The development of Ultra Deep Water (UDW) oil and gas fields, down to 3000 m and beyond, requires high specification flowline and riser systems. At these depths, the flexible pipes must withstand high axial loads and severe dynamic loadings generated by currents, waves and vessel motions. Moreover, the constraints generated by the dynamic loadings are often combined to corrosion issues linked to the presence of CO2 and H2S. In case of sour service application, the structural layers of a classical flexible pipe require the use of steel with reduced mechanical properties compared to a sweet service application. The combination of UDW and sour service applications consequently lead to a riser design of considerable top tension. The main challenges of such applications are the suspended weight and the fatigue / corrosion performances. Carbon fiber composite have demonstrated high specific strength and outstanding corrosion and fatigue damage resistance. The use of carbon fiber composite instead of conventional steel for the tensile armour layers of flexible pipes represents a great alternative for the development of UDW applications combined with sour service conditions. Technip has been engaged for a number of years in the development and qualification program of Carbon Fiber Composite (CFC) Armour. In 2011, an important step has been passed with the successful realization of a full-scale tension-flexion dynamic test. The program of the full-scale dynamic test is based on a representative Brazilian offshore project, a typical UDW application. The CFC prototype structure was designed considering a 9” gas export riser installed at a water depth of 2140m, in free hanging configuration. The riser is made of 2 parts: a top riser with CFC armours and a bottom riser with steel armours. 1.8 millions of cycles were performed without damage, combining internal pressure, tensile loading and bending cycling. The whole test was monitored by acoustic emission to detect the potential damage of the CFC armours. After explaining the advantages of CFC structures compared to traditional steel structures, the paper will focus on the realization of the full-scale dynamic test program. It will detail the design and manufacture of the prototype structure, the construction of the test program representative of the offshore conditions first and then extended to more severe loadings. The paper will also present fatigue analysis and the construction of the CFC fatigue curves.

With the increasing water depth of offshore field developments, the suspendedweight and the fatigue performance of dynamic risers become more and more oftena driving design factor. The use of light weight composite materials is ideallysuited to this challenge as the strength/weight ratio of a typical carbon fibrecomposite material can be an order of magnitude greater than the equivalentsteel solution. The introduction of this technology to a new domain such asflexible risers requires careful consideration of material selection andqualification criteria. When developing a composite material the key point is to perform the rightselection of carbon fibre and matrix, that is able to sustain continuouslyduring twenty or thirty years both the mechanical loading and theenvironment. The exceptional fatigue performance of the Carbon fibre composite under axialloads makes it a material perfectly suited for tensile armour reinforcement indynamic deepwater risers. In case of sour service conditions generated by thepresence of H2S, the structural layers of a classical flexible pipe require theuse of steel with reduced mechanical properties (i.e sour grade). The typicalcombination of sour service conditions and Ultra-Deep Water (UDW) riserapplications generates a design of considerable weight and top tension. Forsuch applications, the advantages of Carbon Fibre Armours (CFA) are twofold dueto the fact that CFA has a higher strength/weight ratio and is also notsensitive to H2S. Technip has been engaged for a number of years in the development andqualification program of CFA. The high level performance of the qualifiedflexible pipe integrating CFA allows now to propose light weight flexible risersolution and also allows to reduce or to remove the requirement for buoyancyelements for UDW configurations. The first part of this paper is dedicated to the comparison of flexible risersystems designed with CFA versus steel armour flexible pipe. The second part of this paper will focus on the material tests and full-scaleflexible pipe prototype qualification. In particular, results generated duringthe last full scale test will be presented with the associated qualifiedapplication domain.

Unbonded flexible pipelines used for offshore fields developments usually rely upon a stainless steel carcass as innermost layer for collapse resistance and a polymeric pressure sheath for internal leakproofness. For some dynamic flexible risers, this pressure sheath is a multi-layer construction made of 2 or 3 layers. In this multi-layer configuration, during operation, the fluid transported in the bore of the pipe can penetrate the annular space between the pressure sheath layers by means of flow paths through the end-fitting. Then, when the pipe bore is depressurized, the pressure of the fluid trapped between the sacrificial sheath and the pressure sheath does not decrease as fast as in the bore. This is due to the small annular flow path between the sheaths. Depressurization of the pipe bore should thus be performed at a limited and controlled rate to avoid an excessive differential pressure between the pipe bore and the annulus that could potentially cause the collapse of the carcass. The allowable depressurization rate is a key parameter the field operators need to know to avoid such an issue. A model was developed and implemented in a numerical software to calculate the evolution of the differential pressure during the depressurization. It is based on a one dimension gas/liquid transient transport model. More specifically, it is composed of the mass and momentum conservation laws. The thermodynamics properties of the fluid are computed with a simple model. In addition, the dynamic behavior of the annulus is coupled with the conservation equations and the pressure of both the bore and the external environment. A Finite Volume Method is chosen to both discretize and keep the conservative properties of the model on a discrete level. In addition, a large test campaign was carried out on a full scale pipe to validate the model. A test consists in a pressure increase followed by a depressurization. The pressures in the bore and in the annulus were permanently recorded using a specific device which enables to know in real time the differential pressure between the pipe bore and the annulus. Various conditions were tested by varying the fluid viscosity, the initial bore pressure and the depressurization rate. After explaining the physics of the depressurization for this specific flexible pipe construction, this paper will present the numerical model developed to calculate the maximum differential pressure and the test campaign performed to validate this model.

No abstract

Unbonded flexible pipelines for deep water field applications are subject to elevated hydrostatic constraints; bore pressure being dropped brings high compressive stresses due to reverse end cap effect. If the external sheath is unsealed, the axial compression loads transmitted to the tensile armors induce their radial expansion, which can lead to their buckling. Specific layers shall be added in order to maintain armors in place: anti-buckling tapes. These layers, designed with high performance fibers, have been introduced in the early 1990's when the use of flexible pipe has grown in deep water. Various failure modes are expected and observed depending on the fiber characteristics, tape construction, tensile armors properties and annulus environment. For each pipe design, an optimum has to be found between the anti-buckling tapes loads and the compressive stress in the armor layers in order to ensure sufficient safety margins against both tapes breakage and different buckling modes of the armors. Anti-buckling tape interaction with the tensile armors and annulus environment main influencing parameters will be explained in the light of full scale tests performed on flexible pipes in hyperbaric tank. Specific laboratory devices have also been developed to be representative of the operating behavior of both armors and high strength tapes. In parallel, this paper will present qualification tests performed in compliance with the requirements of 2014 editions of API standards 17J and 17B. The tests showing tape resistance and flexible pipe behavior have enabled to establish a robust design rule. This process is being scrutinized by an Independent Verification Agent and aims at leading to Type Approval Certification according to the latest edition of API 17J.