The Dynamic Positioning system allows a vessel to keep a precise position and heading during stationing operations in a rough sea by using onboard actuators only. During the design phase, it is mandatory to identify the capability of the system actuators to counteract the environmental forces. Conventional predictions are limited to the estimation of a maximum sustainable wind speed on predefined encounter angles by estimating the corresponding wave parameters with questionable standard deterministic correlations. The proposed approach aims at determining the dynamic positioning performances by using site-specific long-term environmental conditions which are modelled with joint distributions of wind and wave parameters. To this end, the operability of the dynamic positioning system is evaluated as a non-deterministic multidimensional Monte Carlo integration process, based on the sampling of environmental joint distributions. For each environmental condition, a quasi-static dynamic positioning analysis is performed solving the equilibrium between external forces and the vessel's actuators through a non-linear thrust allocation algorithm. The proposed methodology is applied to a reference offshore ship in five different operative geographic areas, highlighting the suitability of the calculation methodology for site-specific operability predictions.

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The importance of comfort during transfer and stationing becomes a key performance parameter for large yacht design, on the same level as propulsive issues. Such a matter extends questions in terms of form and service demand to the motion behaviour of the unit in waves. Relevant studies refer to outdated hull forms not specific to modern large yachts. In this study, five hull forms with different bow concepts represent the most common design solutions for yachts at constant draught and displacement. The preliminary ranking on the effect of alternative bows on comfort requires the definition of internationally accepted comfort standards. Here, the AWI 22834 guidelines for large yachts provide the service and environmental conditions and criteria for the comfort analysis, being the only reference specific to yachts. The calculations employ a strip-theory-based numerical model to provide results of easy understanding for designers during the early design stage. The obtained ranking among the design solutions on a reference large yacht favours the option nested with a bulb, contradicting the expectations in favour of a vertical bow concept. The discussion and conclusions provide a way forward for additional analyses and investigations aimed at proposing suitable multicriterial design guidelines for large yachts. However, the results also show the unsuitability of AWI environmental and encounter conditions for hull form ranking. @en

Installation of jacket platforms requires simultaneous and combined operations of multiple assets. When the whole process has to be planned, it is necessary to predict in a fast and reliable way the possible weather limitations that may occur during the operations. The paper will present the major challenges of this unusual and innovative Dynamic Positioning analysis which has been carried out for Ana Jacket installation. The obtained results show that the Dynamic Positioning system of the core vessel in intact configuration is capable to hold the position for the investigated vessels' arrangements and design operative weather conditions. Lifting, upending and installation of Ana Jacket were carried out successfully in 2021.

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Determining Dynamic Positioning capability for an offshore vessel is mandatory to identify the environmental forces the system can counteract, together with the operability in a specific operational area of interest. Conventional predictions evaluate the capability as a maximum sustainable wind speed at a predefined encounter angle for a given wind–wave correlation, not reflecting the effective wind and waves occurrence at the site. In this respect, a step forward is provided by the scatter diagram approach, allowing the evaluation of operability in a specific sea area, using a simplified method to predict wind speed from wave parameters. Here, using known wind–waves joint distributions for the long-term environmental conditions further improves the scatter diagram approach, assessing the operability of a Dynamic Positioning system through a Quasi-Monte Carlo sampling of the joint distribution. Analysing the results of the Quasi-Monte Carlo process, it is possible to obtain a site-specific capability plot, allowing the identification of critical wind speeds in a way that is familiar to operators in the offshore industry. The application of this novel method in the case of quasi-static calculations both to a reference supply vessel and a pipe-lay vessel shows the flexibility of the proposed approach for site-specific Dynamic Positioning capability predictions.@en

In the offshore industry, the thruster layout for dynamic positioning (DP) is not optimal to reach the maximum station-keeping capability. It is possible to optimise the position of the thrusters since early design stage applying a genetic algorithm-based optimisation procedure. The enhancements due to an optimal thruster configuration can be reported in the standard capability plot format as commonly done in the offshore industry. However, besides standard capability plots, additional calculations can be performed by explicitly considering the vessel’s geographic operational areas. By adopting this novel approach, it is possible to associate to vessel’s DP capability an operability index which is specific for the considered sea area and is in analogy of that adopted for ship motion criteria. In this way, the improvements of the optimised thruster configuration can be represented in terms of the increment of DP operability index in a selected sea area, instead of the capability plot area increase for standard conditions. The procedure is here applied to a reference Pipe-lay vessel, where the thruster layout has been optimised. The improvements are reported through both conventional capability plot and newly proposed DP operability index for the North Sea area.@en

The execution of the so-called extinction tests represents the classical experimental method used to estimate the damping of an oscillatory system. For the specific case of ship roll motion, the roll decay tests are carried out at model-scale in a hydrodynamic basin. During these tests, the vessel is posed in an imbalance condition by an external moment and, after the release, the motion decays to the equilibrium condition. When the damping is far below the critical one, the transient decay is oscillatory. Here a new methodology is presented to determine the damping coefficients by fitting the roll decay curves directly, using a least-square fitting through a differential evolution algorithm of global optimisation. The results obtained with this methodology are compared with the predictions from standard methods. This kind of approach seems to be very promising when the motion model of the system under investigation is established with any level of non-linearities included. The usage of the fitting procedure on the approximate analytic solution of the differential equation of motion demonstrates the flexibility of the method. As a benchmark example, two experimentally measured roll extinction curves have been considered and suitably fitted. The newly predicted results, compared with the ones obtained from standard roll decay analysis, show that the algorithm is capable to perform a good regression on the experimental data.

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For the operability evaluation of an offshore unit, it is usual to adopt specific working limiting criteria dealing with ship motions and accelerations. These criteria given by operators reflect the ability of the vessel (or crew) to accomplish a precise target in an operative scenario with different meteorological conditions. However, an offshore vessel needs also to keep a determined position and heading during working operations. That means designers or operators have to consider criteria associated both with seakeeping and station keeping to evaluate the effective operability of the vessel. Traditionally, early design stage operability analyses do not combine seakeeping and station keeping predictions, which are referring to different working environments. Here, an innovative procedure allows to evaluate the joint operability of an offshore unit, adopting scatter diagrams for local environmental conditions. The method has been applied on a Pipe-Laying vessel, assessing the global operability for different areas of commercial interest.

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For the dimensioning of particular structures like stingers or generic tubular elements for offshore industry, it is usual to predict the extreme values of wave induced loads. Both for model test and calculations, the traditionally adopted methods of analysis are based on Weibull distribution. The necessity to investigate severe sea state conditions together with the increased complexity of the structure is an evident source of non-linearities in the exciting force peaks distribution. In the specific, the adoption of a standard Weibull approach is not indicated for accurately predict the extreme loads. The adoption of more accurate distributions suitable to capture peaks non-linearity will ensure to overcome or capture possible multi-modal behaviour of the considered population. These enhanced techniques can be used not only for model test results analysis, but also for results coming from preliminary hydrodynamic calculations (CFD). In the present work, two different methodologies based on Mixed Weibull and Generalised Pareto distributions will be applied to the results obtained for a stinger geometry, where Morison theory is adopted to evaluate wave loads considering shield effects between the single tubular elements.

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During the design process of an offshore vessel, the DP system is not representing one of the primary constraints for the general arrangement of the vessel. Usually the location of the thruster devices is driven by the internal spaces available, i.e. the thrusters position is not optimised to reach the maximum capability. This is also true for a conversion of an existing vessel. Nowadays almost all offshore vessels require the installation of a DP system on board, means that it could be worthy to study more in detail the arrangement of the thrusters in such a way to maximise the capability of the vessel to keep position with a determined amount of power installed on board. In the present work a procedure aimed to find an optimal thruster location on the hull during early design stage will be presented. Using a quasi-static approach for DP capability evaluation, an optimisation procedure based on genetic algorithm has been developed, considering the constraints given by the vessel geometry. The discussion is supported by a test case on a reference vessel, were the original layout is compared with two possible optimised configurations.

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The analysis of time records, coming from seakeeping experiments in irregular waves, is used to determine the occurrence of extreme events. The common procedure used for data analysis is to assume that the statistics of record’s peaks is following two or three parameters Weibull distribution. For particularly severe sea states it can happen that the peaks assume a multi-modal distribution. In this case, a Weibull distribution is not suitable, also in the three parameters form, to reproduce the peaks population. That means some errors in the estimate of the extreme loads may occur, affecting consequently the vessel/structure design process. To overcome this source of error it is possible to use multi-modal distributions, or to change the peaks extraction technique, adopting a certain threshold. By using this second approach data should be fitted according to a Generalised Pareto distribution. Based on this theory, a data analysis procedure including the threshold selection is here proposed and tested on a set of time records coming from seakeeping model-scale experiments. The results are then compared with the standard Weibull approach.

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Minimisation of power consumption is the primary target of the most commonly used procedures and strategies for the capability prediction of DP systems installed on floating objects and, therefore, dealing with an huge amount of limitations or restrictions that should be satisfied at the same time for different operating conditions. However, in the particular case of the “extreme limit prediction” for a considered system, other factors assume a prominent importance instead of fuel consumption, like the ability to use all the available power to keep the position in heavy weather conditions. When the maximum operating cases for a vessel are analysed, other effects like thruster–hull interaction and thruster–thruster interaction are rarely considered. To take into account the above-mentioned effects, different thrust allocation strategies were studied and implemented in a capability prediction suite. In this paper a new allocation philosophy is presented, with the aim of maximising the DP capability of a vessel operating in heavy whether conditions. An application of the procedure on two existing offshore vessels is also considered to highlight the effects of different allocation strategies and of the interaction effects on the resulting DP capability in terms of “operational rosettes”.

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It is a consolidated procedure to investigate vessel station-keeping capability disjoint from her seakeeping qualities during operations at field in storm conditions. This is mainly due to the fact that the station keeping problem is essentially related to surge, sway and yaw motions, while seakeeping predictions deal at most with heave, roll and pitch motions. The operational limitations on ship motions and accelerations are for sure more restrictive than the DP capability in head sea, and could also be that certain criteria would be reached when the station-keeping capability is no more effective. Referenced theoretical models are fully disjoint as well as the most commonly implemented software codes, which are daily used for design and operability analysis. An integrated approach simultaneously dealing with the two problems of station-keeping and seakeeping has not yet been considered. In this paper, we present a newly developed procedure for integrated station-keeping and seakeeping investigations with the aim of obtaining more realistic predictions on the vessel effective capability of carrying out field operations and maintaining position during different storm conditions. To this aim the final result will be expressed in terms of vessel hydrodynamics statistically evaluated through meteorological data of the geographic area of interest. In this way, an effective index could be evaluated for an offshore vessel operating in a field during heavy weather conditions, taking into account both the above mentioned main topics.

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To predict the capability of dynamic positioning systems onfl oating o bjects, the existing procedures and strategies are primarily focused on the power consumption minimisation, treating of numerous limitations and conditions that should be satisfied at the same time. On the other hand, when the extreme limits of the system must be predicted other factors are of primary importance instead of minimal power consumption. Thruster interaction effect such as axial and transversfle ow acting on the propeller disc, thruster-hull interaction, thruster-thruster interaction and ventilation are relevant and rarely taken into account. These effects, whether they occur separately or in combinations, can cause significant thrust losses, which could consequently degrade reliability of dynamic positioning systems. Different allocation strategies were studied to take the above mentioned effects into account. Here, a smart thrust allocation strategy has been developed to maximise the DP capability of vessels during operations in heavy weather conditions. The main goal of this paper is the quanfitication of the e ffects due to thruster interaction on the resulting DP capability predictions of the vessel. Applications to two existing offshore vessels have been considered to highlight different allocation strategies on the resulting DP capability plots in terms of operational rosettes.@en