Challenges and recent advances in offshore scour modelling
D. Rudolph (Deltares | Delft Hydraulics)T.C. Raaijmakers (Deltares | Delft Hydraulics) B. de Sonneville (Deltares | Delft Hydraulics)
Abstract
In recent years, progress has been made on the understanding and description of scour around offshore structures. For simple geometries, such as cylindrical piles and triangular structures, empirical formulae have been developed. This makes it possible to predict scour depth and time rate depending on site-specific conditions with reasonable accuracy. However, the jack-up industry does not (yet) have validated and generally applicable tools to predict scour development for almost all existing jack-up footing designs. This has triggered the initiation of the joint industry project “OSCAR” (Offshore SCour Assessment and Remedial measures), which has started in autumn 2008. Oil companies, drilling rig owners, designers and research companies joined forces to develop engineering tools for jack-up specific scour assessments.
This paper outlines major challenges in scour assessments for jack-ups and introduces recent developments and innovations in scour modelling. Their successful application to offshore monopiles offers a large potential to tackle and resolve a number of key challenges on jack-up scour:
Online monitoring with cameras allows gathering time-dependent information, the identification of the equilibrium scour depth and insight into the behaviour of scour protection.
Development of transfer functions from model scale to reality and verification of these functions with field data allow for improved reliability of scour predictions.
Fundamental research on the dynamic behaviour of scour protection beyond the threshold for initiation of motion allows to predict the performance of the scour protection and to conceptually design scour protection layouts.
Key words
1 I
NTRODUCTION
In recent years, progress has been made on the understanding and description of scour around offshore structures. For simple geometries, such as cylindrical piles and triangular structures, empirical formulae have been developed [1, 2, 3, 4]. This makes it possible to predict scour depth and time rate depending on site-specific conditions with reasonable accuracy. However, the jack-up industry does not (yet) have validated and generally applicable tools to predict scour development for almost all existing jack-up footing designs. In this case, engineers need to work with crude rules of thumb and experience.
The lack of a profound theoretical basis and empirical relations for jack-up scour prediction can lead to uneconomical application of scour protection or risky situations due to scouring followed by loss of bearing capacity. Therefore, oil companies, drilling rig owners, designers and research companies decided to join forces and initiated the joint industry project “OSCAR” (Offshore SCour Assessment and Remedial measures), which has started in autumn 2008.
This paper outlines major challenges in scour assessments for jack-ups (Section 2) and introduces recent developments and innovations in scour modelling (Section 3). The scope of work of JIP OSCAR is briefly summarized (Section 4).
2 C
HALLENGES IN SCOUR ASSESSMENTS
The SNAME guideline for site specific assessment of mobile jack-up units [5] states that “…there is no definitive
procedure for the evaluation of scour potential and emphasis must usually be placed on previous operational experience…”. In [6], a generic approach was suggested including the main steps to be followed in a site assessment:
1. Site data analysis
2. Compute potential of seabed mobility 3. Scour prediction
4. Scour protection design
Challenges in these main steps are briefly outlined below.
2.1 Site data analysis
Step 1 includes the collection of relevant site-specific data. This is usually a straightforward task. Checklists can be used, which contain essential information on environmental conditions (operational and extreme), seabed composition (original seabed, previous dumps) and the jack-up (footing design, orientation, penetration depth etc.).
2.2 Potential of seabed mobility
Basic concept
Step 2 is usually based on well-validated empirical formulae for the threshold of the initiation of motion of seabed material. Formulae make use of the bed shear stress approach, see for example [7]. The seabed is mobile if the wave- and current-induced bed shear stress exceeds the critical bed shear stress of the seabed. This approach can also be used for the stability assessment of scour protection consisting of loose rock.
The importance of a gradient in sediment transport
Scour can not occur if the seabed is not (sufficiently) mobile. If the seabed is mobile, the development of scour also requires a gradient in sediment transport. If there is no gradient and no net transport of bed material, scour does not occur. This is illustrated in Figure 1. Such a gradient in sediment transport can be caused by a local increase of flow velocities around a structure. The magnitude of seabed mobility is not necessarily a measure for the expected scour depth but it can affect the time rate of scour development.
The effect of the structure – amplification of velocity and bed shear stress
One of the major challenges in step 2 is the quantification of the effect of the jack-up footing on local hydrodynamics. Depending on size and shape of the structure, additional turbulence is generated which enhances the instantaneous velocities at the seabed. It is common practice to work with simplified relations for the local velocity or bed shear stress increase. A factor of 1 represents an undisturbed situation (i.e. no influence of the structure on local flow). According to potential flow theory, an amplification factor of 2 is applicable for a current around a cylindrical pile extending through the water column in current dominated situations. This factor does not take into account turbulent vorticity.
For structures with sharp edges, velocity increase factors of up to 3 have been reported. The uncertainties in these factors have a large influence on the bed mobility assessment because the bed shear stress increases with the square of the velocity.
Actually, a differentiation should be made between an increase of the mean instantaneous flow velocity (e.g. due to streamline contraction) and an increase due to fluctuating velocities (turbulence). We suggest the following definition for the velocity amplification factor, which is an extension of an approach by other researchers [8]:
dis dis vel undis undis U c TKE K = U c TKE with
Kvel … velocity amplification factor [-]
Udis … disturbed mean velocity near the seabed, close to the structure [m/s]
Uundis … undisturbed mean velocity near the seabed, far away from the structure [m/s]
c … empirical coefficient [-] TKE … turbulent kinetic energy [m2/s2]
The term for the mean velocity U can either be the nearbed current velocity (e.g. at 1 m above seabed) or the wave-induced bed orbital velocity. In case of current-dominated situations, the turbulence term plays an important role. In wave-dominated conditions, the velocity amplification Kvel is governed by the first term (Udis/Uundis) and the turbulence
term is less important. This is because the wave dynamics do not allow full development of turbulent flow. For current-dominated conditions, the turbulence term becomes important. This concept for the velocity amplification requires calibration data, especially for the disturbed situation. Without calibration it cannot be applied in engineering practice. Conclusion
It can be concluded that there are validated empirical formulae to compute bed shear stress and seabed mobility for an undisturbed situation, i.e. far away from the jack-up footing. The main challenge is to assess the disturbing influence of the structure. Velocity and turbulence amplifications are dependent on the jack-up footing design. The only way to determine these amplification factors is the execution of measurements (lab or field).
2.3 Scour prediction
The prediction of scour depth and scour pattern (or at least the position where the maximum scour can be expected) requires the availability of empirical formulae or – if not available - the execution of laboratory experiments. Presently, empirical formulae do not yet exist for most of the jack-up footing designs. Available rules of thumb do not provide more than a first rough estimate.
In the last decade, worldwide offshore scour research has focused on scour around cylindrical piles. This is because of the emerging offshore wind energy market and the planned installation of hundreds of monopiles with diameters between 4 and 6m. Progress has been made on a number of aspects:
The effect of the combined action of waves and currents on the maximum scour depth, The time scale of scour development,
The time scale of backfilling of scour holes during mild hydrodynamic conditions The development of scour outside the protected area (“edge scour”),
The stability of a dynamically stable scour protection consisting of small rock and Design recommendations for scour protection layouts.
Scour development around jack-up footings is more complex than cylindrical monopiles. There are a number of aspects that introduce additional challenges:
The duration of a jack-up deployment at a certain location is much shorter than the lifetime of a fixed platform, which makes estimation of time scales related to scour development more important.
The shape of jack-up footings is much more complex. Each design has its own scour characteristics.
The initial penetration is site-specific. The penetration affects the scour potential and determines the acceptable level of scour. The penetration depth can be adjusted (relevelling).
Each site has different seabed conditions including leftovers from previous operations (scour protection, drill cuttings, footprints).
The application of scour protection after jack-up arrival is often hampered by accessibility.
There are additional requirements regarding the type of scour protection, such as limitations to the maximum stone size with respect to future operations and damage to spudcans.
In the last couple of years, Deltares | Delft Hydraulics has continued carrying out scour research – predominately supported by laboratory experiments – in order to derive scour prediction and scour protection formulae:
Structures with triangular base, see [2, 6] Cone-shaped footings, see [9]
Cylindrical monopiles [3, 4]
Rectangular blocks and cylindrical piles with collars and skirt (not yet published)
Although the shapes of these basic structure types are fundamentally different from most modern jack-up footing designs, a number of identified basic trends and modelling techniques are extremely useful also for jack-up scour (see Chapter 3).
2.4 Scour protection design
Scour protection with loose rock
The design of scour protection consisting of loose rock includes the selection of a design event (e.g. a 50 year storm), the computation of hydrodynamic parameters at the seabed (such as the bed orbital velocity) and the required rock size to obtain an acceptable safety level. The rock size can be determined based on the bed shear stress approach. However, in most cases it turns out that a stable scour protection requires rock gradings of more than 20 inch. This grading is often not acceptable in view of potential damage to the jack-up footing.
Figure 2 shows theoretical rock stability computations for 2 inch, 6 inch and 20 inch for a water depth of 30m. The peak wave period was assumed to be a function of the significant wave height: T =4.5× H , both parameters in SI p s units. The influence of a current on the total bed shear stress is small and therefore neglected in this example for simplicity reasons. The plotted relative mobility indicates the relation between the wave-induced mobility and the critical mobility. The threshold for “stable” and “mobile” is at 1. It can be seen that small rock (2 inch, 6 inch) is mobile for typical southern North Sea design conditions (Hs=8-10m).
The exceedance of the critical mobility (relative mobility >1) does not necessarily mean that small rock is not suitable as scour protection: The individual stones are mobile (moving forward-backward with the wave cycle) but the protection as a whole can still be effective. Small rock can serve as sacrificial scour protection and has the advantage that it redistributes itself, covers places where it is needed and accumulates in deep scour holes. The effectiveness depends of the rock size and the volume applied in the effective area close to the structure. This concept has been validated for structures with triangular bases and resulted in an empirical formula that is applicable for this specific type of structure [2]. For other structure shapes (such as most of the spudcan designs) a similar formula for scour protection performance of mobile rock can be deduced in future.
The challenge is therefore to develop formulae to assess the efficiency of rock (which is mobile under the design conditions) as scour protection for a wide range of jack-up footings and hydraulic design conditions.
Scour protection with gravel bags or frond mats
While the basic principle of rock stability and rock displacement is reasonably well understood (without the influence of the structure), other protection measures – such as gravel bags, frond mats and other types of mattresses – generally miss a technical basis. Presently, no formulae and no validated field data are available to design and evaluate the stability of measures other than rock. This huge gap in the engineering toolbox leaves room for future research.
2.5 Summary of challenges
Scour development around most of the jack-up footing types cannot yet be predicted. The available rules of thumb do not provide more than first rough estimates. The challenge is to develop scour prediction formulae, which are
applicable to a wide range of realistic conditions and which provide reasonable extrapolations beyond the validated range.
Presently, the stability of scour protection can only be assessed on the basis of the bed shear stress approach. This approach only quantifies whether stones are stable or not. The commonly applied gradings of loose rock (2 to 10 inch) are not fully stable under typical southern North Sea design conditions (e.g. Hs=8-10m at 30m water depth).
The challenge is to quantify the behaviour and the efficiency of loose rock serving as sacrificial scour protection. There is a lack of fundamental knowledge on the performance of scour protection measures such as gravel bags and mats. The challenge is to understand and to quantify the mechanisms providing stability against wave and current action. Then, it will also be possible to make a comparison with loose rock and to decide on the most efficient scour protection measure.
Scour formulae are usually based on experimental results. Well-documented field data are needed for verification. The challenge is to conduct appropriate surveys, to record hydraulic conditions and to hindcast the scour development by using available formulae.
Numerical models are under development. However, at the present stage there is no program available that can handle all relevant aspects: wave-current interaction, correct schematisation of complex structures, sediment transport, bed friction, turbulence and acceptable computational times. The challenge is to implement these aspects and to validate with lab and field measurements.
3 R
ECENT ADVANCES IN SCOUR MODELLING
3.1 General
Some of the above mentioned challenges have been tackled in the last years. In this chapter we describe a part of recent developments:
An innovative scour monitoring technique with video camera mounted inside a structure The transfer of knowledge from model scale to reality,
Analysis of field measurements for the purpose of calibration of the scour prediction models and 3D mapping of scour patterns
3.2 Innovative scour monitoring technique with camera mounted inside a structure
The need for innovations in scour monitoring techniques was driven by
The demand of the industry to provide scour predictions and to evaluate scour protection performance, The scientific challenges to improve insight into highly complex scour processes and
Practical reasons to reduce costs and efforts of laboratory experiments.
So far, one of the major shortcomings in laboratory experiments in a wave basin was that scour development was not visible during test execution because of the turbidity of water at high sediment concentrations. Scour hole inspection required drainage of the wave basin. Since this process takes one day, it was not yet feasible to carry out tests with many time steps because this would have increased time and costs considerably. Until a few years ago, this practical problem limited the investigation of scour development in time and deformation of scour protection during a storm. The use of video cameras also made it possible to improve the understanding and therefore also the trust in the performance of dynamically stable scour protection (consisting of loose rock) and the behaviour of gravel bags or mats. In the following, we use simple structures such as cylindrical piles and triangular blocks for illustration. Similar techniques are being applied in the presently ongoing Joint Industry Project “Offshore Scour Assessment and Remedial Measures”, which focuses on jack-ups. Reference is made to Chapter 4. The databases, scour formulae and PC tools developed within this JIP are not public and can therefore not be presented here.
The development of the new monitoring technique includes
Recording of images with a high frequency by using a digital camera and a fish-eye lens Automatic detection and processing of the interface between water and seabed on the snapshots Conversion of interfaces into scour-time functions
Figure 3 shows the camera and the fisheye lens mounted inside a structure and an image taken by the camera before test execution. The camera records images with a frequency of 6Hz. This is sufficient to resolve the effect of seabed material brought into suspension by wave motion, turbulent redistribution and settlement. The faces of the triangular structure appear distorted due to the projection of a 3D view on a 2D plane.
Figure 4 presents a camera image and the interface detection based on colour gradients. The coordinates of the interface can be linked to a scour depth via a structure and camera-specific calibration. The interface found in each directional sector (here 5º) and at each time step is transferred to a time series of scour depths. A few positions are selected for the analysis of equilibrium scour depth and time scales of scour development.
The last but not least important step, the interpretation of the scour-time functions and the transfer functions to reality, is described in Section 3.3.
3.3 Time-dependent scour development
The development of scour around most offshore structures is not an instantaneous process but time-dependent. Each combination of hydraulic conditions (waves, current, water depth) has its own characteristic equilibrium scour pattern and time rate to reach this equilibrium. The scour depth increases until equilibrium, until the hydraulic condition changes (which is accompanied with another characteristic equilibrium scour depth) or the depth at which scour affects the leg stability and triggers remedial actions (e.g. relevelling), whichever happens first.
The development of the scour depth with time can generally be described by an exponential law. The scour depth at a certain time step depends on the equilibrium scour depth and the characteristic time for the considered hydraulic condition. eq char S t t =1exp -S T with
S(t) scour depth as a function of time [s] Seq equilibrium scour depth [m]
t time [s]
Tchar characteristic time [s]
The importance of a good understanding of the time rate of scour development is illustrated in Figure 5. In general, the equilibrium scour depth (Seq) increases with the severity of wave conditions and the time to reach this equilibrium
becomes shorter. Whether the maximum acceptable scour depth is exceeded also depends on the duration of the storm event.
The greatest challenge is to obtain a reasonable estimate for the characteristic time. Data fitting on experimental data is relatively straightforward but the transfer from model to prototype actually requires research at various scales and the calibration and verification with field measurements.
Such a step has been successfully taken for cylindrical piles [4]. In 2008, the offshore windpark Prinses Amalia was installed on the Dutch continental shelf in about 25 metres water depth. The windpark comprises 60 wind turbines on monopile foundations (diameter of 4.0m). The seabed material in the windpark area generally consists of fine to medium non-cohesive sand (d50=0.1-0.3mm). After several months leaving the windpark monopiles unprotected,
multibeam echo soundings were conducted. From these bathymetric surveys scour depths were derived. Hydrodynamic conditions between pile installation and scour survey were processed. The development of scour with time was hindcasted with various existing scour formulae. Numerous combinations of formulae and assumptions were checked. Finally, a set of formulae was identified, which provides scour predictions with reasonable accuracy. Figure 6 shows an example of the scour hindcast and the survey results for one pile and the overall agreement between field measurements and hindcasts for the available dataset.
3.4 Dynamically stable scour protection
Two laboratory experiments were conducted to check whether a highly mobile, small amount of scour protection can slow down scouring and whether it can reduce the final scour depth. Figure 7 shows the model set-up. Waves approach the face of the triangular structure (width of 0.50m) perpendicularly. One corner (A) was protected by a small amount of loose rock (extent =1/3 of face length, mean grain size of d50=2.4mm, layer thickness equivalent to 15d50). The other
two corners (B and C) were left unprotected. Corner A and corner B were exposed to the same hydrodynamic load. Corner C was sheltered against waves. Two hydraulic conditions were applied subsequently:
test 1: Hs=0.14m, Tp=2.32s, hw=0.50m, duration of 45 minutes
test 2: Hs=0.18m, Tp=2.77s, hw=0.50m, duration of 45 minutes
The theoretical stability of the rock protection outside of the disturbing influence of the structure was computed on the basis of the bed shear stress approach. The wave-induced mobility parameter was found to be 0.06 and 0.11 (undisturbed), based on Hs and Tp. The critical mobility parameter (or Shields parameter) of loose rock is 0.055. This
means that the applied rock is already mobile at the undisturbed seabed (relative mobility of 110% and 190%, respectively). Assuming a structure-induced velocity amplification factor of K=2, the relative mobility increases by a factor of K2=4. The relative mobility close to the structure is about 4 to 8 (i.e. 400 to 800% of the critical mobility). The scour development was monitored during the tests. The camera images were processed and the time-dependent scour functions were derived, see also Figure 8. The following was observed during the tests:
Scour development occurred at the protected corner (A) but at a lower rate than at the unprotected corner. At the end of test 1, the scour depth at the unprotected corner (B) reached equilibrium. The scour development at the protected corner (A) was still in development but at a low rate.
In test 2, the wave conditions were increased. This was accompanied by an increased equilibrium scour depth for both unprotected (B) and protected corners (A).
With time, scouring of the protected corner (A) continued. At the end of test 2, the scour holes of the unprotected corner (B) reached equilibrium, scour at the protected corner (A) was still developing (see Figure 6).
The sheltered corner (C) hardly experienced any scour.
It was concluded that the equilibrium scour depth reduced by about 1/3 and that the time to reach the equilibrium was delayed (factor of 2) due to the presence of a relatively low volume of small rock. However, it should be noted that the occurrence of scour could not be prevented. The efficiency of this combined effect (depth reduction and slower development) obviously depends on the rock diameter and the applied volume.
3.5 Mapping of scour patterns
Scour around structures such as spudcans is often characterised by shallow and wide depressions. Maximum scour depths do not necessarily occur at the interface between structure, seabed and water but possibly at a few feet away. A camera mounted inside a structure is able to record the scour at the structure but it cannot catch spatial information further away (including global scour and redistribution of scour protection).
Spatial information is essential for
Determination of the position and the extent of maximum scour (which indicates on where to apply protection)
Mapping global scour (in addition to local scour)
Computation of scour protection performance (difference before and after test) Comparison between field surveys and laboratory experimental results
Investigation of morphological patterns, which allows interpretation of hydraulic loads Recent advances in three-dimensional mapping of scour holes have been based on two techniques:
3D Laser scan 3D stereophotography
The 3D laser scan is based on the projection of a fan of laser light on a scour hole. It consists of a hand-held wand, a transmitter and a data acquisition system. The camera views the laser to record cross-sectional profiles. Data
processing of various sweeps over the scour hole leads to a full 3D-image of the scour hole. Figure 9 shows a photo and a laser scan from the same position.
The 3D stereophotography is based on a series of photos taken around the scour hole and calibration markers at various positions (see Figure 10). Two synchronized cameras (“left camera” and “right camera”) are used. At first, the processing software identifies the positions of the calibration markers, the camera positions and a number of camera- and lens-specific parameters. Then, each pair of synchronized photos is analyzed by dense stereo-matching. The inequality (disparity) between pixels on the left image and the right image is a measure for the distance to the synchronised cameras. With the availability of all camera positions and the spatial information of the pixels relative to the camera, also the absolute coordinates of the pixels are known and 3D surfaces can be plotted.
The spatial information obtained with these techniques can be used to visualize scour patterns and to compute differences between various test stages.
4 JIP
OSCAR
–
ESSENTIAL STEPS TOWARDS ENGINEERING TOOLS FOR
J
ACK
-
UP
SCOUR ASSESSMENTS
The above described challenges and recent advances in offshore scour modelling were an incentive to initiate the Joint Industry Project “Offshore Scour Assessment and Remedial Measures” (JIP OSCAR). Oil companies, drilling rig owners, designers and research companies joined forces and started this JIP with particular focus on jack-up footings in autumn 2008. Since confidentiality has been agreed and research is still ongoing, results cannot be presented here. However, we can briefly introduce the scope of work, the chosen approach and the deliverables. The JIP is still open for late participants. The scope of work consists of four work packages (WP), which are briefly described in the following. WP 1: Desk study
The objectives of WP 1 are to summarize existing literature, which is relevant for jack-up scour and to further develop a standard approach for jack-up scour assessments. The literature review focuses on scour depth predictions, time scale of scour development and the evaluation of various types of scour protection measures. Relevant sources are publications on jack-up scour and the most relevant offshore scour papers. Rules of thumb, detailed formulae and graphs are provided, which allows the user to go through the essential steps quickly for a first idea on the scour potential.
WP 2: Physical modelling and derivation of new scour formulae
The execution of systematic model tests provides the basis for the understanding of the governing processes. New scour prediction formulae are derived for three types of jack-up footing. The formulae are set up for a wide range of conditions including variations on water depth, wave conditions, current velocity, relative direction between waves and current, orientation of jack-up footing towards waves and current, penetration depth and seabed material.
WP 3: PC program “OSCAR the scour manager”
The PC program “OSCAR – the scour manager” is stand-alone software, which is intended for use by practical engineers in connection with a site assessment. It follows the main steps of the standard approach for scour assessments and makes use of the new scour formulae for the jack-up footings. In the graphical user interface, presently five structures can be selected: a cylindrical pile, a triangular structure and – exclusively for the JIP OSCAR participants – the three selected jack-up footings.
Scour depth computations can be made for a given steady state condition or a time series of waves and currents. Best estimates and confidence intervals are given. An indication is provided whether the chosen site conditions are well-covered by the range tested in scale models (most reliable), well-covered by interpolation or extrapolation (least reliable). Sensitivity computations can be made to check the effect of uncertainties in the input variables.
WP 4: Field case workshop
The new theoretical knowledge is applied to past operations and hypothetical future practical situations. The scour assessment recipe, rules of thumb, general conclusions on scour-dominating processes and the PC program are used in field cases. Experiences are compared with theoretical predictions.
5 S
UMMARY AND CONCLUSIONS
The assessment of scour at jack-up footings is challenging – from a theoretical and a practical point of view. This is mainly due to complexity of site conditions, complex structure shapes and the time-dependency of scour development. Apart from a few rules of thumb, scour prediction formulae for most of the jack-up footings are not yet available. Recent advantages in offshore scour modelling and their successful application to cylindrical piles offer a large potential to tackle and resolve a number of key challenges on jack-up scour:
Online monitoring with cameras allows gathering time-dependent information, the identification of the equilibrium scour depth and insight into the behaviour of scour protection.
Development of transfer functions from model scale to reality and verification of these functions with field data allow for improved reliability of scour predictions.
Fundamental research on the dynamic behaviour of scour protection beyond the threshold for initiation of motion allows to predict the performance of the scour protection and to conceptually design scour protection layouts.
The above described challenges and recent advances have been dealt with in the presently ongoing Joint Industry Project “Offshore Scour Assessment and Remedial Measures” (JIP OSCAR). The participants are taking essential steps to improve the available tools for reliable and safe scour assessments.
References
[1] Sumer, B.M., Fredsøe, J.: The mechanics of scour in the marine environment, World Scientific Publishing Co. Pte. Ltd., Singapore, 2002
[2] Raaijmakers, T.C., Rudolph, D.: Scour protection of spud cans - a new design approach, International Conference The Jack-Up Platform – Design, Construction and Operations, London, 11-12 September, 2007 [3] Raaijmakers, T.C., Rudolph, D.: Time-dependent scour development under combined current and waves
conditions - laboratory experiments with online monitoring technique, 4th International Conference on Scour and Erosion, Tokyo, November 2009
[4] Rudolph, D., Raaijmakers, T.C., Stam, C.: Time-dependent scour development under combined current and wave conditions – hindcast of field measurements, 4th International Conference on Scour and Erosion, Tokyo, November 2009
[5] SNAME: Guidelines for Site Specific Assessment of Mobile Jack-Up Units, Technical & Research Bulletin, 5-5A, Society of Naval Architects and Marine Engineers, New Jersey, USA, 2002
[6] Rudolph, D., Bos, K.J., Rietema, K., Hunt, R.: Scour management – theory and model scale testing, 10th International Conference The Jack-Up Platform – Design, Construction and Operations, London, 13-14 September, 2005
[7] Soulsby, R.L.: Dynamics of Marine Sands, A Manual for Practical Applications, Thomas Telford, London, 1997
[8] Jongeling, T.H.G., Jagers, H.R.A., Stolker, C.: Design of granular bed protections using a RANS 3D-flow model, 3rd International Conference on Scour and Erosion, Amsterdam, November 2006
[9] Rudolph: Scour around jack-up legs with cone-shaped footings and skirts, International Conference The Jack-Up Platform – Design, Construction and Operations, London, 11-12 September, 2007
Figures
Figure 1 The importance of a gradient in sediment transport for scour development
A scour hole develops if there is a gradient in sediment transport. The arrows indicate the magnitude of sediment transport. A local increase in sediment transport is caused by higher (instantaneous) flow velocities at the structure. With time, a scour hole develops. The change of the local geometry interacts with a change of flow patterns and instantaneous flow velocities. When the scour hole is in equilibrium state, the amount of material moved out of the scour hole is in equilibrium with the amount of material moved towards the scour hole.
Mobility of bed protection at 30m water depth
0 1 2 3 4 5 0 2 4 6 8 10 12 14
significant wave height [m]
rel at ive mob ili ty [-]
2 inch rock (undisturbed) 2 inch rock (at structure, K=1.5) 6 inch rock (undisturbed) 6 inch rock (at structure, K=1.5) 20 inch rock (undisturbed) 20 inch rock (at structure, K=1.5)
Effect of structure: increase of instantaneous velocities, example for velocity amplification of K=1.5. mobility threshold stable mobile
Figure 2 Relative mobility of loose rock (2 inch, 6 inch, 20 inch) depending on significant wave height
Figure 3 Camera mounted inside a monopile with collar (left) and view from inside (right)
Figure 4 Automatic detection of interface between water and sand (left) and processing of time series (right) bottom of pile penetration depth camera with fisheye lens monopile interface sand - water C B D A A D C B
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0 12 time [hours] 24 scou r de pt h [m etre s]
slow scour development fast scour development
This graph indicates relations between duration and scour development. The equlibrium scour depth is the same. The assumed storm duration is 12 hours.
If the acceptable scour depth is 0.8m, this critical value is exceeded within 4 hours (blue line) or it is not reached within the storm duration of 12 hours (red line, after 24 hours).
The time rate would have decided whether scour would have been a problem or not.
storm duration of 12 hours
Seq=1.1m, Tchar=18h
Seq=1.1m, Tchar=3h The characteristic time Tchar is the
time when 63% of the equilibrium
scour depth Seq has occurred.
Figure 5 Scour as a function of time
Figure 6 Scour development at a monopile in the offshore windpark Prinses Amalia. Left: hindcasted time series versus field surveys. Right: overall agreement between hindcasts and measurements. Images extracted from [4].
Figure 7 Experiment with triangular structure protected at one side. Left: before test, right: after test.
A
B
C
waves
A
B
C
0.00 0.05 0.10 0 1000 2000 3000 4000 5000 6000 time [s] scou r de pth [m]
FIT unprotected corner (B) FIT protected corner (A)
MEASUREMENTS unprotected corner (B) MEASUREMENTS protected corner (A)
erroneous interface processing
time lag due redistribution of rock before scouring starts
Tchar=800sec
Tchar=300sec Tchar=400sec
Tchar=700sec
Figure 8 Development of scour as a function of time, laboratory experiment
Figure 9 Scour pattern after drainage of the basin. Left: photo. Right: 3D laser scan.
Figure 10 Scour pattern mapping with stereophotography. Left: block structure with calibration markers. Right: processed 3D bathymetry.
