Comparative strength and stability analysis of conventional and lighter composite flexible risers in ultra-deep water subsea environment

Abstract

The hybrid flexible risers have a multi-layered structure and use thermoplastic composite for the pressure and tensile armour. In contrast, a conventional flexible riser uses heavier carbon steel as armour which significantly contributes to its weight. For shallow-water applications, the conventional risers are widely used in offshore oil and gas industry due to their corrosion resistance properties and low transportation costs. However, the weight of conventional risers is a key limitation in ultra-deep-water applications. This shortcoming can be addressed by including a lightweight carbon fibre reinforced polymer (CFRP) composite as one of the individual layers. The use of CFRP reduces the effective tension at the hang off point which is a key limitation in extending the range of flexible risers. Here, the dynamic stability and functional load interactions of both risers (viz: a thermoplastic CFRP riser and a conventional flexible riser) at a water depth of 3000 m were studied. A global analysis was performed considering the onerous 1000-year hurricane wave with 100-year currents. The investigation considered ±150 m vessel offsets, three vessel headings (viz: 135, 180, 225°) and three vessel draughts (ballasted, empty, loaded). Additionally, a numerical model with a variable bending stiffness was used to capture the orthotropic material behaviour of a flexible riser. Results showed that the buoyancy requirement and effective tension were 2.1 times greater and 2% higher for the conventional riser compared to its composite counterpart. The most onerous case for a conventional riser was at zero offset whereas for its composite counterpart was at –150 m along the length of a riser. It was observed that the heavier masses of a conventional riser aid in aligning the weight vector with the upward direction of the buoyancy force. Contrarily, the composite risers undergo large displacements leading to misalignments and instability. Furthermore, the observed bending radius of the flexible riser was found to be within the allowable minimum bend radius at the hog bend location.

The Effect of Ice Floe on the Strength, Stability, and Fatigue of Hybrid Flexible Risers in the Arctic Sea

“…The smeared buoyancy was used throughout the analysis [31]. All dimensions and material information were derived from previous work [4].…”

“…The loxodromic and geodesic curve analysis established the nonlinear curvature-bending moment relation [32,33]. For this nonlinear bending moment, curvature relations were captured from Equations ( 1) and (2) based on the frictionlimited slippage between each layer [4]. The critical curvature represents the curvature radius of the flexible riser when the helical strip of the tensile layer starts slipping to reduce the effective overall bending stiffness.…”

“…The CFRP bonded pipe (pressure armour) was modelled considering 40 layers of CFRP layers and 1 thick layer of the PA12 polymer layer [4,37]. The thin wear protecting layers (polymer sheaths) were not included in the model (Figure 2b).…”

“…The materials and dimensions of each layer used in such a hybrid flexible riser have not been widely published due to the limited availability of operational data. However, many experimental and numerical studies of such risers have been published recently [4]. The strength and stability of hybrid risers in arctic conditions have not been previously studied.…”

“…The hybrid riser system, incorporating the CFRP composite layer serving as a pressure armour, is instrumental in the conveyance of fluids from the subsea manifold to the vessel located in a deep-water setup [4]. This innovative system, if employed as a key component of floating offshore facilities, can be utilised in deep waters such as the Arctic Sea due to its lightweight per metre length [5][6][7][8].…”