By Ed Jones, Subsea Cable Lead, Cathie Associates
Background
Since the 1850s, submarine cables have played an integral role in international data transmission and are now crucial to the growth of offshore renewables sector. Adequate protection of submarine cables is vital so that they remain safe from threats such as fishing gear, shipping anchors, dredging and other dropped objects for the lifetime of the asset. Majority of existing offshore wind farms have experienced some form of cable problem, most frequently during the construction stage. Although subsea cables account for only 11 percent of CAPEX, nearly 80 percent of insurance claims against offshore wind developments are related to cable damage. Most of these damages are due to fabrication and installation issues. While understanding the threat to a cable is essential in assessing the lifetime risk to the system, understanding the whole life cost of the system including installation cost, insurance, cable repairs or loss of revenue is essential for development decisions.
Cable burial is generally regarded as the optimal protection technique, and first came into play in the 1970s. The standard burial depth was ~0.6 meters for many years, mainly to provide protection against fishing gear. The Burial Protection Index (BPI), developed in the 1990s for fibre optic communication cables, was a best practice for submarine cables for over a decade. However, the method had several limitations including its conservative approach to anchoring and fishing and ignoring the influence of water depth and sediment mobility. Some of these limitations were overcome through the engineering acumen of design team, however, this limited the standardization and repeatability of design and led to over-conservatism from perceived threats.
Completing installation of power cables in a cost-effective manner is essential for future wind farm developments. The Carbon Trust Cable Burial Risk Assessment Guidelines offer a standardized, repeatable and quantitative method to improve risk management of subsea cables for offshore wind farms, improve estimates of risk through reducing undue conservatism, and ultimately reduce the installation and insurance costs for subsea cables. This new method has become an established standard in the U.K., Denmark, Belgium, Netherlands, France and Australia. Emerging markets are also looking at the Carbon Trust method including U.S., Korea and Taiwan. The CBRA method :
- Provides a framework for risk assessment, allowing engagement with all relevant stakeholders;
- Assummes 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.
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 advisors.
Key Lessons
Although existing guidance provides a good framework, it doesn’t provide details for assessing every aspect leaving several uncertainties such as:
Anchor Penetration Depth
This is the most onerous ‘primary’ threat in terms of anticipated penetration depth below seabed level. Current practice is based on generic references Shaphiro, 1997, NCEL, 1987 and BPI, 1997, and recommendations can include 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 that penetration depth in clay is very sensitive to undrained shear strength of the soil. There are several studies currently underway that are seeking to improve anchor penetration prediction in mixed and interbedded sediments, for e.g. Scale Physical Modelling and Discrete and Dynamic Finite Element Anlaysis.
Seabed Mobility
Seabed mobility is a secondary hazard and can result in exposure of cables at seabed, increasing risk from external threats, fatigue and overburial. Common practice is to avoid areas of general mobility, and focus on sympathetic microrouting. Accounting for megaripples and sandwaves can result in very onerous burial depths if traditional methods are used.
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, such as:
- 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).
Acceptable Risk
Risk is difficult to quantify and is subjective. Often the developers’ cable installation package manager specifies the target burial depth and defines the acceptable risk. Even with the benefit of a CBRA report, the level of protection is often decided on the basis of ‘previous experience’, ‘gut feel’ or DNV (pipeline) recommendations. The general perception is that ‘deeper is always safer’, and the risk associated with installation (particularly when installing to onerous burial targets) is generally not considered. Risk can be estimated by considering the following parameters:
- Quantitative Risk Understanding – CBRA guidelines provide a methodology for quantifying risk due to emergency anchoring. Engineering experience 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 to be determined 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.
Cathie Associates’ experience in the field of CBRA over the past 15 years has demonstrated that a good understanding of risk can be used to optimize installation and maintenance strategy. The Carbon Trust Cable Burial Risk Assessment Guidelines, which have become the default Industry best practice, highlight that by accepting greater levels of operational risk we may reduce installation costs and risk. Furthermore, greater understanding of risk can help optimize O&M costs, particularly at sites where shifting seabed sands can lead to changes in cable burial depth over the lifetime of the wind farm. Operators of some of the earlier offshore wind farms, constructed prior to the publication of the CBRA guidelines, are also seeing benefit in undertaking a retrospective CBRA, or Cable Integirty Risk Assessment, using the current method to better charterize the current risk profile of their site and inform their future Asset Management strategy.