Ships and Rig stability for non Naval Architect

If the shell of the ship or offshore rig's pontoon or hull is damaged such as to open one or more

internal spaces or compartments to the sea, flow will take place between the sea and these spaces until stable equilibrium is established or until the ship sinks or capsizes. The loss of watertight integrity could be due to collision, grounding or internal accident such as an explosion. It is impractical to design a ship to withstand any possible damage. The degree to which a vessel approaches this limit is the true index of its safety. To reduce the probability of loss, the hull is divided into a series of watertight compartments by means of transverse watertight bulkheads extending from side to side of the ship.

It is true that the more severe the standard adopted for subdivision and stability, the greater the probability that capital and operating costs will be increased. For example, too close spacing of bulkheads may unnecessarily increase both the first cost and operating costs and may also seriously restrict the vessel’s usefulness. In addition, it might be expected that the more bulkheads the safer the ship. But damage may occur entirely between adjacent bulkheads or may involve one or more bulkheads. Hence, for a given length of damage, any increase in the number of bulkheads may actually increase the likelihood of bulkhead damage, which would reduce rather than increase the chances of survival.

The General Effects of Flooding:

(1) Change of Draft

The draft will change so that the displacement of the remaining unflooded part of the ship is equal to the displacement of the ship before damage less the weight of any liquids which were in the space opened to the sea.

(2) Change of Trim

The ship will trim until the centre of buoyancy of the remaining unflooded part of the ship lies in a transverse plane through the ship’s or rig's centre of gravity and perpendicular to the equilibrium waterplane.

(3) Heel

If the flooded space is unsymmetrical with respect to the centerline, the ship will heel until the centre of buoyancy of the remaining unflooded part of the ship lies in a fore-and –aft plane through the ship’s centre of gravity and perpendicular to the equilibrium waterline. If the GM in the flooded condition is negative, the flooded ship will be unstable in the upright condition , and even though the flooded space is symmetrical, the ship or semi-sub will either heel until a stable heeled condition is reachedor capsize. Trim and heel may result in further flooding through immersion of openings bulkheads, side shell or decks (downflooding).

(4) Change of Stability

Flooding changes both the transverse and longitudinal stability. The initial metacentric height is given by:

GM = KB + BM - KG

Sinkage results in an increase in KB. If there is sufficient trim, there may also be an appreciable further increase in KB as a result. BM tends to decrease because of the loss of the moment of inertia of the flooded part of the waterplane. However, sinkage usually results in an increase in the moment of inertia of the undamaged part of the waterplane, thus tending to compensate for the loss. Also, trim by the stern usually increases the transverse moment of inertia of the undamaged waterplane, and vice versa. For most ocean-going ships the combined effect of these factors is usually a net decrease in GM.

(5) Change of Freeboard

The increase in draft after flooding results in a decrease in the amount of freeboard. Even though the residual GM may be positive, if the freeboard is minimal and the waterline is close to the deck edge, submerging the deck edge at small angles of heel greatly reduces the range of positive righting arm GZ, and leaves the vessel vulnerable to the forces of wind and sea.

(6 ) Loss of Ship or Rig

Where changes in draft, trim and/or heel necessary to attain stable equilibrium are such as to immerse non-watertight portions of a ship, equilibrium will not be reached because of progressive flooding and the ship will sink either with or without capsizing.

Where the maximum GZ in the damaged condition is adequate and where the immersion of non-tight portions of the ship only results in slow extension of flooding, sinking may be quite slow. In such cases, control measures aimed at stopping progressive flooding, either by reducing heel, pumping leakage water or fitting emergency means of checking the flow of water or a combination of such measures may be successful. Therefore, it has been realized that providing the master with an instruction manual outlining damage control measures available to minimize flooding would be a valuable contribution to safety.

Floatation Calculations :

In order to assess the ship’s or rig's ability to withstand damage, it is necessary to determine:

(a) The damaged waterline, i.e. the new draft, trim and heel.

(b) The damage stability, i.e. after flooding.

The floatation calculations can be carried out by either one of two methods, the lost buoyancy method or the added weight method.

1. The Lost Buoyancy Method

In this method the lost buoyancy due to a compartment or compartments being opened to the sea is calculated. This lost buoyancy and its moments are equated to the buoyancy gain and moments accompanying sinkage, trim and heel of the remaining intact part of the ship. In this method, it is assumed that the displacement and the position of the centre of gravity are unchanged.

This procedure is convenient and simple to use if the form of the vessel and configuration of the flooded space are such that the resulting sinkage, trim and heel do not involve extreme or discontinuous changes in the remaining undamaged part of the waterline. Consequently, this procedure is often used for merchant ships.

Compartments of ship  or rig open to the sea do not fill totally with water because some space is already occupied by structure, machinery or cargo. The ratio of the volume which can be occupied by water to the total gross volume is called the permeability μ. For cargo spaces it is taken as 60%, for accommodation spaces as 95% and for machinery spaces as 85%.

The steps of this method are as follows:

1. Calculate the permeable volume of compartment up to the original waterline.

2. Calculate TPC, longitudinal and lateral positions of CF for the waterplane with the damaged area removed.

3. Calculate revised second moments of areas of the waterplane about the CF in the two directions and hence new BMs.

4. Calculate parallel sinkage and rise of CB due to the vertical transfer of buoyancy from the flooded compartment to the layer.

5. Calculate new GMs.

6. Calculate angles of rotation due to the eccentricity of the loss of buoyancy from the new CFs.

 

Semi-sub advantages

§ motion characteristics suited to most (but not all) ocean conditions

§ provides a stable platform for drilling and completion operations

§ Semi-sub disadvantages

§ variable deck load and deck space can be limited

§ relatively slow transit speed

§ may require tow vessels to move (modern DP semis are fully self-propelled)

 

Drillship advantages

§ relatively fast transit

§ most drillship designs can accommodate large loads of casing,mud chemicals and other supplies in below deck holds

Drillship disadvantages

§ less stable than a semi-sub,

§ less suited for harsh environments

§ Response of the vessel to a given seastate is calculated using the RAO –

Response Amplitude Operator

§ function of wave height and wave period

§ varies depending on direction of wave (head, beam, quartering)

§ Heave, surge and sway RAO is dimensionless (m/m)

Wave period is key

§ Typical North Sea rough weather, 10-14 second waves

§ drillship heave response is OK compared to semi-sub

§ drillship pitch response worse than semi-sub

§ North Atlantic Storm, 14-19 second waves

Semi-sub pitch and heave response is relatively good, whereas Drillship pitch and heave response is relatively poor.

Very long ocean swells >19 second period can give the semi submersible problems as wave period approaches the heave natural frequency. Such seas can occur in some open ocean areas

§ if heave RAO = 0.3 m/m, then in 10m wave, vessel will heave 3m

§ Pitch, roll and yaw RAO is °/m

§ if pitch RAO = 0.5 °/m, then in 10m wave, vessel will roll 5°

In deep water and harsh environment/high current location, riser tensioning capacity may be critical requirement

§ riser analysis essential to determine string configuration and top tension requirements

§ riser tensioning capacity varies considerably

§ <1 lbs="" million="" to="">5 million lbs

§ A heavy riser string will place considerable demands on the rig when running BOP

§ string weight in excess of 1.5 million lbs may be required in some situations

§ riser design may not permit fully buoyant string

§ additional acceleration loads due to rig heave

§ derrick and travelling equipment must be up to the task

Many deepwater rigs (most 6th generation) both semi and ship have some degree of dual activity capability

§ significant time savings possible in deepwater

§ drill top hole, run surface casing while running BOP

§ in development drilling, can very efficiently install subsea trees

§ pick up and rack drillpipe, casing offline

§ 20-30% time saving possible, depending on operations

§ can offset day rate premium

§ Best suited to short duration exploration and development drilling

§ high percentage of open water work

Stability_basic principles.pdf

Standards for subdivision and damage stability have been established by international conventions, by recommendations of IMO, by national regulations and by classification society rules. In this chapter we shall confine ourselves to the standards related to the International Convention for the Safety of Life at Sea “SOLAS” and the International Convention for the Prevention of Pollution from Ships “MARPOL”.

The “SOLAS” Standards :

The basic philosophy of these standards is that the true index of safety is the probability of survival after damage occurring anywhere along the length of the ship, between or on a bulkhead.

Fundamentally, three probabilities relate to subdivision and damage stability requirements:

(a) Probability that a ship/rig may be damaged.

(b) If the ship/rig is damaged, the probability as to the location and extent of damage.

(c) Probability that the ship/rig may survive such flooding.

Chapter II-1 of the SOLAS shall apply to ships the keels of which are laid on or after 1 January 2009. These requirements apply to cargo ships of 80 m in length (L) and upwards and to all passenger ships regardless of length. The degree of subdivision shall vary with the subdivision length (Ls) of the ship and with the service, in such manner that the highest degree of subdivision corresponds with the ships of greatest subdivision length (Ls), primarily engaged in the carriage of passengers.

Definitions

1. Subdivision Length (Ls) : The greatest projected length of that part of the ship at or below deck or decks limiting the vertical extent of flooding with the ship at the deepest subdivision draft.

2. Mid-Length : The mid-point of the subdivision length of the ship.

3. Aft and Forward Terminals : The aft and forward limits of the subdivision length.

4. Length (L) : The length as defined in the International Convention on Load lines.

5. Deepest Subdivision Draft (ds) : The waterline which corresponds to the summer load line draft of the ship.

6. Light Service Draft (dl) : The service draft corresponding to the lightest anticipated loading and associated tankage.

7. Partial Subdivision Draft (dp) : The light service draft plus 60% of the difference between the light service draft and the deepest subdivision draft.

8. Permeability (μ) : The permeability of a space is the proportion of the immersed volume of that space which can be occupied by water.

9. Bulkhead Deck : In a passenger ship means the uppermost deck at any point in the subdivision length (Ls) to which the main bulkheads and the ship’s shell are carried watertight. The bulkhead deck may be a stepped deck. In a cargo ship the freeboard deck may be taken as the bulkhead deck.

10. Amidship : At the middle of the length (L).

Permeability :

For the purpose of the subdivision and damage stability calculations of the regulations, the permeability of each general compartment or part of a compartment shall be as follows:

Spaces  -  "Permeability"

Appropriated to stores 0.60

Occupied by accommodation 0.95

Occupied by machinery 0.85

Void spaces 0.95

Intended for liquids 0 or 0.95

For the purpose of the subdivision and damage stability calculations of the regulations, the permeability of each cargo compartment or part of a compartment shall be as follows:

Spaces -- "Permeability at draft ds" ,  "Permeability at draft dp" ,   "Permeability at draft dl"

Dry cargo spaces  0.70, 0.80, 0.95

Container spaces  0.70, 0.80, 0.95

Ro-ro spaces        0.90, 0.90, 0.95

Cargo liquids       0.70, 0.80, 0.95

Ship_Stability_12.ppt

   

Stability of Semi submersible.pdf

Well control for non-drillers ...

Well control is one of the most important aspects of offshore shallow or deepwater drilling

operations. Improper handling of kicks in well control can result in blowouts with very grave consequences, including the loss of valuable resources such as in recent Deep Horizon Macondo incident losing millions in revenue. Even though the cost of a blowout (as a result of improper/no oil well control) can easily reach millions or billions of dollars, the monetary loss is not as serious as the other damages that can occur: irreparable damage to the environment, waste of valuable resources, ruined equipment, and most importantly, the safety and lives of personnel on the drilling rig.

In order to avert the consequences of blowout, the utmost attention must be given to oil well control. That is why well control procedures should be in place prior to the start of an abnormal situation noticed within the wellbore, and ideally when a new rig position is sited. In other words, this includes the time the new location is picked, all drilling, completion, workover, snubbing and any other drilling-related operations that should be executed with proper oil well control in mind. This type of preparation involves widespread training of personnel, the development of strict operational guidelines and the design of drilling programs — maximizing the probability of successfully regaining hydrostatic control of a well after a significant influx of formation fluid has taken place.

One concern is the increasing number of governmental regulations and restrictions placed on the hydrocarbon industry, partially as a result of recent, much-publicized well-control incidents. For these and other reasons, it is important that drilling personnel understand well-control principles and the procedures to follow to properly control potential blowouts.

The key elements that can be used to control kicks and prevent blowouts are based on the work of a blowout specialist and are briefly presented below:

Quickly shut in the well.

When in doubt, shut down and get help. Kicks occur as frequently while drilling as they do while tripping out of the hole. Many small kicks turn into big blowouts because of improper handling.

Act cautiously to avoid mistakes—take your time to get it right the first time. You may not have another opportunity to do it correctly.

Many well-control procedures have been developed over the years. Some have used systematic approaches, while others are based on logical, but perhaps unsound, principles. The systematic approaches will be presented here.

The drilling mud forms the first line of defence against kicks and blowouts. The second, and last, line of defence is the blow-out preventer stack. This is a collection of large, high-pressure valves which is fitted on the top of the wellhead in a vertical tier and which can be controlled remotely from any of several positions on the drilling unit. Although outwardly the BOP stack on a deep-water floater appears fairly unremarkable, it is an enormously expensive precision tool that can withstand pressures of up to 15,000 psi.

Because of the intricacy of its numerous working parts a dedicated ‘sub-sea engineer’ is employed by the drilling contractor to maintain it and its control system in top condition. Through the middle of the BOP stack is a hole wide enough for large drilling tools to pass up and down during the course of normal operations. The width of the opening is determined to some extent by the stage at which the stack is intended to be first used in the well programme. An 18-3/4” stack is quite a popular size, but this can obviously not be used until wide-diameter bits have drilled 36” and 26” hole.
When a kick or blow-out threatens the rig and the BOP controls are operated, large and powerful devices are closed together to seal off the hole and prevent the passage of well fluids up to the rig. Arrangements have to be made for sealing the hole either when drill pipe is in it, or when it is empty,
and different types of preventer are incorporated in the stack for use in everydifferent situation.The topmost preventer in the stack looks like a large steel pot from the outside and is called the ‘annular preventer’ or, sometimes, the ‘bag preventer’or ‘spherical preventer’. This can seal off the annulus between the preventer housing and any type of tubular that happens to be inside it. It can also seal off the hole completely if there is nothing inside running through the preventer at the time.

With the constant-bottomhole-pressure concept, the total pressures (e.g., mud hydrostatic pressure and casing pressure) at the hole bottom are maintained at a value slightly greater than the formation pressures to prevent further influxes of formation fluids into the wellbore. And, because the pressure is only slightly greater than the formation pressure, the possibility of inducing a fracture and an underground blowout is minimized. This concept can be implemented in three ways:

One-Circulation, or Wait-and-Weight, Method. After the kick is shut in, weight the mud to kill density and then pump out the kick fluid in one circulation using the kill mud. (Another name often applied to this method is “the engineer’s method.”)

Two-Circulation, or Driller’s, Method. After the kick is shut in, the kick fluid is pumped out of the hole before the mud density is increased.

Concurrent Method. Pumping begins immediately after the kick is shut in and pressures are recorded. The mud density is increased as rapidly as possible while pumping the kick fluid out of the well.

If applied properly, each method achieves constant pressure at the hole bottom and will not allow additional influx into the well. Procedural and theoretical differences make one procedure more desirable than the others.

Process suitability partially depends on the ease with which the procedure can be executed. The same principle holds true for well control. If a kick-killing procedure is difficult to comprehend and implement, its reliability diminishes.

The concurrent method is less reliable because of its complexity. To perform this procedure properly, the drillpipe pressure must be reduced according to the mud weight being circulated and its position in the pipe. This implies that the crew will inform the operator when a new mud weight is being pumped, that the rig facilities can maintain this increased mud-weight increment, and that the mud-weight position in the pipe can be determined by counting pump strokes. Many operators have stopped using this complex method entirely.

Basic of Well Control.ppt 

Well control Rev1.ppt  

Subsea well control

 

Norwegian Cat J rig to be built by Korean yard

Statoil and its license partners in the Gullfaks and Oseberg area fields in the Norwegian North Sea Unit have acquired two new “Category J” jackup drilling rigs.Both sets of licensees will own the rigs, which are designed to work in harsh environments, in water depths of 70-140 m (229-459 ft), and to drill wells up to 10,000 m (32,808 ft). They are based on proven technology, although optimized, Statoil says, to allow for more efficient drilling and completion of subsea wells compared with existing jackups. The primary role will be in drilling and completion of production wells.

Samsung Heavy Industries will build the rigs and KCA Deutag Drilling Norway will operate them, with offshore operations set to start in 2016-2017. The initial operation contract, valued at NOK 900 million ($155 million), is for eight years, extendable by four three-year periods.

Statoil’s strategy is to rejuvenate its rig fleet, secure long-term rig capacity, and reduce drilling costs to improve recovery rates from its Norwegian fields.

Both Gullfaks and Oseberg have long-term drilling programs, and the new rigs will likely operate at these fields for a long period. Costs are expected to be lower as a result of the ownership model, and this is expected to allow more targets to be drilled that would otherwise not be economical.

Partners at Gullfaks are Statoil, Petoro. Partners at Oseberg are Statoil, Petoro, Total E&P, ConocoPhillips

Cat J --- new design rig

A jack-up is a mobile unit that floats during transport but rest on the seabed during drilling and well operations. This is made possible by lifting and lowering the legs of the unit, and the rig is lifted above the sea surface to minimize influence of waves during operations. The specially-designed category J rig is able to operate at water depths from 70 to 150 meters and drill wells down to 10,000 meters. It will be a workhorse primarily for drilling and completion of production wells. It is a tailor-made jack-up rig for operations in harsh environment on both surface and subsea wells in the shallow-water segments on the NCS. 

Why Cat J? 

The key to maintaining today’s production level on the NCS towards 2020 is improved recovery from existing fields and fast and efficient development of new fields. In order to implement these measures it is vital to secure a rig fleet which is adapted to suit the assignments and which can work more efficient. 

To meet these challenges, sustainable cost competitiveness, drilling efficiency and sufficient rig capacity are key factors. Statoil has therefore developed a new mobile offshore drilling unit concept; cat J. This rig will be 

customised for year-round production drilling in shallow-water depths. The goal is that the new rig will perform operations 20% more efficiently than the conventional rigs. This will reduce field development costs 

and the rig fleet will be rejuvenated. 

Technical aspects 

Hull designers, topside suppliers, construction yards and drilling contractors have participated in the development of the cat J rig concept and will continue to develop this in an innovative design process. 

The cat J conceptual design has the following key elements- 

- Very competitive operational cost compared with existing rig fleet 

- More efficient drilling with quadruple derrick 

- Minimum weather downtime due to vessel motion in operation 

- Low diesel consumption cost 

- Lower wellhead fatigue exposure 

- High flexibility for efficient drilling over wellhead platforms and subsea field development 

- Facilitates for early production drilling on new field development projects 

Main features: 

• GustoMSC CJ70-X150-A concept is based on proven technology and equipment from Aker Solutions, National Oilwell Varco or TTS. 

• Ultra harsh environment jack-up 

• Test/service BOP and x-mas tree on cellar deck 

• Facilitates for new fast-track X-mas tree size and system 

• 1.5 derrick - simultaneous drilling and building stands 

• Trip saver function --- possible to hang off blow out preventer (BOP) and riser (250 tonnes) on Texas deck 

• Designed for Statoil’s subsea system on the NCS 

• X-Y skidding cantilever 110 feet reach 

• BOP riser tension systems are designed for both surface and subsea BOP operations 

• High pressure high temperature (HPHT) operations (15k) 

• Up to 150m water depth 

• Class Notation: DNV 1A1 self-elevating unit and drilling unit (N)

Cat J has a double barrier philosophy for preventing falling objects and ensure well control. It is also designed to prevent environmental spills and assure improved working environment. It features a high level of redundancy in material handling, crane coverage and pipe handling as well as power generation and control systems. 

11697 12101 001 a Principal Dimensions 

Will a basic degree direct the path for a good career ?

Extracted from Straits Times May2013

Will having a basic university degree lead you to the route of a good career?

Our PM Mr Lee Hsien Loong, National Development Minister Khaw Boon Wan, Acting Minister for Social and Family Development Chan Chun Sing and Education Minister Heng Swee Keat - recently spoke on similar themes of how academic qualifications are not a sure ticket to success.

Indeed, Asia "is a bit hung up on that piece of paper", Singapore Management University (SMU) president Arnoud De Meyer tells Insight. A degree helps get a better salary, but it is the experience of learning that is more important in today's age, says Professor De Meyer, who has held top posts at Insead and Cambridge University's Judge Business School.

He also discusses last year's announcement of a 3,000-strong increase in student numbers by 2020, noting that not everybody needs to go to university and there are good jobs that do not require a degree.

· What is your take on the recent debate?Having a good degree helps you to find a better job and a higher salary. It also offers you broader options. If you go for a diploma, usually you're quite specialised.

But there are successful people with no degree, such as (late Apple chief executive officer) Steve Jobs and (Facebook CEO) Mark Zuckerberg.

Even when the PM announced last year that 40 per cent of each cohort will go to university by 2020 (up from 27 per cent now), that still means that 60 per cent will not go to university.

If you look at other advanced countries like the Scandinavian countries or Britain, 40 per cent is in the upper limit.

University degrees offer three things as opposed to a diploma.

First, you get your specialisation, your skills you build up.

Second, it is broad base learning. Students have a lot of flexibility. Most universities now provide the students option to take second major, second degree or electives,etc..

The third,  is "learning to learn". What you learn today may be obsolete five years from now. You need to constantly learn new things. That is also what university education provides - a system of learning.

Some other articles typically note that learning how to learn is a process in which we all engage throughout our lives, although often we do not realise that we are, in fact, learning how to learn. Most of the time we concentrate on what we are learning rather than how we are learning it. The process of learning much more explicit by getting you to apply the various ideas and activities to your own current or recent study as a way of increasing your awareness of your own learning. Most learning has to be an active process - and this is particularly true of learning how to learn. 

The ministers emphasised the value of work experience and entrepreneurship. Can't these be obtained both in and out of university?   Mr Khaw said that it is not about the piece of paper you can hang on your wall but about real experience and the components of your education which is more critical.

It is about being able to immediately start working when you get a job. That is why we believe in internships and nowadays it is compulsory for every student to do an internship as well as international exposure and holistic education - these prepare people better for a job in the real world.

That's what universities need to do more in the future - mix conceptual and theoretical learning with practical exposure.

Asia as a whole is a bit hung up on that piece of paper. It's the experience of learning that is more important. A university is both about skills and things like interactions and discussions with each other, the creativity of working day and night on projects, going out, making new friends.

It is also true that we need to become more entrepreneurial, not necessarily setting up a business but in the way we act within the company. Singapore Institute for Innovation and Entrepreneurship has 55 companies created by our students. They are all small and some of them will fail but you will see that some students have that entrepreneurial attitude.

We have to be realistic. It's not necessary that everybody goes. There are really interesting and good jobs that don't require a university degree and they may be a better fit for people with more practical or artistic aptitudes. We all have different capabilities and should recognise that not everybody will have to go to university.

The second point is, we know from the European experience that if you have too many students going to university, you get graduate unemployment. So underlying the messages from the ministers is, be careful, a degree is no guarantee for a job.

Third, be careful in choosing a line of study. Choose a broad area where there will be demand.

Polytechnic graduates are concerned that in sectors where they compete with university graduates for jobs, starting pay and job progression will differ.

At polytechnics, you get very good people with much more practical skills who can hit the ground running. They start working at 19 for women or 21 for the men, and you could argue that they have four more years of earning money than a university graduate.

University graduates also have to pay their tuition fees. Somehow the market recognises the difference in investments that students have made.

· How has the value of a university degree changed in Singapore? Recruiters ask for more than skills from your studies. They are looking for communication skills and global exposure. They expect us to groom students to be more job- ready.

· How can universities ensure both that their education remains accessible while graduates are employable? By making sure our students are employable.

We have to be realistic. Just because I got a degree in economics today doesn't mean it will be valid 10 years later. That's why our young students nowadays have to think about a broad set of capabilities and skills and to keep improving.

I agree with those who believe it to be essential that for example uni lecturers have expertise in their subjects. But such expertise is not limited to certification: if it were, Tony Blair could hardly have taken up a position lecturing in Politics at Harvard when his BA is limited to Jurisprudence. And who would bother listening to Margaret Thatcher droning on about statecraft when her BSc is in Chemistry? And yet many will sit at their feet, because they have qualification way beyond a framed degree.

Thus lecturers should be an expert, but expertise comes in a number of guises. When lecturing ceases to inspire, the learning ceases to engage. When the learning ceases to engage, little or nothing is learned. What unis' and polys' desperately need are outstanding professionals with a sense of vocation: ten of those liberated to draw out students' faculties and intelligences will eclipse a hundred who have been certified and licensed by the institute to impart a centralised curriculum in accordance with government guidelines for the sake of targets and league tables.