THE JACK UP - OFFSHORE OPERATIONS COURSE - PIDC
OFFSHORE OPERATIONS COURSE SECTION THE JACK UP SECTION THE JACK UP Training to be FIRST OFFSHORE OPERATIONS COURSE SECTION THE JACK UP TABLE OF CONTENTS TABLE OF CONTENTS JU CHAPTER GENERAL ON JACK-UPS CHAPTER ENVIRONMENTAL LOADS ON THE JU 23 CHAPTER BASICS OF SOILS AND MECHANICS 40 CHAPTER THE JACK UP IN ELEVATED POSITION - PART 50 CHAPTER THE JACK UP IN ELEVATED POSITION PART 61 Training to be FIRST OFFSHORE OPERATIONS COURSE SECTION THE JACK UP CHAPTER GENERAL CHAPTER GENERAL ON JACK-UPS 1.1 INTRODUCTION Jack up drilling rigs represent about 60% of the worldwide Mobile Offshore Drilling Units (MODU's) fleet Transocean Sedco Forex has more than 50 JU’s, which represent about 30% of the fleet within the company and 15% of the worldwide fleet These figures show the importance of the JU design for the offshore drilling industry Compared to other type of drilling rigs, the JU is rather special since it involves specific problems such as leg penetration, punch through and moving with the legs fully raised The purpose of this section is to introduce the basic concepts, which have involved the structural and naval architectural aspects of jack ups The recommendations in the Marine Operations Manual are a result of the analysis, which is rig specific Historically the JU was built to operate in mild environments up to 250ft of water depth The modern largest JU's are built to operate world wide with at present up to maximum water depth up to 450ft 1.2 ADVANTAGES AND DISADVANTAGES OF A JACK-UP In comparison to semi-submersibles, a jack-up has some definite advantages: a) Lower construction costs b) Less personnel required to run the rig c) Because of (a) and (b) lower day rates d) The possibility to work over a fixed platform e) It is cheaper for the operator to use a jack-up: Less power full boats to move the rig No mooring system required -no lost time to run anchors Less maintenance costs Surface BOP without sub sea system Simple well head assembly f) Less down time: No wait on weather due to motions Drilling equipment can be handled faster and easier Training to be FIRST OFFSHORE OPERATIONS COURSE SECTION THE JACK UP CHAPTER GENERAL However, the jack-up’s have some disadvantages: a) Limited water depth The maximum water depth for the largest JU is 450ft b) Depends on bottom condition The bottom soil conditions may cause a punch through or deep leg penetration c) In case of a blow-out the rig can not move off location d) More fragile Many incidents and damages during moving and because of a punch through Statistics have shown that over 75% of the incidents occur under tow or during jack-up/jack-down operations e) Safe operations require strict procedures 1.3 TYPES OF JACK-UP's There are two types of jack-up's (Fig 1.1): 1) The independent leg type 2) The mat supported type 3) For both types of rigs the derrick structure can be fixed over a slot in the hull or mounted on a skid to allow the derrick to slide out (cantilever type) over a platform (Fig 1.2) Independent leg jack-up Mat supported JU Fig 1.1 Types of JU Training to be FIRST OFFSHORE OPERATIONS COURSE SECTION THE JACK UP CHAPTER GENERAL Cantilever SSlot Slot type JU Cantilever JU : Typical Extension: 45ft (typ.) Lateral extension: 12ft (typ.) Note: The hook-load/setback depends on cantilever and substructure position Fig.1.2: Slot type versus Cantilever JU 1.4 THE INDEPENDENT LEG JU Most JU's have three legs but some JU's have four legs The leg structure can be the lattice type trusses construction of the K type It is called K type because the tubulars form a horizontal letter K For additional strength, some of the harsh environment jack ups have the X-type trusses The spud cans on the bottom of each leg are generally polygonal shaped structures designed with a heavy point to provide support on the hard seabed as well as to ease the penetration in soft soil The pressure on the seabed by the spud can is between 25t/m² and 35t/m² (Fig:1.3) Training to be FIRST OFFSHORE OPERATIONS COURSE SECTION THE JACK UP CHAPTER GENERAL The independent leg jack-ups are designed for a wide range of bottom conditions including an uneven slanted seabed The leg utilization becomes restrictive when the penetration becomes excessive The design penetration for most rigs is about 30 ft However, some of the TSF rigs have worked with a penetration of 150ft 1.5 THE MAT SUPPORTED JU Mat supported JU's are designed specifically for very soft bottom conditions The legs of mat supported JU are round steel columns connected at the bottom to an A-shaped structure The large bottom contact area with the seabed provides for a much lower bearing pressure between and t/m² and t/m² This type of rig is a logical choice for soft soil conditions Because the legs cannot be adjusted for a sloping bottom the mat supported units are designed to work with a seabed slope of up to 1.5° only Typical soil pressure: Independent leg JU: 25-35t/m² Mat type JU: 2-3t/m² Fig: 1.3 Soil pressure independent leg versus mat supported JU 1.6 TYPE OF JACKING SYSTEMS (Fig.1.4) The jacking systems for most the independent leg units are of the electrically powered rack and pinion type Some designs use an electro-hydraulic system to power the rack and pinions For many mat supported units a hydraulic jacking system is used to operate hydraulic jacks and associates yoke pins to fit in pin holes along the column shaped legs (Fig 1.4) Training to be FIRST OFFSHORE OPERATIONS COURSE SECTION THE JACK UP CHAPTER GENERAL Jack -house and Jacking system Examples of jacking systems gear trains Hydraulic system Joke pin Fixed pin jacking Fig: 1.4 Pictures of jacking house and jacking systems Training to be FIRST OFFSHORE OPERATIONS COURSE SECTION THE JACK UP CHAPTER GENERAL From a structural behavior standpoint, the jacking systems for the independent leg units can be classified into two types: 1) The floating jacking system 2) The fixed jacking system The environmental forces wind wave and current try to overturn the rig These forces cause bending moments, shear forces and axial forces in the legs.(Fig 1.5) Depending on the type of jacking system the reaction forces are resisted with a horizontal or vertical couple at the upper and lower guides LEG INTERNAL EFFORTS WIND WAVE CURRENT H R R Air gap FORCES: H AND R MOMENT: R * L R * L = ROTATION L Water depth Sea bed Penetration R R Fig 1.5 Bending, forces, shear forces and moments on the JU legs P L Moment (torque) = P*L Training to be FIRST OFFSHORE OPERATIONS COURSE SECTION THE JACK UP CHAPTER GENERAL 1.6 THE FLOATING JACKING SYSTEM (Fig 1.6) Environmental forces as well as gravity forces create bending moments on the JU legs In the floating system the leg bending moment is resisted by a horizontal couple of forces acting at the lower and upper guides (Fig 1.6 and Fig 1.7) This creates a large shear force on the leg between the guides Consequently the floating system JU needs heavy leg sections The result is that the moment is mainly resisted by a couple of horizontal forces acting at the upper and lower guide Fig 1.6 The floating jacking system The floating jacking system diagram Effort at Leg-Hull connection AXIAL FORCES PINION LOADS SHEAR FORCES MOMENTS UPPER AND LOWER GUIDES (Horizontal reaction) NORMAL JACKING UNITS Can't jack up with full pre-load Training to be FIRST HEAVY LEGS Truss members are heavy solicited OFFSHORE OPERATIONS COURSE SECTION THE JACK UP CHAPTER GENERAL Fig 1.7 The floating jacking system -horizontal forces at upper and lower guide Typical examples of JU with the floating jacking system are JU's such as the Trident 6, Trident Modec 300 and 400 design Not al floating jacking system have the rubber shock absorbers installed For example on the MLT-116C design (like Trident 2, Trident 4,Ron Tappmeyer, D.R.Steward) where even though the jacking system is welded fixed to the hull, the structural behavior is of a floating system Because in this design the stiffness of the pinion is small in comparison to that of the jack house and guiding structure As a result the moment is mainly resisted by a couple of horizontal forces acting at the upper and lower guides 1.7 THE FIXED JACKING SYSTEM (Fig.1.8) In this system the stiffness of the jacking unit is high Therefore the leg bending moment is mainly resisted by a couple of vertical forces acting at the pinions Strong and heavy build pinions support the vertical reactions As the leg bending moment does not create a large shear force in the leg between the guides, the leg section can be lighter than with the fixed jacking system To show the difference; the leg sections of the Trident IX (fixed system) is 5.7t/m compared to the 7.1t/m on the Trident II (floating system) The rack-chock system is another example of the fixed system As show in Fig 1.9 the rack- shock is the a mechanical device when engaged fixes the rack to the JU hull 10 Training to be FIRST OFFSHORE OPERATIONS COURSE SECTION THE JACK UP CHAPTER THE JU ELEVATED PART Fig 5.18 Deep-seated bearing capacity failure for mat 5.4 STABILITY OF STRUCTURE So far the rig safety discussion covered: • STABILITY AGAINS OVERTURNING • STABILITY OF FOUNDATION In this section, the STABILITY OF THE STUCTURE will be discussed 5.4.1 STRENGTH VERIFICATION The rig structure resists the combined action of gravity and environmental loads as a flexible body The unit endures the loads a stresses The objective of strength verification is to confirm that the stresses caused by the combined loads are within the allowable limits If not, it may be possible to reinforce the structure However if the OTM and foundation calculations are well within the Design Criteria it is not necessary to perform the strength calculation If in doubt the strength calculation will be done 5.4.2 FORCES ON A JACK UP The jack-up structure is mainly composed of a hull and or more legs connecting to the hull at the jack house The rig is modeled as a space frame with beam elements as shown in Fig 5.19 At the connection between the jack house and the legs, the lower pinions take most of the load The torque needs adjustment to equalize the torque between the pinions 85 Training to be FIRST OFFSHORE OPERATIONS COURSE SECTION THE JACK UP CHAPTER THE JU ELEVATED PART EQUIVALENT DIAMETER, THICKNESS, AND HYDRIODANAMIC S DISTRIBUTION FACTOR BETWEEN LEVELS OF JACKING UNIT Y AXIS PINION LOAD X AXIS LEG TO HULL CONNECTION Fig 5.19 Space frame analysis model 86 Training to be FIRST OFFSHORE OPERATIONS COURSE SECTION THE JACK UP CHAPTER THE JU ELEVATED PART The hull model includes beam elements to represent the various bulkheads The models of the legs are tubulars with equivalent diameter, thickness, and hydrodynamic coefficients so that the leg stiffness and generated wave loads will be equal to those of the actual legs A computer analyses and computes the forces Fig 5.20 shows a typical picture of such complicated computer analysis Transverse direction storm forces Oblique direction storm forces Longitudinal direction storm forces Fig 5.20 Picture of computer analysis The analysis computes the storm from (Fig 5.20): • A longitudinal direction • A transverse direction • An oblique direction In most cases, the oblique direction is the most critical one 87 Training to be FIRST OFFSHORE OPERATIONS COURSE SECTION THE JACK UP CHAPTER THE JU ELEVATED PART In the jack house structure, the key part is the leg-hull connection and the jack house with three main leg forces: • The vertical or axial force in the leg • The horizontal or shear force in the leg • The bending moment in the leg These forces, generated upon the environment criteria and the gravity loads, are transferred to the hull through the jack house and the jacking units In addition, the significance of these forces also depends upon the degree of rotational restraint 5.4.3 THE JACK HOUSE LEG-HULL CONNECTION The leg axial force, shear force, and bending moment transfer to the hull through many elements: chords, horizontal and diagonal tubular members jacking units, upper and lower guides, jack house structure It is important to understand the load paths inside this "connection" for many reasons: (Fig 5.21) • Load sharing between the jack house and jacking units to take the leg moment • Load redistribution when one pinion breaks • Reserve strength of rig when there is some local damage of the rig structure The axial force in the leg due to gravity and environmental loads transfers to the hull by the pinions of jacking units The shear force transfers to the hull through the lower guides The leg bending moment transfers to the hull: • Mainly through the pinions of jacking units of a fixed jacking system The leg moment reacts by a couple formed with vertical forces acting on the pinions For example, in the case of the TRIDENT the pinions take 94% of the moment The upper and lower guides take the remaining 6% • Through the upper and lower guides of a floating jacking system The leg moment reacts by a couple formed with horizontal forces acting on the guides For example, in the case of the TRIDENT 12, the pinions take 39 % of the moment The upper and lower guides take the remaining 61 % 88 Training to be FIRST OFFSHORE OPERATIONS COURSE SECTION THE JACK UP CHAPTER THE JU ELEVATED PART Load sharing of pinions in fixed system Reserve strength in case of some local structure damage Load sharing between jack house and jacking units 28% 31% 41% Load sharing if one of the pinion breaks Fig 5.21 Load sharing and reserve strength positions fixed system 89 Training to be FIRST OFFSHORE OPERATIONS COURSE SECTION THE JACK UP CHAPTER THE JU ELEVATED PART In the fixed jacking system the load sharing forces between the pinions at different levels of jacking units is not uniform (Fig 5.21 and Fig, 5.22) For example a three and four level jacking system the load sharing is : Four level system Three level system Percentage of load sharing Percentage of load sharing Upper pinion 28% Upper pinion 19% Middle pinion 31% Upper intermediate pinion 20% Lower pinion 41% Lower intermediate pinion 26% Lower pinion 35% Fig 5.22 Example load sharing between pinions three level system The way the floating jacking system reacts to the leg bending moment as discussed above creates a shear forces between the guides This shear force is much larger than the one due to environmental loads Therefore, the members of the truss leg between the upper and lower guides are highly stressed In the strength analysis, checks these members on buckling As shown in Fig 5.23, when vertical force transferred from the rack to the pinion, the inclined surface of the rack tooth creates a force component in the horizontal direction When the chord of a leg is a tubular, the pinions are located on both sides of the chord; therefore, the horizontal forces from the rack teeth on one side of the tubular cancel the ones from the other side This is not true for mono-rack chord such as the one in the MLT 116C design With only one rack per chord, the horizontal component will create a local bending of the chord and compression in the horizontal members Fig 5.23 Mono-rack horizontal force versus dual pinion 90 Training to be FIRST OFFSHORE OPERATIONS COURSE SECTION THE JACK UP CHAPTER THE JU ELEVATED PART 5.4.4 SPUD CAN FIXITY EFFECT The structure analysis assumes that the bottom of the legs are pin supported in other words, the spud cans are free to rotate Consequently, the soil underneath does not provide any restraint moment This assumption is true for hard bottom and reduced contact area However, in most cases, the legs have sufficient penetration and the spud-cans are fully in contact with the soil The soil will provide some restraint to the rotation of the spud-cans and gives some fixity to the bottom supports Spud can fixity works as if a vertical spring is installed under the spud can The bending moment at the top and bottom is different for each graduation of fixity as show in Fig 5.24 STORM PINNED SPRING CONSTANT Fig 5.24 Bending moment at top and bottom of leg with different fixity The spud can fixity is very important, because it directly affects: • STRENGTH: Spud can fixity creates a load sharing between the lower and the upper part of the legs When the leg is pin-supported, the upper part of the leg has to resist the full leg bending moment As shown in Fig 5.24., with spud can fixity, the lower part of the leg takes a portion of the bending moment; thus, the moment in the upper part will be less Therefore, the strength of the unit is increased 91 Training to be FIRST OFFSHORE OPERATIONS COURSE SECTION THE JACK UP CHAPTER THE JU ELEVATED PART • STABILIZING MOMENT: The overall stiffness of the jack-up is increased Thus, the lateral displacement due to environmental forces is less thus, the stabilizing moment will increase • DYNAMICS: The stiffness of the jack up increases, its natural period reduces This will lessen the dynamic effect • LOAD DISTRIBUTION: On the spud cans, the connection of the leg chord into the spud can is more loaded by the moment due to fixity In the rig strength analysis the spud can fixity is modeled as a spring element The rotation spring stiffness (rocking stiffness) depends on the spud can geometry and soil characteristics Expressed in a formula: Kθ = K ER Kθ = Rocking stiffnes K and E are functions of soil characteristics R = footing radius The formula shows that the rocking stiffness is proportional to the third power of spud can radius A large spud can provides for a considerable restraint As an example, the calculated leg moments for a rig for bottom support pinned are: Mbot= 0t-m Mtop= 38,900t-m For the same rig with sufficient penetration: Mbot= 19,400t-m Mtop= 23,100t-m The example shows a 42% reduction of the moment at the top In other words, without spud can fixity, the upper part of the leg will be overstressed 92 Training to be FIRST OFFSHORE OPERATIONS COURSE SECTION THE JACK UP CHAPTER THE JU ELEVATED PART 5.4.5 THE TILTING EFFECT As discussed, inclination caused by a sudden foundation penetration (punch through) of the unit generates additional overturning moment and leg forces This inclination will cause a horizontal component force of the total gravity loads Combined with the loads due to the environments and gravity, this inclination force may cause excessive deformation and even a fracture of the leg L=116.4 m D=37.68m 1m Fig 5.25 Effect on strength caused by inclination As an example, consider the TRIDENT with meter penetration at the aft leg.(Fig 5.25) If the inclination =1.51 ° Total hull load = 6849t Total leg length = 125m, and leg weight = 3030t Calculate the additional moment Horizontal component F.hul1 = 6849 x sin (1.51) =181t applied at 116.43 meter from the spud can Horizontal component F.leg = 3030 x sin (1.51) = 80 tons applied at half length; i.e 62.5 meters to the spudcan The additional overturning moment due to the tilt is : Mo =181 x 116.43 + 80 x 62.5 = 26,062 t - m 93 Training to be FIRST OFFSHORE OPERATIONS COURSE SECTION THE JACK UP CHAPTER THE JU ELEVATED PART 5.4.6 SPUD CAN ECCENTRICITY EFFECT As discussed a shallow spud can penetration develops a risk of scouring with the result of a non-uniform bearing area underneath the spud can The eccentricity between the vertical load of the leg and the reaction acting at the centroid of the bearing area creates an additional leg moment Storm 2.31m 2.31m eccentricity Fig, 5.26 Relative position of eccentricity The adverse stresses may exceed the strength of the unit For example if the eccentricity on the TRIDENT on one spud equals one third of the spud can radius the additional bending moment is: 94 Training to be FIRST OFFSHORE OPERATIONS COURSE SECTION THE JACK UP CHAPTER THE JU ELEVATED PART 6.93m = 2.31 The max give leg reaction is 4,800t The additional bending moment by eccentricity m = 4,800t ∗ 2.32m = 11,088t - m Radius spud can = 6.93m Eccentricity = The maximum storm leg bending moment = 31,210t - m The total moment caused by storm and eccentricity is : M = 31,210t - m + 11,088t - m In this case this exceeds the allowable leg strength Conclusion this eccentricty is unacceptable The example shows that eccentricity can be dangerous for the structure and should be avoided 95 Training to be FIRST OFFSHORE OPERATIONS COURSE SECTION THE JACK UP CHAPTER THE JU ELEVATED PART 5.4.7 PULLING LEGS Pulling out of jack up is a delicate operation To prevent damage, strict operating procedures are required The rig personnel are under pressure utilize every minute to start the move Chartered tugboats are waiting ready to tow the rig to its new location As a guideline, the pulling operation is restricted to weather limits with maximum 1.5m wave height 5.4.8 INDEPENDENT LEG JACK UP PULLING In soft bottom, the legs may penetrate deep into the soil Penetration of 140 ft have been recorded The pull out resistance comes from: • The overburden soil pressure on top of the spud can • The side friction around the contour of the spud can • The bottom suction of the spud can Jetting reduces the resistance of the bottom suction The streamlined shape of the spudcan top reduces the pull out resistance Fig 5.27 shows two examples of spud can configurations Fig 5.27 Spud can shapes Great care is required when pulling legs to prevent deformation on the structure In general, the Designers Operations Manual does not give detailed information on pulling procedures Much depends on the experience of the responsible person 96 Training to be FIRST OFFSHORE OPERATIONS COURSE SECTION THE JACK UP CHAPTER THE JU ELEVATED PART Some guidelines for pulling legs are: • Apply an overdraft to the hull to pull out the legs Never exceed an overdraft of foot • In the event it is necessary to use the jetting system, the following precautions should be taken., • When the first leg is free, raise it only about ft • When the second leg is free, raise if about 5ft • When the third leg is free, raised all legs to towing position Pulling legs out of the soil can produce significant stresses on structural members Fig 5.28 shows a jack up with leg penetrated deeply into the soil The legs are in compression supporting the platform weight When the unit’s hull starts to enter into the water, the buoyancy force increases with draft At the floating draft, the buoyancy supports the platform weight With an overdraft, the excess buoyancy provides an upward force on the legs (Fig 5.28) This force will break the spud can loose from the soil Total of suction plus weight of rig Overdraft + Buoyancy rig Fig.5.28 Rig with 1ft overdraft If the three legs break loose at the same instant, then the maximum stress the legs will experience is that due to the overdraft However, legs generally loosen one at a time Consider the most unfavorable case where overdraft forces and the jacking units forced two legs out 97 Training to be FIRST OFFSHORE OPERATIONS COURSE SECTION THE JACK UP CHAPTER THE JU ELEVATED PART With the forward leg still stuck in the mud, the port-, and the starboard aft legs free (one after one) above the mud line, the rig to will tilt to the forward leg (Fig.5.29) The buoyancy force created by the overdraft produces a moment to the forward leg This amount of bending moment depends on: • The size of the jack up • The water depth • The amount of penetration • The stiffness of the leg • The amount of overdraft The combination of these factors can induce significant bending moment on the leg(s) with damaging result to the leg structure Another possible failure mode for the leg is the twisting action With two free moving legs above the mud line and the forward leg still stuck in the soil, any lateral force from wind, wave or by the tugs moves the rig sideways and thus twist the leg (Fig 5.29) It is very important to follow the procedure: • When the first leg is free, raise it to about 5ft • When the second leg is free, raise it to about 5ft • When the third leg is free, raise all legs to the towing position immediately Leg twisting Fig 5.29 Leg twisting 98 Training to be FIRST OFFSHORE OPERATIONS COURSE SECTION THE JACK UP CHAPTER THE JU ELEVATED PART 5.4.9 MAT SUPPORTED PULLING Even with the small penetration, sometimes mat supported JU’s experience pulling problems caused by the suction between the mat bottom plate and the soil As for independent leg JU’s, apply overdraft to create excess buoyancy force to overcome the suction The maximum overdraft is generally set at 1ft Once the suction breaks, the entire rigid mat and the three legs lift free from the soil In some cases, it takes more than three days to pull out a mat supported jack-up Although water pumped through suction breaking tubes installed at various locations in the mat may unset the mat, the best way to help pulling out mat is to "play the swell" Even though the weather limit is 1.5m wave, the presence of swell will provide a varying pulling force in addition to that by the overdraft This is very effective to break the suction force 5.5 DYNAMIC EFFECT IN DEEP WATER The discussion around the rig analysis until now disregarded the dynamic effect of the wave forces For deep water JU’s with a water depth capacity between 300ft and 450ft the dynamic the analysis includes the dynamic wave forces Without going into details a few basic principles explains the dynamic effect or also called dynamic amplification • Dynamic effect reduces the safety factor against overturning, increases leg loads that can cause foundation instability and causing higher stresses in the members thus reducing the strength of the unit • The dynamic characteristics of the jack up are governed by the first three modes of vibration They are: • The longitudinal displacement in which the jack up vibrates like a flagpole • Transverse displacement, which is similar to the one above but the vibration, is in a perpendicular axis • Torsional rotation, which is a twisting mode The amplification of the overturning moment is computed with the Dynamic Amplification Factor (DAF) The dynamic effect increases the total overturning moment and decreases the safety factor Of course for deep water operations, the result after application of the DAF must be within the design criteria as established by the authorities and or Classification Societies 99 Training to be FIRST ... FIRST OFFSHORE OPERATIONS COURSE SECTION THE JACK UP CHAPTER BASICS OF SOILS Fig.3.7 The remote wire line vane test (Scissometer) 48 Training to be FIRST OFFSHORE OPERATIONS COURSE SECTION THE JACK. .. during jack- up /jack- down operations e) Safe operations require strict procedures 1.3 TYPES OF JACK- UP'' s There are two types of jack- up'' s (Fig 1.1): 1) The independent leg type 2) The mat supported... OPERATIONS COURSE SECTION THE JACK UP CHAPTER GENERAL Jack -house and Jacking system Examples of jacking systems gear trains Hydraulic system Joke pin Fixed pin jacking Fig: 1.4 Pictures of jacking