Five ways consultancy helps reduce unexploded ordnance risk

Marine UXO present significant risks to offshore wind farm developers. If UXOs are disturbed, this may trigger an explosion, or even a series of explosions. The consequences of this for human life, marine life and wind farm operations would be catastrophic.

We can help developers mitigate marine UXO risks effectively by considering the five areas outlined:

Migration

Mobility is a key consideration for marine UXO. Since their time of disposal, UXOs will have been interacting with the water column and the seafloor. Sea conditions, including waves and currents, can cause UXO devices to move. The mobility can also be facilitated by seabed gradient (on the lee side of a sand wave).

To Mitigate UXO risk, it is critical to understand if and what type of UXO are present at the site. The initial understanding is provided by studying the historical records. However, to determine the UXO risk mitigation strategy it is important to understand if and how far the UXOs have migrated. This is especially important when it comes to low-ferrous mines, which are very difficult to detect. The insights from the migrations assessment can help to reduce the area that is suspected from these mines.

In addition, the insights can help with extending the ALARP validity duration. Many wind farms are designed to operate for 30 years. Typically, ALARP certificates are valid for two to five years, after which they must be reviewed and renewed.

To facilitate requests for ALARP validity extension, we can re-use our existing research to conduct a desk-based assessment. If we can prove that UXO targets will not have moved closer to wind farm structures, this will avoid the need to restart cost- and labour-intensive UXO investigations.

Burial

Over time, many UXO devices become partially or completely embedded in the seafloor. The three main reasons for this are:

• Penetration on impact – wartime bombs dropped from military aircraft may have hit the surface of the water with significant momentum, causing them to dissipate rapidly through the water column. In areas where the water is shallow and the seabed is soft, the devices may have penetrated the seabed on impact;

• Flow disturbance – the presence of UXO can amplify friction on the seabed, resulting in local scouring;

• Seabed mobility – the morphology of the seabed can change during stormy weather or because of waves and currents, causing UXO devices to become buried at greater depths or re-exposed. In fact, large parts of the seafloor are covered by sand dunes that can be up to 6 m high and can move as much as 10 m a year.

The choice of UXO survey method varies according to the depth of burial, so desk research is always our starting point. It helps us determine the reference seabed level – this is the lowest level that will have occurred since the year when the UXO device entered the sea.

We perform most of our UXO surveys using our Geowing®. Pending the size of UXO and how high the Geowing® is flown, the magnetometer has a range of 2-3 metres below the seabed.

Knowing which UXO devices are likely to be buried in the seabed – and their likely depth of burial – allows us to optimise our survey operations, including by:

• Restricting target picking – we know the magnetic signs of the UXO devices buried in the search area;

• Increasing certainty – we know the type of dredging equipment that will be needed during the UXO identification phase;

• Broadening the line spacing – in areas where UXO is buried close to the surface, we can broaden our line spacing and still detect up to the required depth. This practice achieves significant cost and time savings for our clients;

• Reducing infill in troughs of sand waves – underwater troughs can make navigating the ROTV more difficult, so we calculate maximum UXO depth of burial in the trough of sand waves based on three factors: the force of the waves and currents that are driving sediment transportation, the susceptibility of the soil conditions to mobility, and the characteristics of the UXO device, including diameter, length, shape and weight.

In a trough, the maximum depth of burial of a UXO device is often less than 1.5 metres. This insight can greatly improve the survey design, resulting in less infill and ROTV seabed crashed.

Vibration

Marine UXO devices can detonate if they experience levels of vibration that are above their threshold of tolerance.

The construction phase of an offshore wind farm therefore presents a major risk. As monopiles are hammered into the seabed, the energy from the vibrations propagates through the water, radiating as far as 200-300 metres (the ‘influence zone’).

The ALARP certificate requires all potential UXOs within the influence zone to be identified and removed. This is a time-consuming and costly operation. However, it is often possible to reduce the influence zone without compromising on safety. Our in-house experts apply three parameters to calculate the vibration intensity and the optimal size of the influence zone:

• The hammer – its type and the amount of energy it produces;

• The monopile – its dimensions and weight;

• The propagating medium – the Geo-data and soil properties in the target location.

By accurately demonstrating that the radius of the influence zone can be reduced safely from 200 m to 150 m, the UXO search area – and the number of targets requiring identification – can be reduced by for example around 40 %.

It’s also possible to apply these principles in reverse: if we locate a UXO device and know the details of the hammer type and monopole size, we can demonstrate that it is safe to leave it where it is, rather than doing a ‘lift and shift’ or disposing of it in situ.

Noise

Sound travels very quickly in water – and the seabed is an even more efficient propagation medium. The detonation of a UXO device can generate huge underwater sound waves that can propagate across a radius of several tens of kilometres.

Underwater noise pollution can cause disruption and significant harm to sea creatures, so the potential impact of marine UXO explosions must be carefully assessed and mitigated ahead of any UXO clearance campaign.

Fugro has developed an innovative application that models the noise created by the detonation of a UXO target and distance travelled by the sound waves in the water column and the seabed. Our in-house noise model applies multiple parameters including the type of UXO, sound frequency, seabed conditions (sediment type and density) and water depth to produce an effective noise mitigation plan.

Mitigation measures may include the use of a ‘bubble curtain’. Air is a compressible medium, so as the bubbles expand and collapse, they reduce the energy in the sound. Modelling enables us to assess the efficiency of the curtain, to ensure that sound waves do not exceed the target threshold. Our integrated UXO risk mitigation solutions offers a quick turn-around time in calculation and production of noise maps – either in advance or when a UXO device is identified – which is proving invaluable in keeping our clients’ projects on schedule.

Route

Laying a cable route for an offshore wind farm across a corridor where a number of potential UXO targets have been identified is always challenging. One of the many reasons for this is that export cables have a relatively large diameter and do not bend easily. To make this task easier we have developed a unique weighted algorithm that enables us to optimise cable routes for offshore wind farms, avoiding most UXO targets.

Unlike other cable optimisation algorithms, it captures a variety of constraints relevant to the installation phase. Examples include the curvature associated with the cable-laying vessel and the capabilities of the cable-burial equipment. We apply the algorithm to the findings of the initial UXO survey, which looks at magnetic anomalies. It then optimises the cable route in terms of safety and cost-effectiveness, delivering a priority list for UXO targets that require identification and clearance. The algorithm can be updated after the identification phase and continuously refined throughout the project, as more information becomes available.

In conclusion

We believe that the more you know, the smaller the risk. To effectively mitigate UXO risks, a comprehensive approach focusing on migration, burial, vibration, noise, and route optimisation is essential. Understanding UXO migration patterns through historical assessments helps reduce search areas and extends safety certificates. This versatile strategy empowers developers to navigate the intricate challenges of marine UXO risks, securing the safety and sustainability of offshore wind farm projects. Our marine UXO risk mitigation strategy provides developers with the certainty they need to plan, install and operate offshore wind farms, safely and cost-effectively.