More on Drilling Jack-up and some installations on board

The first step in the jack-up rig design is the definition of its configuration. This is based on operational and economic requirements and past design experience. Decisions made at this stage have a significant impact on the behaviour of the structure. The geometry of the configuration developed should have the necessary capacity to accommodate needed equipment, preload tanks and quarters. Preliminary estimates of weights should be made and a naval architect should assess the configuration for the “afloat” mode of the jack-up rig. A configuration for the legs should be developed. The system for connecting the legs to the hull so as to achieve efficient moment transfer should be chosen. A classification society should also be chosen [American Bureau of Shipping, 2001; Det Norske Veritas-Rules for Classification of Mobile Offshore Units]. A preliminary assessment should then be made to ensure that the chosen configuration complies with the requirements of the chosen classification society. After this, the basic design can be developed. The efforts of the structural engineer are important from this stage on. Hull scantlings are the individual elements that makeup the structure.

Due to the numerous complexities associated with jack-ups, it should be remembered that a structural analysis would be based on a number of simplifying assumptions and approximations. Though software is available to execute a non-linear dynamic analysis, the designer may opt for a simple static analysis using wave forces generated from a hydrodynamic analysis applying a linear wave theory (Such as the Stoke’s Fifth Order Potential Wave Theory) to a hydrodynamic model generated for this purpose.

The following steps should serve as a general guideline for the analysis of a jack-up platform:-

Define the environment including water depth, wind speed, wave (type, height, period) and current velocity and its variation with depth. This can be a location specific environment (North Sea, Persian Gulf) or a world wide criteria. The worldwide criterion is a reference benchmark that does not necessarily reflect any particular location. Some of the storm parameters (100 knot wind) are defined per code or classification authority or refer to API.

The results of these environments are then used as reference for the actual unit location. With the exception of very heavy loads (such as cantilever, transom and hold-down reactions, heliport support members, etc.), this may be accomplished by summing all the equipment weight on a deck, a proportion of the variable load on that deck and dead load and distributing this load uniformly over the entire deck. This may be done for all decks. Loads from the drill floor may be applied as concentrated forces at appropriate locations. Usually, the weight is assumed to be balanced equally among the three legs. This is normally achieved by moving the liquids among the various tanks to reach a balanced condition.

Generate a hydrodynamic model of the jack-up platform. This may be a simple model consisting of three “stick” elements that have the same hydrodynamic properties as the trussed leg. The ideal source of the drag values of the unit would generally be determined via wind tunnel models. This takes into account the actual geometry of the unit and the effects of shielding. Usually the product of these studies is a single drag value for the legs and hull. The main problem with this source of parameters is cost and time.

Generate a Global Structural Model: a typical finite element analysis model of a jack-up platform structure and usually the length of leg that should be used in the modelling for a given water depth. For a jack-up platform whose legs have independent spud can foundations, the legs are usually assumed to be pinned at a depth of about 10 ft below the mudline. For a mat supported jack-up, the structure of the mat may be modelled using plate elements and the legs could be fixed to this structure. Per the ABS Rules [American Bureau of Shipping, 20011, the minimum crest clearance to be provided is 4 ft (1.2 m) above the crest of maximum wave or 10% of the combined height of the storm tide plus the astronomical tide and height of the maximum wave crest above the mean low water level, whichever is less between the underside of the unit in the elevated position and the crest of the design wave.

Spud Cans

This is the most common type of jack-up platform foundation in use. Spud cans typically consist of a conically shaped bottom face. The purpose of a spud can is to transfer the jack-up leg loads into the seabed below. The structure of the spud-can should thus have the capacity to resist the resulting shear and bending stresses exerted on it by the leg and the foundation soils. To determine the maximum force on a spud can during the design phase, the total weight of the upper hull during the worst design storm condition and its center of gravity is first established. This weight is then distributed over all the legs of the jack-up platform. From the applied environmental forces, the overturning moment is determined next. The direction of this overturning moment should be so as to cause the maximum compressive force on one leg. An appropriate load factor should then be applied to this force. The area of contact between the spud can and the soil should be sufficient for the weakest chosen soil condition to support this force.

Other criteria that are applied to design the structural strength of the spud can are:

Assume that the entire reaction acts as a concentrated load on the tip of the spud can.

Assume that the entire reaction acts on a circle centred on the tip of the spud can, whose radius is (i) %, (ii) %, (iii) 3/4 and (iv) 1 times the equivalent radius of the can.

The lower plating should be designed for the resulting distributed loads. Spud cans are usually designed to be flooded during operation. To facilitate access to the inside of the can, during the floating condition of the jack-up platform, vents may be provided to a certain height above the top of the can. The upper plating should be designed for a hydrostatic head corresponding to the height of this vent in case the can is not flooded.

Legs

Trussed legs are the most common type on modern jack-up rigs, the other type being cylindrical legs. Legs are subjected to the following forces:

(1) Elevated condition:
(a) Compression forces due to gravity loads on the hull.
(b) Compression forces due to the reactive couple caused by overturning moments on the jack-up.
(c) Bending moments at the hull due to the horizontal displacement of the hull and the moment connection between the leg and the hull.
(d) Horizontal forces on the leg due to wave, current and wind action.
(e) Bending moments due to P-A effect on the leg.
(f) High local stresses due to force transfer and from the pinions. “rack chocks, hull upper and lower guides”.
(2) Afloat condition:(a) Gravity loads on the leg.
(b) Wind force.
(c) Inertia forces due to vessel motions.
(d) Restraining reactions from guide units or other locking devices in the hull that create high moments in the leg.
(e) Fatigue causing cyclic stresses in the lower bays of the legs due to the constant pitch and roll motions of the floating vessel.

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.