STUs address key issues for deepwater dynamic applications

Umbilicals are a key component in the wet tree development of deepwater fields.

Darren Gallagher
Frank Grealish

MCS

Umbilicals are a key component in the wet tree development of deepwater fields. Umbilical failure can result in partial or total loss of production, with resulting severe economic consequences. In many recent deepwater projects, steel tubes for control and injection lines have become the preferred solution over traditional thermo-plastic hose-based designs. This preference is due to enhanced response times and prevention of permeation of fluids (particularly methanol) from the tube. In general, the preferred material for the steel tubes is super duplex stainless steel. The typical size for the steel tubes is between 1/2 in. and 1 in., though some designs are proposed that incorporate a large core tube, e.g., 2-3 in., which is used for gas lift or methanol injection. .

A number of key issues must be considered when selecting a steel tube umbilical (STU) design. These include material selection and qualification, fabrication procedures, cross-section analysis, design and lay-up, riser configuration selection, installation procedures, risk strategies, dynamic analysis, fatigue life assessment, accumulated plastic strain, and low cycle-high strain plastic loading or low cycle fatigue (LCF).

The free hanging catenary has been the preferred dynamic umbilical riser configuration for many deepwater TLP and spar developments, due to cost effectiveness and ease of installation. However, for recent projects offshore West Africa and Brazil employing FPSO solutions, dynamic umbilicals must be designed to withstand comparatively large heave motions. This can produce large compressive loads and instability at the touchdown point in addition to significant fatigue damage. In an attempt to counteract this problem, STU designs are being proposed that include additional weight elements such as steel rods or steel armor wires in either lazy wave or lazy-S configurations.

Key issues

Qualifying the steel tube manufacturing process is critical when selecting an STU for a particular application. Sigma phase is an inter-metallic phase that can occur during steel tube fabrication. It reduces the super duplex's corrosion resistance and toughness. The steel tube supplier should have sigma phase control measures in place to detect if the sigma phase of the super duplex steel is above the allowable value. Eddy current tests and microscopic examinations are among the tests performed by the suppliers as sigma phase control measures.

Analyzing the manufacturing process is important in STU selection. Consideration should be given to which supplier produces the steel tubes and what welding procedures they use. During manufacture, the STU is reeled a number of times onto different carrousels as the multi-component cross section is assembled. This procedure can cause the larger steel tubes to plastically deform and can significantly affect the performance of the steel tubes, particularly in terms of fatigue.

STU cross-section design optimization is required. The advantages of using armoring and weight components should be investigated. Armoring reduces the stresses experienced in the steel tubes, while including weight members means that a required diameter-to-weight ratio can be achieved.

Validating and qualifying fatigue S-N data is critical in the design process. Factors that impact on the S-N curve qualification include tube size, welding process, number of weld passes, weld versus parent material performance, effect of mean stress, and effect of LCF seen in the manufacturing process.

An STU should be designed for a number of contingency scenarios during installation. These scenarios include retrieval and reinstallation as well as suspension of the installation procedure, both of which can add to the plastic strain experienced in the tube. Selecting the installation procedure, vertical lay system, or overboarding chute is also a critical design issue.

As well as achieving von Mises stress and fatigue life criterion, the integrity of the steel tubes must be considered. All possible failure modes, such as buckle propagation, pipe collapse, and pressure containment should be analyzed.

Low cycle, high strain

During the manufacture and installation of an STU, the steel tubes are subject to the curvatures of the manufacturing bobbins, the STU helical radius, the transport reels, and the installation chute. These curvatures induce elastic and plastic strain within the steel tubes. The key issues to be considered are accumulated plastic strain and LCF. The calculations associated with both of these phenomena are based upon the axial and bending strains experienced.

In dynamic STU applications, friction stresses occur in the STU as a result of the different tubes within the STU interacting with one another. Traditional in-place fatigue analyses techniques calculate the STU in-place fatigue damage based on the axial and bending stresses that occur in the steel tubes due to the dynamic motions of the STU. These stresses can be used to calculate the friction stresses within the STU that are then added to the original axial and bending stresses. A new fatigue life can be calculated using these increased values. Including friction stresses can significantly reduce the calculated fatigue life of an STU.

STU friction stresses are largely dependent on the lay-up of the STUs, particularly the steel tube layer radius, the distance from the center of the umbilical to the neutral axis of the steel tube, and lay-angle. Analyses have shown that the smaller the lay angle and the larger the layer radius, the longer the fatigue life. Careful consideration has to be given to how many tubes are acting on one another and at what angle the contact forces are acting relative to the contact surfaces, all of which make the accurate modeling of friction within the STU a complicated procedure.

Including mean stress effects also reduces the STU fatigue lives. In a specimen where there is an alternating stress, the fatigue life is usually calculated based on the amplitude of the alternating stress. In an STU where large tensions exist, i.e., at the hang-off region, it can be argued that the fatigue life of a steel tube is not solely based on the tube stress amplitude but is also a function of the mean stress of the tube. Mean stress effects can be accounted for using a Goodman-Soderberg approach.

Recent analyses have shown that including the effect of both friction and mean stresses can in some cases increase the fatigue damage by up to 140-fold.

STUs represent a complex construction that is subject to significant loads in all stages from initial fabrication through installation to in-place operation. Detailed evaluations are required in the design and verification process, so as to ensure that qualified designs are achieved, particularly for deepwater dynamic applications.

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