Riser Design – API 16Q

1. Riser Components Selection Criteria:

1) Design of a riser system begins with an assessment of expected operating conditions: an engineering analysis to establish parameters such as tensile, bending, and combined stresses (maximum and mean), buoyancy requirements, top tension requirements, vessel RAOs (Response Amplitude Operators), etc.

2) Other factors influencing riser system design include riser length (water depth), dimensional requirements (bore, wall thickness, etc.), internal pressure rating, choke/kill, and auxiliary line specifications, makeup method, storage and handling conditions, operating economy, etc.

3) Once established, these riser system design criteria should permit the selection of riser components that suit the application.

2. Tensioner System:

1) A typical tensioner system is a hydraulic ram with large air-filled accumulator system. The accumulator system supplies the pressure and maintains the pressure.

2) Tensioner systems typically supply constant axial vertical forces all the time and are typically four-part line receiving system, which means that stroke of the cylinder, will be 1/4thof the total vessel heave.

3) Selection Criteria:

- Fleet Angle: Smaller the fleet angle, larger is the vertical component of the force applied by the tensioner system. Increase in fleet angle will lead to increase in the horizontal component of the applied force, which is not what is desired.

- Wireline Life

- Accumulator System: each tensioner system should be equipped with an accumulator unit, which should be large enough to ensure that it can supply the required hydraulic fluid to the piston/ram. Larger the air pressure vessel, lower will be the fluctuation in the pressure as the piston strokes in and out.

- Fluid and Airflow requirement

- Friction and Inertia losses: Seal friction, sheave friction, and inertia of sheaves, wire rope, tensioner rods, and pistons all contribute to variations in the wireline tension

- Dynamic Tension Limit: DTL= P x Acnl x NLPP , = maximumallowable system operating pressureAcn = effective hydraulic areaN,, = number of line parts

- All components in a riser system installation, including piping, should be designed for the maximum allow- able working pressure.

- Thetensioner system should be designed to permit one unit to be out of service for maintenance or repair without jeopardizing the ability of the remaining tensioner units to provide the required tension to the marine drilling riser. A unit may be either a single tensioner or a pair of tensioners, depending on specific design.

- Max. Tensioner rating: 90% of DTL, to ensure that maximum variations are always less than the DTL

- Velocity Limiting Device: sense the abnormally high fluid flowrate and immediately stop or greatly reduce the fluid flow into the tensioner. If a tensioner wireline should break or other failure occur which would allow the tensioner to stroke out at an uncontrolled rate.

3. Diverter System (Surface):

1) Placed just below the Rotary Table and is connected to the upper flex joint on bottom. It is latched into a built-in housing below the Rotary table

2) Opens up diverter lines and closes mud line simultaneously

4. Telescopic Joint:

1) To compensate for the translational movement between the vessel and the riser

2) Riser tensioner ring attached to the top of the telescopic joint outer barrel. Pinned joints at the pedeyes are used to connect the tensioner ring with the tension lines.

3) Selection Criteria:

- Strength- Stroke: the combined stroke must accommodate the combined heave, vessel

offset, tidal change and the maximum vessel excursion in the event of station-keeping failure

- Tensioner- Auxiliary lines- Packing elements: double packer elements are used for redundancy- Handling and storage: telescopic joint is usually longer and heavier than the

normal riser joints, so needs special handling equipment

5. Riser Joints:

1) The main tube is specified by the outer diameter, WT and the material properties2) Some typical riser joint designs:

13 5/8” (346.1 mm) BOP, 16” (406.4 mm) Riser16 3/4” (425.5 mm) BOP, 18 5/8” (473.1 mm) Riser18 3/4” (476.3 mm) BOP, 20” (508 mm) or 21” (533.4 mm) Riser20 3/4” (527.1 mm) BOP, 22” (558.8 mm) or 24” (609.6 mm) Riser21 l/4” (539.8 mm) BOP, 24” (609.6 mm) Riser

3) Riser couplings: i) Dog type ii) Threaded iii) Breech-block iv) Flanged

4) Generally used materials: X-52, X-65 and X-80

5) Outer shell of the riser has support brackets for kill and the choke lines and typically range from 50-75ft in length

6. LMRP (Lower Marine Riser Package):

1) The Lower Marine Riser Package typically includes an assemblage of a riser adapter, flex/ ball joint, one, two, or no annular BOPs, subsea control pods, and a hydraulic connector mating the riser system to the BOP stack. The LMRP provides a releasable interface between the riser and the BOP stack. In addition, it provides hydraulic control of BOP stack functions through the control pods. Jumper hoses provide a flow path around the flex/ball joint for the choke and kill lines.

Riser Design:

1) Look for Appendix-B in API 16Q for the data which may be required to carry out a full riser analysis

2) Some of the things which needs to be considered to carry out a Riser Design analysis:- Vessel station keeping considerations- Riser induced load considerations: the loads induced on the LMRP, BOP, Casing,

wellhead, auxiliary lines, mud boost lines etc. due to riser response- Currents: velocity and the amplitude of the currents- Top tension load requirements: tensile load required to ensure that the riser

does not buckle due to various loadings- Mud fluid density: Top tension requirement should be done for various fluid

densities, from seawater up to maximum anticipated density

3) Operating modes for a riser:- Connected, drilling: this covers all the regular drilling activities, which includes,

drilling ahead, circulation, tripping, casing running etc.

- Connected, Non-Drilling: This covers the activities when the riser is still connected to the BOPs etc., but you cant circulate, trip the pipe. This may be encountered due to deteriorated environmental conditions

- Disconnected mode: this one covers the most sever environmental conditions, under which the riser has to be dis-connected as it is no longer considered safe to drill, stay connected

4) Top Tension: calculate the top tension requirement using the following formula:The minimum top tension, Tti, is determined by:

Tlin = Ts,, N/[R, (N-n)]where,Ts, = Minimum Slip Ring Tension = WBf, - B,fk + A;[d,H, - dvHvl

- The top tension calculation should have enough redundancy to accommodate for any tension line failure

- Top tension requirement is a function of the type/density of the mud

5) Structural Modeling:- Geometric non-linearties must be considered in the analysis if the riser develops

an angle > 10 Deg

- A riser is modeled as a tensioned beam loaded throughout its length with boundary conditions at the end

- Element length in the analysis should be specified considering the loading conditions at that place. Higher variations in loading, smaller the joint/element length to get more accurate analysis

- The dimension of the auxiliary lines such as kill, choke and other supported lines in addition to the main riser tube body must be considered during the hydrodynamic analysis. The weight used in the analysis must be the total weight of the riser tub + all the auxiliary attachments such as choke, kill, support brackets etc.

- Boundary Conditions:

o Top Boundary: generally include top tension, vessel offsets and motions as well as the rotational stiffness of the upper ball/flex joint

o Bottom Boundary: bottom boundary condition may emanate from either connected or disconnected modes. In the connected mode the riser model usually ends at the lower flex/ball joint, in that case the rotational stiffness of that flex/ball joint is a bottom boundary condition and the vertical and the horizontal loads as well as the bottom angle are the output of the analysis. Some people prefer to keep the structural (conductor) casing as the lower boundary in which case the LMRP, BOP etc. must be taken into account and the lower flex/ball joint will be treated as an intermediate joint rather than lower boundary conditions

6) Hydrodynamic Modeling:- Hydrodynamic modeling is done to calculate the effect of hydrodynamic forces

on the drag etc.- Drag and Mass coefficients play a very key role in Hydrodynamic analysis along

with cross-section, Reynolds number, roughness, orientation of auxiliary lines etc. and must be chosen based on experience with great care. Refer to API-16Q for generic values

- Mostly the analysis is done in planar mode, which means that the forces, motions etc., are all considered to be acting in one direction whereas in reality it could be bipolar. If considered must, should be carried out.

7) FEA:- Either a Finite Element or Finite Difference analysis is done

- A local FEA may be considered to account for local details of the riser structure such as flanges, choke, kill, joints etc.

Operating Procedures:

1) The riser operating manual must contain normal running in, operating and emergency disconnect procedures

2) Running in: Calculate the riser length required, specific to the site, wellhead conditions etc.

3) Operating Conditions:

- During normal drilling operations flex/ball joint angle, vessel offset and mud density should be monitored

- If the mean flex/ball joint angle exceeds 3 Deg., and can not be corrected by adjusting riser tension and vessel offset, preparation should be made to suspend any operations and suspend any operations that involve any pipe movement in the well

- Hydraulic Tensioner Failure: if any of the hydraulic tensioner fails, then check of the remaining tensioners can meet the top tension requirement, if yes, then adjust the remaining tensioners to ensure that adequate tension is maintained all the time, else the drilling fluid may be circulated out of the riser to reduce the tension requirements. If the situation worsens then riser should be disconnected, hung-off or pulled

- Loss of buoyancy: loss of buoyancy can be detected by monitoring the lower flex/ball joint angle, tension in an instrumented riser joint just above the lower flex/ball joint or by TV inspection for damage or air leaks

- By adjusting the tension and vessel offset and attempt should be made to keep a small flex/ball joint angle to facilitate the riser disconnect and lower the tension to slightly above the hanging weight of riser and LMRP

- After disconnect the vessel should be moved off the location and the guidelines must be slackened to prevent heaving riser and LMRP striking the BOP stack

- An emergency disconnect is sometimes necessary in case of excessive vessel excursion and in blow out conditions

Special Conditions:

1) Deepwater > 2000 ft, UD > 5000 ft

- As the water depth increases the deck weight and storage requirements for riser systems increases significantly and often represent a significant percentage of the VDL (variable deck load)

- The additional weight is not just because of the extra length but also because of the extra thickness, auxiliary lines and stronger couplings requirement

- For UD the cost and effectiveness of the buoyancy system must be weighted against the weight and the cost of the riser itself

- For UD operations composite fiber materials, titanium are some of the high strength/weight material being used/under consideration Titanium can give a 120Ksi strength, but the E is almost half of that steel and this may lead to significantly higher response compared to the steel risers

- In case of an unwanted drift off or vessel motion the emergency disconnect system should be able to disengage in around 30 sec time from the BOP stack

- An automatic disconnect secures and/or shears the string in BOP, disconnect the riser and activates the anti-recoil system

- With depth the syntactic foam starts becoming ineffective due to requirement of stronger and denser forma with depth. Air can buoyancy systems are more effective for UD operations. Also, they can have a controlled bleed-off system to have a controlled buoyancy control at any particular point of interest

Flowchart:

Decision Tree: - What type of Well control equipment to be used- Surface or Sub-sea

- Surface mean- HP riser system, Sub-sea means low pressure riser system

- What riser type to be designed?

- Response of the platform/vessel to be determined- vessel characteristics

- Estimation of fatigue life?

- Prediction of extreme and operating loads

- Model the riser

o FEA done using COSMOS (Shell’s in-house tool). TIARA is used as well What kind of analysis- time or frequency domain? Analysis of a scenario when one of the tensioner cylinders fail

- Extreme Response Analysiso Selection of extreme condition events such as 100-yr and a 25-yr

hurricane, 25-year loop current etc.- from where to get this data???- Also, the probability of occurrence etc.

o Riser life (5-yrs for drilling vs. 30 years for production risers)o Design Criteria:

Basic allowable stress (what’s is the maximum allowable stress level- 2/3rd of the Yp may be?)

Riser location and wellhead tilt allowance, wherever applicable

- Data required:o Vessel characteristics-who?o Riser tensioner system properties/characteristics-who?o Operating and extreme load conditions-who?o Rotational stiffness of riser system components-who?o Material to be used- properties- -who?

- Output:o Maximum stress levelso Tensioner strokeso Axial stresses in riser for each design evento Bending moments in the riser system and the resultant axial stresses for

each design evento Calculate VMEo Requirement of things such as “stress joint, its length, material etc.”o