Abstract
Industry guidance is provided to support the assessment of seabed mobility for projects planning to install assets on or under the seabed where knowledge is required of expected changes in the seabed profile over the lifetime of a project. Without such knowledge, projects may face an increased risk of assets being more vulnerable to damage or failure. For example, cables are often buried under the seabed for protection against damage from external threats, such as anchors, however, in areas susceptible to a high level of seabed mobility, the probability of buried cables becoming exposed over their operating period is increased.
Application of this guidance aims to improve project awareness of areas with a mobile seabed and the ability to predict future seabed levels, with this knowledge informing requirements for cable burial or provisions to protect other assets which may be vulnerable to a change in the seabed profile. Projects are recommended to consider the potential risks associated with a mobile seabed from the outset of project development and engage in a process to develop improved understanding and reduce uncertainty.
A framework for delivering an assessment of seabed mobility is provided which recommends a two-stage approach.
- An initial seabed mobility assessment where existing data and information establishes a Conceptual Site Model (CSM), maps the degree of seabed mobility across areas of interest, identifies data gaps to target with subsequent surveys (as relevant), and contributes to the project’s risk register. The initial seabed mobility assessment should be undertaken as early as possible during project development. At this time, the assessment is likely to be high level, mainly qualitative and should clearly identify associated levels of uncertainty
- A detailed seabed mobility assessment which should aim to reduce levels of uncertainty from the initial seabed mobility assessment by applying new survey data and relevant analytical methods. The outcomes from the detailed assessment should refine the CSM and seabed mobility maps, develop predictions of future seabed levels and review relevant projects risks and associated mitigation. Recommendations should also be made for onward monitoring of seabed levels during the operational phase.
In some cases, the initial seabed mobility assessment may be sufficient to confirm that an area is not subjected to a level of seabed mobility which places assets at risk and a subsequent detailed assessment is not required. Importantly, this assertion should be associated with an appropriate low level of uncertainty.
For the detailed seabed mobility assessment, the prediction of future seabed levels should be delivered within a probabilistic framework which bounds a best estimate within upper and lower estimates based on associated residual uncertainties. During the operational cycle, onward seabed monitoring should provide a basis to review and refine the prediction of future seabed levels.
Notably, this guidance aims to fulfil the link with other guidance, recommend practice and design standards which call for an assessment of seabed mobility.