TECHNOLOGY Offshore risers must be designed to resist pipeline movement

Design methods are available to counter effects of pipeline movement on subsea riser systems.

Their application depends on environmental conditions, platform structure/constraints, in-service conditions, pipeline approach to the platform, and economics.

Factors for selecting among the methods are discussed here. A hypothetical project, a 24-in. OD pipeline, is presented as an example.

In this example, the seabed is prone to scouring, necessitating seabed preparation around the platform to ensure structural integrity of the riser system.

Additionally, it is as sumed that the system will be operated at high pressure (150 barg) and temperature (80° C.) in 50 m of water.

Pipeline-riser system

A submarine pipeline between two offshore platforms or between a platform and shore is usually tied into the platform via a rigid riser. The riser is fixed to the platform with a varying number of clamps depending on platform height, flexibility of the riser, dynamic behavior of the riser, and constraints imposed by the platform structure.

The riser system may be subject to movements of the pipeline towards the platform. These movements are a function of the submerged weight of the pipe, its operating conditions (pressures and temperatures), and environmental conditions.

If these movements are large, measures must be taken during design to ensure the structural integrity of the riser system during its service life.

Following are the remedial measures usually adopted to reduce effects of pipeline movements at the riser base:

• Using pipe for the riser system that has a thicker wall than the rest of the pipeline.

• Applying extra concrete coating to a length of the pipeline close to the riser base. This approach must be within the laying capability of the pipeline installation vessels.

• In combination with these two measures, relying on the riser pipe's flexibility (first span from the seabed) as a vertical offset.

• Installing the riser and a length of the pipeline below the seabed.

• Dumping rock over a length of the pipeline to reduce expansion.

• Tying-in the riser-pipeline with a dogleg spool, commonly known as an "offset," or an expansion loop such as U-loops.

The technical feasibility of the methods should be reviewed with regard for the stability of the seabed surrounding the platform structure. If the seabed is prone to scouring from combined action of current and waves and the related transportation-erosion of seabed sediments, the scouring pattern of the seabed requires close attention during design.

The problems, solutions, and technical and cost assessments for such pipeline-riser systems have never been completely dealt with. This article is intended therefore to help solve problems faced by designers and offshore operators.

Expansion spool design

Stability and structural design of an expansion device require the following factors to be considered:

• Its wall thickness and steel quality should be able to accept stresses induced in the system during installation and operation.

• Its negative buoyancy should be sufficient to resist horizontal and vertical movements from environmental forces during installation and operation.

• The seabed below the expansion device should be stable; otherwise, seabed preparation will be required to ensure the structural integrity and lifetime operation of the system.

• If rock dumping is selected as an additional means to minimize pipeline expansion, design of the spool should ensure it can withstand extreme environmental forces likely during the service life of the pipeline-riser system.

Winds and waves, water depth, currents, and seabed soils should be considered for the system's stability (OGJ, Feb. 22, 1982, p. 110; Mar. 1, 1982, p. 110). These data should be assessed and environmental design parameters prepared for use in the system's engineering.

For structural design, necessary parameters are internal and external pressures, installation and design temperatures, and hydrostatic test pressures. These parameters are available from the process design, location of the expansion device (for external pressure), and applicable codes, standards and guidelines (for hydrostatic test pressure).^1-4

Assessing the pipeline's axial movement towards the platform requires knowing whether the pipeline is to be trenched or laid on the seabed. This will depend on stability requirements of the pipeline against environmental loads, such external hazards as fishing and dropping and dragging of anchors by construction and other vessels, and statutory requirements.

For example, in the North Sea, trenching pipelines smaller than 16-in. OD is mandatory.

Platform deflections (for 1 and 10-year return periods maximum wave conditions) and elevations of all riser clamps from the seabed must be considered during design. The location of the first riser clamp predominantly governs the flexibility of the whole system.

Design procedures

Design procedures for expansion devices will depend on each project's conditions but can be generally summarized:

1. Assess the stability requirements for the pipeline and establish the pipeline's required submerged weight during operating conditions.

   If lowering the pipeline below the seabed by trenching or burial is necessary, the weight-coating requirements and hence the resulting submerged weight can be reduced.

   Deciding whether the pipeline must lie below the seabed depends on severe environmental loads that necessitate pipeline submerged weights outside the laying capability of the installation vessel, protection of the pipeline against external hazards, and requirements of government agencies.

   Fig. 1 [18312 bytes] shows a relationship between nominal pipe diameters and commercial wall thicknesses (D/t) as a function of three design pressures for API Steel Grade X-60.

   For various pipe nominal diameters, Fig. 2 [32893 bytes] show the relationship between commercial wall thicknesses and submerged weight as a function of weight-coating thicknesses.

   Because of the severe environmental conditions in shallow waters (site dependent), it is usual to select the weight-coating density as shown. This high-coating density, combined with properly chosen reinforcement, will resist the impact of fishing gears and at the same time reduce the coating thickness required.

   For the pipe sizes shown in Fig. 2 [32893 bytes], the selected weight-coating thicknesses (45 mm, 75 mm, and 100 mm) are based on the thickness which can be applied by weight-coating plants and the maximum thickness which is within the laying capability of proven pipelay equipment for 50-m water depth, for example.

2. Depending upon the configuration of the pipeline and known in-service conditions, compute the axial movement of the pipeline towards the platform.⁵

   Fig. 3 [25532 bytes] shows a relationship between pipeline expansion and nominal pipeline diameter as a function of pipeline submerged weight and design temperatures. For example, the curve 2000 (80) indicates a submerged weight of 2,000 Newtons/m (137 lb/ft) and a design temperature of 80° C. (144° F.).

   As shown in Fig. 3 [25532 bytes], various curves have been drawn for a single design pressure (150 barg). Generally, the stresses/strains as a result of internal pressure are much less compared to the temperature-induced stresses/

   strains. Therefore, for lower design pressures, the resulting expansion can be assumed to be 10-25% lower.

3. Investigate whether the calculated axial movement of the pipeline can be absorbed by the riser flexibility to meet the following requirements:
   

   The pipeline system, for example, must be installed where, because of existing pipelines, it is not feasible to install an expansion device. In such situations, the location of the first riser clamp from the seabed is very important.

   In order to facilitate riser-pipeline tie in, avoid installing an expansion device.

4. If the flexibility of the riser can absorb the expected axial movement, the analyses for establishing or verifying riser spans (static and dynamic criteria) and for riser flexibility^1-4 are undertaken. These analyses must consider the functional as well as the extreme environmental loads in combination with applicable platform deflections.

   If the platform area is prone to scouring, seabed stabilization must be considered.

5. If the axial movement of the pipeline is excessive and the riser flexibility cannot absorb it, several available options depend on technical and economic considerations.

   Figs. 4-11 illustrate various options to reduce pipeline and riser axial movements:
   

   • Installing doglegs or expansion loops.
   • For congested areas, installing riser base and a section of the pipeline below a predetermined seabed level.

     The feasibility of this option depends on the seabed soils, availability of the necessary equipment to execute the works, and the impact on the overall project completion.

   • Rock dumping (engineered backfill).
   • Installing a combination of doglegs or expansion loops with rock dumping over a section of the pipeline.

   Fig. 4 [22029 bytes] shows the design of doglegs and U-loops. This design assumes that the wall thickness and steel quality of the expansion devices are the same as the riser-pipeline near the platform.

   Depending on project requirements, data shown in Fig. 4 [22029 bytes] may be used to approximate the size of the expansion devices. The size should then be optimized in the subsequent computer analysis of the riser-pipeline system.

   Fig. 5 [22907 bytes] illustrates the design for rock dump as an expansion-reducing device. This design is based on a practical rock dump cover of 1 m (3.3 ft) over the top of the weight-coated pipeline.

   As in Fig. 3 [25532 bytes], data shown are valid for a design pressure of 150 barg and, as a conservative design, may be also used for lower design pressures. For clarity, curves for lower design temperatures have not been shown.

   For submerged weights, reference is made to Figs. 2a-d [32893 bytes]. It is suggested that in conjunction with Fig. 3 [25532 bytes], the designer should use his or her judgment and experience to arrive at a rational design.

Combination solutions

Table 1 [27123 bytes] shows the expansion-minimizing combination solutions based on installation economics for the considered pipe sizes and their submerged weight ranges in this discussion.

The 8-in. combination solutions are not shown in Table 1 [27123 bytes] because all the possible solutions are covered under the first of four considerations, listed presently, upon which Table 1's [27123 bytes] data are based.

For a 16-in. pipeline, the heaviest submerged weight considered in this discussion necessitates no combination solutions. Table 1 [27123 bytes] for this pipe size, therefore, includes only data for the lightest submerged weight.

Following are the considerations upon which Table 1's [27123 bytes] data are based:

1. For pipeline expansion of 0.5 m, a maximum dogleg length of 24 m, or a maximum U-loop size of 10 m x 10 m or a maximum rock dump length of 50 m may be used, regardless of the pipeline sizes and their corresponding submerged weights.

   If seabed scour is possible, use of rock dump only is advised.

2. For other pipeline expansions of 1-3 m, for the combination solutions depending upon pipe size and applicable submerged weight, a maximum dogleg length of 36 m, or a maximum U-loop size of 24 m x 24 m, and necessary rock dump length are proposed.

3. For combination solutions, it is proposed to install the necessary rock dump past the dogleg or the U-loop, away from the platform, to allow the expansion device to act as a free system. This requires that the stability of the installed expansion device be ensured under extreme environmental conditions.

4. As mentioned earlier, the proposed combination solutions have been established for an extreme design case of high pressure and temperature. Other possible and less extreme cases can easily be estimated from Table 1 [27123 bytes].

Scour prevention

The combination of a severe storm-wave climate, tidal currents, and fine mobile seabed sediments,^6-8 if predominant, can generate seabed scour. In certain shallow regions, scouring of the seabed around platforms is common.

Several scour-prevention and pipeline support methods are available. Some have detailed track records; others, however, offer limited field experience. Solutions discussed here are therefore based on practical experience and track records to demonstrate the success and applicability of any given method.

The three main methods are summarized in Table 2 [53708 bytes] which also includes a brief indication of associated typical costs. Other methods such as use of artificial seaweed tried in the shallow areas of the North Sea, have had little success, based on available reports.

1. Installation of an engineered backfill material (rock dump) is considered to be the most reliable, adaptable, and cost-effective method of achieving large-scale seabed stability, scour prevention, and/or rectification.

2. Discrete pipeline supports such as stabilizing grout bags should, wherever possible, be avoided close to the platforms.

3. If supports are required as a preinstallation feature, they are best achieved by installation of flexible protection mattresses.

Whichever system is selected, the installation contractor should ensure that the support foundation is adequately stabilized so that local scour is not initiated.

Two criteria should be satisfied to ensure the stability of the engineered backfill:

• The lower layers should be suitably graded to prevent the rock fill from sinking into the underlying layer.

• The outer layer should be stable against extreme wave and current action.

Based on technical and practical considerations, it is a common practice to install engineered backfill having a cover of 1 m on top of the pipeline. The lower layers around the bottom of the pipeline can have an average grain size varying 10-60 mm. Economic considerations indicate installing excessive quantities of backfill material should be avoided.

Installation methods

Installation of expansion devices should satisfy the following criteria:

• Installation procedures should conform to accepted offshore safety standards and applicable code and field requirements.

• The installation method and tie-in of a riser-pipeline should minimize the risk of damage to the riser system or the platform and should not jeopardize the integrity of the riser-pipeline system during its operational life.

• The installation method should minimize offshore costs.

• The selected scour-prevention method should be readily incorporated into the overall installation procedures and overall project schedule.

Before installation of the riser-pipeline interface begins, the local seabed and the platform structure should receive a comprehensive survey.

Methods that may be adopted to tie-in the riser and pipeline system are surface tie-in, flanged or mechanical connections, and hyperbaric welding.

The use of surface a tie-in is only applicable to shallow and transitional waters and necessitates a suitably sized vessel capable of lifting the pipeline under tension while also lifting the riser into a position so that it may be welded and finally lowered into position.

Because of the more exact dimensional requirements of flanged or mechanical connections, a spool piece is usually required to facilitate the tie-in between the riser base and the pipeline termination. Flanged or mechanical connections as a general rule must be positioned at locations where the bending moments in the riser-pipeline system are minimal.

Hyperbaric welds are completed by divers within the confines of a welding habitat positioned over the exposed pipeline ends.

On completion of the riser-pipeline tie-in, the pipeline and specifically the riser base should be checked for spans. Any spans in the system in excess of the allowable limits should be rectified by installation of grout-bag supports. Alternatively, local covering with backfill material may also be adopted.

Protecting expansion devices

For safe operation and minimum maintenance of a riser-pipeline system, it is essential fully to establish the seabed preparation required if seabed scouring near the platform is imminent.

The stability and protection of expansion devices should also be addressed during detail design.

Whichever option of seabed preparation and protection of the expansion device is chosen, the following criteria should be satisfied: technical feasibility, operational requirements, installation and safety aspects, reasonable costs, and minimum inspection and maintenance.

Figs. 6-11 illustrate various feasible options.

• Option 1. Design of certain riser-pipeline systems requires the system to be buried before being made operational (Fig. 6 [18476 bytes]).

  If this method is adopted, the pipeline approaches should be buried and backfilled beneath the lowest expected scour profile, thereby ensuring its continuous support.

  Burial deeper than 2 m may be required. Such burial of the pipe to a specified depth will ensure that the expansion effects will be significantly reduced.

  Addition of rock dump material over the platform approaches of the buried pipeline will serve to reduce further this expansion and also minimize scouring effects.

• Option 2. This option relies on the rock dumping of the riser-pipeline section close to the platform (Fig. 7 [21639 bytes]). Burial of the pipeline approaches may be completed as a one-stage operation.

  This operation allows installation of the pipeline approaches directly onto an unprepared seabed; data from the relevant seabed surveys should be reviewed to ensure that no excessive spans are generated.

• Option 3. In this option (Fig. 8 [22485 bytes]), seabed preparation and subsequent protection of the riser-pipeline system are carried out in two stages. The first involves installation of a layer of rock dump material to provide a level seabed and eliminate potential spans.

  The pipeline system may then be installed on the prepared seabed and fully checked to ensure the correct contact has been developed. The rock dump material is subsequently installed over the pipeline which is buried to the required depth.

  The final extent and profile of the rock dump should be checked to ensure compliance with the design requirements.

• Option 4. In this case (Fig. 9 [18217 bytes]), before installation of the pipeline, a preinstalled and leveled rock dump material is covered by an articulated concrete "carpet" which provides a uniform, flat surface over which the pipeline may expand.

  In this case, it is also feasible to install an expansion device such as a dogleg or a U-loop. This will provide increased expansion absorption capability and may be integrated into the system when the vertical expansion offset (riser flexibility) arrangement has reached the limit of its expansion absorption capabilities.

  Consideration should be given to the limited length (dimensions) of the expansion devices which may still be installed by conventional techniques and the stability of the system under extreme environmental conditions.

• Option 5. Previous options have identified various solutions which use rock dump material either to cover or support the pipeline approaches. Under certain conditions, various combinations of these two solutions may be developed to satisfy a given situation.

  Typically, this may include rock dumping a section of the pipeline to limit its expansion, so that the remaining expansion may be accommodated by a suitably sized seabed expansion device. The expansion device would, as in Option 4, also be installed on a preinstalled layer of rock dump material and an articulated concrete carpet.

  The length of the rock dumped section can be determined to limit the expansion of the pipeline system (Fig. 10 [16292 bytes]).

• Option 6. A flexible pipe (Fig. 11 [15082 bytes]) can be used to connect the pipeline to the riser.

  Flexible pipe tie-ins are generally completed with standard and swivel flanged connections. The pipeline and riser systems may be installed as separate independent components which are connected at a later date by the flexible pipe.

The inherent flexibility of these pipes provides two major advantages to the system.

Example

The design example (Table 3 [37715 bytes]) considers a 24-in. gas pipeline of API 5L Grade X-60 steel in an assumed water depth of 50 m.

The design submerged weight in gas-filled condition is based on typical 100-year storm return period environmental data.

It is assumed that during its operating life, the pipeline will rest on the sandy seabed with slight embedment and that the seabed near the platform is prone to scouring. For this purpose, the design considers a 1 m deep and 15 m long scour pit.

The height of the first riser span was checked against vortex excitation criteria and found
acceptable. ²Table 4 [47406 bytes] summarizes the results.

The expansion-absorbing capability of the riser being limited (0.16 m = 0.53 ft), it was therefore necessary to evaluate various expansion-absorbing devices.

Because of the seabed scour, preparing the seabed is necessary for all the six options.

Option 1 would necessitate mobilizing a jetting spread at high cost because the seabed-preparation vessel, which would also be used to cover the riser-pipeline with engineered backfill, would be on standby until the riser and pipeline have been jetted to the desired grade.

Options 2 and 3 can be installed by any of the three installation techniques discussed earlier, after preparation of the seabed.

Options 4 through 6 may become uneconomic because of standby of the rock dumping vessel.

For this example, Option 3 is therefore recommended. For this solution, however, stability of the expansion device is of utmost importance.

References

1. "Rules for Submarine Pipeline Systems," DnV 1981.

2. "Code of Practice for Pipelines," British Standard BS 8010, Part 3, 1993.

3. "Liquid Transportation Systems for Hydrocarbons, Liquid Petroleum Gas, Anhydrous Ammonia, and Alcohols," ASME B31.4, 1992.

4. "Gas Transmission and Distribution Piping System," ASME B31.8, 1992.

5. Palmer, A.C., and Ling, M.T.S., "Movements of Submarine Pipelines Close to Platforms," Proceedings, Offshore Technology Conference, Houston, 1981.

6. U.S. Army Coastal Research Center, "Shore Protection Manual," Vol. 3, 1984.

7. Breusers, H.N.C., et al., "Local Scour around Cylindrical Piers," Journal of Hydraulics Research, Vol. 15, No. 3.

8. Raudkuri, A.J., Loose Boundary Hydraulics, 1976.

The Author