Some design notes for semi-submersible and TLP

Semi-Submersible

In the design of a semi-submersible, and its configuration in particular, a clear idea of the functions it must perform should be in hand. These will strongly influence the configurational choices. Besides function like drilling, other functions include production, heavy lift, accommodations and operational support (surface, subsea). Apart from the mission and support functions, stated simply, there are two essential functions of a semi-submersible, i.e. to stably support a payload above the highest waves, to minimally respond to waves.

These are the principal factors that establish size. It is, however, the mission functions and associated support functions that most significantly contribute to configuration.

The four main configurational components are:   

Pontoons

Stability columns

Deck

Space frame bracing

Virtually, all semi-submersibles have at least two floatation states: semi-submerged (afloat on the columns) and afloat on the pontoons. The pontoons are the sole source of floatation of the semi when not semi-submerged. The stability columns are the principal elements of floatation and floatation stability while semi-submerged.  

Column and Pontoon 

The number and arrangements of pontoons and columns distinguish many configurational variants employed in the evolution of the semi. This has included as few as three to as many as a dozen or more columns. It has likewise included a simple two parallel pontoon arrangement, up to six, and even a grillage of orthogonally intersecting pontoons. 

The twin pontoon preference is principally because of its mobility. A preference for the 6- and 8-columns relates primarily to the twin pontoon option, and is influenced by the use of bracing systems. 

The function of the columns is to provide stability and the critical point of stability is when a semi is submerging,  the flotation undergoes transitions from being afloat on the pontoons to being afloat on the columns. This operation is restricted to mild conditions and requires only that there be “positive GM”. It is common to flare the columns at the pontoons to enhance stability through the critical range of drafts. 

The main problem in semi-submersible design is to adopt the right configuration for the specific functions required. Rigorous hydrostatic, stability, hydrodynamic and structural analyses should be performed once the appropriate shape and size is determined for the initial design.

In sizing of a semi, it is informative to re-examine the most fundamental functions of the type:

- To stably support a payload above the highest waves

- To minimally respond to waves 

The second basic function, “minimum response to waves” relates to the size, shape, and submergence of the pontoons relative to the column waterplane area, and the spacing of the pontoons and columns.

Generally, the fewer the columns, the lower is the cost of the structure, even if the lesser number of columns must be more robust. For the twin pontoon configuration, each column pair at least requires a transverse brace between the columns to resist the squeeze-pry forces. These are usually associated with diagonal bracing. The issue of bracing and deck configuration can be avoided in the initial, parametric stages of design, but must be addressed.

Column and Pontoon 

The number and type of arrangements for the pontoons and columns distinguish many variants employed in the design of the semi. This has included as few as three to as many as a dozen or more columns. It has likewise included a simple two parallel pontoon arrangement, up to six, and even a grillage of orthogonally intersecting pontoons.  Only the 4-, 6-, and 8-column configurations continue to be preferred in new generation semi. Similarly only the twin pontoon and the closed array pontoon arrangements are currently used. A 3-column.

closed array pontoon (triangular) arrangement has been proposed for both FPS semi-

submersible and TLP applications, and offers a steel reduction opportunity  but these designs have not been successful, perhaps because of the more complex deck arrangements.

The twin pontoon preference is principally because of its mobility. A preference for the 6- and 8-columns relates primarily to the twin pontoon option, and is influenced by the use of bracing systems. 

The pontoon-to-column connection is especially important, particularly with regard to structural connectivity. For reasons noted before, the column may be flared at the pontoon, typically rectangularly. If rectangular, aligning the internal bulkheads within the pontoons as continuations of the column sides can significantly reduce stress. Generally there are at least two or four pump rooms in the pontoons under the corner columns depending on the rig whether it is DP2 or DP3 or moored only type. The mooring equipment arrangement is also a significant aspect of the column design, most notably chain lockers (or wire storage), hawsepipes, external fairleaders, and windlasses (or winches) at the top. For DP semi, should thrusters be installed for the dynamic positioning purpose, space and special arrangements must be provided for the thrusters and their internal support systems, not to mention the fuel tanks. The importance of this is the overall space and size, particularly in that this is in or near the column base. 

The height of the deck and columns matter most to weight estimating and meeting stability requirements. As noted previously, the columns should be sufficiently tall to support the deck with sufficient wave clearance. With single deck semi-submersibles, the column tops are flush with the deck. With hull-type decks, particularly if the column is integrally connected, the column tops may be in level with the upper deck.  

Bracing configurations vary considerably and include a transverse bracing, low on the columns, to resist squeeze/pry forces and, with these, a transverse diagonal bracing.  The diagonal bracing is both to support the deck weight and, together with the horizontal transverse, provide the lateral racking strength. Often, a system of the horizontal diagonals is used to provide racking strength against quartering seas.

A bracing system commonly found on many of the earlier generation drilling semis is shown here, where transverse bracing  in heavy dark lines and horizontal diagonals are shown in heavy dashed lines. Where continuous, strong longitudinal pontoons are employed, the longitudinal diagonals are not particularly useful and are rarely used in contemporary designs. As a structural system, the strength of the space frame truss system is typically developed in parallel series of planes between columns, following civil engineering practice, called “bents.” Each bent is a full truss, including the deck as a top chord and the horizontal, transverse brace as the bottom chord, all spanning between a pair of stability columns. Some use an “inverted-V” form of diagonals and some use an “inverted-W.” Except for a deck girder, the members consist of large diameter, thin walled cylinders. 

The Classification rules and the API Codes may require that there be 5ft (1.5m) clearance (“airgap”) between the highest wave crest and the deck. The highest wave, or the crest level above still water, is usually specified with the design seastate data. 

Excessive airgap raises the centre of gravity and thereby impairs the payload performance. Determination of the effective airgap should consider the relative motions of the vessel. For large, long period waves, a semi will tend to rise and fall synchronously with the waves, possibly as much as 20% of the wave height (single amplitude). To recognise this, in initial design, it can be conservatively assumed that the semi rises 10% of the wave height. 

For drilling semi units the, shorter columns are preferred for a lower centre of gravity for large deck loads. Drilling semi achieve deep submergence by ballasting to a deeper draft for drilling, but otherwise deballast to a desirable airgap for severe storms. It is also desirable to minimise the ballasting time and the amount of ballast water to be handled. Consequently, mobile semi-submersibles are no taller than need be, with operating drafts no more than necessary. 

For drilling rig. the maximum drafts could well be in the 70-80ft range, with a relatively small air gap. For the severe storm condition (“survival”), drafts in the 50-60 ft range would be used and a more generous air gap. Example, with a typical hurricane survival condition with Hs, = 40 ft, the extreme crest elevation would be about 45 ft above stillwater. Allowing a 5 ft of rig heave at the crest, and 5 ft crest clearance, a 45 ft calm water airgap should be sufficient. It is also considered undesirable that the pontoon tops be exposed in the trough of extreme waves. Under the same hurricane survival condition, the pontoon tops should be at least 40 ft below still water. With pontoon depth 25-30 ft deep, this would correspond to a 60-70ft survival draft and 85ft of column between the pontoon top and the deck. Correspondingly, the operating draft would be 80-90 ft with 25 ft stillwater airgap. The initial deign of a drilling semi-submersible would be based upon achieving the best drilling performance, and be based upon the shallower operating draft. 

A big issue in semi-submersible design criteria is the rig lifespan, inspectability, future class survey and repair. For permanently sited semi-submersibles, there are site-specific extreme environments and the fatigue requirements and the difficulties in structural maintenance, repair and inspections.

Conversely, mobile units can be dry-docked and can also be inspected and repaired afloat on the pontoons. However, the MODU classification rules do represent unlimited, world class service and this is actually quite severe. Also, most semi-submersibles give 30 years or more in service life. Quite often, the extreme design loads for mobile units are more severe than those of the permanently sited units. The opposite is true for the mooring systems, whereby the permanent structure mooring is usually subject to more severe requirements than the mobile units. 

Local strength is the consideration of whether the structure is sufficiently strong to resist the expected distributed load, particularly the hydrostatic pressures. This applies to the plating, the stiffening, and the framing of all watertight surfaces. It also applies to distributed deck loading as well as a variety of functional concentrated loads. In this connection it is significant that 80-85% of all hull steel is a consequence of local loading. For the column and pontoon shell, and the internal subdivision surfaces, a variety of hydrostatic heads are to be considered as potential controlling design pressures. At a minimum, the shell plating must be designed to resist the static loading for the most extreme operating draft without consideration of internal pressure. However, the water-tight shell must be designed for no less than a 20 ft head and this is where the watertight doors have to be able to take such pressure as well.

Global strength addresses the overall strength of the structure as a space frame, and of the main elements forming it. For a semi-submersible, the elements that form the space frame are the pontoons. columns, and deck and may include bracing. Global strength relates primarily to two types of loading systems: the gravitylbuoyancy load and the environmental loading. The direct loading of waves and the inertial load from consequent response are the principal environmental loads. What is unique to the global strength of the semi-submersibles is the controlling load patterns. 

An idealised distribution of deck load and concentrations of buoyancy forces at the pontoon and column lines is shown in the section view. An additional gravity load is included in the pontoons and the columns. A distribution of gravity loading on the superstructure must be supported by buoyancy concentrated at the extremities, causing a tendency to sag. This causes very large tensions in the horizontal brace to resist the sag. Additionally, the interior parts of the deck weight will transmit directly through the diagonals into the column. This exhibits one important function of the main bracing as primary structure. Particularly important are the end connections at the column, especially the efficiency of the load flow from the diagonal to the transverse. 

In semi design, there must be sufficient buoyancy to balance the weight of the rig and the external forces. The required buoyancy determines the underwater volume, or “displacement.” This comprises the volume of the pontoons, the columns, and, sometimes, the bracing. Displacement is a primary determinant of size and proportions. Consequently, much of the initial design work is devoted to determining all the components of weight.

Although payload and its height above the most extreme of waves were specifically identified as the salient factors in design, payload ( or also call VDL ) is only a part of the total weight. It and all other weights as well as its centre of gravity is needed to proceed with a design, at least a first estimate. This estimate should be continuously refined throughout the designing process.  Weight is made up of two components, “Lightship” (W,) and “Variable Load”. The former comprises all the steel, equipment, and outfitting provided at completion and is usually defined and verified according to regulation. The latter comprises all weight beyond the light ship to be carried by the semi: i.e. variables like the ballast, the consumable liquids, the bulk items, the personnel and effects, 3rd party equipment, fuel, etc. and, as the name implies, varies according to the operating state of the vessel. In addition, there are a variety of external loads to consider (e.g. mooring tensions. riser tension, hook load, etc.).

As for the term "payload", this comprises all of the mission-related equipment, variable load, and external load. The necessary support system weight that is needed, regardless of the mission function (e.g. mooring equipment and other “marine systems”), is not considered to be a part of the payload. But some rig owners may like to consider mooring anchor and its wire as VDL. Payload exclusive of deck structural steel, is referred to as net payload. However, if the deck structural steel is included, it is referred to as gross payload. The net and gross distinction is needed, particularly in comparing designs, because some mission functions can have a high impact on the amount of structural steel and is not an inherent property of the semi design. Such distinctions are particularly important when the same design is used for varied applications and also when conversion and upgrades are to be considered. This distinction is also needed in the evaluation of designs in as much as many designers are not consistent. 

Response Amplitude Operator (RAO)

A wave scatter diagram provides a long-term wave description for only one specific site. Determining the stress Frequency Response Function (FRF) or Response Amplitude Operator (RAO), H (0; an,&) is one of the major efforts in the strength assessment, because it allows the transfer of the exciting waves into the response of structures. This concept of linear dynamic theory is applicable to any type of oscillatory "load" (wave, wind-gust, mechanical excitation, etc.) and any type of "response" (motion, tension, bending moment, stress, strain

etc.).

For a linear system the response function at a wave frequency can be written as

Response(t) = RAO.q(t)

where V(t) denotes the wave profile as a function of time t. The RAO could be determined using theoretical computation or experimental measurement (Bhattacharyya, 1978). Almost all of the theoretical computation has neglected viscosity and used potential flow.

The structure may be envisaged in a general terms as a ''black box", see Figure 3.7. The input to the box is time-history of loads and the output from a structural analysis is time-history of the response. The basic assumption behind the RAO concept is linearity, that allows superimpose the output based on superimpose of the input. In these situations, the response to regular oscillatory loading of any waveform can be obtained by expressing the load as a Fourier series, and then estimate the corresponding Fourier series of the response for each component. A typical RAO is shown in Figure 3.8, that is a roll RAO of a barge in beam seas.

The RAO is given in degrees (or meters/A) of motion amplitude, per meter (or A) of wave amplitude and expressed as a function of wave period (second). The RAO may be calculated using the first order wave theory as wave fkequency response.

Another application of the RAO is to calculate loads in irregular waves. It is suggested that the total response of a vessel in an irregular seaway is the linear superposition of the response to the individual components that may be determined using RAO.

In the calculation of H (o,a&), a suitable range of wave frequency, number of frequency points, and wave headings should be used. The commonly used parameters for an FPSO analysis are:

Frequency increment: 0.05 rad/s

Wave heading: 0" to 360" with 15" increment

If a finite element method is used, the pressure distribution needs to be mapped from a hydrodynamic model onto a finite element model with NAXNFXNH loading cases, where:

Frequency range: 0.20 I o I 1.80 rad/s

TLP- Tension Leg Platform

Tension Leg Platforms have been used exclusively as production and drilling platforms,with the exception of the “East Spar” platform, which is a control buoy. Figure  shows the latest platform P61 recently completed in Brazil

Like semi-submersible, the TLPs consist of columns and pontoons. The unique feature is the mooring system, which consists of vertical tendons (sometimes called “tethers”), which restrain the heave motion.

As with the semi-submersible, the main problem in TLP design is to address the specific functions and rational sizing. Even more so than semi-submersibles, the construction program consideration is a salient design issue. Here too the approach is to initiate the

design with a straight forward process for sizing, and to discourage multiple trial-and-error analyses with inappropriately rigorous detail and methods.

The initial design should include the best working weight estimate, hull displacement, and hydrostatics available.

The key analytic areas for preliminavy analyses of the initial design for a TLP include the following:

Weights and CG’s

Wind Forces

Current forces

Global Performance Analyses

-Motions

-Drift force

-Tendon tensions

Global Strength

With a well executed initial design, a model can be quickly established for the above analyses to rigorously proceed in parallel without the need for additional major design iterations. Results for the preliminary analyses, based upon the initial design, can be used for a reasonably conclusive revision, this being the preliminary design. The initial design is also adequate for the beginning of specialised subsystem (topsides, tendons, riser, installation, etc.) design and analyses to proceed concurrently. The following is about how to develop such a model.

Unlike semi-submersibles, hydrostatics and stability are not salient design issues for a TLP, although they are important considerations for addressing transport and installation. The TLPs to date have been exclusively used for permanently sited production systems, most with drilling or workover functions. They have fewer functions to consider and therefore limited configurational variants.

Although it is implicit in any design, that the construction, transport and installation scenario is a particularly important aspect of TLP design with considerable impact on certain design choices. Usually, the hull and deck are separately fabricated, with either an inshore hull-deck mating or an offshore heavy lift. In either case transit, ballast to the operating draft, stability, and installation of the tendons are key issues in the design of TLP.

Besides the mission and support functions, the essential function of a TLP is to the support a payload above the highest waves. More specifically, the hull is to provide buoyancy, both for the support of weight and to provide tendon tension. It should also be tall enough to give the deck wave clearance in all modes of operation. Tendon tension has as much influence on hull size as the payload.

While the heave motion of a semi-submersible is a salient design issue. vertical motion of a TLP is far less of an issue and entirely different. While the TLP does not heave, it will undergo set-down with offset. Like a semi-submersible, the TLP is laterally compliant and will surge, sway and yaw. In both platform types, there is relatively little design-wise that can be done to affect lateral motions, although steady offset can be minimised by increased tendon tension.

The three main configurational components are:

Pontoons

Stability columns

Deck

Decks of TLPs (and some production semi-submersibles) are unique. Virtually all TLP decks are separately

built from the hull (often on a different side of the world) and joined later, either at dockside, offshore, or in a separate, sheltered location. An underlying issue with this uniqueness is more administrative than technical. With TLP hull installation benefitting from minimisation of topside weight, usually some parts of the deck system are placed after installation of the TLP. The deck-to-column interface is of considerable importance, with many options for load transfer and securing, and many being proprietary.

A semi-submersible is a true, free floating structure, restrained with compliant spread moorings and/or

dynamic positioning, a TLP is kept in place through lateral forces developed by the tendons when the TLP is moved off from centre. The lateral force is dependent upon the tendon tensions. Consequently, a major portion of the TLP buoyancy is devoted to development of tendon tension.

A TLP is highly compliant to lateral forces and. at the same time, is highly resistant to vertical forces. Offset from lateral forces is not altogether different from surge,’sway response of any compliantly restrained floater. However, what is truly different for a TLP is set-down.  Nevertheless, it is noted that tendon response periods higher than say 3seconds should be avoided in the initial design. To achieve this, tendons need sufficient stiffness relative to the TLP mass. This precludes certain material for tendons and tends to pose limits on TLP water depth.

Generally the internal outfitting within the TLP hulls is considerably less than found on semi-submersibles. Typically, there is no mooring equipment. In as much as there is very little active ballast, piping systems are much different. In service, most internal spaces are considered to be voids and may be piped differently than in a semi-submersible. The ballast is used for installation (and removal). but this may be through a temporary system deactivated after installation. Otherwise, any “operable” system must be maintained in working order. Often a less robust system is employed for damage control dewatering, but this depends upon the damage mediation strategy.  The TLP is essentially a fixed-draft, constant buoyancy system and, once installed, does not rely on floatation stability. It was also noted that a large part of the buoyancy is provided to develop tendon tension. While small changes in the sea level (e.g. tide) and set-down do occur. these result in small changes in tendon tension.

Variable load on TLPs is mostly on the deck and includes operating liquids, supplies, drilling and other consumables as well as personnel and effects. Liquid consumables (e.g. fuel, drill water, etc.) may be on the deck or within the hull. Additionally, and significantly, is the tension of top tensioned risers. Hull variable load will include ballast and risers, export in particular, but sometimes subsea well riser systems. Hull variable load on a TLP is usually quite small. While there may be very little difference between the normal operating loading condition and that for a severe storm, there can be significant differences related to the state of production riser deployment. There may, however, be special ballast distributions related to tendon tensions. There also may be special ballast distributions related to tendon failure or internal flooding. Without further detail, transport and installation will address a number of loading conditions, most expedient to the objectives.

Marine HVAC system and notes....

On board every rig or vessel, there is an accomodation module or living quarter which houses the crew of about 200 men or more

working day and night offshore and in this quarter area, there is a set of offshore heating, ventilation and air conditioning, and refrigeration systems providing 24 hours non-stop either cooling ( in summer) or heating ( in winter ) for the comfort of the crew members.

Therefore there is need of professional  marine services and construction personnel to tackle these challenging projects to meet the rig specific requirements, and engineer a set of proper equipment selections.

HVAC/R systems engineering

Involves in design, fabrication , supply and installation of all kinds of HVAC systems. Based on  rig supplied general lay-outs (A-CAD) all required calculations and drawings for design and fabrication, including:

•Detailed calculations for heating, cooling, air-volumes, and humidification

•Air pressure and noise levels evaluation and treatment, carbon dioxide calculations

•P&ID, airflow and electrical diagrams

•Data sheets, material lists, instruction manuals, MRBs and supply chain information

•Commissioning plans and project supervision

Air conditioning systems -

•Water and air-cooled DX / condensing units

•Water and air-cooled chill water units

•Package air-handling and fan coil units

•Package self-contained units

•Cabins units, terminals, diffusers and accessories

•Cold room and freezer evaporators, room controllers and timer panels

•Control systems for platform integration, F&G interface, local AHUs

•Refrigeration valves, separators, condensers, and accessories

•Fire and hydrocarbon rated dampers, blast dampers, local control stations

The system package also involves supplying air conditioning systems specifically designed for the marine and offshore industries with custom-designed marine and explosion-proof chilled water systems (marine chillers) as well as marine condensing units for clients, and these air conditioning systems on board the rig or vessel could either include:
•Direct expansion (DX) cooling systems with environmentally friendly refrigerants such as R134a, R404a, R407c; sea water or fresh water cooled or air cooled

•Chilled water central cooling systems for complete accommodations Integrated or local heating systems

•Electric duct heaters

•Cabin units with integrated re-heaters

•Hot water heating coils (duct or AHU mounted) complete with two-way or three-way control valves

•Steam heating coils complete with two-way or three-way control valves and traps

•Thermal oil heating coils (duct or AHU mounted) complete with two-way or three-way control valves

•Electric unit blast heaters (safe and hazardous areas)

•Thermal oil unit blast heaters (safe and hazardous areas)

A well-designed air distribution system shall result in an efficient air conditioning system. A low-velocity duct system is practical in facilities where space is of secondary importance and a high-velocity duct system is often most practical in a facility where space is at a premium. In this instance, spiral high press duct is used.

1. Due to space constraints and considering the air flow requirements in offshore service, the designer may have to go for high pressure ducting. The pressure drop in supply air ducting will be in the range of 1000 to 1500 Pa and that in return air ducting will be in the range of 500 to 1000 Pa.

2. Ducts that may carry contaminated air or run through areas that may become contaminated shall be gas tight. Duct systems shall be designed within prescribed limits of available space, friction loss, noise level, heat loss or gain, and pressure containment.

3. Circular ducting (machine fabricated by using GS strip bands of 100 or 150 mm width) is recommended, as the helically wound longitudinal joints provide adequate mechanical strength.

4. Ductwork connections to the outside atmosphere and through fire barriers would need to be provided with fire / gas dampers rated to that of the fire barrier penetrated.

5. Ducts shall be constructed in accordance with applicable Sheet Metal and Air Conditioning Contractors National Association (SMACNA) standards.

6. Special attention shall be paid to ductwork connections to fan inlets and outlets in order to maximize the fan performance. See AMCA publication 201.

7. Flexible ducting shall be kept to a minimum and be used only for vibration damping or thermal expansion purposes.

8. Return air shall be ducted to get a proper air balance. Some contractors may use return air plenums above a false ceiling, which results in unbalanced operations within a short period of commissioning, as the space above the false ceiling is being used for other services too.  

HVAC control system :-

The HVAC system shall as a minimum have the following control provisions:

1. Auto/manual operation selecting facilities

2. Start/stop of fans

3. Fan and damper status/alarm
4. Alarm for loss of pressurization/flow

5. Auto/stand-by selecting facilities for fans

6. Temperature status/alarm for temperature sensitive areas

7. The system logic shall be equipped with manual reset

8. Controlled shutdown

9. Emergency shutdown and facilities for safe re-start after an incident

SYSTEM LAYOUT

Careful consideration should be given to the location and layout of HVAC systems and associated plant and components to enable adequate routine inspection testing and preventative and breakdown maintenance to be carried out without prejudicing safety of the installation.

1. Suitable access platforms and routes for entry and removal of expendable components or failed equipment should be provided.

2. Access doors into plant and ductwork would need to be of sufficient size to enable servicing to be adequately carried out.

3. HVAC systems should be laid out with safety aspects in mind. They should be kept clear of areas prone to damage from normal operations. Where practicable, hydrocarbon fuel lines and main power and signal cables would need to be kept clear of HVAC systems

Hull Ventilation systems -

Involves dedicated ventilation systems as well as component equipment such as fans, louvers, and moisture eliminators with filter coalescers in stainless or galvanised, and such Hull ventilation system could be in these machinery spaces :

•Engine room ventilation

•Mud pit ventilation (explosion-proof fans)

•Shale shaker room ventilation (explosion-proof fans)

•Machinery space areas

•Hazardous-area ventilation
•Hazardous-area cooling


Ventilation DESIGN CONCEPTS :-


Area classification enables all parts of the rig installation to be identified as one of the following:

1. Zone 0 (Hazardous Areas), in which an explosive gas / air mixture is continuously present or present for long periods.
2. Zone 1, in which an explosive gas / air mixture is likely to occur in normal operation.


3. Zone 2, in which an explosive gas / air mixture is not likely to occur in normal operation, and if it occurs it will exist only for a short time.

4. Non-hazardous areas - manned and un-manned areas in which an explosive gas / air mixture will not occur in normal operation.

The design of a confined ventilation system shall ensure the desired airflow characteristics when personnel access doors or hatches are open. When necessary, air locks or enclosed trap shall be used to minimize the impact of this on the ventilation system and to prevent the spread of airborne contamination within the facility. The ventilation system design shall provide the required confinement capability under all credible circumstances including a single-point failure in the system.

  Maintenance of a pressure differential between hazardous and non-hazardous areas (generally in the range of 30 to 70 Pa) is essential to prevent ingress of toxic or hazardous gases like H2S or CO or CO2. Hazardous areas (zone 0 and zone 1) shall be at negative pressure whereas the non-hazardous zones shall be at positive pressure. Positive pressurization is achieved by dumping more outside air flow than it is exhausted from the spaces. Requirements include:


1. Living accommodation should preferably be located in a non-hazardous area and shall be at a positive pressure with respect to outside ambient. Usually the passage or corridor is positive against the outside environment to prevent any ingress of gas into the living module.

2. Mechanically ventilated enclosed escape ways shall have overpressure against neighboring areas.

3. All process areas such as mud storage, mixing, chemical storage rooms, shale shakers and pump rooms should be at negative pressure with respect to adjacent lower classification zones. Arrangements shall be made to enclose the various mud handling processes within hoods, booths or enclosures so as to trap fumes, dust and gas at source and exhaust to a safe point of discharge to the outside atmosphere.


4. All areas housing hazardous equipment such as battery rooms shall be maintained at negative pressure.     

Marine-Hvac 1.pdf

Ducts pressure loss.ppt

Testing_of_HVAC_fan.ppt

fan theory.pdf

Hvac.pdf

HVAC-compressor.pdf



More about Offshore drilling .....

The present offshore drilling industry is involved in an unprecedented construction and drilling boom. Hundreds of millions of dollars are being spent to capture the much-needed energy supplies and offshore drilling activities around the world have never slowdown. The technology for drilling and producing in deeper waters and in more hostile environments have been rapidly and continuously expanding with better drilling tools and interphase software technology to enable safer and more productive when carrying out drilling and exploration.

With the advancement in technology, these factors have created an acute shortage of trained and qualified personnel to operate the rigs now under construction by various shipyards over the world. 

Offshore mobile drilling units as we know them today contained alot of sophisticated machineries for the running or drilling into the deep. However, in the old days, original units were simply land rigs taken into shallow waters and placed on a structure for drilling. The same drilling techniques that had been developed on land were used on the first offshore rigs. These techniques worked for some time, but the need to drill in deeper waters created a new type of offshore structural design. And along with the new engineering concepts came the new breed of drilling rigs which we see today.

Following offshore market trends, we find that there are basic types of offshore mobile drilling units: the submersible, the jack-up, the semisubmersible, and the drillship. 

The early breed of jack-ups was primarily designed to operate in the U.S. Gulf of Mexico area in water depths up to 200 feet. Wave heights in the range of 20 to 30 feet with winds up to 75 mph were considered as design criteria for these units. In most cases, in the event of a pending hurricane, the rigs were withdrawn

to sheltered areas. Today's jack-ups, however, are being used in international waters in a range of environmental conditions that many years ago were considered to be unrealistic. For example, a rig designed for 250-350 feet of water will have to meet the following range of criteria:

a. U.S. Gulf Coast- 50foot wave, 125 mph wind, minimal current.

b. North Sea- 75 foot wave, 115 mph wind, 1 to 2 knot current.

c. Southeast Asia- 30 foot wave, 100 mph wind, minimal current. 

As the water depth increases, the criteria rise accordingly and for 400 foot water depths the range becomes:

a. U.S. GulfCoast- 65 foot wave, 125 mph wind, 1 to 11/2knot current.

b. North Sea- 90 foot wave, 125 mph wind, 2 to 21/2knot current.

c. Southeast Asia- 50 foot wave, 115 mph wind, 1/2 to 1 knot current. 

http://kimwhye.blogspot.sg/2010/10/insights-of-jack-up-drilling-rig.html

Semi-submersibles permit drilling to be carried out in very deep waters and they are held on location either by a conventional mooring system or by dynamic positioning thrusters installed at the bottom of the floating pontoons. The conventional mooring system usually consists of 8 anchors placed in a spread pattern and connected to the hull by chain or wire rope, sometimes even a combination of both. The dynamic positioning method is an evolution of the ship sonar system whereby a signal is sent out from the floating vessel to transducers set out on the ocean floor. Dynamic positioning becomes a greater necessity as the water depth increases and is generally considered necessary in water depths beyond 1,000 feet. However, a semi-submersible in the past been contracted for 1,500 foot water depths using the anchor and chain method. Much of the necessary chain will be carried on supply vessels. Nowadays, semi-sub have enough capacity to carry such additional variable load without having to depend on supply vessels. 

Because of the submerged maps of the semisubmersible, rolling and pitching is of a low magnitude, The motion that causes problems for the semisubmersible is heave, or the vertical motion. Because of forces on the drill string when the vessel is heaving, the semisubmersible with a low heave response is considered to be the most suitable. Heave is generated in response to exposed waterplane and is expressed as   T =  2π/√(gt/D)   where T = time in seconds; t = tons per foot immersion; D =displacement in tons. 

Therefore, the smaller the waterplane area, or 't', the lower the heave response. This is achieved in the semisubmersible by submerging the lower hulls ( pontoon ) and floating at the column or caisson level. With the loss of waterplane area to reduce heave response, a reduction in stability follows. Designer must reach a compromise between acceptable heave response and adequate stability. There are, of course, other methods of reducing heave induced forces on drill string. 

In selecting a semisubmersible, it is therefore necessary to consider the following criteria:

a. Water depth.

b. Drilling depth requirement.

c. Environmental criteria.

d. Motion characteristics.

e. Consumables capacity.

f. .Mobility

http://kimwhye.blogspot.sg/2012/08/more-on-ultra-deepwater-drilling-semi.html

Drillships

The last type of mobile drilling unit is the drillship. As the name implies, it is simply a shipshape vessel used for drilling purposes. Earlier drillships were converted vessels, either barges, ore carriers, tankers, or supply vessels. However, although conversions are still being done, there are now many advanced drillships being designed purely for drilling, such as the earlier ones Glomar Challenger or the Offshore Discoverer. Drillships are the most mobile of all drilling units, but they are the least productive. The very configuration that permits mobility results in very bad drilling capabilities. Drillships are being used extensively in the U.S. Gulf Coast to bridge the gap between the jack-up and the semisubmersible.

However, it is the drillship that has drilled in the deepest water, over 1,000 feet ( now 10 times the depth capability). As discussed earlier, heave is the major problem when using a floating vessel. The drillship, because of its surface contact with the sea, develops very large heave response compared to the semisubmersible. It is possible, by means of stabilizing tanks and other methods, to reduce roll on drills hips but heave cannot be reduced. A subsequent increase in "rig downtime" or "lost" time occurs. Because of this there is a bigger demand for the use of compensation devices.

Mooring for drillships is very similar to the methods previously discussed for semisubmersibles. However, there is one additional system that has been developed on a drillship-the "Turret" system. 

http://kimwhye.blogspot.sg/2012/08/offshore-drillship-design-and-building.html

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Drilling Operation Guide 

Drilling Equipment and Operations 

Marine Engineering Knowledge 

China shipbuilders yet to recover from setback while singapore yards still in good shape

With the last few years of shipbuilding and global economy downturn, 2013 we see the global economy with mild recovery but the shipbuilding industry still set with some China big players facing severe credit crunch and some are well-established with strong fundamental chinese shipbuilders. Cosco's share price has plunged 28 per cent to 74 cents while Yangzijiang was down 25.4 per cent to 85 cents as of this month, while they contended with the shipbuilding downturn in China and now the credit squeeze in the mainland banking system.

Going by the gloomy forecasts from analysts on the sector, these Chinese shipbuilders' fortunes may not change for the better any time soon ( while I hold some shares of Cosco two years ago, seems likely it will not be any sooner to see sign of "ROI" with the small sum being set aside to this counter.. . :(   

One long, dark shadow is being cast by another fellow shipbuilder Hong Kong-listed China Rongsheng - once billed as the biggest shipbuilder in the world in terms of tonnage of orders on hand - which faces the grim prospect of insolvency. It has gotten workers to take "holidays" and even had to borrow money from its founder to stave off a cash crunch.

RS also had to contend with laid-off workers who formed a blockade outside the company's Nantong shipyard over a wage dispute, even as it tried to fend off other problems such as a lack of orders this year. Its woeful tale provides a cautionary lesson for investors on the problems plaguing China shipbuilders. China Rongsheng Heavy Industries Group, China's largest private shipbuilder, appealed for financial help from the Chinese government and big shareholders on Friday after cutting its workforce and delaying payments to suppliers.

Analysts said the company could be the biggest casualty of a local shipbuilding industry suffering from overcapacity and shrinking orders amid a global shipping downturn. New ship orders for Chinese builders fell by about half last year.

Hours after China Rongsheng made its appeal in a filing to the Hong Kong stock exchange, where the company is listed, Beijing vowed to bring about the orderly closure of some factories in industries plagued by overcapacity. The statement by the State Council, or cabinet, laid out broad plans to ensure banks support the kind of economic rebalancing Beijing wants as it looks to focus more on high-end manufacturing. It did not mention any specific industries or companies and there was no suggestion it was referring to Rongsheng.

RS said it was expecting a net loss for the six months that ended June 30 from a year earlier, according to the filing. The company reported a net loss of 572.6 million yuan ($93.47 million) in 2012, its worst-ever, despite getting government subsidies of 1.27 billion yuan in that year, its latest annual report shows. It's shares plunged 16 percent to a record low in heavy turnover on Friday, leaving its market capitalisation at just under $1 billion.

Some Investment Research think that Cosco with its large debt burden will be vulnerable. The group's net gearing climbed to 131 per cent as at the end of the first quarter from just 10 per cent at the end of 2010. Cosco's existing $3.4 billion debt would need to be refinanced within the next 12 months. The yard's  free cash flow is also likely to remain negative for the next few years, due to its low net profit margin and increasingly back-end loaded contracts in its order book..

However, one analyst believes that the stress on the Chinese shipbuilding industry from the slowdown in vessel orders may not affect all shipbuilders in the same way. On the flipside, signs of instability at a yard can become a self-fulfilling prophecy, as shipowners withhold progress payments if there is concern that the yard cannot complete the order. As of the first quarter, Yangzijiang's order book was US$3.3 billion, 75 per cent of which is in container vessels. In our view, Yangzijiang's ability to produce high quality, large container vessels and a strong balance sheet with 11 billion yuan in new cash and financial assets makes it a long-term winner in the shipbuilding industry.

By contrast, another Chinese shipbuilder, Singapore-listed Yangzijiang Shipbuilding (Holdings) Ltd, has secured total orders of $1 billion in the first half, Barclays said.

While the Chinese shipbuilding industry faced "unprecedented challenges", China Rongsheng's board was confident management could ease pressure on working capital in the near future and maintain normal operations, the company said in the filing.

The Chinese government has been trying to support the domestic shipping industry since the 2008 financial crisis, and local media reports said this week Beijing was considering policies to revive the shipbuilding business.

The holding orders of Chinese shipyards dropped 23 percent in the first five months of this year compared with a year earlier, according to the China Association of the National Shipbuilding Industry. New orders dropped to a seven-year low in 2012. ($1=6.1258 yuan)

Following the setting up of YOEPL, together with one of our wholly owned subsidiaries, Jiangsu Yangzijiang 

Shipbuilding Co. Ltd (“Jiangsu Yangzijiang”), another new subsidiary named Jiangsu Yangzijiang Offshore Engineering Co., Ltd (“JYOEC”) was set up in Taicang.  On 3 December 2012, JYOEC secured its maiden offshore contract – a Jackup Drilling Rig worth US$170 million. 

Some of YZJ other recent ventures that extend on shipbuilding capabilities include steel fabrication for building facades of petrochemical plants, energy equipment manufacturing as well as other non-shipbuilding activities. 

Beyond shipbuilding and its related activities, they have developed supplementary income streams from conservatively managed businesses such as low-risk financial investments. They also leveraged on strong balance sheet to assist ship owners in ship finance and lease vessels for income. 

YZJ currently manage more than Rmb 11 billion of financial assets that are over and above its Rmb 2 billion cash reserve, which is held for working capital, expansion and dividend payment needs. 

Having supplementary income streams puts them in the favorable position of being able to be selective on shipbuilding contracts during downturns. That means they need not enter contracts on compromised terms and conditions. They expect the shipbuilding environment to remain difficult in 2013 and intend to deliver 42 vessels in 2013, which is lower than the 51 vessels done in 2012. 

The poor shipbuilding market has proven to be an opportunity for YZJ to become more client-oriented and competitive. They will focus on developing vessels that meet ship-owners’ needs and focus on large vessels, for which there is greater demand.  Even though faced with stiff competition as many other shipyards are likewise trying to enter this sector to mitigate the shipbuilding downturn, they will not easily take orders with unfavorable terms just to secure contracts. 

Rather, they intend to secure offshore orders selectively, and work towards a good track record of timely and successful deliveries. During the downturn, they will seize opportunities to generate additional income streams through joint ventures with established players in low-risk business sectors that are synergistic with their Group’s development. 

OIL rig-building yards in China may offer lower prices and more attractive financing, but Keppel Fels remains unfazed by talk of keener competition and tighter margins. The world’s largest rig-builder has its own competitive edge – on-time delivery and costs that are kept within budget.

Keppel Fels told The Straits Times: “Look at our orders. The Chinese story has been there for at least two years now, but today, we’re still getting our B Class orders. This year alone, when the Chinese have been playing in full swing, we’ve already got eight jack-ups. ”

This year, Keppel Fels is delivering a record 20 rigs, well over the previous peak of 13 seen in 2009. http://kimwhye.blogspot.sg/2013/02/keppel-handover-first-few-of-twenty.html

Our Tuas yard was abuzz with activity during a recent visit by The Straits Times, with workers clocking overtime hours and the building docks fully occupied. This amid reports of widespread job layoffs at Chinese shipyards even as they diversify into rig-building to offset their ship order slump. There has also been the recent credit crunch on the mainland.

The B Class is Keppel’s signature rig and is its most popular design. Since 2010, its B Class rigs have accounted for about 45 per cent of the total number ordered among rigs of its class globally. Developed by its technology arm and launched in 2000, the rig is able to operate in water depths of up to 120m and drill to depths of 9,000m.
http://kimwhye.blogspot.sg/2013/04/the-61st-b-class-jackup-ordered-since.html

We just delivered its 45th B Class rig recently – a fitting milestone given that this year is parent company The rig was delivered to Arabian Drilling Company (ADC) 14 days ahead of schedule,  on budget and with a perfect safety record. Keppel FELS was awarded an early delivery bonus of US$210,000. Named ArabDrill 50 at a ceremony yesterday, the rig will be chartered to Saudi Aramco for operations in offshore Saudi Arabia. The innovative and cost-effective KFELS B Class jackup rig has proven to be the trusted, reliable workhorse of the industry, and has performed excellently in major offshore exploration and development programmes in various locations worldwide. It has been employed by some 20 international drilling operators in over 15 countries. With usage rates calculated on a daily basis, and costing about US$150,000 to US$190,000 a day, having a more efficient rig can result in cost savings of several million dollars per drilling project. This could amount to many more millions of dollars, if one considers that the lifespan of a rig is typically at least 20 years.

http://kimwhye.blogspot.sg/2010/10/insights-of-jack-up-drilling-rig.html
http://kimwhye.blogspot.sg/2011/09/more-on-drilling-jack-up-and-some-of.html

Listing 2013 MODU Construction