Active truncation of slender marine structures: Influence of thecontrol system on Fidelity

Performing hydrodynamic model testing of ultra-deep water floating systems at a reasonable scale ischallenging, due to the limited space available in existing laboratories and to the large spatial extent ofthe slender marine structures that connect the floater to the seabed. In this paper, we consider a methodbased on real-time hybrid model testing, namely the active truncation of the slender marine structures:while their upper part is modelled physically in an ocean basin, their lower part is simulated by anadequate numerical model. The control system connecting the two substructures inevitably introducesartefacts, such as noise, biases and time delays, whose probabilistic description is assumed to be known.We investigate specifically how these artefacts influence the fidelity of the active truncation setup, thatis its capability to reproduce correctly the dynamic behaviour of the system under study. We propose asystematic numerical method to rank the artefacts according to their influence on the fidelity of the test.The method is demonstrated on the active truncation of a taut polyester mooring line.acceptedVersio

HYPSO-1 CubeSat: First Images and In-Orbit Characterization

The HYPSO-1 satellite, a 6U CubeSat carrying a hyperspectral imager, was launched on 13 January 2022, with the Goal of imaging ocean color in support of marine research. This article describes the development and current status of the mission and payload operations, including examples of agile planning, captures with low revisit time and time series acquired during a campaign. The in-orbit performance of the hyperspectral instrument is also characterized. The usable spectral range of the instrument is in the range of 430 nm to 800 nm over 120 bands after binning during nominal captures. The spatial resolvability is found empirically to be below 2.2 pixels in terms of Full-Width at Half-Maximum (FWHM) at 565 nm. This measure corresponds to an inherent ground resolvable resolution of 142 m across-track for close to nadir capture. In the across-track direction, there are 1216 pixels available, which gives a swath width of 70 km. However, the 684 center pixels are used for nominal captures. With the nominal pixels used in the across-track direction, the nadir swath-width is 40 km. The spectral resolution in terms of FWHM is estimated to be close to 5 nm at the center wavelength of 600 nm, and the Signal-to-Noise Ratio (SNR) is evaluated to be greater than 300 at 450 nm to 500 nm for Top-of-Atmosphere (ToA) signals. Examples of images from the first months of operations are also shown.publishedVersio

Real-Time Hybrid Model Testing of a Top Tensioned Riser: a Numerical Case Study on Interface Time-Delays and Truncation Ratio

This paper investigates the applicability of real-time hybrid model testing (ReaTHM testing) to the study of offshore systems in deep water. The focus is in particular on slender marine structures connecting floating structures to the seabed, and on how they could be truncated so that a model test setup at a reasonable scale could fit existing hydrodynamic laboratory infrastructures. In this context, ReaTHM testing consists in “substructuring” the slender structures in two parts. At the lower part of the water column, the first substructure is numerical, simulated using a nonlinear finite element method. On the upper part of the water column, the other substructure is physically modelled in an ocean basin. Both substructures interact in real-time through a set of sensors and actuators. This paper addresses through a case study the important issue of accuracy of ReaTHM testing, that is how the behavior of the substructured system varies from that of the emulated system. A top-tensioned riser in 1200m water depth is considered, with two truncation locations: 240m and 600m below the free surface. It is assumed that an artefact is introduced by the ReaTHM test setup, namely a time delay induced by e.g. the numerical calculations, or the actuation system. It is first shown how this artefact influences the accuracy of the setup, and then how the truncation ratio influences the tolerance of the ReaTHM test setup to such an artefact.acceptedVersio

Integration Filter for APS, DVL, IMU and Pressure Gauge for Underwater Vehicles

High-accuracy underwater navigation is important in order to automate motion control of remotely operated vehicles (ROVs). An observer that estimates the vehicle states (position, velocity, attitude and turn rates) is proposed for closed-loop control. Measurements from an acoustic positioning system (APS), a Doppler velocity log (DVL), an inertial measurement unit (IMU) and a pressure gauge (PG) are used in the proposed observer. The observer is divided into an attitude observer and a translational motion observer with interconnections. The attitude observer is an explicit complementary filter (ECF). Results from simulation are presented

Detection of mooring line failures using Dynamic Hypothesis Testing

This article proposes a novel methodology for the detection of mooring line breakage in thruster assisted position mooring (PM) systems, when no measurements of the tensions on the mooring lines are available. For dynamic positioning (DP) of marine vessels moored to the seabed via a turret-based spread mooring system, thrusters provide only complementary assistance to the mooring system, which is responsible for generating a large part of the forces and moments required for station keeping. However, in extreme weather conditions thruster assistance is essential to avoid mooring line failure. Once a mooring line is parted, the remaining lines must withstand an increase in the tension forces required to compensate for the lost tension in the ruptured line. This in turn may lead to a cascade breakage of the mooring lines. Hence, it is of paramount importance to detect any line breakage as soon as it occurs to compensate for the lost tension by proper use of DP thruster assistance. As a contribution to solving this problem, in this paper we propose a methodology that builds on Dynamic Hypothesis Testing (DHT) whereby a set of hypotheses are assessed, at each sampling time, using the measured inputs and outputs of the thruster assisted position mooring system. While the first hypothesis corresponds to the assumption that all mooring lines are intact, the remaining hypotheses are built assuming that a single, or multiple line breakage events have taken place. At each sampling time, the inputs and outputs to the system are used to generate the conditional probability of each hypothesis being true. The conditional probabilities are then used to evaluate which hypothesis is more probable to be compatible with the collected measurements. In addition, we find conditions for any pair of hypothesis to be distinguishable. Numerical simulations, carried out using a high fidelity nonlinear PM simulator, illustrate the efficiency of the proposed methodology.Detection of mooring line failures using Dynamic Hypothesis TestingThis work was supported in part by FCT [UID/EEA/50009/2013] and the European Commission under the H2020-ICT-2014 WiMUST Project (Grant Agreement No. 645141) and was the result of a collaborative effort between ISR/IST and AMOS; the Norwegian Research Council is acknowledged as the main sponsor of AMOS.acceptedVersio

Experimental study of current forces and deformations on a half ellipsoidal closed flexible fish cage

Closed flexible fish cages are proposed as a new concept in marine aquaculture, replacing the conventional net cages in order to meet ecological challenges related to fish lice and escapes. It is important to understand the response of the cage exposed to current loads. Then more knowledge about forces and deformations on the closed flexible fish cage for different filling levels is needed. A scaled model of a closed flexible fish cage shaped like a half ellipsoid was tested in a towing-tank. Global drag forces and bag deformations were measured for four different filling levels between 70% and 100%, and steady current velocities between 0.04 m/s and 0.22 m/s in model scale, corresponding to Reynolds numbers in the range Re=3–17×104Re=3–17×104. Findings from the experiments showed that the drag force increased for decreasing filling levels. This increase was caused by a large deformation of the front of the bag affecting the drag coefficient.Norges forskningsrådacceptedVersio

Adaptive Wave Filtering for Dynamic Positioning of Marine Vessels Using Maximum Likelihood Identification: Theory and Experiments

This paper addresses a filtering problem that arises in the design of dynamic positioning systems for ships and offshore rigs subjected to the influence of sea waves. The dynamic model of the vessel captures explicitly the sea state as an uncertain parameter. The proposed adaptive wave filter borrows from maximum likelihood identification techniques. The general form of the logarithmic likelihood function is derived and the dominant wave frequency (the uncertain parameter) is identified by maximizing this function. To this effect, a bank of Kalman filters is used to evaluate the log-likelihood function for different values of the uncertain parameter. After each identification step a new set of Kalman filters is designed to estimate the dominant wave frequency with better accuracy. The proposed sea state identification technique enables adaptive Wave Filtering (WF) and Dynamic Positioning (DP) systems to operate in different operational conditions and hence, it is a step forward to the development of a so-called all-year marine estimation and control system. The results are experimentally verified by model testing a DP operated ship, the Cybership III, under different sea conditions in a towing tank equipped with a hydraulic wave maker

Adaptive Wave Filtering for Dynamic Positioning of Marine Vessels Using Maximum Likelihood Identification: Theory and Experiments

This paper addresses a filtering problem that arises in the design of dynamic positioning systems for ships and offshore rigs subjected to the influence of sea waves. The dynamic model of the vessel captures explicitly the sea state as an uncertain parameter. The proposed adaptive wave filter borrows from maximum likelihood identification techniques. The general form of the logarithmic likelihood function is derived and the dominant wave frequency (the uncertain parameter) is identified by maximizing this function. To this effect, a bank of Kalman filters is used to evaluate the log-likelihood function for different values of the uncertain parameter. After each identification step a new set of Kalman filters is designed to estimate the dominant wave frequency with better accuracy. The proposed sea state identification technique enables adaptive Wave Filtering (WF) and Dynamic Positioning (DP) systems to operate in different operational conditions and hence, it is a step forward to the development of a so-called all-year marine estimation and control system. The results are experimentally verified by model testing a DP operated ship, the Cybership III, under different sea conditions in a towing tank equipped with a hydraulic wave maker.acceptedVersio

A novel methodology for robust dynamic positioning of marine vessels: Theory and experiments

The paper describes a novel robust adaptive controller for Dynamic Positioning (DP) of marine vessels. The proposed Robust Multiple Model Adaptive Dynamic Positioning (RMMADP) structure consists of a bank of robust controllers designed using the Mixed-μ methodology and an identification unit. The latter is composed by a bank of (steady-state) Kalman filters (KFs) that generate online the output estimation errors (residuals) that are used to generate appropriate monitoring signals. At each sampling time, the monitoring signals are assessed to decide which controller should be selected from the bank of the controllers. The proposed adaptive structure of the RMMADP enables the DP system to operate in different operational conditions and hence, it is a step forward to a so-called all-year marine DP system. Numerical simulations, carried out with a high fidelity nonlinear DP simulator, illustrate the efficacy of the RMMADP techniques proposed. To bridge the gap between theory and practice, the results are experimentally verified by model testing a DP operated ship, the Cybership III, under different sea conditions in a towing tank equipped with a hydraulic wave maker

Controller Analysis in Real-Time Hybrid Model Testing of an Offshore Floating System

This paper presents an experimental study using Real-Time Hybrid Model (ReaTHM) testing of a moored floating cylindrical buoy, conducted in a wave basin. ReaTHM testing is a method for studying the dynamics of marine structures by partitioning the system into numerical and physical substructures that are then coupled in real-time using a control system. In this study, the floating cylinder buoy is modelled physically, and the mooring system modelled numerically. In this paper, the effect of selected controller parameters on the performance of the control system is studied, for both wave frequency and low-frequency ranges. The architecture/design of the control system is presented in the first part of the paper, while results from experimental tests with wave excitation on the physical substructure are presented in the second part of the paper.acceptedVersio