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 In most cases, in the event of a pending hurricane, the rigs were withdrawn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

                           (Source : Youtube )

Drilling Operation Guide 

Drilling Equipment and Operations 

Marine Engineering Knowledge 

One of the workhorses in GOM offshore

In 2011, LeTourneau Technologies announced it has entered into a contract to furnish a kit and license to Lamprell Energy, Ltd. ("Lamprell") for the construction of Lamprell's sixth Super 116E class self-elevating Mobile Offshore Drilling Platform ("jack-up"). This rig will be outfitted with 477 feet of leg designed to operate in water depths of up to 350 feet and will have a 1.5 million pound hook load for a rated drilling depth of 30,000 feet.

The rig will be built at Lamprell's Hamriyah facility located in the United Arab Emirates for Greatship Global Energy Services of Singapore. Rig delivery is expected in the fourth quarter 2012. Lamprell currently has three additional Super 116E class jack-ups under construction under license from LeTourneau. These transactions demonstrate LeTourneau's strong commitment for international growth through the supply of state-of-the-art rig designs, rig kits, and drilling equipment.

LeTourneau's delivery of the kit, which includes the rig's leg components, jacking system, cranes and certain other components, will occur in stages in accordance with the rig construction schedule. The initial delivery is expected to occur in the third quarter of 2011 and the final delivery during the second quarter of 2012.

This marks the 25th order for the LeTourneau Super 116 series of jack-ups, of which 8 others are currently under construction. The Super 116E is one of four current LeTourneau jackup rig designs and an evolution of the industry's original workhorse, the LeTourneau 116-C design. The Super 116E provides over 3,000 kips greater payload than its predecessor along with the enhanced cantilever package offering 70 ft reach and 2,650 kip drill load capacity. The rig is designed to drill in up to 350 feet of water in moderate environmental locations and can be outfitted to handle high temperature/high pressure wells.

LeTourneau built the world's first jack-up drilling rig in 1955 and is an acknowledged leader in the supply of offshore jack-up drilling rigs today. LeTourneau has designed and been involved in the construction of approximately one-third of all jack-up rigs in service.

LeTourneau's vision is to be recognized by our customers as designing, building and supporting the most innovative, capable and reliable equipment for developing the world's natural resources.

Two years on, in DUBAI, January 2013, Lamprell has delivered the newbuild jackup Greatdrill Chaaya to Greatship Global Energy Services.

Work was completed 18 months after initial steel cutting and within budget. The rig left Lamprell’s Hamriyah facility last week for a drilling location offshore India, where it is contracted to ONGC.

Greatdrill Chaaya, weighing 10,394 metric tons (11,457 tons) at delivery, is a LeTourneau Super 116E (Enhanced) design with a self-elevating 477-ft (145-m) leg system.

Lamprell modified the spud can design in order to reduce the soil bearing pressure expected at the offshore location.

The company has now completed construction of 12 newbuild jackups, five of the LeTourneau Super 116 E design and seven of the Friede & Goldman Super M2 design. The company has seven more LeTourneau rigs undergoing various stages of construction.

Rig Designs

More on Ultra-deepwater drilling Semi-rig

Scarabeo 9 is one of the largest offshore drilling rigs in the world. It is the first (and as of 2012 the only) Frigstad Engineering developed Frigstad D90 design rig ever built. The rig is able to operate at the water depth up to 11,811 feet (3,600 m), which is classified by the oil industry as "ultra-deepwater", and its drilling depth is 15,200 metres (49,900 ft). The water depth still suitable for its operations is twice as much as for Deepwater Horizon.The drilling equipment was provided by the Norwegian engineering company Aker Solutions.

Scarabeo 9 has a length of 115 metres (377 ft) and a breadth of 78 metres (256 ft). Its gross tonnage is 36,863, dead weight tonnage 23,965, and net tonnage 11,059 tonnes. The vessel is powered by eight Wärtsilä 12V32 diesel engines. It is equipped with two cranes.

DNV Class Notation:

1A1 Column-stabilised HELDK CRANE ECO F-AM DYNPOS-AUTRO   POSMOOR DRILL

Flag: Bahamas Signal Letters: C6YV7

Port: NASSAU
Owner: Saipem Maritime Asset Management Luxembourg S.A.R.L. GT : 36,863
NT (ITC 69): 11,059

Manager: SAIPEM SPA  DWT: 23,965
Yard: Yantai Raffles Shipyard Co. Ltd. (114020) Year of Build: 2011
Dynamic positioning:  DP class 3.
Eight 4.3 MW azimuth thrusters.
Mooring system: Four double drum winches with eight fairleads for connection to pre-set system.

Ram rig:  Hydraulic dual ram rig. Both main rig and auxiliary rig have 1,000 s.t capacity and full drilling capabilities.  Set back for 50,000 ft drill string/casing in 88-92 ft stands.
Rotary table: Two 60.5” with independent drive.
Top drives:  Two top drives with 1,000 s.t capacity.
Riser tensioners:  Six in line hydraulic cylinder riser tensioners.
BOP:

Two 18.75” x 10,000 psi wp annular preventors.

Five 18.75” x 15,000 psi wp ram preventors.
Riser:  10,000 ft of 21” riser.

The rig includes quarters for up to 200 workers. There is also a helicopter deck suitable for MI8, S61, and EH101 helicopters. The rig has been described by the industry sources as "the latest technology for deepwater drilling operations".

The rig cost US$750 million to build. It was constructed at the Yantai Raffles Shipyard in Yantai, China. The contract was signed on 5 April 2006, the keel was laid on 1 April 2008, and originally the construction was to be completed in September 2009. After several delays at the Yantai Raffles shipyard it was shipped to the Keppel FELS shipyard in Singapore for extra work completion in 2010.

The rig was delivered to Saipem on 25 August 2011. On her maiden voyage to Cuba, Scarabeo 9 was escorted around the Cape of Good Hope by the Fairmount Marine owned tugboat Fairmount Glacier.


More on Semi rig design :

Two years ago, Keppel FELS delivered the third DSS 21 deepwater semisubmersible drilling rig to Maersk Drilling (Maersk) 43 days ahead of schedule and with a perfect safety record, garnering a bonus of $400,000.

Tong Chong Heong, CEO of Keppel O&M, said, “We are proud to be Maersk’s preferred partner for its fleet expansion plans. The safe and early completion of Maersk Deepwater Semi III attests to the astute choice made by Maersk to build the premium fleet with Keppel FELS, as well as the solidarity and calibre of our teams working hand in glove with one another. The success of this Series of semis is built upon our long term partnership with thought-leading operators such as Maersk, who have provided the necessary platforms for our technologies to gain market ascendance".

"The demand for modern drillings rigs has increased over the past years concurrently with the growing technical challenges we are faced with in the drilling industry. The search for new finds is moving to deeper waters and areas with complex soil conditions and more advanced drilling rigs are needed to meet those challenges. Our ultra deepwater semisubmersibles are well equipped to meet this demand,” Mr Claus V. Hemmingsen, CEO of Maersk Drilling added.

DSS Series rigs are highly cost effective exploration units capable of drilling down 10,000 m (30,000 ft) wells and operating at a water depth of 3,000 m (10,000 ft). The Series also features a dynamic positioning system, with the ability to attach to a prelaid mooring system.

DSS Series rigs are particularly well suited to drill deep and complicated wells in areas such as offshore West Africa, Brazil, the Gulf of Mexico and Southeast Asia, and may be further customised to meet the unique challenges of each location.

• Length Overall approximately 117.00 m

• Length of Pontoon 114.50 m
• Breadth Outside Pontoon 78.00 m
• Pontoon Breadth 17.50 m
• Pontoon Height 10.00 m

• Columns 17.00 x 17.50 m

• Longitudinal Distance between CL of Columns 61.50 m
• Transverse Distance between CL of Columns 60.50 m
• Length of Main Deck 78.50 m
• Breadth of Main Deck 78.00 m
• Transit draft 9.7 m

• Survival draft 16.00 m
• Operational draft (min.) 18.50 m
• Operational draft (max.) 20.50 m
• Transit displacement(Approx.) 38,960 Mt
• Survival displacement(Approx.) 48,295 Mt

Semi Rig Equipment1


Courtesy of Maersk Drilling

Electrical Installation Concept on a MODU rig

One of the earliest tasks for the electrical engineer who is designing a power system is to estimate the normal operating rig power plant load. He is also interested in knowing how much additional margin he should include in the final design. There are no ‘hard and fast’ rules for estimating loads, and various basic questions need to be answered at the beginning of a project, for example,

• Is the rig power a new plant?

• How long will the offshore rig power system to exist e.g. 10, 20, 30 years?

• Is the old rig power being extended?

• Does the owner have a particular philosophy regarding the ‘sparing’ of equipment?

• Are there any operational or maintenance difficulties to be considered?

• Is the power factor important with regard to importing power from an external source?

• If a generator suddenly shuts down, will this cause a major interruption to the rig operation?

• Are there any problems with high fault levels?

 

The electrical engineer will need to roughly draft a key single-line diagram and a set of subsidiary single-line diagrams. The key single-line diagram should show the sources of power e.g. generators, utility intakes, the main switchboard and the interconnections to the subsidiary or secondary switchboards. It should also show important equipment such as power transformers, busbars, busbar section circuit breakers, incoming and interconnecting circuit breakers, large items of equipment such as high voltage induction motors, series reactors for fault current limitation, and connections to old or existing equipment if these are relevant and the main earthing arrangements. The key single-line diagram should show at least, the various voltage levels, system frequency, power or volt-ampere capacity of main items such as generators, motors and transformers, switchboard fault current levels, the vector group for each power transformer and the identification names and unique ‘tag’ numbers of the main equipment.

 

Vital loads are normally fed from a switchboard that has one or more dedicated generators and one or more incoming feeders from an upstream switchboard. The generators provide power during the emergency when the main source of power fails. Hence these generators are usually called ‘emergency’ generators and are driven by diesel engines. They are designed to automatically start, run-up and be closed onto the switchboard whenever a loss of voltage at the busbars of the switchboard is detected.

 

Vital AC loads, example below

Emergency lighting
Emergency generator auxiliaries
Helicopter pad lighting
Control room supplies
Vital LV pumps 

 

Essential AC loads, example below

 

Diesel fuel transfer pumps 

Diesel fire pump auxiliaries

Main pump auxiliaries

Main compressor auxiliaries

Main generator auxiliaries

Electric fire pumps

Living quarters
Air compressor
General service water pumps
Fresh water pumps
Equipment room HVAC supplies
Life boat davits
Anti-condensation heaters in panels and switchboards
Security lighting supplies
Control room supplies
UPS supplies
Radio supplies
Computer supplies
Battery chargers for engine starting systems
Instrumentation supplies

Vital DC Loads, example below 

Public address system

Plant alarm systems
System shutdown system
Telemetry systems
Emergency radio supplies
Fire and gas detection system
Navigation aids

Hence each switchboard will usually have an amount of all three of these categories. Call these C for continuous duty, I for intermittent duty and S for the standby duty. Let the total amount of each at a particular switchboard j be Cjsum, Ijsum and Sjsum. Each of these totals will consist of the active power and the corresponding reactive power.

In order to estimate the total consumption for the particular switchboard it is necessary to assign a diversity factor to each total amount. Let these factors be Dcj for Csumj , Dij for Isumj and Dsj for Ssumj . Offshore rig companies that use this approach have different values for their diversity factors, largely based upon experience gained over many years of designing plants. Different types of plants

may warrant different diversity factors.

Electrical Installation Concept on MODU_ Choong
The info  presented in the slides are samples only and may not represent the total correctness of what is being installed subjected to the specifications of the contract scope.    

 

Comparison of US and IEC Nomenclature, eg. below

While there are many similarities and even direct interchangeabilities between U.S. and IEC recognized standards, specific applications must be considered.

Motors may be acceptable under all standards but not necessarily certified under all standards.

The IEC "flame-proof" motor is essentially the same as the U.S. "explosion-proof" motor. Each design withstands an internal explosion of a (specified) gas or vapor and prevents ignition of the specified gas or vapor that may surround the motor. However, construction standards are not identical. The U.S. standard is generally more stringent and acceptability can be based on approval of local authorities.

The U.S. totally enclosed "purged and pressurized," or "inert gas filled," motors are manufactured to similar standards as those of IEC pressurized motors. Each operates by first purging the motor enclosure of any flammable vapor and then preventing entry of the surrounding (potentially explosive

or corrosive) atmosphere into the motor enclosure by maintaining a positive gas pressure within the enclosure.

IEC Type 'e' (Increased Safety) motors are nonsparking motors with additional features that provide further protection against the possibilities of excess temperature and/or occurrence of arcs or sparks.

NEMA and IEEE standards and testing are more comprehensive than the IEC standards. In general, motors designed to NEMA/IEEE standards should be suitable for application under IEC standards from a rating, performance, and testing viewpoint.

 

 

 

Rig Stability during tow or transit

Offshore rig stability is a complicated aspect of naval architecture which has existed in some form or another for past years. Historically, offshore rig stability adopted some of the ship stability calculations and for ships, it relied on rule-of-thumb calculations, often tied to a specific system of measurement. Some of these very old equations continue to be used in naval architecture books today, however the advent of the floaters including rig and ship, model basin allows much more complex analysis.

Stability standards for both ships and offshore rigs ( jackup, semi-submersibles ) are based on a two-tier approach:

• intact stability requirements, designed to ensure that the unit will withstand all expected environmental conditions when in its normal operating or survival condition, and while it remains
undamaged and watertight;

• damaged stability requirements, designed to ensure that the unit will not capsize in foreseeable environmental conditions, after undergoing a limited amount of damage or flooding, and will be capable of returning to the upright condition.

Two alternative approaches are normally adopted when defining damage: damage to any one compartment at any draught, or waterline damage, including breaching of internal watertight divisions between compartments. Both approaches have their strengths and weaknesses. An offshore unit designed to meet the any one compartment standard cannot necessarily be guaranteed to meet the waterline damaged standard, and vice versa.

NMD adopted a three-tier approach. The first two tiers were the established intact and damaged stability philosophies, and the third was a requirement that the unit should withstand loss of buoyancy from either the whole or a major part of one column, but without any requirement to return to the upright position. The objective in this case was to allow the crew time to evacuate the unit. This requirement was expressed in terms of providing a maximum angle of heel after a large loss of righting moment, and a minimum level of reserve buoyancy above the damaged waterline. The concept of providing some level of reserve buoyancy, beyond that necessary to meet basic code requirements, has since been widely accepted.

This information is intended to provide an adequate level of stability during routine operations of floating Installations. The aim is to take account of the most probable damage cases, in particular low energy collisions with supply vessels during loading, towing and anchor handling. Consideration should be given to carrying out an inclining test on the first unit of a design, when as near to completion as possible, to determine accurately the lightship weight and position of centre of gravity. The test will need to be conducted in accordance with an approved procedure.

For successive units of a design which are identical with regard to hull form and arrangement (with the exception of minor changes in machinery or outfit) detailed weight calculations showing only the differences of weight and centres of gravity may be acceptable. However, the calculated changes in weight and position of centre of gravity should be small, and the accuracy of the calculations confirmed by a deadweight survey.

 
 
 
Vessel Stability_1
 
 
 


Stability_Jackup
 

Ship_Stability.pdf

Offshore Rig Power - Diesel engines

What is a diesel run engine gen-set and how does it differ to the industrial engine on which most are based? The following basically explain many of the terms applicable to diesel gen-set design, development, operating and ownership by the rig operator.

MARINE DIESEL ENGINE denotes the engines used either as the propulsive prime mover of a ship or generating electrical power to the consumers onboard the offshore rig or semi-submersible. The consumers not only provide to the drilling equipment but also services to the hotel onboard, fire and safety systems, etc. The term may be extended to include the propulsion of engines that are used for shipboard auxiliary services such as the generation of electric power.

IOPU – Independent Operating Power Unit. These are multi speed non vehicle power units. The are normally sold with radiator, cooling group and fan, and typically share ratings from their off highway derivatives. Typical applications include pumps and compressors.

Operating Speed – Gen-sets are normally governed to fixed speed running. 1500 rpm to produce 50Hz electrical supply for European market and 1800 rpm to produce 60 Hz for US market. 60Hz supply can be achieved at 1200Hz with some alternator sets– this is uncommon.

kWe – Kilowatts electrical is a measure of electrical power produced by a gen-set. 60Hz generator sets are usually marketed in terms of kWe.

kVa – Kilovolt amps is a measure of electrical power produced by a genset. 50Hz gen-sets are usually marketed in terms of kVa. As gensets produce an alternating current P=VI doesn’t hold true. Voltage and current follow sinusoidal wave forms with a phase shift due to the reactance (generated by inductance & capacitance) of the load on the alternator, and hence a power factor is used. Industry assumes a 100% resistive load for which a 0.8 power factor is used. This relates kWe to kVa by the following:     kWe = kVa x 0.8

Fuel Coolers – Gen-sets are normally fitted into a frame, which holds a small fuel or “day tank” for limited time running. If the gen-set operates in elevated ambient temperatures, or the engine has a high fuel spill ratio, the temperature of the fuel will often be controlled by a small fuel cooler (air-to-fuel) mounted on the cooling group. The cooler prevents rises in “day tank” temperatures preventing fuel injector damage.

Alternator Efficiency (ha) – The alternator on the gen-set converts the mechanical energy delivered by the engine into electrical energy, and has an associated efficiency. Typically alternators have an efficiency of 0.95 (95%).

kWm – Gen-sets are marketed in terms of the electrical power which they produce. However engine manufacturers are more interested in the mechanical power which their engine needs to deliver to the alternator to provide the quoted electrical power. This includes fan powers and alternator efficiency:

kWe = (kWm –Fp) x ha or kWe = kWm x 0.90 x 0.95 (<10L engine)

kWe = (kWm –Fp) x ha or kWe = kWm x 0.95 x 0.95 (>10L engine)

Emissions- Genset emissions are complicated and specific to the country in which they operate. Generally requirements are less demanding than other off highway equipment, but are often driven by marketing rather than legislative needs. Legislative limits are complicated, determined by introduction date, engine powers and power rating.

Ambient/Altitude Clearance - Gen-sets are operated in global environments, with extreme ambient and altitude operating environments. Running at higher ambient temperatures adds additional loads on the cooling system, and at elevated altitudes the inlet system struggles to deliver sufficient air for combustion with the lower air density/pressure. Gen-sets are expected to run at altitudes up to 4000m and ambient temperatures of 55 ⁰C, which may require derate. Clearance is defined as the margin on the altitude/ambient performance limiting parameters (such as coolant and exhaust temperature) when tested at standard operating conditions (sea level 25 ⁰C). From the Ambient/Altitude clearance, curves are developed to assist application engineers in sizing appropriate derates for extreme operating conditions.

Governing – Gen-sets are fixed speed applications with governors developed to maintain the desired running speed within careful limits. This is particularly important as electrical equipment powered by the genset may be damaged by supply outside of the normal 50/60Hz limits. Gen-set governing is detailed by ISO 8528.

Load Acceptance – Gen-sets are often used for standby/emergency power, where they will be expected to start-up, run up to running speed and then accept a large % of maximum electrical load. Load acceptance is measured in terms of a % frequency dip and a recovery time, and are defined by ISO 8528-5 and NFPA 99/110. Additional requirements are customer driven demanding typically 80% of the prime rating within 10 seconds of start-up, within ISO 8528-5 limits. Engine load acceptance has been demonstrated as a linear function of trapped mass.

Power Rating - Gen-sets are sold at three main power ratings determined by their application. Power ratings are defined by ISO 8528-1.

An important parameter for a marine diesel engine is the rating figure,usually stated as bhp or kW per cylinder at a given rev/min. Although engine makers talk of continuous service rating (csr) and maximum continuous rating (mcr), as well as overload ratings, the rating which concerns a ship or rig owner most is the maximum output guaranteed by the engine maker at which the engine will operate continuously day in and day out. It is most important that an engine be sold for operation at its true maximum rating and that a correctly sized engine be installed in the ship or rig; an under-rated main engine, or more particularly an auxiliary, will inevitably be operated at its limits most of the time.

Rig or ship owners usually require that the engines be capable of maintaining the desired service while fully loaded, when developing not more than 80 per cent (or some other percentage) of their rated brake horsepower. Such stipulation may leave the full-rated power undefined and therefore does not necessarily ensure a satisfactory moderate continuous rating, hence the appearance of continuous service rating and maximum continuous rating. The former is the moderate in-service figure, the latter is the enginebuilder’s set point of mean pressures and revolutions which the engines can carry continuously.  Normally a ship or semi rig ( with thrusters)  will run sea trials to meet the contract speed or thruster load (at a sufficient margin above the required service speed) and the continuous service rating should be applied when the vessel is in service.

DERATING

An option available to reduce the specific fuel consumption of diesel engines is derated or so-called ‘economy’ ratings. This means operation of an engine at its normal maximum cylinder pressure for the design continuous service rating, but at lower mean effective pressure and shaft speed. By altering the fuel injection timing to adjust the mean pressure/ maximum pressure relationship the result is a worthwhile saving in fuel consumption. Example, the horsepower required for a particular speed by a given ship or semi rig with thrusters is calculated by the naval architect and, once the chosen engine is coupled to a fixed pitch propeller ( in this case of ship propulsion ) , the relationship between engine horsepower, propeller revolutions and ship speed is set according to the fixed propeller curve. A move from one point on the curve to another is simply a matter of giving more or less fuel to the engine.

Diesel Power Choong1

A major boost to engine output and reductions in size and weight resulted from the adoption of turbochargers. Pressure charging by various methods was applied by most enginebuilders in the 1920s and 1930s to ensure an adequate scavenge air supply: crankshaftdriven reciprocating air pumps, side-mounted pumps driven by levers off the crossheads, attached Roots-type blowers or independently driven pumps and blowers.

The first turbocharged marine engines were 10-cylinder Vulcan- MAN four-stroke single-acting models in the twin-screw Preussen and Hansestadt Danzig, commissioned in 1927. Turbocharging under a constant pressure system by Brown Boveri turboblowers increased the output of these 540 mm bore/600 mm stroke engines from 1250 kW at 240 rev/min to 1765 kW continuously at 275 rev/min, with a maximum of 2960 kW at 317 rev/min. Büchi turbocharging was keenly exploited by large four-stroke engine designers, and in 1929 some 79 engines totalling 162 000 kW were in service or contracted with the system.

The turbocharger comprises a gas turbine driven by the engine exhaust gases mounted on the same spindle as a blower, with the power generated in the turbine equal to that required by the compressor.

There are a number of advantages of pressure charging by means of an exhaust gas turboblower system:

- A substantial increase in engine power output for any stated size and piston speed, or conversely a substantial reduction in engine dimensions and weight for any stated horsepower.

- An appreciable reduction in the specific fuel consumption rate at all engine loads.  A reduction in initial engine cost.

- Increased reliability and reduced maintenance costs, resulting from less exacting conditions in the cylinders.

- Cleaner emissions (see section below).

- Enhanced engine operating flexibility.

Larger two-stroke engines may be equipped with up to four turbochargers, each serving between three and five cylinders.


Compared with four-stroke engines, the application of pressure charging to two-stroke engines is more complicated because, until a certain level of speed and power is reached, the turboblower is not selfsupporting.  Two-stroke engine turbocharging is achieved by two distinct methods, respectively termed the ‘constant pressure’ and ‘pulse’ systems. It is the constant pressure system that is now used by all low speed two-stroke engines. For constant pressure operation, all cylinders exhaust into a common receiver which tends to dampen-out all the gas pulses to maintain an almost constant pressure. The advantage of this system is that it eliminates complicated multiple exhaust pipe arrangements and leads to higher turbine efficiencies and hence lower specific fuel consumptions. An additional advantage is that the lack of restriction, within reasonable limits, on exhaust pipe length permits greater flexibility in positioning the turboblower relative to the engine.
The main disadvantage of the constant pressure system is the poor performance at part load conditions and, owing to the relatively large exhaust manifold, the system is insensitive to changes in engine operating conditions. The resultant delay in turboblower acceleration, or deceleration, results in poor combustion during transition periods.

Diesel Engine Turbocharging

Some more insights of a Semi-sub Drilling Rig

The company has secured a US$300 plus million to build a repeat semisubmersible drilling rig for Brazilian drilling contractor group Queiroz Galvão Óleo e Gás (QGOG) and to be named "Alpha Star". The first one built earlier was named Gold Star ( see below rig data taken from QGOG website ). An innovative design, the DSSTM 38 semisubmersible drilling rig is designed to meet the operational requirements in the deepwater “Golden Triangle” region, comprising Brazil, Africa and the Gulf of Mexico.

The rig is rated to drill to depths of 30,000 feet below mud line in just over 9,000 feet water depth. It is 103.5 metres in overall length, with a main deck size of 69.5 metres by 69.5 metres. Its operational displacement is approximately 38,000 tonnes. The rig has accommodation facilities to house a crew of up to 130 men. It has both vertical and horizontal riser storage. The eight 3000kW Azimuthing thrusters configuration are designed to keep the vessel in position. All configurations comply with the  Dynamic Positioned System (DPS-2) requirements.

 Photo


Alpha Star


Gold star



Source : Straits Times, QGOG website.

Jackup Terminologies and Types

In early 1955 ( before I was born in '59 and I started to work only in 1980, see my blog article on pressure vessel design ), the first 3-legged jack-up appeared on the offshore scene. The rig was the R.G. LeTourneau jack-up, the Scorpion, for Zapata Offshore Company. The Scorpion, an independent leg jack-up, used a rack and pinion elevating system on a truss framed leg. The rig worked very successfully for several years but was lost during a move in the Gulf of Mexico. The Scorpion was closely followed by The Offshore Company Rig No. 54. For Rig No. 54, however, a hydraulic jacking system on a trussed leg was used. These jack-ups were followed by Gus II, a mat supported unit using a hydraulic jacking system, which was built by Bethlehem Steel Corporation.

Those early breed of jack-ups were 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. Jack-ups can be either self-propelled, propulsion assisted, or nonpropelled. The majority of jack-up rigs are non-propelled. The self-propelled unit, although very flexible, requires a specially trained crew of operators as well as a better trained rig drilling team.

Jack-ups have been built with as many as 14 legs and as few as 3 legs. As the water depth increases and the environmental criteria become more severe, we find that to use more than 4 legs is not only expensive but impractical. The prime forces on a jack-up are generated from the waves and currents, hence, the less exposure to the waves and currents the fewer the forces being developed on the unit. From this standpoint the optimum jack-up is the monopod or single leg unit.

Problems other than wave forces, however, must be overcome with the monopod type unit. But in areas such as the North Sea with very rough' seas there is a need for the monopod jack-up.

When evaluating which type of jack-up to use, it is usually some of the criterias to consider :

1. Water depth and environmental criteria.

2. Type and density of sea bed.

3. Drilling depth requirement, environmental conditions.

4. Necessity to move or stop during hurricane or storm season.

5. Capability to operate with minimum support.

6. How often it is necessary to move.

7. Time lost preparing to move.

8. Operational and towing limitations of the unit.

The independent leg unit depends on a platform (spud can) at the base of each leg for support. These spud cans are either circular, square, or polygonal, and are usually small. Nowadays, spudcan bottom comes with tips for better holding on ground. The larger spud can being used to date is about 56 feet wide. Spud cans are subjected to bearing pressures of around 5,000 to 6,000 pounds per square foot, although in the North Sea this can be as much as 10,000 psf. Allowable bearing pressures must be known before a jack-up can be put on location.

Jackup Slides Ckw


Le Thourneau rigs have been the majorities in the Gulf of Mexico and most of them operating in the region are coming to thirty years or more in operating life. Some have gone through many upgrades, eg, increasing the cantilever outreach and hook load increase.

Le thourneau jackup

More on Offshore Drilling

A jack-up rig consists of a movable platform which can be jacked up and down the (usually) three supporting legs. The video clip below shows one of the typical designs. These provide a common means of drilling in water, where the water depth is relatively shallow - say, 50 to 400 feet.

Jack-ups will be floated out to location and the legs then lowered independently until they are bedded securely and the platform is level and above wave height. It is clear that their use would be restricted when

there are strong currents or an unstable seabed.

A semi-submersible is a floater drilling rig. In this case, a deck is supported by a tubular structure, and by two hulls to provide buoyancy. Again, the deck carries equipment, accommodation modules, a helicopter pad and typical layout is shown in the below video clip.

Semi-submersibles can move easily from one location to another either by being towed or under their own thrusters power. They are mainly used, therefore, for exploration and appraisal drilling where this ease of movement is essential.

When on location, the semi-sub (as it is often helipad called) takes on water ballast (into the two hulls, etc). This will lower the structure in the water and lower the centre of gravity. In this position it is shielded from the effects of rough water at the surface and achieves a high degree of stability.

A semi-sub can operate in deeper water than a jack-up. Its maximum operating water depth depends on the type of mooring system employed. Some semi-subs use anchors with wire and chain to hold them on station. Others use dynamic positioning which is a system of computer controlled thrusters, to maintain their position. Modern semi-subs using anchors may, in exceptional circumstances, drill in water up to 3,000 feet deep.

Semi-subs using dynamic positioning systems are capable of drilling in even deeper waters, up to 6,000 to 10,000feet deep.

The drilling equipment in the semi-submersible is more sophisticated compare to a jack-up and more costly as the semi-sub has various sea motions while afloat and the drilling system on board has to be able to function at the same time deal with the heave motions while the rig is afloat. Some drilling systems may be able to discoupled from the vessel in case of severe and uncontrollable situation such as unexpected storm or harsh environment and for the safety of the crew, the drilling riser may be released after the sub-sea LMRP/BOP is being shut from the well.

In comparison to semi-submersibles, a jack-up has some advantages:

a) Lower construction costs. Semi-sub usually cost almost double of that of jack-up or more
b) Less personnel required to run the rig. Jack-up has about 120men compared to Semi-sub with 200men onboard
c) Because of (a) and (b) lower day rates.
d) The possibility to work over a fixed platform.
e) It is cheaper for the operator to use a jack-up:
 -Less powerful tug boats to move the rig while it is afloat
 -No mooring system required, no lost time to run anchors. But some rigs now come standard with anchors
 -Less maintenance costs.
 -Surface BOP without sub sea system.
 -Simple well head assembly.
f) Less down time:
 -No wait on weather due to motions.
 -Drilling equipment can be handled faster and easier.

However, the jack-up have some disadvantages:

a) Limited water depth. The maximum water depth for the largest JU is 450ft. Semi-sub could work up to 10,000feet on the latest design.
b) Depends on bottom condition. The bottom soil conditions may cause a punch through or deep leg penetration. Semi-sub does not have such issue as it is afloat while drilling operation is carrying out.
c) In case of a blow-out the rig can not move off location whereas a semi could.
d) More fragile. Many incidents and damages during moving and because of a punch through. Statistics have shown that over 75% of the incidents occur under tow or during jack-up/jack-down operations.
e) Safe operations require strict procedures.

The table below show some of the different jackup designs ( excluding KeppelFELS design of A-class, Super A, B-class, Super-B,etc ) and its capability in terms of size, VDL capacity,etc :

Types of Drilling Rig Capability

Some key personnel on board the drilling rig and their roles and responsibility :-

Tool pusher - In overall charge of rig operations, implementing the drilling plan and compliance with all safety requirements. Reports to the company representative.

Driller - In charge of the drilling process and operations. Responsible for compliance with the drilling plan and for the drilling crew. Reports to the tool/tourpusher.

Assistant driller - Assists the driller. Usually responsible to the driller for the operation of bulk storage equipment (for handling mud chemicals, etc.) and for the mud flowline system. Reports to the driller.

Derrickman - Responsible for the storage and movement of tubulars in the derrick and monitoring the mud systems. Reports to the driller.

Roughneck - Works on the rig floor. Responsible for general rig floor activities under the direction of the driller/assistant driller. Reports to the driller.

Rustabout - A member of the general workforce, assisting with the movement of materials,cleaning, painting, etc.

Drilling Operation and components

Drilling Completion

Formulas for Drilling and Prod



Rig Functions

Drilling Mud Technology

Drilling Mud Technology