Sunday, 14 October 2018

IOT and Ships

Information technology is put to use in almost every activity that humans are concerned about, right from a short message costing a few cents to large ships costing millions. While, it is optimized and scaled, depending on the purpose. IT (information technology) has been widely accepted as an integral factor by mankind for a significantly long time and consequently, humanity has grown into a more advanced and stronger race at exponential rates. We are very much frequently amazed by the cutting-edge technologies that are put out to our service by this exceptional sector of data collection and computational sciences.

Courtesy : Google Images


One such innovative idea that has been here for almost a couple of decades but has recently evolved significantly and still is evolving to have become a buzzword among the technical communities is IOT or the Internet of things.

IOT has been widely accepted as a novel innovation everywhere.It is almost impossible to resist or stay away from its influence given its vast scope and infinite extending possibilities.

What is IOT?


As its name suggests it is connecting things to the internet, things like domestic appliances, electronic actuators, sensing devices, etc. to the internet via certain connectivity hardware. Once connected these devices can be controlled and monitored. The data they generate can be stored and used for big data analyses and research purposes.

Influence of IOT in the shipping industry


Just like any other industry, the maritime industry also extensively uses information services to collectively carry out various operations involved in efficiently maintaining the balance of marine transport and off-shore services. Thus, it is an undeniable fact that the shipping industry has plenty of room for a revolutionary innovation like IOT.

Introduction of IOT in ships will lead to the achievement of a certain level of automation in terms of the process control systems. Further, the shift of controls from the control room to the controller's mobile device will drastically reduce the space requirement for the control room while making the control interface easier and remotely accessible.

 Ships are relatively large floating structures that are in fact sometimes referred to as floating cities sailing in the vast blues. All the data that these ships would generate will be immense in quantitative as well as qualitative terms.


This data can be used in many ways like training optimization models, building autopilot systems and getting insights into the challenges involved in the practical implementation of complete autonomous vessels which in a broader sense has never been done before.

Threats


Given all these possibilities, there exists a few significant threats and dangers that need to be carefully studied, analyzed and countered before the application of IOT on ships.

Some of the serious concerns include DDoS (distributed denial of services), vulnerability towards intentional invasive cyber-attacks, self-generated malware issues and data theft.

As claimed by many cyber specialists there is no single permanent solution to cyber security problems. These systems need consistent and efficient monitoring to maintain secure connectivity and protect stored data.

A number of research-intensive studies are being carried out at many organizations to devise secure systems while ensuring user privacy.

It will not be late that we will witness IOT display its significance and carry maritime sector through decentralized connectivity revolution.

Article By- Zia Ur Rahman

Saturday, 6 October 2018

Decommissioning of Oil and Gas Installations


In a world of depleting fossil fuels, oil and natural gas are one of the most consumed and exploited natural resources. Averaging a consumption of 93 barrels per day and 120 Trillion cubic feet for Oil and natural gas respectively, they can be considered as the most important fuels that the mankind is using right now. The entire human race is focused on using these precious resources and is least bothered about where these resources actually come from or whether their supply meets the demand. With just 55 years or so for these resources to run out we are not even concerned or ready to curb or even put a leash on the staggering consumption rates. With oil wells being the major producer of both oil and natural gas in the world, these are an indispensable factor in the oil and gas industry. So just as how important is the installation and commissioning of these wells, its safe decommissioning is also crucial.

Oil and Gas Installations


The offshore oil and gas industry had its humble beginning in the Gulf of Mexico in the year 1947.It was built 43 miles south-west of Morgan City which gradually sloped to the Gulf of Mexico with a depth of just 18 feet. Vessels were required to aid the platform with supplies, equipment and crew. Though this was a small and humble beginning, it paved the way for an enormous number of platforms to be installed to cater to the ever-increasing need of fuel in the world. Slowly but steadily the number of such rigs and installations increased and reached staggering 389 platforms with the North Sea having the most of it -184. The platforms we see today have changed from what we had. New efficient platforms and equipment were developed and are further being developed for efficient extraction of oil and gas from these rigs.

Image Courtesy:Google Images

 Decommissioning of oil and gas installations


Decommissioning of a constructed rig is as important as its installation. The North sea has seen the most number of decommissioned installations in the past 40-50 years, with almost 1500 decommissioned wells.

The majority of structures in the area have a water depth of around 3 to 300 meters and often encounter huge waves of height exceeding 10 meters. In these conditions the removal or decommissioning process could be highly difficult to execute. A major concern in this process is to carefully select the method to remove the structure from the sea bed and the support availability to aid this process in remote locations.

Image Courtesy:Google Images


So what is the basic process involved?

There are a lot of steps involved when it comes to decommissioning of oil and gas installations :

  1.Project Analysis:

In this step the entire project is evaluated from both a financial and an environmental point of view. Every aspect of the project going to be decommissioned is evaluated and verified to obtain the maximum financial and economic gain.


   2.Permitting and Regulatory agreement

Like any other process this also goes under the scrutiny of a wide legal framework. Obtaining permissions for decommissioning of an oil rig can take up to 3 years . In this time interval evaluations and tests are done on models and simulations. There are many authorities which are directly involved in the legal framework including BOEMRE, US Fish and Wildlife Society, National Oceanic and Atmospheric Administration, Department of Transportation, Office of Pipeline Safety etc.


  3.Platform Preparation

The tanks, piping and processing equipment must be cleaned and the residual hydrocarbons must be removed. Platform equipment like pipes must be removed and proper reinforcement of the structure should be completed.

  4.Wall plugging and Abandonment

This process takes up the major funds in the entire process. This involves plugging of the entire structure to avoid the entrance of water and abandonment of the structure.

  5.Conductor removal

According to many organizations all platform equipment including conductor casing must be removed to at least 15 ft below the ocean floor to attain approval.

 6.Mobilization/Demobilization and Platform Removal

This process is very vital. First the topside is removed and taken into a barge. Various processes are used for this removal. Topside can be removed all in one piece, in groups of modules, in reverse order of installation or in small pieces.  Removing of the jacket is often very costly and is the second step. It can be removed in a single lift or as small pieces based on the size of the installations.

 7.Pipeline and Power Cable Decommissioning

The pipelines and power cables are to be removed if they are considered a hazard during the permit process. The pipelines are first flushed with water and then disconnected from the structure followed by filling them with seawater and then finally the open end is plugged and buried 3 m below the seabed and covered with concrete.

 8.Materials disposal and Site clearance

The platform materials can be reused and refurbished, scrapped and recycled or disposed of in specified landfills.


Reasons for increased decommissioning of installations.

Decommissioning in the oil and gas industry has seen a rapid increase due to a lot of reasons. Many a times in the current scenario running a platform or installation is deemed a loss for the owners. A few of the reasons are listed below.

1) Decline in oil prices

     The price of oil has been in a constant decline for the past few years. With the dollar staying strong compared to other currencies and with the Middle – Eastern countries ready to pump out oil overseas, the price of oil is not expected to rise at a rapid pace anytime soon. The demand of oil has increased due to the availability of different types of fuels which are more economical and has resulted in a situation where the supply has exceeded the demand.

  2) Increased capacity of vessels.

The vessels now that are being built and the one’s already commissioned and running are of higher capacities as that compared to that of the earlier days. With the availability of super tankers now a days the amount of oil that can be carried by a vessel has increased by a huge margin.


 3) Regulations

     With more and more regulations that have come with the installation and operations of a rig the number of people that are involved in this industry are going down. The organizations that are involved are making it more difficult for the players in the market to own and operate a fully functional rig.

Like any other specialized field, sophisticated equipment are required for the decommissioning of a platform. Various specialized equipment and vessels are now available in the market catering to this sector.

Manufacturers like Damen are coming up with equipment which specialize in decommissioning. 

These vessels specialize in 3 aspects of oil and gas decommissioning.
1)     Topside decommissioning and maintenance
2)     Offshore platform removal
3)     Subsea cleaning and removal 

Modules that can be attached to normal ships are also being developed which can aid the decommissioning process. This method is deemed to be highly cost effective. The little tweaks that are involved can reduce the cost of the project by a huge margin. 

The add-ons can include:
1)     ROV spreads
2)     Subsea Cranes
3)     Jacket removal equipment


In the case of vessels that are specialized in the oil and decommissioning projects, the lifting capacity is of great importance. The major vessel available now is the 382-meter-long Pioneering Spirit with a lifting capacity of 48,000 tonnes. She completed her first heavy lift by removing 13,500 tonnes 100 km off the coast of Norway.




Image Courtesy:Google Images

  Article By- Sunand Krishna

Wednesday, 26 September 2018


Case Study : Capsizing of Costa Concordia


It’s said that history often repeats itself . After 100 years from 1912 when the Titanic met its unfortunate fate a similar incident happened with a famous cruise ship making it second in the line of the most infamous shipwrecks.

The ship Costa Concordia was operated by the notable Italian cruise line Costa Conciere which was established  in 1854.Since the year 1947 when the company commenced passenger services it has established a good reputation over the years and ultimately became one of the largest cruise operators in the world. 

However the capsizing of one of its star cruise ships barely 7 years in service not only left a dent on the company’s reputation but also raised serious concerns over international cruising.

Concordia in its final position
(Source: Google Images)

The following article sheds light on  the reasons behind the capsizing as well as lists out the circumstances that led to the disaster.

The Accident


On the night of January 13 , 2012 Costa Concordia struck her port side on an underwater reef near the Italian island of Isola del Giglio .The ship immediately lost all propulsive power and soon after there was a complete blackout as water reached the electrical panels .The breach resulted in a 60 meter long gash in the ship’s hull.This led to the rapid flooding of the watertight compartments and ultimately led to its capsize.

It's difficult to comprehend the fact that such an incident can happen given the availability of advanced technology and instrumentation giving precise and detailed information of every possible circumstance a ship may face. From rough seas to mapping of the ocean floor to high speed winds ; all relevant data is available with a ship at regular intervals. The topography of any area in which the ship sails forth is already available on board a ship.

 Despite all this the ship Costa Concordia struck an underwater reef off the coast of the Italian Island Isola del Giglio and grounded ,finally resting on the rocks near the coast.

What went wrong that night?


 A big question arises as to why was the ship sailing so near the coast in the first place which ultimately led to one of the most horrifying disasters in international cruising.

Most cruise ships perform what is called as a Sail - By Maneuver. The ship takes a slight deviation from its official course to sail near an island just to give the passengers a unique glimpse of it and give a nod to the fellow sailors on land. This is a routine maneuver in cruise lining and has been practiced successfully across the world for years. The ship's deviation course is plotted in such a way so that it is at a minimum safe distance from the island to prevent any situation of grounding in shallow waters or due to the presence of rocks and reefs near the coast. Concordia had earlier also performed it for the same island in its past voyages .

 So what possibly went wrong this time endangering the lives of around 4000 people on board which would ultimately change the cruising industry forever.


Planned Route with Deviation. Red Line shows the route followed by Concordia 
                         on the day of disaster (Source: Wikipedia)


Concordia’s deviation course required it to stay at least 1500 ft away from the island. But as it turned out the ship landed up around 659 meters closer to the coast. This egregious error due to a series of miscommunications between the captain, first officers and the officer at helm is reflected later in the investigative report. 


Comparison with route for the same maneuver performed in 2011(Source: Wikipedia)


Because of the incorrect heading angles , the turning radius of the ship was much wider than it was supposed to be as per the chartered deviation ; ultimately bringing the ship in close proximity to the shore.

Path showing Collision and Grounding
(Source:Wikipedia)


By the time the captain realised the situation and started giving a series of orders for rudder turns it was too late as the ship was already in too close proximity and moving at a speed of 16 knots. The erroneous executions of order by the helmsman and delay in correcting them ultimately left Concordia no chance to pass safely.

Why couldn't the flagship liner maintain its stability?


So why did such a big and modern cruise liner could not maintain its stability and capsized within an hour of the initial impact. 

The 60 meter long gash in the ship's hull allowed seawater to rush inside the watertight compartments at a very fast pace , ultimately flooding 4 of the compartments.

 Whenever a compartment is flooded the buoyancy provided by that compartment is lost. To maintain its stability the ship 'sinks' more on to that side to achieve the required buoyant force to support its weight. As water keeps on flooding the buoyant force is progressively lost and the ship keeps sinking. The change in the transverse center of gravity due to the addition of water lists the ship.

The ship initially listed to the port side and after making a 180 degree turn it listed rapidly to the starboard side finally resting on the shoreline rocks at an 80 degree angle. Whether the ship was deliberately grounded by the captain to avoid complete sinkage in deep waters or it naturally drifted towards the coast due to the underwater currents and winds is still debatable. But the fact that Concordia partially sank resulted in less number of casualties than the case had it sunk completely. 


Timeline of the Disaster( Source : Wikipedia)


Aftermath


The disaster was declared a total loss for the ship which costed around 570 million dollars .The salvage operation alone costed a massive 1.2 billion dollars thus resulting in a total loss of around 2 billion for Carnival Cruises.The wreck was finally gutted and sold for scrap.
The starboard side of the ship was completely crushed under its own weight
(Source: Wikipedia)


Thus human error once again led to a disastrous ramification in the 21st Century just like it happened in the 20th Century with RMS Titanic.The sinking of Concordia pointed out some pretty gaping holes and incompetency of some of the crew members on board.In its annual report Carnival Cruises reflected the annual losses incurred due to the sinking of Concordia. Better safety procedures and robust techniques must be adopted to regain the trust of people willing to go on board cruises and enjoy cruising in the most spectacular way.


Article By - Shivansh Singh





Saturday, 15 September 2018

Relaxations  in Cabotage Law

Introduction

Cabotage traditionally refers to the transportation of goods within the country via the coastal routes by the transporters of the same nation, thereby restricting the foreign flagged transporters to perform these “chores” in that territory.  Cabotage law is a set of guidelines set by a nation to ensure and guarantee that the entitled ones reap the maximum benefits of waters of one’s own land. Following the historical trail, like most of the other countries of the world, India would not allow the foreign flagged ships to carry goods around until now; things have taken a turn with India relaxing the cabotage laws on May 21, 2018 with a Ministry of Shipping notification, in what is said to be a “game changing decision” by benefitting Indian ports and helping India become a transhipping hub.
The relaxations were long awaited. It has been put that one of the major causes the relaxations were given was that the Indian shipping companies could not provide appropriate services as and when required.  They are also unlikely to have adequate number of vessels to cater to the ever-growing demand of the domestic industries. Water transportation being the cheapest, such a situation could not be allowed to be the Achilles heel.

Positive Aspects

 The relaxations will double up its benefits by forming conducive circumstances to help India become a major transhipment hub.  India can now attract cargo loaded on vessels that are originated or destined to foreign ports that can drop cargo at different Indian ports en-route.  More number of vessels would ensure less delay in cargo transportation. Indian ports would be more busy and gain some extra revenue.
 The freight rates would descent consequently. The persisting problem of empty containers being accumulated on the ports due to an imbalance between imports and exports would also be addressed. The foreign flagged vessels would be allowed to reposition these containers from the ports that have extra, unused containers to the ports facing a crunch of containers as opposed to the earlier cabotage law.

Requirement of a  Level Playing Field

While we look at what the relaxations in the cabotage law have to offer to the shipping industry in India and how the foreign players could change the existing scenario of the industry, emphasis must be laid on providing a level playing field to the Indian flagged ships that have invested over 68000 crores INR based on the existing policy. The Asian countries have relatively relaxed the law more than other part of the world. The relaxations in the cabotage law have already been declared as a “retrograde move” by the Indian National Shipowners’ CEO Mr. Arun Devli. The relaxations might also result in the loss of “Right of First Refusal” to the Indian ship owners.  The point is to put the Indian flagged ships at a level equivalent to the foreign flagged ships who have not relaxed cabotage laws at their land but would be enjoying the relaxations here at India ;  being benefitted doubly they could and would cite lower freight rates. Indian ship owners would not be able to match their rates and ultimately, lose the contract.  The difference could be as humongous as 41%, according to a 2015 data. With globalisation it has become integral for the government to protect its subjects not by eliminating foreign competitions but by pitting both the groups at a level playing field.


Creating a Level Playing Field

One way out could be taking away the burden of hefty taxes from the Indian ship owners; taxation is very consuming in India for the mariners. Other way out, the traditional way, higher taxation rates could be levied so as to create evenness as well as a source of income to India. The reverse charging scheme could be beneficial as well where the foreign flagged vessels were asked to pay service tax based on freight rates and commercial value of their goods at a rate of 15% whereas domestic vessels were somewhat relaxed (at a rate of 4.5%)    The Indian Controlled Tonnage scheme is an Indian ship owner friendly step but only when coupled with the Right of First Refusal. According to this scheme, Indian ship owners are allowed to flag their ships outside India if half of their crew are Indians. The foreign ship owners could also be asked to register their ships in India in order to operate here and pay taxes, which is the National Fleet policy, and enter a level playing field.

Conclusion

The overall impact of the relaxations in the cabotage law would be out in some time for everyone to critically analyse and assess the results. In future, it could as well lead to Indian ship owners  getting their vessels flagged at a foreign land.  If the government manages to create a level playing field for the Indian ship owners, who have been growing their fleet progressively, it could benefit the shipping industry to prosper as well as the end customers who would enjoy a cut in delivery cost and time. The ports will certainly attract more traffic and India is set to become a major transhipment hub.  The rail and road transportation will be relaxed and their rates may descend as well.
India has asked for a global exposure for its cargo movement and to improve the water transportation while other countries still haven’t relaxed their cabotage laws. It is up to it to ensure that the local industry does not succumb to the competition. In the 21st century, sooner or later, relaxations have to be made if the supply portion lags in delivering its service and India has put out a difficult yet bold step; the fruits of the seeds sowed....calculated yet awaited.

Article By- Rajiv Ratna Jha

Monday, 10 September 2018

Advanced Marine Vehicles



Many of us are familiar with the principle behind aircrafts - The Bernoulli's Principle. The difference in pressures above and below the aerofoil section of the wing creates lift . But I doubt whether many are apprised of the fact  that the same principle has been applied to ships also over the years! In fact some of the fastest warships in history were able to achieve such speeds by making some specialized sections attached to their hulls "fly" inside the water just like the wings of an aircraft.Racing fascinates us in every medium be it be on land , air or water. Ever wondered what makes the racing boats capable enough to cross speeds of 300km/hr?. Read the article to gain valuable insight about more such developments in making of Advanced Marine Vehicles.




The ocean environment is dynamic in nature and the conventional displacement hull forms are not best suited for all circumstances. Many new hull forms have been developed over the years to serve different purposes. 


High Performance Mono Hulls

WHAT IS A MONO HULL?

A Monohull is basically a boat or a ship with a single hull making it the most extremely used, efficient, versatile hull up to a Froude no. of (1).
Most high speed small monohulls have recently been converted to hard chine structures. They are now provided with greater beam and reduced length, thus giving improved performance in calm water and leading to high vertical accelerations in a seaway. Their ride has been improved by using higher deadrise angles leading to a deep V form.
Ø  For getting a higher Froude no. we need to lift the hull out of the water, by planing the hull.


Ø  Destriero and Jupiter are the names of two famous planing monohulls. Planing is the method of operation for a naval vessel in which its weight is predominantly supported by hydrodynamic lift, instead of hydrostatic lift (buoyancy).
Ø  We can make a hull go faster by making it extremely slender and using a very narrow beam. But if we take it to extreme, it results in an unstable ship, hence some sort of change has to be done to get stability.
Ø  Trimarans belong to a class of vessels properly called Stabilized Monohulls. They are characterized by the extreme slenderness of the main hull.

CATAMARANS

catamaran is a multi-hulled watercraft which has two parallel hulls of equal size. It is a geometrically-stable craft, which derives its stability from its wide beam, instead from a ballasted keel but from a monohulled sailboat. “Catamaran” is abstracted from a Tamil word "Katt-umaram" which basically means logs tied together.
Catamarans typically have less hull volume, higher displacement, and shallower draft (draught) than monohulls of comparable length. It provides large upper deck areas for passenger facilities in ferries for helicopter operations.
The hull separation in a catamaran is about 1.25 times the beam of each hull which helps in its stability and helps to take the manoeuvrability of these vessels to a greater extent. Catamarans are available in all sizes from small (sailing or rowing vessels) to large (naval ships and cargo ferries). 

WAVE PIERCING CATAMARAN

Catamarans have faced many problems in the past mainly related to ride facilities. So as to make the ride quality smooth, “PHILIP CHRISTIAN HERCUS” the famous Australian naval architect came up with the wave piercing hull form.This new hull form design takes the help of a narrow protruding beam piercing the waves instead of rising above one another.
Incat Tasmania ,is an international manufacturer of  high speed crafts, mainly wave piercing catamarans found by “BOB CLIFFORD” and “PHILIP HERCUS”. The company excels in building large vessels using aluminium as a construction material, wave piercing and water-jet propulsion technology. The vessels have been constructed up to a length of 112 metres, with a size of 10,800 Gross Tons and having  cruising speeds up to 58 knots (107 km/h).
Image courtesy: https://photos.marinetraffic.com/ais/showphoto.aspx?mmsi=770576272
Built at Incat's shipyards in Hobart, Tasmania, “FRANCISCO”, this is the world's fastest catamaran ship. With a cargo of over 1,000 passengers and 150 cars, the 99-meter catamaran can travel at speeds of up to 58 knots or 107 km per hour.

HYDROFOILS

The term "hydrofoil" may be sub-divided into two meanings:

> A hydrofoil is a foil which can operate in water. They are same in appearance and purpose when related to aerofoils.
> The term "hydrofoil" is often used to refer the floating vessels that use  the hydrofoil technology.

The term "hydrofoil" is commonly referred to the wing-like structure which is clamped onto the struts below the hull, or across the keels of a catamaran in a variety of boats, which helps to lift the boat out of the water during forward motion, thus resulting in reducing the drag force on hull. Hydrofoils can be artificial, for example the rudder or keel of a boat, the diving planes on a submarine, a surfboard fin. As soon as a hydrofoil-equipped marine craft increases its speed, the hydrofoil elements below the hull develop or produce enough lift force to raise the hull up, out of the water. This results in a great reduction in drag force of the hull, along with a corresponding increase in speed and efficiency in operation with respect to fuel consumption.

Image courtesy: https://upload.wikimedia.org/wikipedia/commons/thumb/5/56/Hydrofoil_types.svg/450px-Hydrofoil_types.svg.png

SURFACE EFFECT SHIP

Surface Effect Ship (SES) or Sidewall Hovercraft is a naval vessel which comprises of an air cushion or pillow, just like a hovercraft, and also twin hulls, as a catamaran. When the air cushion is in practice, a small part of the twin hulls lie in the water. When the air cushion is turned off, the full weight of the vessel is balanced by the buoyancy of the hull pair.
The “SES” has acquired two advantages over a normal hovercraft in case of open sea operation. It encounters more resistance when it slips sideways and it operates in air or sea, and it can use water jets for propulsion since the inlet nozzles are always covered by water.
Surface effect ships are a special type of vessels which are a combination of hovercrafts and catamarans.  They are so designed that they can use both air cushion and conventional propulsion as and when required. This helps them to attain a greater speed on sea water. The main advantage of the surface effect ships is that they retain their stability in any kind of water conditions on the basis of being equipped with both – dual hulls and air springs. The maximum speed that can be achieved is around 80 knots.


Video Courtesy : YouTube

ACVs or HOVERCRAFT

The ACV or Air Cushion Vehicle is a vessel, which is capable of moving in land, sea, snow, mud or other surfaces. It does not have the side hulls of the SES or catamaran. It has got an air cushion like structure, which makes it an amphibious watercraft. It allows very low drag force, thus ensuring speeds to establish high Froude Number. (Froude Number=4)
ACVs are generally noisy, making it a bit uncomfortable, and mechanically complex, but they do possess unique features, such as the ability to fly up over the beach. Large hovercraft have successfully served on the English Channel for over 25 years.
According to my knowledge, I have seen some written materials which state that the hovercraft was also used for airport‐to‐airport service across San Francisco Bay.
Imagecourtesy: https://slideplayer.com/slide/4311856/14/images/32

WING IN GROUND-EFFECT(WIG)

When a normal aerofoil is operationally active in a region close to the ground, its normal pressure distribution is disturbed. Pressure tends to develop to a higher level under the wing and adds to the normal dynamic lift of the aerofoil. This enhanced lift force is popularly known as the ground effect. Wing in Ground-effect (WIG) aircraft, can be referred as "flying boats" made in purpose to operate just above the wave crests in order to avoid occasional water contact during flight. WIG aircraft possess one or more wings which possess three orders of magnitude larger than the foils of hydrofoil craft. When a WIG aircraft has accelerated to a sufficient high velocity through the water,then the aerodynamic lift generated by the wings helps to lift the aircraft entirely out of the water. By remaining close to the water's surface, WIG aircraft faces significantly less resistance than it would face at higher altitudes because of the aerodynamic lift.
Imagecourtesy:https://www.maritimeexecutive.com/media/images/article/Photos/Vessels_Small/Cropped/Wingship%2016x9.jpg

THE SUSTENSION TRIANGLE

The "sustention triangle" is a very popularly used device for categorizing different ship types. It is an imaginary or conceptual device used for the better understanding of the floatation process of boat. Traditional or normal ships float because they are immersed in water and get an up-thrust or buoyant force due to Archimedes' principle. It occupies the lower left corner of the triangle. There are other ways to hold ships up. As in case of hovercraft, for example, where the ship is lifted on a bubble of air.
Hydrofoils and hydroplanes are both dynamic lift marine vessels. It thus occupies the topmost corner of the triangle.
Some vessels occupy the intermediate positions on the edges of the triangle. For example, an SES (mixture of catamaran and hovercraft).

THE SUSTENSION CUBE

It is also an imaginary, conceptual “design space” which consist of mutually orthogonal axes. The result of this consideration leads directly to the sustention cube.
The Contents of the Sustention Cube
The corners are defined by combing the following pairs, to produce eight points:
> Passive or Active
> Hydro or Aero
> Static or Dynamic

Thus the eight corners are:
> Passive Hydrostatics- Conventional ships and barges.
> Passive Hydrodynamics- Hydrofoils.
> Passive Aerostatics- Blimps
> Passive Aerodynamics- WIGs
> Active Hydrostatics- Hovercraft
> Active Hydrodynamics
> Active Aerostatics
> Active Aerodynamics


Article By- Suryadip Ghosh

Monday, 3 September 2018

Cavitation - The constraining factor

Transportation across the oceans has attracted the attention of mankind since the dawn of history. What started with a simple bundle of logs has evolved into a huge sophisticated vessel of today. Earlier, wind (sails) and human power (by oars) were the only means of propulsion but since the advent of 19th century Mechanical Propulsion began to be used in ships. Most of the modern ships have quite similar propulsion arrangements consisting of a wheel rotating about a transverse axis with radial blades or paddles to impart astern momentum to the water around, giving it forward thrust . 




As the size of vessels grew, the thrust required from these screw propellers also increased. Consequently, the rpm of these propellers escalated. When the  propellers’ diameter is restricted and they are required to produce very large thrusts at high rpm’s  the propellers are likely to experience a phenomenon called “Cavitation”.

Cause 

Water begins to vaporise when its vapour pressure equalises the saturation vapour pressure. The vapour pressure of water is 1.704 kN/m^2 at 15 °celsius and 101.325 kN/m^2 (atmospheric pressure) at 100 °celsius. If the pressure inside water falls to the vapour pressure the water at that point begins to vaporise forming voids filled with water vapour. The formation of such vapour filled cavities is termed as cavitation.Cavitation also depends upon the purity of water. The impurities in sea water such as suspended solid particles and dissolved gases act as nuclei for the formation of cavities.Vapour cavities are formed locally but when pressure increases clear of this region the vapour cavities collapse or “implode”. This implosion occurs with a high impact force with stresses nearing 2800 N/mm^2. 


A propeller generates thrust by creating a pressure difference between the face and back of its blades. The pressure on the back of the blade falls below the ambient pressure and the pressure on the face rises above it. Hence, the back face of the blade becomes prone to cavitation.




Types of Propeller Cavitation

The type of cavitation occurring has two bases for classification :

   1.Classification on the basis of the region where cavitation occurs:
  • Tip Cavitation
  • Root Cavitation
  • Boss or Hub Cavitation
  • Leading Edge Cavitation
  • Trailing Edge Cavitation
  • Face Cavitation
  • Back Cavitation
    2.Classification on the basis of the nature of the cavities or appearance:
  • Sheet cavitation
  • Spot cavitation
  • Streak cavitation
  • Cloud cavitation
  • Bubble cavitation
  • Vortex cavitation
Cavitation exhibited in a cavitation tunnel in a stroboscopic arrangement
Hub Vortex Cavitation
Leading  Edge Cavitation
Tip Vortex Cavitation

Effects 


  1. Cavitation in marine propellers results in the increase of blade rpm without any appreciable increase in speed of the ship hence reducing the efficiency and thrust of the propeller. 
  2. When the bubbles implode they generate very strong local shock waves in water which generate a lot of noise and do mechanical erosion of the blades in form of pitting. The frequency of collapsing is generally around 15,000 Hz to 20,000 Hz. The collision intensity depends upon a number of factors-: 
  • Life span of cavitating bubble which is about three milliseconds which makes the event rapid. The faster the surrounding water collides, the greater the energy it possesses.
  • The size of the cavitating vapour bubble. Generally, the vapour bubble formed by cavitation at 68° Fahrenheit is about 35 times larger than the one produced at 212° Fahrenheit. A larger bubble corresponds to a greater amount of water colliding.
  • The quickness and amount of water together represent the total kinetic energy with which the imploding bubble attacks the metal.
  • It also accelerates corrosion in blades due to ‘Cavitation stain’. Cavitation Stain is a phenomenon where the oxidised layer that protects the metal is removed by cavitating bubbles hence making the metal vulnerable to corrosion. Corrosion damage is visually detectable and can eventually destroy the propeller.  
  • In the given figure a series of 18 shots show progressive events of flattening of a spherical bubble and finally imploding with the release of enormous energy.
Series of 18 shots show progressive events of cavitation

Pitting Due to Cavitation
Propeller Pitting due to cavitation

Prevention of Cavitation

Due to the detrimental effects of cavitation propellers are designed so as to restrict its level to a value so that its effects are negligible. It is achieved by following three methods:

  • Increasing the cavitation number.
  • Decreasing the loading on the propeller
  • Designing the propeller for uniform loading according to Keller criteria, which is related to the number of blades of the propeller, diameter, thrust, and the depth of shaft.

Supercavitation and Supercavitating Propellers


Generally it has been observed that it is difficult to avoid cavitation completely in high speed crafts where the propeller rpm crosses 1000 rpm. So instead of avoiding it some propellers are being designed such that they use this property to create a sheet of vapour around the propeller blades which increases the effective diameter of the propeller and thereby increases the thrust as well as the efficiency of the propeller. Cavitation is not a problem for these propellers as they are designed to work in cavitating zone. Although before achieving the design speed (when cavitation occurs) these types of propellers demonstrate very low efficiency due to their non-streamlined (wedge shaped) sections.

Super cavitating propeller with wedge shaped training edges


Supercavitation is even used for torpedoes and other high speed objects in marine environment. The vapour sheet formed due to cavitation corresponding to the high speed of torpedo encompasses the object thereby greatly reducing the drag and hence enabling higher speeds. They are being developed by defence forces in fast supercavitating torpedoes and war projectiles. For instance, the soviet torpedo named VA-111 Shkval, German torpedo Barracuda, weapons like SPP-1 underwater pistol, APS amphibious rifle.


VA-111 Shkval using the phenomenon of cavitation



Thus cavitation though has some serious detrimental effects, it can be used as a beneficial tool in particular cases. It is one of the most important factors determining the diameter and rpm of the propeller blade by placing major constraints. An optimum design and balance needs to be maintained between the blade diameter and rpm to obviate the risk of cavitation.


Article By - Soumya Sameer