bluuebunny/crossref_metadata_2025 huggingface dataset:
Improvements in Heavy Topside Installation Onto Spar Hull by Catamaran Floatover Method
Authors: Luo Michael Y. H. , Edelson David , Wan Jianan (Jay) , Sun Jiulong , Hassanaliaragh Sina
Published: November 2013

The Murphy Kikeh Spar in Malaysia was the first Spar to employ a catamaran floatover method to install the topsides onto the Spar hull at the platform offshore location. The single column hull of the Spar dictates that the most practical way to float a topsides onto the Spar hull is by use of a catamaran system where the topsides structure forms a connection between the twin barges. For the Kikeh Spar the topside was first loaded out onto a single barge before being transferred to the catamaran system. The transfer operation was performed in sheltered waters. After the transfer and installation of the seafastening, the topside was towed to the installation site. The transfer of the topside weight was accomplished by deballasting the Spar and no quick release mechanism was used in the separation of the topside from the catamaran barges.

While the operations described were successfully implemented for the 4,000 MT Kikeh topside in a relative mild environment, improvements are recommended to perform this operation for heaver topsides in harsher environments such as the Gulf of Mexico.

This paper summarizes an internal study by Technip to extend and improve the floatover installation of Spar topsides to the Gulf of Mexico. This paper presents a step-by-step overview of an improved process for the installation of topsides having transportation weights of up to 25,000 ST. Similarities and improvements compared with Kikeh floatover installation will be discussed with particular focus on the following areas:

1. A new method of loading out of the topsides from the fabrication yard directly to the catamaran barges, requiring only one operation and eliminating the need for a transfer barge.

2. Specific Catamaran Ocean tow design and analysis considerations for the Gulf of Mexico. This addresses the effects of the harsher environment on the barges, grillage and topsides structure. A novel method of preloading the catamaran system is presented that reduces or, in some instances, eliminates the requirement for additional topside steel weight to accommodate additional motion-induced dynamic loads on the catamaran system during the ocean tow. The preloading also eliminates the risk associated with the operation of cutting tied-down braces.

3. A quick load release system is described which enables the rapid separation of the barges from the topside following the appropriate level of topside load transfer to the Spar hull.
Stability Requirements of Multi-Purpose Ships for Heavy Lifts
Authors: Rathje Helge , Hympendahl Ole , Kaufmann Jan , Schellin Thomas E.
Published: November 2013

New minimum intact stability criteria are presented to ensure safety against capsizing invoked by sudden loss of crane load during heavy lifts at sea, followed by typical sample stability assessments for a lifting operation on four multipurpose ships. For added stability, two of these ships had a pontoon attached at their sides opposite the lift. Two numerical time-domain methods assessed the transient dynamic heel after a sudden loss of crane load. With the ship at equilibrium, both analyses started by releasing the crane load, simulating a sudden failure of the lifting gear. The first method solved the roll motion equation as a one-degree-of-freedom system; the second method used a Reynolds-averaged Navier-Stokes equations solver. The first method relied on appropriately chosen linearized roll damping coefficients, and the nonlinearity of the righting moment function had to be accounted for. The second method required creating extensive numerical grids to idealize the ship’s hull, including the counter balancing stability pontoon, rudder and bilge keels, as well as all parts of the ship’s superstructure that effect the righting moment at large heeling angles.
Modifications of the PSA Regulations Based on Case Studies of Stability Accidents
Authors: Kvitrud Arne
Published: November 2013

The paper describes available information about eight accidents worldwide related to the stability of petroleum units since 2000. A further seven stability incidents in Norway during the same period are also described. These accidents and incidents are used as cases for an evaluation of the existing Norwegian regulations on the stability of floating offshore petroleum units.

The accidents and incidents are grouped in categories based on the causes, and examples are given for each group.

The paper describes which Norwegian requirements could have prevented the accidents if they are complied with. Most of the accidents have involved deviations from the Norwegian regulations. Compliance with Norwegian regulations and (high-level) standards would cover most of the accident scenarios. However, some changes in the regulations have been found necessary. The paper describes these amendments, and the reasoning behind them.
Experimental Study on the Flow Around a Moving Cylinder
Authors: Gao Yangyang , Tan Danielle S. , Zhang Baofeng , Tan Soon Keat
Published: November 2013

An experimental investigation of flow behavior behind a moving circular cylinder was carried out in an open channel. The cylinder was mounted on a platform and extended to submerge in the water. The platform was rested on two ball-bearing-filled tracks which were designed to reduce the friction between the platform and the tracks. The cylinder was maintained perpendicular to the platform. The platform was free to move in a horizontal plane. The flow fields behind the suspended cylinder were captured using the technique of Particle Image Velocimetry (PIV) and the trajectory of the cylinder was captured using a camera operated in video mode (60 frames per second). Instantaneous flow patterns and orbital responses of the moving cylinder were analyzed and the findings were presented herein.
Efficient J-Based Failure Assessment Diagrams for Engineering Critical Assessments of Circumferentially Cracked Pipes Subjected to Axial Force, Pressure, and Bending
Authors: Chell G. Graham , Lee Yi-Der , Hudak Stephen J.
Published: November 2013

Engineering critical assessments (ECAs) of cracked pipes increasingly involve situations of high strains (e.g., reeling and ratcheting fatigue), multiple loads (combined bending, axial forces, and internal pressure), and multi-axial stressing (due to pressure). In this paper, some of the implications of these loading conditions on ECAs are investigated by generating BS 7910 Level 3C Failure Assessment Diagrams (FADs) from the results of a large matrix of finite element analysis (FEA) J computations for circumferentially cracked pipes. The Level 3C (J-based) FADs (which provide the most accurate FAD approach to ECAs) are compared with the corresponding and more widely employed (but less accurate) Level 2B (material dependent) FADs in order to assess the accuracy of the latter. Use of FEA J solutions in a Level 3C FAD ensures that the effects of material behavior, load type, crack type, crack geometry, and pipe geometry are accurately captured whereas a Level 2B FAD only attempts to accurately capture the effects of material stress-strain behavior. It is demonstrated that under some circumstances a Level 2B assessment will result in non-conservative results compared to the corresponding Level 3C assessment. The current comparison between Levels 3C and 2B addresses the mechanics involved in these approaches and does not take into account the possible differing treatments of material property uncertainties on ECAs within the two approaches. Based on the current results, an efficient J formulation is described that facilitates the practical implementation of a J-based ECA. The novel approach used is based on determining material dependent shift factors that transform Level 3C FADs derived from the fully plastic components of J solutions into Level 3C FADs that represent J behaviors in the linear elastic and fully plastic regimes, and the transition region in-between. This new J formulation treats combined axial forces, pressure, and bending when applied proportionally or non-proportionally and forms the basis of the monotonic and cyclic crack tip driving forces employed in the program FlawPRO. This program performs comprehensive conventional and high strain J-based ECAs that involve reeling, arbitrary strain cycling, ratcheting fatigue, and ductile tearing that are equivalent to a Level 3C FAD approach.
Torpedo Piles: VH Capacity in Clay Using Resistance Envelope Equations
Authors: Kay S.
Published: November 2013

This paper shows that torpedo pile in-place capacity under inclined VH load applied to the pile head can be reasonably assessed using simple modifications to resistance equations originally developed for straight-shafted circular caissons in clay under general vertical, lateral and moment (VHM) loading (Kay and Palix, 2010).

Also, the number of equations defining the VHM resistance envelope has been reduced from three to one. The resulting caisson yield function extends those currently available for surface and embedded foundations in clay under inclined load (e.g. DNV, 1992 and ISO, 2011).
Field Installation Tests of Monopod Suction Pile and Tripod Suction Buckets
Authors: Kwag D. , Oh Myounghak , Kwon OSoon , Bang S.
Published: November 2013

Small-scale field tests on a monopod suction pile with and without the detachable penetration apparatus (DPA) and tripod suction buckets have been performed. One of the objectives of the field tests was to identify the capability of the DPA at small-scale and to obtain useful information for planned future full-scale field tests. The DPA which has been developed to control and increase the accuracy of the verticality of monopod suction piles during installation. In addition, it can also contribute to increase the penetration force under certain conditions. Included in this paper are the details of the small-scale field tests, including the site conditions, suction pile details, DPA details, installation, instrumentation equipment, and test results. Results indicate that the tripod suction buckets have a significant advantage over the monopod suction pile for verticality control and that the newly developed DPA is fully capable of enhancing the verticality control of the monopod suction pile during installation.
Reliability of Buckle Formation Accounting for Bathymetry
Authors: Brunet Philippe , Oliphant John
Published: November 2013

The mitigation of lateral buckling for subsea pipelines is achieved by making the pipe buckle at intended locations in preference to uncontrolled lateral buckles. At these engineered locations, the pipeline can be given enhanced characteristics (pipe tolerances, welds and Non-Destructive Examination (NDE) criteria). Alternatively, the buckle initiation equipment can be designed to distribute evenly the stress along the buckled pipeline and consequently reduce the maximum stress in the buckled pipeline section. As various parameters involved in the buckle formation process are uncertain (e.g. soil characteristics, residual horizontal imperfections after pipelay), a probabilistic approach is used to assess the likelihood that the pipeline will buckle at the intended locations.

Up to now, the existing guidelines focus on in-plane residual Out Of Straightness (OOS) whereas the impact of bathymetry (i.e. uneven seabed) is only addressed in a high level fashion, with no clear methodology.

The authors have therefore developed an original approach to account for the effects of bathymetry on the reliability of buckle formation. In a first step, the effects of bathymetry on buckle formation are discussed from a geotechnical and mechanical perspective. Then, the implementation of the buckling formation formulation in a probabilistic numerical subroutine is presented. Finally, a generic case study is considered to assess the impact of accounting for bathymetry on the buckle formation reliability analysis.
Experimental Investigation on the Stability of Parallel and Oblique Plates as Suppressors of Vortex-Induced Vibration
Authors: Assi Gustavo R. S. , Franco Guilherme S.
Published: November 2013

Experiments have been carried out on models of free-to-rotate parallel and oblique plates fitted to a rigid section of circular cylinder to investigate the effect of plate length and oblique angle on the stability of this type of VIV (vortex-induced vibration) suppressor. Measurements of the dynamic response and trajectories of motion are presented for models with low mass and damping which are free to respond in the cross-flow and streamwise directions. It is shown that, depending on a combination of geometric parameters — such as plate length, plate angle and gap between plates and cylinder — devices might not be able to completely suppress VIV for the whole range of reduced velocities investigated. Plates with larger oblique angles turned to be less stable than parallel plates and induced high-amplitude vibrations for some specific reduced velocities. An undesirable steady lateral force was also observed to occur for all configurations and might be related to the existence of a large gap and oblique angles. Systems may present streamwise vibration due to strong flow separation and reattachment on the outer surface of plates with large oblique angles. Large angles may also increase drag. Experiments with a plain cylinder in the Reynolds number range from 3,000 to 20,000 have been performed to serve as reference. Reduced velocity was varied between 2 and 13.
Research on the Arrangement of the Mooring Lines of the Thruster Assisted Mooring System
Authors: Zou Gang , Wang Lei , Zhang Feng
Published: November 2013

As the offshore industry is developing into deeper and deeper water, station keeping technics are becoming more and more important to the industry. Based on the dynamic positioning system, the thruster assisted mooring system (TAMS) is developed, which consisted of mooring lines and thrusters.

The main function of the TAMS is to hold a structure against wind wave and current loads with its thruster and cables, which is mainly evaluated by the holding capacity of the system. The arrangement of the mooring lines (location of anchor or the mooring line angle relative to platform) will directly affect the TAMS holding capacity because of the influence of the directions of the mooring forces. So finding out an optimum arrangement of the mooring lines is essential since the performance of the TAMS depends greatly on the arrangement of the mooring lines. The TAMS of a semi-submersible platform, which is studied in this paper, consisted of eight mooring lines. By fixing the layout of the thrusters and changing the location of each mooring line for every case, the performances of the TAMS are analyzed. The platform motions, mooring line tensions and power consumptions are compared to obtain the optimum arrangement of mooring lines, and thus a thruster assisted mooring system with a better performance can be achieved. Time domain simulation is carried out in this paper to obtain the results.
Joint Environmental Data at Five European Offshore Sites for Design of Combined Wind and Wave Energy Devices
Authors: Li Lin , Gao Zhen , Moan Torgeir
Published: November 2013

The costs for an offshore wind farm, especially with bottom fixed foundations increase significantly with increasing water depth. If costs can be reduced to a competitive level, the potential for wind farms in deep water is huge. One way of reducing costs might be to combine offshore wind with wave energy facilities at sites where these resources are concentrated.

In order to design combined renewable energy concepts, it is important to choose sites where both wind and wave energy resources are substantial. Such facilities might be designed in ultimate limit states based on load effects corresponding to 50-year wind and wave conditions. This requires a long-term joint probabilistic model for the wind and wave parameters at potential sites.

In this paper, five European offshore sites are selected for analysis and comparison of combined renewable energy concepts developed in the EU FP7 project – MARINA Platform. The five sites cover both shallow water (<100m) and deep water (> 200m), with three sites facing the Atlantic Ocean and the other two sites in the North Sea. The selection of the sites is carried out by considering average wind and wave energy resources, as well as extreme environmental conditions which indicate the cost of the system.

Long-term joint distributions of mean wind speed at 10-meter height (Uw), significant wave height (Hs) and spectral peak period (Tp) are presented for selected sites. Simultaneous hourly wind and wave hindcast data from 2001–2010 are used as a database, which are obtained from the National and Kapodistrian University of Athens.

The joint distributions are estimated by fitting analytical distributions to the hindcast data following a procedure suggested by Johannessen et al. (2001). The long-term joint distributions can be used to estimate the wind and wave power output from each combined concept, and to estimate the fatigue lifetime of the structure. For estimation of the wind and wave power separately, the marginal distributions of wind and wave are also provided.

Based on the joint distributions, contour surfaces are established for combined wind and wave parameters for which the probability of exceedance corresponds to a return period of 50 years. The design points on the 50-year contour surfaces are suggested for extreme response analysis of combined concepts. The analytical long-term distributions established could also be applied for design analysis of other offshore structures with similar environmental considerations of these sites.
Experimental Study on Water Damping Effects of Hybrid Floating Structure
Authors: Jeong Youn-Ju , You Young-Jun , Lee Du-Ho , Park Min-Su
Published: November 2013

In this study, in order to evaluate water damping effects of hybrid pontoon system with cylinders, experimental studies were carried out. At first, in order to evaluate oscillatory motions, three small-scale models of hybrid, tapered, and pontoon were fabricated and tested under the still-water condition. Four acceleration gauges were attached on the top edges and acceleration of top edge were measured during the oscillation. Then, oscillatory motions of oscillation period and stabilizing time to steady-state were analyzed. Finally, based on the oscillatory motions, damping properties of the logarithmic decrement, damping ratio, and natural frequency of damped system were calculated and compared with each other. As the results of this study, it was found that hybrid model presented about 3.67 times higher decay rate of amplitude of the oscillatory motion than the pontoon model. Also, hybrid model presented about 3.67 times higher damping ratio than the pontoon model. Whereas the natural frequency of the pontoon and tapered model were nearly same with the natural frequency of undamped system, that of the hybrid model presented some difference with the that of the undamped system. In addition, periods of floating body at the wet mode presented about 1.5∼3.0 times longer periods than the dry mode, and it was expected that there was not possibility for the resonance. Therefore, it was expected that the hybrid model of this study should contribute to improve serviceability and safety of offshore floating structures as decreasing oscillatory motions.
Ship Security Analysis: The Effect of Ship Speed and Effective Lookout
Authors: Liwång Hans , Ringsberg Jonas W.
Published: November 2013

The threat of piracy to commercial shipping is a concern for the protection and safeguarding of human lives, property and environment. Therefore, ships under piracy threat should follow security measures suggested by the International Maritime Organization (IMO) and the Contact Group on Piracy off the Coast of Somali. It is, therefore, important to choose the proper security measures for the right situation.

This study presents a simulation model that can be used for probabilistic risk assessments regarding the operation of commercial ships. This investigation specifically studies the pirate approach phase and quantifies the effect of ship speed and effective lookout. The purpose of introducing probabilistic risk assessment into the analysis of pirate attacks is to meet safety goals more effectively through a well-balanced combination of proactive and reactive measures whilst keeping focus on the intended over all purpose of the particular ship.

The study presents collected and documented knowledge regarding pirate capability, intention and likelihood to perform attacks. The knowledge is collected from experts with experience from the situation off the Horn of Africa. The collected information is input to an influence analysis that identifies the network of influences that govern the skiff approach. The simulation model describes piracy characteristics and decision making on the threatened ship, the characteristics and countermeasures of the ship under attack, as well as weather.

Based on a comparison with available statistics the overall conclusion of the work is that the threat analysis and the simulation model can quantify and explain how the studied risk control options affect the probability of a successful approach. The result therefore exemplifies how a quantified ship security analysis can support the recommendations in industry guidelines and also enable recommendations that to a greater extent can facilitate an educated decision by the ship operators.
Research Studies on Modeling Soil Erosion Caused by External Factors Near Offshore Structures
Authors: Shchemelinin Leonid G. , Sazonov Kirill E. , Denisov Valeriy I.
Published: November 2013

For development of hydrocarbon deposits located in a shallow part of coastal shelf, offshore structures of gravitational type are widely used. The foundations of such structures are towed to sites where they are flooded and installed directly on sea bed. During operation of offshore structures the sea bed soil around and beneath the structure foundations are exposed to external effects such as sea currents, waves and also propeller streams generated by ships approaching the structures. As a result the soil is washed out from under the structure which may cause reduction of bearing surface area and even lead to loss of stability of the structure on seabed.

The model studies on soil erosion process near foundations of gravitational-type offshore platforms have been conducted at Krylov’s shallow water towing tank since 2000. This towing tank was not originally intended for such experiments, and, therefore, special experimental equipment has been developed for this purpose. These include a sandy bottom, current generation arrangements in the model test zone, instrumentation for measuring the bottom profile. Also specific model test procedures and methods for research on soil scour near the test objects have been developed and verified.

The paper gives a review of these research studies at the Krylov Centre, describe the test equipment, modeling techniques and test data scaling procedures as well as the experience learned from the experiments.

The paper gives some results of research studies of soil erosion near the foundations of offshore structures as a result of influence from sea currents and waves with coincident directions, and also as a result of influence from propeller streams of moored tankers and supply vessels.

The paper presents the model test results regarding the efficiency of a mobile system developed for soil erosion protection near the offshore structure intended for exploratory drilling in shallow waters of the Gulf of Ob in the Kara Sea.

A review of the recent Krylov’s studies in this field of research is given.
Modelling Covariate Effects in Extremes of Storm Severity on the Australian North West Shelf
Authors: Randell David , Wu Yanyun , Jonathan Philip , Ewans Kevin
Published: November 2013

Careful modelling of covariate effects is critical to reliable specification of design criteria. We present a spline based methodology to incorporate spatial, directional, temporal and other covariate effects in extreme value models for environmental variables such as storm severity. For storm peak significant wave height events, the approach uses quantile regression to estimate a suitable extremal threshold, a Poisson process model for the rate of occurrence of threshold exceedances, and a generalised Pareto model for size of threshold. Multidimensional covariate effects are incorporated at each stage using penalised tensor products of B-splines to give smooth model parameter variation as a function of multiple covariates. Optimal smoothing penalties are selected using cross-validation, and model uncertainty is quantified using a bootstrap resampling procedure. The method is applied to estimate return values for a large spatial neighbourhood of locations off the North West Shelf of Australia, incorporating spatial and directional effects.
Numerical Simulation of an Oscillating Wave Surge Converter
Authors: Wei Yanji , Rafiee Ashkan , Elsaesser Bjoern , Dias Frederic
Published: November 2013

It has been known that bottom hinged Oscillating Wave Surge Converters (OWSCs) are an efficient way of extracting power from ocean waves (Whittaker et al. 2007). OWSCs are in general large buoyant flaps, hinged at the bottom of the ocean and oscillating back and forth under the action of incoming incident waves (Schmitt et al. 2012, Renzi and Dias 2012). The oscillating motion is then converted into energy by pumping high-pressure water to drive a hydro–electric turbine.

This paper deals with numerical studies of wave loading on an OWSC using the FLUENT software. In numerical simulation of wave loading on an OWSC using mesh-based methods, the mesh around the flap is required to be updated frequently. This is due to the large amplitude rotation of the OWSC around the hinge. In this work, the remeshing was achieved by using the so-called dynamic mesh approach built in FLUENT. Furthermore, the motion of the OWSC is updated in time using a fourth order multi point time integration scheme coupled with the flow solver. The results for the flap motion and the excited torque on the hinged position were compared with experimental data obtained in a wave tank at Queen’s University of Belfast. The results showed the capability of the numerical model with a dynamic mesh approach in modeling large amplitude motions of the flap. In addition, the pressures at various locations on the flap were compared with the experimental measurements in order to demonstrate the accuracy of the proposed model in capturing local features of the flow as well as the global features.
Carcass Failures in Multilayer PVDF Risers
Authors: Farnes Knut-Aril , Kristensen Claus , Kristoffersen Steinar , Muren Jan , Sødahl Nils
Published: November 2013

Statoil have experienced failures in a number of flexible risers due to collapse, overload, tearing and unspiraling of the carcass in pipe structures with multi-layer PVDF pressure barrier. The paper will discuss the carcass failure modes that are characteristic for multilayer flexible pipes with particular focus on the failures due to carcass tearing. The nature of the carcass tearing problem is explained and suggestions for load model and operational policy for mitigation risk of new failures is presented.
Application of Artificial Clay Samples (Pellets) in Laboratory Testing of Shale/Drilling Fluid Interaction
Authors: Pasic Borivoje , Gaurina-Medjimurec Nediljka , Moslavac Bojan
Published: November 2013

Wellbore instability was and is one of the most frequent problems in petroleum industry, especially in the drilling operations. It is mainly caused by the shale formations which represent 75% of all drilled formations. The wellbore instability problems involve tight hole spots, wellbore diameter enlargement, the appearance of cavings, the inability of carrying out wireline operations, poor hole cleaning, unsuccessful wellbore cementing operations and other. The wellbore instability is the result of mechanical and physico-chemical causes mostly acting concurrently. The shale instability basically comes out of its mineralogical composition (especially clay minerals content) and physico-chemical properties. Shale-mud interaction includes water/ions movement in and out of the shales due to pressure differential, osmosis, diffusive flow and capillary pressure. Many research activities about shale instability causes and shale properties (affecting shale behavior) definition have been carried out by now. Different shale samples, laboratory equipment and inhibitive muds have been used. Laboratory tested shale samples are provided by the wellbore cores, surface sampling or, which is the simplest method, by collecting the samples at the shale shakers during drilling operation. The amount of these samples is not enough for laboratory testing. Another problem is closely connected to sample quality and preservation. There are also differences in drilling fluids used in these laboratory tests, especially in their composition (sometimes containing more than one shale inhibitor). It is difficult to compare test results and conclusions made by different authors. The laboratory study presented within this paper are done with artificial clay samples (pellets) made by compacting the powderish material containing exact quantity of quartz, montmorillonite and kaolinite. The laboratory testing is done by treating the powderish samples inside the desiccator (24 hours), compacting (30 minutes), swelling (24 hours) and drying samples (24-hour). Sample swelling is tested by using different mud types and the sample mass is measured in each above mentioned phase. Special attention is directed to preparation and pellets content definition as a good replacement for the original shale in laboratory testing of shale and drilling fluid interaction. The influence of used muds on the total pellet swelling and swelling intensity, especially at the early phase of testing was determined.
Assessment of Significant Wave Height – Peak Period Distribution Considering the Wave Steepness Limit
Authors: Drago Michele , Giovanetti Giancarlo , Pizzigalli Claudia
Published: November 2013

The physical limit of the significant wave steepness is generally exceeded when assessing the seastate climate and the extreme iso-probability contours, i.e. too short significant wave peak periods Tp are sometimes associated to a certain significant wave height Hs.

The occurrence of not physically consistent Tp is clearly due to a fault in the generally made assumption of a log-normal distribution of the Tp, where the physical limit for the period would be Tp > 0, i.e. the existence limit of the log-normal distribution, which is well below the real physical limit for significant wave steepness.

If this is not a problem for pipeline design, where stability and fatigue are dominated by longer peak periods associated at each significant wave height, loads overestimation could arise for near surface structures, e.g. riser, where the largest loads and fatigue, are caused by the shorter peak periods associated to a certain significant wave height. Hence, the possibility to define a Tp distribution which respects the physical lower bound of the limiting wave steepness has a significant relevance when dealing with design and installation of near surface structures.

The present paper proposes a new methodology for the assessment of the Hs-Tp distribution which respects an a-priori defined wave steepness limit.

This can be done basing on the definition of significant wave steepness Sp = 2πHs/gTp2 which, assessing a limiting steepness Sp, provide an physical lower bound Tplim for the peak period Defining a new variable Tp′ = Tp – Tplim and imposing that Tp′ follows a log-normal distribution, hence having a physical limit Tp′ > 0, is equivalent to assess a Tp distribution which respects the defined significant wave steepness limit Tp > Tplim.

A test case compares results obtained with the ‘old’ and ‘new’ methodologies and shows the implication on the design loads. Moreover, another test case has been investigated to verify the performance and characteristics of the new methodology.
Modeling of Wave-Induced Seabed Response and Liquefaction Potential Around Pile Foundation
Authors: Sui Titi , Zhang Jisheng , Zheng Jinhai , Zhang Chi
Published: November 2013

A 3D integrated model is developed to study wave-induced seabed response and liquefaction potential around square pile foundation. While a Boussinesq wave mode (FUNWAVE) is used to simulate the wave-pile interaction, a seabed mode (WINBED) based on Biot’s poro-elastic theory is solved for the seabed deformation, effective stresses and pore water pressure in soil. After verified with previous model and experimental works, this integrated model is applied to investigate the wave-induced seabed response and liquefaction potential in the vicinity of square pile foundation. The numerical results indicate that wave-induced pore pressure reduces rapidly with an increasing seabed depth, and the maximum pore pressure and largest liquefaction potential can be identified in front of the square pile foundation. It is also found that the phenomenon of liquefaction may occur inside the seabed soil while the upper layer remains un-liquefied.
Evaluation of Directional Analysis Methods for Low-Frequency Waves to Predict LNGC Motion Response in Nearshore Areas
Authors: van Essen Sanne , van der Hout Arne , Huijsmans René , Waals Olaf
Published: November 2013

Because LNG terminals are located increasingly close to shore, the importance of shallow-water effects associated with low-frequency (LF) waves increases as well. The LF wave spectrum in these areas is generally complex, with multiple frequency peaks and/or directional peaks due to LF wave interaction with the shore. Both free and bound LF waves at the same frequency can be present. Since LF waves are potentially very significant for moored vessel motions, it is important to include their effect in an early stage of the terminal design. This requires an efficient and relatively simple tool able to estimate the LF wave spectrum in nearshore areas. The benefit of such a procedure with respect to state-of-the-art response methods is the ability to include the LF free wave distribution in a local wave field in the vessel response calculation.

The objectives of the present study are to identify such a tool, and to evaluate the use of its output as input for a vessel motion calculation. Three methods, designed for the determination of wave spectra of free wave-frequency (WF) waves, were applied to artificial LF wave fields for comparison of their performance. Two stochastic methods, EMEP (Hashimoto et al., 1994) and BDM (Hashimoto et al., 1987) and one deterministic method, r-DPRA (De Jong and Borsboom, 2012) were selected for this comparison. The foreseen application is beyond the formal capabilities for which these three methods were intended. However, in this study we have investigated how far we can take these existing methods for the determination of directional LF wave spectra.

Sensitivity analyses showed that the EMEP method is the most suitable method of the three for a range of LF wave fields. The reconstructed LF wave spectra using EMEP resembled the input spectra most closely over the whole range of water depths and frequencies, although its performance deteriorated with increasing water depth and wave frequency. Subsequently, a first effort was made to use the information in the reconstructed EMEP LF wave spectrum of a representative shallow-water wave field for a first estimate of the motions of a moored LNG carrier. The results were acceptable. This is a first indication that EMEP output might be used to calculate the motions of an LNG carrier moored in shallow water.
DCV Aegir Pipelay Installation Analyses and Capabilities
Authors: Smienk Henk , Karjadi Erwan , Vazquez Gabriel , Doherty Peter , Dooley Patrick
Published: November 2013

Heerema Marine Contractors (HMC) is entering a new era of pipe laying with the new Deep water Construction Vessel (DCV) Aegir, designed to be able to reel/J-lay pipelines for a broad range of pipe dimension and water depth combinations. On the one side this is governed by equipment/vessel limitations (moonpool size, high top tension capacity, stinger component capabilities) and on the other side limited by pipeline design code (e.g. DNV, API) acceptance criteria for reel-lay and J-lay installation.

This paper outlines the pipelay capabilities of DCV Aegir and details the J-lay (with quad joints) and reel-lay installation analyses performed to aid in the design of the vessel pipelay equipment. DCV Aegir has two modes for J-lay installation, which are light J-lay with friction clamps and heavy J-lay with collar clamps in combination with collars in the pipeline. DCV Aegir reel-lay installation from the pipeline in the tensioners down to the seabed will also be explained.

Light J-lay, heavy J-lay and reel-lay have maximum top tension capacities (related to the equipment) of 600 mT, 2000 mT and 800 mT, respectively. The top tension capacity also depends on pipe OD, coating type and thickness. J-lay and reel-lay installation analyses are performed with the non-linear finite element software package Flexcom from MCS Kenny to determine installation capabilities with respect to pipe OD, wall thickness and water depth combinations. Together with that the pipelay equipment design is validated by pipeline installation analyses.

Shallow and deep water normal pipeline installation for all three pipelay options will be discussed. DCV Aegir pipelay equipment includes a retractable hang off module/stinger for deployment of pipelines. The usage and benefits of the hang off module will be documented. For the J-lay installation modes the procedure for lowering a quad joint is analysed in order to optimise equipment usage. DCV Aegir possesses a high capacity abandonment and recovery system (up to 2000mT). Abandonment and recovery analyses description and design review aspects will be discussed. Finally, the pipeline in-line structure installation analyses, together with design review considerations will be documented.
Numerically Modelling the Effect of the Drag Crisis on Vortex Induced Vibration of a Circular Cylinder
Authors: Botterill Neil , Morvan Hervé P. , Owen John S.
Published: November 2013

This paper presents work using commercially available computational fluid dynamics (CFD) software that provides evidence of the effect that the drag crisis has on the dynamic response of a bluff object excited by vortex induced vibration (VIV) from normal flow. Results are presented of simulations of flow past a stationary circular section. A dynamic Large Eddy Simulation (LES) turbulence formulation is used to ensure important features of the drag crisis are captured. Further simulations are presented where the cylinder is free to oscillate in two directions, parallel and normal to the direction of flow. Emphasis is given to the role that the drag crisis phenomenon plays on the locus of oscillations. Key findings are that the drag crisis increases the response by allowing more energy to go into the system on the upstream stroke of the oscillations and that a reversal of drag can occur in both upstream and downstream movements. This suggests the drag crisis phenomenon can affect fatigue performance.
Real-Time Hybrid Testing for Marine Structures: Challenges and Strategies
Authors: Chabaud Valentin , Steen Sverre , Skjetne Roger
Published: November 2013

Within the field of hydrodynamics, it is fairly easy to find examples of model tests whose performance is impaired by only a subpart of the whole system, which may not be the one of interest.

Real-time hybrid testing (RTHT) overcomes this issue by performing scale model testing only on a subpart of the whole structure, the remainder being simulated numerically. The loads acting on the virtual substructure are calculated from online-measured motions of the physical substructure and actuated back on the latter in real-time. RTHT involves data measurement, filtering, force estimation, motion observing and force actuation. The main challenge is to fit all of those items into one time step.

A simple case study is suggested. It consists in a linearized one degree of freedom floating wind turbine, whose floating substructure is physically tested while wind loads are numerically simulated and actuated. Design rules to build the corresponding RTHT set up are then presented.
Study on Influence of Wave-Current Interaction on Prediction of Rogue Wave Taking Viscous Effect Into Account
Authors: Peng Bo , Ma Ning , Gu Xiechong
Published: November 2013

Wave-current interaction is a common phenomenon in real sea and has significant influence on sea conditions, thus threatening the safety of offshore structures. Waves countering current is one of the main reasons for occurrence of rogue waves, which imperils offshore structures and is of much importance for research. Based on Computational Fluid Dynamics (CFD), the simulation of viscous flow for wave generation in a Circulating Water Channel (CWC) has been carried out. In the calculation model, the motion of an oscillating flap type wave maker is simulated to generate specified waves by controlling movement of dynamic mesh in numerical model. Smoothing and local refreshing of dynamic meshes have been done to adapt meshes. Then, viscous flow simulation of wave generation among current in numerical tank is accomplished by using Reynolds-Averaged-Navier-Stokes (RANS) equation, renormalization group method based (RNG-based) k-ε turbulence model and Volume of Fluid (VOF) method as treatment of free surface. Both waves propagating along and against current have been investigated. To validate the numerical model, a part of calculation results are compared with the experimental results in CWC for regular wave propagating along current. Furthermore, calculations based on linear wave theory and modified nonlinear Schrödinger Equation (mNLS) are also performed. For further investigation of occurrence of rogue wave in deep water, focusing technique is adopted by using transient water wave. Results show that wave height and group velocity of wave sequence would change notably when wave propagates along preexisting large-scale current. Wave height decreases as the velocity of uniform current increases and wave sequence arrives at given position earlier when propagating on faster current. On the other hand, on a counter current, waves are steepened and wave height increases dramatically. Based on the numerical results, several characteristics and principal values such as wave height and asymmetry have been discussed. The effects of viscous flow and wave-current interaction on prediction of rogue waves are analyzed and evaluated. Accordingly, some concluding remarks on improving of numerical model of rogue wave are given.
Development of Turbine Access System
Authors: ter Brake Erik , Clegg Andy , Perdrix Frederic
Published: November 2013

The offshore wind industry is growing rapidly and the number of personnel transfers between turbines and work boats is increasing. As this is considered an operation that carries a significant risk, a number of improvements have been developed in the industry. In order to provide a cost effective solution optimised for the safety and convenience of operations, a Turbine Access System (TAS) was developed. The functionality of TAS is based on the principle that the motions of the vessel are actively compensated in heave, roll and pitch in order to provide a stable platform for personnel transfer.

This paper presents the technical design of TAS, including the development of the global design, the hydraulic system and the control systems. This covers deriving the general arrangement of the unit, obtaining the required inverse motions of the gangway, the means of hydraulic cylinder activation and developing the control system.

The control system comprises cascade control with feed-forward and nonlinear compensation in order to minimise trajectory tracking errors. The methodology to derive high degrees of accuracy and smooth operations is discussed alongside the operational logic, monitoring, fault actions and safety features which are accessed through two touch-panel computers.

The first TAS came into operation during the summer of 2012 where it was mounted on a 24m catamaran. The sea trials, offshore commissioning and control system optimisation provided extensive amounts of data on the performance of TAS in waves. The results of the performance in waves has been investigated and documented in order to validate the claims of the functionality of TAS.

Further optimisation of the motion compensation principle is discussed where the main focus lies on the increase of the operability range beyond the current standards. By changing the bow fenders of the work boat to rollers that provide hydraulic energy dissipation, the additional imposed pitch damping is expected to reduce the boat motions and increase the TAS operability. The paper discusses the methodology and logic behind the active pitch damping devices and the effect on the operation of TAS.

The paper discusses the main technical challenges and solutions to achieve a safe, user friendly and comfortable transfer system for the offshore industry.
Importance of Second-Order Difference-Frequency Wave-Diffraction Forces in the Validation of a FAST Semi-Submersible Floating Wind Turbine Model
Authors: Coulling Alexander J. , Goupee Andrew J. , Robertson Amy N. , Jonkman Jason M.
Published: November 2013

To better access the abundant offshore wind resource, efforts are being made across the world to develop and improve floating offshore wind turbine technologies. A critical aspect of creating reliable, mature floating wind turbine technology is the development, verification, and validation of efficient computer-aided-engineering (CAE) tools. The National Renewable Energy Laboratory (NREL) has created FAST, a comprehensive, coupled analysis CAE tool for floating wind turbines, which has been verified and utilized in numerous floating wind turbine studies. Several efforts are underway to validate the floating platform functionality of FAST to complement its already validated aerodynamic and structural simulation capabilities. The research employs the 1/50th-scale DeepCwind wind/wave basin model test dataset, which was obtained at the Maritime Research Institute Netherlands (MARIN) in 2011. This paper describes further work being undertaken to continue this validation. These efforts focus on FAST’s ability to replicate global response behaviors associated with dynamic wind forces and second-order difference-frequency wave-diffraction forces separately and simultaneously.

The first step is the construction of a FAST numerical model of the DeepCwind semi-submersible floating wind turbine that includes alterations for the addition of second-order difference-frequency wave-diffraction forces. The implementation of these second-order wave forces, which are not currently standard in FAST, are outlined and discussed. After construction of the FAST model, the calibration of the FAST model’s wind turbine aerodynamics, tower-bending dynamics, and platform hydrodynamic damping using select test data is discussed. Subsequently, select cases with coupled dynamic wind and irregular wave loading are simulated in FAST, and these results are compared to test data. Particular attention is paid to global motion and load responses associated with the interaction of the wind and wave environmental loads. These loads are most prevalent in the vicinity of the rigid-body motion natural frequencies for the DeepCwind semi-submersible, with dynamic wind forces and the second-order difference-frequency wave-diffraction forces driving the global system response at these low frequencies. Studies are also performed to investigate the impact of neglecting the second-order wave forces on the predictive capabilities of the FAST model. The comparisons of the simulation and test results highlight the ability of FAST to accurately capture many of the important coupled global response behaviors of the DeepCwind semi-submersible floating wind turbine.