SPE 145704 Ultra Deep Water Drilling Riser
Santosh Kumar Das, SPE, Rajiv Gandhi Institute of Petroleum Technology
Copyright 2011, Society of Petroleum Engineers This paper was
prepared for presentation at the SPE Asia Pacific Oil and Gas Conference and Exhibition held in Jakarta, Indonesia, 20–22 September 2011. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.
Abstract In recent years, numerous exploration activities of oil and gas industry have been conducted in ultra-deep water. Global offshore industry is building systems today for drilling in even deeper water, progressively using new technologies, and significantly extending existing technologies. The technology of ultra-deep water risers, which is the main tool in drilling oil, is more and more standard. The riser system is the communication system between the vessel and the subsea well head. Through this riser, down hole equipment is guided in to the well and mud is returned to the surface. The paper addresses the key design issues looking at sizing, design of key components, material selection, analysis methods and installation issues. Limitations of current technology and application are the driving source for innovations in this field. The riser is the key element for drilling in ultra-deep water. Its architecture for deep-water drilling depends on numerous different factors related to operational and environmental conditions. These include water depth, mud weight, auxiliary line diameters and working pressures, sea states and current profiles, and maximum rig offset. All of the above parameters have to be taken into account in the design of the various riser system components including the main tube, the auxiliary lines, the connectors, the distribution of buoyancy modules, and the tensioning system. Major concerns of drilling contractors are to run and retrieve the riser fast and to operate it safely in ultra-deep water. Thus, it leads to developed methodology and new technology for expanding the range of application of risers systems and make them well suited for ultra-deep drilling in very harsh operational and oceanographic environments. The main purpose of the methodology is to optimise the riser design to determine the working envelopes. This technical paper manly focuses on the global analysis of the drilling risers. INTRODUCTION: The most important and challenging aspect of ultra-deep water field development are the selection and design of riser systems for floating production platforms. Development of cost efficient and reliable riser system for large floating production platform in ultra-deep water is critical. The definition of a deep water drilling riser in 1995 was one that could operate safely in water depths between 3,000 and 5,000 ft. Risers were being extended beyond their initial design ratings by lowering the operating limits, combining risers of different strengths, or by simply taking higher risks. Between 1995 and 1997, the oil companies targeted drilling ultra-deep water wells in the Gulf of Mexico and West Africa at depths nearing 10,000 ft. When the industry moves to ultra-deep water, all the riser systems pose technical challenges such as prediction of hydrodynamic behaviour, the potential impact of risers in an array, fatigue durability, method of deployment and extremely high tensions for top tensioned risers in ultra-deep water. At that time, a conscious decision was made by more than one operator to build drilling vessels big enough and risers strong enough to meet these objectives. This paper describes a design and manufacturing methodology used to achieve those goals. This paper presents an overview of the design methodology and manufacturing of Drilling Risers that were developed for ultra-deep water drilling. Time constraints on delivery and dusting off the analysis tools created some special design and manufacturing challenges. Riser design and manufacturing industry standards had been idle for at least ten years. Bringing them forward without time for industry consensus was risky. To meet the challenges, each step of the development process was evaluated for optimum contribution from the design and material availability to manufacturing techniques and quality control.Drilling in ultra-deep water presents a significant challenge. With an increase in drilling operations in harsh environments, drilling riser requirements and limits have become more onerous due to uncertainties involved in response prediction and prolonged drilling programs. Ultra-deep water drilling riser engineering is complex. A high level of understanding is required for the response of the system to various conditions, and the design issues that govern the
system.The riser systems described here based on extensive work pertaining to ultra-deep water drilling and has carried out without any significant failures and responsible for most of the ultra-deep water wells drilled in the Gulf of Mexico, West Africa and Brazil.
STATEMENT OF THEORY AND DEFINITIONS:
A riser is a large diameter pipe that connects the subsea BOP stack to a floating surface rig to take mud returns to the surface. Without the riser, the mud would simply spill out of the top of the stack onto the seafloor. The riser might be loosely considered a temporary extension of the wellbore to the surface or Riser is a pipe that extends from the drilling platform down to the seafloor. Drilling mud and cutting from the borehole are returned to the surface through the riser. The top of the riser is attached to the drillship/rig, while its bottom is secured at the seafloor. A blowout preventer (BOP) placed at the seafloor between the wellhead and the riser provides protection against overpressure formations and sudden release of gas. The riser pipe diameter of up to 21 in. [53.3 cm] is large enough to allow the drill pipe, logging tools and multiple casing strings to pass through. A schematic of processes is shown in Fig. 1(a). And a schematic of typical deep water drilling riser is shown in Fig. 1(b). COMPONENT OF RISER SYSTEM: The riser is not a simple piece of equipment but rather a complex system of component parts. The manufacturers produce individual parts of the subsea riser system but only a few supplies the entire equipment package. Components must be strong enough to withstand high tension and bending moments and have high flexibility to resist fatigue, yet be light a practicable to minimise tensioning and floatation requirements. ? Slip Joint: Slip joint compensate the heave of the vessel and consists of an inner barrel that slides into the outer barrel. The inner barrel is connected to the vessel by some mechanism – ballast joint or through a gimbals that will allow the vessel to pitch and roll .without affecting the riser. Diverter: The mud flow line and diverter system is between the inner slip joint barrel and the rig floor. The outer slip joint barrel supports the entire riser. A diverter is located on the top of the telescopic joint that diverts the gasified mud either to the shale shaker or to the port. A diverter should be installed between the inner ship joint barrel and the vessel. A diverter is low point annular preventer that seals off the riser base. This diverter re-directs the flow of mud and cuttings that would otherwise be blown on the rig floor during a kick when the BOPs are not used. The diverted fluid flows over board. Riser String: The riser string for a floating exploratory vessel is usually made of 50 feet long joints, which can be stacked and stored on deck during transit from one drilling location to another. The ends of each joint have quick disconnecting coupling permanently attached to the joint. Telescopic Joint: The telescoping joint, which is at the upper end of the riser string, is usually designed for a maximum heave of between 15 and 30 feet. There are two basic type of telescopic joints used in the marine risers. The constant tension system (remote axial tensioning system) is most often used because maintenance is easier. This method uses a linkage system at the base of the drilling floor to maintain equal force on the several wire ropes attached to the outer barrel of the telescopic joint. An alternative design of the telescopic joint uses a direct axial tensioning method. This is a procedure where a seals and guide rings on the telescopic joint are designed to compensate the internal pressure so that the telescopic joint has dual function of allowing vessel heave and acting as a direct tensioning piston. Ball Joints: Ball joint or flexible joint allow limited angular motion of the riser. Ball jointsin each of the riser allow for rotation in any direction up to about 7 – 10 degrees. A few operators insist for the use of two balls joint rather than one ball joint due to more reliability but generally are not preferred because of the increased cost and time taking job. The usual arrangement of the floating drilling operations is gimbals under the drilling deck and one ball joint attached to the top of the subsea BOP stack, which sits on the well head. The well head attached to the base template that is set with the conductor pipe at the beginning of the operation. As drilling progresses, casing supported by casing hangers, in the well head is placed in the well bore. At intermediate depths, another casing string of smaller diameter is set inside the first casing string from other casing hangers in the well head. The depth and number of the various sizes of casing string depends upon geological conditions.
Upper Ball Joint: Sometimes in deep water drilling, an upper ball joint is used to decrease the stresses caused by riser motion acting on the transition of stiffeners that occurs when going from the riser to the ship joint. The main body of riser is comprised of riser joint with kill and choke lines integrally attached. Lower Ball Joint: It is located between the bottom riser joint and the BOPs. It allows for limited deflection of the riser and compensates for the surge and sway (offset) of the vessel. Riser Coupling Joint: Riser Joints are seam less pipe with mechanical connectors welded on the ends. Kill/ Choke lines are attached to the riser by extended flanges of the connectors 2 to 6 bolts are used for each connector. The first riser systems had the choke and kill lines scrapped to the riser pipe. Pipe handling problems with this system proved to be extremely time consuming. Most riser systems use integral choke and kill lines, which are permanently attached to opposite sides of the riser and have their own connectors. When the riser joints are stabbed and quick connected, the choke and kill lines are also stabbed and automatically connected at the same time. The BOP requirements are the decisive factor in determining the size and wall thickness of the riser pipe. The riser pipes being thick walled and heavy pipes, they require the tensioner force. In early drilling operations, a two stack 20 inch BOP was used that was changed to 13 ? inch BOP for making the rest of the bore hole. The first single stack were 16 ? and required a 17 ? inch under reamed hole for setting 13 3/8 inch casing. Because of the dissatisfaction with under reaming, a single 21 ? inch BOP was then used. The heavy weight of the system exceeds than 200m tons and also over stressed in rough weather therefore the North sea riser system is a single 18 ? inch stack which weigh half as much as the 21 ? inch system but does not require under reaming for the 17 ? inch casing hole. Choke and Kill Lines: The choke and kill lines run from the deck along there are various schemes such as looped pipes, to get the required flexibility in a jump line arrangement running from the bottom of the riser string (top of ball joint) around the ball joint to the BOP stack. The choke and kill lines control to prevent them from developing in to blow outs. When a kick is detected, the mud is pumped down through kill line at the BOP stack to restore pressure balance in the hole. When excess gases occur, the bag and dram type BOP are closed around the drill string. The gas is relieved at the choke manifold on the BOP stack by running up the choke, displacing more mud and travelling faster as the gas bubble or gas entrapped mud approaches the mean water line. Without the choke, the gas would push out the annulus mud between the drill string and the riser and control from the weighted mud would be lost. Force Tensioner System: Riser tension is maintained through tensioner that is connected by wire rope to tensioning ring that is set against a hub on the top of the outer slip joint barrel. The flexibility of wire rope minimizes the effect of the yaw motion that would otherwise be transferred to the riser. A constant force tensioner system is attached to the top of the fixed outer barrel of the telescopic joint to provide enough axial force to the riser string to prevent buckling. The outer barrel and the riser string have a lateral movement with vessel’s surge and sway but essentially no vertical movement with the vessel heave. The vessel and the inner barrel of the telescopic joint move together vertically with vessel heave. The optimum tension is a fluctuation of water depth and operating condition.
RISER DESIGN DESCRIPTIONS AND APPLICATIONS OF EQUIPMENT AND PROCESSES: It is to be verified that the each riser is capable of withstanding all loads that are reasonably anticipated over its specified design life. The risers are to be designed to meet all applicable design criteria with the following failure modes considered i.e. Burst, Leakage, Yielding, Local buckling, Global buckling, Fatigue, Wear and tear, Cross sectional out-of-roundness etc. Designs of ultra-deep water risers have largely been selected by functional specifications and on a single development basis. Design and manufacturing challenges: Even though a proven and reliable technology have been developed for ultra-deep water field and a well-established record of innovations and success has been achieved as production has move into deeper and deeper water depth, still many technical and commercial challenges lied ahead specially for harsh environment. So in return to that Manufacturers are developing high strength connector bolts according to requirement and giving special consideration to bolt and thread capacity design limits, careful consideration of riser pressure end load effects (PEL), Loop current and VIV effects on the riser, Hurricane abandonment strategies require emergency disconnect systems, riser hang-off and centralizing, Larger mud flow rate requirements because of higher pressure drops across choke/kill auxiliary lines, Gas migration into the riser and deep-water well control issues require gas handling equipment, Riser mass limitations during running and out-of phase heave acceleration
of vessel causing riser buckling after disconnect. As Riser buoyancy method is key component for extending riser for more deep water, Due to Lack of adequate riser buoyancy industry design standards and Strength limitations of shallow water connector designs, industry is more focused at delivering the solution and is ideally positioned to meet these challenges. TECHNOLOGY DEVELOPMENT: Current research and study are to be developing technology to a level whereby it can be applied with confidence. Currents studies are focuses on flexible flow lines in ultra-deep water, reduce riser weight, buoyancy method etc. for extending riser technology in ultra –deep water. Technologies needed for ultra-deep riser are described in Table 1.
ENABLING TECHNOLOGIES (Advanced technology for development): As modern technology is the key component for development of ultra-deep water field. Operators have gone far in this technology field by improving dynamic positioning of ultra-deep-water drilling vessels, improving in thruster electromechanical drives, availing shipyards to design and build tanker hulls as drillships. Also technology has developed for improvements in mooring winch systems for ultra-deep-water, availability of high tolerance wall seamed pipe, more efficient riser design analysis programs and FEA tools, improvements in buoyancy manufacturing quality control, improvements in fabrication of high strength bolts for riser connectors, direct acting riser tensioner technology, building 60-inch rotary table development, improvement in riser gas handling technology, Improvements in subsea control system designs etc. Alternatives looked at for extending riser designs included limiting the mud weight requirement, Running thicker-walled, higher strength (main tube) risers in the high stress regions, Investigations into lighter weight aluminium riser designs, Investigations into composite risers, upgrading existing coupling designs and rebuilding risers. RISER DESIGN FLOWCHART:
RISER SYSTEM DESIGN DESCRIPTION: The typical new generation deep water drilling riser systems consist of the following components from the wellhead up to the rig floor. Wellhead Connector Lower BOP (5 pipe/csg rams) Lower Marine Riser Package (LMRP) w/controls LMRP Connector Dual Annular’s (integrated into LMRP) Riser Flex joint Riser Booster Line Joint Standard Riser Joints Pup Joints (for proper telescopic joint space out) Intermediate Flex joint (drillships only) Termination joint (required w/gas handler) Gas Handler (optional) Telescopic joint Tensioner Support Ring (around telescopic joint) Flex joint (alternately included for riser hang-off) Riser Hang-off Joint (with flex joint installed) Diverter Riser Centralizer (drillship only for hang-off survival) The key critical components that make or break a deep water design are the riser flange design, buoyancy selection, and riserdisconnect safety features. If you can’t disconnect the riser before exposure to an unexpected storm or current condition, you may find yourself in a position of remaining connected to the customer’s wellhead or losing the riser system altogether. Determining the maximum operating limits of the riser is critical to recovering it to the pipe rack prior to the maximum storm event. Riser bolted flange designs were considered to be the only viable type design for the 10,000 ft. water depth application. It was determined that two primary vendors had existing designs in place with a reasonable confidence that their prior connector product lines could be extended beyond the perceived limit of about 5,000 ft. water depth. One development was actually being completed in 1997 for the ultra-deep water application and the other need further design enhancements. The current designs went up to the Type E design which was capable of an API-defined maximum static tension capacity of 2,000,000 lbs. Initial estimates of the 10,000 ft. riser system indicated that a 2,400 kips minimum was required. At that point an API Type F connector was included in the request for proposals. The analysis of a flanged, drilling riser joint should include all the possible loads that could be applied to the riser simultaneously. Those loads are simply: Actual Tension at the Support Ring, TSR Equivalent tensions from bending Equivalent tension from the contents riser Pressure end load effects Buoyancy effects By summing the effect of these loads at each joint along the length of the riser a maximum tension capacity of the connector is determined. Leaving out any one of these loads could be catastrophic in the design of ultra-deep water risers. Tmax at connector = TSR+Tequivalent from bending+Tequivalent from content riser+PEL+Buoyancy ……(0) In Fig.2 is a chart of those loads for a riser with specific buoyancy and specific mud weight, 14 ppg, in 10,000 ft. water depth.It can be seen that that the maximum tension occurs at about 6,000 feet below MSL. The connector and its bolting preload needs to be able to accommodate the stresses from this maximum load condition. The flange itself may need to only meet a lower load condition, say 2,400 kips, but the bolts need to accommodate the 3,100 kip equivalent tension Von Misses stresses for the pipe body are shown in Fig. 3. They are required to be below 2/3 x yield stress of the pipe strength (80 ksi). This design approach may seem simplistic, but the manufacturing of the high strength bolts is difficult and subject to extreme metallurgical scrutiny in the effects from chloride stress corrosion and hydrogen embrittlement at areas of concentrated stress (bolts and thread inserts). These effects are non-trivial in the design of a flanged riser connector. The primary factor governing riser stability and operability is the angle at the flex joint above the BOP and wellhead. High angles cause wear, called keyseating inside the flex joint or riser joints. By keeping the angle at a minimum during normal operations, keyseating can be minimized.
BASIC MECHSANISM OF DRILLING RISER: As riser is the key element for drilling in ultra-deep water. Its architecture for deep water drilling depends on numerous different factors related to operational and environmental conditions. These include water depth, mud weight, auxiliary line diameters and working pressures, sea states and current profiles, and maximum rig offset. All of the above parameters have to be taken into account in the design of the various riser system components including the main tube, the auxiliary lines, the connectors, the distribution of buoyancy modules, and the tensioning system. EFECTIVE TENSION AND TRUE WALL TENSION: Effective tension (Teffective) is fundamental to the calculation of riser behaviour, since it is the tension that governs the curvature and stability of individual riser elements as well as global riser. The difference between effective tension and the true wall tension must first be recalled. For a single pipe the relation between these two tensions can be expressed most clearly as follows: ........... (1) Teffective=Ttw+(?PiSi) ? (?PeSe) Where: (-PiSi) is the “axial tension” in the internal fluid column. (-PeSe) is the “axial tension” in the displaced fluid column. Hence effective tension is the sum of the tensions in the pipe wall, the internal fluid column and the tension in the displaced fluid column. For a riser composed of several tubes, such as a drilling riser, Equation (1) can be used to analyse the stability of individual tubes that make up the riser, such as the auxiliary lines. When analysing the global behaviour of the complete riser Equation (1) must be replaced by the following: T riser effective=∑Ttw +∑(?PiSi)-∑(?PeSe) ..........(2)
The effective tension at any point (z) of the riser can be most easily obtained by considering the top tension (Ttop) and the apparent weight (W) of the intervening section of riser with its contents according to the following equation:
T riser effective(z) =TTop+∑
From Equations (1) and (2) it follows that the effective tension(T riser effective) in the complete riser is the sum of the effective tensions in all the individual tubes from which the riser is made up:
Triser effective = TMP effective + ∑TAL effective…………… (4)
This equation is the key to deriving the tensions in the different tubes (main pipe and auxiliary lines) that make up the riser. Once the effective tension has been found in each tube comprising the riser, then Equation (1) allows the corresponding true wall tension and axial stresses to be calculated in each tube. The lateral static displacement of (y) the riser at height (z) can be derived from the following equation, where E is Young’s modulus of the material, I the moment of inertia of the riser and q (z) the lateral loads induced by the current.
? ? ? ?
For practical riser cases the effective tension must be positive at all points in order to avoid buckling instability. Top Tension: It is important to have an accurate evaluation of riser top tension since it influences the definition of the tensioner system. Top tension (TTop) is the sum of three distinct components as given by the following equation: TTop= + Tbottom…………...... (6)
The three terms of Equation (1) can be calculated separately according to the specifications of the riser components. For example: Wriser= WMP + WAL + Wmud+ ΔBM……………… (7) The required minimum value of the bottom end effective tension Tbottomat the lower flex-joint is determined by consideration of the mean angle at the flex joint which has to be maintained within the limit of 2° specified by API. The minimum tension must also exceed the apparent weight of the LMRP (low marine riser package) in order to assure lift-off in case of an emergency disconnect. In most of the cases, this tension is of the order of 200 kips.
Tension in Auxiliary Lines under Pressure: The auxiliary lines are relatively small-diameter tubes which are fixed to the connector at one extremity of each riser joint and are free to slide at the other end (stab-in connections). Therefore assuming the sealing diameter is equal to the external diameter of the tube, the true wall “tension” in the line is: …………….. (8) Ttw(AL)= -Pi(Se-Si) Hence from Equation (1): TAL effective = -(Pi-Pe)SAL effective ……………. (9) When under pressure, the tubes are in effective compression. Intermediate collars are therefore needed to prevent buckling.
Influence of Auxiliary Line Pressures on the Main Tube Tension: From Equation (3) it can be seen that internal pressure in one or more of the tubes that make up the riser has no influence on the global riser effective tension. However it does influence the distribution of tension (effective and true wall) between the different tubes that make up the riser. As has been seen above, an increase in the internal pressure in an auxiliary line causes the effective tension (TAL effective) in that line to decrease (to a negative value). But as the global effective tension in the riser remains unchanged (Eq. (3)) it follows from Equation (4) that the effective tension in the main pipe must increase. Hence from Equation (1) the true wall tension of the main pipe also increases. If the pressure in the main tube is unchanged, then: (Ttw MP) +δ(TAL effective)= 0 ………. (10)
For real risers, pressure in the auxiliary lines can cause significant additional axial loads in the main pipe of the order of 500 kips (225 t), per line for 15 000 psi (103.4 MPa) working pressure and 4 1/2" (114.3 mm) diameter sealing. These additional loads must be taken into account when designing the riser. Tensile Loads in the Connectors: The effective tension in the riser connectors is given as before by Equation (3). When deducing the true axial force in the connector lugs, dogs, flange bolts or other connecting elements, it is the “seal” sectional area (Sseal) within the connector that must be used in the effective tension equation. Hence that equation becomes: TTrue connector =TMP effective+ (Pi-Pe) * Sseal………… (11) TMP effectiveis normally a maximum at the riser top end. However the effective tension does not generally vary linearly with depth because of two factors. Firstly the main tube wall section is not of constant thickness. It tends to be thinner in the midheight region than at the upper end, where tension is greatest, and the lower end where bursting stresses are greatest. Secondly buoyancy units tend to be concentrated in the upper section of a riser. ThusTtrue connectortends to have its greatest value at an intermediate point along the riser. Drilling-Riser Design Procedure: Design Criteria: The riser has to be designed according to API requirements and in particular maximum von Misses stresses must be limited to less than 2/3 of yield and mean angle at the riser foot has to be less than 2°. No other quantified specification is listed in this recommendation concerning the riser design. In particular, no recommendations are given concerning corrosion, fatigue, and pressure in the auxiliary lines. IFP has proposed a practical methodology for designing drilling risers to meet particular specifications considering riser behaviour in the connected (drilling mode) and disconnected (hung off mode). Connected Drilling Mode: This is the operating mode for which von Misses stresses must be kept below 2/3 of yield. The following situation should be considered: Riser connected to the floating vessel through the slip joint and the tensioner system Riser full of mud with the maximum density All the auxiliary lines under maximum pressure simultaneously The wall thickness of the main pipe 5% less than nominal (due to tolerances) 1/16" (1.588 mm) decrease of the wall thickness due to corrosion 3% buoyancy loss of the flotation modules due to water absorption.
Disconnected Hung off Mode: In this situation the fluctuating axial tension should remain positive when the vessel heaves in order to avoid any slackening or dynamic buckling of the riser. To meet these criteria, the apparent weight of the hanging riser must be greater than the maximum amplitude of variation of the tension in any point. In this mode, the calculation should take into account the following: Riser disconnected from the wellhead with the LMRP suspended at the riser lower end The riser and the auxiliary lines full of sea water The auxiliary lines non-pressurised (hydrostatic pressure) The wall thickness of the main pipe 5% less than nominal Loss of 1/16" (1.588 mm) of the wall thickness due to corrosion 3% buoyancy loss of the flotation modules due to water absorption. During the design phase, iterative calculations are made between the two modes to optimise the architecture. The wall thickness of each riser section is modified to meet the criteria in the connected mode while the buoyancy ratio of the riser is adjusted to ensure that no dynamic compression occurs in the disconnected mode. RISER BUOYANCY: Riser buoyancy standards were not given serious consideration by API until the early 1990’s. Recurring problems associated with loss of buoyancy due to water ingress and limited outer shell strength resulted in the need for a continuous buoyancy module repair and replacement programs. Standards began to creep into the API Committee 16 drilling riser and design standards, but materially there were no significant changes put forward by the manufacturers until it was apparent that a significant market for buoyancy was about to take off with the deep water impetus. Up to this time, buoyancy module lengths were limited by the size of the manufacturers’ ovens. Depth ratings for standard syntactic glass foam composite materials were not fully developed for drilling riser buoyancy beyond about 5,000 ft. The proposal implemented improvements in macro-sphere dimensional tolerances, syntactic material batching processes and quality control. Critical design issues included bending strength of the longer modules during handling of the 90 foot long joints. Joints were positioned on the riser joint in an arrangement that would minimize the bending stresses on any individual module. An FEA analysis performed confirming the strength along the principal axis was sufficient, with utilization factors of only about 50% based a 2 ft. deflection at the riser midpoint. The out-of-plane axis is somewhat weaker. Handling the joints was specified by a field procedure with specific orientation during pickup. Overstressing modules was solved by adding a 12” flat section to the normally round buoyancy module and riser lifting pins designed for insertion in the flange bolt holes. A secondary advantage was that the flat allowed more compact stacking of riser joints and improved safety during handling by preventing the joint from rolling during pickup. A larger weather window of for running and retrieving operations can be achieved where the buoyancy requirements are better understood. Using smaller diameter modules that provide smaller incremental changes in total lift distributed along the total length of the riser may be desirable. Buoyancy Ratio: The buoyancy ratio is defined as follows:
Buoyancy ratio =
The buoyancy ratio is important since it allows the buoyancy module diameter to be fixed. It must be as high as possible so that the top tension can be reduced. However, since the riser must have positive apparent weight in the disconnected hang off mode (to avoid dynamic compression), it is always less than 100%. Influence of Main Operational and Environmental Parameters: In the final stages of the design, sensitivity to operational or environmental parameters has to be examined. Rather than these accidental parameter needs to be considered for safety margin. A list of parameter is described in Table 2. Their influence on the riser behaviour and on the top tension, which is the most important factor with respect to the tensioner capacity, has to be checked. The main parameters, which act directly on the riser, are dealt with below Operational Parameters: Mud Density: According to Equation (1), the top tension depends directly on the apparent weight of the mud (as the bottom tension is generally about 200 kips (~ 100 t), and the apparent weight of the riser may exceed 600 kips (275 t) in very severe conditions to meet the disconnected mode specifications). Risers must be designed with the maximum mud density that may be encountered. The range is from 14 ppg (1.6) to 17 ppg (2.04) in the GOM (Gulf of Mexico) conditions.
For a typical riser in 10 000 ft. (3048 m) water depth, an increase of 1 ppg (0.12) of the mud density induces an increase in wall thickness of the main pipe of 1/16" (1.588 mm), an increase in buoyancy module diameter of 1/2" (12.7 mm), an increase in top tension of 175 000 lbs. (80 t). Similarly a water depth increase of 1000 ft. (305 m) requires 150 000 lbs. (70 t) of additional top tension because of the increase in mud volume considering 17 ppg (2.04) mud density. Hence it may be possible to upgrade the water depth of the riser by reducing the mud density range. For example, the water depth of a riser may be increased more by 1000 ft. (305 m) just by reducing the maximum mud density by 1 ppg (0.12). It should be noted that the maximum von Misses stresses must also be checked, as well as the no dynamic compression criterion, in the disconnected mode. Pressure in the Choke and Kill Lines: The service pressure of choke and kill lines influences riser architecture. The difference between 4"ID (101.6 mm) * 10 000 psi (69 MPa) and 4 1/2"ID (114.3 mm) * 15 000 psi (103.4 MPa) working pressure is significant. For example, in GOM configuration, 10 000 ft. (3050 m) water depth and 15 ppg (1.8) mud density the wall thickness of the main pipe has to be increased by 2/16" (3.175 mm), the riser mass is increased by 1 800 000 lbs. (800 t), the buoyancy module diameter is increased by 6" (152 mm), the top tension is increased by 200 000 lbs. (90 t). Clearly it is not possible to upgrade choke and kill lines from 3" (76.2 mm to 4 1/2"(114.3 mm) ID without making considerable changes to the riser architecture. Disconnected Mode: The required buoyancy ratio of the riser, to avoid inducing dynamic compression at the riser top end, depends on which hang off mode is chosen (hard or soft hang off). With the “hard hang off”, the riser is supported directly on the dogs of the spider (with the telescopic joint removed). The heave motion of the vessel is transmitted directly to the riser and may induce dynamic tensions at the riser top end sufficient to put the riser into effective compression. This situation is unacceptable. This leads to required buoyancy ratio of about 80-85% for 10 000 ft. (3048 m) water depth. Its precise value depends on the response amplitude operator (RAO) of the vessel which varies with the vessel type (dynamically positioned drillship, semi-sub). In the “soft hang off”, the riser is suspended from the tensioners and the heave motion transmitted to the riser is greatly reduced. Thus the buoyancy ratio can be increased significantly (up to ~ 95%) allowing a corresponding reduction in top tension, but leading to an increase in buoyancy module diameter. Hence the architecture of a riser may be significantly different in terms of top tension and buoyancy module diameter, depending on which hang off mode is adopted. However, the “soft hang off” is not studied in this paper since its reliability in extreme conditions has yet to be proved. Buoyancy Module: Buoyancy modules, which may be of lightweight materials or steel tanks, are to be rated to a maximum allowable water depth and are to withstand normal handling, transportation, installation and environmental loads, and at the same time be reliable and easy to operate. The module size is to be determined based on the lift requirements together with considerations to handling and installation requirements. The buoyancy material is to provide the required buoyant lift over the intended service life, accounting for time-dependent degradation of buoyancy. As a minimum, the following parameters are to be considered in the selection of buoyancy coating i.e. environmental conditions, service life, density of the buoyancy, dry weight (mass) in air, submerged weight (mass) in the water, lift force in water and loads acting through all operating phases Influence of the LMRP Apparent Weight: Increasing the apparent weight of the LMRP allows the buoyancy ratio of the riser to be slightly increased and thus the top tension to be slightly decreased. The LMRP apparent weight plays a significant role in the disconnected mode since it reduces the risk of dynamic compression in the hung off riser. See the example (Table 3) for 10,000 ft. (3050 m) water depth, 15 000 psi (103.4 MPa) working pressure and GOM conditions.
Apparent weight of LMRP (lbs.) (t) Buoyancy ratio Top tension (lbs.) (t) 140000 65 80% 2600000 1170 280000 130 83% 250000 1130
Environmental Parameters: Wind: Wind forces are exerted upon parts of risers that are above the water surface and marine structures to which risers might be attached. Statistical wind data is normally to include information on the frequency of occurrence, duration and direction of various wind speeds. For design cases where the riser is attached to a floating installation, it might also be necessary to establish the spectrum of wind speed fluctuation for comparison with the structure’s natural sway periods. Vertical profiles of
horizontal wind are to be determined based on recognized statistical or mathematical models. For wind normal to the riser axis, the following relationship may be used to calculate the wind load: Fw= ρa * Cs * Vy2 *A where Fw= wind load ρa= density of air Cs = shape coefficient (dimensionless, = 0.50 for cylindrical sections) Vy= wind speed at altitude y A = projected area of pipe on a plane normal to the direction of the considered force As an alternative to applying wind loads, the effect of wind can be indirectly accounted for through the modelling of floating installation offset and slow drift movement. Current: Current may be a major contributor to both static and dynamic loading on risers installed at any depth. The current velocity and direction profile at a given location may have several contributions of which the most common are oceanic scale circulation patterns, lunar/astronomical tides, wind and pressure differential generated storm surge and river outflow The vector sum of all current components at specified elevations from the seafloor to the water surface describes the current velocity and direction profile for the given location. The current profile might be seasonally dependent, in which case, this is to be accounted for in the design. For riser design, the total current profile associated with the sea state producing extreme waves is to be used in design analyses. The current velocity and direction normally do not change rapidly with time and may be treated as time invariant for each sea state. The current profiles and intensity (associated with the rig offset) determine the riser top and bottom end angles. According to API RP 16Q , in the drilling connected mode these angles have to be less of 2° in static conditions (no waves) and 4° in dynamic. However for deep-water risers with high top tensions, the dynamic riser foot angle variation is extremely small (typically less than 0.2°). Hence a static analysis is generally sufficient to assess the riser top and bottom end angles. Waves: Wave action influences riser design by contributing to the hydrodynamic loads acting on the riser and affecting the riser top end motions through the rig RAO’s. Waves are a major source of dynamic loads acting on risers and their description is therefore of high importance. Statistical site-specific wave data, from which design parameters are to be determined, are normally to include the frequency of occurrence for various wave height groups and associated wave periods and directions. For areas where prior knowledge of oceanographic conditions is insufficient, the development of wave dependent design parameters is to be performed in cooperation with experienced specialists in the fields of meteorology, oceanography and hydrodynamics. Rig motions are more significant for deep-water riser design. They have a major influence on riser behaviour, particularly in the disconnected mode. LESSONS LEARNED AND DEEPWATER RISER DESIGN GUIDEANCE NOTES: The requirement for a new generation of deep-water drilling risers pushed the limits of the available technology that was stagnant for more than a decade. Accurate prediction of riser performance and tension requirements is necessary to prevent over stressing or under tensioning the riser. Deep-water buoyancy design, testing and fabrication methods need industry standardization. Mud density, mud volume, wet weight of the riser, and pressure end loads are primary drivers for riser connector design ratings Riser design and manufacturing standards need to be more prescriptive. Riser design methods used on shallower risers can be applied to the new generation of deep-water risers, but more stringent standards for fatigue and welding procedures need to be developed. Handling high-pressures from gas intrusion in the riser is feasible, but the impact on the riser design, rig payloads, increased cost and operations is significant. For 10,000 ft. water depth ratings, API 16Q, Type ‘F’ connectors (2,500 kip rating) are limited about a 15 ppg mud density. Type ‘G” (3,500 kips) connectors have a higher mud density capability. For adding riser tension in deep-water, it’s cheaper, pound-for-pound to add buoyancy than additional hydropneumatic tensioning equipment. Length limitation of buoyancy modules is controlled by allowable bending stress, not manufacturer’s oven length. Additional module length can reduce overall weight of long riser joints. Syntactic foam is actually fairly brittle stuff, and controlling stress concentration factors around clamp access holes and riser auxiliary line cut outs is paramount.
Longer riser strings are typically less buoyant than shorter strings, because the concentrated weight at the bottom (LMRP or LMRP+BOP stack) is a smaller percentage of the wet weight to effective mass ratio. This ratio, expressed in terms of ‘g’ force, is a primary consideration for disconnecting the riser in severe weather and compressive heave buckling avoidance. Removing buoyancy at the bottom to enhance the ‘g’ force acceleration ratio has a threshold which is limited by hook load capacity of the rig. The more ‘g’ force safety factor, the more weight added to the bottom of the riser. As a result, deep-water risers require significantly more hook load capacity to be disconnected safely. DESIGN CONCLUSION: Ultra-deep water presents challenging conditions for riser system design and increases proportion of floating production system development costs. Current analysis and design practices need to be rationalized to reduce conservatism and costs. As exploration moves in to deeper and deeper water greater effort is needed to optimize overall riser system configuration to provide satisfactory levels of serviceability and safety. As already discussed, Riser is the key element for drilling in ultra-deep water. Its architecture for deep-water drilling depends on numerous different factors related to operational and environmental conditions. Architectures in ultra-deep water are specific for particular conditions. Studies must be performed to determine whether a riser can be used in other environmental or operational conditions.Other studies should be carried out by considering hyper-static integration (hyper-static integration system leads to enlarged operating envelopes and improved fatigue life of the riser) of auxiliary lines and soft hang off in standby mode. Two factors can radically change the whole architecture of a riser: the integration of auxiliary lines (sliding or hyper static) and the disconnected hang off mode (hard or soft). Studies of these particular points must be carried out in the future to improve the design of risers in ultra-deep water. Studies should perform for advance drilling that is, for further more deep water drilling in recent future to fulfil the world’s need for oil and gas.
REFERENCES: 1. Guesnon J, Gaillard Ch. and F. Richard. 2002. Ultra Deep Riser Drilling riser and Relative Technology, Institutfrancaies du petrole, Oil & Gas Science Technology, Rev. IFP, Vol. 57. 2. Lim Frank and Eyles Tim. 2006, The Need for Standardising Deep water Riser Systems, OFFSHORE DRILLING & DEEPWATER/UNDERWATER TECHNOLOGIES, Kuala Lumpur, MALAYSIA,27-28 November 2006 3. Steve W. Bernard Robert H. Taylor, Thomas A.Fraser. 2004, New Generation Deep Water Risers: A Design methodology, Houston.DP MTS SYMPOSIUM,September 28-30 4. 2006,ABS: Guide for building and classing Subsea riser system, American Bureau of Shipping, ABS Plaza, 16855 Northchase Drive, Houston, TX 77060 USA 5. HariharanMadhu, Thethi Ricky, Drilling riser management in Deep-water environment: 2H offshore inc. Houston, Tx, USA, 6. Chastain T, Stone D. 1986, Deep Water Drilling Riser System, Paper SPE 13479 presented at SPE Drilling Engineering, August. 7. Dr Howells Hugh, 1998, Deep water Drilling riser technology VIV and Fatigue Management: 2H offshore engineering Limited.Presented at Drilling Engineering Association (Europe), 4th Quarter Meeting, Paris 8. Aird Peter. 2010.Deep water drilling riser issue, Kingdom drilling, www.kingdomdrilling.co.uk 9. Wang H. Howard, 2002, Challenging Issues in the Design and Analysis of Deep & Ultra-Deep Water Riser Systems, Deep Water Risers, Mooring & Anchoring Conference, March 18-19. 10. KoziczJohn, 2008, Offshore Drilling Technology Report, Transocean, Offshore magazine, April 11. Childers Mark, Oceanics Atwood,2004, Slim riser an alternate method for deepwater drilling, Drilling Contractor, anuary/ February.
Table 1 TYPE Catenary Steel Lazy Wave Flexible Riser Free Standing Bundled Hybrid Tethered Bundled Tensioned Leg Top Tensioned Steel Conventional Steel Tubing RISER CONFIGURATION Steel Catenary DESIGN CONSIDERATIONS Offset, VIV, Touch Down Response, Welding Offset, VIV, Touch Down Response, Welding Hydrodynamic Collapse Buoyancy Design, Jumper Design Buoyancy Design, Jumper Design Buoyancy Design, Jumper Design Vessel Motions, Top Tensions Vessel Motions, Top Tensions
Wind Waves Current Tides Surge Marine growth Sea ice Seabed subsidence Hydrothermal aging
Weight in air of: - Pipe - Coating - Anodes - Attachments Buoyancy Towing External hydrostatic pressure Internal pressures: - Mill pressure test - Installation - Storage, empty/water filled - In place pressure test - Operation Installation tension (pipes) Installation bending Top tension (risers) Makeup (connectors) Loads due to containment: - Weight - Pressure - Temperature - Fluid flow, surge and slug - Fluid absorption Inertia Pigging and run tools
Impacts from dropped objects Impacts from collision between risers Mooring or tendon failure Loss of floating installation station keeping capability Tensioner failure
Fig 1(a). Schematic of drilling riser
Fig. 1(b): A Typical Deep Water Drilling Riser
Fig. 2 Coupling Design Tension Required in 10,000 ft Water Depth-21” Riser
Fig. 3 Riser Stress profile in 10,000 ft.