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

Material Considerations

Factors to be considered when selecting materials that are fit-for-purpose in a riser system include:

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  yield and ultimate strength;
  
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  material toughness and fracture characteristics;
  
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  Young’s modulus;
  
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  shear modulus;
  
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  Poisson’s ratio,
  
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  S-N fatigue curve;
  
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  internal erosion or wear requirements based on the fluid properties and flow rate;
  
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  H₂S/CO₂ produced water salinity/acidity;
  
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  internal corrosive effects;
  
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  external corrosive effects;
  
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  biofouling;
  
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  operating temperature;
  
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  welding, weldability, and HAZ properties;
  
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  machining;
  
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  manufacturing processes;
  
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  galvanic corrosion.
  

7. Installation, retrieval, and reinstallation of metal risers

It is common to think of metal risers as being held in taut, vertical configurations. However, it is important to recognize that normally metal riser pipe sections may be configured in such a manner where inherent compliance is achieved. In such cases, many of the following considerations would still apply, but special features may lead to treatment as other riser types (see 3.9).

The following sections refer to operations involved with the establishment of metal riser systems for floating production developments. Metal risers include all riser types where the riser sections are normally considered to be solid metal. This includes risers for drilling, completion/workover, production, injection and export/import service. The special features and service demands for metal risers involved in drilling and completion/workover functions make it prudent for the designer to refer to guidance provided in API RP 16Q and API RP 17G.

1. Preparations, testing and required support equipment

Tank testing of ROV, diving and subsea interfaces should be considered. All ROV functions, including installation and other operation and maintenance functions, should be tested by actual equipment operation or by mockups of the ROV interfaces.

All utilities and installation support equipment should be checked and tested. Critical installation equipment should be load tested, if possible.

Step-by-step installation procedures, from transporting equipment from the manufacturing facility to completing installation of all risers, should be prepared. All risers should be considered, as well as plausible combinations of connected risers.

The following should be considered in the installation and operational planning:

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  space out requirements, where special segments may be used in the riser string to achieve a specific overall length between desired connection points;
  
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  accessories that must be attached to the riser during running;
  
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  the method(s) used to guide the riser to the sea floor. In the case of multiple risers, there may be different guidance means required during the different stages of the installation;
  
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  interference with other risers, mooring lines and other obstructions during installation;
  
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  motion compensation requirements for installation of each riser system, during the running, landing and disconnect phases;
  
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  support vessels, if any, used during riser deployment;
  
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  contingency plans concerning actions required for suspending operations if there is riser damage or equipment malfunction;
  
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  whether the riser is installed empty or full of water.
  

Installation procedures (and scheduled durations) should allow for pigging, if appropriate, and hydrostatic testing of the riser after installation.

2. Transportation and handling

Requirements for protecting the riser system during shipment and handling on the rig should be considered. Protective shrouds, end caps and other protective devices should be used if necessary.

The means for transporting riser and associated equipment to and from the site should be evaluated. Motions of the transportation vessels (barges or supply boats) should be considered in the design of seafastenings and tiedowns.

Depending upon the installation method, equipment on the FPS or support vessel should be capable of handling the riser. Crane capacities and reaches at load, derrick capacities and clearances must be considered.

Deck load and deck space requirements during and after installation, including installation tools, should be considered in the installation procedures. Adequate laydown areas should be provided.

Moonpool areas and clearances for moving equipment into and out of restricted areas on the vessel should be considered.

3. Installation considerations

In this phase of the operation, safe riser deployment operating envelopes are needed.

Special running tools may be required to deploy the riser. These tools facilitate connecting, lifting, lowering and riser support. Other tools, such as those used for pressure testing and inspection, may be used as well.

The installation method must allow for a weather window that is large enough to accomplish the work. Contingency procedures must be considered to include suspension and reversal of the installation.

1. FPS deployed

Special tools may be required to assemble the riser component and to lift and lower the riser string. Also, special tools are normally needed to handle the riser joints.

Special appurtenances, such as large buoyancy modules or drag fairings, may be attached to the riser and deployed through the rig's moonpool area.

During running, the riser may be guided either by guidelines, or by the vessel stationkeeping system if it can be used to position the rig and the suspended riser during riser deployment.

Single bore risers and multibore risers that have integral joints are normally deployed similar to a drilling riser on a drilling rig (i.e., joint by joint).

Multibore risers that have non-integral joints are deployed first by running and landing the central structural core and then running each bore separately.

2. Other riser deployment methods

Installation methods developed for pipelines can be used to install the riser. These include the J-lay method, the reel method and the S-lay method. In these methods, the flowline or pipeline is usually laid first, followed by directly attaching and running the riser.

In the J-lay method, riser joints are assembled vertically on a drilling rig or a derrick barge with a suitable frame. The riser can be assembled while the installation vessel is moving toward or away from the FPS with the top of the assembled riser passed over to the FPS at the appropriate time.

In the reel method, the riser is assembled in a continuous length and is coiled onto a large diameter reel. A special vessel then deploys the riser by paying the riser out of the reel and passing the top end to the FPS.

For riser installation by the S-lay method, a pipelay vessel is used to assemble the riser joints and to lay them off the stern of the vessel.

Another method has the riser transported to the site and installed by a tow and upend procedure. In this case the riser is assembled on land (or in protected waters), then towed out horizontally with one or more tugs to the site. At the site, the riser is upended by removing buoyancy tanks or controlled flooding and is then attached to the seabed and the FPS. This technique has been successfully used with TLP tendons, although much care is required in the design of temporary buoyancy and its attachment.

Some of these deployment and installation operations may involve "keelhauling". Such an operation has the riser or riser system components passed from the installation vessel to the FPS with handling lines attached in a manner that allows the FPS to pull the top end of the riser underwater, beneath part of the submerged FPS hull, toward the connection point.

4. Disconnect and Retrieval

Rapid or emergency disconnection of the riser system may be necessary if vessel or well system emergencies occur, the FPS stationkeeping system fails or the weather suddenly and unpredictably deteriorates beyond the riser's operating threshold.

If riser recovery is required following an emergency disconnect response, all wellhead valves should be closed, the production fluids in the riser system flushed and the riser vented before the riser system is recovered. Sufficient storage capacity needs to be available to stack the riser, otherwise it will need to be offloaded.

Some difficulties can arise if workover/drilling activities are being performed at the time of an emergency. In such cases all equipment should be designed to be fail safe to prevent the escape of fluids from the wellbore to the environment.

The following items should be considered for riser retrieval:

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  reverse installation procedures should be altered to suit riser retrieval;
  
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  related vessel ballast control operations should be anticipated and clarified in procedures;
  
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  if mass has been added or removed from the riser by adjusting ballast or any other means, the riser mass may need to be altered before removal of the riser;
  
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  recoil after disconnect needs to be considered and tension adjusted so that no riser or vessel damage is sustained;
  
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  if the riser requires guidelines ( or other guidance systems) for removal and these are not present, then replacements or alternatives should be used;
  
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  rig offset control should be performed, if required (See 3.12.2).
  

5. Reinstallation Considerations

6. Hydrostatic testing

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