Research on flow assurance of deepwater submarine natural gas pipelines: Hydrate prediction and prevention

The formation of hydrate will lead to serious flow assurance problems in deepwater submarine natural gas transmission pipelines. However, the accurate evaluation model of the hydrate blocking risk for submarine natural gas transportation is still lacking. In this work, a novel model is established for evaluating the hydrate risk in deepwater submarine gas pipelines. Based on hydrate growth-deposition mechanism, the mathematical model mainly consists of mass, momentum and energy conservation equations. Meantime, the model results are obtained by finite difference method and iterative technique. Finally, the model has been applied in the production of deepwater gas field (L Gas Field) in China, and the sensitivity analysis of relevant parameters has been carried out. The results show that: (a). The mathematical model can well predict the hydrate blockage risk in deepwater natural gas pipelines after verification. (b). Hydrate is easily formed at the intersection of horizontal pipeline and vertical riser, and the maximum blocking position often occurs in middle of the riser. (c). The hydrate blockage degree and length of hydrate formation region (HFR) decrease with the increase of gas transport rate. (d). The hydrate blockage degree and length of HFR decrease with the increase of gas transport temperature. (e). The hydrate blockage degree and length of HFR increase with the extension of horizontal pipeline. (f). Injecting inhibitors can effectively inhibit hydrate formation and blockage, but the improvement of transmission measures can significantly reduce the dosage of inhibitor. It is concluded that measures such as increasing gas transportation rate and temperature, shortening horizontal pipeline length, optimizing inhibitor injection point and injection rate can play a safe, economic and efficient role in hydrate preventing and controlling.     

“Everything was fine”*: An analysis of the drill crew's situation awareness on Deepwater Horizon

PurposeInvestigation reports into the Deepwater Horizon drilling rig disaster identified issues with the drill crew's situation awareness (SA). The aim was to (1) apply the Driller's Situation Awareness (DSA) model to the cognitive data extracted from accident reports from this event to determine if it could help to explain why the crew erroneously concluded that the well was stable, which would (2) provide a preliminary evaluation of the model's validity.MethodThe DSA model was used for a content analysis of the SA components in the accounts of the crew's actions during two Negative Pressure Tests (NPT), in the hours before the blowout.ResultsThe analysis provided (1) insight into the crew's likely cognitive processes before the blowout. In particular, it revealed issues with their interpretation and mental models of the well state, as well as possible influencing factors including expectation, distraction and experience, emphasising the impact that SA can have on process safety. The categorisation has (2) initially suggested that the DSA model does contain the appropriate components.LimitationsThere are limited first hand reports of this event and thus cognitive processes have to be inferred with a degree of caution.Practical implicationsThe findings give a preliminary validation of the DSA model for further use in training and in investigation of well control events. Recommendations based on the findings are offered for assisting driller SA and consequently, for supporting safe and efficient drilling operations. There is also the opportunity to adapt the DSA model and apply the recommendations from the analysis to similar monitoring positions, where SA is essential, within the process industries.     

长江口邻近水域浮游植物群落动态变化及其环境因子的研究

根据1998-2003年长江口邻近水域(30°-32°N,121°27′-122°30′E),丰水期和枯水期10个航次、20个点位的海洋监测结果,对浮游植物动态变化及其环境影响因子进行了研究。6年内共检出浮游植物238种,其中大部分为硅藻。研究结果表明,深水航道附近水域中,浮游植物种类、丰度和生物多样性均呈下降趋势。相反,航道内浮游植物总丰度呈直线上升(R=0.81,P<0.05)。对航道内12个环境因子做主成分分析,盐度、溶解氧、硅酸盐和浊度为主要环境主要因子。其中浊度和硅酸盐与浮游植物群落变化关系密切,是影响浮游植物群落动态变化的关键因子,并受深水航道工程影响。     

An Experiment with Forced Oxygenation of the Deepwater of the Anoxic By Fjord,Western Sweden

In a 2.5-year-long environmental engineering experiment in the By Fjord, surface water was pumped into the deepwater where the frequency of deepwater renewals increased by a factor of 10. During the experiment, the deepwater became long-term oxic, and nitrate became the dominating dissolved inorganic nitrogen component. The amount of phosphate in the water column decreased by a factor of 5 due to the increase in flushing and reduction in the leakage of phosphate from the sediments when the sediment surface became oxidized. Oxygenation of the sediments did not increase the leakage of toxic metals and organic pollutants. The bacterial community was the first to show changes after the oxygenation, with aerobic bacteria also thriving in the deepwater. The earlier azoic deepwater bottom sediments were colonized by animals. No structural difference between the phytoplankton communities in the By Fjord and the adjacent Havsten Fjord, with oxygenated deepwater, could be detected during the experiment.

Electronic supplementary material

The online version of this article (doi:10.1007/s13280-014-0524-9) contains supplementary material, which is available to authorized users.     

An advanced Driller's Method simulator for deepwater well control

Deepwater hydrocarbon reserves play an important role in the global energy industry. The safety issue which is associated with deepwater well control is one of the most crucial aspects during the well design and construction in a drilling scenario. Limited by the narrow drilling window and the long but small choke line, accurate prediction of the wellbore pressure is required to ensure a successful well control operation in deepwater. This paper focuses on the deepwater Driller's Method, developing an advanced simulator to predict the standpipe and choke pressures in deepwater horizontal well killing on the basis of “dynamic” bottom hole pressure. In order to improve the precision of the simulator, circulation temperature, gas expansion and choke line friction loss have been considered in the model. According to the data of a planned horizontal well X-2, the simulation and analysis of the behaviors of the standpipe and choke pressures during the deepwater Driller's Method well killing operation were performed with the proposed simulator. For further understanding the characteristics of the deepwater Driller's Method, it investigated the influence of some engineering parameters on the surface pressures, including seawater depth, horizontal section length, gas kick volume and killing rate. The results demonstrate that the standpipe pressure increases with seawater depth and killing rate; the peak choke pressure increases with seawater depth and gas kick volume; and the horizontal section length has a little impact on the standpipe and choke pressures. In addition, the application of the well killing case in BY-3 well, which is located in the South China Sea, has proved the validity of the simulator.     

Evacuation, escape, and rescue experiences from offshore accidents including the Deepwater Horizon

When a major hazard occurs on an installation, evacuation, escape, and rescue (EER) operations play a vital role in safeguarding the lives of personnel. There have been several major offshore accidents where most of the crew has been killed during EER operations. The major hazards and EER operations can be divided into three categories; depending on the hazard, time pressure and the risk influencing factors (RIFs). The RIFs are categorized into human elements, the installation and hazards. A step by step evacuation sequence is illustrated. The escape and evacuation sequence from the Deepwater Horizon offshore drilling platform is reviewed based on testimonies from the survivors. Although no casualties were reported as a result of the EER operations from the Deepwater Horizon, the number of survivors offers a limited insight into the level of success of the EER operations. Several technical and non-technical improvements are suggested to improve EER operations. There is need for a comprehensive analysis of the systems used for the rescue of personnel at sea, life rafts and lifeboats in the Gulf of Mexico.