessibility and Equipment transitioning

The transition zone can often mean that the seabed is not deep enough for equipment to be fully submerged, leading to positioning equipment struggling to navigate while constantly being splashed with water. This, in turn, leads to having to transition between surface-level equipment and under the surface, which can be inefficient and add complexity. 

Unstable USBL positioning

Utilising acoustic positioning methods such as USBL (Ultra Short Baseline Positioning) in the transition zone can be challenging as the thermocline and pycnocline layers often found there can cause distortion or bending of the acoustic signals used by USBL systems.

A USBL system can also typically be affected by multipath, reverberation and acoustic interference in this area making positioning unstable and inaccurate. 

Unstable GPS positioning

The vibrations caused by excavation into the seabed and dredging mean that most normal GNSS equipment (which lacks shock mounting for noise or harsh environments), will struggle to provide stable positioning and can result in positional jumps or complete loss of position.

INS instability

An Inertial Navigation System (INS) can be used for positioning in motion applications but is typically not suited to high-vibration environments due to sensor sensitivity. INS systems that can compensate for high vibration can still suffer from drift over time so GNSS corrections via a MiniPod can improve performance and significantly reduce operational downtime.

Safety

Safety is of the utmost importance when working within the transition zone as the hazards are plentiful. Operating heavy machinery poses its own challenges at the best of times, but even more so when used in places with limited access and harsh environments with rapidly changing weather conditions, wave action and tidal forces to contend with.

Cost and Time

Divers, support vessels and heavy specialist machinery all come at a cost, and with the transition  zone posing many operational challenges it can sometimes be hard to fully quantify how long all the supporting elements will be required in place to carry out any works, meaning it can be easy for costs and initial timelines to dramatically increase.

How can applied acoustics’ MiniPods support?

The MiniPod is an excellent example of a technology that can improve the safety and accuracy of transition zone operations. It is designed to be compact yet highly accurate, utilising global navigation satellite systems (GNSS), a far more accurate way of positioning than relying on standard GPS, which GNSS covers and more. 

The MiniPod GNSS positioning system is especially useful for:

• Tracking dredging equipment and subsea assets in dynamic environments

With integrated GNSS receiver and RF transmitter, the MiniPod provides real-time precise surface positioning when operating in the transition zone either wirelessly and/ or via cable to equipment or back to a vessel or shore.

• Underwater installations

The MiniPod’s adaptability enables precise monitoring in variable sea conditions, which is critical when dealing with underwater installations, marine foundation work, or large-scale dredging projects. The design incorporates a double-sealed housing for harsh environment survival, and the quick acquisition time of signals allows for immediate positioning when breaking the surface. The 200 series can be used to send GNSS and additional sensor data wirelessly back to the vessel or shore for status updates of equipment. Its use mitigates risks by offering detailed location data, helping to avoid unexpected costs, reducing post-processing time and anticipate equipment wear and failure in harsh transition zone conditions. 

MiniPod features include:

• Internal and external shock mounts 

Allowing for work to be carried out in areas of high vibration, the 200 series MiniPod is assembly shock-rated to 75G for field endurance in harsh conditions.

• Greater accuracy

For even greater accuracy 2 x 201G MiniPods can be configured in a base or moving base and rover mode can provide accurate corrections to other 201G MiniPods. Both the 100 series MiniPods and the 200 series MiniPods are capable of receiving LBand corrections, the 100 series uses the hemisphere GNSS receiver and benefits from the ‘Atlas’ correction service and the 200 series is fitted with the Septentrio GNSS and can utilise the Marinestar network.

The MiniPods will also support other forms of RTK corrections either directly sent to the MiniPods over serial or via the RFR-101G hardware and MiniPod Receiver software.

• Deep submersion

The 106G MiniPod is designed to survive 6000m immersion, and begins its precise positioning <30s after the vehicle breaks the surface. It also includes an internal battery to assist in the case of temporary power failure so critical motion sensors can remain accurate for up to 1 hour of operation, ideal for automated vehicles or assisting vehicle recovery.

• Flexible power options

The MiniPods can be powered by our range of primary cell and rechargeable subsea power packs, the outputs or which can be connected in parallel to double the life of the battery packs or to maintain operations while packs are switched out for Hot Swapping.

Conclusion

Working in the transition zone still remains one of the most challenging aspects of offshore operations. The harsh conditions accelerate corrosion, fatigue, and marine growth, posing significant risks to both structures and personnel. However, with the advent of cutting-edge technologies like the MiniPod, offshore construction companies can rest assured that they are working in a safer, more efficient, and cost-effective way.

For more information on the MiniPods be sure to watch this helpful MiniPod video or visit the MiniPod product page.