Flow past a fixed and freely vibrating drilling riser system with auxiliaries in laminar flow

Drilling risers used in oil and gas operations are subjected to external loads such as wave and current. One of the phenomena that arise from the external loads is the Vortex-Induced Vibration (VIV), which affects the performance of the riser due to excessive vibration from the vortex shedding. A significant factor influencing the VIV is the design of the drilling riser and its auxiliary lines. Until now, the optimum geometrical size and gap between the auxiliary and the main riser are still very scarcely studied. In this paper, the main objective is to study the effects of the gap ratio (G/D) on the vortex shedding phenomenon on a fixed and freely vibrating riser. The riser system was modelled with a main drilling riser and six auxiliary lines with a constant diameter ratio (d/D) of 0.45 and gap ratio (G/D) = 0 to 2.0 in the laminar flow regime with Reynold Number, Re = 200. The simulations were conducted for Single Degree of Freedom (SDOF) using Computational Fluid Dynamics (CFD) software, Altair AcuSolve. It was found that the freely vibrating riser experienced higher lift and drag forces as compared to the fixed riser due to the synchronization (lock-in) of the shedding vibration and the natural frequencies. The lock-in phenomenon is normally observed on the drilling riser at different current directions. The forces are reduced when G/D is higher. The vortex shedding was significantly reduced for auxiliaries between 0.3 to 1.4. It is confirmed that by modifying the interaction of the vortices in the wake region with auxiliary lines, the hydrodynamic forces will be decreased. Finally, this fundamental study could potentially be used in the designing stage of an optimum drilling riser system by considering significant governing factors

Investigation of the corrosion factor to the global strength of aging offshore jacket platforms under different marine zones

This paper investigates the corrosion factor to the global strength of aging offshore jacket platforms in different marine zones. The time-dependent corrosion wastage model from Melchers, an active corrosion expert, by considering different marine zones, i.e., atmospheric, splash, and immersion. In addition, three aged jacket platform models of the originally installed platform were selected to investigate the effect of corrosion wastage on global strength. The jacket platform's global strength is calculated by the Reserve Strength Ratio (RSR) value. RSR is obtained via nonlinear static pushover analysis and is always used to estimate overall structure strength reserves. Safety assessment is evaluated by the PETRONAS Technical Standard (PTS). According to this study, immersion zone corrosion is the most influential zone after splash zone corrosion. Furthermore, the study also predicts the lifetime of each jacket platform, and it is useful for structural engineers to estimate corrosion allowance at the early design stage

Risk Assessment of Fishing Trawl Activities to Subsea Pipelines of Sabah and Labuan Waters

Introduction. Trawling is a method of catching fish in a large volume where fish nets are pulled through water using one or two boats. Bottom trawling is where the nets are pulled over on the seabed. The gear of the bottom trawling would impact the exposed subsea pipeline, on the seabed. Subsea pipelines transport crude oil and gas from the offshore platform to shore facility. This study assesses the risk of fish trawling activities to the subsea pipelines at Sabah and Labuan offshore. The specification of trawl equipment used by local trawlers in Sabah was determined by the on-site survey. The frequency of a fish trawler crossing over the pipelines was calculated based interview on operation and site survey. The calculation of the pull-over load of the otter board was calculated using the DNVGL algorithm. The severity and frequency index of the risk matrix was developed based on literature review. Results showed that the pull-over load of the otter board would not damage the pipelines. The risk posed by the fish trawler activity to the pipelines is low and moderate

Modified wake oscillator model for vortex-induced motion prediction of low aspect ratio structures

Floating structures, such as Spar platform, are likely to experience significant motions while operation due to Vortex-induced Motion (VIM). The main features of VIM include aspect ratio and mass ratio of the structure. This paper focuses on further development of a previously published semi-empirical method called wake oscillator model (WOM) for VIM, which aims to derive a mathematical correlation in modelling all features of the phenomenon. Analytical solution is proposed to coupled WOM and structure oscillator model based on the Van der Pol equation by integrating mass ratio, m* = 4.36, and various low aspect ratios between 0.3 and 2.0. Additionally, a new approach was initiated to embed several empirical parameters linked to the aspect ratio in the existing WOM. It was found the aspect ratio parameter in the modified WOM significantly affected the performance of the model for estimation of VIM on floating structures

A fundamental CFD investigation of offshore structures for artificial coral reef development

Rigs-to-Reefs (R2R) is an alternative for oil and gas industry for decommissioning purpose. The program will benefit marine life as an artificial reef for a sustainable solution to promote ecosystem growth. For any oil platform considered for reefing, an extensive study on the suitability and performance of the artificial reef is essential. These studies will evaluate the stability and relevance of the structure as an artificial reef when deployed on the seabed. This paper presents the research on dynamic response and flow characteristics of a conceptual proposed R2R platform. The Computational Fluid Dynamics (CFD) simulation is intended to study the dynamic responses and flow characteristics analysis of a jacket oil platform for optimal settlement and growth of corals. Artificial reef performances were evaluated by calculating the performance indices of the upwelling and back eddy profile. It was found that the upwelling efficiency index is higher at current direction normal to the platform due to the higher frontal area exposed to the incoming flow. Meanwhile, back eddy efficiency index shows a significant low value for all directions. The CFD results presented will be discussed in terms of the possibility of better performance of an artificial reef which considering engineering and biological aspects

Reefing Viability Index for Rigs-to-Reefs (R2R) in Malaysia

Decommissioning of the offshore platform as an artificial reef, known as Rigs-to-Reefs (R2R), has become a sustainable approach for oil companies. The platform was reused to serve the underwater ecosystem as an artificial reef for a new marine ecosystem which helps to tackle food security issue. This paper presents the findings of the formulation of the reefing viability index to recognize an offshore region that can be used for R2R projects within the South China Sea. The combined effects of spatial data, numerical modelling, and geographic system (GIS) are proposed to study the relationship of spawning ground coral reefs, diversity, and planula larvae in the process of colonization to establish a map of the reef potential environment. Coral connectivity and spawning behaviour were studied to determine the possible source of coral seedling released during the spawning season, twice a year. A geographic reef viability index was established consisting of seven parameters which are coral larval density, pelagic larval length, sea currents, temperature, chlorophyll-a, depth, and substrate availability. The ocean hydrodynamic model was designed to resemble the pattern of larval scattering. By using the simulations and rankings, there were 95 (21%) sites which could probably be used for in situ reefing, whereas 358 (79%) sites were likely ideal for ex situ reefing. Validation of the viability index was carried out using media footage assessment of remotely operated vehicle (ROV)