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Subsea cable burial is not only about selecting a trenching tool or nominating one depth for the whole route. The right burial strategy needs to balance external threats, seabed conditions, installation feasibility, cable limits, and residual risk. This is where the Burial Assessment Study (BAS) and Cable Burial Risk Assessment (CBRA) work together.
The Carbon Trust CBRA guidance provides a structured and repeatable approach for assessing cable burial risk and defining a suitable Depth of Lowering (DoL). A BAS then tests whether that DoL is achievable along the route and identifies the most practical burial or protection method for each section.
CBRA: What Protection Level Is Needed?
A CBRA focuses on risk. It considers what could damage the cable during its operational life and what level of burial or protection is required to reduce that risk to an acceptable level.
- Anthropogenic threats such as anchors, fishing gear, dredging, mooring, construction activity and third-party interaction.
- Natural and seabed-related threats such as mobile sediments, scour, sand waves, hard ground, boulders, slopes and exposure risk.
- Route sensitivity, cable importance, consequence of failure and acceptable residual risk.
- Recommended minimum DoL and target DoL for each route section, with assumptions clearly stated.
In simple terms, the CBRA asks: how deep does the cable need to be, or what additional protection is required, to manage the identified threats?
BAS: Can the Target Burial Be Achieved?
A BAS focuses on feasibility. It uses the geophysical, geotechnical, metocean and route data to assess how the target DoL can be achieved in practice. It also identifies where burial may be difficult, where the target depth may not be achievable, and where alternative protection should be considered.
In simple terms, the BAS asks: given the seabed and soil conditions, what method can realistically achieve the required burial, and where do we need a different strategy?
Study | Main Question | Typical Output |
CBRA | What is the required burial or protection level to manage risk? | Recommended minimum DoL, target DoL, residual risk and protection requirement. |
BAS | Can the target burial be achieved with available methods? | Burial feasibility by segment, tool suitability, expected performance and fallback protection. |
Integrated Strategy | What is the most practical route protection plan? | Route sections, burial method, protection schedule, data gaps and installation risk controls. |
The BAS Workflow
A clear BAS should be built in steps. The purpose is to turn survey data into a practical burial strategy that can be used by the cable engineer, installer and client.
BAS Step | Purpose |
1. Define the burial objective | Confirm the cable type, project risk criteria, required survey standard, installation tolerance, target DoL terminology, route limits, crossings and client acceptance requirements. |
2. Build the route ground model | Review DTS findings, charts, bathymetry, MBES, SSS, SBP, magnetometer data, seabed features, CPT/core/lab results, metocean conditions, AIS and fishing activity. |
3. Segment the route | Break the route into practical engineering sections based on seabed morphology, shallow geology, soil strength, sediment mobility, rock risk, boulders, slopes, existing assets and construction constraints. |
4. Bring in the CBRA outputs | Use the CBRA to define the recommended minimum DoL, target DoL and residual risk requirement for each route section. Figure 1 shows why these depth terms must be clearly defined and not mixed. |
5. Assess burial tool suitability | Compare seabed conditions and target DoL against realistic burial methods such as jetting, ploughing, mechanical cutting, trenching, dredging, pre-sweeping or surface protection. Figure 2 shows the concept: different tools suit different soil density and strength ranges. |
6. Check achievable burial | Estimate whether the target DoL can be achieved in each section, including expected number of passes, burial tolerance, trench stability, cable handling risk, vessel limits, current, weather and operational constraints. |
7. Identify gaps and protection needs | Where target burial is not achievable or not reliable, recommend a change in route, adjusted burial target, pre-treatment, rock placement, mattressing, split pipe, concrete protection, HDD/shore approach treatment or other protection method. |
8. Produce the burial strategy | Deliver a route-by-route summary showing target DoL, recommended burial method, expected achievability, residual risk, protection requirement, assumptions, data gaps and recommended survey or engineering follow-up. |
How BAS and CBRA Work Together
The CBRA should not sit separately from the BAS. The CBRA identifies the risk-based protection requirement; the BAS confirms whether that requirement is technically and commercially achievable. If it is not achievable, the burial strategy should be updated and the residual risk reviewed.
This feedback loop is important. A target DoL that looks suitable in the risk assessment may not be practical in dense sand, stiff clay, cemented layers, rock, boulder fields or areas with high current and poor trench stability. Likewise, a section that is easy to trench may not need excessive burial if the actual external threat level is low.
QOffshore Approach
QOffshore approaches BAS and CBRA as one route engineering workflow. We start with the decision the client needs to make: route selection, target DoL, burial method, protection requirement, installation risk, or residual risk acceptance.
- Desktop review of route, hazards, AIS, seabed mobility, existing infrastructure and available survey data.
- Integration of geophysical and geotechnical data into practical route segments.
- Assessment of target DoL, achievable burial and suitable burial tools by segment.
- Identification of areas requiring route adjustment, additional survey, pre-treatment or external protection.
- Clear outputs that can support client review, installer discussions, risk registers and construction planning.
Key Takeaways
- CBRA defines the risk-based burial or protection requirement.
- BAS tests whether the target burial can realistically be achieved and how.
- DoL, trench depth and depth of cover must be clearly defined before design or installation decisions are made.
- Tool selection depends on seabed conditions, soil strength, target depth, cable limits and installation constraints.
- The best result is an integrated burial strategy that balances risk, feasibility and residual protection.
Reference basis: Carbon Trust Offshore Wind Accelerator Cable Burial Risk Assessment Guidance and Application Guide for the Specification of the Depth of Lowering. Figures provided for draft blog layout.
