Bernt J. Leira

DYNAMIC RESPONSE AND FLUID/STRUCTURE INTERACTION OF SUBMERGED FLOATING TUNNELS

Abstract Alternative approaches to stochastic dynamic response analysis of submerged floating tunnels subjected to wave loading are presented. For the purpose of establishing force, damping and mass coefficients for structural elements with three-dimensional flow conditions, fluid/structure interaction is modeled as finite element implementation of the Navier–Stokes equation. The numerical examples emphasize the effects of wave direction, shortcrestedness, damping, geometrical stiffness and frequency dependence in mass and damping coefficients.

A reliability-based control algorithm for dynamic positioning of floating vessels

Abstract The present paper is concerned with utilization of reliability methods in relation to on-line control of dynamic systems. The particular application is to dynamic positioning of marine vehicles in connection with reliability of mechanical subsystems. The present focus is on top and bottom angles of marine risers which are suspended between the seabed and the floating vessel. These angles are of crucial importance during, e.g. drilling and workover operations. The relationship between surface floater motion and angle responses is first considered. The possibility of reducing the maximum angular response levels by dynamic positioning of the floater is then investigated. Typically, and somewhat dependent of variation of current with depth, minimization of one of the riser top and bottom angles by adjusting the vessel position will take place at the cost of increasing the other angle. Hence, an optimum position should be defined by considering both angles but with different weight functions. An attractive approach is to determine these weights as functions of the respective reliability indices for each of the two angles. A further possibility is to apply an object function (loss function) which is purely expressed in terms of reliability indices. The viability of different schemes of this type is explored by numerical simulation for a specific riser configuration.

Reliability-based analysis of a stretch-bending process for aluminium extrusions

Abstract The paper presents reliability analyses of a plastic forming process. Stretch-bending of extruded aluminium square hollow sections was studied through finite element analysis. The profiles were axially fixed and given a specified curvature by means of a bending die. Important response parameters in stretch-bending of extruded profiles are flange sagging and elastic springback. The sensitivity of the predicted response parameters was investigated with respect to variations in the material properties of the aluminium alloy, i.e. yield stress, strain hardening and plastic anisotropy. The material parameters were modelled as random variables with given statistical properties. The probability of the response parameters staying within specified limits could then be calculated by means of response surface methods and FORM/SORM. Finally, the consequences of the specified response limits on allowable variations of the material properties were established. The paper shows how reliability-based methods can be used to establish relations between the variation of model parameters (e.g. material properties, cross-sectional geometry and process parameters) and the resulting variation of characteristic response parameters for a given forming process.

Analytical and numerical modelling of oil spill from a side tank with collision damage

The objective of this paper is to study oil spills from damaged tanks during ship collision. The work comprises development of analytic models for estimation of oil spill and computational fluid dynamics (CFD) simulation with the Fluent software for verification of the simple models. In previous papers, oil leakage from a damaged tank with an opening in the bottom was investigated. The purpose of the present study is to extend the scope to cover oil spills for the case of collisions where the openings occur in the sides. In such cases, the gravitational force is very important as for the grounding scenario. However, for collision scenarios, there is a local imbalance due to different densities of the fluids even when the internal pressure is equal to the external pressure. A combination of water inflow and oil outflow through the opening may occur. Analytical calculations and time-domain simulations are applied to calculate the volume of oil outflow and the outflow rate versus time. Good agreement is obtained between the analytic model and the CFD analysis of the oil spill.

Brief paper: Ensuring mooring line integrity by dynamic positioning: Controller design and experimental tests

This paper addresses the dynamic positioning of surface vessels moored to the seabed via a turret based spread mooring system, an operation referred to as the position mooring. While the mooring system keeps the surface vessel in place most of the time, thruster assistance is needed in severe weather conditions to avoid mooring line failure. Traditionally, this is done by keeping the vessel within a predefined geographical region. We present a conceptually new controller for position mooring operations. By using a structural reliability measure for the mooring lines, the new controller protects the mooring system whenever needed as a result of severe weather conditions and high environmental loads. This is done by maintaining the probability of mooring line failure below a preset value. In particular, the excessive use of thrusters caused by conservatively defined safety regions in conventional PM systems is avoided, giving a fuel optimal operation. The feasibility of our controller is successfully verified in laboratory experiments.

Analysis Guidelines and Application of a Riser-Soil Interaction Model Including Trench Effects

A general overview of design aspects related to metal catenary risers is first given. Response characteristics of these types of riser configurations are considered. It is subsequently focused on models for soil-pipe interaction, which have a strong influence on computed riser stresses in the touch-down region. The so-called CARISIMA models for riser-soil interaction in relation to horizontal and vertical motion (i.e.horizontal resistance and suction) are presented. Inherent limitations of the models are summarised. Basic input parameters are described, considerations relevant for establishment of initial trench profile are given, and examples of riser response analysis are presented. Typical behaviour of the numerical models which is observed in the example analyses is also reported.Copyright

Analytical Prediction of Oil Spill From Grounded Cargo Tankers

The objective of this paper is to study the oil spillage process which corresponds to various tank designs. For this purpose, analytical calculations and time domain simulations were applied. Numerical simulations have been performed to verify the models. Good agreement has been found between the results obtained from the simplified analytic model and those from the CFD calculations for predicting the oil spill from different hull designs. The volume and efflux rates of oil that are based on the suggested model are comparable to those obtained by means of numerical simulations.Copyright

System Reliability Analysis by Monte Carlo Based Method and Finite Element Structural Models

In principle, the reliability of complex structural systems can be accurately predicted by Monte Carlo simulation. This method has several attractive features for structural system reliability, the most important being that the system failure criterion is usually relatively easy to check almost irrespective of the complexity of the system. However, the computational cost involved in the simulation may be prohibitive for highly reliable structural systems. In this paper a new Monte Carlo based method recently proposed for system reliability estimation that aims at reducing the computational cost is applied. It has been shown that the method provides good estimates for the system failure probability with reduced computational cost. In a numerical example the usefulness and efficiency of the method to estimate the reliability of a system represented by a nonlinear finite element structural model is presented. To reduce the computational cost involved in the nonlinear finite element analysis the method is combined with a response surface model. [DOI: 10.1115/1.4025871]

Statistical Change Detection for Diagnosis of Buoyancy Element Defects on Moored Floating Vessels

Abstract Floating platforms with mooring systems are used extensively in off-shore operations. Part of the mooring systems are underwater buoyancy elements that are attached to the mooring lines. Loss or damage of a buoyancy element is invisible but changes the characteristics of the mooring system and alters its ability to provide the necessary responses to withstand loads from weather. Damage of a buoyancy element increases the operation risk and could even cause abortion during an oil-offloading. The objective of this paper is to diagnose the loss of a buoyancy element using diagnostic methods. After residual generation, statistical change detection scheme is derived from mathematical models supported by experimental data. To experimentally verify loss of an underwater buoyancy element, an underwater line breaker is designed to create realistic replication of abrupt faults. The paper analyses the properties of residuals and suggests a dedicated GLRT change detector based on a vector residual. Special attention is paid to threshold selection for non ideal (non-IID) test statistics.

Dynamic Positioning of Moored Vessels Based on Structural Reliability

This paper addresses dynamic positioning of surface vessels moored to the seabed via a spread mooring system, referred to as position mooring. In normal weather conditions, the mooring system constrains the vessel and the controller applies thruster force for motion damping and heading control, only. In harsh weather, however, the mooring system becomes inadequate and thruster force is also needed for positioning in order to avoid damage to the mooring lines. While traditional position mooring systems apply thruster force based on constraining the vessel to lie within a predefined geographical region, the controller presented in this paper instead employs structural reliability measures to restrict movement. These structural reliability measures become an intrinsic part of the controller, automatically adjusting the allowed geographical region based on current weather conditions and structural properties of the mooring lines