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.