Under the Skin: Part 1

Have you ever wondered what lies within the skin of the prodigious engineering marvels such like ships? You may be surprised to learn that ships or vessels like we humans and other living creatures have an intricate 'skeleton' underneath! Analogous to the complex network of bones and ligaments in living beings, they have a well-engineered arrangement of structural members in varying attributes.

Fig. 1 A high-definition picture of Queen Mary 2 (Courtesy: www.largestships.com Archives)

You may still be in some doubt!

Large number of big and small components make up the hull structure which is the primary phase of manufacturing of any vessel and is the most tedious task. It is believed that the safety and sustainability of the vessel during its service is a chunk dependent on the ingenuity of the structural members and thereby the entire structure. Failure of any single component leads to disastrous results. So, it is a challenging task to undergo detailed selection, arrangement, analysis and testing methods to these structures.With due respect to the traditional mechanical analysis methods of materials along with empirical probabilistic methods,modern computational testing methods accompanied by Finite Element Analysis and other precision software have eased the cumber of the olden days providing a wider scope of accuracy and precision. 

SHIP'S HULL- AN OVERVIEW 

                    Fig.2 Cross-Sectional View of a General Cargo Carrier                

                                

A ship's hull is comprised of a criss-cross network of perpendicularly placed plates or members which are fixated to each other at all possible degrees of freedom. Some crucial members include extensive varieties of plates, struts, columns, bars, beams, stanchions, angles, brackets, knees, elbows and so on.

Must be wondering the arrangement of all of these in a mere vessel? And that too which floats and treads in water for such a long span of time!

The answer is quite simple. There is a holistic placement of all of them in accordance to their capabilities which complies with their strength, scantlings, locations, loads/stresses and their utilities. A ship, as we know is a large structure aimed at overcoming all hindrances of distance, carriage and cargo, sea-states, environmental vagaries, titanic loads accompanied by the pressing issue of safety of life as well as property (cargo/machinery). Naval architects, shipbuilders and designers owe a considerable amount of showmanship regarding design, lines plan, materials, scantlings, strength and model testings, intricate architecture of each component, related calculations and final accuracy in materialization of the arduous job. An obvious question arises at this juncture: What should be the apt choice for material selection as far as the hull is concerned? 

What is the material used?

Mild steel and wrought iron has been a sought-after choice of shipbuilders around the globe after the departure of the yesteryear wooden/timber vessels.However, in the recent times, wrought iron is losing its popularity owing to its corrosive nature and being bulkier.Mild steel earns the reputation of its abundance, ease in assemblage, lightweight along with required strength, ductility and malleability. However it is prone to failure under high stresses and hull shocks. It also has a degree of unreliability when it comes to their service as girders, struts or internal supporting members, with due respect to larger ships of today having massive tonnage. Furthermore it is highly corrosive in nature and is also prone to other problems like fouling. So, another breed of steel, High Tensile Steel/High Strength Steel with increased yield points have stolen the limelight. They include grades of steel like AH,DH,EH. Their yield point lies in the range of 315-350 MPa!

Fig.3: A very rare pic of The Titanic in construction. Wrought iron and mild steel were the chief materials used along with rivet joining (Courtesy: www.amberonline.com)     

But as it is said, everything has its pros and cons. High Steel Strength, despite its high degree of toughness has poor response when it comes to the problem of flexural bending. As longitudinal bending is inevitable in most of the large ocean-going vessels, they often pose the risk of fracture due to intensive bending moments at uncongenial sea states. The advent of composite fabricated members in the modern shipbuilding has changed the scenario. Though the SOLAS requirements and a majority of classification societies were dubious about its utility in the superstructures or other deck areas, thanks to its reduced fire resistance, the mid-1980s saw a gradual upraise of composite fibers in the industry. As usual, supporting reasons existed!

• Lightweight. This helped in easily meeting the critical dead-weight barrier assigned to a ship. So this, up to a certain extent  helped in greater "inclusion" of added mass on board which was a constraint if material was a bulky one like steel. 
• It has a lower life cycle cost coupled with cheaper maintenance and durability.
• Due to its light weight, it amounted to lower fuel consumption. 
• Higher stiffness and accorded to greater flexural bending moments by the virtue of its elasticity.
• Was tough and rigid with minimal scope of cracking. Offered greater resilience.
• Often use of composite material allowed higher superstructure and more accommodation space in passenger vessels designed for a given displacement. 
• Its resistance to corrosion and fouling when present as a component in the hull structure was a boon.

FRP-sandwich panels/GRP-sandwich panels and PVC enforced steel plating found great predominance. Polymeric and non-polymeric materials found equal importance. However, composite had disadvantages as well like being highly flammable or being difficult to manufacture along with yielding at very high temperatures. Thus the decisive step is being taken as to optimize between steel and composite materials in suitable proportions for the best results.

Fig. 4 (Copyright:Linkedin)

Apart from these, forged steel and cast steel are used in secondary structural components like rudder posts, stern frame and stem according to the specifications of the latest classification societies. Some classification rules even give higher adherence to the existing Aluminium Alloys as the principal material for the construction of deckhouses, superstructures, hatch openings, covers etc. over composite materials. 

BASIC/FUNDAMENTAL STRUCTURAL MEMBERS

After a brief insight into the material properties commonly used, let us have a glance at the big and small structural members that make up a ship's hull!

              Fig. 5 General steel plating arrangement of the ship's hull (Courtesy: wikipedia.org) 

Plating: 

They are the primary building blocks to any vessel just as bricks are to any house ! Right from the day when wooden vessels were made of wooden boards to the present day where steel/composite plating form the hull. Plates of varying dimensions or scantlings are used depending on the functionality, general arrangement and stress considerations. The final placement of the plates give rise to the structural truss which along with the outfitting and ancillary components give rise to the entire vessel form. Platings broadly referred to the outershell  plating as well as the inner walls of the hull. They may also be found in the modified form of bulkheads or as components in decks and other outfitting. Platings prove their versatility in the role of taking up any shape as per the criteria, like round or being curved.

Well, a question which is definitely going to arise in your minds at this juncture: How are these mere plates joined or concatenated to give rise to the final shape?

  Fig. 6 Ongoing welding jobs within a ship's hull (Courtesy: www.gettyimages.com)

The answer is quite simple. Joining is very catalytic component in shipbuilding not only a ship's production but also its subsequent productivity, performance and safety is highly dependent on the joining methods. The total length of structural joints in a large cruise ship is of the order of 400 -500 km! Even that too has an evolution like the ship itself. 

Earlier,the method of caulking was used to make the seams in wooden boats and ships watertight, by driving fibrous materials into the wedge-shaped seams between boards.Till the World War era, it's successor riveting was the chief technique employed which was the . The entire ship was exacted by joining cogs and nuts at every plate/board itself. This was a tedious task which made making of a ship very cumbersome and time-taking.

However, post-World War II, the modern techniques of Welding took over. Welding as we know is a safe,convenient and firm way of joining metals. Without delving into the details some of the various methods of welding involved are :

1. Electric-Arc Welding
2. Electro-Slag Welding                                    
3.  Shielded Metal Arc Welding
4. Submerged Metal Arc Welding
5. Gas Metal Arc Welding
6. Ceramic Welding
7. Laser Welding

                            

 Fig. 7 Underwater welding repair of hull (Copyright: www.telegraph.co.uk)

While each of them have their own pros and cons, other methods of metal joining like Adhesive Bonding and Mechanical Joining Techniques are often used as well.

  Keel :

          Fig. 8 Typical closeup view of keel network (Courtesy: googleimages)

All of you must be familiar with this term. It is often referred to as the backbone of any ship. It is the chief structural member in the form of the center plane girder that runs longitudinally from fore to aft, generally a beam around which the entire hull is supported. This could be worth saying that the entire hull grows from strength to strength about the keel at the base. The structural strength and integrity of the keel, is a key determinant to the safety and the performance of the ship.

Maybe, this is why keel laying ceremony is celebrated in such an aura in any shipbuilding project!

The keel forms are divided into three principal types: 

• Flat keel. The commonest form of every keel. A highly strengthened, flat beam is placed parallel to the ground. Most of the large ocean-going vessels and other bigger ships nowadays have this type of keel. This accounts for lower resistance, accurate draught; but is susceptible to grounding.

                

          Fig.9 Keel laying of an old design flat-plate keel (Courtesy: wikipedia.org)

• Bar keel: An old design of keel, still used in many smaller vessels and boats involves a rectangular cross-section flange poised over the bottom-most part of the hull. They are becoming rarer these days, thanks to its added weight problems which increase draft without increasing the displacement. However, having the unique property of jutting out below the main hull form closure ,bar keels are still alluded as a ready-hand solution to excess rolling.

• Duct keel is the hollow form of the keel floor in some ships, generally running from collision bulkhead to engine room bulkhead with the provision of allowing piping systems throughout its expanse.

                                  

 Fig. 10 Inside a duck keel passage of an LNG carrier (Courtesy: http://www.fsharris.co.uk/gallery/29.jpg)

Strakes: 

They specifically refer to the bottom and side shell plating which are the supposed points of maximum stresses. Strakes are categorized as Bottom Strake, bilge strake and Sheer Strake. The bottom shell plating follows a unique system of nomenclature in almost all ships,i.e. in the form of successive alphabets with the keel as reference (e.g. A strake, B strake etc.). 

The first stake in the order of appearance is also termed as Garboard Strake.

Similarly, the strake situated at the "turn of the bilge" is referred to as the Bilge Strake. 

The upper-most strake near the deck edge is the Sheer Strake. It may be worth saying that as these are the critical points of high stresses, additional strengthening is provided to these plating to sustain high amounts of unpredictable loads. 

Other members: Even if the ship looks complicated, its structural components are not that much complicated or massive as they might seem.The items are very basic like columns, struts, beams, flanges, angles, brackets and stanchions. Maybe these are not all. Small to negligible members exist in every minuscule of the vessel to give rise to the proper functionality of the structure. The nomenclature of all such members are different according to their role and location.Their role may be variant in the either of the following forms:

• Construction
• Support
• Strengthening
• Stiffening

            

          Fig. 11 Diagram of all the basic structural parts of an arbitrary hull section (Copyright: United States Naval Academy student archives)

Although strengthening and stiffening are not exactly same, we simplify our topic of discussion by the convenience that the corresponding structures employed in ship construction are one and the same. We next a brief insight into the stiffening members.

 STIFFENING AND STIFFENERS

The word 'stiffening' essentially suggests the purpose of providing extra stress-bearing capacity or rigidity to the existing structural member. 

Does simply erecting four successive plates in a mid-ship section, for instance serve the purpose?

The answer is a big no. Every structural member requires a stiffening in some form not only to be stable itself, but also to provide resistance to the various amount of internal and external causal agents leading to stress concentrations on the structure. These stress concentrations as you know are fatal, even if neglected once! Thus stiffening from grass root level is of utmost mandate for every single component irrespective of size, location, form and purpose. Stiffeners are these secondary structural components which conjoin with the principal members giving rise to the complete framework of stiffened panels.These panels in unison all throughout the designated length, beam and depth form the final structure of the ship's hull. The stiffeners also have their own classification: 

• Longitudinal Stiffeners: As the name suggests, they provide longitudinal stiffening,viz., retain the rigidity against the longitudinal bending and buckling of the ship. As the waves in the open seas are unpredictable and maybe sometimes highly precarious, failure. Longitudinal stiffening mainly focuses on the length-wise stiffening parallel to the center line. The primary objective running through the mind of all designers and naval architects are the basic concepts of shear force and the resultant bending moment which may be given as:                                                         

       All of them are attributed to either of the three groups:                      

1. Longitudinals- They are the stiffeners running longitudinally along the bottom of the ship (parallel to the baseline) from fore to aft. They stiffen the bottom-shell plating of the hull, hence preventing it from the external forces such as the wave loads and the internal forces such as loads of cargo or the other contents of the hull. Essentially they are girders of specified scantlings depending on the applicability of the vessel type.
2. Stringers-If we concentrate on the longitudinal stiffening at the side shell girders, stringers are the answer. They are like longitudinals, sideshell plating strengthening members on the hull. Even as the sideshell platings are prone to high amount of transverse wave stresses, they are also reserved to the maximum degree of rigidity
3. Deck Girders/ Longitudinals: Longitudinal stiffening underneath the main deck, i.e, in conjunction to the inner deck plating. They deck is prone to various types of loads, like passenger/crew, live loads, deck equipment, superstructure, green water etc. Thus deck girders running fore to aft serve the bulk of the purpose. However, they are not having high section modulus as lower bottom shell plating as the deck is considered relatively less affected by high stress as compared to the bottom.  

• Transverse Stiffeners. They provide transverse stiffening across the breadth/beam of the ship. They are mutually orthogonal to the longitudinal stiffeners. Akin to their longitudinal counterparts, they too can be segregated into three forms based on their functionality.

1.  Frames are the larger portions of these transverse stiffeners which run from the keel to the main deck uninterrupted. Frames occur at specified intervals throughout the length of the ship. The spacing between the frames is dependent upon the dimensions and the operation of the vessel along with its type. One crucial thing that must be kept in mind is that they are not to be confused with Stations nor Bulkheads. While the former is an imaginary hypothesis assumed by naval architects while shaping up their lines plan, the latter is a composite structural feature. But frames only align themselves with the side shell and the bottom shell plating without interfering with the inner aspects providing an integral contribution in stiffening plating transversely.
2. Floors assumed to be the continuation of the frames at the base. In general, they can be simply interpreted as the transverse stiffening of the bottom-shell plating. In the case of a single bottom ship, frames are sometimes connected directly to floor plates. The lower ends of tween deck frames are connected directly to the deck plating or are extended beyond deck-head and fixed at brackets.
   However, in most of today's ships, the concept of double hull has been imbibed. Hopefully, all of you know what a double hull represent? For the novice, it is just enough to know that they are two-layered system of bottom plating, i.e an outer shell plate and another inner bottom shell plate with some clearance. Floors are generally sandwiched between the inner and outer shell plating to provide sturdiness to the bottom hull. Floors even have their type like solid floors and bottom floors, about which we refrain to comprehend at this juncture. 
3. Deck Transverse They, like deck girders stiffen the deck-shell plating, but across the allowable beam. However, their strength and spacing depends on the type of the vessel and the 'superstructure loads'. 

                  Fig. 12 A typical cross-section of a 5000 DWT coastal tanker showing all the essential stiffeners and resultant stiffened panels ( Courtesy: Ship construction by D.J. Eyres)

Reiterating the last point on Deck Transverses, it may be worthwhile to say that the placement and the number of the longitudinal or stiffener framing system is solely dependent on the type of vessel, capacity, service, sea-states, cargo optimizing the owner's economical constraints with due adherence to the Factors of Safety. 

It may be suffice to know that in most of the smaller ships having Length-to-Breadth ratio not very high, the number of transverses are more than the number of longitudinals. The spacing inbetween the transverse stiffening members are also very less; they are crowded! This type of framing system known as Transverse Framing System is mostly deployed in the smaller vessels where longitudinal bending is not much of an issue. Transverse stiffeners provide resilience against transverse forces such as those induced by the side waves which can cause unwanted Racking and Torsional motions in the ship. 

                           Fig. 13  Profile of the sideways wave forces and the transverse deflection/deformation inducing racking motions (Courtesy: United Stated Naval Academy student archives)

On the contrary, in large/lengthy vessels such as general cargo ships or the majority of the passenger liners which are prone to longitudinal bending, buckling, flexure, there is more number of longitudinal stiffening induced in all along its length. The spacing between the longitudinals are reduced considerably. A ship of this type may be reckoned as a 'Longitudinally Stiffened Ship'. 

TO BE CONTINUED

      Fig. 14 Ongoing Ship Construction (Courtesy :www.wikipedia.org)

So far we had talked about the basic components of the hull in brief. Maybe that is not enough. Similar to the universal process of concatenation, the basic structural elements join in various forms and types to form what is known as the 'Grillage'. The hierarchy of evolution from simple elements to complex ones follows a definite algorithm of assemblage. One thing that must be kept in mind is that the composite form of the ship's hull is basically a resultant of three different kinds of formation, regardless of the modern designs as well as evolution of the composing materials. 

• Stiffened Panels 
• Frameworks
• Blocks

All of you will be provided an insight to the more detailed process of assemblage of individual materials in a well-defined hierarchy in the next article. For the time being, it is just the idea that the ship's hull is just like any other indigenous engineering output where basic elements of given material composition are arrayed in a predefined manner, of course keeping in mind the strength, reliability, utility, safety and economy into consideration.LSD   

Fig. 16 Freedom of The Seas (Courtesy: www.usatoday.com)  

Article By: Subhodeep Ghosh

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

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 gist 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.

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