The bursting phenomenon is a common feature of turbulent boundary layers irrespective of the wall roughness condition. Consistent with Theodorsen's conjecture, recent direct numerical simulation studies have shown that the dynamics of this bursting process over smooth walls is directly linked to the presence of powerful vortical structures with a general horseshoe-type configuration. The present paper describes the results of physical experiments that demonstrate that these vortex structures are also present in turbulent boundary layers over rough walls. They appear to form the central element in a recurring, highly nonlinear cycle of turbulence and burst generating instability. Novel velocity measurement techniques were used in the investigation that, for the first time in physical experimental fluid mechanics research, allowed quasi-instantaneous vortex lines to be traced through a three-dimensional block of flow space to reveal the vortical structures embedded in the shear flowfield. Preliminary test results are also presented that indicate that, as in the smooth wall case, the instability structures over rough boundaries have a preferred spanwise wavelength that scales with the roughness dimension.

It has been recognized for some years that the bursting phenomenon is a common feature of turbulent boundary layers irrespective of wall roughness condition. In confirmation of Theodorsen’s original conjecture, there is now a convincing body of evidence, particularly deriving from recent direct numerical simulation studies, that the dynamics of this bursting process over smooth walls, is directly linked to the presence of powerful vortical structures with a general horseshoe-type configuration, embedded in the wall flow field. The present paper describes the results of physical experiments which demonstrate that these vortex structures are also present in turbulent boundary layers over rough walls and that they are similarly linked to the bursting events. Novel velocity measurement techniques were used in the investigation which, for the first time in physical fluid mechanics research, allowed quasi-instantaneous vortex lines to be traced through a three-dimensional block of flow space to reveal the vortical structures. The preliminary results from a second investigation are also presented which demonstrate that, just as in the smooth wall case, the spanwise flow structure adjacent to a rough boundary exhibits a well defined cross-flow wavelength. The measurements indicate that this wavelength, which reflects the spanwise scale of the near-wall vortex structures, is directly proportional to the size of the boundary roughness elements under fully rough conditions. Consistent with Townsend’s ‘attached-eddy’ hypothesis, the vortical structures are observed to increase in scale with increasing wall distance while remaining attached back to the vorticity generation zone at the boundary. The paper commences with a review of the status of current knowledge relating to coherent vortical structures in turbulent boundary layers and its interpretation in the context of large scale geophysical flows.

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This paper presents detailed learnings into the dissection of a flexible riser section, which had been operating for seven (7) years and subjected to high rates of fatigue damage, after operational field changes. Detailed global and local fatigue analyses were performed to assess the response of the riser system taking into account significant changes between the design and the operational history. These environmental and operational changes were shown to have a negative impact on the fatigue performance of the risers, in particular the top section located inside the bend stiffner. Fatigue analyses are performed using field measured data such as vessel headings, internal pressures and environmental conditions, aiming to replicate the riser response history and calculate fatigue usage to date. Actual field data are shown to be more onerous than design data and therefore a detrimental fatigue response is expected. Results from detailed fatigue analysis shows that flexible risers fatigue usage were above the maximum allowable of 0.1, as outlined in API [1,2]. The most critical location was the pressure armour wires of the top section, located inside the bend stiffener. A mitigation plan was proposed, including an immediate reduction of internal pressure to reduce the risk of riser failure, and the removal of the fatigued riser top section. After removal of the fatigue critical section, the riser was re-terminated and its fatigue performance reinstated. The pipe section removed from operation was carefully dissected. Following the dissection, each riser layer was investigated to determine possible signs of degradation. Additionally, fatigue testing of both pressure and tensile armour wires were performed to determine potential fatigue degradation and confirm analyses findings. Dissection and fatigue tests have demonstrated that the pipe condition was better than expected, highlighting conservatisms in design and analyses methodologies. In-service inspection of flexible riser internal layers is highly complex, with no detailed insight of all the respective layers. Dissection of a flexible riser, in service for seven (7) years and exposed to high rates of fatigue damage, provides valuable information about the state of the different layers comprising the cross section. This is particularly useful since analytical work have shown the riser to have exceeded the fatigue limit of 0.1 as defined per API. The good state of the flexible riser, on the contrary to the prediction, highlights a good level of conservatism in flexible riser design and methodology.