Diffraction/Radiation theory is used to calculate the wave kinematics and the motions of a floating body in area of varying bathymetry. The bathymetry is modeled as a second body, which, without special measures, leads to spurious reflection at the edge of the mesh. A modified formulation of the Boundary Element Method is introduced to model partially transparent panels. Those panels, when properly used to smoothly extend the actual (opaque) bathymetry, allow much more accurate computation. The efficiency of the method is tested with regards of several parameters concerning the bathymetry size and the way to smooth the truncation. Numerical results are satisfactorily compared with a 3D shallow water code based on Green-Naghdi theory. The sensitivity to the slope on the ship response is then investigated (motion, added mass, radiation damping and second order loads). The differences with the constant depth calculations are significant, due to the modified incident wave field, but also due to modified added mass and radiation damping terms. The method presented here could be useful in the context of LNG terminals where the depth is quite shallow and the bathymetric variations significant.
As offshore units near the end of their design life, Operators are often faced with the requirement of extending the operating life of the facility, either to continue producing from an existing and still productive field, or to pursue new opportunities and/or defer new CAPEX as is often the case with MODUs, construction and support vessels and leased production units. Life extension presents a unique engineering challenge from the perspective of risk management; requiring a complete structural re-assessment of the facility, from its current, as-is condition in order to determine the remaining life of the unit, along with any repairs required in order to achieve the desired additional service life. This paper describes a methodology, developed by Bureau Veritas, for the engineering reassessment of aging offshore units that will help operators with decision making relating to life extension. It will focus on the two main degradation mechanisms affecting the structure: corrosion and fatigue. Resulting directly from Bureau Veritas's digital transformation strategy, the methodology is built around the concept of a 3-D "digital twin" model of the asset, which is updated periodically and automatically with critical inspection data collected by connected surveyors. This facilitates continuous updating of the model to ensure a "real-time" equivalence to the "as-is" physical asset. Furthermore, interfaces between the "digital twin" and conventional structural and hydrodynamics analysis software enable accurate condition assessment analyses to be performed, at any time, on the current state of the asset; providing critical information to be used in the assessment of feasibility of life extension and in determining actual expected remaining life. In the event of extended life operations, the output of these condition assessment calculations can also be used to develop a Risk-Based Inspection (RBI) programme, based on the criticality and condition of each structural element, to monitor the critical degradation mechanisms, and measure and predict on-going degradation and calculate any consequences to the actual remaining life, using the continuously updated "digital twin". This approach maximizes the useful life of any facility while minimizing the overall cost of inspection, without compromising safety. While the focus of this paper is life extension of existing assets, this methodology is equally applicable and valid for application on new-build assets, allowing risk-based and just-in-time management of maintenance and repair operations as well as real-time determination of existing asset life from the earliest stages of an assets deployment.
The influence of current in sea-keeping problems is felt not only for first order quantities such as wave run-ups in front of the structure, but also mainly for second order quantities. In particular, the wave drift damping (which is expressed as the derivative of drift force with respect to the current) is of special interest for mooring systems. The interaction effects of a double-body steady flow on wave diffraction-radiation is studied through a decomposition of the time-harmonic potential into linear and interaction components. A boundary integral method is used to solve the first order problem. Ultimately, a far-field method is proposed to get access to second order drift forces.
As offshore activities are growing, the marine operations are becoming more complex involving the presence of few or several vessels in proximity to each other which increases the risk associated to those operations. Shuttle tankers, PSVs, floatels are often equipped with DP systems for maintaining position. The capability of these systems is defined during design phase by the DP manufacturer based on the assumption of standalone operation and considering environmental load cases prescribed in Industry standards (ex. wind, wave and current all aligned). During a realistic operational condition, however, the presence of other unities may significantly alter the loads acting on the DP vessel which will affect somehow its station keeping capacity. Furthermore, in some areas of the world, the misalignment between the environmental loads and the presence of several wave trains from different directions (ex. off-shore Brazil) shall be considered in the sake of safety of the operation. In order to provide the clients means to simulate these complex operations (including moored vessels), a DP module has been integrated to Bureau Veritas multi-body mooring software, ARIANE. In this paper, the case of a DP floatel vessel operating close to a turret moored FPSO in Brazilian waters is analyzed and the differences in the DP capacity under realistic conditions with respect to the original DP capability are presented and discussed.
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