Ultra-deepwater Applications Research Articles

Stress triaxiality as a postponer of the collapse pressure of thick-walled submarine pipes

One of the main concerns of pipeline design engineers for ultra-deepwater applications is to predict the pipes’ collapse capacity due to buckling caused by the hydrostatic pressure. DNV-ST-F101 is one of the most used guides for such systems, and presents the equations for the computation of the collapse pressure of submarine pipelines considering a limited diameter-to-thickness (D/t) range, not taking into account thick-walled structures. This work aims to investigate the accuracy of DNV-ST-F101 equations for submarine pipes with low D/t ratios. We show that, for these specific configurations, the analytical equations provided tend to underestimate the collapse capacity, which might result in the design of unnecessarily thick structures. Using finite element analyses we conclude that thick-walled pipes show a high level of stress triaxiality inside its wall at the moment of collapse, making it necessary a different analytical approach than that of DNV-ST-F101. Finite element simulations of four different metallic materials and five pipe configurations (D/t ratios) are performed to develop an alternative approach for the computation of the plastic collapse of pipes under hidrostatic pressure that takes into account the stress triaxiality at the moment of the pipes’ collapse. Our results improve the accuracy of the original DNV design equation for pipe diameter-to-thickness ratio within the range commonly found in the pre-salt offshore exploration.

Multi-perspective Data Modelling in Cyber Physical Production Networks: Data, Services and Actors

In recent years, Cyber Physical Production Systems and Digital Threads opened the vision on the importance of data modelling and management to lead the smart factory towards a full-fledged vertical and horizontal integration. Vertical integration refers to the full connection of smart factory levels from the work centers on the shop floor up to the business layer. Horizontal integration is realised when a single smart factory participates in multiple interleaved supply chains with different roles (e.g., main producer, supplier), sharing data and services and forming a Cyber Physical Production Network. In such an interconnected world, data and services become fundamental elements in the cyberspace to implement advanced data-driven applications such as production scheduling, energy consumption optimisation, anomaly detection, predictive maintenance, change management in Product Lifecycle Management, process monitoring and so forth. In this paper, we propose a methodology that guides the design of a portfolio of data-oriented services in a Cyber Physical Production Network. The methodology starts from the goals of the actors in the network, as well as their requirements on data and functions. Therefore, a data model is designed to represent the information shared across actors according to three interleaved perspectives, namely, product, process and industrial assets. Finally, multi-perspective data-oriented services for collecting, monitoring, dispatching and displaying data are built on top of the data model, according to the three perspectives. The methodology also includes a set of access policies for the actors in order to enable controlled access to data and services. The methodology is tested on a real case study for the production of valves in deep and ultra-deep water applications. Experimental validation in the real case study demonstrates the benefits of providing a methodological support for the design of multi-perspective data-oriented services in Cyber Physical Production Networks, both in terms of usability of the data navigation through the services and in terms of service performances in presence of Big Data.

Corrosion-Fatigue Crack Growth Performance of Titanium Grade 29 Welds in Tapered Stress Joints

Abstract Design of a steel catenary riser requires the use of connection hardware to decouple the large bending moments induced by the host floater at the hang-off location. Reliability of this connection hardware is essential, particularly in applications involving high pressure and high temperature fluids. One option for this connection hardware is the metallic tapered stress joint. Titanium (Ti) Grade 29 has been identified as an attractive material candidate for demanding stress joint applications due to its “high strength, low weight, superior fatigue performance and innate corrosion resistance”.2 Titanium stress joints for deepwater applications are typically not fabricated as a single piece due to titanium ingot volume limitations, thus making an intermediate girth weld necessary to satisfy length requirements. As with steel, the potential effect of hydrogen embrittlement induced by cathodic and galvanic potentials must be assessed to ensure long-term weld integrity. This paper describes testing from a joint industry project (JIP) conducted to qualify titanium stress joint (TSJ) welds for ultra-deepwater applications under harsh service and environmental conditions. Corrosion-fatigue crack growth rate (CFCGR) results for Ti Grade 29 flat welding-groove weld (1G/PA) gas tungsten arc welding (GTAW) specimens in seawater under cathodic potential and sour brine under galvanic potential are presented and compared to vendor recommended design curves.

Environmental testing for TRL 4 qualification of a novel subsea valve actuator for ultra-deep water applications

Environmental testing for TRL 4 qualification of a novel subsea valve actuator for ultra-deep water applications

Development of next generation subsea production system (NextGen SPS) design and analysis for ultra-deepwater applications

The present paper describes a new offshore field development solution, Next Generation Subsea Production System (NextGen SPS), that aims to overcome the technical and commercial limitations of the current offshore field development concepts (dry tree or subsea tree) in ultra-deep water (more than 1500 m). The key developments of the NextGen SPS, including its main characteristics, stability characteristics and optimal design on the riser system, are presented and discussed. The series of studies demonstrates that the NextGen SPS offers improved technical and commercial performance, higher levels of safety, reduced interface complexity and improved development flexibility for field development in ultra-deep water.

Dynamic response of offshore triceratops with elliptical buoyant legs

Oil and gas exploration in ultra-deepwaters necessitates innovative geometric forms for offshore steel structures. Offshore triceratops is one of a new steel compliant structures found suitable for ultra-deepwater applications. It consists of three buoyant legs attached to the deck by ball joints, which partially isolate the deck from the buoyant legs and constitute the novelty. The present study discusses a detailed numerical analysis of triceratops with buoyant legs of elliptical cross section. The focus is to assess the hydrodynamic diffraction characteristics of the platform caused by the change in cross section of buoyant legs from a conventional tubular one to the elliptical ones. Three elliptical sections with varying eccentricities are considered in the dynamic analyses to assess the response behavior of triceratops under regular waves. The results showed an increase in the total force in the buoyant legs in sway degree of freedom with the increase in the eccentricity of the cross section. Besides, reduced transverse vibrations and increased stability are also observed in both the deck and elliptical buoyant legs. Based on the numerical results, it is recommended that elliptical buoyant legs with eccentricity close to 2 are highly suitable for offshore triceratops in ultra-deepwaters.

Response of triceratops to impact forces: numerical investigations

Triceratops is one of the new generations of offshore compliant platforms suitable for ultra-deepwater applications. Apart from environmental loads, the offshore structures are also susceptible to accidental loads. Due to the increase in the risk of collision between ships and offshore platforms, the accurate prediction of structural response under impact loads becomes necessary. This paper presents the numerical investigations of the impact response of the buoyant leg of triceratops usually designed as an orthogonally stiffened cylindrical shell with stringers and ring frames. The impact analysis of buoyant leg with a rectangularly shaped indenter is carried out using ANSYS explicit analysis solver under different impact load cases. The results show that the shell deformation increases with the increase in impact load, and the ring stiffeners hinder the shell damage from spreading in the longitudinal direction. The response of triceratops is then obtained through hydrodynamic response analysis carried out using ANSYS AQWA. From the results, it is observed that the impact load on single buoyant leg causes periodic vibration in the deck in the surge and pitch degrees of freedom. Since the impact response of the structure is highly affected by the geometric and material properties, numerical studies are also carried out by varying the strain rate, and the location of the indenter and the results are discussed.

Offshore Triceratops Under Impact Forces in Ultra Deep Arctic Waters

In the recent years, offshore oil drilling and production is moving towards ultra-deep Arctic region which demands an adaptable structural form. Apart from the environmental loads, offshore structures in Arctic region will also be subjected to impact forces arising due to ship platform collision. Such loads may endanger the safety of the platform due to the combined effect of reduced temperature and impact forces on the material and geometric properties of the structure. Thus, there is a need to understand the behaviour of offshore structures under impact forces in low-temperature conditions. Offshore Triceratops is one of the recent new-generation compliant platforms proved to be suitable for ultra-deepwater applications. The main aim of this study is to assess the response of triceratops under impact forces in Arctic environment numerically. As the buoyant legs of triceratops are susceptible to impact forces arising from ship platform collision, the numerical model of a buoyant leg is developed using Ansys explicit dynamics solver. The impact analyses is then carried out with rectangular box-shaped indenter representing the stem of a ship, under both ambient conditions and Arctic temperature (− 60 °C) and the local response of the platform is studied through force deformation curves and stress contours. In order to study the global response of the platform, the numerical model of triceratops is developed in Ansys Aqwa solver and analysed under the action of impact load time history obtained from explicit analysis of buoyant leg. The impact load on the buoyant leg resulted in the continuous periodic vibration of the platform. Furthermore, parametric studies were also carried out to investigate the effect of indenter velocity, size, and location on the impact response of triceratops under Arctic temperature, and the results are discussed.

Collapse analyses of sandwich pipes under external pressure considering inter-layer adhesion behaviour

Sandwich pipes composed of two relatively thin concentric steel pipes and a thick and flexible core in the annulus, are viewed as a significant potential for deepwater and ultra-deepwater applications in oil and gas transportation because they can simultaneously meet mechanical and thermal requirements. This paper presents a collapse capacity prediction of sandwich pipes with various inter-layer adhesion behaviour under external pressure. The solid polypropylene with favourable thermal insulation capacity and high compressive strength is used as the core layer material. The stress-strain curves of the polypropylene material are measured through the uniaxial tension and compression tests. The tests of simple shear specimen and sandwich pipe section specimen are conducted to evaluate the stick-slip levels of epoxy resin and 3M-DP8005 adhesives in two surface conditions of steel pipe, respectively. Then, the dedicated finite element model is developed and an extensive parametric study is conducted to explore the influences of geometric configuration, initial imperfection, material property, and inter-layer adhesion behaviour on the pressure capacity and deformability of sandwich pipes. It is observed that the inter-layer adhesion behaviour has a strong influence on the collapse capacity of sandwich pipes.

Numerical investigation of friction joint between Basalt Fiber Reinforced Composites and aluminum

Flexible risers are used in the offshore oil industry for exporting hydrocarbons from subsea equipment to floating production and storage vessels. The latest research in unbonded flexible pipes aims to reduce weight by replacing metal components with composite materials. This would result in lighter and stiffer flexible risers, which would be well suited for ultra deep water applications. This paper develops a new finite element model used for evaluating the efficiency of anchoring flat unidirectional fiber reinforced tendons in a mechanical grip. It consists two flat grips with the fiber reinforced tendon in between. The grips are pressed against the composite and the pullout force is ensured through friction. The novelty of the paper is represented by the detailed investigation of the influence between the coefficient of friction and the pullout force. By comparing numerical and experimentally obtained results, it is possible to show the importance of friction decay in the grip. Improper contact between the grips and composite is also taken into account and leads to good agreement between numerical and experimental results. This study shows how to avoid over-estimating the efficiency of such grip by using dry friction in finite element models.

Riser-System Design in Water Depths Greater Than 3000 m

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 25840, “Frontier Deepwater Developments: The Impact on Riser-System Design in Water Depths Greater Than 3000 m,” by N. Saglar, B. Toleman, and R. Thethi, 2H Offshore, prepared for the 2015 Offshore Technology Conference, Houston, 4–7 May. The paper has not been peer reviewed. The offshore industry anticipates the need for production-riser systems in ultradeepwater fields. The development of these fields [this paper considers a field located in the central Gulf of Mexico (GOM)] leads to many challenges with respect to the selection of the riser concept; in some instances, such applications may require extending riser technology beyond its current limits. This paper evaluates the feasibility of a number of production-and export-riser configurations for ultradeepwater applications. Riser-Wall-Thickness Sizing Please note that riser-design criteria, methodology, and data (riser functions and associated pipe sizes; riser internal-fluid properties; and riser-strength assessment) are provided in the complete paper. Production Risers. Steel-catenary-riser (SCR) wall-thickness sizing is initially carried out when considering X65 line pipe. For a design pressure of 5 ksi, the wall thickness of the production riser is primarily driven by collapse because of external hydrostatic pressure. The maximum wall thicknesses required for 8-, 10-, and 12-in. pipes are 1.51, 1.85, and 2.17 in., respectively, and are driven by burst owing to the 15-ksi internal design pressure. It should be noted that these wall thicknesses are designed to resist only the burst and collapse pressures. The longitudinal-load and combined-load design checks are performed after the sizing is performed for collapse and burst. For ultradeep water, the longitudinal-load design criterion becomes a limiting requirement. For 3000-m water depth, the production risers meet both the longitudinal-load and combined-load design criteria. Buckling caused by combined bending and external pressure is also checked with the calculated wall thicknesses. The allowable bending strains of all the production risers are determined to be greater than the assumed maximum bending strain of 0.5%. The field-proven thickest wall to date for conventionally welded X65 single-pipe wet-tree production-riser systems is 1.65 in. In this paper, the maximum wall thickness that can be welded for offshore fatigue-sensitive riser systems is assumed to be 1.9 in., on the basis of existing tests. Only the wall thickness required for a 12-in. production riser with a design pressure of 15 ksi exceeds the 1.9-in. limit. Export Risers. SCR wall-thickness sizing is initially carried out considering X65 line pipe with a design pressure of 5,000 psi. The wall thickness of the export riser is first calculated to resist the burst and collapse pressures. It is determined that the wall thickness required for collapse is higher than the sizing based on burst for all water depths and outer diameters. However, the wall thickness driven by collapse does not meet the 0.5% allowable bending strain considering the combined bending and external-pressure criterion. The wall thicknesses of the export risers are increased to maintain the allowable bending strain of 0.5% in the pipe. Therefore, the wall thickness of the export risers is driven primarily by buckling because of the combined bending and external-pressure criterion. The wall thicknesses of the export risers are shown in Fig. 1. The maximum wall thicknesses required for 16-, 20-, and 24-in. pipes are 1.55, 1.95, and 2.32 in., respectively, and occur at a water depth of 4500 m. When considering the longitudinal load from the static tension in the flooded condition at the top of the riser in water depths of at least 3750 m, the effective tension exceeds the 60%-of-yield tension capacity. For ultradeep water, the longitudinal-load design criterion becomes the limiting requirement.

Design and Performance Testing of a Subsea Compact Separation System for Deepwater Applications

ExxonMobil Upstream Research Company recently completed a technology development and qualification program, which included performance tests on an integrated subsea compact separation system for ultradeepwater applications.

Numerical solutions for nonlinear large deformation behaviour of deepwater steel lazy-wave riser

To meet the increasing challenges of simple steel catenary riser on large hang-off tension and fatigue damage of the touchdown point (TDP) in deepwater or ultra deepwater applications, the steel lazy-wave risers (SLWR) have gained a viable solution. This paper is focused on the static equilibrium of SLWR which is of great importance to the basic configuration design of the riser. A more accurate model based on nonlinear large deformation beam theory is proposed to simulate the suspended part of SLWR, having evident advantage over previously applied small deformation beam or catenary theory which ignores the riser's bending stiffness and is hard to simulate the rapid change of the inclination angle. The model also deals with the pipe–soil interaction for simulating the TDP. The finite difference method is applied to obtain the numerical solutions. The results are comparatively in agreement with OrcaFlex. As an application, parametric analysis of the SLWR is carried out to investigate the influences of arc length of buoyancy section, arc length of upper section and soil stiffness on the static performance. The proposed model can be a basic reference for the dynamic analysis of the deepwater SLWR.

Bending capacity of sandwich pipes

A sandwich pipe (SP) is an effective design alternative, providing effective load carrying capacity and thermal insulation to pipelines, especially when the pipe is going to be utilized in deep and ultra-deep water applications. However, the design and development of a reliable SP requires an in-depth understanding of the behavior of such a system under various loading conditions. In this paper, the behavior of SPs subject to pure bending, which is one of the governing loading conditions for offshore pipelines, is investigated.In order to perform this investigation, a series of numerical parametric models, using the finite element (FE) method, was developed. The linear eigenvalue buckling analysis and the nonlinear post-buckling analysis were conducted to explore the systems' response. The influence of several significant structural parameters on the pre-buckling, buckling and post-buckling responses of SPs was investigated. The parameters investigated included various combinations of several geometrical and material properties, as well as the consideration of various possible intra-layer adhesion mechanisms. Moreover, the recent high strength steels used in formation of oil and pipes exhibit certain degree of yield anisotropy; it is therefore crucial to understand the effect of material's yield anisotropy on such system's response. This issue has also been considered in this research.

Strength Analyses of Sandwich Pipes for Ultra Deepwaters

Design requirements for pipelines regarding both ultimate strength and flow assurance in ultra deepwater scenarios motivated the development of a new sandwich pipe which is able to combine high structural and thermal insulation properties. In this concept, the annulus is filled with low cost materials with adequate thermal insulation properties and good mechanical resistance. The aim of this research work is to perform small-scale laboratorial tests and to develop a finite element model to evaluate the structural performance of such sandwich pipes with two different options of core material. After calibrated in view of the experimental results, a three-dimensional finite element model incorporating nonlinear geometric and material behavior is employed to perform strength analyses of sandwich pipes under combined external pressure and longitudinal bending. Ultimate strength envelopes for sandwich pipes are compared with those generated for single-wall steel pipes with equivalent collapse pressures. The study shows that sandwich pipe systems with either cement or polypropylene cores are feasible options for ultra deepwater applications.