Dynamic Tension of Marine Cables During Laying Operations in Irregular Waves

A numerical approach for predicting motion and tension of marine cables during laying operations in a rough sea is presented. The solution methodology consists of dividing the cable into straight elements, which must satisfy equilibrium equation and compatibility relations. The system of obtained nonlinear differential equations is solved by Runge-Kutta method, taking explicitly into account the effect of regular and/or irregular waves. Illustrative applications of the method are given for a cable-lying ship of approximately 100 m length, designed to install and repair submarine cable systems. The results are presented as transfer function and response spectrum of cable dynamic tension for two different types of cable, several ship velocity-heading combinations. The obtained responses make it possible to determine order statistics of the cable lying process

Two-Dimensional Non-linear Dynamic Analysis of Cable Laying Operations

A numerical approach for predicting motion and tension of marine cables during laying operations in a rough sea is presented. The solution methodology consists of dividing cable into straight elements, which must satisfy equilibrium equation and compatibility relations. The system of obtained nonlinear differential equations is solved by Runge-Kutta method, taking explicitly into account the effect of regular and/or irregular waves. Illustrative applications of the method are given for a cable-lying ship of approximately 100 m length, designed to properly install and repair submarine cable systems. The results are presented as transfer functions and response spectra of cable dynamic tension for two different types of cable and several heading angles. The obtained responses make possible to determine order statistics of the cable lying process. The cable element angles, cable configuration and cable top force are presented for the case of ship under acceleration

Ship-Pipe Interaction during Laying Operations

Recent offshore activities are continuously moving towards deeper and deeper waters with increased problems for all the related above and underwater support activities. This general trend will not change in the near future and it is foreseeable that it will challenge the capabilities of the existing technologies to positively answer to the demand. For the development of the new ones a better understanding of the design constraints due to ultra-deep water scenario is therefore mandatory. The pipe laying operations are certainly one the most challenging aspects of the ultra-deep water activities and several new vessels are now under constructions. Such a design requires the optimization of seakeeping behavior of the vessel with the aim of reducing the motions of the stinger and thus increasing the operability in storm conditions. The related J-Lay tower design is affected by the induced dynamic accelerations and the increased tension demand coming from the pipe during laying operations. Unfortunately, a theoretical model able to manage both the vessel and pipe dynamics simultaneously is yet not sufficiently developed and in addition it has not been thoroughly investigated. The availability of a prediction tool able to highlight the effects of the ship-pipe interaction at the design stage will facilitate the selection of alternative technological solutions and rationally support final technical decisions. The theoretical model developed by the authors for a cable laying vessel has been recently enhanced and suitably adapted for simulating the pipe laying operations in deep water conditions. The model allows one to analyze all the significant quantities of the pipe as for example the local curvature, stress and maximum tension at stinger. The dynamics of the ship-pipe interaction is explicitly considered and the effect on the ship motion evaluated. It has been shown that for large pipe diameters and increased water depth this effect cannot be neglected. The results of the experience gained during the design of a pipelay vessel is here presented

Sensitivity of Thrust Efficiency Loss in Dynamic Positioning Predictions

Existing strategies and proposed methods for optimal thrust allocation in dynamic positioning systems of floating objects are primarily focused on minimization of power consumption with treating of numerous limitations and conditions that should be satisfied at the same time. On the other hand, one can notice that thruster interaction effects such as axial and transverse current, thruster-hull interaction, thruster-thruster interaction and ventilation are rarely taken into account. These effects, whether they occur separately or in combinations, can cause significant thrust losses, which consequently degrade reliability of dynamic positioning systems, decrease accuracy, increase response time and power/fuel consumption, etc. The main goal of this paper is the quantification of the effects due to different allocation methods on the resulting DP capability predictions of the vessel. Application to an existing offshore vessel has been considered to highlight their sensitivity on the final result in terms of operational rosettes

The Methods of Added Resistance Estimation for Ships in a Seaway

Reliable evaluation of added resistance in a storm conditions allows to quantifying the real overload on the propeller and the engine power needed to achieve the required service speed. The added resistance of a ship in a seaway has been studied for several decades. The essential concern of ship designer and operator lies in how a vessel will perform at sea; to wit, in her sustained sea speed under probable environmental conditions. However, even today the added resistance often is accounted for by increasing engine power between 15 and 30 percent to calm-water resistance. New computational methods of ship motion prediction have raised the prospect of new and advanced added resistance prediction techniques. This paper tends to review the present day methods of added resistance estimation and their development through years. Some of these methods are very simple and based only on main ship characteristics and the others are quite demanding in computational sense. The results of their practical application to a modern ro-ro ship are presented

Impact of Thruster Interaction Effects on Optimal Thrust Allocation in Dynamic Positioning Systems

Existing strategies and proposed methods for optimal thrust allocation in dynamic positioning systems of floating objects are primarily focused on minimization of power consumption with treating of numerous limitations and conditions that should be satisfied at the same time. On the other hand, one can notice that thruster interaction effects such as axial and transverse current, thruster-hull interaction, thruster-thruster interaction and ventilation are rarely taken into account. These effects, whether they occur separately or in combinations, can cause significant thrust losses which can consequently degrade reliability of dynamic positioning systems, decrease accuracy, increase response time and power/fuel consumption, etc. The main goal of this paper is quantification of selected aforementioned effects as well as the proposal of their implementation in optimal thrust allocation strategies based on Moore-Penrose pseudoinverse matrix. Moreover, the thrust allocation procedure here considered has been performed with and without thrust loss effects together with comparative analysis of these two approaches