bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of Wave Induced Second-Order Forces in Semi-Submersible FOWT Mooring Design
Authors: López-Pavón Carlos , Watai Rafael A. , Ruggeri Felipe , Simos Alexandre N. , Souto-Iglesias Antonio
Published: November 2013

AZIMUT project (Spanish CENIT R&D program) is designed to establish the technological groundwork for the subsequent development, of a large-scale offshore wind turbine. The project (2010–2013) has analysed different floating offshore wind turbines (FOWT): SPAR, TLP and Semi-Submersible platforms were studied. Acciona, as part of the consortium, was responsible of scale-testing a Semi-submersible platform to support a 1.5MW wind turbine. The floating platform geometry considered in this paper has been provided by the Hiprwind FP7 project and is composed by three buoyant columns connected by bracings. The main focus of this paper is on hydrodynamic modelling of the floater, with especial emphasis on the estimation of the wave drift components and their effects on the design of the mooring system. Indeed, with natural periods of drift around 60 seconds, accurate computation of the low-frequency second-order components is not a straightforward task. As methods usually adopted when dealing with the slow-drifts of deep-water moored systems, such as Newman’s approximation, have their errors increased by the relatively low resonant periods, and as the effects of depth cannot be ignored, the wave diffraction analysis must be based on full Quadratic Transfer Functions (QTF) computations. A discussion on the numerical aspects of performing such computations is presented, making use of the second-order module available with the seakeeping software WAMIT®. Finally, the paper also provides a preliminary verification of the accuracy of the numerical predictions based on the results obtained in a series of model tests with the structure fixed in bichromatic waves.
Arctic Pipelines Strain Demand Prediction
Authors: Xu Jianfeng , Abdalla Basel , Mckinnon Colin , Audibert-Hayet Annie , Coche Edmond , Gaffard Vincent
Published: November 2013

Strain prediction is a preliminary requirement for the strain based design of arctic pipelines subject to frost heave. It involves modeling of thermal, geotechnical and mechanical of both pipe and soil behaviors. The objective is to predict frost heave and the consequences on pipeline stress-strain state.

This study involves the development of three numerical models: a geothermal model to simulate the heat transfer processes in the soil; a frost heave model to simulate the coupled heat transfer and displacement of soil; and a pipe-soil interaction model to calculate the pipe stresses and strains due to soil displacement. The frost heave prediction in the present study is based on two-dimensional finite element (FE) calculations integrating the so-called “porosity rate function”. The advantage of this approach over the other models stems from a formulation consistent with continuum mechanics.

The frost heave model is first validated with literature data. Then, calculations are performed to predict the heat transfer, frost heave, and pipe strain under design configurations and conditions.

This study presents a good application of porosity rate function in pipeline engineering design. The developed models can be further used to investigate different design options of a chilled pipeline buried in the Arctic environment.
Vortex-Induced Vibration Experiment Research of Two Cylinders in Tandem Arrangement
Authors: Kang Zhuang , Liu Weixing , Qin Wei
Published: November 2013

The vortex-induced vibration of tandem arrangement of two cylinders compared with the single cylinder is more complicated, The double cylinder arranged in tandem, which is free to move in two degrees of freedom respectively, and which has low mass and damping. The present study shows that a critical centre-to-centre spacing can be used to distinguish the far and near wake interference. The streams in this test were uniform flow, ranging from 0.2m/s to 0.8m/s with the interval of 0.1m/s. The Re numbers are ranging from 22000 to 88000. The mass ratio of cylinder is low. For far wake interference, the downstream cylinder shows large amplitudes of response, therefore the wake induced vibration (WIV) is found. For near wake interference, both the upstream cylinder and downstream cylinder are exposed to an evident phenomenon of VIV, but the amplitude of upstream and downstream are less than that of single cylinders in cross-flow direction and in-line direction. We found the critical spacing to be 3.4 to 4.9.
A Power Equation Model for Vortex-Induced Vibration of Circular Cylinder
Authors: Qin Wei , Kang Zhuang , Song Ruxin , Sun Liping , Wu Ying
Published: November 2013

Based on a semi-empirical model of vortex-induced vibration of an elastically mounted circular cylinder, the general equation of dynamics is put forward to establish the power equation for free vortex-induced vibration in this paper. The derivation of the governing equation is the general method to analyze fluid-structure interaction without priori assumptions about the form of the governing equations, which is of good/strong applicability and expansibility. To estimate the reliability of power equation in free vibrations, the power equation acquiring flow field provided by CFD is solved and compared with the experimental results. Results show that the power equation is effective to predict the response of structure in vortex induced vibration.
On the Predication of Pipeline As-Laid Embedment Using a Cycle-by-Cycle Approach for Deepwater Application
Authors: Sun Jie , Chang Gwo-Ang , Liu Xiang
Published: November 2013

In deep water, pipelines are generally laid directly on the sea bed and a pipeline’s as-laid embedment is a major source of uncertainty influencing the assessment of pipe-soil resistances, pipeline lateral buckling formation as well as lateral buckling response, end expansion and walking behavior. In order to have good control of the uncertainties in pipeline as-laid embedment, pipe laying effects, including static and dynamic pipe-soil vertical contact loads and the likely degree of soil softening due to pipeline oscillation at touch down zone should be reliably predicted.

In this paper a new procedure is introduced that uses OrcaFlex [1] to determine pipe laying effects and a cycle-by-cycle approach to predict pipeline as-laid embedment. Validation of the procedure is also presented in this paper by comparing pipeline as-laid embedment results calculated from the procedure with upper and lower bound values developed from field observation data. Very good agreement is obtained. In the study, it is found that pipe-soil vertical contact load amplitude, pipe horizontal oscillation, and duration of the oscillation at touchdown zone are important to soil softening as well as pipeline as-laid embedment.

In addition, results calculated from the procedure were also compared with those evaluated from the SAFEBUCK guideline [2]. It was found that results from the SAFEBUCK guideline, in general, underestimates the pipeline as-laid embedment.
A Two-Dimensional Approximation of a Floating Fish Farm in Waves and Current With the Effect of Snap Loads
Authors: Bardestani Mohsen , Faltinsen Odd M.
Published: November 2013

Wave and current loads on the floater, netting and sinker tube of a floating fish farm are discussed. High flexibility of netting, floating collar and sinker tube and their structural interaction are important effects. Regular wave and uniform current load induced response of a section of a floater with net structure and sinker tube are analyzed experimentally and numerically in order to simplify the problem. The sectional models of the floater and the sinker tube are considered rigid. Experiments were done in a wave flume with model scale 1:10 for three net solidity ratios (Sn) and three sinker tube weights. Hydrodynamic loads on the net structure and its motion are calculated based on the screen force model proposed by Kristiansen and Faltinsen [1] and are coupled with floater and sinker tube motions in a time domain simulation. Good agreement is achieved in terms of net shape, tensions and floater motions. Large snap loads due to the elastic behavior of the net structure and the relative motion between the floater and the net are observed experimentally and predicted numerically. This type of cyclic loading has to be considered in terms of net structure failure.
Numerical Simulation of Flows Past Partially-Submerged Horizontal Circular Cylinders in Free Surface Waves
Authors: Bihs Hans , Ong Muk Chen
Published: November 2013

Two-dimensional (2D) numerical simulations are performed to investigate the flows past partially-submerged circular cylinders in free surface waves. The 2D simulations are carried out by solving the Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with the k-ω turbulence model. The level set method is employed to model the free-surface waves. Validation studies of a numerical wave tank have been performed by comparing the numerical results with the analytical results obtained from the linear-wave theory. Wave forces on the partially-submerged cylinders have been calculated numerically and compared with the published theoretical and experimental data under regular-wave conditions. The free-surface elevations around the cylinders have been investigated and discussed.
Model Test of the Aker Wave Energy Converter Concept
Authors: Ersdal Svein , Moe Anders M.
Published: November 2013

The results from a model test of a wave energy converter of the articulated body type are presented and compared with the analytical results from a linear frequency domain analysis. The focus is on the rotational response of the arm connecting the bodies, since this is the motion used for energy extraction. Using a servo motor with programmable torque vs. angular velocity characteristics constant, linear and quadratic relationships could be modeled in the test. The comparison with the numerical model shows that the presence of walls in the test tank influences the response, thus some uncertainty in the results is found. Still, the capture width is found to be above 60% of the width of the device for the most common waves. For long and large waves the efficiency is very low, which means that the PTO system is not overloaded in storm conditions. Comparison of the response with different characteristics of the PTO show that a quadratic relation gives an effective energy capture over a wide range of sea states with no tuning of parameters.
Hydrodynamic Interactions in Multiple Body Array: A Simple and Fast Approach Coupling Boundary Element Method and Plane Wave Approximation
Authors: Singh Jitendra , Babarit Aurélien
Published: November 2013

The hydrodynamic forces acting on an isolated body could be considerably different than those when it is considered in an array of multiple bodies, due to wave interactions among them. In this context, we present in this paper a numerical approach based on the linear potential flow theory to solve full hydrodynamic interaction problem in a multiple body array. In contrast to the previous approaches that considered all bodies in an array as a single unit, the present approach relies on solving for an isolated body. The interactions among the bodies are then taken into account via plane wave approximation in an iterative manner. The boundary value problem corresponding to a isolated body is solved by the Boundary Element Method (BEM). The approach is useful when the bodies are sufficiently distant from each other, at-least greater than five times the characteristic dimensions of the body. This is a valid assumption for wave energy converter devices array of point absorber type, which is our target application at a later stage. The main advantage of the proposed approach is that the computational time requirement is significantly less than the commonly used direct BEM. The time savings can be realized for even small arrays consisting of four bodies. Another advantage is that the computer memory requirements are also significantly smaller compared to the direct BEM, allowing us to consider large arrays. The numerical results for hydrodynamic interaction problem in two arrays consisting of 25 cylinders and same number of rectangular flaps are presented to validate the proposed approach.
Large Deformation Finite Element Analysis of the Anchor Line Embedded in Seabed Soils
Authors: Zhao Yanbing , Liu Haixiao
Published: November 2013

For most anchors in offshore engineering, such as the anchor pile and the drag anchor, the attachment point will generally be significantly below the seafloor surface, and therefore a portion of the anchor line will be deeply embedded in seabed soils. Complicated interaction happens between the anchor and the embedded anchor line, and the tension at the attachment point and the reverse catenary shape of embedded anchor line play a key role in assessing the capacity and reliability of the anchorage system. Previous investigations on the force distribution and the reverse catenary shape of embedded anchor line involve numerical incremental methods, closed-form theoretical expressions and laboratory tests. In the present work, a large deformation finite element analysis using the Coupled Eulerian-Lagrangian technique is developed to investigate the tension and profile of embedded anchor line. Parametric studies are performed to evaluate the effects of the shear strength of clay, depth of attachment point, diameter of embedded anchor line, self weight of soil, self weight of anchor line, and frictional coefficient between the embedded anchor line and soil. By comparing with theoretical and numerical integration solutions, the FEM simulation results are well verified. The present study demonstrates that the CEL technique is effective for simulating the anchor line-soil interactional problems.
Calculation of Ship Hydrodynamic Propulsion in Rough Seas by Non-Linear BEM With Application to Reduction of Energy Losses in Waves
Authors: Belibassakis K. A. , Gerostathis Th. P. , Politis G. K.
Published: November 2013

Environmental conditions corresponding to realistic sea states (which can be rarely considered calm) significantly affect ship propulsion due to added wave resistance, wind resistance and other factors, as e.g., continuous rudder motion for steering in adverse conditions. In addition, external factors such as ocean currents, which determine the actual flow on the ship, critically affect the actual behavior of the propulsion system. All the above cause significant additional energy losses that sometimes could drive the propulsion system of a ship at its limits. On the other hand, the operation of ship propellers and thrusters in real sea conditions is quite different from their design specifications, usually considered in calm conditions. For example, the vertical stern motion of the ship significantly affects propeller efficiency and becomes dramatically worse if emergence of propeller occurs in high waves. Operation of the ship propulsion system in random waves causes significant variations in performance. In this work we examine in detail the effects of wave-induced motions of the ship on the modification of propulsive thrust and efficiency. Our analysis is based on the non-linear Unsteady Boundary Element Modeling Code UBEM which is applied for the analysis of an unsteadily moving propeller in a wake field, in conjunction with seakeeping analysis in regular and irregular waves. Results from the present hydrodynamic analysis, in conjunction with predictions of added resistance, are used to illustrate applicability in the case of an AFRAMAX tanker, investigating the benefits of small regulation of ship speed and engine RPM from the point of view of optimizing ship’s propulsive performance and reduction of energy losses. The present analysis could support the development of ship monitoring and decision support systems, integrated with engine control systems, aiming to maximize operating efficiency in realistic sea conditions.
Fatigue Analysis of Free Spanning Pipelines Subjected to Vortex Induced Vibrations
Authors: Van den Abeele F. , Boël F. , Hill M.
Published: November 2013

Vortex induced vibration is a major cause of fatigue failure in submarine oil and gas pipelines and steel catenary risers. Even moderate currents can induce vortex shedding, alternately at the top and bottom of the pipeline, at a rate determined by the flow velocity. Each time a vortex sheds, a force is generated in both the in-line and cross-flow direction, causing an oscillatory multi-mode vibration. This vortex induced vibration can give rise to fatigue damage of submarine pipeline spans, especially in the vicinity of the girth welds.

In this paper, an integrated numerical framework is presented to predict and identify free spans that may be vulnerable to fatigue damage caused by vortex induced vibrations (VIV). An elegant and efficient algorithm is introduced to simulate offshore pipeline installation on an uneven seabed. Once the laydown simulation has been completed, the free spans can be automatically detected. When the fatigue screening for both inline and cross-flow VIV indicates that a particular span may be prone to vortex induced vibrations, a detailed fatigue analysis is required.

Amplitude response models are constructed to predict the maximum steady state VIV amplitudes for a given pipeline configuration (mechanical properties) and sea state (hydrodynamic parameters). The vibration amplitudes are translated into corresponding stress ranges, which then provide an input for the fatigue analysis. A case study from the offshore industry is presented, and sensitivity analyses are performed to study the influence of the seabed conditions, where special emphasis is devoted on the selection of pipe soil interaction parameters.
Structural Development of LNG Fueled Large Container Ship
Authors: Kim Jeong Hwan , Choi Joong Hyo , Park Sung Gun , Lee Dong Kwon , Oh Yeong Tae
Published: November 2013

There is a growing interest in the appearance of LNG fueled large commercial ship with rising oil price and stronger emission regulation of these days. Though small or middle size LNG fueled vessels have been already operated, the application of LNG fuel to large commercial vessels is now under developing in stage. LNG fueled propulsion system can be characterized by three differences than typical propulsion system. First one is 2-stroke dual fuel engine, second one is high pressure fuel supply system and the third is LNG fuel tank. Among these three components, this paper is focused on the LNG fuel tank which was developed by DSME so called ‘ACT-iB’ (ACT-iB: Aluminum Cargo Tank Independent type B). This paper describes engineering works such as structural analysis, crack propagation & leakage analysis and temperature distribution analysis for the application of ACT-iB to LNG fueled vessel. From this development, it was concluded that LNG fueled technologies are ready to large container ship application in structural point of view.
Experimental and Numerical Investigations Into the Station Keeping and the Offloading Performances of Hyundai LNG-FPSO
Authors: Lee In-Hye , Park Moon-Kyu , Moon Joong-Soo , Sim Woo-Seung , Shin Hyun-Soo
Published: November 2013

Hyundai Heavy Industries Co., Ltd. has developed a LNG-FPSO with production capacity of 2.5 MTPA to meet the demand of the offshore LNG well development in near future. This paper presents the experimental and the numerical study on the mooring and the offloading performances of the developed LNG-FPSO.

In the FEED design stage, the turret mooring system and the offloading mooring system with side-by-side arrangement were designed using the quasi-dynamic method and verified through the model tests using the truncation method in the basin. It is known that the quasi-dynamic method is very efficient to calculate many environmental combinations for the evaluation of the turret mooring system and the offloading availability. And the numerical parameters for the truncated system are calibrated with the model test results with the same truncated set-up and it is expanded into the full depth model.

In this study, it was verified that the safety of the station keeping system under the extreme environmental condition and the offloading performance satisfied the design requirement. Especially, the hydrodynamic and mechanical couplings between LNG-FPSO and LNG carrier were considered as the important parameters in the offloading availability with ship-to-ship arrangement. Finally, the design parameters such as the relative motion, line tension and fender load of offloading system were evaluated with considering the dynamic positioning system.
3D Wave Transformation Through Openings in Coastal Structures
Authors: Katsardi Vasiliki , Tsoukala Vasiliki K. , Belibassakis Kostas A.
Published: November 2013

Standard openings in coastal structures are the flushing culverts at breakwaters, allowing periodic exchange of the harbor basin water leading to improved water quality. These openings involve sudden water depth changes occurring when the incident waves meet these openings and transmitted into the harbour. The wave transformations during wave propagation through flushing culverts are dominated by 3D diffraction effects due to sudden water depth changes, along with the finite width of the culvert. A new coupled-mode model, based on eigenfunctions expansions of the Laplace equation, is developed and applied to the numerical solution of the local 3D wave flow problem at the opening. The harmonic wave field is excited by incident parallel waves. The numerical solution converges rapidly, permitting the series truncation at its first terms. The proposed method fully accounts for the 3D diffraction effects and produces information to couple with mild-slope models describing efficiently wave propagation and transformation in coastal regions.
Optimizing Pile Driving Fatigue for Offshore Foundations in Very Dense Sand: A Case Study
Authors: Ozsu Erdem , Ta An-Ninh , Stuyts Bruno , Jaeck Christophe
Published: November 2013

With the rapid development of offshore wind energy in Europe, a large number of piled structures are being installed. Driven pipe piles are adopted as a foundation solution for the majority of offshore wind turbine support structures. In soils consisting of very dense sand, pile driving induces large-amplitude stress cycles in pile material, which have to be accounted for in fatigue calculations. These stress cycles can be calculated using one-dimensional wave equation analysis. Different ways of reducing pile driving damage are presented. Depending on the soil surrounding the pile and the target penetration depth, an optimum driving sequence can be established which minimises pile damage. As damage depends more on induced stresses than on the number of hammer blows, reducing the hammer energy at some point during driving can be beneficial for reducing the accumulated damage. In this paper, an optimum driving sequence is developed for a generic soil profile consisting of very dense sand. The pile driving damage calculated with the optimum sequence is compared to the damage calculated when driving close to maximum hammer efficiency. Additionally, using a larger hammer can also be beneficial for reducing induced stresses when keeping the transmitted energy at a similar level. The paper also highlights the advantages of using pile driving monitoring or pile driving back-analysis for verifying the stress levels in the piles during driving. Offshore design standards allow a reduction of the damage fatigue factor for inspected members. This principle may be extended to monitored piles. The differences between data from pile driving monitoring and data from pile driving back-analysis are discussed and the potential impact on the damage fatigue factor is highlighted. Finally, the potential conflict of pile driving fatigue requirements and pile capacity requirements is discussed. Both considerations should eventually lead to an optimized design which satisfies the required design equations.
Reliability Based Pressure Containment Uprating
Authors: Dib Elie , El-Gebaly Sherif , Drennan Frank
Published: November 2013

Over the design life of long tiebacks, project requirements may change in order to accommodate new prospects. These new prospects may have design conditions exceeding those used for the rest of the field and the existing facilities may not be able to withstand the potential increases in pressure arising from new wells. For smaller and accidental increases, the existing system may be proven to be fit for purpose by using a probabilistic approach, or a structural reliability assessment. This approach may eventually reduce the cost of the new prospect by, for example, removing the need for an over-pressure protection system, or removing the need for a dedicated pipeline system. This paper investigates the potential increase in pressure capacity of a main pipeline for an accidental over-pressure condition using a structural reliability analysis and probabilistic approach using as-built data gathered from several projects. The variations on the as-built pipeline properties, the current status of the existing pipeline, and the corresponding operating conditions are taken into account using a Monte Carlo simulation. The results present a burst pressure with an indication of a safety factor and associated with a probability of failure. A comparison between the final results and the pipeline codes target probability of failure is also performed and presented as part of this paper.
Ultra Heavy Wall Linepipe X65: Severe In-Service Straining of Lined Pipes
Authors: Di Vito Luigi , Ferino Jan , Amato Stefano , Mannucci Gianluca , Crippa Stefano , Tintori Federico , Darcis Philippe
Published: November 2013

Tenaris and Centro Sviluppo Materiali (CSM) carried out a Joint Industrial Project aimed at developing heavy wall line pipes. The suitability for very severe applications, involving high service pressures and temperatures, the latter causing large strain fluctuations, in presence of an aggressive sour environment, is analyzed both theoretically and experimentally, including small and full pipe models and tests. Five papers have been already presented on this project, in previous OMAE conferences.

The present paper focusses on Lined Heavy Wall Pipes for the adoption in presence of extremely aggressive conveyed fluids. As in-service large strains are involved in the JIP envisaged scenarios, the risk of liner buckling is necessarily concerned. To evaluate the suitability of lined heavy pipes in presence of in-service severe straining, a finite element study has been performed aimed at quantifying the limits for pipe deformability without occurrence of liner buckling.

Two full scale tests on lined pipe strings have been also performed, imposing the very severe straining sequence previously determined as extreme for pipeline resistance. The sequence has been applied both in pure axial (tensile / compressive) loading and in bending conditions. The latter has been performed in very low internal pressure conditions to conservatively verify the resistance to liner buckling. In both cases, the lined heavy wall pipe resisted the severe straining sequence without any liner buckling, pipe excessive ratcheting or any other damage compromising the serviceability of the pipe.
A Comparison Study of Pressure Vessel Design Using Different Standards
Authors: Askestrand Frode Tjelta , Gudmestad Ove Tobias
Published: November 2013

Several codes are currently available for design and analysis of pressure vessels. Two of the main contributors are the American Society of Mechanical Engineers providing the ASME VIII code, Ref /4/ and the Technical Committee for standardization in Brussels providing the European Standard, Ref /2/.

Methods written in bold letters will be considered in the discussion presented in this paper.

The ASME VIII code, Ref /4/, contains three divisions covering different pressure ranges:

Division 1: up to 200 bar (3000 psi)

Division 2: in general

Division 3: for pressure above 690 bar (10000 psi)

In this paper the ASME division 2, Part 5, “design by analysis” will be considered. This part is also referred to in the DNV-OS-F101, Ref /3/, for offshore pressure containing components.

Here different analysis methods are described, such as:

Elastic Stress Analysis

Limit Load Analysis

Elastic Plastic Analysis

The Elastic Stress Analysis method with stress categorization has been introduced to the industry for many years and has been widely used in design of pressure vessels. However, in the latest issue (2007/2010) of ASME VIII div. 2, this method is not recommended for heavy wall constructions as it might generate non-conservative analysis results.

Heavy wall constructions are defined by: (R/t ≤ 4) with dimensions as illustrated in Figure 1.

In the case of heavy wall constructions the Limit Load Analysis or the Elastic-plastic method shall be used. In this paper focus will be on the Elastic-plastic method while the Limit Load Analysis will not be considered.

Experience from recent projects at IKM Ocean Design indicates that the industry has not been fully aware of the new analysis philosophy mentioned in the 2007 issue of ASME VIII div.2. The Elastic Stress Analysis method is still (2012) being used for heavy wall constructions.

The NS-EN 13445-3; 2009, Ref /2/, provides two different methodologies for design by analysis:

Direct Route

Method based on stress categories.

The method based on stress categories is similar to the Elastic Stress Analysis method from ASME VIII div. 2 and it will therefore not be considered in this paper.
Hydrodynamic Moonpool Behavior in a Monocolumn Hub Platform With an Internal Dock
Authors: de Oliveira Allan C. , Vilameá Eduardo M. , Figueiredo Sílvia R. , Malta Edgard B.
Published: November 2013

One of the possible uses of a Mono Column Platform is its application as a Hub Platform for people transportation to very far offshore production platforms. Considering a trip from shore to the Hub by a High Speed Vessel (HSV) and by helicopter from the hub to the production platforms, the use of this kind of unit can provide small motions for helicopter operations and a sheltered area for people landing. The use of the moonpool area as an internal dock for the HSV docking was studied in this paper considering an opening in the Mono Column hull to allow the access of the HSV to the moonpool area. The hydrodynamics aspects of the moonpool behavior were assessed considering both experimental and numerical tools and a comparison between these approaches had been also performed.
Structural Integrity Assessment of Aging Fixed Steel Offshore Jacket Platforms: A Persian Gulf Case Study
Authors: Golpour Hamid , Zeinoddini Mostafa , Khalili Hadi , Golbaz Ali , Yaghubi Yashar , Adl Mahmood Reza , Ranjbar Pooya , Saberi Reza , Matin Nikoo Hamid
Published: November 2013

The existing knowledge on the structural integrity assessment of offshore platforms may benefit from case studies on the life extension evaluations of aging structures. This paper presents a case study for the structural integrity assessment of an existing 8 legged aging drilling platform located in the Persian Gulf. The platform is now 42 years old and the objective of the study is to check its fit for purpose for a life extension of 25 years beyond 2012. The structural model is based on the best estimates of the existing conditions of the platform.

A number of analysis approach such as i) assessment based on the previous exposures, ii) linear (elastic), iii) equivalent linear (or the linear global analysis with local overload considerations), and iv) non-linear analysis methods have been used to estimate the structural integrity of the platform.

The paper provides further background, clarifications and proposed updates to API-RP 2A-Section 17. The paper is divided into three parts. Section 1 is a discussion on the background of the previous assessment study and perspective view on why the case study platform needs to be assessed. Section 2 and Section 3 include the finding of the code-noncompliance points of the platform based on the recommendations of API RP 2A-2007. Section 4 presents the remedy actions recommended for the fit for purpose of the platform.
Vortex-Induced Motion Evaluation on Semi-Submersible Platforms Using a Time Domain Methodology
Authors: de Lacerda Thiago Ângelo Gonçalves , Ellwanger Gilberto Bruno , Senra Stael Ferreira
Published: November 2013

As exploitation activities moves into fields located in deep water, the industry has been addressing studies aiming at concepts of platforms that reduce the influence of environmental loads on risers. The deep draft semi-submersible platform is one of these concepts [1]. The semi-submersible deep draft platform is described in a simplified way as a deck supported by columns connected by pontoons. This particular geometry can make this structure susceptible to a significant low frequency vibration movement caused by vortex shedding. This phenomenon is usually known as VIM (Vortex Induced Motions). The amplitude of these motions depends on some parameters such as current velocity, hydrodynamic diameter of the column, natural frequency of the platform due to the mooring system and structural damping (hull, mooring lines and risers). One of the consequences of these motions is the increase of the fatigue damage on the risers connected to the platform.

In a previous work presented by the authors in 2009[2], a time domain methodology based on a Van der Pol equation was described. This methodology was developed in order to represent the vortex shedding phenomenon. In this paper the Van der Pol methodology is improved by considering the VIM phenomenon acting on a multicolumn structure.
The Effect of Inclusions on the Fracture Toughness of Local Brittle Zones in the HAZ of Girth Welded Line Pipe
Authors: Moore Philippa , Nicholas Joanna
Published: November 2013

A programme of testing and research has been carried out to deliberately identify the causes of, and measure the fracture toughness of, potential local brittle zones (LBZs) in the heat affected zone (HAZ) of girth welds in a 20mm thick API 5L X70 grade pipe. This involved determining the most susceptible test temperature and notch location within the HAZ of two different welds by Charpy testing to define the T40J. Subsequent fracture toughness testing of a large set of fracture mechanics specimens notched into this region and tested at a low temperature based on T40J was able to achieve occasional low fracture toughness results. Post-test metallography identified that the inclusions in the parent metal affected the risk of brittle fracture initiation in two ways; firstly large and clustered inclusions acted as local brittle zones and/or fracture initiation sites; secondly, the large inclusions had reduced the effect of grain boundary pinning, as would be expected from well-dispersed smaller inclusions. This then resulted in significant grain coarsening in the HAZ fusion line, which could also act as a low toughness fracture initiation region. The inclusion types were chemically analysed, and recommendations were made to the steel producer in order to further improve the fracture toughness of the X70 pipe at increasingly lower temperatures.
VIV Safety Factors for Drilling Risers: Propagating Model Uncertainty in a Long-Term Reliability Analysis
Authors: Tognarelli Michael A. , Panicker Nikhil , Campbell Mike , Winterstein Steven R.
Published: November 2013

Previous work [1] has given an approach to estimate vortex-induced vibration (VIV) factors of safety (FOS) for deepwater drilling risers. This work compared fatigue predictions from the industry-standard fatigue analysis software SHEAR7 with full-scale measured data. The field data have been collected over the past several years from five drilling risers without VIV suppression devices. These risers are on rigs under contract to BP worldwide, at sites susceptible to high currents. Collectively, the data correspond to 9,600 10-minute field measurements, equivalent to 0.18 years of continuous monitoring.

A worked example for a deepwater drilling riser in the Gulf of Mexico was used to show how the FOS methodology can be applied in the case of multiple design currents each with a different annual probability of occurrence. However, the paper cited future work that included development of a mathematical approach that would permit a designer to calculate a modified FOS that accounts for the number of events (currents or wave seastates) analyzed.

This paper documents a mathematical approach developed that can be used to calculate the riser design FOS based on multiple events of varying fatigue damage amplitude and probability. Note that the approach is not limited to VIV. It can be used for long-term wave fatigue life for example, assuming the designer has an understanding of the statistical differences between software predictions and actual field response.
Gap Effects at Side-by-Side LNG-Transfer Operations
Authors: Clauss Günther F. , Dudek Matthias , Testa Daniel
Published: November 2013

The current demand of liquefied natural gas (LNG) from remote marine locations pushes the design of floating LNG (FLNG) liquefaction or regasification facilities, where LNG is transferred between shuttle carrier (LNGC) and terminal. Even if the tandem configuration is the primary choice for LNG transfer at rough offshore locations, side-by-side configurations would be the preferred option because of existing midship coupling manifolds on the present carrier fleet (no need for manifold modifications) as well as standard mooring systems and transfer-process-chains similar to oil-transfer. Therefore, the operation conditions at rough seas have to be improved to allow side-by-side LNG-transfer and to reduce offloading downtime.

Within the SOTLL-project, side-by-side LNG transfer up to HS = 3 m is reached as a transfer limit using a new flexible pipe design, the advantages of sheltered areas at the leeside of the terminal barge and an optimized ship transfer position due to a flexible longitudinal offloading position. In addition to the evaluation of the hydrodynamic characteristics of this multibody system, one key aspect is the analysis of the exciting forces and motions due to wave amplification between the ships. In the gap between the hulls, the incoming wave field is amplified and changes dramatically. Depending on gap width, longitudinal offset, wave heading and length, large wave amplifications, standing waves and other resonance phenomena are observed which may result in high relative motions and increased forces of the entire mooring system. In this paper, the gap effects are investigated in detail with numerical approaches in frequency domain, validated by model tests at TU Berlin. A typical offloading scenario with barge and carrier is investigated for different gap sizes to identify suitable transfer configurations and ensure safe LNG offshore transfer up to HS = 3 m.
Statistics of Extreme Wind Speeds and Wave Heights by the Bivariate ACER Method
Authors: Naess Arvid , Karpa Oleh
Published: November 2013

In the reliability engineering and design of offshore structures probabilistic approaches are frequently adopted. They require the estimation of extreme quantiles of oceanographic data based on the statistical information. Due to strong correlation between such random variables as e.g. wave heights and wind speeds, application of the multivariate, or bivariate in the simplest case, extreme value theory is sometimes necessary.

The paper focuses on the extension of the ACER method for prediction of extreme value statistics to the case of bivariate time series. Using the ACER method it is possible to provide an estimate of the exact extreme value distribution of a univariate time series. This is obtained by introducing a cascade of conditioning approximations to the exact extreme value distribution. When this cascade has converged, an estimate of the exact distribution has been obtained. In this paper it will be shown how the univariate ACER method can be extended in a natural way to also cover the case of bivariate data. Application of the bivariate ACER method will also be demonstrated at the measured coupled wind speed and wave height data.
The Collision Time of Free-Floating Multiple Vessels in Waves
Authors: Guo Fei , Fu Shixiao , Wei Wei , Zhang Weidong , Chen Xiqia , Xu Yuwang
Published: November 2013

Resonance behavior and collision could possibly occur in the process of material replenishment and/or rescue procedures between multiple vessels in ocean environment. In this paper, a numerical model consisting of a free-floating barge and a cargo ship in parallel is built up to study the collisions between them in waves. Hydrodynamic interactions and wave-exciting forces on the vessels are calculated based on 3-D linear potential theory, and near field approach is adopted to deal with the second-order drift forces, which are found to be critically distance-dependent and wave direction-dependent. By synthesizing both the first- and second-order response, the relative motion trajectory of two vessels is obtained, which is applied to calculate the collision time in regular waves with different spacing distances and wave angles. Second-order drift forces are found dominant with respect to collision time. Results indicate distance between vessels and wave direction play the sensitive roles in prediction of collision time while the influence of various collision points can be ignored.
Effect of Pipeline Thermal Expansion on Direct Electric Heating Cable
Authors: Brany Ahmad Makintha , Damsleth Per , Gil Cyprian
Published: November 2013

Subsea Direct Electrical Heating (DEH) of pipelines has been in operation for more than 10 years and is therefore considered by most Operators as proven technology. Aside from the electrical aspects, there are mechanical design challenges with installing a cable piggyback fashion onto a HPHT flowline that expands and contracts with the temperature of the flowing wellstream. The cable may also need protection against fishing gear which becomes an integral part of the DEH pipeline system.

Depending on the pipeline operating temperature, the DEH cable may experience excessive axial tensile strain due to thermal expansion and global buckling unless sufficient cable overlength is incorporated during pipeline installation. The cable will be subjected to additional strains at a thermal buckle location when subject to trawl gear pullover. During DEH cable operation, the flowline will initially be cold and contracted but the cable expands due to heating of the cable itself, adding to any installed compression to provide overlength. The following study has been performed to investigate strains in the cable and pipeline due to their responses to thermal and trawlboard loads and to determine the required overlength to keep the cable within strain limits.

Finite Element models of the flowline with piggybacked DEH cable was developed with both ANSYS and ABAQUS in order to analyse their interaction during installation and operation on the seabed. The influence of friction between cable and protection system, and position of the cable on the flowline was simulated in order to quantify the required overlength to be installed during pipelay to avoid overstraining the cable in tension as well as compression.
The Effect of Measured Whipping and Springing on LNG Vessels
Authors: Storhaug Gaute , Oma Narve , Blomberg Bjarne , Hirota Kazuhiro
Published: November 2013

Strain measurements of structural members onboard two LNG vessels have been collected for a period of 5 years. The vessels have been sailing in the world wide trade except for the North Pacific area. The time spent in the North Atlantic is about 40%, which is higher than for typical LNG vessels. The vessel speed has been relatively low in average and well below the service speed, still the effect of springing and whipping has been significant on the fatigue and extreme loading. Previously, results from the strain sensors have been presented versus the fatigue and extreme loading on a general level, independent on the environmental conditions. In this paper the focus is more towards how the vessel behaves in wind/waves with respect to springing and whipping in order to understand more of the relationship between accumulated fatigue damage, heading and loading condition.

The vessels have been equipped with wave radars and wind sensors. The effect of whipping and springing on accumulated or part fatigue damage versus relative heading is demonstrated. The effect as a function of the wave height/wind speed is shown for selected headings. One of the objectives is to check if the wind sensor can be a useful alternative to the wave sensor to capture the physics. Some of the data is studied for ballast and cargo condition separately in order to see if these loading conditions can be merged, which is desirable from an assessment point of view.
Proposals for Integrity Management of Flexible Pipe
Authors: Yuan Shuai , Tang Gao , Bai Jie , Badaruddin Mohd Fauzi
Published: November 2013

The increasing use of flexible pipes in subsea with high pressure/high temperature brings about much more challenges, for example flexible riser fatigue, bird-caging and armour wire disorganization, development of flexible pipes with smooth bore and ones with anti-H2S layer, which demand operators to adopt an effective integrity management program including every phase of flexible pipe industry to avoid reduction of production. To date, much research work has been carried out on this topic.

In this paper, the applicable inspection and monitoring measures are presented as proposals to develop the integrity management of flexible pipes. Meanwhile, this paper takes a practical project as an example to show the whole process of integrity management study clearly. The process can be used as reference for the other similar integrity management projects.
Cool-Down Time Estimation Through Numerical Analysis for Partially Insulated Offshore Pipe-in-Pipe Field Joints
Authors: Chinello Luca , Cooper Philip , de Haas Ramon , Boyd Helen
Published: November 2013

Offshore pipe-in-pipe systems require high performance thermal insulation to maintain high fluid temperature at arrival and to avoid hydrate formation during the cool-down process that follows a pipeline shut-down. At field joints, it might be difficult to achieve the design insulation performance due to installation challenges. In these cases, the insulation layer partially fills the gap between the inner and outer pipes and thus “cold spots” could potentially arise at field joints during the pipeline operation and cool-down. In this paper the impact on the thermal performance of partially insulated pipe-in-pipe field joints is evaluated through Computational Fluid Dynamics (CFD). Thermal convection is included in the fluid model for the pipe content and the air gap between the inner and outer pipes. Comparison is also made between the numerical analysis and simplified lumped-parameter models. Results from numerical simulations show that for the case considered no cold spot arises due to a lack of field joint insulation and length-averaged Overall Heat Transfer Coefficient (OHTC) can be used to predict the pipeline cool-down time. Numerical predictions have been compared to simulated service test results, which confirm the length-averaging effect on the OHTC. Further studies are recommended to assess potential cost savings that could be achieved for uninsulated field joints.