Offshore exploration for oil and gas is being performed in even more challenging waters, with fields being developed in water depths of 2000 meters and greater. To recover hydrocarbons from these depths, a number of technical challenges are presented to the designers of riser and offloading systems. Metocean characteristics and relatively low reservoir temperatures compound the challenges. This paper discusses how unbonded flexible pipe technology has overcome the challenges of floating production systems in 2000 meters water depth and how its continued development enables solutions for ultra deep water riser systems. During the discussion of the status of flexible pipe technology, an overview is given to the advancements in flow assurance technology and integrity monitoring, both of which can evolve as integral parts of the flexible pipe. These achievements in technology are applied to the design of large diameter offloading and export systems that are being considered for large field developments where crude oil is being offloaded to other floating structures and tankers. The paper details the design and detailed analysis of the unbonded flexible pipe solutions developed by Wellstream to overcome the deepwater challenges associated with large diameter export riser systems. Design comparisons are developed with the alternative products currently proposed. Finally, the paper concludes with an economic appraisal of why flexible pipe systems should be considered a cost effective solution in the overall CAPEX and OPEX of deepwater fielddevelopments. Introduction The exploration of oil and gas offshore is accelerating to even greater depths with each new discovery. Deepwater drilling in the US GOM reached a record high[1] in the final week of 2000. A total of 40 rigs were drilling in the region compared to 26 a year previous. Activity in the ultra-deep is also growing with seven rigs working at the end of the year. The deepest well was drilled by BHP Petroleum at a water depth of 2,545 meters. The recovery of hydrocarbons at such depths presents challenges to the designers and installers of riser and off-loading systems. Since unbonded flexible risers are an enabling solution for floating production and storage systems, being a proven technology spanning three decades, these challenges have already been overcome for the smaller diameter pipe structures in water depths up to 2000 meters. Continued developments will have much larger internal diameters qualified to these water depths in the near future. Unbonded flexible pipe is a multi-layer structure of helically wound metallic wires, tapes and extruded thermoplastics. Each layer has a unique role in the flexible pipe. Refer to Figure 1. The inner metallic layer, which is interlocked, provides the collapse resistance. An extruded thermoplastic layer provides the seal for the fluid to be transmitted through the pipe. An interlocked steel layer wrapped over this thermoplastic layer provides resistance to internal pressure and radial compression. A dual layer of counter-wound tensile armors provides the tensile strength to the pipe structure.
Pipelines in the service of conveying hot fluid will tend to expand due to pressure and differential temperature. However, since the flowline is generally fixed at the end terminations to rigid structures or equipment, such an expansion will be restricted in longitudinal direction. This is particularly the case for the section remote from the pipe ends, and results in an axial compression in the pipe section. In many cases, a subsea flowline has to be trenched or buried for the purposes of protection and thermal insulation. Consequently, the lateral movement of a flexible flowline is greatly limited, and an upward displacement is encouraged that may become excessive. Eventually, the flowline may lift out of the trench when the uplift resistance provided by the backfill cover and self-weight of the flowline is gradually overcome by the strain energy built up in the flowline. For flexible pipe, it is this excessive upward deformation being termed as the Upheaval Buckling, which can be prevented by employing adequate downward restraint, such as sand bag/rock dump or by designing a subsea pipe route to overcome this phenomenon. In this paper a case study of the full three-dimensional finite element analysis of a trenched but unburied 6.0-inch production flowline is presented following a description of Wellstream Finite Element Method (FEM) based methodology for Upheaval Buckling analysis of flexible pipes. The effect Bending Stiffness Hysteresis and Upheaval Creep–unique to flexible pipe characteristics, is considered in addition to the general loads such as the flowline self-weight and backfill, pretension, pressure, temperature distribution and prescribed forces (either concentrated or distributed) and displacements. The effects of environmental loads, such as the action of currents that would result in scouring off the backfill, can also be addressed. The finite element analysis program package ANSYS was chosen for this case study due to its special feature of ANSYS Parametric Design Language (APDL) and contact/target elements; and the general three-dimensional shell and solid elements were used to represent the flexible pipe and trench soil respectively.
Unbonded flexible pipe has a proven track record in the offshore oil and gas industry for more than 20 years. The product is synonymous with the use of floating production systems spanning the water column and connecting subsea structures to facilitate the retrieval of hydrocarbons, provision of water injection systems and the export of processed or semi-processed fluids to main trunk pipelines or onshore. Unbonded Flexible pipe is a technically complex multi-layer structure of helically wound metallic wires and tapes and extruded thermoplastics. In 1996 Wellstream was awarded a major contract for the supply of flexible risers and flowlines as part of the Norsk Hydro Troll Olje Gas Province Development located in 350m water depth 80km west of Bergen. The development consists of two main fields, Troll East (31/3 and 31/6) and Troll West (31/2) which together have an estimated production life in excess of 50 years, making it one of the worlds largest offshore developments. Norsk Hydro is responsible for the development and operation of the production facilities. The scope of supply included 15-inch internal diameter, 213 barg design pressure, dynamic risers for the export of oil and gas from the platform to shore. At contract award, Wellstream was finalising the location of their European Manufacturing site, a facility which would have the capability of manufacturing unbonded flexible pipe with external diameters up to 24-inches. The design, manufacture and qualification of a large diameter oil and gas export riser for service in the Norwegian sector of the North Sea, considered to be one of the most severe environments in the offshore industry, provided unique challenges and attributes. These risers have now been in service for over two year, following an extensive qualification programme. This paper provides an insight into the integrated approach adopted during qualification with the successful application of finite element technology to aid full-scale testing. During a full-scale test program a finite element simulation of a 15 metre long prototype pipe was performed with special emphasis on the evaluation of contact forces between the flexible pipe and a bend limiting structure. The finite element analysis program package ANSYS is chosen for this simulation due to its special feature of contact/target elements. The paper illustrates that the use of Finite Element Modelling is indeed capable of predicting the observed behaviour of prototype risers, which are subjected to a series of dynamic load cases, in a Dynamic Test Rig (DTR). Finally, the paper concludes that focus should now be given to the advantages of using finite element tools that are verified by full scale testing to reduce development costs and schedules.
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