On Efficient Surrogate Model Development for the Prediction of the Long-Term Extreme Response of a Moored Floating Structure

2020 ◽  
Vol 143 (1) ◽  
Author(s):  
HyeongUk Lim ◽  
Lance Manuel ◽  
Ying Min Low
Keyword(s):  
Wave Train ◽  
Offshore Structures ◽  
Surrogate Models ◽  
Offshore Structure ◽  
Sea State ◽  
Peak Period ◽  
Frequency Wave ◽  
Extreme Response ◽  
Term Response

Abstract This study focuses on the development of efficient surrogate models by polynomial chaos expansion (PCE) for the prediction of the long-term extreme surge motion of a moored floating offshore structure. The structure is subjected to first-order and second-order (difference-frequency) wave loading. Uncertainty in the long-term response arises from contrasting sea state conditions, characterized by significant wave height, Hs, and spectral peak period, Tp, and their relative likelihood of occurrence; these two variables are explicitly included in the PCE-based uncertainty quantification (UQ). In a given sea state, however, response simulations must be run for the associated Hs and Tp; in such simulations, typically, a set of random amplitudes and phases define an irregular wave train consistent with that sea state. These random amplitudes and phases for all the frequency components in the wave train introduce additional uncertainty in the simulated waves and in the response. The UQ framework treats these two sources of uncertainty—from Hs and Tp on the one hand, and the amplitude and phase vectors on the other—in a manner that efficiently yields long-term surge motion extreme predictions consistent with more expensive Monte Carlo simulations (MCS) that serve as the “truth” system. To reduce uncertainty in response extremes that result from sea states with a low likelihood of occurrence, importance sampling is employed with both MCS- and PCE-based extreme response predictions. Satisfactory performance with such efficient surrogate models can help in assessing the long-term response of various offshore structures.

On Efficient Long-Term Extreme Response Estimation for a Moored Floating Structure

2018 ◽  
Author(s):  
HyeongUk Lim ◽  
Lance Manuel ◽  
Ying Min Low
Keyword(s):  
Wave Train ◽  
Offshore Structures ◽  
Offshore Structure ◽  
Floating Structure ◽  
Sea State ◽  
Peak Period ◽  
Frequency Wave ◽  
Extreme Response ◽  
Term Response

This study investigates the use of efficient surrogate model development with the help of polynomial chaos expansion (PCE) for the prediction of the long-term extreme surge motion of a simple moored offshore structure. The structure is subjected to first-order and second-order (difference-frequency) wave loading. Uncertainty in the long-term response results from the contrasting sea state conditions, characterized by significant wave height, Hs, and spectral peak period, Tp, and their relative likelihood of occurrence; these two variables are explicitly included in the PCE-based uncertainty quantification (UQ). In a given sea state, however, response simulations must be run for any sampled Hs and Tp; in such simulations, typically, a set of random phases (and deterministic amplitudes) define a wave train consistent with the defined sea state. These random phases for all the frequency components in the wave train introduce additional uncertainty in the simulated waves and in the response. The UQ framework treats these two sources of uncertainty — from Hs and Tp on the one hand, and the phase vector on the other — in a nested manner that is shown to efficiently yield long-term surge motion extreme predictions consistent with more expensive Monte Carlo simulations, which serve as the truth system. Success with the method suggests that similar inexpensive surrogate models may be developed for assessing the long-term response of various offshore structures.

A Discussion of Long Term Response Versus Mean Maximum Response of the Selected Design Sea State

2007 ◽  
Author(s):  
S. Haver
Keyword(s):  
Offshore Structures ◽  
Wave Period ◽  
Sea State ◽  
Safety Level ◽  
Characteristic Response ◽  
Annual Probability ◽  
Extreme Response ◽  
Quadratic Response ◽  
Term Response

The overall aim of the design process is to ensure that a structure can withstand with sufficient margin all foreseen load events. Offshore rules and regulations will typically define a set of rules to be followed by the designer. By fulfilling these it is tacitly assumed that the aimed safety level is achieved. The governing regulations and rules and, not the least, their interpretation may vary between various offshore regions. This may be the case both for the load side of the problem and the strength side of the problem. In connection with design of offshore structures, the largest variability and uncertainty is typically associated with the load side of the problem. The purpose of this paper is to discuss the consequences of two rather common definitions regarding the characteristic environmental load to be used for design purposes: i) The characteristic response is taken as the load for which the annual probability of exceeding it is q (here referred to as the q-probability load), ii) The characteristic response is taken as the mean extreme response of a sea state for which the annual probability of exceeding it is q (q-probability sea state). Results are shown for some generic response cases varying from a linear response case being rather insensitive to wave period to a quadratic response problem associated with a critical wave period band.

Auto-control model building using machine learning regression for extreme response prediction

2020 ◽  
Vol 60 (1) ◽  
pp. 155
Author(s):  
Darrell Leong ◽  
Anand Bahuguni
Keyword(s):  
Machine Learning ◽  
Model Building ◽  
Response Prediction ◽  
Offshore Structures ◽  
Practical Implementation ◽  
Monte Carlo Data ◽  
Sea State ◽  
Extreme Load ◽  
Extreme Response

The long-term forecast of extreme response presents a daunting practical problem for offshore structures. These installations are subject to varying sea conditions, which amplify the need to account for the uncertainties of wave heights and periods across a given sea state. Analysis of each sea state involves numerically intensive non-linear dynamic analysis, leading to massive computational expense across the environmental scatter diagram. Recent research has proposed several effective solutions to predict long-term extreme responses, but not without drawbacks, such as the limitation to specific failure locations and the absence of error estimates. This paper explores the practical implementation of control variates as an efficiency enhancing post-processing technique. The model building framework exhibits the advantage of being fully defined from existing simulation results, without the need for external inputs to set up a control experiment. A composite machine learning regression model is developed and investigated for performance in correlating against Monte Carlo data. The sampling methodology presented possesses a crucial advantage of being independent of failure characteristics, allowing for the concurrent extreme response analyses of multiple components across the global structure without the need for re-analysis. The approach is applied on a simulated floating production storage and offloading unit in a site located in the hurricane-prone Gulf of Mexico, vulnerable to heavy-tailed extreme load events.

Efficient Analysis of a Catenary Riser

2006 ◽  
Author(s):  
Elizabeth Passano ◽  
Carl M. Larsen
Keyword(s):  
Operating Conditions ◽  
Stochastic Simulations ◽  
Worst Case ◽  
Nonlinear Structure ◽  
Tension Response ◽  
Extreme Response ◽  
Analysis Strategies ◽  
Low Tension ◽  
Term Response

The paper deals with the challenge of predicting the extreme response of catenary risers, a topic of both industry and academic interest. Large heave motions introduced at the upper end of a catenary riser can lead to compression and large bending moments in the region immediately above the touch down area. In the worst case, dynamic beam buckling may occur. The focus of the paper will be on understanding the riser behaviour in extreme, low-tension response and in establishing suitable analysis strategies to predict the extreme response. Results from long nonlinear stochastic simulations of many sea states with varying environmental and operating conditions may be combined to describe the long-term response of a nonlinear structure such as a catenary riser. However, this theoretically straight-forward approach is very demanding computationally and ways to limit the extent of nonlinear stochastic simulations are therefore sought. The usefulness of simpler methods such as regular wave analysis to improve understanding of the physical behaviour and to aid in concentrating the nonlinear simulations to where they are most useful, will be demonstrated.

Downtime Analysis Techniques for Complex Offshore and Dredging Operations

2004 ◽  
Author(s):  
Remmelt J. van der Wal ◽  
Gerrit de Boer
Keyword(s):  
Wind Waves ◽  
Weather Forecast ◽  
Offshore Structures ◽  
Operational Mode ◽  
Offshore Structure ◽  
Hydrodynamic Models ◽  
Time Step ◽  
Made In

Offshore operations in open seas may be seriously affected by the weather. This can lead to a downtime during these operations. The question whether an offshore structure or dredger is able to operate in wind, waves and current is defined as “workability”. In recent decades improvements have been made in the hydrodynamic modelling of offshore structures and dredgers. However, the coupling of these hydrodynamic models with methods to analyse the actual workability for a given offshore operation is less developed. The present paper focuses on techniques to determine the workability (or downtime) in an accurate manner. Two different methods of determining the downtime are described in the paper. The first method is widely used in the industry: prediction of downtime on basis of wave scatter diagrams. The second method is less common but results in a much more reliable downtime estimate: determination of the ‘job duration’ on basis of scenario simulations. The analysis using wave scatter diagrams is simple: the downtime is expressed as a percentage of the time (occurrences) that a certain operation can not be carried out. This method can also be used for a combination of operations however using this approach does not take into account critical events. This can lead to a significant underprediction of the downtime. For the determination of the downtime on basis of scenario simulations long term seastate time records are used. By checking for each subsequent time step which operational mode is applicable and if this mode can be carried out the workability is determined. Past events and weather forecast are taken into account. The two different methods are compared and discussed for a simplified offloading operation from a Catenary Anchor Leg Mooring (CALM) buoy. The differences between the methods will be presented and recommendations for further applications are given.

Reliability of TLP Tendons Under Storm Sea States

2005 ◽  
Author(s):  
Federico Barranco Cicilia ◽  
Edison Castro Prates de Lima ◽  
Lui´s Volnei Sudati Sagrilo
Keyword(s):  
Reliability Analysis ◽  
Limit State ◽  
Nonlinear Dynamic Analysis ◽  
Analytical Models ◽  
Cumulative Probability ◽  
Wave Forces ◽  
Ultimate Limit State ◽  
Sea State ◽  
Extreme Response

This paper presents a methodology for reliability analysis of Tension Leg Platform (TLP) tendons subjected to extraordinary sea state conditions like hurricanes or winter storms. A coupled approach in time domain is used to carry out TLP random nonlinear dynamic analysis including wind, current and first and second order wave forces. The tendons Ultimate Limit State (ULS) condition is evaluated by an Interaction Ratio (IR) taking into account dynamic combination among tension, bending and hydrostatic pressure. Expected long-term extreme IR is obtained through the integration of cumulative probability functions (CPFs) fitted to response maxima associated to individual short term sea states. The reliability analysis is performed using a time-integrated scheme including uncertainties in loads, tendon strength, and analytical models. Failure probabilities for the most loaded tendon of a TLP in Campeche Bay, Mexico, considering a 100-yr design sea state and the 100-yr extreme response generated by long-term observed storms are compared.

The Impact of Climate Change on the Long-Term Response of Offshore Structures: A Study Case

2019 ◽  
Author(s):  
Irvin Alberto Mosquera ◽  
Luis Volnei Sudati Sagrilo ◽  
Paulo Maurício Videiro
Keyword(s):  
Climate Change ◽  
Global Climate ◽  
Global Climate Model ◽  
Offshore Structures ◽  
Global Climate Models ◽  
Wave Parameter ◽  
Greenhouse Emissions ◽  
The Impact ◽  
Term Response

Abstract This paper discusses the influence of the climate change in the long-term response of offshore structures. The case studied is a linear single-degree-of-freedom (SDOF) system under environmental load wave characterized by the JONSWAP spectrum. The wave parameter data used in the analyses were obtained from running the wind wave WaveWatch III with wind field input data derived from two Global Climate Models (GCMs): HadGEM2-ES and MRI-CGCM3 considering historical and future greenhouse emissions scenarios. The study was carried out for two locations: one in the North Atlantic and the other in Brazilian South East Coast. Environmental contours have been used to estimate the extreme long-term response. The results suggest that climate change would affect the structure response and its impact is highly depend on the structure location, the global climate model and the greenhouse emissions scenario selected.

A Monte Carlo Approach to Prediction of Extreme Response Statistics of Drag Dominated Offshore Structures

2007 ◽  
Author(s):  
A. Naess ◽  
O. Gaidai
Keyword(s):  
Monte Carlo ◽  
Weather Conditions ◽  
Offshore Structures ◽  
Drag Forces ◽  
Wave Elevation ◽  
The Monte Carlo Method ◽  
Strong Current ◽  
Highly Nonlinear ◽  
Extreme Response

The focus of the present paper is the extreme response statistics of drag dominated offshore structures subjected to harsh weather conditions. More specifically, severe sea states both with and without strong current are considered. The nature of the hydrodynamic forces acting on the structure becomes highly nonlinear. Additionally to the drag forces, the so called inundation effect due to the wave elevation, corrected to include second order waves, is also taken into account. In the present paper the Monte Carlo method along with a special extrapolation technique is applied. The proposed method opens up the possibility to predict simply and efficiently long-term extreme response statistics, which is an important issue for the offshore structures design.

Application of the Contour Line Method for Estimating Extreme Response in Mooring Lines of a Two-Body Floating Wave Energy Converter

2012 ◽  
Author(s):  
Made Jaya Muliawan ◽  
Zhen Gao ◽  
Torgeir Moan
Keyword(s):  
Oil And Gas ◽  
Oil And Gas Industry ◽  
Contour Line ◽  
Mooring Line ◽  
Sea State ◽  
Gas Industry ◽  
Line Method ◽  
Extreme Response ◽  
Two Bodies ◽  
Term Response

One of design criteria that have been used for the mooring system design for floating platforms in the oil and gas industry is the Ultimate Limit State (ULS). The 100-year level response in the mooring line should be applied for this ULS design check, which is ideally estimated by taking into account the dynamic mooring line tension in all sea state available in the operational site. This is called a full long-term response analysis using the all sea state approach. However, this approach is time consuming. Therefore, it is proposed to use the contour line method to estimate the 100-year response by primarily studying the short term response for the most unfavorable sea states along the 100-year environmental contour line. Experiences in the oil and gas industry confirmed that the method could give good prediction if the response at higher percentile than the median is used. In this paper, the mooring system of a two-body wave energy converter (WEC) is considered. Since this system involves interaction between two bodies, the estimation of the ULS level response by using the all sea state approach will be even more time consuming. Therefore, the application of the contour line method for this case will certainly be beneficial. However, its feasibility for a WEC case needs to be documented first. In the present paper, the ULS level response in the mooring tension that is predicted by the contour line method is compared with that estimated by taking into account all sea states. It is achieved by performing the coupled time domain mooring analyses using SIMO/Riflex for six cases with different mooring configurations and connections between two bodies. An axi-symmetric Wavebob-type WEC is chosen as the object of investigation and the Yeu site in France is assumed to be the operational site of the WEC. Hydrodynamic loads including 2nd order forces are determined using WAMIT. Finally, the applicability of the contour line method to predict the ULS level mooring tension for a two-body WEC is assessed and shown to yield accurate results with proper choice of percentile level for the extreme response.

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