bluuebunny/crossref_metadata_2025 huggingface dataset:
The Floatel Superior Loose-Anchor Incident and its Significance for Design and Operation of Semi-Submersibles
Authors: Andersen Terje Leenhart , Kvitrud Arne , Jensen Jan Erik
Published: September 2014

The combination of dynamic positioning (DP) and conventional anchoring of semi-submersible platforms in the oil industry has recently lead to a serious incident with large loss potential: the Floatel Superior platform suffered severe damage to its hull from a loose anchor in heavy seas during the night of November 6th–7th, 2012. This paper discusses the root causes leading to the incident. The investigation report from the Petroleum Safety Authority Norway (PSA) concludes that damage leading to the loose anchor had developed over several months, and various warning signs during that period were not heeded. The main reason for the damage was rooted in the combined operation mode where position-keeping by use of DP resulted in large environmental loads on the anchors while they were stowed in their bolsters along the vessel side in rough weather when the installation was at survival draught and operating on DP. The anchors on Floatel Superior had also been exposed to environmental loads in excess of the design capacity during extended transit two years prior to the incident. The incident itself was handled efficiently by the crew and emergency preparedness organizations, with 336 people evacuated by helicopter and the remaining 38 crew members transporting the damaged vessel to shore for repairs.

In this paper, an immediate recommendation for removing anchors from the splash-zone while operating on DP is presented as relevant for other installations with similar class notation. In a longer perspective, the investigation found that both well-established design codes and their application in the actual design had shortcomings which played a role in the cause of events. The vessel design appears to have been performed with a principal focus on sub-units (anchor, winch, bolster, pendant-wire attachment) and to an insufficient extent on the combined system use under which it was to be operated. A long-established method of design for connections of auxiliary structures to the main hull structure also showed shortcomings in the event: the doubler plates or pads used to attach the bolsters to the hull did not function as weak links as intended.
Advantages of Polyester Mooring for Deepwater Floaters
Authors: Xu Lixin , Chen Jieyan
Published: September 2014

Since first installed at a water depth of 4,660 feet in 1997, polyester mooring systems have now been used on floating platforms in the Gulf of Mexico (GOM), Brazil and other regions. The Mad Dog Spar was the first floating production system (FPS) with permanent deepwater polyester moorings in the GOM. After the Mad Dog Spar, the deepest water depth is 7,800 feet in which a polyester mooring system was installed on the Perdido Spar. The polyester mooring systems have performed favorably, e.g., in the GOM, experiencing hurricanes without incident. The polyester rope in general is more advantageous than steel wire in deeper water due to reduced weight (and tension), high strength, durability (better fatigue and no corrosion), and improved floater global performances (less offset, etc.). Moreover, while a floating production platform is designed to support riser systems, fatigue damage of risers due to Vortex Induced Motions (VIM) of the platform are important design drivers particularly in the GOM. The polyester mooring system has a higher restoring force in horizontal (thus a higher lateral stiffness) in currents resulting in a significantly better fatigue performance (less current bins with VIM lock-in) than the steel mooring system does.

The paper herein presents a comparative study with two kinds of mooring systems (polyester ropes and steel wires) for the same platform. Differences between the polyester and steel mooring systems are evaluated for various aspects, such as the mooring system configuration and performance, installation risks, operations, and impact on hull and riser system design and performance. The results also indicate cost savings for the polyester mooring equipment and the overall production system.
Experimental Study of Turbulent Flow Induced by Regular and Irregular Waves Above a Rock-Armored Slope
Authors: Galani Konstantina A. , Dimou Giannis D. , Dimas Athanassios A.
Published: September 2014

The aim of the present work is the experimental study of the turbulent flow induced by waves above a physical model of a rock-armored slope of 1/3. The armor consisted of two layers of rocks with characteristic diameter D50 = 4.4cm. Measurements of the instantaneous velocity fields were conducted using an underwater planar PIV system. Four cases of incoming waves were tested, two cases of regular waves of 1st order Stokes theory with wave period of 1.134s and wave heights of 0.04m and 0.08m, respectively, and two cases of irregular waves, generated from a JONSWAP spectrum, with a peak period of 1.134s and significant wave heights of 0.04m and 0.08m, respectively. For the regular waves, the period-averaged velocity profiles show the existence of a strong undertow current heading towards deep water, while turbulence is not homogeneous with larger horizontal fluctuations. The phase-averaged horizontal velocity profiles present systematically larger values during wave trough passage than during wave crest passage. Furthermore, as the depth becomes smaller, the waveform loses its symmetry, with the wave trough becoming wider and the wave crest steeper. For the irregular waves, the mean velocity profiles show the existence of an undertow current weaker in magnitude than the one in the regular waves, while turbulence is still not homogeneous with larger horizontal fluctuations. For both wave cases, spanwise vorticity, which is generated at the rough surface of the rock-armored slope, is transported landward by the turbulent velocities.
Stochastic Space-Time Extremes of Wind Sea States: Validation and Modeling
Authors: Barbariol Francesco , Benetazzo Alvise , Bergamasco Filippo , Carniel Sandro , Sclavo Mauro
Published: September 2014

Damages and accidents occurred to offshore structures and routing ships raise questions about adequacy of conventional time domain analysis of short-crested sea waves. Indeed, experimental and field evidence showed that during such wave states, typical of storms, the maximum sea surface elevation gathered at a single point in time, i.e. the time extreme, tends to underestimate the actual maximum that occurs over a surrounding area, i.e. the space-time extreme. Recently, stochastic models for the prediction of multidimensional Gaussian random fields maxima, e.g. Piterbarg’s theorem and Adler and Taylor’s approach, have been applied to ocean waves statistics, permitting to extend extreme value analysis from time to space-time domain. In this paper, we present analytical and numerical approaches aimed at supporting applicability of such models, which is limited by the knowledge of directional spectrum parameters. Firstly, we validate stochastic models against stereo-photogrammetric measurements of surface wave fields. Then, we investigate the dependence of space-time extremes upon physical parameters (wind speed, fetch length, current speed) in the context of analytical spectral formulations, i.e. Pierson-Moskowitz and JONSWAP, and by using spectral numerical wave modeling. To this end, we developed two sets of closed formulae and a modified version of the SWAN model to calculate parameters of analytical and arbitrary directional spectra, respectively. Finally, we present preliminary results of a 3 years Mediterranean Sea hindcast as a first step towards operational forecasts of space-time extremes.
Ultra Long Range DTS (>300km) to Support Deep Offshore and Long Tieback Developments
Authors: Gyger Flavien , Chin Sanghoon , Rochat Etienne , Ravet Fabien , Niklès Marc
Published: September 2014

The continuous demand for energy supplies leads operators to explore and exploit more remote offshore oil fields characterized by deeper deployments, longer tiebacks and subsea completion. These offshore developments either lie in tropical regions subject to stormy weather or in extreme arctic conditions. They imply new requirements in terms of operational efficiency and predictive maintenance for cost optimization as well as environmental footprint minimization.

Consequently, leak detection and flow assurance are required over distances exceeding hundreds of kilometers. And the increasing trend of building all electric completion and heated flowlines impose the use of equally long high voltage power cables.

Distributed temperature sensing (DTS) is a widely adopted technology for leak detection and localization, for heated flow line monitoring and for power cable thermal rating. Optical fiber cables are installed alongside the structure providing detection with meter accuracy localization and degree thermal variation sensitivity. However, there is currently no commercially available DTS instrument capable of reaching a 100km single span without any means of repeating the signal, let alone longer range. Thus, a 300km pipeline would normally require marinized equipment, which is often not a practical solution.

The Omnisens DITEST, a DTS instrument based on Stimulated Brillouin Scattering (SBS) is intrinsically capable of the longest available sensing length, now reaching a sensing distance of around 70km at once; in addition, it is compatible with optical amplifier technology as found in any submarine network. In the present work, we show that, by combining 5 spans of slightly above 65km and 4 optical repeaters together, it is possible to achieve a total of 330km sensing from a single DITEST. Measurement time over the full distance was typically 100minutes, providing a temperature resolution of typically 1K for a spatial resolution of 3m at the end of the 330km long sensing fiber. Shorter measurement time, more suitable with the application could be achieved using optical coding.

As the optical repeater technology is similar to that of telecommunication, efficient cost reduction could be achieved by sharing the repeaters between the sensing system and the needed communication system. At the end of each sensing span, the sensing cable is connected to the repeater for amplification purposes and then goes back to the pipeline for monitoring the next span. The repeaters themselves are electrically powered by conductors embedded in the communication infrastructure.

A viable and cost effective solution to meet the requirements of subsea long distance leak monitoring is thus demonstrated.
Efficiency of Ice Management for Arctic Offshore Operations
Authors: El Bakkay Badr , Coche Edmond , Riska Kaj
Published: September 2014

The exploration and production of polar oil and gas fields, which are technologically challenging due to extreme weather conditions, are also constrained by strong environmental issues. Safe and economical activities in such hostile and fragile regions require very insightful engineering. The presence of sea ice is representing a triple challenge: economical, technological and environmental. This makes the Arctic exploration and production activities complex.

Ice Management (IM) is one of the tools that could efficiently assist to develop Arctic reserves. However, for each project that uses IM operations, a preliminary study is required to evaluate the efficiency of these support operations and to estimate the possible extension in the season of operation of a field.

Efficiency of an IM philosophy can be estimated in a global view based on the extension in the operability window. In a more detailed view, it can be assessed taking into account the optimal number of icebreakers, the IM patterns, the available time for eventual disconnection, and the floe size reduction (leading to ice load reduction). For this study, we will focus on the ice floe size and loads reduction.

The most common approach for physical management of sea ice is the one where icebreakers reduce floe size of the drifting ice upstream the floating platform (ref. Moran et al. [1], Coche et al. [2]). This paper describes this philosophy and demonstrates based on real-time simulation that its benefit is limited to mild ice scenarios such as unidirectional ice drift. A more efficient way to manage sea ice is (1) to identify the most hazardous events (e.g. big ridges); (2) prioritize these events; and (3) deal with them starting by the most hazardous one.
Flexible Armor Wires: Fatigue Load Frequency Effects and an Accelerated Pitting Methodology
Authors: Krishnan Venkat R. , Asher Stefanie , Doynov Krassimir , Zhang Yan-Hui
Published: September 2014

Corrosion fatigue in the tensile armor layer is a design consideration for both flexible risers and flowlines offshore. Recently, the industry has experienced a handful of in-service flexible pipe replacements due to corrosion fatigue of armor wires. That experience motivated the work effort summarized herein. This paper presents the preliminary results from an experimental program undertaken by ExxonMobil to evaluate and suggest improvements to the currently established fatigue testing methodology for armor wires in corrosive environments. In particular, the results from a frequency scanning test program for armor wires and a methodology for artificially generating pitted armor wire specimens for fatigue endurance tests are presented.
Preliminary Structural Design of OCAES Vessel
Authors: Xiao Jinfu , Gabr M. , Rahman M. S.
Published: September 2014

A vessel for Offshore Compressed Air Energy Storage (OCAES) is being considered as a part of the emerging technology to store intermittently generated energy from offshore renewable sources. The basic idea is to submerge the storage vessel into the ocean at a large water depth, convert the energy into compressed air, store it with using the hydrostatic water pressure to balance the compressed air pressure without requiring the storage vessels to resist high internal air pressure. An additional advantage is that the interface of air and water acts as a piston to maintain the air pressure level.

In this study, a rectangular configuration and a cylindrical configuration are proposed for the OCEAS vessel. Stress controlled analyses are performed for the preliminary design of the OCAES vessel with emphasis on characterization of applied cyclic load induced by the air/water exchange in the inflation/deflation process. The stress distribution and deformation pattern in the OCAES vessel at different loading stages are studied by using the multi-physics FEM program COMSOL. Key parameters including storage volume, geometry of OCAES vessel, and material selection are discussed.
Numerical Simulation of Wave Propagation Over Submerged Composite Breakwaters Using the Immersed Boundary Method
Authors: Koutrouveli Theofano I. , Dimas Athanassios A.
Published: September 2014

The two-dimensional flow induced by waves over submerged breakwaters of two different shapes is studied by means of a two-phase (water and air) Navier-Stokes equations solver. A time-splitting method is used for the temporal discretization, while the spatial discretization is based on the use of finite differences in a Cartesian staggered grid. The implementation of the boundary conditions at solid surfaces, as well as the treatment of the free surface is performed using the immersed boundary method where the breakwater, the seabed and the free surface are boundaries immersed in the numerical grid. The numerical model was applied on the propagation and breaking over a constant slope beach, as well as on the propagation and nonlinear transformation of waves over two types of submerged breakwaters, i.e., trapezoidal and composite (with berm in the up-slope side). The results of the numerical model reveal that the presence of the berm reduces the transmission coefficient and this reduction increases with the decrease of the berm depth of submergence.
Residual Compressive Strength of Dented FPSO Side Shell Panel
Authors: Amante Diogo do Amaral , Chujutalli John Alex , Estefen Segen F.
Published: September 2014

This paper presents a non-linear finite element study to obtain the ultimate compressive strength of dented FPSO side stiffened panels. The finite element program ABAQUS was used to perform numerical analysis. The analysis is carried out in two steps. First the supply collision damage is imposed using the ABAQUS explicit code program. After the indentation, a compressive load is applied and then the panel residual compressive strength is obtained. A parametric study was accomplished to evaluate the influence of the dent depth on the stiffened panel strength. Damages occurring on the stiffener and between the stiffeners are compared.
Fluid-Structure Interaction Modeling of a Subsea Jumper Pipe
Authors: Elyyan Mohammad A. , Perng Yeong-Yan , Doan Mai
Published: September 2014

Flow-induced vibration (FIV) is one of the main reasons for subsea piping failure, where subsea pipes, which typically carry multiphase flow, experience large fluctuating forces. These fluctuating forces can induce severe vibrations leading to premature piping failure. This paper presents a transient numerical study of a typical subsea M-shape jumper pipe that is carrying a gas-liquid multiphase flow subject to a slug frequency of 4.4 Hz, starting from rest to include the start-up effect as part of the study. 3-D numerical simulations were used to capture the fluid-structure interaction (FSI) and estimate pipe deformations due to fluctuating hydrodynamic forces. In this paper, two FSI approaches were used to compute the pipe deformations, two-way coupled and one-way decoupled. Analysis of the results showed that decoupled (one-way) FSI approach overestimated the peak pipe deformation by about 100%, and showed faster decay of fluctuations than coupled (two-way) FSI analysis. The assessment of resonant risk due to FIV is also discussed.
Human Injury Probability During Water Entry of Free-Fall Lifeboats: Operational Criteria Based on Long-Term Simulations Using Hindcast Data
Authors: Fouques Sébastien , Sauder Thomas , Reinholdtsen Svein-Arne , van Dam Esra , Uittenbogaard Jeroen
Published: September 2014

The paper addresses the safety of occupants in free-fall lifeboats launched from turret-moored floating production, storage and offloading (FPSO) vessels. It presents a methodology for assessing operational limits with respect to acceleration-induced loads experienced by the passengers during water entry. The probability of being injured is estimated by means of numerical simulations for several seat rows and in various sea states described in terms of significant wave height and mean wind velocity. Those results are therefore practical for on-site decisions regarding the use of the free-fall lifeboats.

The numerical simulations performed to estimate the 6-degrees of freedom (6-DOF) water entry accelerations in the lifeboats are based on more than 50 years of hindcast metocean data. These consist of sea state parameters provided every third hour and including the significant wave height, the peak period and the direction of both wind-sea and swell as well as the direction and mean velocity of the wind. In a first step, the motion of the FPSO is computed for the whole time period covered by hindcast metocean data, using a state-of-the art numerical model validated against experimental data. The model includes nonlinear excitation forces, a dynamic positioning system with a realistic heading control strategy, mooring line forces as well as turret-hull coupling. The obtained FPSO motion is then used in Monte Carlo simulations of lifeboat launches performed for selected time windows in the original metocean hindcast database corresponding to selected intervals of the significant wave height and mean wind velocity. In addition to the 6-DOF skid motion, the lifeboat launch simulations account for the effects of wind and waves diffracted by the FPSO hull.

Finally, a probabilistic model describing the joint-distribution of several injury types and water entry acceleration parameters computed through the launch simulations is used to evaluate the injury probability. The results are presented in term of seating matrices showing critical seat rows, in which the probability of being injured exceeds a pre-defined threshold.
The Hydrodynamic Performance of a Twin-Hull FLNG Concept
Authors: Schut Xavier , de Boom Wim
Published: September 2014

This paper presents the results of a numerical and experimental study concerning the hydrodynamic performance of the SBM midscale Twin-Hull FLNG concept. SBM’s innovative design consists of two LNG carriers joined together, and converted into a single FLNG facility. This option provides both sufficient deck space and storage capacity while having the benefit of a conversion project, being reduced delivery schedule and lower costs. This results in an unusually wide vessel for which only very limited number of precedents exist.

Important hydrodynamic performance indicators such as first order motions, green water and bottom slamming are investigated by means of sea keeping tests. In these tests the scale model of the FLNG is subjected to moderate and harsh environmental conditions from various headings. The results are used to verify the numerical predictions of the global motions, wave drift forces and internal loads and to assess green water and bottom slamming loads.

Current coefficients were calculated by computational fluid dynamics (CFD) and validated with wind tunnel experiments. All load components (wind, wave, current) are then used to evaluate the ability of the vessel to weathervane. This paper presents the investigation of the quality of numeric predictions by comparing model test data against numerical data and hence it summarizes the hydrodynamic performance of the Twin-Hull.
A Complete Failure of a Floatover Installation With Strand Jack Lifting Scheme and a Great Success of Float-On and Float-Off Rescue Operation
Authors: Wang Alan M. , Wang Tao , Li Youcheng , Yu Changsheng , Tao Fuwen , He Chen
Published: September 2014

An innovative and complicated strand jack lifting technology was applied to floatover installation of DPP integrated topsides in South China Sea. Due to initial failure to lower the four outer support legs, an unfortunate chain of events occurred thereafter, thus yielding a complete failure of the floatover installation never happened in history. A rescue operation was carried out in the two following days to successfully salvage both the topsides and the floatover barge stranded inside the jacket slot in extraneous circumstances. This paper presents the floatover systems and the failure events of the innovative low-deck floatover technology, as well as the successful rescue operations by floating on and floating off the topsides onto the eight-legged jacket. Big lessons learned are also addressed here.
CFD in the Loop: Ensemble Kalman Filtering With Underwater Mobile Sensor Networks
Authors: Hackbarth Axel , Kreuzer Edwin , Schröder Thorben
Published: September 2014

In marine environments, sparse in-situ measurements can be used for the estimation of the fluid dynamic field. To make best use of a mobile sensor network in an environment whose dynamics can be described by the Navier-Stokes equations, we developed a framework for data assimilation with motion-constrained underwater vehicles, that takes the physical field properties into account while sampling. Our algorithm uses an ensemble Kalman filter that propagates hundreds of slightly varied coarse fluid dynamic simulations through time. Flow and scalar measurements from the mobile sensors are integrated into all ensemble members. We implemented a model predictive controller to calculate covariance minimizing paths from the estimated flow field and motion primitives of the vehicles, which are affected by a strong current. Thereby, we were able to indirectly track dynamically changing wall temperatures through measurements of flow field variables.
Seismic Reliability of a Fixed Offshore Platform Against Collapse
Authors: Dyanati Mojtaba , Huang Qindan
Published: September 2014

As many jacket type steel platforms have been constructed in the highly active seismic area, seismic reliability evaluation of such structures is desirable. Ultimate limit state (ULS) with base shear capacity and demand can be used to estimate seismic performance of fixed offshore platform against collapse. Base shear capacity is evaluated from pushover analysis on a 3D finite element model of the offshore structure using different load patterns. Base shear demand is calculated from spectral acceleration at a given site and the total mass of the platform. Uncertainties are considered in both capacity and demand evaluations.

With the limit state function, seismic fragility of a prototype structure is assessed using reliability analysis. The results indicate that various load patterns affect the seismic performance evaluation. It is also found that the steel yield stress is a critical parameter in the reliability of the steel jacket platforms.
Surface Characterisation and Fatigue Strength of Corroded Armour Wire
Authors: Berge Stig , Wang Ting , Langhelle Nina
Published: September 2014

Tensile armour wire from two flexible risers was fatigue tested for assessment of SN-curves. Both risers had been subjected to sea water flooding of the annulus for several years. The wire was corroded in the form of pits, however, the cross section was not significantly reduced. Reference wire from the same batches of material that had not been subjected to corrosion was also tested. Testing was carried out in air, to investigate the effect of accumulated corrosion damage in terms of a fatigue notch factor. The fatigue strength of the corroded wire was significantly reduced, corresponding to a fatigue notch factor in the range 1.5–1.7. The wire was examined by optical and by electron scanning microscopy and the surface defects were characterised.
Generation of Multivariate Wave Conditions as Input for a Probabilistic Level III Breakwater Design
Authors: Mercelis Peter , Dufour Marc , Alvarez Gebelin Ariel , Gruwez Vincent , Doorme Sarah , Sas Marc , Leyssen Gert
Published: September 2014

For an offshore LNG project situated in the estuary of the Rio de la Plata nearby Montevideo, Uruguay, it was required to verify the deterministic design of the protective rubble mound breakwater and the jetty infrastructure with a level three probabilistic design. Therefore, in first instance extreme site conditions were required both in front of and behind the breakwater.

To obtain these conditions, the first step is to extrapolate the offshore variables in order to translate them to the breakwater location. All the possible combinations of extreme wind, water level and waves are quantified with a probability of occurrence. A combination of univariate extreme value distributions, copula’s and regression is used to describe the multivariate statistical behaviour of the offshore variables. The main variable is the wind velocity, as in the area of concern extreme wave conditions are wind driven. The secondary variable is water level. Wind velocity and water levels are only correlated for some wind directions. For these directions, wind velocity and water level extreme value distributions are linked through a multivariate Gumbel Copula. The wave height at the model boundaries was taken into account by a regression function with the extreme wind velocity at the offshore location and the wave period by a regression function with the wave height.

This way 1515 synthetic events were selected and simulated with the spectral wave model SWAN, each of which a frequency of occurrence is calculated for. However, due to refraction and diffraction effects of the approach channel (in the area of concern water depths are limited to about 7 m and the navigation channel has a depth of about 14 m), the port basin and the breakwater itself, the spectral wave model SWAN is not sufficient to accurately calculate the local wave conditions in the entire area of interest. Therefore a non-linear Boussinesq wave model (i.e. Mike 21 BW) was set up in addition, using input from the spectral model at the boundary and including the navigation channel of more than 12 km long. Combining both models, significant wave heights are obtained on both the seaward side and the leeside of the breakwater with corresponding frequencies of occurrence. This approach allows the determination of conditional return periods and generates the site conditions required for a probabilistic level three design of the breakwater and the jetty infrastructure taking for example the joint probabilities between waves and water levels fully into account as needed for overtopping or failure calculations.
Incorporating Engineering Criteria to the Synthesis and Optimization of Submarine Pipeline Routes: On-Bottom Stability, VIV-Induced Fatigue and Multiphase Flow
Authors: Baioco Juliana Souza , Stape Philip , Granja Marcelo , Albrecht Carl Horst , de Lima Beatriz Souza Leite Pires , Jacob Breno Pinheiro
Published: September 2014

In offshore field development it is extremely important to plan the submarine pipeline route for the hydrocarbons transportation. To ensure an expedited project, researchers from Petrobras and LAMCSO/COPPE/UFRJ have been involved in the development and implementation of a computational tool, based on Evolutionary Algorithms, for the synthesis and optimization of submarine pipeline routes. In this tool, each candidate route is evaluated in terms of several criteria that are incorporated in an objective function, to take into account the relevant aspects that should be considered in the design of a route, such as the pipeline length, bathymetry data, obstacles, number and length of free spans and others.

Previous works described the incorporation of engineering criteria related to on-bottom stability and VIV-induced fatigue in free spans for pipes under the action of hydrostatic and hydrodynamic environmental loads of current and waves. Now, this work presents more examples of applications related to this latter criterion, and focus on the incorporation of a production criterion related to the pressure drop due to multiphase fluid flow, according to correlations for inclined pipelines.

The goal is to obtain a robust tool to reduce the design time and minimize the cost of the pipeline route, leading to an optimal route configuration complying with various engineering criteria that, on a traditional design procedure, would be checked separately only after the definition of the route.
Lifetime Assessment of Flexible Pipes
Authors: Eide Jorgen Thomas Wold , Muren Jan
Published: September 2014

This paper presents a methodology for performing a lifetime assessment of flexible pipes applicable to re-qualification during the original design life and at life extension. A systematic approach is developed, providing flexible riser and flowline engineers a standardized methodology for determining the current integrity- and risk level. The objective is to provide methodology that is easy to implement, thus enabling consistent assessments of all flexible pipes in the operator’s portfolio. The methods described are taken from work performed in a recent JIP run jointly by MARINTEK/NTNU and 4Subsea, and is based on substantial experiences with lifetime assessment combined with a review of relevant guidelines and standards. Key areas are suggested for industry improvement and recommendations to further developments, to increase both efficiency and quality of the lifetime assessment process.
The Inverse KdV-Based Nonlinear Fourier Transform (KdV-NLFT): Nonlinear Superposition of Cnoidal Waves and Reconstruction of the Original Data
Authors: Brühl Markus , Oumeraci Hocine
Published: September 2014

Since 2008, at Leichtweiß-Institute for Hydraulic Engineering and Water Resources at TU Braunschweig a KdV-based nonlinear Fourier transform is implemented and successfully applied to numerical and hydraulic model test data of solitary wave fission behind submerged reefs [1]. The KdV-NLFT is the application of the direct and inverse scattering transform for the solution of the Korteweg-deVries equation. This approach explicitly considers both solitons and oscillatory waves (cnoidal waves) as spectral basic components for the decomposition of the original data. Furthermore, the nonlinear wave-wave interactions between the nonlinear spectral basic components are explicitly considered in the analysis. The direct KdV-NLFT decomposes the original data into cnoidal waves and provides wave heights, wave numbers or frequencies, phases and the moduli which are a measure of the nonlinearity of cnoidal waves. Details of this procedure are given in Brühl & Oumeraci [2]. The interpretation of the nonlinear spectral basic components is described in Brühl & Oumeraci [3]. The inverse KdV-NLFT which is addressed here calculates the nonlinear wave-wave interactions between cnoidal waves and provides the original data by superposition of cnoidal waves and their nonlinear interactions. The practical application of the KdV-NLFT for the analysis of long-wave propagation in shallow water is presented in Brühl & Oumeraci [4].
Experimental Analysis of a Vertical and Flexible Cylinder in Water: Response to Top Motion Excitation and Parametric Resonance
Authors: Franzini Guilherme R. , Pesce Celso P. , Salles Rafael , Gonçalves Rodolfo T. , Fujarra André L. C. , Mendes Pedro
Published: September 2014

Experiments with a flexible and submerged cylinder were carried out to investigate fundamental aspects of risers dynamics subjected to harmonic excitation at the top. The flexible model was designed aiming a high level of dynamic similarity with a real riser. Vertical motion, with amplitude of 1% of the unstretched length, was imposed with a device driven by a servomotor. Four values of the ratio between the exciting frequencies and the first eigenfrequency were investigated, namely ft: fN,1 = 1 : 3; 1 : 1; 2 : 1 and 3 : 1. Cartesian coordinates of 43 monitored points positioned all along the span were experimentally acquired by using an optical tracking system. A simple Galerkin’s scheme applied for modal decomposition, combined with standard Mathieu chart analysis, led to the idenfication of parametric resonances. A curious finding of this paper is that the Mathieu instability may simultaneously occur in more than one mode, leading to interesting dynamic behaviors, also revealed through standard power spectra analysis and displacement scalograms.
Development of Time Domain Model for Synthetic Rope Mooring Systems
Authors: Kaasen Karl E. , Lie Halvor , Wu Jie , Falkenberg Erik , Åhjem Vidar , Larsen Kjell
Published: September 2014

Mooring of offshore structures in very deep water has been made possible through the use of lines made of fibres of synthetic material. The mechanical behaviour of synthetic ropes is considerably more complex than that of steel wire rope and chain, due to the visco-elastic and visco-plastic properties of the synthetic material. In particular, the gradually developing permanent increase in rope length will affect the offset and motion of the moored structure and make its characteristics change from one storm to the next.

For design and analysis of offshore mooring systems incorporating synthetic ropes it is valuable to have good models that can be used for response simulation in the time domain. The paper describes the development of a time domain model for synthetic rope. The model structure and the values of the parameters are determined from experimental data using a system identification technique. The resulting model is implemented in an existing computer program for analysis of mooring and riser systems. In particular, the permanent elongation of the synthetic rope appears to be well represented.
Experimental Validation of a Numerical Model for a Dry-Tree Semisubmersible in Benign Environments
Authors: Martin Bruce , Rijken Oriol , Davies Kent
Published: September 2014

The offshore industry has spent the last several years developing semisubmersible platforms capable of supporting both drilling and production activities. The production trees are located on top of the top tensioned risers (TTRs) on a Dry Tree Semisubmersible. A key challenge in the design of these vessels is to reduce the heave motion as much as possible to enable the use of state-of-the-art riser tensioners. A model test campaign was executed as part of the developmental program. The primary objective of this campaign was to improve the accuracy of the numerical tools to be used in the design process.

Riser tensioners are typically hydro-pneumatic devices, with a nonlinear tension-stroke relationship. A riser tensioner was developed at model scale which had a similar nonlinear behavior to the prototype. Examining the effect of this tensioner on the global motions was an additional objective of this test campaign. The techniques used to model this nonlinear spring is described, and its effect on global motions investigated.

A key challenge in model testing platforms intended for ultra-deep water (e.g. greater than 7000 ft) is the modeling of the mooring and riser system. The premise for the design of the model mooring system is 1.) maintain as best as possible the force-offset relationship of the mooring lines and 2.) be able to describe the model test configuration in the numerical tools to be used for global design. The near taut behavior of the prototype mooring system is modeled using heavy chain and a high-catenary mooring line.
Numerical Simulation of VIM Response of a Submersible Buoy Using a Semi-Empirical Approach
Authors: Tsukada Raphael I. , Shiguemoto Denis A. , Morooka Celso K.
Published: September 2014

Nowadays, discoveries of petroleum reservoirs are located in ultra deep-water depths. In this scenario, risers systems generally demand submersible buoys to support the riser, in order to reduce weight in floating platform and riser tensioners. Usually, these buoys are installed below 200 meters depth to avoid the wave forces. However, in this condition the sea current cause the Vortex-Induced Motion (VIM) of the buoys, which can increase significantly the riser fatigue damage. Although the VIM phenomenon is similar to Vortex-Induced Vibration (VIV), it generally occurs in rigid bodies with low aspect ratio, where end effects causes tridimensional flow behavior. Therefore, the vortex wake characteristics and the hydrodynamics coefficients found for VIV is no longer valid for VIM. It makes complex the prediction of VIM in buoys. In this paper, a semi-empirical model using hydrodynamic coefficients found for low aspect ratio cylinders are presented. In order to verify accuracy of numerical simulations, results are compared with experimental data presented in the literature and a good agreement is found.
An Enhanced 1-Way Coupling Method to Predict Elastic Global Hull Girder Loads
Authors: Ley Jens , el Moctar Ould
Published: September 2014

This paper introduces a numerical method to predict global hull girder loads of sea-going vessels, taking into account the structural elasticity. A field method based on a Finite Volume discretisation is applied to simulate the nonlinear rigid ship motions and provides the external loads at the hull surface. The structural response is computed in a full transient 3D-Finite-Element Analysis. The lowest global structural mode shapes and eigenfrequencies are covered by the 3D-FE model. The mapping between the Finite Volume mesh and Finite Element grid, is performed by the Mesh-Based Code Coupling Interface (MpCCI). As long as only global vertical bending modes are considered, simplified beam models may sufficiently cover the structural response. However, the use of the 3D-FE model is motivated by the prediction of the global torsional and local loads that are influenced by hydroelastic effects. A 1-way coupling method is applied. To account for hydromass effects, the Finite-Element model is enhanced by acoustic elements. Acoustic wave equations are solved to simulate the sound wave propagation in water and to obtain realistic eigenfrequencies of the wetted hull. Structural and hydrodynamic damping is controlled by the Rayleigh-Damping method. Simulations are performed for an ultra large container vessel sailing in regular head waves. The computed time histories of the vertical bending moment are compared with experimental data and with numerical simulations using a strong 2-way coupling simulation that employs a Finite-Element Timoshenko-Beam.
Integrated Optimization of Floating Wind Turbine Systems
Authors: Sandner Frank , Schlipf David , Matha Denis , Cheng Po Wen
Published: September 2014

The purpose of this paper is to show an exemplary methodology for the integrated conceptioning of a floating wind turbine system with focus on the spar-type hull and the wind turbine blade-pitch-to-feather controller. It is a special interest to use a standard controller, which is easily implementable, even at early design stages. The optimization of the system is done with adapted static and dynamic models through a stepwise narrowing of the design space according to the requirements of floating wind turbines. After selecting three spar-type hull geometries with variable draft a simplified nonlinear simulation model with four degrees of freedom is set up and then linearized including the aerodynamics with the blade pitch controller in the closed-loop. The linear system allows conventional procedures for SISO controller design giving a theoretically suitable range of controller gains. Subsequently, the nonlinear model is used to find the optimal controller gains for each platform. Finally, a nonlinear coupled model with nine degrees of freedom gives the optimal solution under realistic wind and wave loads.
Model Test Data Correlations With Fully Coupled Hull/Mooring Analysis for a Floating Wind Turbine on a Semi-Submersible Platform
Authors: Koo Bonjun , Goupee Andrew J. , Lambrakos Kostas , Lim Ho-Joon
Published: September 2014

The DeepCwind floating wind turbine model tests were performed at MARIN (Maritime Research Institute Netherlands) with a model set-up corresponding to a 1:50 Froude scaling. In the model tests, the wind turbine was a scaled model of the National Renewable Energy Lab (NREL) 5MW, horizontal axis reference wind turbine supported by three different generic floating platforms: a spar, a semi-submersible and a tension-leg platform (TLP) (Ref. [1] and [2]). This paper presents validation of the MLTSIM-FAST [3] code with DeepCwind semi-submersible wind turbine model test results. In this integrated program, the turbine tower and rotor dynamics are simulated by the subroutines of FAST [4], and the hydrodynamic loads and mooring system dynamics are simulated by the subroutines of MLTSIM. In this study, fully coupled hull/mooring dynamics and second-order difference-frequency response are included in MLTSIM-FAST. The analysis results are systematically compared with model test results and show good agreement.
Pipeline Remaining Life Estimation Based on Concepts of Structural Reliability and Bayesian Inference
Authors: Bisaggio Helio da Cunha , Netto Theodoro Antoun
Published: September 2014

In this paper, structural reliability concepts are used in conjunction with DNV Recommended Practice RP-F101 [1] formulation to establish the limit state functions of corroded pipes. The model takes into account the natural spread of material properties, geometric and operational parameters, and the uncertainties associated with the sizing of eventual corrosion defects to determine the probability of failure at a given time. Bayesian and reliability concepts are used to estimate the evolution of a pre-defined distribution of defects obtained, for instance, from an inspection campaign. By comparing the predicted probability of failure with the reliability acceptance criteria the operator can schedule defect repairs and establish inspection intervals with more confidence. Thus, a simple method to predict the probability of failure of a corroded pipeline along its operational life is proposed to provide the basis to develop a risk based maintenance strategy.
A Simplified Method for Quantitative Reliability and Integrity Analysis of SCRs
Authors: Mousavi Mir Emad , Reza Zaqie , Upadhye Sanjeev , Vijayaraghavan Vishnu , Haverty Kevin
Published: September 2014

Quantitative reliability and integrity analysis of Steel Catenary Risers (SCR) can provide important information about their safety and towards their cost-effective and optimal design. SCRs are one of the commonly used riser systems in offshore production stations. The consequence of a SCR failure is significant; however, the overall safety of the riser is typically not quantified. Especially, because of the uncertainties associated with environmental conditions and structural capacities, quantitative reliability methods can take advantage of available data and developments in computing technology to provide a strong basis for their reliable engineering decision making. This paper presents a simplified approach for assessing the strength and fatigue reliability of SCRs, accounting for the uncertainties with their yield-strength and fatigue capacities as well as the environmental conditions. Moreover, the integrity-based optimal design of riser strength limit state for a target annual probability of failure is discussed. The fatigue reliability of the SCR system is also assessed in component and system levels. The proposed method is then applied to a typical SCR attached to a semi-submersible vessel under Gulf of Mexico conditions. Results of dynamic (time-domain) analyses under various environmental conditions are used to quantify the SCR safety and integrity and to optimize its design for a target annual probability of strength failure. By estimating the riser system probability of strength and fatigue failure in its lifetime, the strength and fatigue integrity indices, and the optimality factors of the riser sections for the strength limit state, suggestions are provided to improve the riser design. For example, it was found that considering the two main limit states of strength and fatigue failure of the SCR system, a strength failure at the taper stress joint is the likely mode of failure in this riser system, which has a probability of 0.0035 in its 25 years lifetime.
Development of a Training Simulator for Oil Spill Response
Authors: Vileti Vinicius L. , Neto Albino Ribeiro , Sales Joel S. , Esperança Paulo de Tarso T. , Sphaier Sergio H. , Antunes Maciel Leonardo S.
Published: September 2014

The paper describes the development of a training simulator for boats used on Oil Spill response at sea. The simulator models the dynamics of tug boats pulling an oil boom under waves, wind and current. The boom is modeled as a flexible line connected by lumped masses and its flotation characteristics and loads are calculated by Morison equation. A simplified model is used to simulate the oil itself and its interface with the boom line. The verification process was applied and its outcomes are discussed. Also, some case study scenarios are presented and the results are used to evaluate the applicability of the simulator as a training system.
A Nonlinear Dynamic Substructuring Approach to Global Dynamics of Flexible Riser Systems
Authors: Majed Arya , Cooper Phil
Published: September 2014

Standard riser global dynamic analysis software packages utilize line element models that cannot capture the complex behavior of flexible risers. This paper presents a computationally efficient nonlinear dynamic analysis methodology capable of incorporating detailed finite element models and scalable to global dynamic simulations of entire flexible riser systems. Subject methodology captures the global geometric nonlinear effects and its coupling to stick-slip friction — a clear requirement for accurate armour stress predictions. In addition, the method enables the formulation of stress transformation matrices which allow the direct recovery of armour stresses from the global simulations. A demonstration problem involving the nonlinear dynamic simulation of a 500m flexible riser system is presented.