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Safety and Reliability

2005;():3-8. doi:10.1115/OMAE2005-67009.

The risk analysis of an oil complex require a careful estimation of failure consequences and, as an important component, the indirect loss may play a relevant role, especially for high production facilities. In this work, the interaction between the industrials sectors is considered uncertain by modeling it through random variables. The contribution of the five most important sectors is considered to be normally distributed and the value of its loss, as reported by INEGI (1986), is considered to be the mean value and a coefficient of variation of 0.1 is assumed for each sector loss. By using Monte Carlo techniques, the loss for each of the five sectors is simulated and the total indirect loss is calculated. The production loss is obtained by assuming that time to restore the production rate in the five platforms is 33 months. For each trial, the I/O matrix is obtained, the vector of predisaster products is calculated and the production loss is estimated. Then, the demand and the production level post-disaster are assessed. Finally, the second round and total losses are estimated. With the sample of indirect losses obtained, its probability distribution is estimated. The probabilistic description of the indirect loss includes, among other epistemic uncertainties, the one related to the sector interactions, which are considered as linear in a simplified way.

Commentary by Dr. Valentin Fuster
2005;():9-16. doi:10.1115/OMAE2005-67017.

A key principle for achieving Tolerable Risk under the UK Health and Safety Executive’s (HSE) approach is the reduction of risks to “As Low As Reasonably Practicable” or ALARP. This principle is founded on the ideal of reducing risks to a point of diminishing returns where additional risk reduction would cost “disproportionally” more than the risk reduction benefit achieved. The HSE approach of estimating the degree of disproportionality between the cost and benefits associated with a potential risk reduction measure is a variation of the Cost Effectiveness approach to ALARP Evaluation. A probabilistic cost-benefit analysis method is outlined for evaluating design options and establishing whether the conditions of ALARP are met. This paper demonstrates the value of probabilistic cost-benefit analysis for the selection of a target safety level and also presents an example of its practical application by way of a case history.

Commentary by Dr. Valentin Fuster
2005;():17-23. doi:10.1115/OMAE2005-67023.

The objectives of the study are to quantify the reliability of a boil-off gas (BOG) reliquefaction system for liquefied natural gas (LNG) carriers and to verify design improvements based on the reliability analysis. The system is broken into subsystems and then further into components. Failure rates are collected from generic references, primarily from the OREDA handbook. The reliabilities of the subsystems are estimated, and a reliability block diagram for the whole system is established. The subsystems are classified into three ranks according to their reliability: the high reliability rank comprises the subsystems ‘BOG preparation system,’ ‘Seawater intake system,’ and ‘Buffer N2 reservoir system’, the medium reliability rank the subsystem ‘BOG liquefaction system’, and the low reliability rank the subsystems ‘BOG compression system,’ ‘N2 cooling system’, and ‘Buffer N2 generation system.’ The reliability and availability are estimated for various process configurations where some of the low reliability category have a standby. The ‘bare’ system without any redundancy fails to attain an availability higher than 0.96. Addition of redundancy to one of the least reliable three commonly results in an increase in reliability improvement, to around 0.97. If all of the three subsystems have standby units, the system reliability improves to 0.99. It is recommended that maintenance efforts should be concentrated on the rotating machines that caused the subsystems to have a low reliability.

Commentary by Dr. Valentin Fuster
2005;():25-35. doi:10.1115/OMAE2005-67025.

The objective of this study is to obtain basic data for discussing the structural integrity of aged ships, especially hold frames of aged bulk carriers. Firstly, shapes of corrosion pits observed on hold frames of bulk carriers have been investigated. It was shown that the shape of the corrosion pits is a circular cone and the ratio of the diameter to the depth is in the range between 8 to 1 and 10 to 1. Secondly, a series of tensile tests has been conducted to investigate the effect of pitting corrosion on tensile strength. It was pointed out that the tensile strength decreases gradually and the total elongation decreases drastically with the increase of thickness loss due to pitting corrosion. Thirdly, a series of 4-point bend tests with structural models which consist of shell, web and face plates simulating hold frames of bulk carriers has been carried out to investigate the effect of pitting corrosion on collapse behavior and lateral-distortional buckling behavior. Following the experiment, a series of non-linear FE-analyses has been also made. In the case where tensile load acted on the face plate, cracks were initiated at the bottom of the pits when pitting concentrated on the web near the face plate. On the other hand in the case where compression load acted on the face plate, lateral-distortional buckling has been observed and the ultimate load of the structural models where pitting developed regularly on the web was found to be almost the same as that of the structural models where the web has uniform corrosion corresponding to the average thickness loss.

Commentary by Dr. Valentin Fuster
2005;():37-44. doi:10.1115/OMAE2005-67034.

This paper describes why and how the authors have developed an online distance learning package specifically designed with the objective of raising awareness of Metocean amongst Shell’s discipline engineers around the world. Metocean technology is applied across the oil and gas business in design, operational planning and in everyday operations. It has a significant high value impact. Changes in the working and business environment have highlighted a clear need to raise awareness amongst users of Metocean technology — both for the business and for individual competence development. The disciplines identified are project management, structures, floaters, pipelines, subsea, drilling, operations, civil, geomatics and seismic. The paper explores ways to raise this awareness, leading to the choice of a tailored online distance learning course. It describes its design and development, and the incorporation of a number of learning innovations. It also presents the feedback both from line managers and participants on the course, and identifies lessons learnt and areas for improvement. The paper concludes with suggestions for the future and how others can develop bespoke awareness programmes most effectively to add value to their businesses. The significance of this paper is that it focuses on how best to communicate Metocean technology to a broad range of users, and it describes how various online distance learning techniques and innovations were tailored to achieve this. Not only do participants learn about Metocean principles and conditions around the world, but also they select their own business related course assignment. The course design encourages interaction and sharing amongst participants, and includes a variety of case studies and peer reviews. Amongst participants, the course has received high completion, satisfaction and business value scores.

Commentary by Dr. Valentin Fuster
2005;():45-50. doi:10.1115/OMAE2005-67083.

The tendons of a tension leg platform are critical structural components in maintaining the system integrity. A common tendon design limit is associated with the minimum tendon tension. Most TLP reliability studies in the past defined the minimum-tension limit state as the loss of tendon tension. However, studies have shown that temporary tension loss at a corner of a TLP will not necessarily jeopardize the system integrity if the tendon is allowed to stroke down during a loss of tension event. Hence, we define the limit state associated with minimum tension event as the down-stroke where the tendon unlatches itself. An approximate method is proposed in this study to evaluate the probability of tendon unlatching. Sensitivities to the key assumptions are also investigated.

Commentary by Dr. Valentin Fuster
2005;():51-59. doi:10.1115/OMAE2005-67112.

This paper presents a general axi-symmetrical solid element to be used mainly for the calculation of added masses of water surrounding members of offshore structures, and in general, for multi-purposes such as analyses of shells of revolution, circular beams and plates, axi-symmetrical structures and soils, plane stress/strain problems. Since one element type is used for modeling of different media such as structures, soil and water, the element is very suitable to solve interaction problems. The element is derived parametrically so that changing values of parameters can generate flexible geometrical shapes in exact forms. In the element formulation, a constant shear locking is used to solve bending problems of beam like structures. A similar fluid element is also formulated to analyze fluid-structure interactions and to determine added masses of co-vibrating water. The added mass is calculated from hydrodynamic pressures, which are produced by fluid-structure interactions. In the paper, a special solution algorithm is presented for the coupled eigenvalue problem of the interaction. An analytic calculation of the added mass is also presented for members along which a constant variation of hydrodynamic pressure occurs. A couple of examples are provided to demonstrate applications of the elements explained. Added mass coefficients of offshore structural members (tubular members) are investigated for practical uses.

Commentary by Dr. Valentin Fuster
2005;():61-68. doi:10.1115/OMAE2005-67123.

Ship safety under normal and severe weather conditions is determined by ship design, approval, and operation. Numerical simulation of ship motions has proven to be a valuable tool for ship design evaluation — not only for accident investigations and studies addressing fundamental stability related phenomena, but also during the design process. A sufficient number of simulations provide a data basis for polar plots to judge the ship’s situation in a sea state — defined by significant wave height and characteristic period — with respect to load case, encounter angle and ship speed. Application of data provided by numerical tools has to take into account the validity range of the model and has to be validated sufficiently by model test data. For providing useful validation data, the exact correlation of wave excitation and ship motion in model testing is indispensable. In the framework of the German research project SINSEE, this is achieved by a fully automated test procedure with a free running ship model in combination with deterministic generation of tailored realistic wave sequences and their transformation to the moving reference frame of the cruising ship. The resultant wave train can be directly correlated with time series of motions — registered by an optical system — and forces. In this paper, these methods are applied to investigate pre-simulated seakeeping scenarios in the model basin and compare the results directly to the simulation results.

Commentary by Dr. Valentin Fuster
2005;():69-78. doi:10.1115/OMAE2005-67163.

To ensure the safety and reliability of ship structures during their service lifetime, inspections are essential to evaluate corrosion or fatigue damage and schedule maintenance or repair. This paper aims to propose a cost-benefit model of risk based inspection and repair planning for ship structures subjected to corrosion deterioration. The models of expected costs and expected benefit are formulated. The benefit-cost ratio is adopted to select the optimal inspection and repair planning. The planning problem is formulated as an optimization problem where the total benefit-cost ratio in the expected lifetime is maximized with a constraint on the minimum acceptable reliability index. A component subjected to corrosion deterioration is presented to illustrate the proposed approach. In addition, sensitivity studies are presented to account for the effects of uncertainties in basic parameters on the benefit-cost ratio and the optimal inspection and repair planning. The results indicate that the proposed approach can effectively integrate the economy with reliability of the inspection and repair planning. Furthermore, a balance can be achieved between the risk cost and total expected inspection and repair costs, which is very effective in selecting the optimal inspection and repair planning. Parametric studies show that the benefit-cost ratio is sensitive to the time of inspection and repair, the reference period, the repair criterion and the failure criterion.

Commentary by Dr. Valentin Fuster
2005;():79-86. doi:10.1115/OMAE2005-67170.

This paper presents a methodology for reliability analysis of Tension Leg Platform (TLP) tendons subjected to extraordinary sea state conditions like hurricanes or winter storms. A coupled approach in time domain is used to carry out TLP random nonlinear dynamic analysis including wind, current and first and second order wave forces. The tendons Ultimate Limit State (ULS) condition is evaluated by an Interaction Ratio (IR) taking into account dynamic combination among tension, bending and hydrostatic pressure. Expected long-term extreme IR is obtained through the integration of cumulative probability functions (CPFs) fitted to response maxima associated to individual short term sea states. The reliability analysis is performed using a time-integrated scheme including uncertainties in loads, tendon strength, and analytical models. Failure probabilities for the most loaded tendon of a TLP in Campeche Bay, Mexico, considering a 100-yr design sea state and the 100-yr extreme response generated by long-term observed storms are compared.

Commentary by Dr. Valentin Fuster
2005;():87-96. doi:10.1115/OMAE2005-67186.

An efficient time-variant reliability formulation for the safety assessment of an aging Floating Production Storage and Offloading (FPSO) vessels with the presence of through-thickness cracks (i.e. long cracks), is presented in this paper. Often in ship structures, cracks are detected by means of close visual inspection when they have already propagated through the thickness. The propagation of long cracks in stiffened panels is therefore considered, as they may be present in critical details of the deck and/or bottom plating of the vessel. The mean stress effect due to the continuously varying still-water loading as well as residual stresses is explicitly accounted for in the crack growth calculation. An analytical model is established for determining the equivalent long-term stress range. Although it has been found that stiffened panels are tolerant to fatigue cracking, the probability of brittle fracture of an aging hull structure becomes more relevant with the presence of long cracks. The continuously varying still-water load effects due to the operational nature of FPSOs introduce additional uncertainties in the estimation of fatigue damage as well as in the likelihood of fracture failure mode. These uncertainties are accordingly considered in the analyses.

Commentary by Dr. Valentin Fuster
2005;():97-105. doi:10.1115/OMAE2005-67199.

Many parameters influencing corrosion degradation are time-variant and the corrosion process is thus subject to temporal variability, the real characteristics of which are commonly neglected in reliability assessment. After a short overview on the quantitative modeling of corrosion loss, a comparison is made between different temporal models of corrosion degradation and the consequences of applying an inappropriate model are investigated. The effect of temporal variability is then investigated in detail and illustrated on an example considering CO2 corrosion in pipelines. It is demonstrated how the time-variant corrosion process can be consistently represented by constant parameters, using equivalent values of the corrosion rate. Finally the influence of temporal variability on the reliability updating following inspections is investigated and it is shown how this effect can be accounted for in inspection planning.

Commentary by Dr. Valentin Fuster
2005;():107-114. doi:10.1115/OMAE2005-67236.

The state-of-the-art representation of the horizontal motions of e.g. a TLP in random seas is in terms of a second order stochastic Volterra series. Until recently, there has been no method available for accurately calculating the mean level upcrossing rate of such response processes. Since the mean upcrossing rate is a key parameter for estimating the large and extreme responses it is clearly of importance to develop methods for its calculation. The paper describes numerical methods for calculating the mean level upcrossing rate of a stochastic response process represented as a second order stochastic Volterra series. Since no approximations are made, the only source of inaccuracy is in the numerical calculation, which can be controlled.

Commentary by Dr. Valentin Fuster
2005;():115-122. doi:10.1115/OMAE2005-67241.

The present paper reviews and outlines the interpretation of uncertainties with a view to the various different categorizations introduced in the literature. A framework is then presented for risk based decision making taking basis in the Bayesian decision theory and recent methodical developments in risk assessment. It is emphasized that in principle all types of uncertainties should be included in formal decision analysis and that not doing so corresponds to informal decision analysis the quality of which may be difficult to judge. The controversial problem in engineering decision making of how to take into account uncertainties associated with the definition of the system being analyzed is outlined. For the typical situation where a discrete set of possible system representations is possible it is shown how a decision problem may be formulated for the identification of the optimal system to be considered as basis for decision making. The presented decision framework takes into account all prevailing uncertainties, epistemic as well as aleatory. Examples related to structural design and assessment problems relevant for offshore engineering are given illustrating how not to account for all types of uncertainties leads to sub-optimal decision making.

Topics: Decision making
Commentary by Dr. Valentin Fuster
2005;():123-131. doi:10.1115/OMAE2005-67266.

Relatively accurate techniques are available to assess structural behavior under given loads, yet the loads themselves remain an estimate based in part on field measurements, in part on professional logic and experience, and in part on trial and error. The design of piled foundations for fixed offshore platforms must consider operating and extreme weather conditions. In the operating condition, the magnitude of live loads on open areas of topside structure is an important consideration. Unfortunately, the design live load intensity that applies to open areas on offshore platforms is not identified in international codes and standards. There does not appear to be any consensus on the value to be adopted in the industry. Some operators suggest the open area live loads need not be considered for pile foundation design, while others stipulate values such as 10 kPa. This is partly due to the variability associated with the different live loads sources. The objective of this study is to obtain a better understanding of open area live loads on offshore platforms and develop a methodology to obtain the long-term and extreme open area live load. A load survey was conducted for the purpose of this study, and a probabilistic analysis was carried out to derive the maximum axial load on piles that is expected during platform lifetime. The results of this study indicate that the use of a single value for the open area live load (OALL) may not be appropriate and suggest appropriate values for Load Resistance Factor Design (LRFD) or Working Stress Design (WSD) methods.

Commentary by Dr. Valentin Fuster
2005;():133-142. doi:10.1115/OMAE2005-67269.

Offshore sites in the Arabian Gulf are characterized by the presence of calcareous sediments. Research work on driven piles in calcareous sediments has been developing for over 40 years. Yet, international codes and standards do not provide, exploit or quantify guidance for driven piles in carbonated sediments. Lack of design methods is partly attributed to inability of conventional soil mechanics to predict appropriate engineering parameters in calcareous sediments. Further, the characteristics of the sediments vary between the geographical regions depending on the geological history forming that region. As a result, current industry practice follows a conservative and subjective approach at the mere mention of carbonated sediments. Consequently, reassessment of an existing platform may falsely indicate the need for expensive construction intervention. In this study, we reviewed current practice for assessment of piled foundation in the calcareous sediments of the Arabian Gulf, collated a database of actual pile driving records, developed and implemented a detailed back-analysis procedure and implemented to derive actual pile capacities. The statistics show that the use of a single capacity value, as implied by the deterministic method of codes and standards, is insufficient to describe the various conditions surrounding the as-installed driven piles in calcareous sediments.

Commentary by Dr. Valentin Fuster
2005;():143-149. doi:10.1115/OMAE2005-67292.

A new analytical solution for the return period of a sea storm in which the largest wave height exceeds a fixed threshold is obtained, by applying the Boccotti’s Equivalent Triangular Storm (ETS) model. An expression is then given for the probability that a wave, which is both higher than a fixed threshold and the highest of its own storm, will occur during a sea storm with a given intensity (the storm intensity being the maximum significant wave height during the storm). In the applications it is shown that the new solution has a simpler form than the Boccotti’s original one (two integrals to solve numerically, compared with the four integrals of the Boccotti’s solution), and that they give identical results. Finally it is achieved that the extreme waves of given height H, with large probability will occur in a sea storm with maximum significant wave height Hs max close to 0.5H.

Topics: Waves , Storms , Seas
Commentary by Dr. Valentin Fuster
2005;():151-159. doi:10.1115/OMAE2005-67329.

The scientific developments carried out in the last years with spaceborne synthetic aperture radar (SAR), as well as the comparisons with in-situ sensors, have demonstrated that SAR is a reliable remote sensing tool to study wave fields on the open ocean. In their so-called SAR image mode, SAR systems on board satellites are able to scan ocean areas about 100 × 100 km2 with a spatial resolution about 20 × 20 m2 . These SAR images are able to provide information about the spatial variability of wave fields, as well as other phenomena that occur on the ocean surface, such as the local wind field. This work investigates the capabilities of spaceborne SAR to extract sea state information for those areas close to coastal locations, where the incoming wave fields present high spatial inhomogeneities. For this purpose, ERS-2 SAR and ENVISAT Advanced SAR (ASAR) images acquired over the North Sea and the Bay of Biscay are used to study ocean waves in shallow waters under different climate and oceanographic conditions. In addition, the potential of ENVISAT ASAR capability to scan the ocean surface with dual polarizations (horizontal, HH, and vertical, VV) is analyzed in this work. The relevance of these investigations for different practical applications concerning the analysis of ocean waves is discussed.

Commentary by Dr. Valentin Fuster
2005;():161-168. doi:10.1115/OMAE2005-67330.

Structural reliability is of an intense significance for the evaluation of failure probability and safety levels of offshore structures in their structural design stage, especially when the variables are eminently random. Hence, the environmental conditions are considered among the most important parameters in the design of offshore structures whose effect of great contribution to structural safety. The 100 year storm wave height and current velocity constitute the majority of uncertainty in the environmental conditions. Eventually, these values are used as stochastic values i.e. mean values in addition to standard deviation. This paper uses the stochastic values of 100 year storm wave height and current velocity to estimate the probability of failure. These simulations had been encountered in the structural analysis and design on practical case study representing an offshore structure located in the Mediterranean Sea to predict the reliability level. The outcome of these simulations uses the Response Surface Method with design point technique for both stress and displacement limit states. The results of this Reliability method lead to an acceptable level for reliability analysis and risk assessments.

Commentary by Dr. Valentin Fuster
2005;():169-180. doi:10.1115/OMAE2005-67382.

An extension of joint omnidirectional probabilistic model including combined seas and possibility of environmental parameters approaching from different direction is presented. The following parameters are considered: mean wind speed, wind direction, significant wave height (sea and swell), spectral peak period (sea and swell), wind sea direction, swell direction, current speed and current direction. Two approaches for wind sea and swell modelling are proposed. The joint description is developed for three locations at the Norwegian Continental Shelf and for the West Shetland. Uncertainties related to it are discussed. The model is meant to be applied in reliability analysis of offshore structures, in particularly FPSO units, for which a proper account of directional effects and combined seas is important.

Topics: Seas
Commentary by Dr. Valentin Fuster
2005;():181-188. doi:10.1115/OMAE2005-67408.

This paper provides a selection of findings on the reliability and risk analysis of submarine blowout preventers (BOPs) achieved in recent studies, which the author has participated among technicians in Petrobras and Rio de Janeiro Federal University - UFRJ. Petrobras is a deepwater E&P leading company and special attention has been given on the deepwater and ultra deepwater scenarios. Particular factors on the BOP for dynamic positioning (DP) rigs need to be accounted as far as risks on safety and downtime are concerned. The analyses have considered a comprehensive BOP reliability database that covers more than 12 years of experience, as well as, a collection of DP experience has been accessed during work. The riser safety margin tends to be unavailable as the water depth becomes deeper, and, so, in an emergency disconnection, the safety isolation of the well tends to be relied on the BOP mechanical barriers only. In this context, risks on different configurations of the BOP are compared and their results discussed with focused attention on main factors of risk. Finally, there has been visible progress on BOP reliability within recent years and a number of good operational practices in place are also mentioned in the paper.

Commentary by Dr. Valentin Fuster
2005;():189-201. doi:10.1115/OMAE2005-67412.

This research has focused on the behavior, under seismic conditions, of already existing steel storage tanks of large capacity, located in high risk zones. From the revision of analysis and design criteria concerned with thin walls structures, it has been prepared a procedure based on a numeric modeling where the mechanic characteristics of the materials and the real geometrical measures have been considered. Numeric analysis by FEM have been used in different conditions: empty tanks vibration, full tanks where fluid-structure interaction is considered to the case of flexible walls, in order to take into account the pressure distribution of the liquid. To estimate the response, real seismic records originated in the Mexican Region, have been used. Finally the numerical results obtained of the empty tanks with those calculated analytically are compared and it is observed that a good correlation between both approaches. For the results obtained of the fluid-structure interaction models with the selected seismic registry is observed that given its great dimensions and the rigidity that provides the ring to them in the top part of the tanks, the effect of the surge is not very significant due to the fluid system — structure is excited in the first seconds, reason why the action of the hydrostatic pressure on the walls of these is sample to be dominant.

Topics: Storage tanks
Commentary by Dr. Valentin Fuster
2005;():203-210. doi:10.1115/OMAE2005-67429.

This paper deals with time variant overload reliability analysis of a mooring system due to corrosion deterioration. A probabilistic model for uniform corrosion is adopted to predict the strength degradation. A simplified method and nonlinear finite element analysis are used to calculate the breaking strength of the chain link and comparison is made. The strength of one mooring line is modeled by a weakest link system. The effect of correlation in corrosion models for different chain links in one mooring line and the effect of higher corrosion rate in the splash zone are discussed. The annual failure probability of the most loaded mooring line of a semi-submersible is calculated. The first and second order motions of the semi-submersible and the corresponding line tensions are found by a simplified analysis. The time variant reliability analysis is performed by approximating the degraded strength by a piece-wise constant model. The annual failure probability is obtained for different years. It is found that the annual failure probability increases significantly as the chain is corroded.

Commentary by Dr. Valentin Fuster
2005;():211-217. doi:10.1115/OMAE2005-67432.

In the present paper, fatigue safety factors for flexible risers are assessed. A procedure for reliability analysis of wave-induced fatigue is first described. The procedure is based on performing a number of parametric studies with respect to variables that influence the fatigue lifetime. The results of these parametric studies are subsequently combined with models describing the statistical scatter of the same parameters. By application of this procedure, the safety factors which are required in order to reach specific target reliability levels can be computed. Such safety factors are computed for three specific flexible riser configurations. Different SN -curves which correspond to different corrosive environments are considered. The percentwise contribution from each parameter to the total statistical variation of the limit state function is also quantified.

Commentary by Dr. Valentin Fuster
2005;():219-224. doi:10.1115/OMAE2005-67458.

Accidents related to production, processing, storage and transportation of oil and its by-products have been studied extensively because of their social and environmental impacts. However, researchers have largely ignored accidents related to the construction of oil facilities, perhaps because such accidents involve a small group of people and result in smaller-scale environmental impacts. In this paper we discuss the major potential risks in every phase of an offshore unit construction. We identified the limitations of traditional safety management systems in coping with the critical problems related to safety issues. Many companies are using integrated management systems as the major tools to control risk. Such systems cover health, safety and environmental issues (HSE). PETROBRAS, the largest Brazilian oil company and one of the leading oil companies in the world, has adopted the HSE system. The system focuses on employee participation in implementation of the HSE system. We highlight leader commitment as one of the keys to good results. In many cases, management commitment is low, implying a need to convince them of the importance of health and safety performance to the future prosperity of their company. Contractual directives are baselines to build good relationships during the project. Another relevant point is to demand a management system like OHSAS 18001 from contractors. Awareness of health risks is generally below that of safety risks and emphasis should be placed on increasing understanding of the causes.

Commentary by Dr. Valentin Fuster
2005;():225-234. doi:10.1115/OMAE2005-67461.

In the present work, a nonstationary stochastic model, which is suitable for the analysis and simulation of multivariate time series of wind and wave data, is being presented and validated. This model belongs to the class of periodically correlated stochastic processes with yearly periodic mean value and standard deviation (periodically correlated or cyclostationary stochastic process). First, the time series is appropriately transformed to become Gaussian using the Box-Cox transformation. Then, the series is decomposed, using an appropriate seasonal standardization procedure, to a periodic (deterministic) mean value and a (stochastic) residual time series multiplied by a periodic (deterministic) standard deviation. The periodic components are estimated using appropriate time series of monthly data. The residual stochastic part, which is proved to be stationary, is modelled as a VARMA process. This way the initial process can be given the structure of a multivariate periodically correlated process. The present methodology permits a reliable reproduction of available information about wind and wave conditions, which is required for a number of applications.

Topics: Waves , Modeling , Wind
Commentary by Dr. Valentin Fuster
2005;():235-244. doi:10.1115/OMAE2005-67476.

Real options models are currently available as one of the best tools for the assessment of investment projects. This is so mainly due to the capability of the real options models to: (1) account for uncertainties in financial variables that are crucial to the investment project; and (2) quantify the value of the possibility to make a decision on whether to defer, abandon, expand or reduce the project at one or several points along time. Recently, some researchers have proposed the use of real options models for the assessment of infrastructure projects for hydrocarbon exploitation from an economics point of view. The objective of this work is to develop real options models for decision making regarding inspection, maintenance and decommissioning of offshore facilities taking into account the financial and technical aspects of the project. In all cases it is considered that at some point in the future, within the service lifetime of the structure, the decision maker will have an option to carry or not an inspection, and take or not a maintenance or decommissioning action, which will determine the structural and financial performance of the project for its remaining lifetime. The in-service times with no structural failure and the rehabilitation times are modeled as random variables. The cash flows are modeled as stochastic processes considering interruption of operation due to repairs after failure. Analytical expressions are derived for the computation of structural reliability and availability depending upon maintenance actions. An example is given for a jacket platform subjected to fatigue deterioration and damage. Simple and compound options of maintenance and decommissioning options are analyzed. The value of the project is computed by means of an approach similar to that of Black and Scholes for financial options [2]. The results are compared to those obtained under the traditional Net Present Value approach.

Commentary by Dr. Valentin Fuster
2005;():245-252. doi:10.1115/OMAE2005-67481.

In risk based inspection planning the number of planned inspections directly depends on the uncertainty on the load history. In this paper it is shown how this uncertainty can be reduced by modal based fatigue monitoring. The technique is based on continuously measuring of the accelerations in few points of the structure. The accelerations are integrated to obtain the displacement and a modal decomposition of the obtained displacements is performed. The relation between the modal coordinates and the stress history in an arbitrary point of the structure is established by a calibrated finite element model. The technique has been applied on a wind loaded lattice pylon structure and compared with strain gauges measurements, showing that the uncertainty on the presented technique is quite small. It is shown that if the technique is applied to an offshore steel structure, then the number of required inspections may be reduced with more than 50%.

Topics: Fatigue , Steel , Inspection
Commentary by Dr. Valentin Fuster
2005;():253-261. doi:10.1115/OMAE2005-67491.

This paper discusses the effects of corrosion and slamming on Floating Production Storage and Offloading (FPSO) structures in the North Sea by using a time variant structural reliability based approach. An FPSO structure currently in operation in the North Sea is analysed, an analytical approach is suggested and used to determine the Ultimate Strength of the FPSO structure and compared with other published methods. A non-linear corrosion method is then used to determine the effect of corrosion on the ultimate strength and subsequently in the reliability of the structure. Slamming loads on the midship section of the FPSO are calculated using closed form expressions. The effect of slamming and green water load on the overall vertical bending moment applied on the structure is combined with the vertical wave bending and still water loads using the Ferry Borges- Castanheta method and the SRSS rule, to provide the distribution and variation of a loading component for structural reliability analysis to be performed. Time variant reliability analysis using Monte Carlo Simulation is then used to determine reliability indices and probabilities of failure for the structural system analysed on a component level.

Commentary by Dr. Valentin Fuster
2005;():263-270. doi:10.1115/OMAE2005-67531.

Heat exposure of process equipment is a complex problem area that has gained interest over the last years. The reason is that existing standards vague on the subject and operate with too low heat load compared to what has been found in experiments. It has also been shown that the subject is more complex than expected when the existing standards were formed. It has been clear lately that simple formulas will not cover all the aspects and variation of the phenomenon. It seems that multi physic simulations are needed to be able to predict the behaviour satisfactorily. VessFire is such a simulation system that can handle this problem and has been widely used the last years by several companies and projects. One of the major parameters that need to be modelled is the heat transfer phenomenon inside the equipment shell. This is a difficult task and it is not easy to find data for verification. This paper describes a number of experiments performed for verification purposes. The project was supported by The Petroleum Safety Authority Norway and is a result of the authorities’ need for better means in safety management as new technology is introduced. The simulation results show good agreement to the experiments and indicate that the simulation system gives a reasonably good representation of the phenomena.

Topics: Heat , Simulation
Commentary by Dr. Valentin Fuster
2005;():271-276. doi:10.1115/OMAE2005-67534.

The paper presents an effective way of incorporating the human, organisational and management factors in order to improve integrity of offshore marine operations. The aim of the paper is to establish safety barriers and integrate human, organisational and management factors into these barriers and define performance shaping factors that could erode the barrier function. This can be achieved by using bow tie analysis. Furthermore, the quantification of the risk model, if required, can be achieved by direct application of human reliability methods for operational errors and by defining the failure rate multiplier based on the assessment of management and organisational characteristics.

Commentary by Dr. Valentin Fuster
2005;():277-283. doi:10.1115/OMAE2005-67535.

The Single Point Mooring (SPM) jacket platforms have been developed over the last several decades and are widely applied in the exploitation of offshore oil-gas resources. The SPM jacket platforms are continuously subjected to environmental loads due to wave, current, wind and mooring force. As the uncertainty of those loads, the system reliability research of SPM jacket platforms is one of the most important research areas. A methodology of system reliability-based assessment of the SPM jacket platform was introduced in this paper and applied for the assessment of BZ28-1 SPM jacket platform. According to nonlinear mechanical analysis, the mechanics characteristics of SPM jacket platform subjected to extreme loads were analyzed on considering pile-soil-structure nonlinear interaction. Based on the FEM model, with Monte Carlo simulation, the reliability of SPM jacket platform was studied, and the system reliabilities in 20-year-based-period and 50-year-return-period were calculated respectively. The results show that the statistical analysis of parametric variables based on 50-year return period is appropriate, and the mooring forces due to wave are the predominant loads for SPM jacket platforms. The system reliability-based assessment method can provide important and reliable referential suggestions for the design, maintenance and repair of SPM jacket platforms.

Topics: Reliability , Mooring
Commentary by Dr. Valentin Fuster
2005;():285-293. doi:10.1115/OMAE2005-67545.

Four formulations for dissipation by whitecapping in discrete spectral wave models have been compared, and their applicability in combined swell-sea systems has been investigated. To obtain workable formulations, a sensitivity study has been carried out, followed by a simple calibration. Finally, the effect of a small background swell on the growth of a wind sea has been studied for all formulations.

Topics: Seas
Commentary by Dr. Valentin Fuster
2005;():295-309. doi:10.1115/OMAE2005-67571.

The sea elevation at a fixed point is modelled by means of a second order model, which is a smooth algebraic function of a vector valued Gaussian process. Asymptotic methods, presented first in [1], are used to estimate the mean upcrossing intensity μ+ (h). The intensity is then used to determine the density of crest height in a second order sea. Numerical examples illustrate the method. The proposed approximation is used to estimate the design crest height for a specified return period.

Commentary by Dr. Valentin Fuster
2005;():311-317. doi:10.1115/OMAE2005-67572.

The oil and gas industry has, over the recent years, invested heavily in multi-billion dollar projects aimed at developing an exploration and production infrastructure in the deepwater Gulf of Mexico. The integrity management of such facilities will be critical for the industry in the coming years. Competency of Personnel stands out as the most important of these areas. Although office-based personnel plays an important part, the personnel working offshore such as OIMs, supervisors, ballast operators and topsides operators will make critical decisions that may affect the safety of an entire production unit. Alliances between offshore industries, contractors and community colleges generate curricula exclusively tailored to provide specific training and retraining to new hires on the duties and tasks performed by the oil and gas production technician and encumbered workers of the offshore industry. Offshore oil and gas extraction organizations are faced with unique challenges, when needing competent, skillful, safe, and well trained workers. Liabilities from non-trained personnel, and the understanding that safety is priority #1 on the platform, are paramount in concepts of the offshore workforce. Unlike other areas of the world, production in the Gulf of Mexico has been predominantly on the shallow water continental shelf and many organizations are faced with the challenges of developing new sets of skills for different types of production facilities used in deep water. Training of technicians, who will be operating an offshore deepwater platform, should cover topics that include ballast control and stability management, vessel station-keeping, riser management, response to hurricanes and loop currents, hydrocarbon and operations safety, process equipment and systems, troubleshooting, sub-surface geology, petroleum engineering and drilling technology. This paper discusses collaborative initiatives and discusses specific solutions. Based on a multi-year experience generating of such curricula, recommendations are provided to industries that want to establish such training programs for their technical personnel.

Commentary by Dr. Valentin Fuster
2005;():319-328. doi:10.1115/OMAE2005-67576.

In the maritime industry the International Maritime Organization (IMO) is the UN organization responsible for developing international safety and environmental protection regulations. IMO has now developed the second version of ‘Guidelines for Formal Safety Assessment (FSA) for use in the IMO rule making process’. The Guidelines are available as circulars both from the Marine Safety Committee (MSC) and the Marine Environmental Protection Committee (MEPC). This standard is, as far as the author knows, the first risk assessment standard adopted in an UN organization. The work with developing this standard was initiated in 1995 at IMO based on an UK initiative. As there have been some attempts to develop internationally accepted risk assessment and risk management standards also in other industries, this paper tries to describe some of the experience and lessons learned from developing and implementing FSA at IMO. Paralleling the development of the guidelines there has been a number of applications of the guidelines, recently focusing on bulk carrier safety. Relevant studies have been carried out by UK, by Japan, by Norway and International Confederation of Free Trade Unions (ICFTU), and by the International Association of Classification Societies (IACS). These studies will be briefly reviewed with respect to methods used, assumptions made and conclusions drawn. The entire process from the initial terms of reference formulated by IMO to the final decisions is considered. The main conclusion is that the maritime industry has made a lot of progress, quite fast, in the use of risk assessment as part of the decision making process. This being the case, despite the many communication problems that arises in discussing risk issues in international forums. Furthermore, the FSA has helped balancing the often conflicting interest of the flag states and non-governmental organizations present in IMO. In 2004, a new initiative was taken on developing Goal Based Standards at IMO. This initiative was taken by Greece and Bahamas, and has now been debated at three meetings of MSC. The paper will also discuss the relationship between GBS and FSA based on the experience gained.

Topics: Safety , Regulations
Commentary by Dr. Valentin Fuster
2005;():329-336. doi:10.1115/OMAE2005-67579.

The corrosion wastage of deck plates of ballast and cargo tanks is analyzed based on a non-linear corrosion model. This model is able to describe an initial period without corrosion due to the presence of a corrosion protection system, a transition period with a nonlinear increase of wastage up to a steady state of long-term corrosion wastage. This model is applied to corrosion wastage data of deck plates collected by the American Bureau of Shipping. The objective of this work is to fit this corrosion wastage model to the service measured data, determining the values of the model parameters that represent the best fit to the data so as to describe how corrosion wastage varies in time as a result of generalized corrosion.

Commentary by Dr. Valentin Fuster
2005;():337-345. doi:10.1115/OMAE2005-67584.

Parametric and non-parametric spectral descriptions are used to estimate the wave spectra from wave frequency motions of a vessel at low advance speed, using minimization procedures based on a constrained gradient search and a genetic algorithm. The parametric description provides smooth power spectral energies as a result of the analytic spectral description. The non-parametric requires a smoothing post processing if very good readability is needed. It has been concluded that the non-parametric description is faster, more robust to model errors and likely to be a better formulation for adaptive algorithms.

Commentary by Dr. Valentin Fuster

Ocean Space Utilization

2005;():349-355. doi:10.1115/OMAE2005-67057.

This paper describes theoretical formulations for a prediction of hydrodynamic forces on the aircushion type large floating structure and shows corresponding results. Therefore, a new program code is developed. The structure is supported by large mono-aircushion. The present theory is based on the three-dimensional singular point distribution method. Effect of the aircushion is considered in this. Then, a processing of a singular integral of the boundary integral method is shown and its validity is confirmed. In conclusion, the draft effect to the pressure within the aircushion is discussed. As a result, when width of the skirt is thin, the draft effect is little. However, in case that the width is large, the draft effect is enlarged.

Commentary by Dr. Valentin Fuster
2005;():357-364. doi:10.1115/OMAE2005-67084.

Finite element method is applied to investigate the influence of varying sea-bottom topography on the hydroelastic response of the Very Large Floating Structures (VLFS). A floating structure is modeled as a thin plate with uniform stiffness. The hydroelastic responses of a pontoon-type VLFS are analyzed by the mode superposition method. To enhance the satisfaction of free edge condition a set of orthogonal function is adopted as the mode function instead of well-known free-free beam mode. An influence on the hydroelastic response by the sea-bottom is observed in the numerical computations and the experimental measurements.

Commentary by Dr. Valentin Fuster
2005;():365-372. doi:10.1115/OMAE2005-67089.

Design requirements for a Semi-submersible Type Mega-float are safety and functionality, and it has been shown that solutions which satisfy the requirements exist. In this paper, in order to find the optimal configuration of a semi-submersible type mega-float, an efficient optimization procedure and an objective function based on concept of risk were proposed. This function unifies the structural weight and the requirements of safety and functionality such as motion and elastic response. To reduce heavy computational load, a simplified analysis model with sufficient precision and short computational time was developed and used for the optimization. The simplified analysis model was developed based on the theory of plate. Using this modeling and the objective function, the structure was optimized and more efficient configuration of a semi-submersible type mega-float was obtained.

Commentary by Dr. Valentin Fuster
2005;():373-379. doi:10.1115/OMAE2005-67094.

It is important to estimate both added mass and radiation damping of floating artificial base, which is floating structure, in fluid-structure interaction problem. When a floating artificial base shows elastic behavior, its damping contains both structural damping and radiation damping. Therefore it is difficult to estimate the damping. We aimed to grasp tendencies of damping, when a floating artificial base shows elastic behavior, by both experiment and field measurement. The experimental modal analysis using elastic plate models was carried out and tendencies of damping ratio of experimental models were grasped. Besides, damping ratio of an existent floating artificial base was calculated from measured vibration data by the Random Decrement Technique (RDT). In our experimental results, the damping ratio became larger value in lower vibration mode, and then it decreased in higher vibration mode. In higher vibration mode, we could not confirm radiation damping. It is thought that structural damping accounts for nearly all of obtained damping ratio. Consequently, it is appropriate that damping ratio of the floating artificial base is evaluated as Rayleigh type damping.

Topics: Damping
Commentary by Dr. Valentin Fuster
2005;():381-389. doi:10.1115/OMAE2005-67101.

Importance of hydroelastic analysis in design of VLFS is well recognized since considering deformation of the structures is very important for such a huge structure. The ultimate goal of hydroelastic analysis is to insure structural integrity as well as operational effectiveness, so it is necessary to predict wave loads and induced structural stresses accurately. In the present paper, accuracy of hydroelastic analysis up to shear stress is investigated by comparing various hydroelastic analysis models.

Topics: Stress , Waves
Commentary by Dr. Valentin Fuster
2005;():391-396. doi:10.1115/OMAE2005-67175.

At present, coastal cities in advanced countries are losing their vitality and their sprawl has been depriving these cities of the unique attraction provided by their coastal position with seaside spaces. For the vitalization of coastal cities, it is important to increase the visiting population to seaside spaces from other areas. While the rapid development of advanced countries in the modern era has been founded on the economic order originally established by the Industrial Revolution, the phenomenon of an aging society with fewer children which is currently in progress makes such economic foundations vulnerable, constituting a social problem which must be urgently dealt with. This study features the planning of signs as a technique for the development of a universal design in consideration of the imminent increase of the number of the elderly and disabled and as a measure to increase the visiting population to vitalize coastal cities while focusing on the planning of signs as a system to assist ocean recreational activities. The underlying concept is that these activities have the potential to secure or even increase the number of visitors to seaside spaces which have a variety of resources and characteristics. As the introduction of a new strategic concept for the planning of signs as a measure to increase the number of visitors is essential, the focus is placed on pictogrammes which are visual signs with a superior information conveyance function to other types of signs. The outcomes of this study presented here are (i) techniques to configure pictogrammes which are easy to understand by all and which are capable of inducing people’s desire for actions and (ii) concrete pictogramme designs.

Commentary by Dr. Valentin Fuster
2005;():397-408. doi:10.1115/OMAE2005-67296.

The authors proposed a concept of “ECO-PLATFORM” for solving environmental problems that people have been facing and will face in and around the coastal area, setting their sights on the contribution from the viewpoint of the sea [1]. The Eco-Platform is a complex on a floating platform and has functions for solving social and environmental problems. The functions to be installed are generation of heat energy and electric power from combustible waste without introducing fossil energy, cascade use of heat energy, application of marine biotechnology for processing the sewage and for purifying eutrophic seawater, reproduction/regeneration of material and energy resources from manmade waste and sewage. All the functions to be installed are integrated systematically and efficiently. To ensure the realization of the concept, it is necessary to study/investigate the feasibility of systematic combination of functions and facilities installed on the Eco-Platform. The authors have been studying the required functions and size of each facility, probable waste/sewage/energy/resource input and output relations. After the studies on the desirable combination and arrangement of installed facilities, they also studied the energy and material balance of the total system from the technical standpoints to implement the Eco-Platform system. They will show as the results of the study that the Eco-Platform is technically feasible introducing technologies mainly developed in the latter half of the 20th century and will show possible combination of functions/facilities supposing waste/sewage disposal in Sakai-city, Osaka, Japan.

Commentary by Dr. Valentin Fuster
2005;():409-418. doi:10.1115/OMAE2005-67365.

A multi-connected floating body system composed of many connected small-scale floating body units has a number of advantages with regard to construction and performance. To realize the multi-connected floating body system, a new type of mechanical connecting device was developed. It is composed of multi-degrees of freedom connecting mechanism and restricting mechanism to control a restricting condition between floating body units, and its effectiveness was confirmed by model basin test in waves. Next, friction and wear tests in the seawater environment led to the discovery of suitable materials for oscillatory sliding parts in multi-degrees of freedom connecting mechanism. Finally, a field test performed for two floating body units connected by use of the mechanical connecting device proved that the developed mechanical connecting device was of practical use.

Topics: Floating bodies
Commentary by Dr. Valentin Fuster
2005;():419-423. doi:10.1115/OMAE2005-67368.

The technology of generating an artificial upwelling by constructing a seabed mound to increase the primary production is being examined off Ikitsuki Island, Nagasaki, Japan. A numerical simulation model is being developed in order to evaluate the effects and environmental influence of the artificial seabed mound. At the first stage of the modeling, tidal current is calculated for the coastal region including the artificial upwelling area. Since most of the boundaries of the computational domain are open to the ocean, it is necessary to give incoming tidal wave all over them, which is not easy because of complicate topography. A newly developed inverse method using the observation data at tidal stations along the coastline was applied to determine the boundary conditions of tide. The calculated tide by the method agreed fairly well with the observation data. Currents near the seabed mound was also investigated, whose result is indicated that internal wave induced by the mound makes the mixing larger.

Commentary by Dr. Valentin Fuster
2005;():425-432. doi:10.1115/OMAE2005-67397.

A competition model among mussels (Mytilus galloprovincialis) was developed to predict the environmental impacts of mussels under a coastal floating platform, which is called Mega-Float. The model describes the dynamics of mussels as controlled by competition for space and food availability. The model consists of a physiological growth submodel based on the Dynamic Energy Budget (DEB) model and a competition submodel for space and food. First, the parameter values in the physiological submodel are calibrated by using observations on growth of mussels (Mytilus galloprovincialis) cultivated in the north-west coast of Spain. Then the competition submodel for space and food among mussels is described as a function of the mussel density, and is calibrated by using observations on time variation in the population number of the mussels on the cultivation ropes. The population number of mussels starts with 5,000 individuals per meter and some mussels are shoved to the inner layer of the mussel bank as mussels grow. This undoubtedly leads to food shortage and starvation for them due to their unfavorable position. As a result, some mussels are starved to death and about half of the remaining mussels are inactive in the inner layer of the mussel bank. The competition model can predict well the decrease in the population number of mussels at the cultivation ropes. Finally, the competition model is combined with three-dimensional marine ecosystem model and numerical simulation is conducted to predict the growth of the mussel bank on the under-surface of an imaginary Mega-Float, which is anchored in the head of Tokyo Bay. It was revealed that about two-thirds of mussels are inactive in the inner layer of the mussel bank and do not contribute to food ingestion rate of the mussel bank.

Commentary by Dr. Valentin Fuster
2005;():433-443. doi:10.1115/OMAE2005-67557.

Hydroelastic response of VLFS under inhomogeneous sea environment is an important issue in offshore engineering applications. The most typical inhomogeneous ocean environment in offshore engineering is the inhomogeneity caused by the uneven sea bottom. In this paper, the hydroelastic response of VLFS due to the variation of water depth is studied experimentally and numerically. Experiments were performed in the state key laboratory of ocean engineering at Shanghai Jiao Tong University (SJTU). Different shoals were set on the bottom of the wave basin to simulate the uneven sea bottom. The cases tested in the Lab were studied by extending the traditional hydroelastic method in constant water depth to the varied water depth. Comparisons between the experimental measurements and the numerical results show good agreement. It is found that the inhomogeneous ocean environment has some effect on the hydroelastic response of VLFS.

Topics: Seas
Commentary by Dr. Valentin Fuster

Ocean Engineering

2005;():449-456. doi:10.1115/OMAE2005-67041.

A hybrid technique, based on the coupled-mode theory developed by Athanassoulis & Belibassakis (1999) and extended to 3D by Belibassakis et al (2001) and Belibassakis & Athanassoulis (2004), which is free of any mild-slope assumption, is used, in conjunction with a boundary integral representation of the near field in the vicinity of the body, to treat the problem of hydrodynamic analysis of floating bodies in the presence of variable bathymetry. Numerical results are presented concerning floating bodies of simple geometry lying over sloping seabeds. With the aid of systematic comparisons, the effects of bottom slope on the hydrodynamic characteristics (hydrodynamic coefficients and responses) are illustrated and discussed.

Topics: Floating bodies
Commentary by Dr. Valentin Fuster
2005;():457-466. doi:10.1115/OMAE2005-67045.

In this work, numerical simulation is used to study the stability enhancement of high speed supercavitating hydrofoils. Although supercavitation is known as one of the most effective methods for drag reduction, producing the cavity, either by ventilation or by cavitator at front of the body, may cause some instabilities on cavity surface and thus on the projectile’s motion. Therefore removing these instabilities comes as an important point of discussion. First of all, we calculate the sources of instabilities and measure respective forces and then present some approaches that significantly reduce these instabilities. One of these methods that could produce more stable supercavities is injecting of the air into the cavity unsteadily which varies through the projectile’s surface. This approach is provided by arrays of slots distributed on the projectile’s surface and unsteady injection is modeled over the surface. Furthermore, the position of ventilation, dramatically affects the stability like those in aerodynamics. In all approaches it is assumed that the supercavity covers the whole of the body, however the forces caused by the wakes, formed behind the body are taken into account. The calculation is performed at three cavitation numbers with respective velocities of 40 m/s, 50 m/s, 60 m/s.

Commentary by Dr. Valentin Fuster
2005;():467-473. doi:10.1115/OMAE2005-67049.

For the deterministic analysis of wave/structure interaction in the sense of cause-reaction chains, and for analyzing structure responses due to special wave sequences (e.g. three sisters phenomenon or other rogue wave groups) methods for the precise generation of tailored wave sequences are required. Applying conventional wave generation methods, the creation of wave trains satisfying given local wave parameters and the generation of wave groups with predefined characteristics is often difficult or impossible, if sufficient accuracy is required. In this paper we present an optimization approach for the experimental generation of wave sequences with defined characteristics. The method is applied to generate scenarios with a single high wave superimposed to irregular seas. The optimization process is carried out in a small wave tank. The resulting control signal is then transferred to a large wave tank considering the electrical, hydraulic and hydrodynamical RAOs of the respective wave generator in order to investigate wave/structure interaction at a large scale.

Topics: Waves , Optimization
Commentary by Dr. Valentin Fuster
2005;():475-482. doi:10.1115/OMAE2005-67054.

The exciting force induced by vortex shedding on the hull of bluff body floaters, such as Spars, may cause response near the resonant period of any of the 6 rigid body modes of response of the floater [1]. Usually the sway vortex induced motion (VIM) is of particular concern for mooring and riser integrity. Most efforts to study this phenomenon to-date have focused on uniform current loading over the depth of the platform, such as one would find in the eddy/loop currents in the Gulf of Mexico. Under these conditions, tow tests with the model mounted horizontally or vertically and restrained with a spring system have been sufficient to characterize the VIM behavior [2, 3]. VIM responses of a Classic Spar measured in the field after the passage of a hurricane in what is referred to as the “hurricane inertial current” event has raised the awareness of the possibility of platform VIM response in sheared currents and in the presence of waves. In order to evaluate this condition, model scale Truss Spar VIM tests were conducted at the MARIN Offshore Basin in the summer of 2004. Uniform current, “shear” or two-layer current, and wave plus current conditions were simulated and compared with benchmark uniform current tow test results. The tests were done in current alone, in waves alone, and with waves in-line with the current, and, waves transverse and oblique to the current. Comparisons are made of VIM response in uniform and shear current conditions, and, response with and without waves. One of the key findings is that with waves in-line with the current, VIM is generally reduced; however, at some headings the response in waves plus current in transverse waves is larger than in current alone.

Commentary by Dr. Valentin Fuster
2005;():483-492. doi:10.1115/OMAE2005-67064.

Due to the pronounced dynamic behavior of wind turbines, fatigue load effects may be quite sensitive to the precise modeling of the frequency content of the wave loading. As the offshore wind turbine technology progresses, larger and larger turbines will be placed at still deeper waters, causing the resonant frequency of the first eigen mode of a traditional bottom-fixed support structure to be typically in the range from 0.25 Hz to 0.35 Hz. The deeper the water and the larger the turbine, the lower the frequency will be. As an example, wave measurements from the offshore wind farm Bockstigen show a second peak at approximately 0.3 Hz in the wave spectrum. Thus this peak, or similar peaks realized at shallow water sites, may very well be dynamically amplified in the response. This second peak cannot be modelled with a linear wave model, and a wave model taking non-linearities into account has to be used. In the presented work, both a linear and a non-linear wave model are used to study the fatigue in an offshore wind power plant and the difference is compared. Time series of irregular linear and non-linear waves are calculated, and structural calculations of an offshore wind turbine with a slender support structure are used in the analysis of the fatigue loads. The forces on the structure are calculated using Morison’s equation, integrating along the structure and lumping the loads in nodes for the structural calculations. The difference between the wave models is significant and the non-linear model yields higher fatigue damage than the linear one.

Commentary by Dr. Valentin Fuster
2005;():493-498. doi:10.1115/OMAE2005-67070.

It is well known that air compressibility is likely to influence the performance of oscillating water columns (OWC’s) and this has previously been modelled and estimated using linear aero-hydrodynamic lump-mass models. In addition, the optimum performance of specific OWC’s has been determined, with the effect of air compressibility included within the optimisation. This paper extends this work to determine the optimum performance of any OWC, defined by its hydrodynamic coefficients and serves as a reminder to experimentalists of the importance of considering air compressibility in their models. Moreover, the optimisation is analytical and so produces explicit expressions for maximum power capture, optimum damping, etc. The results indicate that for the majority of OWC’s the maximum power capture decreases with increasing plenum chamber volume, accompanied by a reduction in the optimum turbine damping and an increase in optimum water column motion. However, the results also show that air compressibility will increase maximum power capture for OWC’s when the incident wave period is shorter than the natural period of the water column. To date, air turbine hysteresis has been studied as a purely aerodynamic phenomenon and its effect on the optimum hydrodynamic performance of OWC’s has not been investigated. This paper uses a linearised phase shift to model an approximation of turbine hysteresis that shows that turbine hysteresis will have a small but significant influence on hydrodynamic performance, where the magnitude of influence is similar to the effect of subtle blade profile and blade sweep angle modifications on aerodynamic performance. The results indicate that hysteresis should be considered in turbine design, especially if a turbine modification improves aerodynamic performance whilst reducing hydrodynamic performance due to a change in the turbine hysteresis.

Topics: Turbines , Water
Commentary by Dr. Valentin Fuster
2005;():499-506. doi:10.1115/OMAE2005-67075.

The problem of transformation of the directional spectrum of an incident wave system over a region of strongly varying three-dimensional bottom topography is studied, in the context of linear theory. The Consistent Coupled-Mode Model (Athanassoulis and Belibassakis 1999, Belibassakis et al 2001) is exploited for the calculation of the linear transfer function, connecting the incident wave with the wave conditions at each point in the field. This model takes fully into account reflection, refraction and diffraction phenomena. The present approach permits the consistent transformation of any incident directional wave spectrum over a variable bathymetry region and the calculation of the spatial evolution of point spectra of all interesting wave quantities (free surface elevation, velocity, pressure), at every point in the domain. This approach can be extended to treat weakly non-linear waves.

Commentary by Dr. Valentin Fuster
2005;():507-516. doi:10.1115/OMAE2005-67076.

This paper presents a physically based calculation model to simulate the behavior of a cage system considered as moored flexible structures. The flexible structure described is composed of a network of mass points and springs, the behavior of which is calculated using the implicit integration method of the fundamental law of dynamics. Also, to increase the accuracy of calculations, the reduction ratio of flow speed due to shielding effects is acquired by using the flume tank experiments. By applying the reduction ratio of flow speed to the numerical calculation, the calculation accuracy was increased, and it was found that the result of the simulated cage movements was very close to that of the model cage. In addition, the dynamic behavior of an actual fish cage system under the current and waves was simulated.

Commentary by Dr. Valentin Fuster
2005;():517-525. doi:10.1115/OMAE2005-67077.

The stabilizing effects of a water-entrapment plate at the keel of a small three-legged semi-submersible platform are determined using laboratory experiments and time-domain simulations. Motion predictions were carried out in the time-domain using coupled-analysis between the vessel and its mooring, linear diffraction-radiation theory, and an empirical wave-viscous interaction model. Model tests were conducted at the U.C. Berkeley Ship Model Testing Facility to determine the validity of the numerical model.

Topics: Water
Commentary by Dr. Valentin Fuster
2005;():527-532. doi:10.1115/OMAE2005-67090.

More than ever before, operators are advancing mobile jack-up units into deeper waters and harsher environments. Constraining their use is the capacity of the shallow foundations (typically inverted conical spudcans) to withstand the larger wave, wind and current loads of these environments. The use of foundation skirts (or caissons) is often touted as an alternative, though today only a small percentage of jack-up units use caisson foundations. One restraining feature is limited understanding of their behaviour when compared to conventional spudcan. This paper addresses this by comparing the results of push-over experiments of a three-legged jack-up with similarly sized caisson and spudcan foundations. The tests were conducted on a 250:1 scale jack-up on an overconsolidated clay sample. With improved understanding of the overall response of a jack-up platform with caisson foundations, operators should have greater confidence to use these footings.

Commentary by Dr. Valentin Fuster
2005;():533-538. doi:10.1115/OMAE2005-67103.

The 105K COT (105,000 DWT Crude Oil Tanker) is being continuously constructed at the ground yard of HHI (Hyundai Heavy Industries), Ulsan, Korea as the first ground construction for ship type structures. She is loaded out transversely from the quayside by skidding onto the DBU (Double Barge Unit). The DBU will then be towed to a pre-determined site and ballasted down to float-off the 105K COT from the DBU. This paper describes the motions, mooring forces, collision possibilities and constraint forces of DBU / 105K COT during load-out, towing and float-off of the 105K COT.

Topics: Construction , Stress , Ships
Commentary by Dr. Valentin Fuster
2005;():539-546. doi:10.1115/OMAE2005-67107.

In the last decade within several European and bi-national (German-Italian) projects a set of equipment and technology for benthic stations, and their deployment and recovery has been developed. Starting with a single prototype station which operated autonomously for several months in the Mediterranean Sea (GEOSTAR2 – Geophysical and Oceanographic Station for Abyssal Research – Deep Sea Mission, 1999–2001, /1/) with a near-real-time-communication-system including a acoustic link from station to buoy and a shore radio link. This concept has been extended to realize a sub-sea network of several benthic stations (ORION- Ocean Research by Integrated Observation Networks, 2002–2005, /1/). This includes an extended version of the GEOSTAR acoustic underwater communication system between the stations and the buoy unit for direct shore communication. For the deployment and recovery of these benthic stations the deep-sea shuttle MODUS (Mobile Docker for Underwater Sciences) has been developed and used (operation depth 4000 m). All operations up to now were successfully conducted. The ORION-network is under operation at a water depth of 3700 m North of Sicily/Italy since December 2003. In addition to this MODUS has been used to install the first long-term benthic station directly cable connected to shore. Thus, the first European long term benthic station has been established, with the aim to demonstrate a technology for the ongoing plans for a European deep-sea shore network (ESONET – European Seafloor Observatory Network, /1/). The deployment took place east of Sicily using MODUS and a ROV for the realization of the cable connection between the station and the cable termination in January 2005.

Commentary by Dr. Valentin Fuster
2005;():547-555. doi:10.1115/OMAE2005-67110.

This paper concerns the description of large waves in intermediate and shallow water depths. In deep water it is well known that the evolution of the largest waves is governed by linear dispersion. In contrast, as the water depth reduces the effects of wave dispersion are weakened and the relative significance of wave modulation shown to be increasingly important. This leads to very different extreme wave groups, the properties of which are critically dependent upon the directionality of the wave field. The paper also concerns the water particle kinematics arising beneath these nonlinear wave groups and contrasts fully non-linear predictions based on a state-of-the-art wave model with the results of the commonly applied design wave solutions. To explore these effects, and to provide a physical explanation for their occurrence, two wave models are employed. The first, proposed by Bateman, Swan & Taylor [1, 2], allows fully-nonlinear descriptions of the evolution of large waves in realistic seas, involving a significant spread of wave energy in both frequency and direction. The second is a wave evolution equation based upon the early work of Zakharov [3] and written in Hamiltonian form by Kasitskii [4]. This model is only valid to a fourth-order of wave steepness, but has the over-riding advantage that it gives physical insight into the evolution process.

Commentary by Dr. Valentin Fuster
2005;():557-562. doi:10.1115/OMAE2005-67115.

In this paper the diffraction of a plane first-order solitary wave by a vertical permeable breakwater with calculation of the wave loading is investigated. The interaction between long non-linear water waves and dissipative/absorbing coastal structures that are commonly used in coastal engineering is investigated using both analytical and numerical technique. The breakwater consists of a vertical permeable surface-piercing elliptical cylinder fixed in the ground. The analytical model herein is based on the application of Boussinesq equations to describe the diffraction of the first-order solitary wave by an elliptical breakwater. The method of solution is based on perturbation theory, using a perturbation parameter defined in terms of surface geometry of the cylinder. The analysis includes terms up to the first-order in this parameter, where the zeroth-order solution corresponds to a circular cylinder. The velocity potentials at the zeroth and first orders are expressed as eigenfunction expansion involving unknown coefficients that are determined through the boundary conditions on unperturbed cylinder. The flow through the porous breakwater is assumed to obey Darcy’s law. The total force onto the elliptical cylinder is obtained by integration of the pressure over permeable cylindrical wall. The analytical solution is obtained by means of a Fourier transformation technique. The effects of porosity, relative wave length and the incident wave angle are discussed. Numerical results compare well with previous predictions for the limiting cases of a permeable cylindrical breakwater.

Topics: Waves , Breakwaters
Commentary by Dr. Valentin Fuster
2005;():563-570. doi:10.1115/OMAE2005-67146.

New developments in instrumentation for ocean environmental engineering are allowing unprecedented levels of trace contaminant measurement in the deep ocean. With funding from the U.S. National Science Foundation (NSF), our engineering design team constructed a new mass spectrometer-based in situ analysis system for work in the deep ocean environment over prolonged deployment periods. Our design goals were a depth capability of up to 4,000 m water depth (400 bars hydrostatic pressure) and autonomous operation for periods of up to six months to a year, depending upon the type of external battery system used or other deployment circumstances, e.g., availability of a power cable or fuel cell power source. We chose a membrane introduction mass spectrometry (MIMS) sampling approach, which allows for dissolved gases and volatile organics introduction into the mass spectrometer vacuum system. The MIMS approach and the hydrophobic, silicon-coated membrane chosen both draw upon our previous experience with this technology in the deep ocean. The membrane has been tested to 400 bars in a series of long-term hydrostatic pressure tests, which extend the 200-bar working depth rating of this membrane by a factor of 2. Long-term deployment capability of the moderately powered, approximately 100 W system, was accomplished by power management of the embedded computer system and custom electronics with Windows-based and custom software now fully-developed and bench tested. The entire system fits within a 6.5-inch outside diameter pressure housing that is approximately five feet long. It consists of a 1 to 200 amu range quadrupole mass spectrometer equipped with Faraday and electron multiplier detectors, compact turbo-molecular and backing diaphragm vacuum pumps, internal rechargeable batteries, and internal waste vacuum chamber. Sample routing past the MIMS is accomplished by computer-controlled solenoid valves. We designed the pressure housings of both 6AL4V and type 2 titanium alloys that are rated to working depths of >4,000 m and are essentially corrosion proof over long-term deployments. We designed and integrated a fail-safe valving system for both rapid response to high-pressure MIMS failure and a pressure-switch circuit and high-pressure solenoid valve to detect and protect against slow leaks of the MIMS. To route sample waters to the MIMS-based instrument, we also designed and built a rugged plastic plenum that couples to the face of the sampler head, the latter of which consists of the MIMS inlet and a full-ocean rated thermister temperature probe with an operational range from −5 to 50°C. These instrumentation innovations will be described in the paper.

Commentary by Dr. Valentin Fuster
2005;():571-580. doi:10.1115/OMAE2005-67158.

This paper concerns the nonlinear, higher-harmonic, wave-forces acting on a vertical surface-piercing cylinder. New laboratory data is presented which confirms that in the case a vertical cylinder, the diameter of which is large but not sufficiently large that the body lies within the linear diffraction regime, the second- and third-harmonic forces are not well described by existing models. This is particularly apparent when the incident waves are steep and have a relatively small wave period. Indeed, under these conditions the second-, third- and fourth-harmonic forces are shown to be comparable in size. This is clearly at odds with the results of a traditional perturbation expansion. An explanation for this lies in the nature of the scattered wave field, particularly the high-frequency waves identified by Sheikh & Swan [1]. The phasing of these scattered waves are, at least in part, dependent upon the motion of the fluid around the circumference of the cylinder and will not therefore be captured by a series solution based solely on the harmonics of the incident wave motion. The paper considers several test cases, fully exploring the correlation between the nonlinear forcing and the high-frequency scattering. The practical implications of these results are also addressed.

Commentary by Dr. Valentin Fuster
2005;():581-591. doi:10.1115/OMAE2005-67165.

We are studying numerically the problem of generation and propagation of gravity long-crested waves in a tank containing an incompressible inviscid homogeneous fluid initially at rest with a horizontal free surface of finite extent and of infinite depth. A non-orthogonal curvilinear coordinate system, which follows the free surface is constructed which gives a realistic “continuity condition”, since it tracks the entire fluid domain at all times. A depth profile of the potential is assumed, and employed to perform a waveform relaxation algorithm to decouple the discrete Laplacian along dimensional lines, thereby reducing it’s computation over this total fluid domain. In addition, the full nonlinear kinematic and dynamic free surface boundary conditions are utilized in the algorithm, and a suitably tuned numerical beach is used to avoid reflections. It is well known that instability, in the form of generated spurious “sawtooth waves”, plagues this problem, leading to numerical overflow. This makes it very difficult to generate steep waves for sufficiently long simulation times. The authors have struggled with this problem for some time, with significant success, by employing an “aliasing filter”. This paper outlines our ongoing study of the stability of the model, including an analysis of the possible nature of the underlying causes including compatibility conditions. We conclude by giving a simple practical technique for greatly improving the stability.

Commentary by Dr. Valentin Fuster
2005;():593-599. doi:10.1115/OMAE2005-67181.

It is a well-known fact that long-term time series of wind and wave data are modelled as nonstationary stochastic processes with yearly periodic mean value and standard deviation (periodically correlated or cyclostationary stochastic processes). Using this model, the initial nonstationary series are decomposed to a seasonal (periodic) mean value m(t) and a residual time series W(t) multiplied by a seasonal (periodic) standard deviation s(t), of the form Y(t) = m(t) + s(t)W(t). The periodic components m(t) and s(t) are estimated using mean monthly values, and the residual time series W(t) is examined for stationarity. For this purpose, spectral densities of W(t) are obtained from different seasonal segments, calculated and compared with each other. It is shown that W(t) can indeed be considered stationary, and thus Y(t) can be considered periodically correlated. This analysis has been applied to model wind and wave data from several locations in the Mediterranean Sea. It turns out that the spectrum of W(t) is very weakly dependent on the site, a fact that might be useful for the geographic parameterization of wind and wave climate.

Commentary by Dr. Valentin Fuster
2005;():601-609. doi:10.1115/OMAE2005-67193.

This paper presents a summary of the work done by the authors regarding the design, construction and test of vertical axis hydro turbines to exploit tidal currents. Double Multiple Streamtube (DMS) model and Vortex model have been used to predict turbines performances either with fixed blades or with self-acting variable pitch blades. Within the DMS model, VAWT and VAWT_dyn codes have been developed to analyze steady and dynamic performances; within Vortex model, VAT-VOR3D code has been developed. Theoretical analysis and numerical predicted performances have been compared and validated with experimental test results on both model and real scale turbines. A comparison between DMS and Vortex model results has been presented. Moreover, the recent activities in terms of numerical investigations on the flow curvature effects are presented.

Commentary by Dr. Valentin Fuster
2005;():611-618. doi:10.1115/OMAE2005-67194.

Reassessment of existing offshore structures needs rational aid tools to update new information (metocean data, new regulations, ...). This paper focuses on marine growth management on Jacket offshore structures. After years of exploitation, marine growth thickness reaches up to the initial beam diameter. Inspection and cleaning are actions to be optimized, keeping in mind that safety must be preserved according to cost reduction. The paper first presents the building of an exhaustive data base in view to use the whole available information. An illustration is given for structures placed in Gulf of Guinea: main species, colonization process as well as modeling of thickness increasing are discussed. A modeling of hydrodynamic coefficient is then suggested according to kinematics field and physical response surface.

Topics: Inspection , Stress
Commentary by Dr. Valentin Fuster
2005;():619-626. doi:10.1115/OMAE2005-67195.

After decades of developments of methods and software, reassessment of jacket structures is still a challenge. This paper focus on marine growth effects in case of fatigue behavior for Jacket offshore structures. Two main items are pointed out: selection of marine growth profile and modeling of hydrodynamic coefficients in presence of marine growth. A complete approach is provided and completed with both sensitivity and uncertainty studies.

Topics: Fatigue , Modeling
Commentary by Dr. Valentin Fuster
2005;():627-636. doi:10.1115/OMAE2005-67215.

The scientific and economic significance of underwater technology is considerable, particularly as the oceans are becoming more and more important not only as a source of energy, raw materials and food but also due to their influence on climatic changes. Underwater technology, especially at great depths, represents a very particular challenge which is being tackled by ATLAS ELEKTRONIK Autonomous Underwater Vehicles (AUVs) DeepC© , SeaOtter Mk1 and SeaOtter Mk2. Many actions that would normally require high cost ROV systems or which are not possible (e.g. under ice investigations) can now accomplished using AUVs, at less cost and manpower. DeepC © is a high efficient AUV system for ultra deep missions (4000m), with long endurance (40–60h) and for a lot of applications. The development objectives are: Low vehicle weight, modular design, high efficient energy generation, autonomous mission execution, obstacle avoidance, in mission re-planning capability, precise navigation, advanced diagnosis and fault recovery system, high manoeuvrability, hover capability and containerized payload. The system is final integrated and ready for water tests and demonstrations till end of 2004. The paper deals with the system performance and shows the development status. The AUV SeaOtter Mk1 , based on the ATLAS Maridan M600 vehicle and adapted to modern requirements in navigation software and mission management, is the ideal payload sensor carrier for military and commercial purposes. It ensures precise positioning, stability and manoeuvrability for accurate data collection. The on-board system provides all necessary position, attitude, status and time information to ensure effective and efficient performance throughout the mission. High positional accuracy and accurate stabilization of the vehicle’s attitude throughout the collection of data allows the mapping of raw data to be implemented directly, without post-processing, to obtain the necessary correlation of the survey mosaic. The advanced version of the SeaOtter AUV, the Mk2 is designed for various purposes, strictly adhering to a modular approach with regard to superstructure, propulsion, energy, communication, navigation and payload. The basic vehicle can easily be extended by the means of one or two extension modules to carry heavier payloads. The paper shows the SeaOtter Mk1 performance and gives an overview over the Mk2 Philosophy and the development status.

Commentary by Dr. Valentin Fuster
2005;():637-645. doi:10.1115/OMAE2005-67226.

In this paper, we consider the minimum time problem for underwater vehicles. Using Lagrangian mechanics, we write the equations of motion for marine vehicles with 6 degrees of freedom as a controlled mechanical system. We then apply the necessary conditions from the maximum principle for a trajectory to be time optimal. Using techniques from differential geometry we analyze the resuls. Finally we supplement the theoretical study with numerical simulations.

Commentary by Dr. Valentin Fuster
2005;():647-654. doi:10.1115/OMAE2005-67227.

The paper presents a systematic study of the structural loads induced by abnormal waves on a FPSO. This work is a follow-up of a previous investigation that explored the possibility of using freak, abnormal or episodic waves as additional wave load conditions to be considered in the design of ships and offshore platforms. In the previous work, a procedure was developed and implemented to adopt deterministic time series of wave elevation, which may include abnormal waves, as reference design conditions to calculate the wave induced structural loads on ships. An application example was presented for a FPSO subjected to the well known New Year Wave trace that was measured during a severe storm in Central North Sea. In the present paper, the same procedure is applied to obtain the wave induced structural loads on a FPSO, but a systematic investigation is carried out by using a large set of wave traces. These wave traces have been measured at different occasions in the North Sea, additionally at one location in the Central Gulf of Mexico, and they all include episodic freak waves. In this way it is possible to assess the influence of realistic rogue wave characteristics on the wave induced structural loads. Finally, and based on the platform responses to all wave traces, some statistics are produced, regarding the platform responses and structural loads induced by rogue waves.

Topics: Stress , Waves , FPSO
Commentary by Dr. Valentin Fuster
2005;():655-663. doi:10.1115/OMAE2005-67235.

The use of Triaxys directional wave buoy and acoustic Doppler current profiler (ADCP) for wave measurements are relatively recent. The US National Oceanic and Atmospheric Administration’s (NOAA) National Ocean Service (NOS) acquired these instruments in 2001 and systematic laboratory and field tests were conducted during 2001–2002. This paper describes the field tests conducted near the US Army Corps of Engineers’ Field Research Facility (FRF) ocean pier and near the Barren Islands in the Chesapeake Bay. At the FRF site, Triaxys buoy wave measurements were compared with FRF’s field standards of pressure sensor arrays and Datawell Waverider buoy. For the Bay test, ADCP was compared with the Triaxys buoy. There are significant numbers of outlier in the Triaxys peak periods at both test sites. In the Chesapeake Bay, which is dominated by high frequency and low energy waves, there is much scatter in the Triaxys data for significant wave heights below 0.2 m. Detailed analyses were performed after these outliers and noisy data were removed. Statistics of differences in significant wave heights, peak periods and directions between each comparative pair were computed and characteristics of frequency and frequency-direction spectra were examined. Overall, correlations between each instrument pair are very good in significant wave heights, fair in wave peak periods (except the ADCP/Triaxys pair), and marginal in wave directions. Triaxys buoy compared better with Waverider buoy than with others. Both ADCP and FRF pressure sensor array exhibit higher resolution in detecting multi-modal and multi-frequency waves. In most cases, energy distribution of spectral peaks in Triaxys buoy data differs significantly from those obtained from FRF pressure sensor array and ADCP.

Commentary by Dr. Valentin Fuster
2005;():665-675. doi:10.1115/OMAE2005-67240.

Supercavitation is known as the way of viscous drag reduction for the projectiles, moving in the liquid phase. In recent works, there is distinct investigation between cavitation flow and momentum transfer far away from the cavity surface. However, it seems that there is strong connection between overall flow and what takes place in the sheet cavity where a constant pressure distribution is assumed. Furthermore as we’ll see, pressure distribution on cavity surface caused due to overall conditions, induct nonaxisymetric forces and they may need to be investigated. Primarily we describe how pressure distribution into the cavity can cause separation of the aero boundary layer. Then we present some approaches by which this probable separation can be controlled. Comparisons of several conditions exhibits that at very low cavitation numbers, constant pressure assumption fails particularly for gradient shaped profiles and separation is probable if the flow is sufficiently turbulent. Air injection into the NATURALLY FORMED supercavity is found as an effective way to delay probable separation and so significant pressure drag reduction is achieved. In addition, the position of injection plays a major role to control the aero boundary layer and it has to be considered. Moreover, electromagnetic forces cause to delay or even prevent separation in high pressure gradient flows and interesting results obtained in this regard shows significant drag reduction in supercavitating vehicles.

Commentary by Dr. Valentin Fuster
2005;():677-683. doi:10.1115/OMAE2005-67248.

In this paper, the mechanism of local scour around submarine pipeline is studied numerically based on a Renormalized Group (RNG) turbulence model. To validate the numerical model adopted in this paper, the equilibrium profiles of local scour for two cases are simulated and compared with the experimental data. It shows that the RNG turbulence model can give an appropriate prediction for the configuration of equilibrium scour hole, and it is applicable to this situation. The local scour mechanism around submarine pipeline including the flow structure, shear stress distribution and pressure field is then analyzed and compared with experiments. For the further comparison and validation, especially for the flow structure, a numerical model of Large Eddy Simulation (LES) is also developed in this paper. The numerical results of RNG demonstrate that the critical factor governing the equilibrium profile is the seabed shear stress distribution in the case of bed load sediment transport, and the two-equation RNG turbulence model coupled with the law of wall is capable of giving a satisfying estimation for the bed shear stress. Moreover, the piping phenomena due to the great difference of pressure between the upstream and downstream parts of pipeline and the vortex structure around submarine pipeline are also simulated successfully, which are believed to lead to the onset of local scour.

Commentary by Dr. Valentin Fuster
2005;():685-693. doi:10.1115/OMAE2005-67259.

An important property used in the design stage of floating systems is the RAO (Response Amplitude Operator), the transfer function, for motions, forces and so on. This importance has motivated the development of several analytical, numerical and experimental tools to obtain the hydrodynamic behavior of platforms and ships. Experimental model tests in wave tanks are advisable for the accurate evaluation of the body movements. Three known techniques are used to obtain the RAO curves: tests with regular, irregular and transient waves. In the present work, special attention is given to the technique used to perform model testing with one type of transient wave: the Gaussian Wave Packet. The advantages of using such technique are discussed and results are also presented for a semi-submersible model during tests carried out at Laboratório de Tecnologia Oceânica - LabOceano/COPPE/UFRJ, Brazil. Numerical calculations and tests with regular and irregular waves are used for validation and comparison.

Topics: Wave packets
Commentary by Dr. Valentin Fuster
2005;():695-702. doi:10.1115/OMAE2005-67265.

The present paper contains results of experimental study on a semi-planing catamaran. The aim of this study is to find the effect of hydrofoil usage on resistance of semi-planing catamaran and achieving optimal attack angle, arrangement of hydrofoils and displacement center of gravity. Tests have been done without and with hydrofoils in certain arrangement with different angles of attack then the results are compared. Results show that considerable resistance reduction obtained in semi-planing vessels which equipped with hydrofoil in optimized angle of attack in service speed range. Meanwhile the hydrofoils arrangement and the angle of attack are important so unsuitable design causes the instability in the motion and may be increases hydrodynamic resistance.

Commentary by Dr. Valentin Fuster
2005;():703-712. doi:10.1115/OMAE2005-67286.

This paper introduces a new second generation wave energy converter concept named SEAREV [Systeme Electrique Autonome de Recuperation d’Energie des Vagues]. The working principle and linearized equations of the device are described. It is shown how energy absorption depends on the shape of the external floating body and on the mechanical characteristics of the moving mass. This allows to numerically optimize the geometry of the device. Latching control is used to further improve the capture width of the system, with success in regular waves.

Commentary by Dr. Valentin Fuster
2005;():713-718. doi:10.1115/OMAE2005-67288.

The series expression of random wave drag forces is studied in this paper by means of nonlinear spectral analysis. A new method is proposed in this study since the derivative of the wave drag force is discontinuous. The characteristics of random process are used to obtain the coefficients of the series in this method. Numerical verification indicates that the series expression given in the paper approaches the original expression satisfactory. The maximum error is less than 4% when measured in the peak values of the time series and less 3% when measured in the root mean square values. That is to say, the series expression given in the study has reasonable accuracy and can be used in engineering computations. The application example indicates that the spectral densities of wave forces, as well as the root mean squares of wave forces, can be easily obtained without lose much accuracy using the series expression of wave drag forces.

Commentary by Dr. Valentin Fuster
2005;():719-727. doi:10.1115/OMAE2005-67315.

A concept was developed by the Seabasing Innovation Cell within the Center for Innovation in Ship Design (CISD) at the Naval Surface Warfare Center Carderock Division (NSWCCD). The study was undertaken during February–May 2003 with funding provided by the Office of Naval Research (ONR). The concept was developed in summer 2003 with further CISD funding and has been chosen for the 2004 Senior Year Design, Build and Test project by Florida Atlantic University’s (FAU) Ocean Engineering Department. The concept, known as the Deep Water Stable Craneship (DWSC) consists of two entities, a catamaran craneship and a detachable spar, which when connected form a self-deploying, open ocean capable trimaran. The spar can be rotated through 90 degrees, from horizontal to vertical, using seawater ballast. When vertical, partial de-ballasting ‘lifts’ the catamaran clear of the water surface allowing the system to operate as a spar and take advantage of the superior seakeeping afforded by the small waterplane area. The concept was inspired by the “ONR owned” and “Scripp’s Institute operated” FLIPSHIP, and was developed as a potential solution to the Seabasing goal of transferring containerized cargo at sea between large and small vessels in seastate four (significant wave-heights 1.25–2.5m). Current crane operations at-sea are limited to seastate two (significant wave-heights 0.1–0.5m), largely due to pendulation of the load. This paper presents the development of the concept, its performance and alternative uses. In addition, a 1:15 scale ‘demonstrator’ is discussed as is a proposal for a FLIPSHIP-II.

Topics: Spar platforms
Commentary by Dr. Valentin Fuster
2005;():729-735. doi:10.1115/OMAE2005-67316.

The hydrodynamic interaction of multiple floating bodies in waves has been computed in the frequency domain based on the panel-free method developed earlier for the time-domain analysis. The integral equations are first desingularized by removing the singularity in the Green function and then discretized by Gaussian quadrature over the exact geometry. Non-uniform rational B-splines (NURBS) surfaces are employed to represent the exact body surface. Robustness and accuracy of the method has been demonstrated by its application to vertically floating cylinders. Computed motions, hydrodynamic coefficients and wave exciting forces due to interactions are presented and compared with other published results. The numerical method has also been applied to the performance study of a wave energy converter. The computed results were compared with the experimental ones.

Topics: Waves , Computation
Commentary by Dr. Valentin Fuster
2005;():737-741. doi:10.1115/OMAE2005-67324.

The turned mass damper (TMD) system has found extensive applications in vibration control nowadays. The main disadvantage of the system is to add additional mass to the structures. In order to make use of its merits and overcome its disadvantage, the developed turned mass damper (DTMD) vibration control system is studied in this paper. It uses the equipment set on the structure instead of additional mass to control the vibration of structures. The theory analysis shows that the frequency range is enlarged and the resonant band is reduced with the mass ratio increasing. The FEM simulation shows that the DTMD vibration control system could absorb more vibration energy when the frequency of the DTMD is near the excitation frequency. The platform has better vibration control effect when the mass ratio between 3.75 and 5 and when the frequency ratio is in the range of (0.3,0.35) and (0.35,0.5). The whole system, including the platform and the DTMD, has the smaller vibration response when the frequency ratio is between 0.5 and 2. The results of the simulation also showed that the DTMD is more effective in the vibration control for the jacket platforms.

Commentary by Dr. Valentin Fuster
2005;():743-749. doi:10.1115/OMAE2005-67326.

The problem of steady internal waves in a weakly stratified two-layered fluid is studied analytically. We discuss the model with a constant density in lower layer and exponential stratification in the other one. The long-wave approximation using a scaling procedure with small Boussinesq parameter is constructed. The nonlinear ordinary differential equation describing large amplitude solitary waves and internal bores is obtained.

Topics: Fluids , Waves
Commentary by Dr. Valentin Fuster
2005;():751-757. doi:10.1115/OMAE2005-67338.

The paper aims at presenting a numerical model to predict performance characteristics of tight moored vertical axisymmetric wave energy converters that are allowed to move in heave, pitch and sway modes of motion. The hydrodynamic characteristics (exciting wave forces, hydrodynamic parameters) of the floats are evaluated using a linearized diffraction–radiation method of analysis that is suited for the type of bodies under consideration. According to this method matched axisymmetric eigenfunction expansions of the velocity potentials in properly defined fluid regions around the body are introduced to solve the respective diffraction and radiation problems and to calculate the floats’ hydrodynamic characteristics in the frequency domain. Based on these characteristics, the retardation forcing terms are calculated, which account for the memory effects of the motion. In this procedure, the coupling terms between the different modes of motion are properly formulated and taken into account. The floating WEC is connected to an underwater piston that feeds a hydraulic system with pressurized fluid. Numerical results showing parametrically the performance characteristics in terms of the expected power production for several types of floats that are exposed to the wave climate conditions commonly encountered in the Mediterranean area are presented and discussed.

Commentary by Dr. Valentin Fuster
2005;():759-766. doi:10.1115/OMAE2005-67347.

On May 29, 2003, the KAIKO’s secondary cable snapped, and the KAIKO’s vehicle has been missing. JAMSTEC (Japan Agency for Marine-Earth Science and Technology) has lost a method to survey more than 6,500m deep areas where submersible “Shinkai6500” cannot reach. JAMSTEC decided to remodel the vehicle of UROV7K system instead of the lost vehicle in order to continue the scientific activities. This system is called “KAIKO7000” system. After the accident, safety measures of the KAIKO system must be reconsidered in order to improve the safety measures of the KAIKO7000 system. The KAIKO7000 system was finished remodeling in March 2004, and the sea trials have been started in April 2004. Then, we succeeded in the maximum operation depth test, which was reached at a depth of 7031 m. This paper describes on the development of the KAIKO7000 system.

Commentary by Dr. Valentin Fuster
2005;():767-774. doi:10.1115/OMAE2005-67371.

An oscillating water column device enables the conversion of wave energy into electrical energy via wave interaction with a semi-submerged chamber coupled with a turbine for power take off. This present work concentrates on the wave interaction with the semi-submerged chamber, whereby a shore based oscillating water column (OWC) is studied experimentally to examine energy efficiencies for power take-off. The wave environment considered consists of plane progressive waves of steepnesses ranging from kA = 0.01 to 0.22 and water depth ratios varying from kh = 0.30 to 3.72, where k, A and h denote the wave number, wave amplitude and water depth respectively. The key feature of this experimental campaign is a study on the influence of geometrical parameters of the front wall on the OWC’s performance. More specifically, these parameters include: front wall draught; thickness; and aperture shape. We make use of a two-dimensional inviscid theory for an OWC for comparative purposes and to explain trends noted in the experimental measurements. The work undertaken here has revealed a broad banded efficiency centred about the natural frequency of the OWC. The magnitude and shape of the efficiency curves are influenced by the geometry of the front wall. Typical resonant efficiencies of the OWC are in the order of 70%.

Topics: Water
Commentary by Dr. Valentin Fuster
2005;():775-785. doi:10.1115/OMAE2005-67374.

The loads on both a model of a smooth cable and a helical strake (also known as scrouton spiral) were measured in a wind tunnel. The coefficients of drag, lift and transverse force were ascertained for an angle of attack between 0 and 360 degrees. Results show that the drag coefficient increases by the spiral in each case whereas the lift coefficient changes only insignificantly. A defined transverse force is caused by the spiral. To find an explanation of this force by the circulation a simple mathematical model is given. Finally the behaviour of a twisted rope in water calculated by a known code is predicted in case of steady current.

Topics: Stress , Ropes
Commentary by Dr. Valentin Fuster
2005;():787-792. doi:10.1115/OMAE2005-67383.

Floating fish cages are the main means of production in the multi billion euro salmon farming industry. Despite its pivotal role in production safety, protection of values, as well as in protection of the environment, neither verified structural design procedures nor computer tools for structural analysis and design have received much attention. To a large extent they can be regarded as not being in accordance with the current state-of-the-art of structural analysis and design of slender marine structures. A momentum to move towards a more scientific based design approach has been created by the requirements of the recently introduced Norwegian certification criteria and the accompanying design code NS 9415. A prototype for analysis and design of floating fish cages has been developed and is described herein. The tool is based on an object-oriented framework for general FE analysis. The framework has among other things been developed with ease of extensibility for the software developer in mind. The prototype is thus intended for iterative extension of the functionality. In the first development iteration, described here, the FE framework has been extended, with hydrodynamic load models and a user interface for analysis of floating fish farms. The development of the prototype shows that by building on an object oriented FE framework, specialized and focused applications for aquaculture can be developed with limited effort. As an example analysis — and a possible benchmark — a simplistic model of a steel frame is chosen. Comparison of results obtained with different load formulations indicated that the buoyancy load was more important than the hydrodynamic load.

Commentary by Dr. Valentin Fuster
2005;():793-795. doi:10.1115/OMAE2005-67390.

In order to verify the theoretical work considering very low speed permanent magnetized generators for marine current power conversion, an experimental setup is under construction. Earlier 2-dimensional Finite Element Method (FEM) simulations have been performed and in this paper focus is on the practical considerations regarding the rotor construction of the generator as well as the total system configuration with motor, frequency transformer and generator. A rotor configuration with milled grooves for magnet fixation along with a power feedback option with a resistive load is chosen for the first experimental setup.

Commentary by Dr. Valentin Fuster
2005;():797-801. doi:10.1115/OMAE2005-67391.

A novel wave energy converter concept is developed at Uppsala University, Department of Engineering Science. The concept is based on a synchronous permanent magnet linear generator, placed on the seabed. The piston of the generator is directly connected to a surface-floating buoy with a rope. The tension in the rope is maintained with springs that pull the piston downward. The three-phase current induced in the stator coil has a varying amplitude and frequency and a conversion is therefore necessary. Research has been carried out in three main areas: generator design, dynamic behavior and grid connection. The generator is modeled by full physics numeric simulations, based on a 2-dimensional finite element formulation of the time dependent electromagnetic field. A first set up is built to experimentally verify the simulated results. The impact of different parameters are estimated with mathematical models and verified by experiments. This paper includes both simulated and experimental results.

Commentary by Dr. Valentin Fuster
2005;():803-809. doi:10.1115/OMAE2005-67414.

Site selection for wave energy power plants is a complex procedure. In addition to enjoy good resource, the site should obey to certain geomorphologic and environmental conditions, comply with a number of restrictions to avoid conflicts of use, and enjoy close availability of specific infrastructures. Information on those topics is usually spread out through a number of institutions making their collection a time consuming task. In this paper a preliminary version of PEMAP, a database containing required information for this purpose for mainland Portugal, is presented. This database is implemented in a Geographical Information System. It contains information on wave energy resource, bathymetry, slope and seabed coverage of the mainland continental shelf and electrical grid among others. Near future development concerning engineering conditions such as information on harbors and shipyards, and marine biodiversity and fisheries is planned.

Commentary by Dr. Valentin Fuster
2005;():811-818. doi:10.1115/OMAE2005-67424.

Taweelah Power and Desalination Plant was built in Abu Dhabi Emirate, United Arab Emirates in 1995. The capacity of the plant was increased in 1998. At present, it was decided to carry out the second phase of the capacity extension to satisfy the rapid urban and industrial developments in the emirate. A comprehensive study was carried out to investigate the increased recirculation of salinity and temperature from the plant outfall to the intake due to the proposed extension. The study was carried out using a 3-dimensional numerical flow model simulates the tidal flows and heat transport in the vicinity of the plant. Recirculation of temperature at the plant intake was also obtained from the available long-term seawater temperature measurements at the intake and in the ambient seawater. The resulting recirculation of seawater temperature from the measurements was compared with the model results.

Commentary by Dr. Valentin Fuster
2005;():819-824. doi:10.1115/OMAE2005-67434.

This paper presents a different approach to the work developed by Cruz and Sarmento (2005), where the same problem was studied in the frequency domain. It concerns the same sphere, connected to the seabed by a tension line (single point moored), that oscillates with respect to the vertical direction in the plane of wave propagation. The pulsating nature of the sphere is the basic physical phenomenon that allows the use of this model as a simulation of a floating wave energy converter. The hydrodynamic coefficients and diffraction forces presented in Linton (1991) and Lopes and Sarmento (2002) for a submerged sphere are used. The equation of motion in the angular direction is solved in the time domain without any assumption about its output, allowing comparisons with the previously obtained results.

Commentary by Dr. Valentin Fuster
2005;():825-832. doi:10.1115/OMAE2005-67451.

A Norwegian Standard NS 9415 (NAS, 2003) has been introduced to the offshore fish farming industry in Norway. This is the first standard dealing with offshore fish farm facilities. The main objective of the standard is to reduce environmental pollution by fish escape. The work process leading to NS 9415 revealed the need for research work in several areas to enhance design criteria with the objective of having a consistent safety level through out the life cycle of a fish farm facility. This paper presents results from a government supported research project with the objective of enhancing criteria for design and operation of fish farm facilities. A case study of a fish farm facility representative for the majority of polyethylene based fish farms in Norway is presented and the sensitivity of such fish farms to variation in the mooring system is shown and discussed for design relevance. The sensitivity of net cage volume to current and weights is presented and discussed. Possible hazards from operational conditions are listed.

Topics: Design
Commentary by Dr. Valentin Fuster
2005;():833-839. doi:10.1115/OMAE2005-67462.

The wave induced yaw drift moment on floating structures is of particular interest when the lateral yaw motion of the structure should be controlled by moorings and/or active dynamic positioning systems. In the present paper, the estimation of the yaw drift moment in the modeled natural wave environment is conducted by application of a nonlinear time domain numerical method accounting for the motion of arbitrarily shaped floating bodies in waves. The computational method is based on linear potential theory and includes the non-linear hydrostatic terms in an exact way, whereas the higher-order wave-induced effects are partly approximated. Despite the approximate modeling of the second order hydrodynamic forces, the method proved to satisfactorily approach the dominant part of the exerted hydrodynamic forces enabling the calculation of drift forces and of other drift effects in irregular waves. Hence, the subject yaw drift moment in the modeled natural wave environment is derived, resulting to a basic reference for the design of similar type floating structures.

Commentary by Dr. Valentin Fuster
2005;():841-849. doi:10.1115/OMAE2005-67490.

In this paper, three nonlinear methods are described for artificially generating operational sea state histories. In the first method, referred to as Translated Gaussian Process the observed time series is transformed to a process which is supposed to be Gaussian. This Gaussian process is simulated and back transformed. The second method, called Local Grid Bootstrap, consists in a resampling algorithm for Markov chains within which the transition probabilities are estimated locally. The last models is a Markov Switching Autoregressive model which allows in particular to model different weather types.

Commentary by Dr. Valentin Fuster
2005;():851-857. doi:10.1115/OMAE2005-67494.

In this paper, we propose a numerical simulation procedure of moored ship motions due to initial attack of large scaled tsunamis and investigate the effects on the motions and mooring loads. The effect of methodology on selection of tsunami wave components and of the drag forces are then considered by using the numerical simulation method, applying to several case studies for LNG-carrier. Large ship motions and excessive mooring loads beyond the safe working loads are induced by the resonant tsunami wave components in the sway and surge motions, and drag forces.

Topics: Motion , Mooring , Ships
Commentary by Dr. Valentin Fuster
2005;():859-866. doi:10.1115/OMAE2005-67503.

In this paper, ringing phenomenon in a drag dominated marine platform is studied numerically using linear transverse and nonlinear coupled axial and transverse beam models. The effects of inertia, added mass, and relative velocity of the structure on ringing are investigated. It is found that, for the structure considered, inertia and added mass have little effect on ringing, while the relative velocity had a significant effect on its duration. The maximum ringing amplitude predicted by the nonlinear beam model is similar to that predicted by the linear beam model. When a smaller second impact is introduced, ringing stops when the nonlinear beam model is used.

Commentary by Dr. Valentin Fuster
2005;():867-873. doi:10.1115/OMAE2005-67512.

MonoBr is the name of a concept of a mono-column structure with a moon-pool developed by PETROBRAS to operate in deep water. A set of tests has been carried out at LabOceano / COPPE / UFRJ to analyze its behavior in waves. Different configurations of the moon-pool entrance have been tested. The main objective of the measurements carried out is to determine the influence of different restrictions on the behavior of the vertical motion of the structure. Results of these measurements are presented and discussed in the paper.

Topics: Waves
Commentary by Dr. Valentin Fuster
2005;():875-883. doi:10.1115/OMAE2005-67539.

This paper presents experimental and theoretical results obtained during the hydrodynamic study of a multi-cylinder system. The main focus of the study was to quantify hydrodynamic interactions between heaving vertical cylinders of a conceptual wave energy conversion system. Several identical circular cylinders representing platforms in an energy conversion system and a parabolic shaped wave reflector were tested in a wave flume tank. Wave heaving forces, radiation and diffraction effects were studied experimentally and numerically. The theoretical calculations were carried out for hydrodynamic coefficients, the radiation and diffraction effect analysis. Experimental results for multi-cylinders were compared with the numerical solutions by a panel-free method in the frequency domain. One main objective of the experimental tests was to calibrate the experimental set up, obtain validation data for numerical calculations. The diffraction studies showed that the hydrodynamic interactions could be constructive or destructive for heave wave forces. The positive magnification of the wave exciting force can be significant if a parabolic shaped reflector is used. It was observed that the wave force magnification and the wave energy absorption depend on incoming wavelength, and the cylinder to wavelength ratio. It has been found in the radiation tests that heave added mass and damping coefficients compare well with the calculations based on potential flow calculations.

Topics: Waves
Commentary by Dr. Valentin Fuster
2005;():885-890. doi:10.1115/OMAE2005-67554.

The flat solitary wave with the behavior of conjugate flow, generated by a submerged body moving in a two-layer fluid, is investigated. A criteria about the existence of weakly nonlinear weakly dispersive flat solitary wave is given. The condition of the stable system of conjugate flow is obtained. The solution of the flat solitary wave satisfying the criteria is numerically verified to be unique. Theoretical analysis is qualitatively consistent with the experimental results obtained by the authors.

Topics: Fluids , Waves
Commentary by Dr. Valentin Fuster
2005;():891-896. doi:10.1115/OMAE2005-67559.

This paper presents results from a programme of centrifuge experiments which examined the effects of drained preloading on the stiffness and load carrying capacity of shallow square footings founded on an overconsolidated clay. The increases in stiffness and bearing capacity induced by various levels of preloading are quantified and compared with standard design guidelines and previously published numerical predictions.

Commentary by Dr. Valentin Fuster
2005;():897-903. doi:10.1115/OMAE2005-67561.

This paper presents results of numerical modeling of an oceanographic mooring system and makes comparisons to loads measured on a deployed test mooring near Monterey Bay, California. The numerical modeling solves the non-linear equations of motion of the cable in the time-domain. The deployed system is instrumented to monitor environmental loading and the resulting tensions in the mooring cable below the buoy and above the anchor. Comparison of the numerical results to the measured results is useful to refine the accuracy of the model, allowing its use in determining fatigue life of the system and for designing similar systems to be deployed in new locations. This study is part of a project to develop and improve mooring systems for oceanographic use that include an electro-optical-mechanical mooring cable that delivers power and data communication to a network of sea-floor instrumentation. The modeling and test results highlight the engineering challenges associated with designing these systems for long lifetimes.

Topics: Cables , Modeling , Mooring , Buoys
Commentary by Dr. Valentin Fuster
2005;():905-909. doi:10.1115/OMAE2005-67570.

Most floating fish cages consist of a floating cage collar, mooring system, and net cage hanging freely down from the floater. Any current moving through the net will deform the net. This deformation depends on the current velocity, the nets ability to attract forces, and the gravity forces from weights at the lower end of the net. The forces on the net will vary with the deformation. This paper shows that for this kind of cage, a reduced velocity Vred = U·(ρ/2G)1/2 , where G is the equivalent weight per area of the net, will be sufficient to estimate the drag force and deformation of the net. The present work is analytical, combined with computer simulations. The reduced velocity parameter could be used to determine the needed bottom weights. The quick estimation of the drag force on the net could be used as an input to mooring design.

Commentary by Dr. Valentin Fuster

Polar and Arctic Sciences and Technology

2005;():913-917. doi:10.1115/OMAE2005-67285.

The use of drilling platforms for exploration drillings in ice-covered shallow water is expected to increase in the future. Jack-up platforms (JU) are regarded to be most suitable for exploration drillings due to their high mobility, their facile transportation and installation. Although having proved their capability for harsh environments, e.g. in the North Sea, jack-ups have neither been especially designed for nor have been operated frequently in icy waters. The problems associated with the operation of jack-ups in ice waters are related to structural integrity as well as to operational limitations. One objective of the research project MATRA-OSE was to check the design of JU for exploration and oil production in ice-covered, shallow-water areas. The global stability of JU as well as local and global loads acting on a three-legged jack-up drilling unit were investigated. The design loads were determined by analytical approaches. Different ice load cases were defined as the basis for the detail design. These load cases include local loads on single members of the platform legs like chords or braces as well as global loads, which may act on one individual leg or on several legs of the platform. Vertical forces, which result from a change of the water level (up-surge and down-surge) in case ice sheets are frozen to the platform legs were considered. Also the impact of ice being jammed between the platform legs was investigated. It has been found out that the most critical load case results from the ice being jammed between the platform legs. It should therefore be tried in practice to avoid the ice jamming by proper ice management, as the large ice loads calculated for this scenario may result in problems for the stability of a JU. Ice breakers could be used to pre break the ice and should try to clear the site from broken ice pieces before it starts to accumulate.

Commentary by Dr. Valentin Fuster
2005;():919-921. doi:10.1115/OMAE2005-67300.

Ice force is an important factor to be taken into account for offshore structures in cold region, and the calculation method of the ice force is meaningful for the offshore structure design. Cone is now used as optimal ice-resistant structure because it can cause bending failure of the ice sheet. The interaction between ice sheet and conical structure is studied in this paper and Croasdale’s model is modified based on the field observations. The newly built model separates the ice sheet into emersed part and floating part, and the equilibrium analyses are carried out respectively. The bending moment distribution of the ice sheet is analyzed to determine the position of bending failure, which serves as a supplementary restriction. Analytic solution of ice force on conical structure is got and it is verified by the experimental data of previous researches.

Topics: Force , Ice
Commentary by Dr. Valentin Fuster
2005;():923-930. doi:10.1115/OMAE2005-67327.

According to the present Finnish-Swedish Ice Class Rules (FSICR) the formulas for the required main engine power for tankers led to much bigger main engines than it is needed for the demanded open water speed. Therefore model tests may be performed in order to verify the vessel’s capability to sail with less required power in brash ice channels compared to the calculations. Several model test runs have been performed in order to study the performance of crude oil tankers sailing in brash ice. The tests were performed as towed propulsion tests and the brash ice channel was prepared according to the guidelines set up by the Finnish Maritime Administration (FMA). The channel width was 2 times the beam of the tanker. The model tests were carried out at a speed of 5 knots. For the tests a parental level ice sheet of adequate thickness is prepared according to HSVA’s standard model ice preparation procedure. After a predefined level ice thickness has been reached, the air temperature in the ice tank will be raised. An ice channel with straight edges will be cut into the ice sheet by means of two ice knives. The ice stripe between the two cuts will be manually broken up into relatively small ice pieces using a special ice chisel and if required the brash ice material will be compacted. Typically the brash ice thickness will be measured prior the tests at 9 positions across the channel and every two meter over the entire length of the brash ice channel with a special device, which consists of a measuring rule with a perforated plate mounted under a right angle at the lower end of the rule. As a result of the tests it could be demonstrated that tankers with a capacity of more than 50 000 tons require 50% and even less power compared to calculations using the present FSICR formulas.

Commentary by Dr. Valentin Fuster
2005;():931-936. doi:10.1115/OMAE2005-67416.

Propeller-ice interaction experiments were conducted in the ice tank at the National Research Council of Canada’s Institute for Ocean Technology. A podded propeller was used in “Puller” mode and loads on an instrumented blade were measured. During the propeller-ice interaction, hydrodynamic loads and ice milling loads were acting on the propeller blade. This paper focuses on the ice milling loads both in water and in air. The ice milling loads, however, cannot be separated from the hydrodynamic loads perfectly. Even if the blade is milling the ice within an ice block, it is still experiencing hydrodynamic loads designated as inseparable hydrodynamic loads. The non-dimensional ice milling loads including inseparable hydrodynamic loads on the blade are presented against advance ratio with varied depths of cut. The results help to reduce the gap of knowledge for interaction between ice and propeller and give information about significant variables acting on the propeller blade during interaction.

Topics: Stress , Ice , Blades , Milling , Propellers
Commentary by Dr. Valentin Fuster
2005;():937-943. doi:10.1115/OMAE2005-67516.

A combined system consisting of a forecast model to predict ice drift and a viewer tool for ice management, IceMS were developed for providing ice information during the Arctic Coring EXpedition (ACEX). The main task during the multi international expedition ACEX was to retrieve sediment cores from the Lomonosov Ridge in the central Arctic Ocean. Large ice floes or thick ice that drifts towards the drill ship threaten the operation by forcing the drill ship from its position. The gathered ice information was imported into IceMS and presented to the ice management team. Risk analysis for continued drilling operation was made, and instructions of where the Icebreakers Oden and Sovjetski Soyuz should break ice in the drilling area were carried out. The software IceMS contains tools to update the ice conditions, history lines of ships and buoys, and includes the possibility to import track lines showing the ice drift forecasts. The key concept is the combined visualization of map data and up to date imagery from satellite, airplane photos and ice charts, together with results from an ice drift forecast model. It is possible to move the images in IceMS map according to the observed ice drift recorded by the buoys placed on drifting ice floes. An overlay to mark and edit polygons, e.g. representing areas with certain ice classification, can be shown on top of the images. The edit overlay can be exported to file, which enables sharing of judgments and forecasts to other units. The model to predict ice drift is a state of the art ice drift model that is developed for describing rapid changes such as circular motion with a period of about twelve hours called inertial motion. The ice drift forecast was based on weather forecast and measured ocean currents near the drill site. The combined system of IceMS and the forecast model is being further developed for supporting ice management teams on offshore platforms and other constructions in ice infested areas. Results from Ice Management will be presented, showing examples of how IceMS presented ice information and validation of the ice drift model during the Arctic Coring EXpedition (ACEX). New tools in IceMS will also be presented.

Commentary by Dr. Valentin Fuster
2005;():945-949. doi:10.1115/OMAE2005-67547.

A new Arbitrary Lagrangian Eulerian (ALE) Finite Element (FE) model of ice scour was recently developed by the authors. It is based on continuum representation of the soil. It was shown in recent papers that such a model can characterize the mechanics of the ice-soil-pipeline interaction without requiring any of the assumptions that the Winkler models depend on. The model utilizes soil properties obtained by conventional laboratory testing. In a recent paper, this model was used to show that the subscour deformations and ice-soil interaction forces are very sensitive to ice ridge geometries for shallow slope ice features. In this paper, the ALE FE ice scour model is utilized to study the effects of the pipeline trench on the scour process and the forces transmitted to the pipeline. Two different infill soil properties and two different ice ridge geometries are analyzed with a 36 inch diameter pipe buried to in a trench of 1.5 m cover. It is shown that the scour process near and in the trench is significantly different than in the ambient seabed soils and the recognition of this may present some potential advantages for the protection of the pipelines not recognized by the Winkler models. It is also shown that the pipeline loads generated during the scour process are cyclic. They build slowly as the ice moves over the trench and then reverse as the ice ridge moves away from the trench. This is in contrast to monotonic and rapidly growing loads predicted by the Winkler models. The paper shows that the loads transferred to the pipeline depend on the infill soil properties placed in the trench. It is shown that loads experienced by the pipeline are less for the softer infill than stiffer soils.

Topics: Ice
Commentary by Dr. Valentin Fuster

LNG Specialty

2005;():953-962. doi:10.1115/OMAE2005-67096.

The paper addresses the problem of determining characteristic extreme values of sloshing pressures for structural design. This involves estimating ship motion in a long-term period, fluid motion in the tank, excited pressures and relevant structural responses. Ship motion analysis is based on linear strip theory. In order to investigate the dependence of the sloshing response on sea conditions, a multimodal approach for an initial prediction of pressures in the tank is utilized. However, an appropriate theoretical / numerical approach which can be used for realistic prediction of the most extreme pressure has not yet been developed. Thus, experiments are utilized for the most severe sea states for a chosen tank filling level. In the analysis of experimental short term pressure distribution the choice and fit of probability distribution models is addressed, with due account of different physical mechanisms causing impacts. The most critical tank areas for sloshing loads are briefly discussed. Appropriate dynamic response of the tank structure needs to be investigated by accounting for temporal and spatial distribution of sloshing loads. These two factors are also addressed in the paper.

Commentary by Dr. Valentin Fuster
2005;():963-973. doi:10.1115/OMAE2005-67105.

The Soft Yoke Mooring and Offloading system (SYMO® ) has been developed in SBM to allow the tandem mooring of an LNG carrier to the stern of an LNG FPSO or a Floating, Storage & Regasification Unit in harsh environments. The SYMO® system consists of two mooring legs supported by a crane. A yoke structure is suspended at the bottom of the two mooring legs by uni-joints. The tip of the yoke is connected to the bow of the LNG carrier via roll, pitch and yaw articulations. When connected the yoke structure is horizontal. Upon disconnection, the yoke settles at an equilibrium position characterized by a pitch angle of 35° to 40° with respect to the horizontal. Owing to the low friction articulations, the yoke oscillations decay very slowly and the yoke may come too close to the LNG carrier bow. A passive system has been developed to improve the SYMO® dynamics at disconnection whereby a small proportion of the overall yoke weight is replaced by water. This water is allowed to move in a suitably designed tank network thus shifting around the overall centre of gravity and yoke equilibrium angle as pitch oscillations take place. The above principle has first been investigated using a kinematics program before being model tested at a large scale of 1:16. A large matrix of tests has been performed to investigate the effect of the water ballast volume and the layout of the tank network. Once the principle was confirmed experimentally, CFD computations were performed in a blind fashion and compared to the experiments.

Commentary by Dr. Valentin Fuster
2005;():975-982. doi:10.1115/OMAE2005-67190.

The LNG-FPSO concept is receiving much attention in recent years, due to its active usage to exploit oil and gas resources. The FPSO offloads LNG to an LNG carrier that is located close to the FPSO, and during this transfer process two large vessels are in close proximity to each other for daylong periods of time. Due to the presence of neighboring vessel, the motion response of both the vessels will be affected significantly. Hydrodynamic interactions related to wave effects may result in unfavorable responses or the risk of collisions in a multi-body floating system. Not only the motion behavior but also the second order drift forces are influenced by the neighboring structures due to interactions of the waves among the structures. A study is made on the time domain analysis to assess the behavior and the operational capability of the FPSO system moored in the sea having an LNG carrier alongside under environmental conditions such as waves, wind and currents. This paper presents an analysis tool to predict the dynamic motion response and non-linear connecting and mooring forces on a parallel-connected LNG-FPSO system due to non-linear exciting forces of wave, wind and current. Simulation for the mooring performance is also investigated. The three-dimensional source-sink technique has been applied to obtain the radiation forces and the transfer function of wave exciting forces on floating multi-bodies. The hydrodynamic interaction effect between the FPSO and the LNG carrier is included to calculate the hydrodynamic forces. For the simulation of a random sea and also for the generation of time depended wind velocity, a fully probabilistic simulation technique has been applied. Wind and current loads are estimated according to OCIMF. The effects of variations in wave, wind and current loads and direction on the slowly varying oscillations of the LNG and FPSO are also investigated in this paper. Finally, some conclusions are drawn based on the numerical results obtained from the present time domain simulations.

Commentary by Dr. Valentin Fuster
2005;():983-991. doi:10.1115/OMAE2005-67219.

For future offshore LNG terminals tugs are planned to assist LNG carriers during berthing and offloading operations. A model test study was carried out to better understand the tug behaviour in waves and to make a first step in the quantification of the related weather limits. Scale 1:35 model tests were performed in the two important ‘modes’ of a tug during this type of operation: the ‘push’ mode and the ‘pull’ mode. Realistic weather conditions were used and the tugs were working at the unshielded and shielded sides of the LNG carrier. Based on the results presented in this paper, it can be concluded that the motions of tugs in waves are significant, even in wave conditions that are considered to be mild for the berthing and offloading LNG carriers. The resulting push or pull loads may hamper these tug operations significantly. Special measures are necessary to take this behaviour into account in tug design, LNG carrier design and development of operational procedures and equipment. The paper gives insight in the typical tug behaviour in different weather conditions. One should be careful, however, to generalize the present results: with an optimised tug design and operation the tugs can be used in more severe conditions.

Commentary by Dr. Valentin Fuster
2005;():993-999. doi:10.1115/OMAE2005-67398.

A study was made to estimate the transit time of time-critical cargo (LNG) from Western Australia to the Far East (e.g. Japan) or the southern U.S. west coast. Unexpected delays during transit can result in stiff financial penalties imposed by LNG buyers. Conversely, quicker than expected voyages will underutilize expensive tankers. Since there was no prior experience along the two routes using the proposed new-build vessels, the SafeTrans computer simulation program was used to calculate the voyage duration. SafeTrans is a comprehensive routing simulator that accounts for weather routing, ship dynamics, and weather forecast errors. Basic voyage duration statistics are presented and indicate important differences between a) outgoing and returning voyages, b) the two routes, and c) weather-routed and non-weather-routed scenarios. These differences are examined and reveal that voyage duration estimate is significantly impacted by weather routing, and a comprehensive ship motion model and weather database. Finally, our results demonstrate an application of SafeTrans that goes beyond the original purposes of the program, and suggests it might well be useful for estimating the voyage duration for other types of time-critical cargo.

Commentary by Dr. Valentin Fuster

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