Saturday, 31 October 2015

Ballast Free Ship Design

INTRODUCTION:


Ballast water is fresh or seawater, held in tanks and cargo holds of ships to increase stability and manoeuvrability during transit. Ballast water is essential to the safe and efficient operation of modern ships, providing balance and stability to un-laden ships (often returning empty during return voyages) as well as loaded ships. Its superb operational advantages, however come at a cost. 

 It poses serious ecological and health threats due to transfer of a multitude of marine species (non- native species) into an altogether different host environment containing different native species. 

Didn’t quite get that?

e.g.- The ballast water is taken from coastal port areas(source point) and transported inside the ship to the next port of call(destination point) where it may be discharged, along with all the surviving organisms. This way, the ballast water may introduce organisms into the port of discharge which do not naturally belong there. These introduced species are called exotic species. Populations of exotic species may grow very quickly in the absence of natural predators. In this case they are called ‘invasive’. However, most species can’t survive in the new surroundings – temperature, salinity etc. (Remember, Survival of the fittest?) being less than optimal. Thus only a few species are ‘successful invaders’, however those that do survive, establish a population and have the potential to cause major harm! Aquatic invasions are considered the second greatest threat to global bio-diversity after habitat loss, are virtually irreversible, and increase in severity over time. If that is the case, then one can’t even imagine the damage, caused by transfer of 3 to 5 billion tons of ballast water each year.
  

SOLUTIONS:


NO BALLAST SHIP (NOBS) CONCEPTS:


There are mainly three projects in which the concept of a ship with zero ballast water has been developed:
  • Delft University of Technology (DUT)-‘Monomaran Hull’.
  • Det Norske Veritas(DNV)-‘Volume Cargo Ship’
  • Daewoo Shipbuilding and Marine Engineering(DSME)-‘ Solid Ballast Ship’
  1. ‘The Monomaran Hull’ – An unloaded rolling ship (without ballast water) requires adequate stability. DUT proposed a monomaran hull by adopting a catamaran shape to the base of the broad single hull.
  2. ‘Volume Cargo Ship’ - DNV proposed a design similar to DUT but with a trimaran hull shape thus imparting high level of stability.
  3. ‘Solid ballast ship’ – In this case, the ballast water is replaced by 25 tonne Solid ballast in standard containers. However the application of this method is limited to container ships only. The hull form (size) remains the same.
Another solution to this problem is the Yokohama buoyancy control compartment concept, which converts conventional ballast tanks into a series of buoyancy control compartments. 






(Fig. 1: Comparison between conventional ballasting and ballast free ship design. (Courtesy: www.nsdrc.com/Publications-"Development of a ballast free ship design" by Avinash Godey, Prof. S.C. Misra, Prof. O.P. Sha)


Each compartment is flooded to provide adequate draught in the unloaded condition then continuously flushed at normal voyage speeds to ensure efficient exchange without the need for pumps. Each compartment is fitted with intake and outlet valves that are optimally designed and positioned for each compartment so as to maximize its flushing rate during normal voyage speeds.

Although ballast water treatment is an effective way of tackling ballast water issues.
The details of it will not be discussed in this article.
   

The Ballast Free Ship (BFS):


When a ship moves forward it produces regions of increased water pressure near its bow and reduced water pressure at its stern. This pressure differential is utilised to drive water through a set of these below-waterline corridor (trunks) without the need for pumps. Although this leads to slight increase in the resistance of the ship, the discharge of the trunk flow into the upper half of the propeller disc tends to smooth out the inflow to the propeller, allowing the propeller to operate at higher propeller efficiency and thus compensate for the added resistance to some extent.


Fig.2:Rendition of the concept behind ballast free ship design (Copyright: Learn Ship Design)


Fig.2:Rendition of the buoyancy control compartments with provisions for flushing water at normal voyage speeds. (Copyright: Learn Ship Design)

However, it also has to overcome some other challenges like:

1.) Loss of cargo carrying capacity- due to ballast water volume restraint. As it’s quite difficult to sustain the cargo carrying capacity and also the same ballast water volume

2.) Loss of ship strength- There would be a total redesign of the double bottom. As the conventional transverse framing will create difficulty for ballast water to flow through the tunnels. Hence this elimination will enhance the flow of ballast water at the cost of ship’s strength.
Classification societies might not permit the elimination of all of these frames. Moreover, watertight trunk boundaries will be required at transverse locations. Longitudinal stiffeners could be replaced with sandwich panels, thereby improving the flow. Thus compensating for the loss of strength.

3.) Increase in ship’s resistance- due to disturbance from discharge of ballast water into the flow around the propeller – The introduction of a plenum at the bow and stern of the ship, as well as the location of the plenums will affect the resistance of the ship, increasing fuel consumption.

Also, the increased ballast water flow velocity at discharge location will increase resistance as shown experimentally

Equipped with such technology, we can hope to minimize our environmental footprint to the greatest possible extent in the different spheres of conflict with the marine ecosystem.LSD

Article By: Vishal Kumar Jha

Saturday, 19 September 2015

An Interview with Dr. Jan Emblemsvåg

Jan Emblemsvag
(Image Courtesy: www.emblemsvag.com)
"The dichotomy of theory versus practice is an artificial one – all practice is based on theory whereas not all theory is based on practice, which is why this dichotomy arose. The difference between modern approaches to leadership and management and other approaches lies in their relation to reality. Modern approaches are fact-based and driven by reality, and so am I." says a note on his website

Jan Emblemsvåg is the SVP of Ship Design and Systems at Rolls-Royce Marine. The acclaimed corporation which has delivered the most ship designs over the years for the world offshore market, major designs in fishing vessel technology and merchant vessels. The corporation has also ventured into special purpose vessels and ship conversion projects.

In this interview with Learn Ship Design, we get to know about Rolls Royce Marine and it's operations. He speaks about Managerial challenges, his work on the Life Cycle Costing Approach, keeping pace with changing trends and what makes Rolls Royce Marine a leader in it's game.

Rolls Royce Marine seems to provide top notch solutions, be it with regard to pure raw power of drill-ships or the demanding precision of research vessels. But, it also amazes to witness the prowess in mainstream application of alternate fuels and the proposed mass automation (drone ships) in ship design. What drives this immense initiative? Please tell us about your experience.

Rolls-Royce Marine (RRM) has a large network in the market so we quickly hear about new ideas, new needs or new trends in general.  This, in combination with highly skilled employees, we have a very good breeding ground for innovation.  Why there are so many, is a good question. It is probably a mix of a willingness to try out new things and the fact that RRM indeed has very wide product portfolio so that there is a large opportunity for innovation.

Would you like to comment on how your work at Rolls Royce Marine and the unique challenges faced in the shipbuilding industry is honing your managerial skills in its own unique way?

The shipbuilding industry, for which we deliver ship design and equipment, is a volatile industry with focus on personal relations (due to the risk level – do you trust your business partner to deliver?) and complex projects with significant risks. RRM is a wholly owned subsidiary of Rolls-Royce plc which is traded at the Financial Times index in London. Stock markets typically want steady growth etc. This creates a unique mixture of apparent contradictions, which I personally believe is a great opportunity of innovations.  As a manager I must therefore try to handle a very large variety of issues – something I find very challenging, interesting and stimulating personally.  My level of performance must someone else talk about. 

Can you give our readers a brief jest about the Life-cycle costing approach and the Monte Carlo method and how we can use them in the maritime industry?

Activity-Based Life-Cycle Costing builds on Activity-Based Costing (ABC) and Life-Cycle Costing (LCC) and it uses Monte Carlo methods to power the approach, so to speak.  From ABC we get the focus on process, correct cost assignment, correct handling of overhead costs and more, from LCC we get the life-cycle perspective and the importance of handling risk and uncertainty and the Monte Carlo methods allow us to make huge models and realistically model risk and uncertainty as well as tracing key success factors.  The approach can be used for all kind of applications to calculate costs, profitability and so on.  Concrete examples can be to calculate the profitability of a contract for a ship owner, use the model and tie it to design changes so that design can be improved before they are built.  This can concern ships, oil rigs and any asset expensive enough for the cost of creating such a model.  The most important aspect of what it can be used for, however, is the availability of data.  This said, the Monte Carlo methods reduce the need for accurate data as long as the data is somewhat consistent. 

What are the main aspects one should keep in mind while evaluating the profitability of a ship/offshore design?

The single most important element in this industry is risk due to its volatility, and therefore flexibility/robustness is critical in the design particularly for offshore- and special purpose vessels.  The risk element is the reason why personal relations are so important – ship owners must feel they can trust the shipyard, the designer and the equipment maker.  There is too much at stake – this is critical the more expensive the assets. 

Given a possibility in the future, how would Rolls Royce Marine respond to providing solutions for a cruise industry linking India, knowing its untapped potential and exciting challenges?

We have had projects in India for several decades so it is a very interesting market.  With my experience with the maritime sector in India, however, cruise vessels do not seem to be a wise place to start.  We are developing designs that are medium spec’ed and easier to build while maintaining many of the quality hallmarks.  We are happy to assist Indian shipyards building UT- and NVC designs. 

While Rolls Royce Marine has been doing reasonably well like we said earlier, there is still a lot of potential remaining to be tapped. Will it be able to match the prowess of the aerospace division in the coming years?


The two markets are fundamentally different.  Aerospace is an industry with very high entry costs so that once you are established you are among very few.  This makes the industry easier to be big in, then in the marine industry where entry costs are much lower and margins are under heavy pressure these days due to the low oil price.  However, we have a lot of untapped potential but matching aerospace will probably be hard.  

Ship design is still transitioning from a rule based design methodology to risk based design methods. How far has your organization been able to adopt the factors of risk assessment into the process of ship design?

We use it to some extent, but the adoption rate varies considerably except where it is mandated by class for which we fully comply, of course. Class notations are vital in our business. 

On a different note, please tell us about your time with students of Management Science. Especially about the skills you deem are vital among management personnel in the field of ship design.

Irrespective of industry, I think an important trait of a good manager is the willingness to always learn something new, constantly reading and updating himself so that he does not become outdated and falls behind. Another important trait is the ability to use facts to solve problems instead if going with the guts all the time. Sure, gut feeling will be an element in many cases, but it has to be aided by facts.  However, what must never be underestimated is the ability to handle people.  Brilliance in subject-matter, but lack of people skills, will imply that this person is suitable for technical work and not management.  Therefore, I look for students that are fact-based and willing to question things and learn new things.  Good humour at own expense and humility are also important traits. Leadership potential is severely restricted in people that constantly have to prove how smart they are because I know from experience that such people will fail as leaders. 

Last question to you, the questions for this interview have been framed by eager students and budding engineers in the field of Naval Architecture, who have looked forward to interacting with you through this interview. What message do you have for them?

I hope you all find something useful in my answers, but more importantly; keep on learning the rest of your lives about management, philosophy and other topics NOT related to engineering or naval architecture– then you can contribute more widely as well and build your career.  

To know more about Dr. Emblemsvåg and his work, you can visit his website:  www.emblemsvag.com

The picture used above does not belong to LSD, and full credit for the same goes to the respective owners. 

Sunday, 6 September 2015

Types of Offshore Structures

Last time, we were introduced to the design and operational considerations of offshore structures. Now, we will take look at the different types in existence today. Before we go into that, take a recap of the previous article here. Also, you might want to see how fierce the environmental conditions can be in this video given below:



Broadly, offshore structures maybe of two types:

  1. Floating - These offshore platforms are floating in free surface and are movable from one location to another. Examples include semi-submersibles, SPAR platforms, Floating-leg platforms, drill-ships, FPSOs(Floating Production, Storage and Offloading systems)
  2. Fixed Platforms: They are immobile and fixed permanently to one place. These sorts of offshore units have greater strength and relative durability as compared to floating ones. These are operationally irreversible as once installed they cannot be relocated . Examples are Concrete/Gravity Platforms, Jacket Platforms,Tension-leg platforms(TLPs), Jack-Up Rigs etc. 
Fig.1:Image of  a deep-sea offshore installation (Courtesy: www.nyborgfan.com/)
Surveillance of the sea-bed and that of the underlying rocks helps in the development of the strategic sites for the optimum extraction of the resources. Furthermore, the oceanographic and climatic conditions around that region are assessed. Then the topographic conditions pertaining to that particular region along with the critical risk factors probable to the given sea-state gives the outline for the apt installation for the site. The offshore structures maybe mobilized from one site to another, may be erected after compilation of the raw materials and parts or maybe made from scratch at the purported site itself depending on its nature. 
As these offshore structures are high-budget, heavy-duty and long-term projects, umpteen care is taken to ensure its livability in hostile sea state and its long term life span.


Fig.2:Image of  a semi-submersible offshore installation (Courtesy: www.offshoreenergytoday.com/)



Jacket Platforms
                                       
These platforms are quite common and comprise of an outer jacket-like structure of welded tubular steel. Steel jackets are vertical sections made of tubular steel members, and are usually piled into the seabed. The pipes are about 1 to 2 m in diameter and have a maximum penetration of about 100 m. They weigh about 20000 tonnes. This external steel jacket has many functions ranging from stability and supporting the entire structure to protecting the internal piping and drill equipment from external disturbances.

Design Parameters 

These steel jackets have tapering, truss-like structure with high strength, durability and fatigue resistance.Loading and life cycle apart from diameter, penetration, thickness and spacing are taken into account. 
  • In a typical design , the cross-sectional dimensions are about 70*65 at the seabed and 56*30 at the top. 
  • They have capacities of resisting forces up to 50 MN in compression and 10 MN in tension and has a large vulnerability to lateral loads like expansion and hydrodynamic stresses. 
  • They also have a maximum permissible overturning moment up to 10 GN.m in most cases.   
The jacket design and the pile design are the crucial factors in determining the total performance of the structure and the total cost in its design and maintenance can devour up to 40-50% of the total cost estimation. 
One point to ponder upon in jacket structure is the protection of the steel from rust and corrosion, a "disease" of all marine vehicles and structures which is principally to be taken care of . This can be done up to a certain extent by using high-grade "stain free" steel and by repeated cathodic protection. 

Jack-Up Rigs

Jack-Up drilling rigs are a type of self-elevating, mobile platform capable of raising the hull part which is obviously buoyant above the sea level and penetrating its steel legs into the sea-bed for support for deep sea drilling operations. The name "jack-up" is derived from versatile nature of the legs or jacks which can be suitably jacked above or below the hull accordingly when not in operation and when in operation. 
These Legs maybe piled into the sea-bed or maybe placed on large footings for better grip and stability. There maybe three or four legs for jacking up the buoyant hull above the sealevel for operations. After a stipulated time period after termination of the operations at a place, the legs are dismantled from the sea floor and are jacked above the hull and the entire unit is mobilized to another destination maybe to some jetty/port or to another drilling site. They are not self-propelled and usually depend on tugs of Heavy Lift Ships for towing from one place to another.
Well when and where can jack up rigs be used? The answer IS that they maybe used in shallow waters as pre-operation and post-operation mantling and dismantling of the jack ups are very bleakly feasible in deeper waters. The steel legs are of high strength HSS steel and are  designed to confront wave dynamics even in rough seas up to a certain limit. Care must  be taken for their maintenance in terms of corrosion, fatigue, stresses. In case of sea or external parameters going beyond the tolerance level, these are immediately "jacked up" and are shifted to safer locations or kept in a safe nonoperational, dormant mode. 
Jack -Up rigs are used specifically for oil well drilling purposes and has limited or no storage capacity. Storage or bulk transport may be through oil tankers or the multi-purpose FPSOs (Floating Production, Storage and Offloading Units).  

 Fig.3: Basic Components of a Steel Jacket Offshore 
Structure (Copyright: Maersk)    
 

Concrete Platforms/Gravity Platforms

Concrete Structures are purely composed of concrete and is considered the safest mode of offshore industrial operations. They maybe directly fixed or molded to the sea floor on a permanent basis or maybe floating. 

Fixed ones are also known as Gravity-based structures or the Caisson type. The entire structure is based on a submerged island-like solid structure which may serve as a foundation structure as well as storage or oil or other by-products. The entire load of the structure acts directly on the subsequent  layers of the sea-bed eradicating problems even of the extreme wave disturbances or seafloor scour. On the other hand the floating ones are freely float-able and has six degrees of freedom under a proper mooring system.  

Some designs of these concrete structures are Condeep, ANDOC, Doris and so on. The design parameters are guided by the number of supporting columns and the diameter of the legs. Constructing is a tedious process where the entire structure or its components are towed and assembled with proper ballasting and de-ballasting measures. They have almost negligible maintenance and high durability. Can be also used for greater depths. 
             
               

Fig.4: Garvity based Offshore Structure 
(Courtesy: www.paroscientific.com/)
Fig. 5: Concrete fixed platform (Courtesy: www.wind-energy-the-facts.org)

Compliant Towers 

Whenever the word "compliant" comes to our mind we mean a sense of complying or yielding. These are tall, slender, single isolated structures with high slenderness ratio.
They are comprised of specialized flex tubes of 2 to 7 metres in diameter comprising the space-time frame-like truss  with high degrees of flexibility and can withstand high amounts of lateral loads up to 10 feet  due to waves or oceanic disturbances. 

The rig consists of narrow, flexible (compliant) towers and a piled foundation supporting a conventional deck for drilling and production operations. They are mainly operated in depths upto 1500 to 3000 feet. They inherently have a frequency lower than the natural frequency of the waves, such that with the emergence of any wave disturbance can make it oscillate with a resultant frequency safer for the loads and eradicate the probability of any resonating conditions. They work on a principle of de-amplification of waves dissipating the energy responsible for creating greater mayhem. Hence they are applicable for high tide or even the worst sea conditions.The lower part of the frame depends on the pilings and can penetrate hundreds of feet below the mud line. 


 Fig.6: Different Compliant Structures through the years (Courtesy: www.atp.nist.gov/)

Tension-Leg platforms

These are floating facilities which stay afloat and remain in position with the help of specialized steel tubes called tethers or tendons. These are nothing but the supporting legs of the floating platform which by the virtue of their upward tension takes care of the position, loading and the functionality of the extraction system. For all such tension leg platforms there is not much vertical oscillatory motion of the platform how rough the sea conditions or the average wave height may be. This facilitates in tying with the wellheads and the piping system for extraction of oil without much distortion. for all such systems there is always an additional buoyant force which keeps the platform afloat. Thus for all such installations,


Tension =Buoyancy - Weight

Here topside facilities and the basic parameters like the number of risers have to be fixed at pre-design stage. Generally the platform is manufactured on the shore and is towed to the desired location with the help of tugs. This structure is apt for depths up to 1200 metres and has limited or no storage facility. As of common usage, these have a high maintenance and surveillance cost of the tethers and under unavoidable circumstances the structure may be prone to irreversible damage. 
          
                                            

Fig. 7: Structure of a typical tension-leg platfrom (Courtesy: www.slideshare.com)


Semi-Submersible

They are floating installations which rest on four or six pillar-like legs called columns with a equal weight distribution on each. These columns or legs are in turn attached to large basements called pontoons floating on the water surface. These pontoons may be ballasted or de-ballasted accordingly on and off operations. Often these pontoons delve deeper under the water surface and maintain the buoyancy and position of the floating system. There is always a greater draft. Thereafter the operational deck is kept well aloof from the wave disturbance or the rough seas.However due to small waterplane area the structure is sensitive to load variations and must be trimmed accordingly. They by the virtue of their equivalent weight distribution and a high draft, it has a greater stability than normal ships. The number of legs, pontoon design , the situation of the risers and drill equipment are decided at pre-design stage. They are generally instrumental in Ultra-deep waters where the fixed structures pose a problem. Their position is maintained generally by a catenary mooring system or sometimes in modern structures by Dynamic :Positioning System. These structures are gigantic and may be towed from one location to another by the virtue of a kind of ships called Heavy Lift ships. 

                                                                                  
Fig. 8: Semi-submersible Offshore Platform (Source: patentimages.storage.googleapis.com)


                                               
Fig. 9: Heavy Lift  Ship (Source: www.motorship.com)

SPARS


These have a large diameter cylindrical deck supporting the main deck .They are generally developed as oil platforms for deeper waters as an alternative to the conventional systems. Most of the drilling and oil extraction equipment are situated within them. The inside part of the cylindrical truss is filled with some material denser than the sea-water to lower down the centre of gravity and hence maintain the vertical stability of the structure.  The deep draft design of spars makes them less affected by wind, wave and currents and allows for both dry tree and subsea production. The cylindrical structure is surrounded by helical strakes to avoid vortex-induced motion. 

The first prototype of the SPAR Platform was Neptune laid off the US coast in 1997. 

There are basically three types of spars, namely, truss spar, classic spar and cell spar. 

It is generally equipped with taut catenary mooring and the heave natural period is generally below 30 seconds. Generally , the  spar is employed for depths up to 2300 m. The number of risers is restricted generally due to the limited space of the core cylinder. These are highly stable and hence are negligible unaffected even on adverse sea conditions or sharp environmental vagaries.  

                                      

                                                   
Fig. 10: A typical deep sea SPAR Platform (Courtesy:www.maritimeconnector.com)


FPSOs

FPSOs stand for Floating, Production, Storage and Offloading Systems. It is a specialized vessel for the purpose of offloading, processing, storage, distribution of oil and other hydrocarbon resources. One important point to wonder is that these installations does not have anything to do with the extraction purposes and can be merely reckoned as a carrier with specialization. These vessels eradicate the need of long pipelines for transfer of oil and petroleum to the shore. FPSOs are preferred in frontier offshore regions as they are easy to install, and do not require a local pipeline infrastructure to export oil. FPSOs can be a conversion of an oil tanker or can be a vessel built specially for the application. A vessel used only to store oil (without processing it) is referred to as a floating storage and offloading vessel (FSO). These can operate extensively in remote and deep waters and also in marginal wells where fixed platform or piping is technically or economically not feasible. 
In majority of the cases the vessel may be buoyed to a vicinal drilling platform. Also the oil or other petroleum products maybe transported to the mainland by pipelines or by a conventional mooring system. The vessel may be fixed in a particular position by mooring lines or by a dynamic positioning system (DPS). 
                                    
                                                   
Fig.11: The ongoing operations to an FPSO and the production process

Fig.12: Floating production storage and offloading unit (Courtesy: www.alibaba.com)

These ships have an integral storage capability inside their voluminous hull and have a calculated freeboard and draught. They are operational in adverse weather  and have a high maintenance cost. 

Drill ships

It is a mobile offshore installation akin to a ship which is specialized for exploration and drilling of oil and gas resources more specifically for scientific exploratory purposes. Mostly, it is used for deep and ultra deep applications involving exploratory and drilling purposes. The first drillship was the CUSS I, designed by Robert F. Bauer of Global Marine in 1955. The CUSS I had drilled in 400 feet deep waters by 1957.Apart from regular drilling purposes, the drillship may also be used for maintenance and surveillance purposes such as casing and tubing installation, subsea tree installations or wellhead capping. Drillships though expensive and of high precision in operating and maintenance, is often believed to be the most proficient mode of offshore technology trend where like any conventional ship, it can be transferred from one place to another anytime and that too without the use of any tugs , heavy lift ships or any other towing system. However a mandatory mooring system should be kept or a DPS positioning system  should be employed. Generally, underneath the derrick through the hull is the moon pool which connects the deck to the sea directly. Sounds pretty interesting, isn't it ? The moonpool is generally an opening of the deck floor with the water in any marine research vessel, drillship, sometimes icebreakers, diving support vessel for easy exposure to the underwater environment or for the easy steup of the drilling equipment as done here. Generally moonpools are situated in four different positions, above the waterline, at the waterline, underneath the waterline or deep submerged depending upon the requirements, the vehicle or the design. So, for a drillship the moonpool provides easy accessibility for the drill equipment or manpower for the purpose of maintenance of the drill equipment or underwater survey. However immense care has to be taken during the design stage to ensure the moon pool compensates the loss of strength in the deck or hull, maintain stability and trim and avoid flooding or leakage. 

                                                              
Fig. 13: Different types of moonpools (Courtesy: www.wikipedia.com)




A simple way to understand what a drillship is to do in order to drill, a marine riser is lowered from the drillship to the seabed with a blowout preventer (BOP) at the bottom that connects to the wellhead. All this is done through the moonpool intricate to the hull. Drillships also have their own storage and processing system . However, in terms of positioning and stagnancy during drilling operations, the semi-submersibles have an upper hand. 

Fig. 14: A TIGER Series Drillship (Courtesy: www.offshoreenergytoday.com)

This was all about the common types of offshore structures in existence today.LSD


Article By: Subhodeep Ghosh

Sunday, 30 August 2015

Know A Ship- Heavy Lift Ships

How would you move a gigantic oil rig from one place to another after installation or after dismantling? Or how would you transport a large passenger vessel with its hull damaged beyond vulnerability and exhausted of the very chances of mending it on-spot? Well, the first impression that enthuse us is the applicability of tugs or similar towing systems. However, everything has its own limitations and sometimes the situation demands that we bank on other options more inventively. Necessity is the mother of invention. So, one of the most tantalizing outcomes in the shipping world is the Heavy Lift Ships.By definition, they are specialized ships that carry out tedious heavy load in-taking operations that cannot be handled by normal or regular ships. They are segregated into two types : semi-submerging vessels capable of lifting another ship out of the water and transporting it; and vessels that augment or cater to heavy unloading facilities at inadequately equipped ports. 
Fig.1 Heavy Lift Ship carrying a cylindrical SPAR Platform(Source: www.dnv.com) 

A Brief History of Time

In the early stint of the previous century, most of the existing offshore technologies like rigs, platforms, oilwells, floating drydocks, drilling rigs,etc. were directly towed across the seas in what was known as "wet tow". The mobilization was long, tedious,cumbersome and often posed risks of on-voyage damage. By the end of the 1960s,the towage companies realized that  if they could manage a large barge-like vessel that would carry on themselves all these bulk items in what was conceptualized as the " dry tow", things could be easier. There was this ideal of carrying a floating cargo as a cargo on another floating object! All these had a bottom reaction" having a capability of a submergence stern resting on the seabed.  In 1976, a renaissance was sparked off when the first heavy-lifter semi-submersible  barge  'Ocean Servant I' entered the heavy lifting market. the induction of the buoyancy casings at the four corners enabled it to remain afloat and submerged without the need of bottom reaction. It had two 500-hp omni-directional propellers and was still self-towed. In 1979,the 1st self-propelled semi-submersible mighty barge 'Super Servant' came into the limelight.
It had a depth of 15 m , 6.5 m water level above the deck and had ballasting and de-ballasting features automated. Further down the timeline, in 1983, three 'Mighty Servants' with more advanced mechanisms and larger cargo-carrying capacity formed the most versatile class of the Heavy-lifters. A stark feature of these trio was the presence of three buoyancy casings which were removable. April 1985 saw the birth of two more of its breed namely, the 'Dan Lifter' and 'Dan Mover', which were rechristened Super Servants 5 & 6. By that time, technology had strode further with Heavy Lifters gradually acquiring the stake of major heavy lifting operations which also saw a further up-soar in the field of offshore like more semi-submersibles entering the deals. With doing away with stowage, mightier structures could easily be shouldered by these marvels and that too much easily by having the deck margin line absolutely parallel to the keel of the cargo. In April 1990, the ultramodern Heavy Lifter made by Russia 'Transshelf' created an upheaval in the revolution of modern Heavy Lifters. 


Why heavy lift ships and how?

Heavy lift ships to be precise are hefty and ginormous special-purpose vessels that are about 3000 times the capacity of a blue whale! Sounds quirky, isn't it? They miraculously have a lifting capacity of over 10000 tons. These ships have a large, wide and longish carriage deck which is mainly allowed to submerge below the water line and large bodies like oil rigs or whole ships are allowed to float on it or mounted on a dry dock. The alternation in freeboard and buoyancy is carried out by suitable ballasting and de-ballasting practices. The deck, henceforth comes out or remains submerged in water in such cases. Probably, you may wonder out of curiosity as to how the ship still remains unsinkable even after carrying such brawny loads. Well, we do give a quick glance at one of the most quintessential aspects naval architecture namely, draught, floatation and buoyancy .

A brief recall of Buoyancy and Displacement


We all since our good old school days are not alien to a fundamental concept of Archimedes' Principle. According to the statement, it is said that : "When a body is completely submerged in a fluid, or floating so that it is partially submerged, the body is subject to an upwardly acting buoyant force, which in turn is equal to the weight of the fluid displaced by the body."
In other words, irrespective of the shape, size and other parameters of the body this upthrust or Buoyant force acts vertically upwards opposite to its weight. This force acts through the geometric centroid of the body and this particular point is christened as Centre of Buoyancy  of the body. Though I do not delve much into the mathematical content and its diversities for different scenarios, one important thing we must confide is that not just a light paper but even the heavy lift ships weighing over 10000 tons obeys this classic principle thereby accounting for its habitability. Hence, if you do have a myth about these structural giants, do get rid of it! Even these hefty giants about 3000 times the size of a blue whale float like a normal paper-boat!Finding it a bit weird to be true? The phenomenon is the same old buoyancy governing all other marine vehicles. The displacement or the volume of water substituted by the submerged body accounting for the validity of Archimedes' Principle is the main governing factor.So, heavy lift ships having a large scale displacement of the order of 10^5 square ms.have a large upthrust and the draught varies within certain limits depending on the temperature and salinity parameters of the water and also the load. 
                                               
                                                       
   
 Fig. 2 Buoyancy and Archimedes' Principle(Image Courtesy:Google Images)


For high stability and to prevent heeling of the vessel sideways, stability is a quintessential aspect in such heavy lifting operations about which will be discussed shortly. However for a simple piece of reasoning that a ship does not lose its trim and tilt at one end, there should be a uniform weight distribution. Hence for a Heavy Lift Ship, the load is generally kept on the larger span of the weather deck which on many occassions remain submerged beforehand to handle larger displacements(and hence larger load capacities). 

                         
                                                        
  Fig. 3 Floating of a ship and its loss of floatation (Image Courtesy:www.schoology.com)


                                               
Fig.4 Heavy Lift Ship carrying a smaller vessel.Has a submerged main 
deck (Image Courtesy:www.amusingplanet.com)

Stability factors in Heavy Lifting Vessels

Loading and heavylifting operations in vessels are not only a matter of how much load a ship can carry but how much does it survive under this load. If you are simply allowed to troll on a walkway with a medium sized brick you may do that easily. But what if the load is being increased persistently and if you are asked to carry a tree trunk ? The probable answer from the majority is that can't.But if momentarily you imagine yourself to be a giant and somehow manage to lift it up, what will be your status? Will you manage to keep yourself upright? And even if so, till how long? Suppose, someone gives you a slight push. Will you be there like before or tumble over? Well same is the case of ships. Stability is an inherent factor for a small boat as well as a heavy lift vessel. In a completely undisturbed condition, or in an equilibrium case, a a given displacement the buoyant force balances the weight acting vertically downwards. Hence in such a case , the centre of gravity(C.G.), the centre of buoyancy(C.B.) and the geometric centroid( which in almost all the normal cases coincides with the centre of gravity) lie on the same line. In such a case, the ship is said to be stable. Considering some external disturbances like wind or some high waves, the stability is said to be lost. Although, by property the C.G. remains intact the position of the C.B. changes. The point of intersection of the original line passing through the C.G and the initial C.B. with that of the line passing through the final position of C.B is termed as the metacentre M.
  
 
                                            


Fig.5 The position of the various points in an unstable  condition (Courtesy: www.marineinsight.com)



In all such heavy lifting errands, like in heavy lift ships, there is a rigorous change in C.G due to loads and also due to operations like lifting through cranes or derricks. This uneven distribution of weight in all such ProjectCargoes often cause the C.G. to be on either side of the centreline. Hence the ship has a high probability to heel towards that side. On normal day-to-day ferries or other ships, the stability mainly gets lost due to two reasons; either shift of C.G due to uneven loads or direct and abrupt change of C.B. due to external conditions leading to heeling. However, in heavy lifting operations, mainly, the culprit is the biased CG.

On one side that causes the vessel to list henceforth. As depicted in the above figure, there is a normal GZ, from the initial C.B to the line passing through the final C.B and is called the righting arm. This can cause a righting moment to the ship and may attain back its position. A negative GM is always uncongenial for the ship which may lead to its capsizing of the ships( which is a pertinent disaster !). Also by the virtue of a reverting counter buoyancy, the ship may, in certain chances attain back its stability,i.e, become upright. This tendency of a negative inherent stability is measured in terms of a certain Angle of Loll. It is the state of the ship which is unstable when upright. It has a dramatic property property to lurch to another side on the action of external forces producing the same value of Angle of Loll on the opposite side. Remember, do not confuse it with list!! Listing is caused by the sifting of cargo and material grain causing a change in C.G of the body. But this counter event  called Loll is just a reaction force in counter to Rolling. In case of a suspicion that the vessel might be subject to Loll, we must do something which may sound quirky! Ballast or do some weight additions to that side which has the loll or the greater inclination downwards. This dramatically depletes the severity of the Angle of Loll, but provides a boon at the end.The draft increases drastically and the freeboard decreases along with the so-called free-surfce effects.  The C.G. immerses further and the greater submersion of weight increases the "Buoyant Reaction", which in turn creates a negative Angle of Loll at that condition. 
                       
                                                        
Fig.6: Angle of Loll(Source:Googleimages)

Another simpler implication is to just undo the cause for this loll.Mainly, the majority of these events are caused by heavylifting activities on the superficial weather deck that drastically shifts the Centre of Gravity to the heavy load carrying side. Cranes, Derricks or other facilities carrying out these operations are the other culprits, which along with the weights they are carrying act as moment arms, causing a resultant tilt. Maybe, the following image explains it better!



Fig.7.: Heavylifting operations on a ship and the resultant shift of C.G (Courtesy:www.sphlashmaritime.com)

So, for a heavy lift ship, care must be taken while loading and offloading operations in the deck such that the ship is free of loll. As a Heavy Lifter is designed specifically for loading purposes, it is not as if heavylifting operations should be avoided. But the weight should have even distributions and as it involves weights in the order of 10000 tonnes, things have to be in line! Most of these have modern ballasting technologies,thrusters and Dynamic Positioning Systems. Heavy Lifters, an alternative to the older technology of heavy lifting cranes or projection cargoes straddle the cargo(jackets, rigs, oil wells, semi-submersibles, ships etc.) uniformly on both sides and allow proper suspension of weight in a manner that avoids Loll or list under normal circumstances.

The Cargoes and its Sea keeping moves

The versatility of the Heavy Lifters coupled with its mammoth capabilities enabled it to be the workhorse for several varieties of cargo like the floating plants, drydocks, rigs, Pressure vessels,cranes, jackets just to name a few. Even the navy vessels which were of high precision were often transported by the virtue of these which substantiated their flexibility. The serviceability limit weights by the order of 4000-5500 tons which leapfrogged to 21,000 plus tons for the largest rigs in the modern days. Marine technologies which has contributed significantly in the revolution of these vessels, has further diversified along with the modern day computational software for complex motion and vibration responses. These ships are subjugated to a high ordeal of strength and endurance testing which affirms its competence in their operations. Well, model testings are an indispensable part in any form of engineering and large scale productions. Heavy lifters are not an exception with motion response tests which are objected to mainly test the non-linear behavior of the ships in rolling motion. 
Some of the important cargoes which have been transported so far by these ships are:


·  Jack-ups ' Norbe 2 and 5 :

 In 1988,  these two mighty jack-ups were transited from Salvador, Brazil to Mumbai( erstwhile Bombay)in India. They had a projection lengths of 4.5 metres and had spud can diameters of 14 metres. If not for these ships, then the net transit time would have been a matter of 150 days. The footings posed a great deal of trouble and in case of direct stowage would have resulted in a drag speed of 3-4 knots which would have made things cumbersome. These were carried out by the 'Mighty Servants 3' and made it possible in a matter of a little over three weeks( almost a month!)The legs were for stability placed on the cribbling blocks for the ease of dynamic loading. The lower deck was cut off to accomodate the rigs upto the safe submersion limit and seafastenings were provided on the aft legs of the rigs. 

·  Tension-Leg Wellhead Platform : 


In 1989, the first ever Tension Leg was transported by the Heavy Lift from the U.S Gulf. It was carried out by the same 'Mighty Servant I' was a float-on type of loading mechanism. The truss deck was carried out by four columns each of 12.2 metres in diameter, 46.2 in height and was a space frame structure of spacings in the order of 42.2 metres. Pontoons, about 7 metres in diameter connected the columns near the base. 

·  Challis Field SALRAM : 


The Challis mooring system consisted of a 121 metres riser attached to a 34 by 34 gravity base structure. This base structure weighing about 4500 tons was put to board on 'Mighty Servant 2' around mid of August 1989. To facilitate the proper spacing, the ship's starboard buoyancy casing was shifted forward to accommodate the entire structure. 

·  Damaged Tanker 'Imperial Acadia' :


You all must have pondered upon the speculation of the operational ships, vessels and structures from one place to another as per the convenience. Have you ever wondered what would happen if a heavy, expensive and high-precision vessel is stalled amid the sea? Or some parts or crucial machinery are broken down? Wet Towage across the seas would be a pursuit of money, time and most importantly a factor of greater risk. If physically some parts of the structure is seriously damaged, forcefully towing it across the rough waters would exalt the chance of damaging it further. Many of the naval vessels or crafts when damaged are carried up by the Heavy Lift Vessels as they are of indigenous design and mechanisms worth millions. A lot of U.S Navy minesweepers have been barged up on 'Mighty Servants I, II and III'. The demobilization of the Canadian giant 'Imperial Acadia' de remains a milestone in the achievements of Heavy Lifters. With a length of 135 metres, this high capacity Canadian Tanker was caved 1.5 to 2 metres on the lower starboard side in the event of a frivolous sea-storm near a french island. Mighty Servant  I performed the tedious task of safely towing it on-board to a shipyard in Halifax for hull repairs.


·  Floating Drydock 'Karish' : 


Drydock facilities are developed generally for still water conditions. Hence forth their mobilization in wavy, gushy seawaters account the dynamic water loads acting on them. So mobilization through the Heavy Lifters through 'dry tow' which affixes it above water eradicates the need of greater steel strengthening or severe damage in case of accidental loads. The offloading of 142 metres long, 7500 tons heavy the 'Karish' dry-dock at Batangas, Philippines by Transshelf Heavy Lifter is reckoned as a notable event in the history of Heavy Lift Ships. Today several such drydocks or floating jetties, ports are shouldered upon by them in case of their relocation from one place to another.


The Reigning Giant-Dockwise Vanguard

The Dockwise Vanguard is the largest Heavy Lift Ship which is owned and operated by the Dockwise B.V. A sprawling 70 by 275 metres  flat upper deck, it can lift upto 110000 tonnes of weight. Its propulsion is by the virtue of 2 giant propellers, 2 retractable azimuth thrusters and a powerful bow thruster. Launched recently on 7th October 2012, the Vanguard has earned its repute by transporting huge offshore rigs,gas facilities and other enormous vessels. It is a semi-submersible vessel that can immerse its principal deck up to 52 feet below the water level in case of deep draught cargoes. It is equipped with four strong buoyancy casings and can handle cargoes at varying depths by their ballasting measures. Some of the structural giants which were pillion on Vanguard were the Chevron's St. Malo Oil Platform, the Goliat FPSO and the Aasta Hasteen Platform, the latter two being born to Hyundai Heavy Industries Ltd. 

                                                   
Fig. Dockland Vanguard (Source: Googleimages)

Next but not the least- MV Blue Marlin


MV Blue Marlin, as you must have heard is the second largest ship owned by the Netherlands Shipping corp Dockwise. It has a length of 224.8 meters, beam of 63.1 meters and a permissible draught of 13.1 meters. It has a deadweight of over 76000 Tons dead weight. Some of the notable cargoes of this giant were the American Destroyer USS Cole, oil platform Thunderhorse and the amphibious Australian warship HMAS Canberra, just to name a few.LSD
          
                                                        
 Fig. MV Blue Marlin (Source: Googleimages)

Article by: Subhodeep Ghosh