Offshore floating platform configurations often consist of geometrically simple and symmetrical shapes which are made complicated by the presence of appurtenances such as helical strakes, tendon porches, steel catenary riser (SCR) porches, pipes, chains, fairleads and anodes on the surface of the hull. Previous studies mainly on spars show that these hull external features affect the Vortex Induced Motion (VIM) performance of the platform significantly. This is to be expected since VIM is controlled by the flow separation on the hull surface and the resulting vortex shedding patterns. Scale effects may also play a role in model tests for bare cylinders or hulls with bare cylindrical columns, whereas previous studies have shown less Reynolds dependence when appurtenances are modelled. This study investigates the effect of hull appurtenances on VIM of a multi-column floating platform, i.e. a Tension Leg Platform (TLP) designed for Southeast Asian environment. Significant difference in VIM behaviors is expected between spars and TLPs since the column aspect ratios are very different and TLPs do not have helical strakes that are commonly fitted on spars. Model testing and Computational Fluid Dynamics (CFD) simulation are used in this VIM study, with the former being the emphasis of this paper. Descriptions of the respective experimental and numerical methodologies are presented and the comparison of the results is made. Further work required to improve the model test set-up and the CFD simulation are suggested. From this study, it is shown that the effect of appurtenances on TLP VIM simulation is important and must be taken into account to obtain realistic results.
In deepwater development areas of Southeast Asia, the current is strong and relatively more persistent compared to other deepwater regions. Top tensioned risers (TTR) are critical submerged components of offshore platforms, constantly exposed to currents. These currents cause unsteady flow patterns around the risers i.e. vortex shedding. When the vortex shedding frequency is near the riser’s natural frequency, undesirable resonant vibration of the riser also known as Vortex Induced Vibration (VIV) occurs. Several types of VIV suppression devices are used in the offshore industry. Among them, the U-shaped fairing claims to have the capabilities of reducing VIV effectively as well as lowering drag loads. This study investigates the effectiveness of a U-shaped fairing in suppressing riser VIV. The model test was successfully performed in a towing tank facility located at Universiti Teknologi Malaysia (UTM), Johor Bahru, Malaysia. This study is a significant collaboration between a local academic institution and the offshore oil and gas industry, aligned with the industry’s initiative of increasing local capabilities for research and development. In this study, the VIV of two risers in tandem is simulated using scaled test models. The current flow is simulated by towing the vertically submerged test models with a moving carriage. The riser with fairing models are attached to a pair of custom-designed test rigs which are able to measure the forces and also allow movement of the test model during towing tests. The two test rigs are attached to a steel structure under the carriage which accommodates different tandem riser configurations and spacings. Two different sizes of risers and fairings are tested to check for Reynolds number effects. For each tandem riser configuration, three different riser conditions are tested, i.e. (a) bare risers without fairings; (b) risers with weathervaning fairings, and (c) upstream riser with fairing stuck at different orientations and downstream riser with weathervaning fairing. The test results show significant reduction in drag and VIV for the risers with weathervaning fairings in different tandem configurations. Interesting motion characteristics are shown in some of the stuck fairing cases highlighting the adverse effects should the fairings fail to perform normally in the field. Effective mitigation of VIV in risers using fairing suppression devices could lead to improved riser fatigue life and overall a more economical platform design. These benefits are highly applicable to local deepwater developments for the oil and gas industry.
Floating vessels with dry tree production equipment are large capital expenditure projects with multiple interfaces often spanning several entities, including the Operator, Engineering Procurement and Integration Contractor (EPIC), Rig Builder, Production and Drilling Top-Tensioned Riser (TTR) supplier, Subsea Wellhead supplier, and Surface Wellhead/Tree supplier. Design and Interface Management play a significant role in the success of a project of this magnitude. The Shell Malikai Single Combo Casing Top Tensioned Riser (SCCTTR) was successfully deployed and commissioned on the first TLP in Malaysia. The Malikai SCCTTR is an industry first and an example of innovative design configuration coupled with rigorous project execution to deliver a system that is an enabling technology for current industry market conditions. The SCCTTR is a single riser used first for drilling and then remains in place for the full field life. This provides significant cost savings, both in hardware and operational time for large field developments, but introduces technical challenges to meet the requirements of both traditional drilling and production/water injection risers. This paper presents key design challenges to meet the requirements for ‘multi-purpose’ configuration and identifies how they were addressed to accomplish the project goal of flawless delivery. Project management was a key factor in achieving this goal. Shell chose a one-stop solution for delivery of the subsea wellhead and SCCTTR which provided advantages in management of interfaces and overall delivery. While this minimized interfaces, it still required a significant effort to coordinate components from multiple continents to deliver a single system. In addition to project management, the system required innovative design, as illustrated by the tensioner system, which introduced new features such as deflection absorbing bearing pads to limit the impact of the SCCTTR on the TLP hull design. Even with the additional features, the new tensioner system was designed, manufactured, and qualified within the project schedule. The Malikai SCCTTR stands as a testament to what can be accomplished when innovative design approach is coupled with robust project management and collaborative efforts between operator and equipment suppliers.
The Southeast Asia metocean environment is characterized by moderate wind and waves with relatively strong and persistent currents. The design of a Tension Leg Platform (TLP) for Southeast Asia is strongly affected by the current and resulting drag loads, particularly if many risers are required. To reduce vortex-induced vibrations (VIV) of offshore tubular members i.e. top tension risers (TTRs), vortex suppression devices such as helical strakes and fairings are used. This paper gives an insight into a set of riser VIV model tests which is carried out to study the effectiveness of an open-back “U”-shaped fairing in suppressing riser VIV. The splitting loads and the drag loads of the fairings are also investigated during these tests. The tests are being done at the Marine Technology Center (MTC), Universiti Teknologi Malaysia (UTM) in support of this application, and also in part to increase local technological capabilities. Two sizes of risers are being tested in three configurations: (a) bare riser without any fairings; (b) riser with weathervaning fairings (normal condition) and (c) riser with fairings fixed at different headings (abnormal condition). The test results show that in the normal condition, the fairing is effective at suppressing VIV.
Malikai Project made a cost-driven decision to adopt the 2-body TLP-DTV configuration to Drill and Complete the Wells offshore. By opting for a Tender Assisted Drilling approach to drill and complete the Well instead of fabricating a permanent rig package, the project could save capital expenditure by building a comparatively smaller TLP. Also cost-driven, for the first time in the industry, the project decided to employ Single-Casing Combo Top-Tensioned Riser (SCTTR) technology as opposed to using the combination of Drilling Risers, and a separate Dual-Casing Production TTR. This is however with a caveat that wear accumulation during drilling needs to be managed and kept to a minimum. Combining the above, the project team needed to manage TLP-DTV marine operations paying attention to station keeping performance required by the SCTTR whilst ensuring the hardware on TLP-DTV (e.g. hawser lines, mooring lines, Personnel Transfer Bridge etc.) is operating safely within their design limits and maximizing the uptime of the offshore activities. To address the specific joint marine operations on TLP and DTV during the Tender Assisted TTR installation, Drilling and Wells Completion phase of the project, an interdisciplinary taskforce comprising the TTR team, Station keeping and Global Performance team, Wells and TLP Operations teams was created for a fully integrated and focused operational procedure. This procedure was aimed atmanaging and assuring TLP-DTV repositioning and station keeping activities which meet the above constraints/objectives, complete with operation decision organization, a clear communication protocol, and comprehensive decision flowcharts to handle all possible scenarios during the operation. An onshore support group located within the well operations team is tasked with monitoring and predicting (based on forecast weather) of the performance of the TLP-TAD system for safe and effective station-keeping performance. The TLP-DTV-TTR operations is built upon the basis that the integrated team is able to plan the TTR installation / Wells construction operations in advance using forecast weather as inputs to continuously simulate numerically TLP-DTV performance - checking against repositioning criteria, as well as the continuous real-time monitoring of TLP-DTV performance against their operating windows using live data obtained from the MIS/MAS system (performed by both the Onshore team and Offshore team). Lastly, to ensure the overall effectiveness and accuracy in executing the operations, the team had created a suite of purpose-designed software to streamline the activities performed by the Onshore and Offshore Team respectively i.e. advanced activity planning via automated continuous forecast simulations of TLP-DTV and TTR performance to indicate operations go/no-go, as well as clear displays of key parameters constantly updated with live real-time data for operations monitoring.
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