Saturday, August 31, 2013

Vessel machinery operations

Ships and offshore rigs are large, complex vessels which must be self-sustaining in their environment for long periods with a high degree of reliability. A vessel ( rig or ship ) is the product of two main areas of skill, those of the naval architect and the marine engineer. The naval architect is concerned with the hull, its construction, form, habitability, stability, strength and ability to endure its designed offshore environment.
The marine or ship engineer is responsible for the various systems which propel, power and operate the vessel or rig. More specifically, this means the machinery required for power, propulsion, anchoring and securing, cargo handling, HVAC air conditioning, marine systems generation and its distribution. Some overlap in responsibilities occurs between naval architects and marine engineers in areas such as propeller design, the reduction of noise and vibration in the ship's or rig structure, and engineering services provided to considerable areas of the vessel.
A vessel might reasonably be divided into three distinct areas: the cargo-carrying  and ballast tanks, the accommodation and the machinery spaces. Depending upon the type each vessel will assume varying proportions and functions. An oil tanker, for instance, will have the cargo-carrying region divided into tanks by two longitudinal bulkheads and several transverse bulkheads. There will be considerable quantities of cargo piping both above and below decks. The general cargo ship will have various cargo holds which are usually the full width of the vessel and formed by transverse bulkheads along the ship's length. Cargohandling
equipment will be arranged on deck and there will be large hatch openings closed with steel hatch covers. The accommodation areas in each of these vessel or rig will be sufficient to meet the requirements for the crew, provide a navigating bridge area and a communications centre. The machinery space size will be decided by the particular machinery installed and the auxiliary equipment necessary. A passenger ship, however, would have a large accommodation area, since this might be considered the 'cargo space'. Machinery space requirements will probably be larger because of air conditioning equipment, stabilisers and
other passenger related equipment. 

Three principal types of machinery installation are to be found offshore sea today. Their individual merits change with technological advances and improvements and economic factors such as the change in current oil prices. The typical layout may involve the use of direct-coupled slow-speed diesel engines, medium-speed diesels with a gearbox, and the steam turbine with a gearbox drive to the propeller.
A propeller or thruster, in order to operate efficiently, must rotate at a relatively low speed. Thus, regardless of the rotational speed of the prime mover, the propeller shaft must rotate at about 80 to 100 rev/min. The slow-speed diesel engine rotates at this low speed and the crankshaft is thus directly coupled to the propeller shafting. The medium-speed diesel engine operates in the range 250—750 rev/min and cannot therefore be direct coupled to the propeller shaft. A gearbox is used to provide a low-speed drive for the propeller shaft. The steam turbine rotates at a very high speed, in the order of 6000 rev/min. Again, a gearbox must be used to provide a low-speed drive for the propeller shaft.

Slow-speed diesel engine

Direct-drive diesel engine may be used in the machinery arrangement. The auxiliaries visible are a diesel generator on the upper flat and an air compressor, below. Other auxiliaries within the machinery space would include additional generators, an oily-water separator, an evaporator, numerous pumps and heat exchangers. An auxiliary boiler and an exhaust gas heat exchanger would be located in the uptake region leading to the funnel. Various workshops and stores and the machinery control room will also be found on the upper flats.

Geared medium-speed diesel 

Medium-speed (500rev/min) diesels could be used in the machinery layout. The gear units provide a twin-screw drive at 170rev/min to controllable pitch propellers. The gear units also power take-offs for shaft-driven generators which provide all power requirements while at sea.
The various pumps and other auxiliaries are arranged at floor plate level in this minimum-height machinery space. The exhaust gas boilers and uptakes are located port and starboard against the side shell plating.

The diesel engine is a type of internal combustion engine which ignites the fuel by injecting it into hot, high-pressure air in a combustion chamber. In common with all internal combustion engines the diesel engine operates with a fixed sequence of events, which may be achieved either in four strokes or two, a stroke being the travel of the piston between its extreme points. Each stroke is accomplished in half a revolution of the crankshaft.

Four-stroke cycle
The four-stroke cycle is completed in four strokes of the piston, or two revolutions of the crankshaft. In order to operate this cycle the engine requires a mechanism to open and close the inlet and exhaust valves.
Consider the piston at the top of its stroke, a position known as top dead centre (TDC). The inlet valve opens and fresh air is drawn in as the piston moves down. At the bottom of the stroke, i.e. bottom dead centre (BDC), the inlet valve closes and the air in the cylinder is compressed (and consequently raised in temperature) as the piston rises. Fuel is injected as the piston reaches top dead centre and combustion takes place, producing very high pressure in the gases. The piston is now forced down by these gases and at bottom dead centre the exhaust valve opens. The final stroke is the exhausting of the burnt gases as the piston rises to top dead centre to complete the cycle. The four distinct strokes are known as 'inlet' (or suction), 'compression', 'power' (or working stroke) and 'exhaust'

Two Stroke Engine

The two-stroke cycle is completed in two strokes of the piston or one revolution of the crankshaft. In order
to operate this cycle where each event is accomplished in a very short time, the engine requires a number of special arrangements. First, the fresh air must be forced in under pressure. The incoming air is used to clean out or scavenge the exhaust gases and then to fill or charge the space with fresh air. Instead of valve holes, known as 'ports', are used which are opened and closed by the sides of the piston as it moves.
Consider the piston at the top of its stroke where fuel injection and combustion have just taken place. The piston is forced down on its working stroke until it uncovers the exhaust port. The burnt gases then begin to exhaust and the piston continues down until it opens the inlet or scavenge port. Pressurised air then enters and drives out the remaining exhaust gas. The piston, on its return stroke, closes the inlet and exhaust ports. The air is then compressed as the piston moves to the top of its stroke to complete the cycle. 

The main difference between the two cycles is the power developed. The two-stroke cycle engine, with one working or power stroke every revolution, will, theoretically, develop twice the power of a four-stroke engine of the same swept volume. Inefficient scavenging however and other losses, reduce the power advantage to about 1.8. For a particular engine power the two-stroke engine will be considerably lighter—an
important consideration for ships. Nor does the two-stroke engine require the complicated valve operating mechanism of the four-stroke.
The four-stroke engine however can operate efficiently at high speeds which offsets its power disadvantage; it also consumes less lubricating oil.
Each type of engine has its applications which on board ship or rig have resulted in the slow speed (i.e. 80— 100 rev/min) main propulsion diesel operating on the two-stroke cycle. At this low speed the engine requires no reduction gearbox between it and the propeller. The four-stroke engine (usually rotating at medium speed, between 250 and 750 rev/min) is used for auxiliaries such as alternators and sometimes for main propulsion with a gearbox to provide a propeller speed of between 80 and 100 rev/min.
There are two possible measurements of engine power: the indicated power and the shaft power. The indicated power is the power developed within the engine cylinder and can be measured by an engine indicator.
The shaft power is the power available at the output shaft of the engine and can be measured using a torsionmeter or with a brake. 

Fuel oil supply for a two-stroke diesel

A slow-speed two-stroke diesel is usually arranged to operate continuously on heavy fuel and have available a diesel oil supply for manoeuvring conditions.
In the system, the oil is stored in tanks in the double bottom from which it is pumped to a settling tank and heated. After passing through centrifuges the cleaned, heated oil is pumped to a daily service tank. From the daily service tank the oil flows through a three-way valve to a mixing tank. A flow meter is fitted into the system to indicate fuel consumption. Booster pumps are used to pump the oil through heaters and a viscosity regulator to the engine-driven fuel pumps. The fuel pumps will discharge high-pressure fuel to their respective injectors.
The viscosity regulator controls the fuel oil temperature in order to provide the correct viscosity for combustion. A pressure regulating valve ensures a constant-pressure supply to the engine-driven pumps, and a pre-warming bypass is used to heat up the fuel before starting the engine. A diesel oil daily service tank may be installed and is connected to the system via a three-way valve. The engine can be started up and
manoeuvred on diesel oil or even a blend of diesel and heavy fuel oil. The mixing tank is used to collect recirculated oil and also acts as a buffer or reserve tank as it will supply fuel when the daily service tank is empty.
The system includes various safety devices such as low-level alarms and remotely operated tank outlet valves which can be closed in the event of a fire. 

Cooling of engines is achieved by circulating a cooling liquid around internal passages within the engine. The cooling liquid is thus heated up and is in turn cooled by a sea water circulated cooler. Without adequate cooling certain parts of the engine which are exposed to very high temperatures, as a result of burning fuel, would soon fail. Cooling enables the engine metals to retain their mechanical properties. The usual coolant used is fresh water: sea water is not used directly as a coolant because of its corrosive action. Lubricating oil is sometimes used for piston cooling since leaks into the crankcase would not cause problems. As a result of its lower specific heat however about twice the quantity of oil compared to water would be required.

Fresh water cooling system
A water cooling system for a slow-speed diesel engine is shown. It is divided into two separate systems: one for cooling the cylinder jackets, cylinder heads and turbo-blowers; the other for piston cooling.

The cylinder jacket cooling water after leaving the engine passes to a sea-water-circulated cooler and then into the jacket-water circulating pumps. It is then pumped around the cylinder jackets, cylinder heads and turbo-blowers. A header tank allows for expansion and water make-up in the system. Vents are led from the engine to the header tank for the release of air from the cooling water. A heater in the circuit facilitates warming of the engine prior to starting by circulating hot water.
The piston cooling system employs similar components, except that a drain tank is used instead of a header tank and the vents are then led to high points in the machinery space. A separate piston cooling system is
used to limit any contamination from piston cooling glands to the piston cooling system only.

Sea water cooling system
The various cooling liquids which circulate the engine are themselves cooled by sea water. The usual arrangement uses individual coolers for lubricating oil, jacket water, and the piston cooling system, each cooler being circulated by sea water. Some modern ships use what is known as a 'central cooling system' with only one large sea-water-circulated cooler.
This cools a supply of fresh water, which then circulates to the other Individual coolers. With less equipment in contact with sea water the corrosion problems are much reduced in this system.
A sea water cooling system is shown. From the sea suction one of a pair of sea-water circulating pumps provides sea water which circulates the lubricating oil cooler, the jacket water cooler and the piston water cooler before discharging overboard. Another branch of the sea water main provides sea water to directly cool the charge air (for a direct-drive two-stroke diesel).

One arrangement of a central cooling system is shown.
The sea water circuit is made up of high and low suctions, usually on either side of the machinery space, suction strainers and several sea water pumps. The sea water is circulated through the central coolers and then discharged overboard. A low-temperature and high-temperature circuit exist in the fresh water system. The fresh water in the high-temperature circuit circulates the main engine and may, if required, be used as a heating medium for an evaporator. The low-temperature circuit circulates the main engine air coolers, the lubricating oil coolers and all other heat exchangers. A regulating valve controls the mixing of water between the high-temperature and low-temperature circuits. 

Starting air system
Diesel engines are started by supplying compressed air into the cylinders in the appropriate sequence for the required direction. A supply of compressed air is stored in air reservoirs or 'bottles' ready for immediate use. Up to 12 starts are possible with the stored quantity of compressed air. The starting air system usually has interlocks to prevent starting if everything is not in order.
Compressed air is supplied by air compressors to the air receivers. The compressed air is then supplied by a large bore pipe to a remote operating non-return or automatic valve and then to the cylinder air start valve. 
cylinder air start valve will admit compressed air into the cylinder. The opening of the cylinder valve and the remote operating valve is controlled by a pilot air system. The pilot air is drawn from the large pipe and passes to a pilot air control valve which is operated by the engine air start lever.
When the air start lever is operated, a supply of pilot air enables the remote valve to open. Pilot air for the appropriate direction of operation is also supplied to an air distributor. This device is usually driven by the engine camshaft and supplies pilot air to the control cylinders of the cylinder air start valves. The pilot air is then supplied in the appropriate sequence for the direction of operation required. The cylinder air start valves are held closed by springs when not in use and opened by the pilot air enabling the compressed air direct from the receivers to enter the engine cylinder. An interlock is shown in the remote operating valve line which stops the valve opening when the engine turning gear is engaged. The remote operating valve prevents the return of air which has been further compressed by the engine into the system. 

Control and safety devices

The principal control device on any engine is the governor. It governs or controls the engine speed at some fixed value while power output changes to meet demand. This is achieved by the governor automatically adjusting the engine fuel pump settings to meet the desired load at the set speed. Governors for diesel engines are usually made up of two systems: a speed sensing arrangement and a hydraulic unit which operates on the fuel pumps to change the engine power output.

Mechanical governor
A flyweight assembly is used to detect engine speed. Two flyweights are fitted to a plate or ballhead which rotates about a vertical axis driven by a gear wheel. The action of centrifugal force throws the weights outwards; this lifts the vertical spindle and compresses the spring until an equilibrium situation is reached. The equilibrium position or set speed may be changed by the speed selector which alters the spring compression.
As the engine speed increases the weights move outwards and raise the spindle; a speed decrease will lower the spindle.
The hydraulic unit is connected to this vertical spindle and acts as a power source to move the engine fuel controls. A piston valve connected to the vertical spindle supplies or drains oil from the power piston which
moves the fuel controls depending upon the flyweight movement. engine speed increases the vertical spindle rises, the piston valve rises and oil is drained from the power piston which results in a fuel control movement. This reduces fuel supply to the engine and slows it down. It is, in effect, a proportional controller 

Electric governor
The electric governor uses a combination of electrical and mechanical components in its operation. The speed sensing device is a small magnetic pick-up coil. The rectified, or d.c., voltage signal is used in conjunction with a desired or set speed signal to operate a hydraulic unit. This unit will then move the fuel controls in the appropriate direction to control the engine speed. 

Crankcase oil mist detector
The presence of an oil mist in the crankcase is the result of oil vaporisation caused by a hot spot. Explosive conditions can result if a build up of oil mist is allowed. The oil mist detector uses photoelectric cells to measure small increases in oil mist density. A motor driven fan continuously draws samples of crankcase oil mist through a measuring tube. An increased meter reading and alarm will result if any crankcase sample contains excessive mist when compared to either clean air or the other crankcase compartments. The rotary valve which draws the sample then stops to indicate the suspect crankcase. The comparator model tests one crankcase mist sample against all the others and once a cycle against clean air. The level model tests each crankcase in turn against a reference tube sealed with clean air. The comparator model is used for crosshead type engines and the level model for trunk piston engines.
Explosion relief valve
As a practical safeguard against explosions which occur in a crankcase, explosion relief valves or doors are fitted. These valves serve to relieve excessive crankcase pressures and stop flames being emitted from the crankcase. They must also be self closing to stop the return of atmospheric air to the crankcase.
Various designs and arrangements of these valves exist where, on large slow-speed diesels, two door type valves may be fitted to each crankcase or, on a medium-speed diesel, one valve may be used. One design of explosion relief valve is shown.
A light springholds the valve closed against its seat and a seal ring completes the joint. A deflector is fitted on the outside of the engine to safeguard personnel from the outflowing gases, and inside the engine, over the valve opening, an oil wetted gauze acts as a flame trap to stop any flames leaving the crankcase. After operation the valve will close automatically under the action of the spring. 

Ship Mooring and berthing

In order to design a ship's mooring system, the environment loads likely to act upon the ship must first be
determined. These can be highly variable from terminal to terminal. To ensure a minimum standard is met for mooring equipment on ships engaged in worldwide trades, the Standard Environmental Criteria given below should be assumed. The Standard Environmental Criteria apply to the design of the ship mooring system and are not criteria for pier design nor a required operating capacity for a pier/ship mooring plan. These parameters are not intended to cover the worst possible conditions, since this would be neither practical nor reasonable.
"Mooring" refers to the system for securing a ship or vessel to a terminal or quayside of a yard. The most common terminals for ships are piers and sea islands, however, other shipboard operations such as mooring at Single Point Moorings (SPM's), Multi-Buoy Moorings (MBM's), emergency towing, tug handling, barge mooring, canal transit, lightening and anchoring may fall into the broad category of mooring and thus require specialised fittings or equipment.

The use of an efficient mooring system is essential for the safety of the ship, her crew, the terminal and the environment. The problem of how to optimise the moorings to resist the various forces will be dealt with by answering the following questions:
• What are the forces applied on the ship?
• What general principles determine how the applied forces are distributed to the mooring lines?
• How can the above principles be applied in establishing a good mooring arrangement?

The term 'mooring pattern' refers to the geometric arrangement of mooring lines between the ship and the berth. The most efficient line 'lead' for resisting any given environmental load is a line oriented in the same direction as the load. This would imply that, theoretically, mooring lines should all be oriented in the direction of the environmental forces and be attached at such a longitudinal location on the ship that the resultant load and restraint act through one and the same location.

The effectiveness of a mooring line is influenced by two angles: the vertical angle the line forms with the pier deck and the horizontal angle the line forms with the parallel side of the ship. The steeper the orientation of a line, the less effective it is in resisting horizontal loads.

• Mooring lines should be arranged as symmetrically as possible about the midship point of the ship. (A symmetrical arrangement is more likely to ensure a good load distribution than an asymmetrical arrangement.)
• Breast lines should be oriented as perpendicular as possible to the longitudinal centre line of the ship and as far aft and forward as possible.
• Spring lines should be oriented as parallel as possible to the longitudinal centre line of the ship.
• The vertical angle of the mooring lines should be kept to a minimum.

The 'flatter' the mooring angle, the more efficient the line will be in resisting horizontally-applied loads on the ship.

Head and stern lines are normally not efficient in restraining a ship in its berth. Mooring facilities with good breast and spring lines allow a ship to be moored most efficiently, virtually 'within its own length'. The use of head and stern lines requires two additional mooring dolphins and decreases the overall restraining efficiency of a mooring pattern when the number of available lines is limited.

High winds and currents from certain directions might make it desirable to have an asymmetrical mooring arrangement. This could mean placing more mooring lines or breast lines at one end of the ship.
The other factor to consider is the optimum length of mooring lines. It would be desirable to keep all lines at a vertical angle of less than 25°. For example, if the ship's chock location is 25m above the shore mooring point, the mooring point should be at least 50m horizontally from the chock.
Long lines are advantageous both from standpoint of load efficiency and line-tending. But where fibre ropes are used, the increased extension can be a disadvantage by permitting the ship to move excessively, thereby endangering loading arms.

The terminal or quayside can utilize a number of concepts in modern mooring management to reduce the possibility of ship break-out.
These are:
• To develop guidelines for the safe mooring of vessels for the operating environment existing at the terminal.
• To obtain information from the ship prior to arrival concerning the ship's mooring equipment
• To examine the ship's mooring equipment after berthing to determine what modification, if any, must be made to standard guidelines in view of the state of maintenance, training of crew, etc.
• To inspect line lending periodically either visually or by the instrumentation of mooring hooks.
• To take whatever action is deemed appropriate to ensure stoppage of cargo transfer, disconnection of loading arms and removal from berth of the ship should the ship fail to take appropriate measures to ensure safety of mooring.

As soon as practicable after berthing, it is recommended that terminals have their representative board the vessel to establish contact with the Master or his designated representative. At this meeting the Terminal Representative should provide information relating to shore facilities and procedures.
In addition he should in concert with the Ship Representative:
• Complete the Ship/Shore Safety Check List in line with guidance given and, where appropriate, physically check items before ticking off.
• Obtain details of moorings and winches, including state of maintenance.
• Review forecasted weather and arrange for the Master to be advised of any expected changes.
• Assess freeboard limitations.
• Assess type and condition of ship mooring equipment and its ballasting ability.

Overloading of mooring lines is evidenced in a number of ways; for example, by direct measurements of mooring line loads, by direct observation of the moorings by experienced personnel, or by predictions made by those having a knowledge of the effects of wind and current on the ship mooring system or by winch slippage.

The following precautions are likely to apply:
• Harden-up on the winch brakes, Do not release brakes and attempt to heave in
• Discontinue cargo operations
• Reduce freeboard by taking-on ballast if loads are due to high wind conditions.
• Disconnect loading arms and gangways.
• Call out crew, linemen, mooring boats, tugs and put ship's engines on readiness.
• Run extra moorings as available together with any shore mooring available to augment ship's equipment.
• In emergencies place winches in gear.

Sunday, August 25, 2013

Some design notes for semi-submersible and TLP


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:   
Stability columns
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
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
-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:
Stability columns

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.

Sunday, July 21, 2013

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


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.