As oil exploration and production moves into deeper waters, riser designers have to face new challenges. In partic-ular, subsea production in ultra-deep-water means:
- Increased installation difficulties
- Higher payload due to longer riser
- Soft soil creates difficulties at the riser/soil interface.
- Flow assurance is of great concern.
This last point presents a major risk of hydrate formation and wax deposition, and demands a high level of thermal insulation, with heat transfer coefficients in the region of 1 W/sq m/K.
A simple and cost effective approach to provide a connection between a subsea well and the surface is the use of steel catenary risers (SCR), which are made of standard steel pipe and can be installed on site with a S-lay or J-lay barge. Unfort-unately, the SCR has a number of drawbacks:
- To ensure the required thermal insulation, the SCR needs a thick coating (for instance polyurethane). This increases the outside diameter and consequently the lateral excursions and stresses when subject to current and waves.
- Should several risers be installed in close proximity, risks of overlap and damage are high.
- Deepening water depth increases the SCR length and its resulting load on the FPSO. The FPSO hull has to be designed to provide enough buoyancy to compensate for this increased payload. In addition, if all the catenary risers are attached to one side of the FPSO, they induce a horizontal load that has to be balanced by additional mooring lines.
- The soil in deepwater seabeds is generally low strength and cohesive, so riser self-trenching is a key issue that must be carefully evaluated with respect to riser storm response and fatigue behavior.
An alternative to the SCR is the laze wave steel catenary riser, which aims to reduce the riser movement on the seabed. However, this concept requires expensive buoyancy modules and the system becomes more sensitive to transverse current.
Flexible pipe has been considered for deepwater, but suffers from a high cost due to its complex structure. It also has limited collapse and tension capacity, and relatively poor thermal performance and hydrogen sulfide resistance.
By selecting "hybrid" tower risers for the Girassol project, a major step forward was made in that it provides the following:
- A flexible section, linking the FPSO to the top of the submerged tower about 50 meters below the surface.
- A rigid section (the tower itself).
The tower contains production risers, gas lift and water injection lines shrouded by the insulating foam. The top of the riser tower is a large capacity buoyancy can which provides the necessary tension to keep the riser upright and compliant with the FPSO movement. Such a design brings the following advantages:
- The tower eliminates the complex riser/soil interaction (a drawback of catenary risers).
- The flexible jumpers accommodate large dynamic relative motion between the FPSO and the tower.
Girassol riser towers are the second generation of hybrid riser towers. The Placid Green Canyon riser was the first. Bouygues SHR'ewd concept is the next generation of hybrid riser tower.
The hydrid riser consists of a steel vertical tensioned riser (VTR) anchored to the seabed and a free hanging steel catenary riser (CR) between the FPSO and top of the VTR.
The hydrid riser consists of a steel vertical tensioned riser (VTR) anchored to the seabed and a free hanging steel catenary riser (CR) between the FPSO and top of the VTR. This combination has the benefit of a low stress level in the CR sagbend, thus alleviating any potential stress or fatigue problems. Consequently, a lower grade steel pipe material may be used.
Stresses at the terminations and junctions of CR and VTR are reduced by the use of flexpoints, which have been used in deepwater risers. Two flexpoints are installed on the CR, one at each extremity. A third one provides a hinge at the seabed end of the VTR. The mid-depth CR flexpoint is integral with a gooseneck spool piece equipped with a male stab for automatic self-locking subsea connection to the VTR top.
Design of the hydrid riser is a simple sequential routine, without a long and interactive loop process. Pipelines are primarily and independently designed to meet client specifications (pressure, temperature, and insulation needs) and in accordance with riser and pipeline codes. The design temperature and insulation needs allow the use of pipe sections, such as single line or pipe-in-pipe (PIP).
Then, the main riser design parameters are installation driven. For cost efficiency, other solutions could have been used depending on the required heat transfer coefficient for the riser.
The combination of efficient pipe-in-pipe for the VTR and a more standard polyurethane insulation coating for the CR is a typical cost optimization to be evaluated on a project by project basis.
As PIP produces a higher linear pipe weight, it has been considered that proving the feasibility of the hybrid riser with a full PIP design would cover the other cases where less bouyancy would be needed.
Verification of the hybrid riser's dynamic response has been carried out by 2H Offshore Engineering. The design was optimized for: a typical West African environment, a 1,350 meter water depth, and a 100 year design wave spectrum with Hrnax = 7m and mean period of Tm = 12 s (beam sea in line with CR).
The surface vessel to which the hybrid riser is to be attached is a multifunction barge developed by Bouygues. The hybrid riser combines the drilling, production, process, storage, and offloading functions. The multifunction barge is a spread moored vessel. The offsets are in the range of +1-3% to +/-5% of the water depth (intact and damaged mooring line). The hybrid riser was designed to meet the following specifications:
- Suspended CR length - 98% of water depth (maximum)
- Von Mises stresses, according to API-RP-2RD.
- Flexpoints dynamic angle not to exceed +20 degrees and the middle flexjoint angle range not to encroach a 30 degree sector from the tower axis (no interference between the CR and the VTR).
- CR hangoff load at vessel < 200 tons.
The J-lay method was selected by Bouygues as the most suitable installation scheme for deepwater developments (minimal pipe preparation, minimal of residual stresses in pipelines). All elements are designed to render an installation procedure even more efficient. The installation principle is to have the CR free-hanging from the FPSO and the VTR freestanding on the seabed. Thus, the CR is designed to be slightly shorter than the water depth, which allows an easy transfer and hangoff of the CR to the FPSO.
The VTR length is then calculated to minimize the sagbend stresses in the CR, which defines the elevation of the top of the VTR around mid-water depth where currents and waves effects are small. Thus, the VTR is subjected to small current loads and therefore, VIV (vortex-induced vibration) suppression devices should not be necessary.
The VTR is tensioned by buoyancy elements made of syntactic foam and several ballast vessels. In the base case described in this paper (single riser), two buoyancy elements are used. Each is composed of a main cylinder made of syntactic foam, and surrounded by several smaller steel cylinders, which can be ballasted and connected or disconnected by ROV.
This design has been selected to ease the installation procedure. In addition, this buoyancy design allows several independent compartments that enable it to remain in damaged condition (the VTR remains in an acceptable configuration even when one or two cylinders are flooded).
These two buoyancy cans are connected by swivel joints or chains to a steel tubular frame, so that no moments are transmitted from the buoys to the frame. As an alternative to the base case, the bouyancy can be designed with a single tank surrounding the VTR top end. The two main functions of the steel tubular frame are to provide:
- Mechanical support to the VTR to transfer the necessary tension
- Hang-off point to the CR lower flexjoint.
The bouyancy up-thrust is transmitted from the frame to the VTR through an anchor flange and a guiding sleeve. Located at the VTR top is a female half of a subsea connector for the connection of CR gooseneck spool piece. Under the frame anchor flange, the VTR is fitted with a small stress joint, which is needed to withstand local stresses due the CR dynamics.
Lastly, the bottom flexjoint of the VTR is locked in an angled receptacle on the foundation (which could be, for instance, a suction-embedded pile or a gravity-based template). Then, the link to the flowlines is provided by a spool piece fitted with a standard mechanical subsea connector, which allows thermal expansion.
The VIV response of the hybrid riser is conducted using a simplified model extending from the top to bottom of the CR. Eleven current profiles are considered to provide a suitable representation of the long term current flow. No VIV suppression devices are used for the analysis.
Assuming the service life of the riser is 20 years, the fatigue life requirements will be 200 years, assuming no inspection is carried out. The results show that the minimum fatigue lives near the top and bottom of the CR are in excess of 6,000 and 750 years, respectively, which satisfy the design requirements. From the results, VIV suppression devices does not appear to be needed.
First order fatigue
First order fatigue analysis was carried out using the optimized riser configuration for head and beam sea loading conditions. Low fatigue lives are observed near the top CR region, sagbend and bottom CR, but the computed fatigue lives are in excess of 200 years with a minimum of 2,400 years observed near the top of the CR.