Elsevier

Ocean Engineering

Volume 121, 15 July 2016, Pages 472-491
Ocean Engineering

Induced wave forces on a ship manoeuvring in coastal waves

https://doi.org/10.1016/j.oceaneng.2016.06.001Get rights and content

Highlights

  • Experimental captive manoeuvring tests in calm water and in regular waves.

  • Stationary and harmonic yaw tests with and without waves for two water depths.

  • Investigation of the steady second order wave forces in coastal waves.

  • Study of the superposition principle applied on manoeuvring in waves.

  • Discussion of ship and wave tank walls interaction.

Abstract

The need for a more realistic prediction of the ship manoeuvres has stressed the importance to incorporate wave effects in mathematical manoeuvring models. Wave effects are not only restricted to open sea; also in coastal areas, considering the important influence of shallow water and the lateral restriction in access channels, the incorporation of such effects is of critical importance. Up to date, several methods have been proposed to simulate manoeuvring in waves and a limited number of experimental research has been performed. Considering this, the present experimental study extends the discussion on the wave induced phenomena on a manoeuvring ship. In this article the focus is put on model tests designed for gentle manoeuvres in restricted areas, for instance ship entrance to ports. The investigation comprises captive manoeuvring tests in calm water and in regular waves for two different water depths.

In addition, the limitations regarding such experiments and the discussion of the superposing principle used in most of the theoretical works are presented. The superposition principle is investigated using numerical results obtained from Hydrostar.

Introduction

The need for a more realistic prediction of a ship's manoeuvring behaviour in a more complex environment, such as navigating and manoeuvring in harbours and access channels which are subjected to incident waves, questions the continuous use of conventional manoeuvring simulation models developed for calm water conditions. Waves will induce motions and forces, the effect of which on the manoeuvring performance needs to be incorporated in the simulator model to guarantee that the ship’s safety is fully assessed in simulation studies. Wave effects should not only be restricted to open sea, as in coastal areas they may be of importance as well. Taking into consideration the manoeuvring issues in shallow water and the limited dimensions of access channels, manoeuvres may be more critical. Implementation of wave forces in manoeuvring simulator models is therefore not only of importance for design and feasibility studies, but also for training activities in more realistic scenarios.

The study of wave effects on ship manoeuvring has recently received more attention. To the authors’ best knowledge, one of the first works addressing the problem was presented by Hirano et al. (1980), who used experimental data for the mean wave drift forces and moments (obtained at zero forward speed) in their manoeuvring model in waves. Several works followed years after, to mention some: Ankudinov (1983), McCreight (1986), Ottoson and Bystrom (1991), and more recent works of Bailey et al. (1998), Skejic (2008), Seo and Kim (2011), Carrica et al. (2012) and Mousaviraad et al. (2012).

One of the main concerns of a ship manoeuvring in waves are the mean wave drift forces; such forces will displace the ship from its expected course in calm water. This effect was also observed by Hirano et al. (1980) and Yasukawa (2006a) in their experimental research with free running ship models in waves. This force component has been extensively studied during the last decade with numerical methods; however, only a limited number of experimental studies have been performed to validate such methodologies.

The main wave effects that have been investigated are the mean wave drift forces and moments for their impact on manoeuvring. However, only considering these steady components and neglecting other effects such as the induced harmonic motions and forces might be insufficient. For instance, in coastal areas characterized by limited water depths and channel widths as well as dense traffic at the entrance to ports, motion responses represent an important constraint to safe ship operations. To have a better overview of the developments regarding manoeuvres in waves, the following will describe the applied methodologies for simulating manoeuvres in waves and the experiments performed up to date.

Only a limited number of methodologies to predict the manoeuvring behaviour in waves are present in the literature. Their approaches can be divided mainly in four types: a convolution integral method (expressed in integral differential form or as a set of ordinary differential equations), a direct calculation of hydrodynamic loads in the time domain, a two-time scale method, and studies conducted by computational fluid dynamics. The first two are considered in ITTC (2014) as the unified method because both merge the seakeeping and the manoeuvring problem; however, one must bear in mind the different approaches in the merging process, hence, the distinction made in the present work.

In addition to the methods mentioned above, experimental studies have been conducted to obtain the mean wave drift forces. The results have either been directly used for manoeuvring models, or have first been employed to tune theoretical prediction of wave loads for later use in manoeuvring models. Such approaches do not differ significantly from the methodologies already mentioned, e.g. Otzen and Simonsen (2012) used experimental studies to tune their seakeeping programme which was later used in their manoeuvring model, and Hirano et al. (1980) used the measured second order forces as input to the manoeuvring problem. However these studies differ in the estimation of the wave effects; the method used to incorporate them in manoeuvring models are similar to the two-time scale described in Skejic (2008). Hereby, experimental studies are not considered as an additional methodology for the simulation problem of manoeuvring in waves.

The convolution integral method is based on the application of the Cummins equations (Cummins, 1962) and the equivalence between the frequency domain characteristics and the time domain impulse response function as described by Ogilvie (1964). Hence, first the frequency domain coefficients are evaluated; subsequently they are transformed into their time domain pairs. To the authors’ knowledge, this methodology was first introduced in the manoeuvring problem in waves by Ankudinov (1983) where he used an equivalent form expressed by a set of ordinary differential equations. The same approach was later used by McCreight (1986). A more detailed analysis of the equivalence between the hydrodynamic forces while manoeuvring in calm water and wave loads was presented by Bailey et al. (1998), who introduced a convolution integral equation incorporating the simulated viscous influence by a ramp function. During the last years, the estimations of second order forces have been improved in the convolution method based on the estimation of the nonlinear wave loads associated to the restoring forces and Froude–Krylov forces acting over the exact wetted surface, see for instance Schoop-Zipfel and Abdel-Maksoud (2011).

The direct calculation of the hydrodynamics loads in the time domain also unifies both manoeuvring in calm water and seakeeping studies, similar as the convolution integral method. However, in such approach the transformation of the frequency coefficients are bypassed and the computations are conducted directly in time domain. Hence, the manoeuvring and the wave loads are evaluated every time step of the simulations. Examples of such an approach are the studies presented by Sutulo and Guedes Soares, 2006, Sutulo and Guedes Soares, 2008 and, more recently, the work of Subramanian (2012).

Following the third approach, the two-time scale method, a direct superposition of the wave effects in the manoeuvring force components is performed. The studies of seakeeping and of calm water manoeuvring are carried out separately, and then the results are exchanged for the next computation. The mean second order wave forces are transferred to the manoeuvring model while the kinematic parameters and incoming wave angle return to the seakeeping problem. The work of Hirano et al. (1980), although not theoretical, made use of such subdivision of the problem introducing directly the mean wave drift forces in the manoeuvring model. This method has been widely used during the last years by researchers such as Ueno et al. (2003), Yasukawa, 2006a, Yasukawa, 2006b, Skejic (2008) and Seo and Kim (2011).

Perhaps, one of the most promising approaches to understand the hydrodynamic problem of manoeuvring in waves, within a good level of detail, is by means of computational fluid dynamics (CFD). CFD has been enhanced significantly in recent years and numerous studies have been presented for steady problems such as resistance and stationary manoeuvres. Only recently, CFD studies have addressed the problem of non-steady conditions such as direct manoeuvring simulation in calm water (see Carrica et al. (2012), Carrica et al. (2016)) and tests in oblique waves (see Akimoto (2010)). It is obvious that manoeuvring in waves is a more time and resources demanding problem because of its complex nature. From the literature only the recent work of Mousaviraad et al. (2012) has been found addressing this problem. As mentioned by Skejic (2013), the progress of CFD studies for manoeuvring cases is clear; however, considering the required resources and level of complexity of the problem, CFD studies with an accurate prediction in real time are not yet foreseen in the near future, certainly not for the shallow water case.

Scale model experiments are the most suitable method to investigate the effect of wave forces on manoeuvring. By studying the obtained results, a better understanding of the wave effects can be gained.

One of the first publications on experimental studies of manoeuvring ships in waves is the work of Hirano et al. (1980), who studied the turning trajectory of a ro–ro ship in regular waves. Years later, the concern regarding ship safety and ocean environmental protection increased, hence promoting research on manoeuvring performance under wave forces. Motivated by the safety concerns regarding manoeuvring in waves, Hirayama and Kim (1994) performed zig-zag tests in irregular waves with a tanker model. Ueno et al. (2003) carried out a more extensive experimental programme including straight course, turning and stopping tests in regular waves with a VLCC ship model. Shortly after, Yasukawa, 2006a, Yasukawa, 2006b performed similar studies for straight course and turning motion in regular and irregular waves with a S-175 ship model. More recently, Yasukawa (2015) conducted zig-zag and turning motions tests in irregular waves using a KVLCC2 ship model.

All the previous works were conducted with free running model tests for deep water scenarios and all have been motivated by studying the manoeuvring characteristics in the presence of waves. In spite of the importance of such works, the question regarding how the wave forces are subjected to the ship is still quite unclear. To answer this question would require captive model tests during which the forces can be measured, and the effect of the water depth should be taken into account. Unfortunately, in the literature only the experimental studies conducted in deep water of Ueno et al. (2000), Yasukawa and Adnan (2006), and Sung et al. (2012) are found. Ueno et al. (2000) studied captive model tests in oblique and turning motion in regular waves for a VLCC ship model; Yasukawa and Adnan (2006) investigated the ship’s motions and induced steady drifting forces on an oblique moving ship in regular waves, and in Sung et al. (2012) a study, consisting of harmonic sway, harmonic yaw and stationary drift tests with a KCS ship model, was performed in following waves for one moderate forward speed. In Sung et al. (2012) no clear second order wave effects in the sway force and yaw moments were reported, and they suggested that tests at low speed would be able to capture such forces.

Manoeuvring in waves has been of high interest during the last decades. A large number of theoretical studies have been presented and further validated, mostly independently, with data obtained from experiments of manoeuvring in calm water and seakeeping, and not with experimental data including both cases. The limited number of available experiments is rather not surprising considering the large number of parameters involved, such as the ship’s speed, water depth (relevant for the shallow water problem), frequency and amplitude of the waves, as well as the ship’s responses, e.g. large pitch and heave motion would induce bottom touching in shallow water. Additionally to the motion restrictions in shallow water, an increase of the hydrodynamic forces in medium deep water (3>h/Tm>1.5) and shallow water (1.5>h/Tm>1.2) is observed. This is because the clearance decreases between the bottom and the keel hindering the flow around the hull. The above used water depth distinction is taken from PIANC (2012).

Taking into account the limitation regarding experimental studies on manoeuvring in waves, the present work attempts to provide experimental data for a manoeuvring ship in waves aiming at a better understanding of the wave induced phenomena on the manoeuvring problem. In contrast to other experimental studies, in the present work, the tested conditions fall within gentle manoeuvres in restricted areas, hence, simulating the ship entrance to ports. The experimental programme consists of captive model tests for two different water depths, and regular waves falling within the range of intermediate to shallow water waves. Tests were conducted with zero and low ( Fr=0.025) forward speed. The discussions of the results focus on the non-zero forward speed tests.

While performing manoeuvring in waves in a towing tank an important constraint is related to the interaction between the ship and the tank walls, which may affect the motion responses. The present paper will also discuss such problems.

In addition, it has been observed from the review that the majority of methods use the superposition principle in order to evaluate the manoeuvring performance in waves. Hence, such a methodology will be investigated in the present work by comparing the measured forces in waves against numerical results. It is understood that the main wave effect on manoeuvring are the mean drift forces, however, such a direct comparison is not always possible because of the order of magnitude of the forces involved. For such scenarios an indirect comparison is performed. In addition to the study of the forces, wherever possible, the motion responses will also be presented. The numerical computation of motions, first and mean wave drift forces and moments will be obtained from Hydrostar, a 3D boundary element method that incorporates the speed effects by employing the so-called “encounter-frequency” approximation based on the use of the Green function associated to the encounter frequency (see Bureau Veritas (2012)).

Section snippets

Theory

The present section describes the theoretical concepts used to solve the hydrodynamic problem related to wave-structure interaction. However, the following theoretical description is limited to monochromatic waves, first-order velocity potential, and second-order effects related to first-order quantities.

In the present study, Hydrostar has been used for comparison against the experimental results. Hence, the theoretical aspects described in the following subsections are related to the approach

Manoeuvring tests in regular waves

The literature revealed that experimental data concerning wave induced forces and motions on a manoeuvring ship are scarce, and even more uncommon if the shallow water problem is considered. On the contrary, a large amount of experimental studies can be found in the literature regarding tests in waves for seakeeping studies. For such conventional seakeeping test setups, the body can be free, partly or fully restrained depending on the wave effects of interest. For instance, to investigate the

Numerical model

As observed in the literature review, the methods to estimate the mean wave drift forces are mainly based on potential codes. Then, these forces and moments are introduced in their respective manoeuvring models. Hence, to test the validity of such methodologies, the experimental data is then compared with a 3D numerical package, Hydrostar, to investigate the superposition principle used in different studies. The theoretical description on how Hydrostar deals with the hydrodynamic problem can be

Conclusions

An experimental study of wave effects on a container ship model in forced motion tests in regular shallow water waves has been presented. Captive model tests in a towing tank were conducted in calm water and in regular waves with varying period and amplitude. Both stationary straight-line tests and harmonic yaw tests were performed. The wave reflection from the tank’s side walls restricts the parameter variation of captive model tests in a towing tank. It was found that disregarding tests based

Acknowledgements

The research described in this paper is performed in the frame of project WL_2013_47 (Scientific support for investigating the manoeuvring behaviour of ships in waves), granted to Ghent University by Flanders Hydraulics Research, Antwerp (Department of Mobility and Public Works, Flemish Government, Belgium).

For the calculations a Hydrostar licence was put at the main author’s disposal by Bureau Veritas through their Antwerp and Paris offices, which is highly appreciated.

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