On Approximations of the Wave Drift Forces Acting on Semi-Submersible Platforms With Heave Plates

The horizontal wave drift force acting on a vertical floating column, without then with a heave plate, is considered. Computations are performed with a diffraction-radiation code and through the Morison and Rainey equations. Focus is on wave frequencies around the heave resonance where the drift force may be significant, even though the scattered wave-field being weak. It is found that the Morison equation overpredicts the drift force while Rainey equations perform rather well.

A Study on Small Scale Associated Gas FLNG Utilizing Small Scale LNG Carriers to Near-By Onshore LNG Plant

This study describes a FLNG specifically designed to monetize Associated Gas (AG) of producing oil fields located within convenient distance of an existing LNG Plant or Port with LNG storage facility. Limited production capacity combined with short range small capacity shuttles and limited LNG storage capacity, provide a cost effective means for LNG production. This FLNG is designed to service an existing industry and does not require development of stranded gas discoveries.

Chain Out of Plane Bending (OPB) Fatigue Joint Industry Project (JIP) Static Test Program and OPB Interlink Stiffness

In 2002, several mooring chains of a deepwater offloading buoy failed prematurely within a very small time frame. These chains were designed according to conventional offshore fatigue assessment using API recommendations. With this first deepwater buoy application, a new mooring chain fatigue mechanism was discovered. High pretension levels combined with significant mooring chain motions caused interlink rotations that generated significant Out of Plane Bending (OPB) fatigue loading. Traditionally, interlink rotations are relatively harmless and generate low bending stresses in the chain links. The intimate mating contact that occurs due to the plastic deformation during the proof loading and the high pretension of the more contemporary mooring designs have been identified as aggravating factors for this phenomenon. A Joint Industry Project (JIP), gathering 26 different companies, was started in 2007 to better understand the Out of Plane Bending (OPB) mooring chain fatigue mechanism and to propose mooring chain fatigue design recommendations. This paper summarizes the quasi static OPB stiffness measurement campaign and the post processing work to derive the OPB interlink stiffness.

Experimental Investigation on the Dynamic Response of a Three Legged Articulated Type Offshore Wind Tower

Demand for renewable energy sources is rapidly increasing since they are able to replace depleting fossil fuels and their capacity to act as a carbon neutral energy source. A substantial amount of such clean, renewable and reliable energy potential exists in offshore winds. The major engineering challenge in establishing an offshore wind energy facility is the design of a reliable and financially viable offshore support for the wind turbine tower. An economically feasible support for an offshore wind turbine is a compliant platform since it moves with wave forces and offer less resistance to them. Amongst the several compliant type offshore structures, articulated type is an innovative one. It is flexibly linked to the seafloor and can move along with the waves and restoring is achieved by large buoyancy force. This study focuses on the experimental investigations on the dynamic response of a three-legged articulated structure supporting a 5MW wind turbine. The experimental investigations are done on a 1: 60 scaled model in a 4m wide wave flume at the Department of Ocean Engineering, Indian Institute of Technology, Madras. The tests were conducted for regular waves of various wave periods and wave heights and for various orientations of the platform. The dynamic responses are presented in the form of Response Amplitude Operators (RAO). The study results revealed that the proposed articulated structure is technically feasible in supporting an offshore wind turbine because the natural frequencies are away from ocean wave frequencies and the RAOs obtained are relatively small.

Bilge Keel Induced Roll Damping of an FPSO With Sponsons

The bilge keel induced roll damping of an FPSO with sponsons is investigated numerically and experimentally. The influence of the bilge keel size, on the roll damping is studied. Free decay tests of a three-dimensional ship model, for three different bilge keel sizes are used to determine roll damping coefficients. The dependency of the quadratic roll damping coefficient to the bilge keel height and the vertical location of the rotation center is studied using CFD. A Navier-Stokes solver based on the Finite Volume Method is adopted for solving the laminar flow of incompressible water around a section of the FPSO undergoing forced roll oscillations in two-dimensions. The free-surface condition is linearized by neglecting the nonlinear free-surface terms and the influence of viscous stresses in the free surface zone, while the body-boundary condition is exact. An averaged center of rotation is estimated by comparing the results of the numerical calculations and the free decay tests. The obtained two-dimensional damping coefficients are extrapolated to 3D by use of strip theory argumentations and compared with the experimental results. It is shown that this simplified approach can be used for evaluating the bilge keel induced roll damping with efficiency, considering unconventional ship shapes and free-surface proximity effects.

Response of a CALM Buoy Moored Vessel in Squall Condition

This paper reviews the response of a hawser moored vessel to squalls and addresses a novel method for obtaining statistically reliable design loads. Industry paradigms related to squall selection for analysis input are reviewed and renewed. A benchmark database consisting of more than 15,000 unique squall-wave-current induced extreme values enables the validation of a range of less computationally demanding analysis and squall selection methods. Extreme values are extrapolated to a design value using a Peak Over Threshold (POT) method to fit a Generalized Pareto Distribution (GPD). The influence of associated metocean conditions and squall characteristics on the vessel response is presented. By means of bootstrapping a satisfactory population size for design purposes is studied. The findings challenge common design practices currently employed throughout the industry.

Vortex Induced Vibrations of Pipelines With Non-Linear Seabed Contact Properties

A promising time domain model for calculation of cross-flow vortex induced vibrations (VIV) is under development at the Norwegian University of Science and Technology. Time domain, as oppose to frequency domain, makes it possible to include non-linearities in the structural model. Pipelines that rest on an irregular seabed will experience free spans. In these areas VIV is a concern with respect to the fatigue life. In this paper, a time domain model for calculation of VIV on free spanning pipelines is proposed. The model has non-linear interaction properties consisting of discrete soil dampers and soil springs turning on or off depending on the pipeline response. The non-linear model is compared to two linear models with linear stiffness and damping properties. One linear model is based on the promising time domain VIV model, while the other one is based on RIFLEX and VIVANA, which calculates VIV in frequency domain. Through four case studies the effect of seabed geometry, current velocity and varying soil damping and soil stiffness is investigated for a specific pipeline. The results show that there is good agreement between the results produced by VIVANA and the linear model. The non-linear model predicts smaller stresses at the pipe shoulders, which is positive for the life time estimations. Soil damping does not influence the response significantly.

Re-Evaluation of Suction Anchor Holding Capacity Based on Back Analysis of Undrained Shear Strength

Suction anchors are used within mooring systems for a range of deep water floating structures. The number of suction anchor systems has increased rapidly over the last two decades due to ease of installation and a well-defined design approach. As with other offshore installations, anchor systems may be subjected to re-assessment during their operational lifetime, due to changes in loading, codes or other issues. Such assessment was performed recently for a floating storage unit (FSU) presently in service in the Norwegian Sea, where the geotechnical design of the anchors was re-evaluated. The re-assessment of this storage unit allowed for a number of changes compared to the original design, including updated loads and load histories as well as changes to the safety factors. The geotechnical aspects of the design were also updated, including more refined analyses and a detailed review of the design soil profiles based on back-analysis of installation data. This paper presents the work done related to the geotechnical holding capacity of the anchors during the assessment project. The work performed highlights the benefit of efficient FE analysis and a thorough evaluation of the soil properties and engineering behaviour.

Hydrostatic Stability and Global Performance Analysis for Set TLP

SET (Samsung Enhanced hull for Tendon) TLP (Tension Leg Platform) has been designed as an innovative TLP hull form with optimized number of tendons compared to conventional TLP design. SHI (Samsung Heavy Industries) designed the SET TLP to have the minimized hull weight with narrow and thin pontoon which results in the less number of tendons without any outboard extension of column or pontoon. The SET TLP has 8 tendons which are evenly attached along with the octagonal shaped-ring pontoon. The unique shape of octagonal ring pontoon distributes the wave load and concentrated tendon loads in larger areas, which can minimize the structural reinforcement. To verify the feasibility of the proposed hull concept with regard to the hydrostatic/hydrodynamic characteristics and tendon design, the numerical analyses for the hydrostatic stability and global performance are carried out. Hydrostatic stability is investigated for all the possible loading cases such as float-off, wet transit, tendon installation and operational conditions, and the proper tank compartments are achieved for all the scenarios without using any temporary stability module. The global performance is validated for all the possible combinations of wave, swell, wind, current and/or squall for a site in Western Africa. Through the frequency-domain analysis and nonlinear time-domain analysis as well, the essential items such as the maximum offset/set-down/top tendon tension, minimum bottom tendon tension are examined and confirms that the certain design criteria of TLP operation are satisfied.

Neural Networks for Tubular Joint Optimization in Offshore Jacket Structures

The use of jacket structures to support offshore installations has for a long time been a popular choice in places with appropriate water depths. In recent years the use of jacket structures as offshore wind turbine foundations has also attracted increasing attention and is becoming a feasible alternative to traditional monopile foundations. One of the key challenges in jacket design is optimizing tubular joints in terms of fatigue resistance. As it is not practically possible to include detailed FEM joint models in global jacket models designers are forced to look for alternative methods to obtain realistic joint representations. This is done by calculating influence factors (INF) and stress concentration factors (SCF) to be applied to simplified models of relevant tubular joints in global models in order to achieve a realistic force flow in the structure. One simple and widely used method is to apply parametric formulas, e.g. those suggested by Efthymiou. However, these approximating formulas have a fairly limited validity range. Therefore, on complex joint the most reliable way to determine INF’s is by setting up refined FEM models of relevant joints and then subsequently using the calculated factors in the global model. This strategy is computationally demanding and hence, very time consuming, as a new detailed FEM analysis of the tubular joint must be conducted for each step in the optimization process. The present paper demonstrates how this time consuming procedure can be avoided by use of artificial neural networks (ANN) trained to estimate INF’s on tubular joints. The neural network is trained on a pre-generated library of detailed FEM joint models and is then able to predict INF’s on joints that are not part of the library — and thereby providing a significant reduction in calculation time during the jacket/joint optimization process. The analysis is conducted on a typical joint on a three legged jacket structure. The joint is located on a jacket leg and has two incoming braces. Such a joint has a finite number of free design variables, e.g. chord diameter/thickness, brace diameter/thickness, brace angle, gap etc. Each of these free variables can be considered as a dimension in the joint design space. Having a sufficient number of FEM joint models in the library the neural network can be trained to recognize and predict underlying patterns in this design space. The method is demonstrated on a limited number of design variables but should easily be extended to cover all variables as the joint library is expanded to include all dimensions.