Beyond CBRA – Defining Acceptable Risk

In our earlier blog on An overview of Cable Burial Risk Assessment Methods, we discussed how cable burial is regarded as an optimal protection technique and the evolution of cable burial risk assessment methods over time to current Carbon Trust Cable Burial Risk Assessment Guidelines.

The Carbon Trust Cable Burial Risk Assessment (CBRA) Guidelines offer a standardized, repeatable and qualitative method to improve risk management of subsea cables, improve estimates of risk through reducing undue conservatism, and ultimately reduce the installation and insurance costs for subsea cables. Cathie Associates were one of the lead authors of the Carbon Trust CBRA guidelines and were then retained by the Carbon Trust Offshore Wind Accelerator to act as technical advisers.

CBRA method :

  • Provides a framework for risk assessment, allowing engagement with all relevant stakeholders
  • Assumes that it is impractical to protect a subsea cable from all possible threats
  • Adopts a probablilistic approach for anchoring to justify reducing overly conservative depth of lowering
  • Provides an understanding of residual risk

Key Lessons

Existing guidance provides a good framework for undertaking clear and repeatable assessments, however, they don’t always provide a significant detail for assessing every aspect and several uncertainties remain such as:

Seabed Mobility – Seabed mobility is a secondary hazard and can result in exposure of cable at seabed thereby increasing risk of other hazards or possibly overburial which could result in thermal related problems. Common practice is to avoid areas of general mobility, and focus on sympathetic microrouting – targeting the troughs of bedforms.

Accounting for megaripples and sandwaves can result in very onerous burial depths if traditional methods are used. Understanding the degree and rate of mobility can provide a better understanding on the potential risk of exposure and enable an optimised mitigation strategy to be developed.

The Reference Seabed Level (RSBL) Concept involves determining a reference ‘non-mobile’ level beyond which the seabed will not fall within the lifetime of the wind farm. A  RSBL combines a comparison of different geophysical survey data sets with an assessment of metocean and seabed sediments to infer potential mobility, and analyses bathymetry and slopes to determine reference depth.

Understanding the reference depth and the potential degree of mobility can allow a more detailed assessment of potential mitigation options, for example:

  • Routing away from significant features
  • Excavation/dredging of significant mobile features
  • Specifying an economic burial depth and plan for future survey and/or remedial works in areas of potential mobility (if risk assessment indicates this is practical)

Anchor Penetration Depth – This is the most onerous ‘primary’ threat in terms of anticipated penetration depth below seabed level. Current practice is generally based on generic references Shaphiro, 1997, NCEL, 1987 and BPI, 1997, and recommendations can result in the requirement for significant burial depth even in competent soils. A preliminary sensitivity study has indicated that 1 fluke length in sand is a conservative estimate and penetration depth is very sensitive to clay strength. There are several known studies currently underway that are seeking to improve anchor penetration prediction is mixed and interbedded sediments e.g. scale physical modelling and Discrete and Dynamic Finite Element Analysis of the problem.

Acceptable Risk – Risk is difficult to quantify and is subjective. Often the developers’ cable installation package manager specifies the target burial depth and the defines the acceptable risk. Even with the benefit of a CBRA report, this is still often decided on the basis of ‘previous experience’, ‘gut feel’ or DNV recommendations for pipeline protection. General perception is that ‘deeper is always safer’, and risk associated with installation is generally not considered.

Conequences of failure should be considered as in the assessment of oil pipelines however, economic consequences for pipeline failure are orders of magnitude higher than offshore wind farm cables:

 For Oil Pipelines –

  • Loss of production ~$6,000.000 per day
  • Ecological damage $18.7 billion (BP Deep Horizon)
  • Longer repair lead times and more complex subsea operations

Offshore Wind –

  • Wind farm design life is generally 25-30 years with estimated repair cost ~$7 million and estimated downtime cost~$55,000/MW/month. Inter-array cable spur down for 3 months can cost ~$14 million while single export cable (50MW) down for 3 months can cost ~$32 million.

 

  • Quantitative Risk Understanding – CBRA guidelines provide methodology for quantifying risk due to emergency anchoring and estimating risk cost. Engineering experience and risk knowledge would also allow a quantitative assessment of other risks such as on-bottom stability, fishing gear interaction and seabed mobility.
  • Burial Cost Estimation – Experience allows an early cost estimate for achieving various burial depths based on required trencher and support vessel and offshore time. It can also include potential risk costs to account for increased risk due to deeper burial.
  • Cost Benefit Comparison – Compares ‘Risk Cost’ and ‘Protection Cost’ for a range of depths to find the optimum burial depth.

 

In conclusion, a good understanding of risk can be used to optimise installation and maintenance strategy. Accepting greater levels of operational risk may reduce installation costs and risk, and greater understanding of risk can help optimise O&M costs.