Flexible Riser

Flexible Riser Global Configuration

Flexible risers can be installed in a number of different configurations. Riser configuration design shall be performed according to the production requirement and site-specific environmental conditions. Static analysis shall be carried out to determine the configuration. The following basis shall be taken into account while determining the riser configuration:

• Global behavior and geometry
• Structural integrity, rigidity and continuity
• Cross sectional properties
• Means of support
• Material
• Costs

The six main configurations for flexible risers are shown in Figure 22.1. Configuration design drivers include a number of factors such as water depth, host vessel access / hang-off location, field layout such as number and type of risers and mooring layout, and in particular environmental data and the host vessel motion characteristics.

– Free Hanging Catenary

This is the simplest configuration for a flexible riser. It is also the cheapest to install because it requires minimal subsea infrastructure, and ease of installation. However a free hanging catenary is exposed to severe loading due to vessel motions. The riser is simply lifted off or lowered down on the seabed. A free hanging catenary under high vessel motions is likely to suffer from compression buckling at the riser touch down point and tensile armor wire ‘birdcaging’. In deeper water the top tension is large due to the long riser length supported.

– Lazy wave and steep wav

In the wave type, buoyancy and weight are added along a longer length of the riser, to
decouple the vessel motions from the touch down point of the riser. Lazy waves are preferred to steep waves because they require minimal subsea infrastructure. However lazy waves are prone to configuration alterations if the internal pipe fluid density changes during the riser lifetime. On the other hand, steep wave risers require a subsea base and subsea bend stififener, and yet are able to maintain their configuration even if the riser fluid density changes.

Buoyancy modules are made of syntactic foam which has the desirable property of low water absorption. The buoyancy modules need to be clamped tightly to the riser to avoid any slippage which could alter the riser configuration and induce high stress in the armor wires. On the other hand the clamping arrangement should not cause any significant damage to the external sheath of the riser as this might cause water ingress into the annulus. Buoyancy modules tend to lose buoyancy over time, and wave configurations are inherently designed to accommodate up to a 10% loss of buoyancy.

– Lazy S and steep S

In the lazy S and steep S riser configuration there is a subsea buoy, either a fixed buoy, which is fixed to a structure at the seabed or a buoyant buoy, which is positioned by e.g. chains. The addition of the buoy removes the problem with the TDP, as described above. The subsea buoy absorbs the tension variation induced by the floater and the TDP has only small variation in tension if any.

‘S’ configurations are considered only if catenary and wave configurations are not suitable for a particular field. This is primarily due to the complex installation required. A lazy-S configuration requires a mid-water arch, tether and tether base, while a steep-S requires a buoy and subsea bend stiffener. The riser response is driven by the buoy hydrodynamics and complex modeling is required due to the large inertial forces in action. In case of large vessel motions a lazy-S might still result in compression problems at the riser touchdown, leaving a steep-S as a possible alternative.

– Pliant wave

The pliant wave configuration is almost like the steep wave configuration where a subsea anchor controls the TDP, i.e. the tension in the riser is transferred to the anchor and not to the TDP. The pliant wave has the additional benefit that it is tied back to the well located beneath the floater. This makes well intervention possible without an additional vessel.

This configuration is able to accommodate a wide range of bore fluid densities and vessel motions without causing any significant change in configuration and inducing high stress in the pipe structure. Due to the complex subsea installation that is required, it would be required only if a simple catenary, lazy wave or steep wave configurations are not viable.

Flexible Riser Design Analysis

The essential tasks for design and analysis of flexible risers are similar to those described for other types of risers, see below.

Design Basis Document:

The document should as minimum include the following

• host layout and subsea layout;
• wind, wave and current data and vessel motion that are applicable for riser analysis;
• applicable design codes and company specifications;
• applicable design criteria;
• porch and I-tube design data;
• load case matrices for static strength, fatigue and interference analysis;
• applicable analysis methodology.

FE Modeling and Static Analysis:

a finite element model is built and a nonlinear static analysis is carried out assuming the vessel is in NEAR, FAR and CROSS positions.

Global Dynamic Analysis:

A global regular wave dynamic analysis is carried out assuming the vessel is in NEAR, FAR and CROSS positions. A sensitivity study is performed on critical parameters such as wave periods, effect of marine growth and hydrodynamic coefficients.

Interference Analysis:

A dynamic regular wave analysis is carried out to check the minimum clearance between the risers and with the vessel system along the water column, for various predefined load cases. The interference analysis shall confirm that selection of riser hang-off angles and departure angles etc.

Cross-sectional Model:

A detailed cross-section model is built to calculate key cross sectional properties such as bending stiffness, axial stiffness etc., FAT pressure etc.

Extreme and Fatigue Analysis:

The wire and tube stresses are calculated at design pressure. An extreme response analysis is carried out using regular wave theory to estimate tensions and cyclic angles etc. The cross-sectional model is then used to perform to fatigue analysis.

Design Review:

This includes check of global configuration, bell mouth design, interference and fatigue design etc. In some special situations, upheaval buckling and onbottom stability of flexible flowlines are also checked, following pipeline design practice.

Typically, a detailed design of flexible pipe is carried out by the supplier for the flexible pipe materials. A 3^^^ party, normally a riser engineering company, is engaged to carried out a verification of the design, as aforementioned.

Source:

Bai Yong, Qiang Bai. 2005. Subsea Pipelines and Risers. UK: Elsevier Ltd

Periodical Inspection – Pipeline

The aim of the pipeline inspection survey is to verify that the design conditions are fulfilled, including:

• seabed stability (e.g. scour, sand waves);
• integrity of pipeline support (e.g. for free spanning pipeline sections);
• integrity of protection cover (e.g. mattresses, sand bags);
• integrity of weight coating;
• pipeline stability (e.g. excessive lateral pipe movements, upheaval buckling);
• pipeline expansion;
• extent of marine growth;
• security of mechanical connections, clamps and anodes;
• performance of external corrosion protection system;
• internal corrosion;
• integrity of valves, anchors, tee- and wye-connections.

A visual inspection of exposed pipeline sections is carried out to determine the general condition of the pipeline, and to locate areas that may require a more detailed inspection, including the detection and mapping of:

• mechanical damage to the pipe (NDT inspection may be required for the detection of cracks);
• damage to coating, insulation, possible jacket sleeves, field joints, etc.;
• anode consumption and condition, including integrity of electrical connections;
• seabed condition with respect to scouring or the build-up of seabed deposits;
• evidence of lateral and axial movements;
• buckled pipe sections (e.g. upheaval buckling);
• free pipeline spans;
• leaks;
• marine growth.

Concerning the last item, it is worth noting that the type and amount of biological activity may be used to assess the probable exposure time of otherwise buried lines.

The performance of the external corrosion protection system should be verified at regular intervals. Electric potential measurements of a cathodic protection system are normally carried out within one year of pipeline installation. The subsequent surveys may be carried out at greater intervals, provided the system is performing satisfactorily.

Measurements of the pipe wall thickness may be required where there is reason to believe that the pipe is subjected to severe corrosion or erosion.

Source:

Braestrup, Andersen, Bryndum, etc. 2005. Design and Installation of Marine Pipelines. UK: Blackwell Science Ltd

Pipeline Commissioning

The subsequent commissioning activities prior to operation comprise functional testing of the system and equipment, and activities associated with the initial filling of the pipeline system with the fluid to be transported. In particular, it is important to test the equipment used for monitoring and control of the pipeline system. This includes safety systems associated with pig-trap interlocks, pressure-, temperature- and flow monitoring systems, and emergency pipeline shutdown systems. Furthermore, it should be checked whether valves are operating satisfactory.

Filling of the pipeline system with the fluid to be transported should be carried out according to written start-up procedures. Prior to introducing the fluid into the system it should be ensured that all functional testing is complete and accepted,  and that operational procedures are in place. Also, a formal transfer of the pipeline system to those responsible for its operation should be completed.

It is important that critical product parameters are kept within the specified design limits during operation of the pipeline system. In particular, the design pressure and temperature should not be exceeded during normal operation. The maximum allowable operating pressure (MAOP) should be established with a sufficient safety margin, taking into account the accuracy tolerances of the pressure control devices. Furthermore, surge pressures should not exceed pre-defined values (e.g. 1.05 times the internal design pressure), and it should be documented that the specified valve closure time does not result in transient pressures exceeding the maximum allowable surge pressure.

The following product parameters should at least be considered:

• pressure control and monitoring along the pipeline system;
• temperature control and monitoring along the pipeline system;
• dew point control and monitoring for gas lines;
• product composition, flow rate, density and viscosity;
• parameters as defined in the corrosion management system;
• concentration of toxic product constituents.

The safety equipment in a pipeline system should be tested and inspected at agreed intervals to verify that it can properly perform the safety function and that the integrity of the safety equipment is intact. This includes pressure control and over-pressure protection devices, emergency shut down systems, automatic shut down valves, and other safety equipment.

Check valves are normally kept open by the fluid flow, and will close in the event of an upstream pressure drop. A mechanical override (operated manually or by ROV), allows the valve to be opened, but it cannot be operated against a significant pressure differential. Therefore a check valve is provided with a bypass for pressure equalisation, which may be a separate (small diameter) bypass line or may be built into the valve. The check valve can be fixed in the open position to allow bi-directional flow and pigging. The operating position normally permits uni-directional pigging, although some pigs may damage the valve if it is not fixed in the open position.

Ball valves can be opened and closed without pressure equalisation, either manually (by diver) or by ROV, or by means of an actuator, which may be remotely controlled. Subsea isolation valves (SSIV) are always remotely operated. Ball valves normally allow bi-directional pigging.

Figure 11.1 shows a typical valve assembly at a riser base, with the check valve at the riser side. This is because in the event of rupture of the riser the check valve will close as a result of the differential pressure, but it can then be isolated by closing the ball valve and opened by the mechanical override. If it was on the downstream side of the ball valve it could not be opened again  without depressurising the entire pipeline. In shallow water there is also the possibility of replacing the check valve, which has a shorter design life than the ball valve.

Source:

Braestrup, Andersen, Bryndum, etc. 2005. Design and Installation of Marine Pipelines. UK: Blackwell Science Ltd

 

Pipeline Decommissioning

A pipeline that will be out of service for an extended period may be decommissioned. However, the pipeline should still be properly maintained and cathodically protected. The decommissioning work should comprise removal of service fluids from the pipe bore and isolation of the pipeline section from other parts of the pipeline system left in service. Liquids may be pumped, or pigged, out of the pipeline using water or an inert gas, considering:

• buoyancy effects if gas is used to displace liquids;
• compression effects which may result in ignition of fluid vapour;
• possible asphyxiating effects of the used inert gases;
• disposal of the pipeline fluids;
• drainage of ‘dead sections’, e.g. valve cavities;
• combustibility of the displaced fluids.

If a pipeline is to be abandoned it should be decommissioned, disconnected from other installations and left in a safe condition. Local legislation may require that the abandoned pipeline be finally removed.

Source:

Braestrup, Andersen, Bryndum, etc. 2005. Design and Installation of Marine Pipelines. UK: Blackwell Science Ltd

Thermal Expansion Design

Thermal expansion is an important issue in deepwater flowlines design since flowlines normally carry very high pressure and temperature fluid, unlike export pipelines. The thermal elongation is a function of the pipe material’s thermal expansion coefficient (α), differential temperature (δT) between the conveyed fluid temperature and the ambient temperature when the pipe is welded, and the pipeline length (L). If a 1.0 miles of carbon steel pipe (α = 6.5 x 10-6 /oF) is operated at 100oF differential temperature, the pipeline end elongation (δL) will be:

However, the pipe/soil friction force resists the pipeline expansion, so the above estimated pipeline end elongation will be reduced significantly. The thermal expansion analysis is not simple and FEA (finite element analysis) tools are commonly used to handle sea bottom irregularities, flowline route curvatures, and pressure and temperature variance along the route. Snaking (lateral displacement) or upheaval buckling (vertical displacement) can occur due to excessive flowline enlogation when both ends are restrained and are not allowed to move freely.

To control or mitigate the thermal expansion problems, such methods can be adopted as follows (also see Figure 10.2):

• Snake lay
• Expansion loop
• Flexible jumper
• Inverted “U” or “M” shape rigid jumper
• Sliding PLET
• Random buckle initiators (sleepers, buoyancies, etc.)
• Random buckle arrestors (random rock dumping, burial, anchor, etc.)

Flowline tends to expand (elongate) to each end of the flowline while the soil holds the axial movement of the flowline. At a certain point, the soil friction resistance equals or exceeds the flowline expansion load. Beyond this point, called a virtual anchor point, the flowline will not move. The flowline walking can occur when the virtual anchor point moves between when flowline is warmed (operation) and when it is cooled down (see Figure 10.3). Repeated shutdowns and startups cycles may cause the axial walking and require anchor pile to hold back the flowline from walk-away. Otherwise, a steel catenary riser (SCR) may buckle due to reduced sag bend radius at seabed due to accumulated pipeline walking.

 

Source:

Lee, Jaeyoung. 2007. Introduction to Offshore Pipelines and Risers