Vessel Stern Research Articles

Launch and recovery of a work class ROV through wave zone in small offshore service vessel

The deployment of remotely operated underwater vehicle (ROV) from a small offshore service vessel (OSV) based on single point mooring system (SPMS) method is recently adopted in offshore renewable energy sector. However, the tension spike in wire, also known as snap load, often occurs when the ROV passes through the wave zone in launching and lifting operation of deployment. In this study, a practical numerical model for predicting wire tension during launch and recovery of ROV is developed and validated by wave flume test of a 1:10 scaled model. The numerical simulations reveal that the ROV deployment at vessel stern along with an appropriate reduction of horizontal distance from the hull are reliable safety strategies for reducing wire tension. By adopting the new deployment strategy, the annual operational capacity can be expanded by approximately 6% when the safe operational limit of ROV under a significant wave height of 1.25 m. Based on the comprehensive numerical simulation, the newly developed safe operating envelope provides a further guidance for onboard ROV operation in the O&M of offshore wind farms.

Study of the influence of the vessel stern shape on its hydrodynamic characteristics

Determining the main dimensions of the small vessels while designing is an important and difficult task due to the large number of mutually exclusive factors affecting the hydrodynamic characteristics of such vessels. The existing patterns for determining the length, width and draft of small vessels are quite general. This complicates their usage for the vessel of a specific design. In this case, it is necessary to check technical solutions, one of the ways that may be implied is the usage of computational fluid dynamics (CFD) methods. This paper presents the results of the numerical simulation of boat movement in the transient mode with Froude numbers up to 1.2. The process of preparation and numerical modeling of the aerodynamics of a composite wing in the NUMECA/FineMarineTM software package is described, the choice of the turbulence model and the parameters of the computational grid, including the resolution of the boundary layer, is justified. The study of the influence of three variants of the width and shape of the aft tip on the change in draft, trim angle, wetted surface and resistance of the boat is carried out. An increase in the width of the flat planing part of the aft tip has a favorable effect on the hydrodynamic characteristics of a small vessel, even when moving in transition mode at L/B from 3.5 to 3.99. At the same time, the use of the extension of the underwater part of the aft tip requires a more detailed study.

Numerical Study of the Effect of Angle and Location of the First and Second Steps on the Performance of Planing Hull and Their Optimization Using the Taguchi Statistical Method

Providing steps at the bottom of a high‐speed craft could decrease the resistance and enhance the longitudinal stability by dividing a single pressure hump into several ones and extending the hydrodynamic pressure towards aft of the vessel. In the present paper, the effects of the fore and aft steps’ locations and angles on double‐stepped hulls are investigated systematically via a numerical approach. The parent model investigated in this study is a model of a high‐speed recreational craft called Cougar, with a 1 : 5 scale. The three‐dimensional analysis of the model is conducted via the finite volume method, and the free surface is tracked using the volume of the fluid technique. The computational domain is discretized by the overset technique, and the calculations are validated via experimental data. Overall, 27 numerical tests designed by the Taguchi experiment design method are simulated, and the resistance, trim, and rise‐up values are calculated at 8 to 10 m/s speeds. The findings support the notion that the resistance increases by increasing the distance between the fore step and vessel stern. In addition, increasing the distance between the aft step and vessel stern and the increase of the angle of the fore step leads to reducing the vessel resistance. However, the results showed that finding a sensible relationship between the angle of the aft step and resistance requires more investigation. The findings revealed that the interactional effects between the angles of the aft and fore steps are the most considerable among all studied interactional effects. Among the all studied configurations, the lowest resistance was experienced by the model with an aft step at 25% of the vessel’s length to the stern, having an angle of 150°, and a fore step at 40% of the vessel’s length to the stern, having an angle of 180°.

Влияние капролоновых дейдвудных подшипников на рабочее состояние гребного вала

The paper considers the conditions for reliability of the ship rudder-propeller complex, whose operation depends on the status of the stern bearings of the ship shafting. Stern bearings are plain bearings located in the vessel's stern tube, they support the propeller shaft of the ship’s shafting. The stern tube is similar for all types of ships, inside which there are located the bearings and a sealing device that prevents ingress of outboard water inside the vessel. It is pointed out that the stern bearings made of caprolone have certain advantages over other traditionally used materials. The numerical values of the main parameters of caprolone stern bearings are presented on the basis of normative and regulatory documents. It is stated that physical and mechanical properties of caprolone in almost all respects are higher than those of other materials currently used for this purpose. Along with strength and high load capacity, caprolone has a low friction coefficient (0.04-0.08). To study the contact of the propeller shaft with the stern bearing there is considered a design diagram (a beam resting on an elastic base with a stiffness coefficient k). The stiffness coefficient of the elastic foundation characterizes the elastic and mechanical properties of the stern bearing. It is proved that caprolone used in stern bearings allows increasing the reliability of the ship’s shafting by full contact with the propeller shaft, reducing vibration and noise, resisting the erosion wear, and increasing safety

Sampling and onboard analytical methods for determining subnanomolar concentrations of zinc in seawater

AbstractZinc (Zn) is an essential micronutrient for bacteria and phytoplankton in the ocean. However, seawater samples for Zn measurements are highly prone to contamination due to the ubiquity of Zn in laboratories and research vessels. To identify a potential Zn contamination problem during a research cruise to the subarctic North Pacific, we compared three operational sampling approaches for trace Zn in seawater. Acid‐cleaned Niskin‐X samplers were: (1) deployed on a conductivity–temperature‐depth carousel multisampling system (CTD‐CMS) launched at the vessel's starboard side, (2) attached to a Kevlar wire lowered at the vessel's stern, and (3) attached to a Ti wire lowered from a starboard position. Zn contamination in the case of Kevlar wire hydrocasts was problematic for samples from ≤400 m because Zn is used as a sacrificial anode for the ship's propeller shaft and for the pipes of the seawater cooling system. In deep waters, consistent Zn concentrations were found using the three different sampling methods. Based on the clean seawater sampling with CTD‐CMS and the onboard analytical method, vertical profiles of dissolved Zn were then obtained. These profiles were compared with those of Si in the subarctic North Pacific. Both Zn and Si concentrations showed a clear west–east gradient in shallow waters.

IDENTIFICATION OF SHIP PROPELLER TORSIONAL VIBRATIONS

The article presents identification methods of ship propeller torsional vibrations in dimensional space and ways for their dumping. Propeller torsional vibrations are caused by static, dynamic and hydrodynamic unbalance of propeller itself and its operation in non-uniform velocity field of ship hull. The propeller working in uneven velocity field of the hull creates cyclic thrust forces, peripheral vibrations of shaft as well as flexural and torsional load of propeller shaft. Vibrations cause additional thrust load on shaft bearings and shaft bending moment. Vibrations of ship hull, particularly its stern section cause variation of pressure field created by propeller. Amplitude of particular parts of the hull comes from the action of cyclic forces created by working ship propeller. They can be very large. Therefore, the ways to reduce vibrations caused by running propeller are very important. Given in the article methods of vibration dumping calculations conduct to determination of vibration dumping coefficients. Calculation results are comparable to measuring results in spite of fact they were determined under assumption of different velocity fields of water flowing through propeller circle. Amplitudes of vibrations depend on number of propeller blades and pulsate with the basic frequency equal to product of the rotational speed and number of blades. Causes of such vibrations and additional load of vessel stern are described in the article. An attention was paid to assure the greatest possible uniformity of velocity field by adequate design of stern shape and accessories. The ways to reduce torsional vibrations of propeller by the appropriate selection of blades number and their accurately machining are also given in the article.

Does propeller turbulence affect the sink rate of baited hooks and their availability to seabirds in pelagic longline fisheries?

ABSTRACT Experiments were conducted on three pelagic longline fishing vessels on the effects of propeller and hull turbulence on baited hook sink rates. Baited hooks were deployed to land in the sea: (i) close to the vessel's stern in the centre line of the propeller; into the wake zones on both (ii) upswing and (iii) downswing sides of the propeller; and outboard of wake zones on vessel (iv) upswing and (v) downswing sides. Two branch line deployment methods – the ‘lead sinker first’ and ‘hook‐and‐sinker together’– were also assesssed. Branch line deployment method made no difference to the sink rates in each of the five bait landing positions. The sink rates within bait landing position varied among vessels as did the order of the sink profiles (fastest to slowest), indicating that results cannot be combined for all vessels. Within each of the three vessels, baited hooks in the centre position reached 3 m depth 16%, 19% and 30% slower than those in the next slowest position. Mean sink times in propeller upswing and downswing zones were virtually the same. Sink times to 3 m in these two positions and in the two positions outboard of vessel wakes varied by only 2 s (0.02 m s‐1). Gear sank fastest outboard of vessel wakes on the downswing side of the propeller, but the advantage in deploying to this area was minor. To reduce bait availability to seabirds, hooks should be set to avoid the area of maximum propeller upwelling astern of vessels (i.e. the centre position). The absence of strong effects with the other bait landing positions dictates that rigging position of bird scaring streamer lines takes priority over landing position, and baits be set to areas that provide the most effective coverage (both laterally and distance astern) by streamer lines. Copyright © 2013 John Wiley & Sons, Ltd.

Effect of line shooter and mainline tension on the sink rates of pelagic longlines and implications for seabird interactions

Abstract1. The likelihood that seabirds will be hooked and drowned in longline fisheries increases when baited hooks sink slowly. Fishermen target different fishing depths by setting the mainline through a line shooter, which controls the tension (or slackness) in the line. An experiment was conducted in Australia's pelagic longline fishery to test the hypothesis of no difference in sink rates of baited hooks attached to mainline set under varying degrees of tension.2. Mainline was set in three configurations typically used in the fishery: (a) surface set tight with no slackness astern; (b) surface set loose with 2 s of slack astern; and (c) deep set loose with 7 s of slack astern.3. Tension on the mainline had a powerful effect on sink rates. Baited hooks on branch lines attached to tight mainlines reached 2 m depth nearly twice as fast as those on the two loose mainline tensions, averaging 5.8 s (0.35 m s−1) compared with 9.9 s (0.20 m s−1) and 11.0 s (0.18 m s−1) for surface set loose and deep set loose tensions, respectively.4. The likely reason for the difference is propeller turbulence. Tight mainline entered the water aft of the area affected by turbulence whereas the two loose mainlines and the clip ends of branch lines were set directly into it about 1 m astern of the vessel. The turbulence presumably slowed the sink rates of baited hooks at the other end of the branch lines.5. The results suggest that mainline deployed with a line shooter (as in deep setting) into propeller turbulence at the vessel stern slows the sink rates of baited hooks, potentially increasing their availability to seabirds. Unless mainline can be set to avoid propeller turbulence the use of line shooters for deep setting should not be promoted as an effective deterrent to seabirds. Copyright © 2010 John Wiley & Sons, Ltd.

Effectiveness of fish waste management strategies in reducing seabird attendance at a trawl vessel

The risk of seabird bycatch in trawl fisheries is increased by high numbers of seabirds attending vessels to feed on fish waste discharged. We conducted an experimental test of whether mincing fish waste prior to its discharge from a factory trawler reduced the number of seabirds attending the vessel. The trial was conducted on a mid-water trawler targeting hoki ( Macruronus novaezelandiae) in New Zealand waters, and the experiment compared three treatments (1) discharging ‘unprocessed’ waste (fish offal and whole discards), (2) mincing all waste to a small particle size before discharge, or, (3) converting all waste to fishmeal and reducing discharge to sump water. The response to the experimental treatments was determined using seabird abundance within a 40 m-radius semi-circular area centred on the vessel stern. Mincing reduced the numbers of large albatrosses ( Diomedea spp.) feeding astern of the vessel, but had no significant effect on other groups of seabirds. In contrast, reducing discharge to sump water resulted in a significant reduction in numbers of all groups of seabirds. In particular, the abundance of the small albatross group (principally Thalassarche spp.), and some smaller procellarids (e.g. sooty shearwater, Puffinus griseus, and white-chinned petrel, Procellaria aequinoctialis), was reduced to less than five percent of the number that were within the sweep area when unprocessed discharge was released. While mincing significantly reduced the abundance of large albatrosses at the vessel stern, relatively small numbers of these birds attend trawl vessels in New Zealand waters and associated bycatch rates are low. In contrast, reducing the quantity of fish waste discharge by mealing resulted in reduced abundances of a wide range of seabirds at the vessel. Therefore, compared to mincing, we recommend fish waste retention as a more effective management strategy for reducing seabird bycatch.

A Brief History of Nautical Logs to A.D. 1800

The name log, to denote a device for measuring a ship's speed or the distance she has made through the water, appears to have come down to us from the Saxon log—‘a hewn trunk or branch of a tree’. Harboard amplifies this by remarking that the log is so named from the word ligen (Dutch liggen=to lie down) because it lies, as it were, immovable. It therefore seemed appropriate that the primitive contrivance for measuring a ship's speed—a wooden board attached to a line—should be called a log. The principle of this—the so-called common log to distinguish it from more sophisticated contrivances—is simple: a floating object cast from a ship tends to remain stationary after entering the sea. The line attached to the log-ship is allowed to render freely as the vessel sails away after the log is hove from the vessel's stern into the sea. A specified length of line which runs out in a measured interval of time or, what became more common practice, a measured length of line which runs out in a specified interval of time, is equivalent—at least approximately—to the distance travelled by the vessel in that interval.

The Deformation of Ship's Hull under Construction (No. 4)

In this report, three problems of hull deflection during construction will be dealt with. First the deflection of that part of the vessel's bottom which overhangs port & starboard of the concrete building berth will be discussed. Secondly the deformation of superstructure decks during the completion of work after launching will be discussed.The third and major topic of discussion will be the problem of the deformation of the vessel's bottom and inner bottom in way of machinery room in connection with the problem of propeller shaft centerline alignment and/or boss boring. In regard to this problem, the following findings will be shown : (1) The rate of rising of the stern is greatest during the earlier period of construction and decreases gradually as the work progresses. The amount of eccentricity of stern tube bossing when ploted against progress of work gives the “Sine” curve.(2) The athwartship deflection of the vessel's stern either to port or starboard is strongly influenced by seasonal, climatic conditions.(3) The fore & aft shrinkage of the stern part of vessel's hull is greatest during early stages of construction (as is the case of the stern rising). The rate of shrinkage of the main deck does not necessarily coincide that of the vessel's bottom.Finally, the shrinking force resulting from welding and heating will be discussed together with the result of some experimental model tests to confirm the fact that the fore & aft shrinkage of the vessel is caused to far greater degree by the welding on the ship's bottom and its vicinity than the welding on the main deck and above.