Tc 67/sc 4 n date: 2005-03-9 iso/wd XXXXXX ISO tc 67/sc 4/wg 6 Secretariat: Design of dynamic risers for offshore production systems Élément introductif — Élément central — Élément complémentaire  Warning

Functional considerations

Functional requirements for risers associated with FPS systems can vary significantly, depending on the application. Functional requirements are also determined by the primary duty or service of the riser. A riser for a single subsea well, for example, will typically have fewer functional requirements than a riser handling produced fluids for multiple wells. The number of functional requirements generally correlates with system complexity.

(Move Appendix B - Functional Considerations here)

Find Appendix B of API-2RD-1998 First Ed.

Added for new revision:

The functional considerations for each of the principal types of riser (export/import risers, production risers, injection service risers, drilling risers, workover and completion risers) are listed below.

1. Catenary export risers

2. Catenary import risers

3. Top tensioned production risers

4. Top tensioned injection service risers

5. Drilling risers

6. Top tensioned workover and completion risers

4. Operational considerations

   1. General

• 
  the designer to take into account all realistic conditions under which the riser will be operated,
  
• 
  operations personnel to be aware of the riser's safe operating limits (This information needs to be communicated to operations personnel in a understandable manner).
  

• 
  risers installed and left for (many) years until subsequent retrieval,
  
• 
  risers run and retrieved many times during their service life, which may also accommodate drilling and/or workover operations.
  

A riser’s operational requirements should be distinguished from its functional requirements. Functional requirements define how the riser must function to achieve its intended purpose. Operational requirements define how the riser should be operated to assure a safe operation.

Operational requirements are typically documented in a Riser Installation and Operation Manual. The manual, which should be prepared jointly by the designer and the operator, defines how to safely install, operate and maintain the riser and its component systems.

2. Operating philosophy

• 
  vessel motions and environmental limits;
  
• 
  manning;
  
• 
  control centers and ancillary support;
  
• 
  riser deployment and retrieval;
  
• 
  in-service operations;
  
• 
  inspection and maintenance philosophy.
  

3. Vessel motions & environmental limits

4. Manning

5. Vessel interfaces

Utility support such as electrical power, hydraulic power, air and water may be required to operate a riser and should be interfaced with the vessel systems.

Ancillary support equipment such as diving, ROVs, video monitoring and subsea positioning systems should be considered early and interfaced with the riser and vessel systems.

6. Riser deployment and retrieval

7. In-Service Operations

The riser may be used for more than one purpose, in which case the riser operating procedures should consider the riser's other functions. For instance, the production riser may be used for minor and major well workovers, injection, completion and other purposes. Drilling risers may be used for well completion and testing.

1. Simultaneous drilling and production

   1. Safety considerations

A Simultaneous Operation consists of any combination of two or more of the following activities executed independently and concurrently which may interact to increase operational complexity and risk:

• 
  producing,
  
• 
  drilling,
  
• 
  workovers and completions,
  
• 
  wireline work,
  
• 
  testing,
  
• 
  pigging,
  
• 
  construction and major maintenance,
  
• 
  supply operations,
  
• 
  offloading.
  

• 
  incompatible activities if work groups have different goals, skills and/or supervision;
  
• 
  hydrocarbon release potential, both controlled and uncontrolled, may increase;
  
• 
  more ignition sources may exist;
  
• 
  facilities design and safeguards may not be ideal for conditions experienced during simultaneous operations, e.g. structural integrity, available work space, protection for processing equipment, etc.
  

2. Limitations and restrictions

Limitations and restrictions on operating conditions and activities should consider:

• 
  the type of production equipment on the platform;
  
• 
  means for shutting-in wells;
  
• 
  the type of wells (oil or gas), shut-in and flowing tubing pressures, and annular pressures;
  
• 
  maximum and expected oil, water and gas production rates for individual wells and for the total facility;
  
• 
  the presence of H₂S in the gas;
  
• 
  gas lift and whether used for sustained production or only to kick-off wells;
  
• 
  controlled well kill operations used prior to starting a workover or for reversing out a backsurge;
  
• 
  drilling or workovers in close proximity to live wells;
  
• 
  repair and maintenance on safety systems, including well control system, ESD, fire or gas detection systems, fire fighting system and ventilation systems or drain systems that affect area integrity;
  
• 
  whether well testing is performed;
  
• 
  whether heavy jarring is performed;
  
• 
  Any flaring or gas venting to atmosphere;
  
• 
  Well unloading and swabbing;
  
• 
  Running or pulling other risers;
  
• 
  Repairs to process facilities which may impair normal operations.
  

3. Interference considerations

If a guidelineless system is employed, additional wells may need to be shut in to further ensure that a failure of the positioning system during reentry operations is not likely to damage a flowing well.

Consideration may be given to using ROVs or tugger lines to steady or guide risers as the connector approaches the wellhead, whether guidelines are used or not.

In the presence of deeply penetrating currents that can cause significant horizontal riser deflections, particularly during deployment, riser running operations may have to be curtailed to avoid potential damage to adjacent risers or wellheads. In some cases, even with a lateral mooring system or dynamic positioning system, it may be difficult or impossible to safely guide the riser.

Disconnects must be carefully made, with or without current, to avoid damaging adjacent wellheads or risers. As a general practice, the riser angle at the lower connector should be away from neighboring wells such that the riser will swing free immediately after the disconnect.

Interference can occur in the water column with a connected riser as well as with a disconnected one. Careful analyses of all anticipated conditions should be made by the design team prior to conducting operations. Interference in the presence of deeply penetrating currents is very sensitive to the tension distribution, contents and buoyancy in adjacent risers. A drilling riser adjacent to production risers poses an especially difficult situation due to major differences in the risers' hydrodynamic drag as well as tension and weight. Risers with significantly different dynamic characteristics will respond at different frequencies and phases during severe current or storm conditions.

In some cases, it is difficult to detect the possibility of riser interference or collision in the water column without TV cameras or an ROV. When close approaches of hardware are expected in the water column, some monitoring means is desirable.

2. Well completions and workovers

3. Through-riser operations

• 
  wireline operations,
  
• 
  coiled tubing operations,
  
• 
  major workover,
  
• 
  drilling and sidetracking considerations,
  
• 
  through flowline (TFL) operations,
  
• 
  flushing,
  
• 
  pigging and riser bore maintenance (scraping).
  

Well control functions must be designed to perform properly with the various risers and in the various modes of operation.

The riser bore should be adequately designed for these through-bore operations. The size, tolerances, material, curvatures and presence of dog-legs are primary considerations.

Using the production riser for workover operations will require proper interface at the top. Workover BOPs, and other equipment not normally used in production operations may be attached.

Wireline or rotary drilling operations may produce severe tracking or keyseating when "dog-legs" (bends) are present in the riser bore. In this case, consideration should be given to using a replaceable wear bushing and limiting vessel offsets during these operations to mitigate this problem. Pipe rubbers and downhole motors can also be used to minimize wear caused by rotation. Calipers should be run in the riser following any operation requiring rotation in the riser. Analysis tools are available to assess the significance of this situation to planned operations.

4. Monitoring

In some cases, such as closely-spaced risers in high current situations, careful riser monitoring of tension, stroke and pressure may be used to minimizing the possibilities of riser-to-riser collisions. Each of these measurements can be set up with alarms for high or low limits.

Riser tension can be monitored by measuring the pressure in the tensioner cylinders and applying correction factors for riser angle, tensioner wire and friction. Strain gages can be used in load pins, on the riser body, on the tensioner structure or in special load cells connecting the tensioner system to the riser.

Riser angles can be measured with inclinometer devices. Some units may be sensitive to the surge and sway of the FPS and require motion corrections.

Tensioner or riser stroke measurement can be made with something as simple as a string potentiometer. The inclination of the tensioner arm can be used to infer riser stroke.

Standard pressure transducers can be used to monitor riser pressure if they are suitable for the hazardous area classification of the area where they are used.

5. Operational support

• 
  ensure that all utilities required for riser operation are available,
  
• 
  ensure that all required tensioner maintenance or repair work has been performed,
  
• 
  check the condition of the tensioner wire ropes and ensure that they set in their respective pulley grooves around the wireline tensioners and turndown sheaves,
  
• 
  function check all alarm systems and indicator lamps,
  
• 
  test any emergency disconnect systems,
  
• 
  test any riser monitoring system.
  

6. Control

The various control stations may need to be manned and communication lines established before these operations take place. Some of these control stations may include the following:

• 
  the bridge of a ship,
  
• 
  ballast control room,
  
• 
  operations control room,
  
• 
  production control room,
  
• 
  moonpool control room,
  
• 
  ROV and diving control center.
  

7. Manning

8. Riser limitations

• 
  moonpool clearance range before impact,
  
• 
  lower flex joint limitations,
  
• 
  upper flex joint limitations,
  
• 
  riser tension,
  
• 
  internal conditions (pressure, weight of internal strings or fluids, etc.),
  
• 
  vessel heading,
  
• 
  current profile,
  
• 
  water depth,
  
• 
  flexible flowline (jumper) length.
  

8. Rig movements and station keeping

1. Stationkeeping considerations

A lateral mooring system is the most common type of adjustable stationkeeping system. However, a dynamic positioning system may be a viable alternative.

2. Strategy for vessel offset control

The movements should be carefully calculated in advance of each adjustment of the vessel's positioning system (e.g., moorings). Avoiding overshooting the target is important in minimizing the possibility of impacting adjacent wellheads.

A safe disconnect from a well will require that the disconnected package, when released, swings a minimal distance away from the well and away from adjacent wells, if possible. The lower riser angle must therefore be biased in the desired direction of motion by a precalculated amount.

9. Storm and Contingency Operations

1. Riser operating limitations

Riser operations can be divided into four main classifications as follows:

latched operations where the riser system is latched to the seabed equipment;

hang-off, with riser detached from the seabed;

parking the riser by attaching the lower marine package to a parking stump located a suitable distance away from critical equipment and leaving the riser free-standing with appropriate topend buoyancy;

retrieval/redeployment of the riser.

Operating limitations should be established for each of the selected riser operating modes.

1. Latched operations

The operating envelopes should be developed based on realistic combinations of various conditions of current, wellbore fluids and Metocean conditions.

Riser operating limits may also be determined by use of a riser management program, where an operator can input actual condition, and the program will determine whether the conditions are acceptable.

2. Riser hangoff mode

When the riser is disconnected and hanging-off, vessel offset is generally not a governing criteria. Riser impact with the vessel hull is. The onset of impact is dependent on vessel heading and motions such as roll, pitch, surge and sway together with current profile. The length of riser suspended beneath the hull is also an important factor.

Operating conditions for the hang-off mode should be developed based on realistic combinations of conditions that affect riser performance.

The hang-off condition may cause increased fatigue damage to the riser and should be assessed. A detailed log should be kept on the length of time, sea state and configuration in which the riser is hung-off. This log can be reviewed regularly to assess the need for riser joint inspection requirements. Durations of and levels of exposure in any/all modes should be logged for keeping track of fatigue status.

3. Riser deployment and retrieval

The following should be considered regarding storm disconnection operations:

• 
  the lower riser angle should be monitored to ensure that it does not exceed maximum angle criteria;
  
• 
  the riser tension should be above minimum allowable tension determined during the design;
  
• 
  with high currents or severe storm conditions, extreme care should be taken and the riser tension should be carefully maintained.
  

2. Disconnect considerations

• 
  a complete production shutdown of all process equipment,
  
• 
  closure of all subsea and riser valves,
  
• 
  an emergency riser unlatch which will unlock the riser.
  

If possible, riser recovery after a rapid or emergency disconnection can be carried out in the normal manner, according to developed operating procedures. Otherwise, the riser will have to remain hungoff until such time as it can be recovered.

See 3.7.4 and 3.8.4 (Changed?) for considerations regarding riser disconnection.

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