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