Subsea Survey

What is a subsea survey?

Before any offshore infrastructure can be installed, it’s imperative to understand the seabed environment where the structures will be placed. Subsea surveys provide a detailed map of the seabed, identifying potential hazards, optimal routes, and areas requiring further investigation or intervention.

Offshore infrastructure projects, such as laying subsea cables and building wind turbine arrays, are complex endeavors that require meticulous planning and execution.

One of the most critical phases of these projects is the subsea survey, which serves as the foundation for all subsequent activities. This article explores the importance of subsea surveys, the key aspects they uncover, and the subsequent steps that ensure safe and efficient installation of subsea cables and wind turbine arrays.

Conducting these surveys is essential for several reasons:

1. Safety: Identifying potential hazards, such as unexploded ordnance (UXO) or boulders, is crucial to ensuring the safety of installation crews and preventing damage to expensive equipment.

2. Cost Efficiency: Detecting and addressing issues before installation begins can save significant costs associated with delays, equipment damage, or unexpected challenges during construction.

3. Environmental Protection: Understanding the seabed’s composition and ecology helps minimise the environmental impact of the installation process.

Key Survey Findings: What Do Subsea Surveys Identify?

Subsea surveys are designed to identify a wide range of seabed features and potential hazards that could impact the installation process. Some of the critical elements they detect include:

1. Unexploded Ordnance (UXO): Particularly in areas with a history of military activity, UXO can pose a severe risk to offshore operations. Surveys are conducted to locate and map these hazards so that they can be safely removed.

2. Boulders and Other Large Obstacles: Large rocks or debris can obstruct cable routes or turbine foundations. Identifying these obstacles early allows for planned removal or avoidance strategies.

3. Seabed Composition: Understanding the composition of the seabed is vital for determining the best installation methods. For instance, soft sediment areas may require different anchoring techniques compared to rocky substrates.

4. Buried Objects: Objects buried beneath the seabed, such as old shipwrecks or pipelines, need to be identified to prevent accidental damage or interference with the installation.

5. Geohazards: Features like sand waves, mudflows, or fault lines can affect the stability of the infrastructure. Surveys help in mapping these hazards so that designs can be adjusted accordingly.

Over-the-side pole deployment method.

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Post-Survey Activities: From UXO Removal to Boulder Clearance

After the survey has been completed, the data can be used to identify which subsequent tasks are required before the main project, such as laying a subsea cable, can be completed. These could include: 

1. UXO Removal: If unexploded ordnance is identified, specialized teams are deployed to safely remove or detonate these items. This step is crucial to ensure the safety of the installation process.

2. Pre-Lay Grapnel Run (PLGR): A PLGR is conducted to clear the proposed cable route of debris, including fishing gear, wires, and other obstructions that could interfere with cable laying operations.

3. Boulder Removal: Large boulders identified during the survey may need to be relocated to prevent damage to cables or to provide a stable foundation for turbine structures.

4. Seabed Preparation: In areas where the seabed is unsuitable for installation, additional preparation might be required. This could include levelling the seabed, adding protective layers, or installing rock berms to stabilise the area.

Sonar Technology in Subsea Surveys: Over-the-Side Deployment Methods

Sonar technology plays a pivotal role in subsea surveys, providing the detailed imagery and data needed to map the seabed accurately. One of the most common methods used is over-the-side deployment, where sonar devices are lowered into the water from a vessel.

Types of Sonar Used in Subsea Surveys:

1. Side-Scan Sonar: This sonar type emits sound waves to the sides of the device, creating a detailed image of the seabed. It is particularly effective at identifying objects lying on or near the seabed surface, such as boulders, debris, or UXO.

2. Multibeam Echosounder (MBES): MBES emits multiple sound beams downward, providing a high-resolution map of the seabed’s topography. This is crucial for identifying geohazards and determining seabed composition.

3. Sub-bottom Profiler: This sonar penetrates the seabed, providing information about the layers beneath the surface. It’s used to detect buried objects and understand the geological structure of the seabed.

Over-the-Side Deployment:

Over-the-side deployment methods are favored for their flexibility and ability to provide accurate data even in challenging conditions. Devices are typically attached to a pole or arm extended from the side of the survey vessel. This method allows for real-time data collection and immediate adjustments if an area of interest is identified.

Conclusion: The Foundation of Successful Offshore Projects

Subsea surveys are the bedrock of any successful offshore infrastructure project. By identifying hazards, mapping the seabed, and enabling precise planning, these surveys help prevent costly delays and ensure the safety of installation teams. The combination of advanced sonar technology and careful seabed preparation allows for the smooth execution of projects that are critical to our energy infrastructure and connectivity.

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