Hybrid Risers

Hybrid risers (HRs) consist of a vertical bundle of steel pipes supported by external buoyancy. Flexible jumpers connecting the top of the riser and the vessel are used to accommodate relative motion between the vessel and riser bundle. The first use of hybrid risers was a deepwater production riser installed in 1988 in the Green Canyon Block 29 (GC29) by Cameron (Fisher and Berner).

In this subsection, we present the current hybrid riser concept including the following main components:

• Riser base foundation;
• Riser base spools;
• Top and bottom transition fogging;
• Riser cross section;
• Buoyancy tank;
  File:Hybrid Riser Towers at the Girassol Field.png

  Hybrid Riser Towers at the Girassol Field
• Flexible jumper between the top of the riser and the hang-off point in the FPS.

Riser Foundation

The foundation structure for the HR consists of a steel suction anchor (and/or a gravity base) that will support the HR installation aids (such as pulling winches) and the flex element (a roto-latch connector is proposed). Two riser base foundation approaches are possible:

• Pinned riser base: This type of riser base allows free riser rotation.
• Fixed riser base: This type of riser base resists riser rotation.

Riser Base Spools

The riser base spool is designed to allow for relative displacements between the HR base and the flowline that may be caused by the thermal expansion of the flowline or/and angular movements (due to environmental loading) at the HR base. For the common use of a pinned riser base, the receptacle for the flex element is inclined to optimize the design of the bundle connection to its base. At the bottom of the hybrid riser, a forged Y-shaped tee allows:

• Vertical sections to be connected to the flex element;
• Lateral branch to be ended by a CVC to the rigid jumper spool.

Top and Bottom Transition Forging

Transition forging on the bottom of the riser connects the standard riser to the flowline spool. A short thickened-wall section of pipe (adapter joint), typically 3 m long, provides a transition between the standard riser pipe and the large stiffness of the riser top assembly. A flanged connection mates with the flow spool that connects the flexible jumper and the standard riser.

Riser Cross Section

The two most important factors for describing the riser cross section are the wall thickness of the riser pipe and corrosion coatings and anodes.

Buoyancy Tank

A steel structured buoyancy tank is designed based on the following functional requirements:

• To thrust the HR dead weight.
• To provide sufficient pulling tension for the dynamic equilibrium of the HR.
• To limit maximum angular deflection of the HR with regards to its static position.

The riser is tensioned by means of an air or nitrogen-filled buoyancy tank. The tank contains a number of individual compartments separated by bulkheads. The bulkheads contain stiffeners arranged on the underside of each bulkhead plate to provide additional reinforcement. The buoyancy tank is designed to be pressure balanced with the external pressure of the water; this allows the thickness of the buoyancy tank skin to be limited to a minimal wall thickness.

Flexible Jumpers and Hook Up to Vessel

Flexible jumpers connecting the top of the riser and the vessel are used to accommodate relative motion between the vessel and riser bundle. Flexible pipe jumpers are used to connect goosenecks, located immediately below the air can and the vessel.

Sizing of Hybrid Risers

The key design issues for the arrangement/sizing of the hybrid riser include:

• Overall arrangement: footprints at bases, hang-off locations/spacing;
• General arrangement:
• Riser thickness determination;
• Riser top: jumper with the characteristics and geometry of end fittings, geometry for the buoyancy tank;
• Riser base: forging and foundation;
• Sizing of buoyancy tank and suction anchor;
• Sizing of rigid jumper spool (at hybrid riser base) connected to the gas export pipeline via collect connectors.
  File:Gooseneck Assembly.png

  Gooseneck Assembly

The riser system is sized so that:

• Tethering tension is equivalent to what can be obtained from available equipment.
• Distributed buoyancy is equivalent to what is needed for neutral buoyancy in production mode.
• The upper air can provides enough buoyancy to give nominal overpull at the riser base.
• Flexible jumper hoses have sufficient length to accommodate relative vertical movement between the riser and platform at maximum drift-off position.

Sizing of Flexible Jumpers

Flowline jumpers are used to connect the riser base with the flowline base. The flowline jumpers are conventional subsea technology requiring five diameter bends for pigging.

File:Lifting of a Flexible Jumper from a Barge.png

Lifting of a Flexible Jumper from a Barge

Preliminary Analysis

The key objective of the preliminary analysis is to get an indication of the requirements for:

• Buoyancy and tension distribution along the riser;
• VIV suppression devices.

Preliminary analysis is performed in two steps:

1. Static analysis to determine response to currents and vessel displacement conditions;

2. Time-domain regular wave analysis to determine response to timevarying loading.

Strength Analysis

The strength analysis should be conducted to optimize the following items for a range of possible loading conditions:

• Distributed buoyancy requirements;
• Top tension;
• Tethering tension (if any);
• Foundation loading.

The loading conditions include:

• Extreme waves;
• Extreme currents;
• Extreme vessel drift;
• Winds and currents from opposing directions.

Fatigue Analysis

Like any other riser analysis, a fatigue analysis for a hybrid riser should include vessel drift motions, first-order wave action, and VIV and installation-induced fatigue. VIV analysis of hybrid risers may consider current profiles of varying levels, up to and including 100-year return currents. The total VIV fatigue damage can then be calculated using the damage from each profile and the associated percentage occurrence.

Riser Hydrostatic Pressure Test

A pressure test must be carried out before any riser commences operation. The filling, cleaning, gauging, batching, logging, dewatering, and hydrostatic pressure testing operations of the riser system should be performed in accordance with the requirements of DNV OS F101 and other relevant codes.

The sequence of pressure testing operations for a riser system is as follows:

• Filling;
• Cleaning and gauging;
• Hydrotesting, including temperature stabilization, pressurization, air contents check, and hydrostatic test/holding period;
• Post-testing, including depressurization and documentation;
• Rectification activities (if required), including leak location during test, dewatering for rectification, and rectification of defects;
• Final/repeat hydrotesting;
• Testing, certificates, and witnessed signature.

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