Mooring Line Design Research Articles

Effects of Different Influence Factors on Static and Dynamic Analysis of Mooring Line

Many analytic factors affect predicted results of mooring line's static and dynamic responses associated with the platform motions. The paper presents a study of analytic factors that influence static and dynamic responses of a mooring line. These factors include mooring line characteristics, environmental conditions, and external excitations such as platform motions. In this paper, a finite element model for a mooring line system is built for a comparative study on computational efficiency and convergence of different initial values for static analysis. The paper also presents investigation of mooring line dynamic responses under different environmental conditions and external excitations, and discusses various influences of the above mentioned factors on the mooring line dynamic tension. Some conclusions are drawn, which can be the reference for mooring line design and analysis.

An integrated methodology for the design of mooring systems and risers

Traditionally, the design of mooring lines and risers of floating production systems (FPS) has been performed separately, by different teams, employing uncoupled analysis tools that do not consider the nonlinear interaction between the platform hull and the mooring lines and risers. Design processes have been focused on fulfilling the design criteria of the respective component (mooring/riser) alone, with few or no consideration to the other component, and little interaction between the design teams. Nowadays the importance of employing analysis tools based on coupled formulations is widely recognized, and analysis strategies have been proposed to consider feedback between mooring lines and risers within their respective design processes.In this context, this work details a proposal of one single and fully integrated design methodology for mooring systems and risers for deep-water FPS. In this methodology, the design stages of both risers and mooring lines are incorporated in a single spiral, allowing the full interaction of different teams; mooring design implicitly considers the riser integrity, and vice-versa, leading to gains in efficiency and cost reduction.Different analysis strategies are employed, taking advantage of uncoupled and coupled numerical models. The models generated at the initial/intermediate design stages can be reused in subsequent stages: simpler models are used in the initial stages, and more refined models are gradually introduced, to reach an ideal balance between computational cost and accuracy of results. In the advanced stages, the exchange of information between mooring/riser also allows the definition of criteria for the selection of governing/critical loading cases to be revised and verified in detail. This leads to the reduction of the original loading case matrix, allowing a feasible use of time-consuming fully coupled analysis.Results of a case study illustrating the application of some of the main processes of the methodology are included.

ANN-based surrogate models for the analysis of mooring lines and risers

This work presents a new surrogate model based on artificial neural networks (ANNs), comprising a rapid computational tool for the analysis and design of mooring lines and risers. The goal is to obtain results nearly as good as those provided by expensive finite element (FE)-based nonlinear dynamic analyses, with dramatic reductions in processing time. The procedure proposed here associates an ANN with a Nonlinear AutoRegressive model with eXogenous inputs (NARX). Differently from previous models based purely on exogenous inputs (i.e. the platform motions), the NARX model relates the present value of the desired time series not only to the present and past values of the exogenous series, but also to the past values of the desired series itself. Case studies are presented to determine the best configurations for the model, and to evaluate its performance in terms of accuracy and computational time.

Risk assessment for structural design criteria of FPSO systems. Part II: Consequence models and applications to determination of target reliabilities

The paper presents applications of the framework for generic risk assessment for FPSO systems described in a companion paper [1]. Consequence models for structural risk assessment are developed for failure of hull girder sections, hull components and mooring lines using Bayesian Probabilistic Networks (BPN). The estimation of conditional probabilities required in the BPN models, such as the probability of a combined failure of several components and system failure conditional on components failure, are discussed. The risk assessment framework is applied for decision making on target reliabilities for ULS design of mooring lines. The target reliabilities are found by optimizing expected life-cycle costs and verifying compliance with risk acceptance criteria developed according to the Life Quality Index (LQI) principle. It is shown that the optimal reliability index, which minimizes total expected costs, may not always lead to an acceptable solution from the point-of-view of the LQI criterion.

Fatigue Design of Mooring Lines of Floating Type Combined Renewable Energy Platforms

This paper presents the concept design procedure of a floating-type combined renewable energy platform based on hydrodynamic analyses and is focused on the fatigue design of taut-type mooring lines of the platform. Two types of combined renewable energy platforms are considered: a combination of wind turbine, wave turbine and photovoltaic energy plant and a combination of wind turbine, current turbine and photovoltaic energy plant. The basic configurations are conceptually determined from the understanding of floating offshore plants, while the main dimensions have been determined based on a hydrostatic calculation. Fully coupled hydrodynamic analyses have been carried out to identify the motion characteristics of the floating body and the tension histories of the mooring lines. The tension history is used for the fatigue life prediction based on the rain-flow cycle counting method. For the fatigue life prediction, tension life curves from API and the Palmgren-Miner rule are employed.

Reliability-based comparative study for mooring lines design criteria

The characteristic environmental load effect for the design of mooring systems of floating units can be defined by means of three procedures: (a) the one associated to an extreme sea state with a given return period, (b) the worst one from a set of sea states on a contour line associated to a return period or (c) the extreme one based on response statistics for a long-term period. This work presents the result of a reliability-based partial safety factor calibration study for a LRFD mooring line design criteria considering the three approaches mentioned above. The calibration exercise is applied to three FPSOs considering North Sea environmental conditions and different water depths: 200, 800 and 3000 m. The mooring systems investigated take into account mooring lines made up of chains and polyester ropes. It is shown that, among all cases investigated, the design procedure based on the long-term response is the one that presents less scattered reliability indices around the target level.

Concerning Wire Ropes for Anchoring Semisubmersibles

If you're about to build yourself a semisubmersible, don't assume that your old Boy Scout Manual will tell you all you need to know about the anchor lines. There's more to the rope tricks than meets the eye, so call on an expert. And call on him early Introduction The expansion of offshore drilling operations during the past few, years has greatly increased our knowledge of anchoring requirements and techniques for offshore drilling units. This experience has been gained under many and varied conditions throughout the world, and particularly in the North Sea, where the severest conditions continue to be encountered. It is true that far more violent conditions are sometimes experienced in other parts of the world, but they are comparatively short-lived; it is the accumulative effect of sustained poor weather that makes the North Sea one of the worst operating areas for offshore units. As a result of earlier experiences with rigs in the North Sea, a new breed of bigger and better rig has been designed and is under construction. Naturally, as the general fabrication increases in size and efficiency, so too must all the component parts. Most of the rigs of this new breed are of the semisubmersible type and therefore mooring members, whether of chain or rope, will obviously play a major role in the total efficiency of the unit play a major role in the total efficiency of the unit and so must be designed with care. Requirements for Good Anchor Lines In the much earlier days of the offshore rig, mooring systems involving wire rope were similar to those used for mooring conventional ships. However, the new sophisticated rigs designed to operate in heavy seas and at greater depths have presented far different problems. It has been necessary to carefully reconsider those designs, fabrication methods, and physical properties. physical properties. Experience has shown that the most important attributes to incorporate into the design of mooring lines for offshore rigs are (1) high strength, (2) high elongation coupled with high elastic limit, (3) corrosion resistance (using maximum effective galvanized coating), and (4) environmental protection (using suitable lubricants and service dressings). The choice of rope for a given application is generally a matter of compromise. In the case of anchor lines, one must also consider resistance to fatigue, abrasion, and deformation. Achieving Strength The strength of a wire rope derives principally from (1) the cross-section of steel area and (2) the tensile strength of the material used. The diameter of anchor line to be used to anchor a given offshore unit will naturally depend upon the size and deign of the unit and upon the forces that will be imposed upon this structure by the wind and the sea. Anchor lines presently being used in the North Sea vary between 2 1/2 and 3 1/2 in. in diameter, and the construction details referred to here are based on this size range. The tensile strength of the material used should always be as high as possible, depending on the construction of the rope employed and the application and work involved, and commensurate with good rope making. JPT P. 659

Embedded suction anchors for floating breakwaters

Embedded suction anchors were used to provide necessary resistance for mooring a floating breakwater system. The embedded suction anchors and the mooring lines were designed using the methods developed by the US Navy. The validity of the mooring line design method was previously made through experimental field and centrifuge model tests. The validity of the design method for the embedded suction anchor resistance was made through centrifuge model tests as part of this study.