Design of Self Propeller

8/13/2019 Design of Self Propeller

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The Design of a Self-propelled Jack-up Drilling Rigfor the Chukchi Sea

John Bandas, Sarah Schlosser, Sean Finn, Nathan Garza, Andy Lister, and Jeff PhillipsOcean Engineering Program, Zachry Department of Civil Engineering

Texas A&M UniversityCollege Station, TX 77843-3136

Abstract- ConocoPhillips asked the team to design a selfpropelled jack up drilling rig for exploratory work in theChukchi Sea, during the warm water season, at a location that isapproximately 131 feet (40 meters) in water depth. This wasaccomplished using computer programs including StabCAD,SolidWorks, AutoCAD, and Visual Analysis. The legs of the jackup were designed to withstand ice collisions with the aid ofpatrolling ice breakers. The jack up rig had to be capable oftraveling at speeds up to 11 knots (5.65 m/s). The stability duringtransit was analyzed for an intact condition as well as a damagedcondition (assuming two ballast tanks are damaged). The centersof gravity and buoyancy were calculated, as well as metacentricheight. A geotechnical analysis was performed on the spud cansof the rig . The rig was designed to comply with all safetyregulations specified by the American Bureau of Shipping (ABS),the Mobile Offshore Drilling Unit (MODU) Rules, theInternational Maritime Organization (IMO) rules, and the T&R5-5A Design Criteria set by the Society of Naval Architects andMarine Engineers (SNAME) and Marine Pollution Act (Marpol73/78).

I. I NTRODUCTION

A jack up drilling rig was designed to operate in theChukchi Sea. The rig meets the requirements of ConocoPhillips and is capable of surviving the open water season.The drilling location is 71N, 165W between Alaska andRussia where the water depth is approximately 131 feet (40meters).

The jack up is intended to be used during the open waterseason which is from June to mid December when the ice isminimal in this location. The rig is capable of operating in

broken ice conditions that are typical during the beginning andend of the open water season in the Chukchi Sea, holdingenough fuel and supplies as well as have enough storage forthe entire season, and accommodating 110 people. The jack uprig was designed in accordance with the ABS Class Rules aswell as the Site Specific Requirements to SNAME T&R 5.5

criteria [1].Some major design considerations include the effect of theice and the extreme temperatures. The ice is a major designconsideration due to the catastrophic consequences of icecolliding with the jack up rig. The rig was designed to be ableto sustain minor collisions. The comfort of the crew was alsotaken into account. With temperatures reaching as low as -20degrees Celsius, the crew needed to be able to function inthese extreme temperatures [2]. Operations on the deck were

designed to be functional despite any heavy gear worn by thecrew.

To ensure that this jack up rig can be built with the supportof the different classification societies the followingregulations were complied with during design. The jack up rigis classified by ABS as a Self-Elevating Drilling Unit [3]. Thisunit is capable of floating freely to the desired location underits own power or tow, and raises itself on its legs to adetermined elevation. Particularly, this drilling unit had to

encounter first- and second-year ice over its operation. Thisrequired special accommodations as shown in the ABS SteelVessel Rules [4].

The vessel had to meet the Mobile Offshore Drilling Unit(MODU) rules which are created by ABS and reference theInternational Maritime Organizations (IMO) stabilityrequirements. These requirements entailed an intact stability in100 knot (51.5 m/s) winds, and damaged stability in 50 knot(25.8 m/s) winds. The final waterline could not submerge anywatertight opening, and the righting moment had to equaltwice the heeling moment at a certain angle [3].

Environmental loading rules are given in Reference [3].These rules cover loads due to wind, waves, and currents, aswell as phenomena such as vortex shedding and gravityloadings due to the unit resting on the seabed. Additionally,the design loads and pressures encountered by ice aredescribed in Ref. [3].

Fire and safety measures are also described. The bulkheaddivisions are characterized along with the various means ofescape. The overall structure arrangement is defined byMODU and SOLAS safety guidelines. Other requiredguidelines include procedure for escaping in case ofemergency, and are followed by making sure all theequipment needed for proper evacuation are within the jack uprig [2].

Fire fighting systems were arranged to protect the generalarea of the rig as well as the drilling area [5]. Additionally, firewater stations are located along the drilling area per ABSguidelines. All fire hoses are collapsible and are within therequired length of 30m (100ft) [3]. Portable fire extinguisherswere provided in accordance with National Fire ProtectionAssociation (NFPA) standards in type and size [6].

In the event of an evacuation, the rig was designed to provide multiple routes of escape. Stairways and ladders are provided to be used during evacuation. In machine areas,vertical ladders were installed to ensure a quicker and more

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practical means of exiting the areas. These machine areasconsist of two vertical ladders each which are insulated to

provide a safe escape and fire shelter [3]. For evacuation purposes, five 25 man life rafts are on both the starboard and port side and numbered even and odd, respectively. There are300 life jackets on site to account for the crew with halflocated throughout the rig itself, and the other half locatednear the life rafts. A three minute evacuation plan is also in

place to satisfy Safety of Life at Sea (SOLAS) guidelines [6].Through site research, the team determined this is rarely ever

pack ice, meaning it is usually just relatively small piecesfloating, therefore not requiring the use of a special icecapable life raft.

The superstructures for the hull, decks and deckhouses areconstructed of ASTM A53 Grade B steel which can withstandtemperatures as low as -60C and is used primarily in offshorestructures [8]. Steel was chosen due to its high modulus ofelasticity and is also used for framing and ceiling constructionas well. Other areas such as control and service spaces areconstructed using material with minimal flame-spreadcharacteristics.

The bulkhead divisions of the rig were designed inaccordance with MODU 3-4-1 and SOLAS Regulation II-2/3.3 [2][6]. The selected division for the structure is to be A-60 class division, subdivided by space classifications rangingfrom control stations to sanitary spaces to the fire integrity of

bulkheads separating adjacent spaces [2]. When cables andother pipes penetrate through the hull, the open spaces aremade air tight in order to prevent smoke and fire fromspreading. To prevent oil spills several safety precautions areimplemented on the rig. Containment modifications to thehull design are implemented as well to avoid hydrocarbonsentering the surrounding environment [8]. Pressure reliefvalves are placed into the process line to avoid over pressureof the oil. In case the oil pressure does supersede the set

pressure the valve will discharge into the secure gutter area.Additionally, vapor depressurizing is used to drop the pressureto 50 percent of the design pressure to bring the system to asafe operating condition. Emergency shutdown stations are

placed in critical locations around the rig such as thehelicopter deck, process deck and control station. This shouldallow for the crew to halt operations when the process area isexperiencing severe over pressure and other methods such asthe pressure relief valves and depressurizing do not suffice.

Additional safety guidelines are followed in the event ofan actual oil spill. Curbing at deck level is implemented to

prevent any oil spill. Protective walls along the gutter are in place to prevent the oil from draining into the environment.Recessed drip pans are installed to collect any oil spilled in thedeck area.

II. E NVIRONMENTAL CONDITIONS

The total pressure acting on the legs, during drilling, is acombination of static and dynamic pressure. The static

pressure is a function of height and increases linearly with thedepth from the mean water level (MWL). The static pressure

is derived from MODU 3-1-A2 and is found using (1) [3].

(1)Here pressure is a function of density , gravity g, and

depth d. The dynamic pressure applied to the structure takesinto account the wave number and varies exponentially withdepth as shown in (2). H refers to wave height, while k is thewave number.

(2)The dynamic and static pressures were superimposed to

determine the total pressure profile. At a depth of 131 feet (40m), the total pressure is 731.3 kips (3250 kN).

The wind loadings were calculated using the beam and bow profiles of the jack up rig. The projected areas of the rigfrom both views were calculated and a shape coefficient isgiven to each section. The two profiles of the jack up werethen taken and divided further and given a height coefficient.The height coefficient changes as the height increases every51.5 ft (15.7 m).

The annual frequency of occurrence of wind in theChukchi Sea was obtained from reports by ConocoPhillips

[10]. From statistical analysis of this table the average windconditions were found as well as storm conditions for up to a100 year storm. The average wind conditions are presented inFigure 1 and the direction they normally propagate in anygiven year is shown in Figure 2. The average wind speeds inthe Chukchi Sea range from 8.2 ft/s to 24.6 ft/s (2.5 m/s to 7.5m/s) as shown in Figure 1 and this wind is coming for thenortheast nearly 30% of the time. This data is useful when

positioning the rig as tests can be run to determine whichangle allows for the least drag when drilling.

According to ABS MODU rules the rig must be designedto withstand storm conditions of a 100 year storm [3].Through statistical analysis, conditions for 3 different stormcases: 1 year, 10 year and 100 year, were performed and are

presented in Figure 3.The wind calculations pertaining to this jack up rig were

calculated in the bow, beam and quartering seas directions. Adesign wind speed of 14.5 knots (7.5 m/s) was used since itwas the highest average wind speed encountered at this site.Under this design wind, the wind load is 203.5 kips (905.2kN), 232.6 kips (1034.7 kN), and 291 kips (1294 kN) for bow,

beam, and quartering directions, respectively.The current forces applied to the structure were calculated

using Ref. [3]. The current profile was determined using (3)from the surface to a depth of 16.4 ft (5 m), which is

Figure 1. Occurrence of Given Wind Speed


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Figure 2. Occurrence of Wind Direction

considered the boundary layer. From the boundary layer to theseabed (4) was used to calculate the current profile becausethe wind does not affect the current below the boundary layer.In these equations, V t denotes the tidal velocity, V s the stormtide velocity, V w the wind driven current, and w the windvelocity.

(3)

(4)

The wind driven current velocity was determined using africtional coefficient which is derived from the Coriolis parameter. The Coriolis parameter is dependent on the latitudeof the drill site. The average current force was calculated, andwas found to be approximately 2.04 kips (9 kN).

Regarding environmental loads, the rig needed to be ableto withstand the worst case scenario of loading which led tothe load being applied from three different directions, bowseas, beam seas and obliquely. The current load wascalculated for one leg of the jack up. These loads werecalculated as 2.1 kips (9.2 kN) for bow and beam seas, and 1.4kips (6.2 kN) for oblique seas.

The jack up was designed with an air gap sufficient to prevent waves from hitting the deck, however properconsideration was needed for the waves hitting the legs. Tocalculate this load Morrisons equation, was used, shown in(5). F I and F D correspond to the inertial and drag forces,respectively, while C S and C D are inertial and dragcoefficients . This equation was specified in the MODU rules[3].

(5)For the Chukchi Sea, the design period was given as 5.5 s

at a water depth of 131 ft (40 m). Using this information wecan solve (5). The structure was found to be drag dominant,with a drag force of 2.25 kips (10 kN), a inertial force of 0.65

kips (2.9 kN), for a total wave force of 2.90 kips (12.9 kN).This jack up rig is outfitted with an ice radar system which

tracks ice flow near the rig and has two ice breakers assistingin minimizing ice collisions. The ice breakers operate at a

perimeter of 20 nautical miles (37 km) to break any pack icethat penetrates that initial boundary. In the case of ice

breaching a boundary of 15 nautical miles (28 km) the crewwill continue normal operations while preparing to secure the

Figure 3: Wind Speed, Wave Height, and Peak Period for Storms

well. If the ice approaches 10 nautical miles (18.5 km)drilling will be stopped and the rig will enter survivor mode.

Further, the rig is designed for minimal ice collisions, inthe event the detection system falters. The ice loadingcalculations were performed as stated in Ref [3]. To determinethe load, the design pressure due to the ice was calculatedusing (6).

(6)The K 1 and K 2 coefficients were determined based on the

class of ice encountered. D ice corresponds to the displacementweight of the ice chunks. The ice class that the rig is designedto encounter is B0, which can be very large when assissted byan ice breaker. This corresponds to a value of K 1 0.165 and avalue of K 2 0.38. The angle of the structure in the ice belt,which is 90 for this rig, was used to determine the K 3 coefficient (7). The flare angle, of the structure in the ice is 90degrees. This angle resulted in a K3 value equal to 0.6.

(7)When (6) was computed, the ice loads were calculated to be1072 ST (9.54 MN).

III. JACK UP DESIGN

The arrangement of the equipment and material on thedeck of the rig is very important to the stability of the facilityand is shown in Figure 4. The center of gravity needed to be asclose to the center as possible to avoid having a rotatingmoment on the deck. The deck itself, from a starboard view,has a trapezoidal profile to minimize the amount of drag onthe hull during transit. The top of the deck is 232.9 ft (71 m)from bow to stern 177 ft (54 m) from starboard to port. The rigis required to accommodate 120 people and researchingexisting crew quarters. The quarters are 4 stories tall, withdimensions of 84 ft (25.6 m) by 42 ft (12.8 m) and 52 ft (15.9m) in height with a total weight of approximately 200,000 lbs(890 kN) [11]. The helipad is attached to the crew quarters toallow quick access in the event of an emergency and isoctagonal in shape with a diameter of 72 ft (22 m). Analuminum deck was chosen, to alleviate some weight, with thecapacity to hold a S92 helicopter [12]. The center of thetopside contains a pit where the casing and piping used fordrilling are stored which cuts into the hull of the topsideapproximately 10 ft (3m) to negate the wind forces on thematerials. In order to reach the entire span of the topside, four


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Figure 4: General Arrangement of Structures on Deck

cranes are required. The cranes chosen are capable of lifting300 tons and are mounted on pedestals on the deck. They are13.1 ft by 13.1 ft (4 m by 4 m) [13].

The drilling derrick is located on the cantilever which can be retracted during transit to keep water from flowing into the pit containing the pipes and casing. The cantilever is 100 ft(30.5 m) in length and can reach a distance of 70 ft (21.3 m)away from the deck. The entire structure will be supported bytriangular legs that are 361 ft (110 m) in length. The legs weredesigned with an inverted K truss to add strength and stability.The square holes in the hull allow the spud cans to be retractedinto the hull during transit to reduce drag. The spud cans areoctagonal in shape, 26.2 ft (8 m) from point to point.

In order for the rig to be stable during transit and for preloading purposes, ballast tanks were required. The rig isdesigned to have twelve ballast tanks, two of which are filledduring transit; they are labeled in Figure 5. In order to stabilizethe rig during transit the two ballast tanks are filled to 4.8 ft(2.1 m). The center compartments in Figure 5 are used formud and portable water storage.

The distribution of the weight on the deck is veryimportant in relation to the stability and moments on thefacility. The weight of one crane and pedestal isapproximately 493 kips (2200 kN) and was treated as a static

Figure 5. Ballast Tank Schematic

load in the analysis of the center of gravity [13]. Thealuminum helipad was chosen for its lightweight properties,weighing approximately 48 kips (214 kN) [12]. The cantileverand drilling derrick were combined into a total weight of 360kips (1600kN). The legs are made of ASTM A53 Grade Bsteel and weigh approximately 905 kips (4026 kN) total. Thematerials needed for drilling have an approximate weight of14000 kips (62000 kN) giving the entire rig an estimated

weight of approximately 19500 kips (86000 kN).With the different weights being arranged around the deck,

a moment was created about the longitudinal axis. Themoments were taken using the distance from the datum, whichis the center of the deck for every axis. Ballast tanks wereused to offset this moment during transit. The hull containstwelve ballast tanks in total and flooding two of these tanksdiminished the moment about the longitudinal axis by

bringing the longitudinal and transverse center of gravity tothe center. This eliminates the overturning moment duringtransit and caused the vertical center of gravity to be 11.8 ft(3.6 m) due to the legs being completely retracted.

One of the design requirements for the jack up is that it

should be self-propelled at a speed of 11 knots (5.66 m/s). Inorder to find the required power to propel the ship, theresistance of the hull was calculated. There are several typesof resistance, including frictional, wave-making, form, andappendage resistance. Without model tests or programs tocalculate the resistance, all forms of the resistance save for thefrictional resistances are extremely difficult to calculate. Tosimplify the calculation of resistance, it was assumed that thefrictional resistance is equal to or greater than all other forms;that is, the total resistance was calculated as simply twice thefrictional resistance. The frictional resistance was calculatedusing (8), where S is the surface area and V is the ship'svelocity [14].

(8)Using these calculations, the resistance of the hull is found

to be approximately 50.4 kips (224 kN). Multiplying by thevelocity yields a power requirement of 1700 hp (1.3 MW).This kind of power can be achieved using retractable thrusters,which retract into the hull when not in use (when the legs aredeployed). Thrusters of this type were found that can achieve600 hp (447 kW) [15]. Four thrusters are installed on eachcorner of the ship. Coupled with the weight of a power plant,this setup weighs approximately 3.3 million pounds (1.5million kg).

Considering the high bearing capacity of the soil in the North Chukchi Sea, spud cans were chosen as the foundation

support for the jack up rig as opposed to a mat footing whichis generally used for unstable soils [16]. A design is shown inFigure 6 where the initial diameter was set at 26.2 ft (8 m).The projecting tapered point on the bottom helps to reduce therisk of slipping in the case of shallow penetration, which istypical in soils with high bearing capacities [16].

When the legs are retracted during transit the spud cans are


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Figure 6. Spud Can Profile Drawing

able to fit into the hull to reduce drag. To calculate the bearing area, the spud can was treated as a flat octagonal plateas shown in the plan view of Figure 6 to simplify thecalculations. This bearing area was used in calculating thevertical reaction force the soil provides to the structure. Thelateral area, shown in the profile view of Figure 6, was used inthe horizontal reaction force provided by the soil. The spudcans are made of plates of ASTM A53 grade B steel with a .5

in (13 mm) thickness.The SNAME T&R 5.5A required a check on backflowover the spud can when fully penetrated to determine if itshould be factored into the preloading requirements. Thecriteria of (9) was used to determine if this rig experiencedany backflow [1]. If (9) is satisfied there is no backflow,which was the case with this jack up rig. In (9 ), D soil is the

penetration depth, N cu is a bearing capacity factor, and ' is theeffective soil weight.

(9)SNAME also required a design to be checked against any

squeezing of the clay (10) [1]. If this equation is satisfied thereis no clay squeezing which was the case for this jack up.

(10)where B is the diameter of the spud can and T soil is the depth ofthe clay layer.

In the northern Chukchi Sea, the soil was classified asclayey silt according to the geotechnical report provided byConocoPhillips [10]. Clay tends to be very plastic regardlessof water content meaning it can be deformed without

breaking, cracking, or changing in volume. Clay possessesvery high strength properties while silt tends to have little orno plasticity giving it very little strength [17]. The first 15 ft

(4.5 m) of soil in the northern Chukchi Sea is very soft clayeysilt while the soil down to 75 ft (22.9 m) tends to be stiff tovery stiff clays.

With these denser soil properties spud cans were used fora foundation as opposed to using a mat which is generallyused in softer soils. Using these soil properties the bearingcapacity of the soil was calculated using (11) [1]. Thisequation is specific to the non-squeezing and no backflowcases discussed earlier.

(11)where cu is the undrained shear strength, S c is the shape factor,d c is a depth factor, and p' o is the effective pressure at the

penetration depth.The un-drained shear strength varies over a range; and the

average of this range was used in the calculations. The bearingcapacity factor was found using (12). Here, is the latitude of

the drilling location.

(12)The shape factor was calculated using (13), where the

bearing capacity factor is given in the SNAME code to be 5.14and the foundation length is equal to the diameter of the spudcan due to its octagonal shape.

(13)To calculate the depth factor, the depth to diameter ratio

had to be evaluated; since it was less than 1, (14) was used.

(14)The effective pressure at depth was calculated using (15).

A is the projected area of the spud can.

(15)The vertical and horizontal reaction forces could then be

calculated using (16) and (17), respectively. To calculate thehorizontal reaction force, the lateral projected area of the spudcan was needed. This area was determined by looking at a sideview of the spud can and calculating the area.

(16)

(17)All of these equations were specified by Ref. [1]. Thediameter of the spud can was designed to be 26.2 ft (8 m).Using this diameter and a penetration depth of 9 ft (2.75 m),the vertical reaction from the soil was calculated as 7065 kips(31.4 MN) and the horizontal reaction force was 19 calculatedas 18600 kips (82.7 MN). These values create factors of safetyin the horizontal and vertical directions of 240 and 1.45,respectively.

To calculate the preloading requirement for installation themaximum loading possible was calculated and magnified toinclude safety factors. This maximum loading was comparedwith the vertical leg reaction which has been reduced by 10%due to the built in safety factors. To calculate the preloadingamount, (18) was used [1].

(18)While preloading, 7200 kips (32000 kN) must be loaded

on each leg to ensure proper stability in the soil. When the rigis installed it will lower two legs, diagonal of each other, andfill ballast tanks until this preloading amount is reached. Whilethe tanks are filling the legs will begin to penetrate the soil


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until the preload is met, and they should be reaching theirmaximum penetration of 9 ft (2 m).

An overturning moment is created at the spud can due tothe environmental loads which is then counteracted by arestoring moment due to the soil. ABS requires a minimumfactor of safety of 1.1 for these moments. They overturningmoment was calculated using Visual Analysis and was foundto be approximately 91.8 k-ft (125.6 kN-m). The restoring

moment was calculated using (19) [16]. This restoringmoment was calculated to be 116.1 k-ft (158.8 kN-m) creatinga factor of safety of 1.26 which satisfies ABS rules.

(19)where M s is the restoring moment, M so is the restoring momentassuming a rigid structure, n is the number of legs, P is theaxial load in the legs, and e is displacement.

The vessel must meet the MODU stability requirements asstated by ABS. This entails an intact stability in 100 knot(51.5 m/s) winds, and damaged stability in 50 knot (25.8 m/s)winds. The final waterline should not submerge any watertightopening, and the righting moment must equal twice theheeling moment at a certain angle according to [3]. Following

the specification on the jack ups topside arrangements; themodel for StabCAD was completed as shown in Figure 7. ForStabCAD analysis, the vessel appendages were configured fora freely floating condition, the period of deployment betweentransit and jacking up. In accordance with the ABS MODUrules, the vessel must be stable during every stage ofdeployment; since the center of gravity is highest with the legsup, this configuration is good for design. Note that the craneand leg trusses are modeled here as cylinders. StabCADallows for such an approximation by means of a shapecoefficient, specified by the user, which converts the

properties of the cylinder to that of the desired truss shape[19]. For this analysis, the shape coefficient is left at a default

value of 0.5, as suggested by the StabCAD manual forapproximating trusses.According to the MODU rules, the area under the wind

righting moment curve must be 40% larger than the area of thewind heeling moment (have an area ratio of 1.4), over a rangeof inclination from rest to the down flooding angle of thevessel [3]. Also, the angle between where the moments areequal and the down flooding (the range of stability) must beabove 7 degrees. The down flooding angle is the angle towhich weather tight integrity is preserved. The inclination istaken about the axis most susceptible to down flooding; in thiscase, diagonally across the hull, or 40.6 degrees from inline.

For the intact condition, the vessel must meet the abovecriteria of moment areas in both calm and storm conditions.For normal operations, the vessel is designed for 70 knot(36m/s) winds, while the design for severe storms is 100 knots(51.5 m/s) [3]. The program is run with a KG of 32.8 ft (10m). The longitudinal and transverse center of gravity isassumed to be in the center of the hull and a draft of 9.19 ft(2.8 m) is used.

StabCAD computes the righting and heeling moments,and automatically generates a down flooding angle and area

Figure 7. StabCAD Model

ratio. The stability curves for the normal conditions are shownin Figure 8.

This is the lowest wind speed allowed for design, and themoment area ratio of 6.52 far surpasses the criteria required of1.4 specified by the MODU rules. Note that in this figure, theallowable KG is stated to be 36.1 ft (11 m); this is not the true

KG. The true allowable KG is retrieved from StabCAD byforcing the area ratio to equal 1.4. For the normal conditions,the allowable KG is 290 ft (89.1m). Even in the severe stormcondition shown in Figure 9, the area ratio is 3.18, and theallowable KG is found to be 290 ft (88 m). For both windspeeds, the range of stability is 8.25 which is greater than the7 requirement according to ABS MODU rules.

For the damaged stability, MODU requires that the vesselmaintain a range of stability of 7 degrees when the hull is

penetrated to a horizontal depth of 5 ft (1.4 m) [3]. The windcondition for damaged stability is 50.2 knots (25.8 m/s) [3].The worst case is a puncture at the bulkhead boundary,flooding two compartments. When comparing this drawingwith the general arrangement of bulkheads, Figure 5, it showsthat two of the ballast tanks would be damaged.

With the damaged tanks, the vessel has a static angle aboutthe inclination axis of 0.58 degrees, found in Figure 10.However, the range of stability is 7.23 degrees, which issatisfactory and the allowable KG is well above the calculatedKG of 33 ft (10 m).

The rig was designed to have triangular-shaped legs due tocost effectiveness. The truss form of the legs are a reverse Karrangement, which helps stability and reduces the amount ofmaterial. Figure 11 is a 2-dimensional model of the rig withthe cantilever retracted. There are plates attached at the bottomto simulate the spud cans. The main deck consists of steel

plate sections which include the ballast tanks and bulkheads asnoted in the general arrangement. The total height of the legsis 361 ft (110m); the excess length helps provide reservestrength for the lowersections which are subjected to variousenvironmental loads.

The wind loads are 788.21 lb/ft (36.4 N/m) distributedlinearly along the top side legs and start dissipating at 10 ft(3.05 m) above the main deck to 0 at the deck line. The


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Figure 8: Moments of Inclination in 70 knot (36 m/s) Winds

cumulative topside weight is 510.5 kip/ft (2270.8 kN/m) and placed 86.12 ft (26.25 m) from the bow. This configurationrepresents the cantilever retracted; for the operational casewhere the cantilever is extended the deck load is 87.63 ft

(26.71m) from the bow. The operational water depth is 131 ft(40m) with a calculated 49.2 ft (15 m) air gap placing the

bottom of the deck at 180.45 ft (55 m). Each test used 616.86lb/ft (9 kN/m) wave and current loads at the mean water leveland linearly decreasing to 0 at 16.4 ft (5 m) below the meanwater level.

With the rack and pinion system used to jack up the legsand support the deck, the strength and stability of theconnecting joints was a very important design consideration.With the two cantilever scenarios, the maximum momentswere 7.04105 kipft (9.54105 kNm); the operationalcantilever slightly changed the location of the center of gravity

but had minimal effects on the joint reactions and member

forces.The total deck and topside produces a maximum verticalforce of 18.14 kips (80.70 kN) on each leg; this required therack and pinion supports to be able to withstand these forcesas well. The moments experienced on the spud cans were

Figure 9. Moments of Inclination in 100 knot (51.1 m/s) Winds

Figure 10. Moments of Inclination in Damaged Condition

maximized at 92.6 kipft (125.60 kNm).In order to ensure the stability of our structure the rack and

pinion system had to be able to withstand the above mentionedmoment and the soil had to withstand the forces created at thespud cans.

The primary goal of structural analysis was to prove thatthe structure as a whole can withstand the applied loadswithout failure. A 3-dimensional model was constructed andthe loads applied. The topside structures in the model weremodeled as plates to help get a more accurate computation ofthe wind loading. The increased surface area subjected to thewind loads will have a greater impact on the resulting momentexperienced by the connecting supports between the legs andmain deck and the rack and pinion system. In order to simulatethe weight of the topsides a uniformly distributed load of0.0215 ST/ft2 (2.06 MPa) was placed on the deck plates. Thisvalue was calculated by dividing the total estimated topside

weight (7496 ST) by the operational area of the deck.The results of the 3-dimensional loadings are applied tothe bow (x-direction) are shown in Figure 12. The deckexperienced minimal deflections demonstrating the current

Figure 11. 2D Model Depicting Retracted Cantilever


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bulkhead configuration was adequate for the environmentalloading.

The members angled out just above where the deck islocated are basic models of the rack and pinion system used tosupport and jack up the hull. As expected, the maximumstresses are experienced within the internal truss members.The max stress for an internal member is 8.75 ksi (60.3 MPa).Maximum shearing and moments were both found within the

members of the simplified rack and pinion system. A maxshearing force of 6.23 kips (27.7 kN) and moment of 18.95kipft (25.7 kNm) were both located at the base of the rackand pinion member. Most of this force is attributed to windloading; the actual members will be shielded from theelements therefore only needing to withstand the deck loadsand topside wind loading.

The results of the 3-dimensional loadings were alsoapplied in the beam direction (y-direction), as shown inFigure 13. As with the previous case, most of the stresses werefound in the internal members of the truss and the forceslocated in the rack and pinion system. The maximum stressand axial stresses were found to be 21.47 ksi (148 MPa) and

0.26 ksi (1.8 MPa), respectively. Both of these were locatedon the upper part of the leg and resulted solely from windloading. The upper limits of the shearing forces and momentswere 10.68 kips (47.5 kN) and 49.12 kipft (66.6 kNm),respectively. These two forces were experienced on thesupport members of the rack and pinion system once again.

Due to all of the extreme forces were experienced on thestarboard wind loading model, proper consideration wasneeded to determine if the steel used in construction is capableof handling such stresses. All of the steel members are circular8 XS pipe with an outer diameter of 9 in (.023 m) and a 1.02in (.026 m) thickness of ASTM A53 Grade B steel for the coldweather conditions. The minimum yield stress for this steel isFy=35 ksi. The maximum stress and axial stress are bothwithin the yield stress limit giving a factor of safety of 1.63.With a design K-value of .65 and effective length of 25.78 ft(7.86 m), the available strength in axial compression ( c Pn )is 373.47 kips [18]. The shearing force of 10.68 kips is well

below the allowable 373.47 kips, giving

Figure 12. Member Forces of Bow Loading Results

Figure 13. Member Forces of Beam Loading Results

a factor of safety of 34.97. Also, the available flexural strengthof this steel is 81.4 kipft (110.36 kNm) found in AISC Table3-15. The steel is able to withstand the maximum moment of49.12 kipft with a factor of safety of 1.66. All of the structuralmembers withstand the applied environmental loading withadequate factors of safety with respect to the failure modes ofthe ASTM A53 Grade B steel pipe used for the structure [18].

IV. COST A NALYSIS

The cost of the rig was determined using modern day shipyard estimates. It was found to be around $900 million withthe funds allocated as shown in Figure 24. The contingencycost of the rig is about 10% of the total cost bringing the totalcost up to $1 billion. From this figure one can see that thetopside equipment and hull steel take up the bulk of the costwith a total of $643 million. This cost is higher than the

average for todays jack up rigs which cost around 650 to 800million dollars due to its required different steel and sheersize.

V. FINAL DESIGN SUMMARY

The jack up drilling rig designed in this report was foroperation in the northern Chukchi Sea in a water depth of 120ft (40m). The jack up works in the warm water season whenice is at a minimum; however, to deal with ice, the jack up hasradar detection systems and an escort of ice-breakers. Thedesign calls for four triangular truss legs that are 20 ft (6 m)wide and sit atop spud cans that are octagonal in shape with a

diameter of 26.2 ft (8 m). The top of the deck is 232.9 ft (71m) from bow to stern and 177 ft (54 m) from starboard to port.The hull houses 4 cranes, accommodations for 120 people,

helideck, and piping for two different drilling locations.Additionally, the ship is self-propelled, up to a speed of 11 kts(5.66 m/s). The total weight of the jack up is 19 million

pounds, or (8.6 million kg). The jack up has been designed tocomply with the MODU, ABS and SNAME rules in theirentirety as well as accommodate the extreme climate issuesthat arise with drilling in the Arctic Ocean.


9/9

The forces on the ship were calculated for wind, current,wave, and ice conditions. The wind force was based off 100year storm conditions and quartering seas are taken as theworst case scenario. For a 100 year storm with wind speed of24.6 ft/s (2.5 m/s), the wind force was 291 kips (1294 kN).Thecalculated value for a current load on a single leg is 2.1 kips(9.2 kN). The wave force was 1.47 kips (6.45 kN), fromMorrisons Equation. For ice, the MODU rules list equations

that yield an ice force of 217.8 metric tons (240 short tons) foreach leg; the pressure is calculated to be 16 psi (1104 MPa)for each leg. The area of the leg occupying the ice belt isfound to be 235 ft2 (21.6 m2) and with four legs to multiply

by, the total force was found to be 1072 short tons (9.54 MN)on the structure.

The in situ geotechnical analysis was performed byMcClelland Engineers and the data was provided to the team

by ConocoPhillips [10]. The bearing capacities werecalculated using the dimensions of the spud can, un-drainedshear strengths and penetration depths. From this data, thespud cans are able to withstand horizontal and vertical forceswith factors of safety 240 and 1.45, respectively. The

maximum penetration depth is 9 ft (2 m).The vessel must meet the MODU stability requirements as published by ABS while in the freely floating condition, the period of deployment between dry-towing and jacking up. The program StabCAD was utilized to calculate the moments,hydrostatics, and down flooding angles of the vessel. The rigwas analyzed for an intact stability in 100 knot (51.5 m/s)winds, and damaged stability in 50 knot (25.8 m/s) winds [3].The vessel was found to meet all the stability requirements.

In order to support the topsides, deck weight and drillingequipment and have the ability to withstand the harshenvironmental loading; this rig has four triangular reverse Ktruss legs for support. Using computer modeling this designhas been found to have minimal deformation. To ensure thestructural strength of the steel members, 8 XS pipe with anouter diameter of 9 in (.023 m) and a 1.02 in (.026 m)thickness of ASTM A53 Grade B steel was used forconstruction of the truss legs. After calculations using theAISC Steel Code this material contains the needed strength tosupport this vessel with acceptable factors of safety [18].

Based on current shipyard estimates and steel prices, thisrig is estimated to cost approximately $1 billion.Approximately 75% of the total cost is attributed to topsideequipment and steel. This estimate is slightly higher than most

jack up rigs, but this is attributed to ice fitting the vessel withspecial steel and extra safety precausions.

ACKNOWLEDGMENT

The authors thank Dr. Robert E. Randall, Dr. CharlesAubeny, and Dr. Terry Kohutek from Texas A&M Universityfor their guidance. Thanks to Mr. Peter Noble and Mr. RandallShafer of ConocoPhillips for access to geotechnical data andguidance on the drilling design process. The authors alsothank Alberto Monrad, of Global Maritime, and J. AndrewBreuer, Pao-Lin Tan, and Han Yu of the American Bureau ofShipping.

R EFERENCES

[1] Society of Naval Architects and Marine Engineers (SNAME) T&R 5-5A(2002). Site specific assessment of mobile jack-up units . Society of

Naval Architects and Marine Engineers , New Jersey. [2] "Weather & climate | alaska.com." Alaska Travel, Jobs, Homes, Fishing,

Hiking and more... | alaska.com. 22 Feb. 2009.

[3] American Bureau of Shipping (ABS). Rules for Classing and Building Mobile Offshore Drilling Units. Vol. 3. Houston: ABS, 2001.

[4] American Bureau of Shipping (ABS). Steel Vessel Rules . Houston:ABS, 2007.

[5] American Petroleum Institute (API). RP 14C: Recommended practice for analysis, Design, Installation and Testing of Basic Surface SafetySystems on Offshore Production Platforms. 3rd ed. Houston: API, 1984.

[6] National Fire Protection Association. Fire Code . Quincy, Ma: NFPA,

2009.[7] International Maritime Organization. SOLAS . London: IMO, 2004.[8] "Steels for Cryogenic and Low-Temperature Service." KEY to METALS

Steel -- Welcome to the World's 39 Most Comprehensive Steel Properties Database. 05 May 2009.

[9] International Maritime Organization. MARPOL . London: IMO, 2006.[10] Noble, Peter. Chukchi Sea Geotechnical Report. Rep. McClelland

Engineers, 26 Jan. 2009.[11] "Living Quarters / Bunk Houses | GML - General Marine Leasing."

GML General Marine Leasing - An Oil States International Company -Offshore & Onshore portable workforce accommodations, auxiliaryequipment, & support services. 19 Feb. 2009.

[12] "Standard Helidecks | Aluminium Offshore - Aluminium SafetyHelidecks, Helideck Support Frames and Offshore Structures."

Aluminium Offshore - Pioneer in the Design and Production of Aluminium Alloy Structures | Aluminium Offshore - Aluminium Safety Helidecks, Helideck Support Frames and Offshore Structures. 13 Feb.2009 .

[13] HUISMAN - Home (en) . 3 Mar. 2009 .[14] Tupper, Eric C. Introduction to Naval Architecture . 4th ed. Oxford:

Elsevier, 2007.[15] "Retractable Thrusters." Thrustmaster of Texas . 03 May 2009

.

[16] Tirant, P. Le. Design Guides for Offshore Structures (Collection Patrimoines) . Minneapolis: Editions Technip, 1993.

[17] McCarthy, David F. Essentials of Soil Mechanics and Foundations : Basic Geotechnics . Upper Saddle River: Pearson Education, 2006.

[18] American Institute of Steel Construction (AISC). Steel Construction Manual . 13th ed. Chicago: AISC, 2005.

[19] StabCAD User Manual . Engineering Dyanmics Inc, Rep. Version 4.3(2003).

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