Section 6 Scour and deposition

6.1 General

6.1.1 Scour, or erosion, results in the removal of soil around a foundation and thus may affect the performance or capacity of that foundation. Depending on the extent of the scour it may require remedial action. Scour is assumed to occur in sand whereas erosion is considered to occur in clay.

6.1.2 Deposition is the process by which soil is deposited around a foundation or structure. Occasionally deposition may become problematic where it results in a structural component being loaded in a manner for which it was not intended. An example of this may be a jacket structural member near to mudline that becomes inundated with soil and due to the movement of the structure the member dissipates load into the deposited soil. This loading induces fatigue stresses in the member that would, in absence of deposition, not normally be present.

6.1.3 It is necessary to consider scour through the concept to detailed design and construction stages and thus it is important to define the philosophy or approach to scour at an early stage of the development as it influences requirements for site investigation and definition of engineering assumptions.

6.1.4 Scour is predominantly caused by a combination of current and wave actions though it may also be caused or exacerbated by the effect of the structural movement or cyclic degradation of soil strength. Other possible causes of scour could be pumping induced, for example where structural movement causes flow of water inside a jacket leg-pile space, or vessel propeller induced scour (e.g. by a maintenance vessel in shallow water); or by placement of a jack-up nearby to the foundation. All potential sources of scour and deposition should be identified and accounted for in the design process.

6.1.5 As per Figure 3.6.1 Global versus local scour around a structure and foundations, the following definitions are associated with scour:

seabed level change is a change in seabed level that would occur irrespective of whether or not a structure is installed. Bed level change may be seasonal, or longer term, and might be associated with the passage or evolution of seabed features such as sandwaves or sandbanks;
general scour occurs as a result of the influence of the overall structure footprint on the seabed; and
local scour occurs around localised areas, or members, of a structure (e.g. corners of a gravity base or piles).

Figure 3.6.1 Global versus local scour around a structure and foundations

6.2 Scour assessment

6.2.1 A high level indication of the likelihood and magnitude of scour can be achieved by the use of indicators of potential mobility and erosion as described by Whitehouse et al. (2011) and presented in Table 3.6.1 Examples of seabed hazards and mitigation. This high-level assessment may then be followed by a more detailed assessment.

Table 3.6.1 Examples of seabed hazards and mitigation

Sand – indicators of mobility	Clay – indicators of erosion
Ripple marks	Longitudinal furrows or grooves
Megaripples	Obstacle marks – scour around rocks or other debris on the seafloor
Sandwaves
Obstacle marks – scour and deposition around rocks or other debris on the seafloor

6.2.2 If sandwaves, or other mobile features, are identified in the development area it is important for future decision making to understand how these features may change with time and what impact they may have on the foundations. For example, the speed and direction of movement of sandwaves should be assessed; potentially by comparison of different surveys.

6.2.3 Following the recommendations of Whitehouse (2006), a scour assessment should aim to assess whether or not scour may occur for certain return period events. If scour is a significant risk, then a more detailed assessment should be performed to determine the depth and extent of scour and how quickly it will develop. The assessment should consider that scour may occur either during, or as a result of, the construction process itself, soon after installation or during operation.

6.2.4 Whitehouse (1998) presents a general approach to assessing scour hazard that considers input of wind, waves and current in combination with soils data and structural characteristics to provide an overall scouring status. A combination of results on likelihood and magnitude of scour give an assessment of scour hazard or risk.

6.2.5 This assessment may be supplemented by the use of techniques such as physical modelling or computational fluid dynamics to give improved predictions.

6.2.6 Where the process of scour assessment establishes that scour hazard is significant, it is likely to be necessary to define a scour management plan. An example of a scour management plan for a gravity base structure is given by Whitehouse et al. (2011), as presented in Figure 3.6.2 Scour management plan (reproduced from Whitehouse et al., 2011).

6.2.7 Generally speaking, scour can be classified into one of the categories below:

significant scour, erosion or deposition is unlikely; therefore, no action in design or construction is required. Routine structural inspection may be used to confirm that this remains the case throughout an asset's life. If inspection does identify scour beyond that included in the design, then further engineering or remedial action may be required; and
significant scour, erosion or deposition is expected. This may either be accounted for by use of a design scour allowance(s) scour protection, or a combination of both.

6.2.8 When setting design scour allowances the following aspects should be considered:

sensitivity of structure capacity and performance to scour; combined with the likelihood and potential impact of scour exceeding the design scour allowance;
a design scour allowance is likely to form the main basis for inspection criteria;
ability to detect and respond to excessive scour before it proves problematic to the foundation performance or capacity;
whether different design scour allowances may be applicable for difference design conditions – for example, FLS, ULS or ALS; and
a scour management plan also requires definition of inspection during installation and in-service with the use of remedial works where scour is found to be excessive; or it is determined scour may exceed the limits set during design. It is important to note that scour magnitude may increase or decrease with time; therefore, an annual inspection may not necessarily capture the full extent of scour that has occurred.

Figure 3.6.2 Scour management plan (reproduced from Whitehouse et al., 2011)

6.3 Scour mitigation, remediation and protection

6.3.1 Where design scour allowances are not practicable or economical, it may be necessary to employ scour mitigation, protection or seabed preparation works. These measures should be subject to detailed design to ensure that they can be constructed properly and will provide sufficient protection against scour for the design life of the foundation.

6.3.2 Scour mitigation may be features that inhibit or limit the effect of scour on a foundation (e.g. the use of skirts around shallow foundations). Skirts may be partitioned internally so that if scour affects the outer skirt the internal skirts may limit the extent of scour underneath the foundation. Another potential scour mitigation measure is to ensure foundations are placed below the potential troughs of seabed features such as sandwaves.

6.3.3 Scour protection requires design to ensure it will function as intended and scour protection may itself be subjected to scour and this should be accounted for in specifying the nature and extent of scour protection. Scour protection will need to be appropriate specified so that it remains stable under the expected current and wave conditions as well as other conditions such as seismic. This may also include the use of filter layers and consider whether any damage or loss of rock is allowable. CIRIA (2012) presents design methodology and practicable aspects of the design of rock for use in marine or offshore projects.

6.3.4 Where scour protection measures are used they should also be subject to inspection to confirm that they remain sufficiently intact to perform their purpose. Nielson et al. (2014) report a case where scour protection was sinking into the underlying seabed and thus was not performing as expected.