A newly developed Tri-Helically Grooved drilling riser buoyancy module design was tested in the towing tank of SINTEF Ocean in June 2017. This new design aims to reduce riser drag loading and suppress vortex-induced vibrations (VIV). Objectives of the test program were two-fold: to assess the hydrodynamic performance of the design allowing for validation of previous computational fluid dynamics (CFD) studies through empirical measurements, and, to develop a hydrodynamic force coefficient database to be used in numerical simulations to evaluate drilling riser deformation due to drag loading and fatigue lives when subjected to VIV. This paper provides the parameters of the testing program and a discussion of the results from the various testing configurations assessed. Tests were performed using large scale, rigid cylinder test models at Reynolds numbers in the super-critical flow regime, defined as starting at a Reynolds number of Re = 3.5 × 105 – 5.0 × 105 (depending on various literatures) and continuing until Re = 3 × 106. Towing tests, with fixed and freely oscillating test models, were completed with both a bare test cylinder and a test cylinder with the Tri-Helical Groove design. Additional forced motion tests were performed on the helically grooved model to calculate lift and added mass coefficients at various amplitudes and frequencies of oscillation for the generation of a hydrodynamic force coefficient database for VIV prediction software. Significant differences were observed in the hydrodynamic performance of the bare and helically grooved test models considering both in-line (IL) drag and cross-flow (CF) cylinder excitation and oscillation amplitude. For the helically grooved model, measured static drag shows a strong independence from Reynolds number and elimination of the drag crisis region with an average drag coefficient of 0.63. Effective elimination of VIV and subsequent drag amplification was observed at relatively higher reduced velocities, where the bare test model shows a significant dynamic response. A small level of expected response for the helically grooved model was seen across the lower range of reduced velocities. However, disruption of vortex correlation still occurs in this range and non-sinusoidal and highly amplitude-modulated responses were observed.

Offshore drilling risers, top-tensioned risers and many production risers are top tensioned, connecting the vessel and seabed via joints. External loads such as currents, waves and vessel motions introduce cyclic loads and motions on riser sections, which may shorten the service life due to accumulated fatigue damage. Dynamic responses under combined currents and waves are more complicated than vortex-induced vibrations (VIV) due to pure currents, and it is not fully understood. Several model test campaigns on top-tensioned riser (TTR) have been carried out at SINTEF Ocean (former MARINTEK) during the past decades. Currents, waves and vessel motions were modelled, and the riser model responses were measured. In this study, selected cases from such model tests are analysed, and used to validate a semi-empirical time domain VIV prediction tool — VIVANA-TD. A better understanding of the dynamic responses of TTR under combined currents and waves has been achieved. By comparing the results from numerical simulation using VIVANA-TD and model test measurements, validity and limitation of the time domain tool have been investigated. Important features that need to be considered are discussed. The experience gained from the present study establishes a good basis for VIV and wave load prediction of full-scale TTRs under combined currents and waves where the uncertainty of VIV prediction is further reduced.

Conventional vortex-induced vibration (VIV) prediction tools are semi-empirical, in other words, based on several empirical parameters extracted from model tests in laboratory. Generally, the lab tests are costly, include small scale test conditions and with a limited test matrix. The extracted empirical databases are not directly applicable to full-scale VIV predictions of various slender marine structures. Therefore, large safety factors have been used by industry for VIV prediction in the past decades. To reduce the uncertainty (e.g. over-conservatism) related to semi-empirical VIV prediction tools, the NLPQL algorithm for parameter optimization of a semi-empirical time-domain prediction tool has been investigated. This methodology was demonstrated on pure cross-flow VIV prediction in an earlier study. It was shown that by setting appropriate constraints and cost functions of the optimization algorithm, this method is feasible to improve the VIV prediction accuracy. In this study, the NLPQL optimization algorithm was applied for combined cross-flow and in-line VIV predictions using time domain numerical model. Selected cases from field measurements representing multi-fidelity data were used to validate and verify the method.

Vortices generated when flow pass offshore slender structures may cause structural vibrations due to fluid-structure interaction, termed vortex-induced vibrations (VIV). VIV can cause severe accumulated fatigue damage, which need to be considered in the structure design. To capture the non-linearities of different riser systems and time varying flow, time domain models are better alternatives than frequency domain models to predict VIV. The input hydrodynamic parameters of time domain models are semi-empirical. However, versatile databases of hydrodynamic parameters are not well established, due to the large variations of the flow conditions, riser systems, and corresponding sub-structures. An optimization procedure could be implemented as an alternative to conventional experimental methods for establishing adaptive hydrodynamic parameter databases capable of accurately handling changes in the underlying structure of the system or flow regime. Literatures on application of optimization algorithms for various offshore engineering problems have been reviewed. VIV represents a constrained non-linear programming problem, where the sequential quadratic programming (SQP) algorithm – NLPQLP has been applied in this study. This paper proposes an optimization procedure of input hydrodynamic parameters for the time domain VIV solver - VIVANA-TD [21], for VIV prediction of various riser systems. A workflow integrating optimization and time domain VIV simulation has been established in the software work bench SIMA. By investigating the VIV prediction of typical laboratory model tests with optimized parameters, the feasibility of optimization using sequential quadratic non-linear programming algorithm - NLPQLP has been demonstrated.

Qualification of new technology for deepwater offshore oil and gas exploration and production is needed for the industry adoption process of new equipment and methodologies. Comprehensive programs can alleviate numerous risks associated with new technology through the application of appropriate industry practices such as DNVGL-RP-A203: Qualification of New Technology. Once qualified, innovative designs can provide effective sources of required sustained cost reductions for field developments creating new economic viability. The objective of this paper is to describe the systematic approach developed for the qualification of macro buoyancy spheres used in loose configuration as adjustable buoyancy in deepwater transport shuttles for subsea facilities and chemical transport. The qualification process is designed to examine macro buoyancy sphere structural stability under representative conditions of offshore transport shuttle loading, storage, and unloading at the surface and at depth. An overview of empirical testing procedures, equipment, and the configurations through which sphere performance will be quantified against application requirements is presented. Macro buoyancy spheres have been used for decades in a number of offshore and subsea buoyancy applications such as drilling riser buoyancy modules on marine drilling risers, mid-water distributed buoyancy on production risers and umbilicals, and installation buoyancy for subsea equipment. Their performance in these applications is well understood with wide acceptance in the market. Mass manufactured from high strength, low density materials, sphere performance specifications can be tightly controlled while still being produced in a very cost-effective manner. Once qualified, under the program described in this paper, these same buoyancy spheres have an innovative re-purposed extension within deepwater transport shuttle delivery systems. The readily adjustable and controllable nature of this innovative approach with loose buoyancy spheres, versus a fixed buoyancy system, allows a single subsea transport shuttle to be utilized for a wide variety of loads and service conditions. The buoyancy can be adjusted to match specific payload needs, reliably positioned when and where required, then re-used multiple times in a service delivery mode. This patented feature (patents published and provisional US and world-wide) will enable subsea shuttle transport barges to economically convey equipment from the sea's surface to the mudline and vice versa, especially in remote locations for smaller subsea projects where a typical heavy-lift vessel may prove to be cost prohibiting.