Single independent risers offer a simple, reliable, and cost-effective system

With the booming deep and ultra-deepwater field developments, the need for innovative riser technology to cover this market has increased very quickly. Saipem has worked to develop new concepts specifically adapted to operators’ needs.

The Single Hybrid Riser (SHR) is a field-proven riser concept for deepwater fields and consists of a vertical, rigid steel riser that is tensioned at the top through a buoyancy tank via a tethered connection, and anchored at the riser base with a pile foundation (Fig. 1). However, technical challenges appear as the water depth increases, and conventional SHR solutions are not suitable because of the size of the required buoyancy tank.

SAIPEM developed a new type of deep and ultra-deepwater riser concept called Single Independent Riser (SIR) that addresses drawbacks of the SHR concept. This SIR concept is adaptable to any floater, even in harsh environments.

Along with these solutions, Saipem has also developed an innovative buoyancy and insulating material called “Glass Bubble Gum” (GBG). It offers good insulating properties as well as low density and can be easily manufactured locally in large quantities.

The SIR is a hybrid riser with an almost-vertical tensioned rigid steel line and a steep wave flexible jumper termination. The SIR is a kind of steep wave configuration where all the buoyancy is either distributed or continuous, thus removing the need for a buoyancy tank.

In addition, the SIR concept makes use of field-proven technology, leading to a simple, highly reliable, and cost-effective system. There is no water depth limitation to the SIR application. Each section is self-compensating, meaning that increasing the water depth only requires increasing the number of sections.

The dynamic response of the SIR is very low, as the flexible section of the riser decouples vessel motions from the SIR. Compared with an SHR, the SIR is very compliant and easily accommodates imposed displacements. This is due to the fact that the tension within an SIR decreases upward from the bottom, while tension increases upward in an SHR.

When comparing the stress RAOs of the SIR to the ones of an SHR at their most critical location, it appears that stresses within the SIR are significantly lower than the ones in an SHR.

[IMAGE: Saipem 3, graph and Saipem 4, graph, CAPTIONS: Saipem 3: Tension along SIR and SHR for a 1000m and 1700m project. Saipem 4: SHR [red] and SIR [blue] stress RAOs at most designing location.]

The SIR was initially designed using distributed buoyancy modules. However, using the modules has two drawbacks: they are difficult to produce locally, and raw materials are quite expensive. Therefore, Saipem focused on developing a material to address these two issues.

The company is developing an insulation and buoyancy material that could be easily produced in large quantities and fitted on complex structures, such as bundles. This material is composed of a filler and glass bubbles trapped into a polymer matrix. The filler has very low thermal conductivity, while the glass bubbles decrease the density of the mix, making it a very light and a good insulator. The ratio of the components can be adjusted so that the density is as low as possible while the viscosity remains low.

The material is created in two steps:  First, mix a monomer, solvent, and glass bubbles; Second, add a polymerization agent to trigger the formation of the matrix, trapping both glass bubbles and solvent.

GBG main characteristics

A wide qualification campaign is currently underway. This test campaign comprises mechanical tests (density; modulus of elasticity; shear modulus; Poisson coefficient; allowable stress); thermal tests (thermal conductivity; heat capacity; expansion coefficient; potential life); and aging tests.

So far mechanical, thermal and potential life tests have been completed. Aging tests are currently on-going and are expected to be completed this year. The measured properties of the GBG are presented in the table below.

Onsite

Local content is a growing requirement for operators, especially in Brazil and West Africa. The fabrication process for the SIR using GBG allows it to be assembled onshore in a local yard as illustrated below.

The steps to fabricate the SIR include: welding the joints together; setting multiple joints in-line with previously fabricated sections and welding together; setting spacers in place and skin with it injection ends; pull forward pipe; inject GBG into the annulus between the pipe and skin. The injection zone features a slight slope to ensure that GBG properly spreads around the pipe.

Note that using GBG with conventional fabrication methods (J-laying for instance) is still possible.

The SIR can be installed using a standard towing technique. The tow can be performed with or without the flexible jumper in place.

Analyses on the up-ending procedures have also been performed. A clump weight is attached to the bottom of the SIR to compensate for its buoyancy. Winch cables are used to control the up ending. An illustration of the various steps of the up-ending is given below.

If the FPSO is not yet in place, it is possible to use a stand-by buoy for either temporary storage of jumpers.

In case the jumper is installed after the up-ending, a specifically developed and qualified connector will be used.

The SIR proposes an evolution of SHR and pushes forward the water depth limit. This is made possible by simplifying the structures (no top nor bottom assembly) and proposing a solution that is almost insensitive to water depth. The analyses that were performed on actual case studies have so far proved the SIR to be a solution well-adapted to the industry needs.

The GBG is an enabling technology allowing for cheap buoyancy and insulation in large quantities and complex structures such as bundles. The simplicity of the industrial process associated to this material makes it particularly fit for local production.

The combination of the two technologies allows for maximizing the local content. OE