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DOI: 10.1016/j.apor.2024.104342
Abstract:
As a result of armed conflicts, huge amounts of Unexploded Ordnance devices (UXO) and Discarded Munition Material (DMM) are expected to be located on the seafloor, especially in coastal regions. During Offshore Construction, strategical site monitoring and systematic remediation activities, the behaviour of such objects in waves and currents is of huge interest as potential mobilization of objects after a survey or clearance activity could change the situational picture again. From findings and reports it often is assumed that UXO and DMM tend to migrate over the seafloor for long distances. Here, anthropogenic effects like fishing or dredging activities are underestimated. More scientific approaches clearly show that mobilization and migration of UXO over long distances does not occur. However, theoretical analysis remain theories until they are proven by experiments. For this reason, three large representative objects were investigated under real scale wave conditions in the Delta Flume of Deltares. The objects represent real scale models of UXO found in the North Sea as well as academically shaped objects. All objects as well as the Flume Tank were intensively instrumented to measure the full environmental conditions as well as the behaviour of the objects. The sediment represents typical sand as found in the North Sea and the seabed morphology and soil conditions were closely monitored during the experiments. The experiments support the theoretical models that predict burial but no mobilization also under extreme wave conditions.
DOI: 10.1016/j.apor.2024.103946
Abstract:
Unexploded Ordnance (UXO) and Discarded Military Munitions (DMMs) are found in many coastal regions. These objects are constantly exposed to environmental influences such as saltwater, which causes corrosion, and waves and currents as well as mobile seabed sediments. The basic assumption to decide if an object is mobilised by waves and currents or not is to compute the critical loads that are needed to move it from its stable position on the sediment bed. The loads on the object are induced by hydrodynamic and hydrostatic forces as well as by friction, gravitation, and inertia. In this study, the hydrodynamic forces were calculated by analytical approaches and numerical simulations (CFD). These forces have been validated for the current-induced mobilisation by wind tunnel experiments. Finally, the dependency of the dimensionless coefficients for lift, drag and added mass of the Reynolds number and the burial depth is transferred into a resulting equation.
Abstract:
Unexploded ordnance devices (UXO) pose a potential threat to human life and material during offshore construction activities. Extensive survey activities are conducted to locate, identify, and clear these objects as necessary. For the period thereafter, it is necessary to investigate whether areas that have already been cleared, or even objects that remain in place, may be affected by mobilization under tidal currents or waves, and could thus have an impact on operation and maintenance during the lifetime of the offshore installation. In this study, model simulations based on fluid mechanics are described to derive the loads on the objects caused by currents and waves and combined with knowledge of the known burial condition of the objects. Within the model, the hydrodynamic and hydrostatic loads on the object caused by waves and currents are balanced with inertia and rolling resistance. Thus, the critical current velocity and critical wave conditions for the mobilization of different objects are calculated and compared with the environmental conditions prevailing in the North Sea. As a result, a recurrence interval for the potential mobilization of objects on the seafloor is given, which can now be used to optimize route surveys and thus help accelerate offshore construction work. It is shown that currents are not able to mobilize the objects investigated in the study in almost all regions of the North Sea. Waves can mobilize certain objects in very shallow and extreme conditions.
Abstract:
Offshore construction works are increasing permanently. Especially since offshore wind energy is developing very rapidly, the presence of UXO on the seafloor is an issue for any type of construction works offshore. Expensive and time-consuming UXO campaigns are done to locate, identify, and remove these objects. The release of a site for construction documented in a sign-off certificate is then often given for a limited time assuming potential UXO migration on the seafloor caused by hydrodynamic loads. As a consequence, UXO measures have to be repeated after the expiring of the sign-off certificate. This problem affects subsea cable installation, the installation of offshore pipelines, offshore mining activities, offshore construction works and activities of authorities and navies. A model for current- and wave-induced mobilisation of objects on a sandy sea floor, based on the Morison equation, is presented
Abstract:
Power cables for floating offshore wind farms are installed freely suspended in the water column and are thus directly exposed to high environmental loads. Even with cable protection systems (CPS) in place, harmful stresses, specifically with respect to fatigue loads, cannot be entirely avoided. Accordingly, the fatigue lifetime needs to be predicted and, if necessary, optimised in advance. This is typically done by time domain simulations of design load cases. However, most simulation tools used consist of comparatively simple structural mechanical models. As a result the stress distribution within the cable is not predicted correctly and does thus not lead to an accurate fatigue lifetime prediction. This paper presents an advanced approach to the modelling of submarine cables. Therefore, stresses are calculated using fully flexible, non-linear time domain simulations based on the floating frame of reference formulation in conjunction with the finite element method. First results of verification load cases are presented.
DOI: 10.1016/j.oceaneng.2018.06.047
Abstract:
The aim of this work was to quantify the physical conditions, required for the mobilisation of unexploded ordnance devices (UXO) on the sea floor. As a basis for this, the hydrodynamic processes around UXO were measured in the wind tunnel and the flume tank in terms of conditions, especially Reynolds numbers. From these experiments, the typical shape of a sandy sea floor in the close vicinity of an object, due to current-induced burial was determined. Knowing this, the model for current-induced mobilisation of objects was developed. The model assumes a critical dimensionless Moment Factor MF=a⋅Reb, where the parameters a and b had to be investigated. This was accomplished by performing a total number of 287 numerical simulations, wind-tunnel testings and flume tank experiments at different geometric scale factors (1:10, 1:5, 1:2 and 1:1). With the parameter values so determined, the model describes the critical situation of an arbitrary shaped cylinder-like object regarding the incident flow velocity, the immersed mass and the burial depth, as well as the length and the volume-averaged diameter of the object.
DOI: 10.1109/OCEANSE.2017.8084925
Abstract:
The presence of a huge amount unexploded ordnance devices (UXO) on the sea floor is one of the major problems during installation and maintenance of offshore wind farms and other offshore structures. To identify all targets, time and cost consuming campaigns are necessary. The clearance of UXO is often limited by time due to the idea that UXO can migrate on the sea floor. Therefore the UXO-measure has to be revisited in some cases before installation or repair. Thus it might be possible, that additional UXO-measures are necessary before starting any operations touching the seabed. Insofar a better understanding of the migration of objects on the sea floor may help to improve currently used methods of time-dependent clearance by more knowledge-based decisions. The aim of this study is to investigate the requirements of initial movement of objects on the sea floor. This has been done by the analysis of literature, previous investigation to scour and burial of such objects, wind-tunnel experiments as well as experiments in a water channel. Additionally, numerical simulations allowed comparisons, combinations and a generalization of experimental results. A new mathematical model developed and validated allows a prediction of the incident fluid velocity that is necessary for an inertial motion of defined cylindrical and spherical objects. This model allows a reliable prediction of the initial migration of objects on a sandy sea floor. It is based on physical parameters, which depend on predictable and measurable events like currents, tides and indirectly on the weather conditions.
DOI: 10.1109/OCEANS-Bergen.2013.6607970
Abstract:
The prediction of scour and burial processes on the seafloor is the major goal of the presented investigations. As this task is extremely complex, single effects which cause scour and burial have been inspected separately. Therefore scour in the immediate vicinity of different objects and its changes under the influence of constant incident flow have been analyzed in the water channel. The observed scour patterns are the result of fluid mechanical effects. The measurements have been carried out by using the particle image velocimetry (PIV) and maintaining the Reynolds number. These investigations yield an explanation for the observed scour patterns and therewith verify the validity of the experiments in the water channel. Numerical simulations of scour around a cylindrical object are the first step to achieve the prediction of scour and burial of objects.
DOI: 10.1063/1.2747420
Abstract:
The existence of oxygen‐rich saltwater in the deeper basins of the Baltic Sea is mainly caused by sporadic inflow‐events of salty and oxygen‐rich saltwater from the North Sea into the Baltic Sea. These inflows take place over the narrow and shallow Drogden Sill into the first basin, the Arkona Sea. Actually different offshore wind farms are planned in this region, which opens a whole string of questions about the ecological influence of offshore wind farms on the mixing of both layers. To answer these questions, numerical simulations of the mixing processes in the wake of wind turbine bases have been carried out. For the evaluation and quantification of these mixing processes a laboratory‐experiment with a simplified model of the natural configuration has been realized. For this purpose a new water‐channel has been build. This channel allows to simulate the inflow of saltwater in a size‐scale of 1:100 to reality by keeping the densimetric Froude‐Number. The experimental configuration consists of a long circular cylinder with a diameter of 8 cm in a 10 cm thick saltwater‐layer flowing under a stationary fresh‐water layer of 30 cm thickness. Focus point of this investigation is the wake of the cylinder in the stratified flow and the mixing‐processes in the shear‐layer due to the influence of the cylinder. The stratified flow around the cylinder induces the typical Karman‐vortex wake, horseshoe‐vortices at the bottom and in the shear layer and Kelvin‐Helmholtz‐instabilities in the shear‐layer. Nonintrusive optical measurements were taken with planar laser‐induced fluorescence (PLIF) combined with two dimensional particle imaging velocimetry (PIV). The combination of both techniques allows the determination of instantaneous velocity components u and w from PIV‐measurements, the salinity s from PLIF‐experiments, their variations u′, w′, s′ and the correlations of those like Reynolds‐stress terms (u′u′, u′w′, w′w′) and turbulent‐ or Reynolds‐flux terms (w′s′, u′s′). Especially the vertical Reynolds‐flux w′s′ is the characteristic parameter to evaluate entrainment‐velocity and entrainment‐coefficient.
Anne Menzel
Daniel Klembt
Peter Menzel
