Reliability Prediction for Offshore Renewable Energy: Data Driven Insights

Accurately quantifying and assessing the reliability of Offshore Renewable Energy (ORE) devices is critical for the successful commercialisation of the industry. At present, due to the nascent stage of the industry and commercial sensitivities there is very little available reliability field data. This presents an issue: how can the reliability of ORE’s be accurately assessed and predicted with a lack of specific reliability data? ORE devices largely rely on the assessment of surrogate data sources for their reliability assessment. To date there are very few published studies that empirically assess the failure rates of offshore renewable energy devices [1]. The applicability of surrogate data sources to the ORE environment is critical and needs to be more thoroughly evaluated for a robust ORE device reliability assessment. This paper tests two commonly held assumptions used in the reliability assessment of ORE devices. Firstly, the constant failure rate assumption that underpins ORE component failure rate estimations is addressed. Secondly, a model that is often used to assess the reliability of onshore wind components, the Non-Homogeneous Poisson Power Law Process (PLP) model is empirically assessed and trend tested to determine its suitability for use in ORE reliability prediction. This paper suggests that pitch systems, generators and frequency converters cannot be considered to have constant failure rates when analysed via nonrepairable methods. Thus, when performing a reliability assessment of an ORE device using non-repairable surrogate data it cannot always be assumed that these components will exhibit random failures. Secondly, this paper suggests when using repairable system methods, the PLP model is not always accurate at describing the failure behaviour of onshore wind pitch systems, generators and frequency converters whether they are assessed as groups of turbines or individually. Thus, when performing a reliability assessment of an ORE device using repairable surrogate data both model choice and assumptions should be carefully considered.

On Maintainability of Winterised Plants Operating in Arctic Regions

In Arctic regions, oil and gas (O&G) operations are adversely affected by harsh weather conditions and severe meteorological phenomena such as icing storms and, in certain regions, polar low pressures. Potential solutions, such as implementing winterisation concepts, are explored in the design and even operation phases in order to overcome such obstacles. Simply, the main aim of winterisation is to provide the crew and equipment units with a range of normal environmental and working conditions through, for instance, insulating equipment units, installing heat tracers, enclosing working areas, providing the crew with adequate clothing, etc. There are, however, some concerns about the efficiency of such winterisation measures and potential changes in operation risk level, of which the changes in plant downtime, production loss, and plant maintainability are the focus of present study. The issue of complex effects of winterisation measures on maintainability analysis of O&G plants operating in the Arctic offshore has gained little attention in the literature. In this study, different aspects of winterisation from the viewpoint of equipment maintainability are discussed. Further, a mathematical framework for maintainability analysis of equipment units subjected to winterisation measures is proposed. The impact of winterisation-related downtimes on plant downtime is analysed as well by employing a Monte Carlo system simulation technique. The application of the proposed framework is illustrated by a case study. The results are further compared with those for a non-winterised system designed for normal-climate regions.

Applications of the Static Condensation Technique to Nonlinear Structural Analysis of Floating Offshore Structures

A cost effective finite element (FE) procedure is proposed for analysis of load-carrying structures with nonlinear contact and frictional behaviors between large floating offshore structures. The key of the procedure is to use the static condensation technique developed from the Guyan model reduction method. The time for computing contact and friction forces on contact interface areas can be dramatically reduced compared to nonlinear analysis with a full FE model. Two representative applications to offshore projects are presented. One is a problem with nonlinear contact of independent tank support in FLNG hull structures and the second is a sea-fastening system used during offshore dry transportation. The reliability and computing efficiency of the proposed analysis procedure are investigated. It is conclusively confirmed that the proposed procedure is practical for application to actual design of offshore projects.

Extreme Response Prediction of Steel Risers Using a Four Parameter Distribution

Recently, a four-parameter distribution known as the shifted generalized lognormal distribution (SGLD) has been presented in the literature. One of its main advantages is that it covers regions of skewness-kurtosis not covered by other distributions of common use in engineering. In this paper, the performance of this distribution is evaluated in the extreme values’ estimation of the utilization ratios of steel riser sections. Three alternatives for using SGLD are investigated in two case studies of different dynamic behavior. The first one is a SLWR (steel-lazy wave riser) connected to a turret-moored FPSO in 914m water depth, and the second is a SLWR connected to a spread-mooring FPSO in a water depth of 1400m. The results obtained by the SGLD-based analysis, which considered several simulation lengths, are compared to those obtained by means of an extreme value distribution fitted to episodical extremes obtained from many distinct realizations. The results of a traditional Weibull-fitting approach to the response peaks and those obtained with and Hermite transformation-based model are also presented for comparison.

A Fundamental Study on the Dynamic Response of Hull Girder of Container Ships due to Slamming Load

The purpose of the present study is to fundamentally investigate dynamic hull girder response due to slamming load. A series of time domain FE-simulation is carried out using a non-uniform finite element beam model of a 8000 TEU container ship where slamming load is applied at the bottom of the bow. The ship is modeled by elaso-plastic material with equivalent ultimate strength and strain rate effect is considered. Hull-girder vertical bending moment as well as deformation modes, bending stress are investigated by varying the time duration of the slamming load which is modeled by sinusoidal impulse. In order to obtain post vibration after the first slamming load explicit analysis is adopted instead of implicit analysis with considering gravity and buoyancy. Buoyancy is modeled by inelastic spring elements. It is found from the present study hull girder vertical bending moment is dependent on time duration of slamming load. Especially if time duration is smaller than natural period response bending moment may become smaller than applied bending moment. Moreover effect of inertia at fore and aft is also investigated in detail.

Risk-Based Multi-Objective Optimisation of a Monopile Offshore Wind Turbine Support Structure

The present work carries out a multi-objective design optimization of a monopile offshore wind turbine support structure. Three objective functions are created related to the minimization of the total construction cost of the monopile support structure, fatigue damage, and permissible stress ratio. The construction cost takes into account the costs associated with welding and labor. The constructional limitations in the offshore industry take into consideration in the selection of the upper boundaries of the design variables. The reliability index is employed to identify the topology of the structure as a part of the Pareto frontier solution in reducing the failure probability for the critical limit states and satisfying the target reliability level. A risk-based assessment of the optimal designs is performed and the output is used to update the life-cycle cost assessment. The ultimate optimization target is deemed to be the minimization of the levelised cost of energy, which is estimated based on the discounted cash-flow method considering the life-cycle costs constituting CAPEX and OPEX.

Transverse Deformation of Pressurised Pipes With Different Axial Loads

Pipelines residing on the seabed are exposed to various hazards, one of them being denting, hooking and release of the pipeline by e.g. anchors or trawl gear. As a pipeline is displaced transversely in a hooking event, an axial tensile load resisting the displacement builds up in the pipeline. This study examines the effect of applying three different axial loads (zero, constant, and linearly increasing) to a pipe while simultaneously deforming it transversely. A fairly sharp indenter conforming to the prevailing design codes was used to deform the pipes. These three tests were repeated with an internal pressure of about 100 bar for comparison. Adding an axial load appeared to increase the pipe’s stiffness in terms of the force-displacement curve arising from deforming the pipe transversely. The internal pressure also increased the stiffness, and produced a more local dent in the pipe compared with the unpressurised pipes. All tests were recreated numerically in finite element simulations. Generally, the results of the simulations were in good agreement with the experiments.

Ultimate Strength Assessment of Semi-Submersible Platform Under Different Load Conditions

The nonlinear finite-element method has been widely used in evaluating the ultimate strength of stiffened plates and part of hull girders, considering the effect of boundary conditions, geometrical initial imperfection and welding-induced residual stress in recent years. However, available research on the ultimate strength of large-sized structures, especially of semi-submersible platform is limited. New large-sized semi-submersible platform has been designed with lateral brace structure and square cross-section columns. The investigation of ultimate strength of the whole structure is of paramount importance in assessing the safety and design of such large structure. Therefore, in this paper, a three-dimensional nonlinear finite element model was developed to investigate the ultimate strength of a new generation of semi-submersible platform under different load conditions and its behavior after collapse using explicit dynamic solvers. Results showed that the time dependent dynamic explicit method was reliable and feasible for the calculation of ultimate strength of such complicated structure. For the target platform, the bracings and upper hull structure were the main bearing component and were critical for the ultimate strength of the whole structure. High stress occurred in connection areas and special attention shall be paid for.

Investigation of Ultimate Limit State Safety Margins in the Structural Design Rules

A study to document the Ultimate Limit State (ULS) safety margins built into the DNV GL rules for Bulk and Tanker is presented. Critical structural members were identified together with the load level at which these members start to develop permanent buckling sets exceeding normal fabrication tolerances. These critical load levels are then compared with the local ULS rule strength limits in order to have a measure for the structural safety margins and hull redundancy. Non-linear finite element (NFLE) analyses were performed to estimate the structural response for different focus areas (critical structural members). Typically, critical members in bottom, deck, transverse bulkhead and hopper were chosen. Cargo hold models were developed both with linear finite elements, [1,2] and non-linear finite elements, [3]. In the non-linear FE analysis, the structural safety factor for ULS was defined as the load level giving permanent plastic deformation equal to the permissible distortion (production tolerances) for structural members. The non-linear FE results were compared with the maximum permissible load level with respect to buckling and yielding according to DNV GL Ship rules [1] inclusive Common Structural Rules for BC&OT (CSR) [4]. The structural safety factor shows a typical value of 1.2–1.4, and for most cases the plate is governing dimensioning structural member. This study has identified significant structural safety margins, typically 20–40% above rule acceptance level for typical highly utilized local areas in Bulk and Tanker hulls. It is to be noted that global Hull Girder Capacity is not addressed in present paper.

Ultimate Bearing Capacity Assessment of Hull Girder With Asymmetric Cross-Section

The objective of this study is to investigate the variation of neutral axis of ship hull girder due to asymmetric geometry or asymmetric load, and its influence on the ultimate strength of hull girder. In order to account for asymmetric geometries and loads of hull girders, the force equilibrium and force-vector equilibrium criteria together with a minimum convergence factors (error) method, are employed to determine the translation and rotation of neutral axis plane of symmetric or asymmetric hull cross-section in the application of Smith’s method at each step of curvature of the hull girder. The ultimate strengths of Dow’s 1/3 frigate model with three predefined structural integrity states, one intact and two damaged respectively, are investigated by the improved Smith’s method for a range of variation of heeling angles. The influence of asymmetric geometry and load on the motion of neutral axis plane and on the ultimate strength are analyzed and discussed. The results show that the improved iteration strategy together with the MCFM is self-adapting and more accurate in searching the translation and rotation of neutral axis plane. Finally, the envelope curves of the bending moments in the three predefined integrity states are obtained, which can be used for assessing ultimate strength of hull girders under combined vertical and horizontal wave bending moments.