A considerable number of fixed offshore platforms around the world have either already surpassed their design life or are approaching it. In the Middle East, more than 70% of the platforms are operating for more than 25 years; some of the assets are even operating for more than 40 years. High oil price and advancement in technology planned to increase the productivity of reservoirs have led to significant investment in terms of cost and resource to manage these assets. There is still a lot of recoverable oil and gas in the reservoirs hence, there is an increased need to extend the life of these facilities while managing the associated risk. This technical paper details a methodology for assessment of remaining life and details some of the degradation/ life extension issues for fixed offshore assets. As it is extremely difficult to cite all the degradation issues, some of the critical degradation issues based on experience and knowledge has been covered in this paper.
This paper describes an original approach to reduce drastically the analysis time needed for fatigue design of structures by using machine learning techniques. The approach is applied to the spectral fatigue analysis of various structural details on a converted FPSO hull, where design iterations are usually time consuming. For the structural detail example used in the present study, numerical results show that by including the right inputs to the machine learning algorithm, the predicted fatigue life could compare well with the spectral fatigue analysis output with a score of up to 0.997. For the critical elements with high fatigue damage, the predicted fatigue life is found up to 2.5 times of the actual value. Overall an estimated 5.5 hours (out of 6 hours) are saved for one iteration of spectral fatigue analysis.
Ocean developments for oil and gas and renewables involve site specific floaters designs. For such assets, it is required to perform detailed structural analysis at the design stage to consider all the specificities of the structure and the environments in the design and make sure the floater can operate safely throughout its design life. For fatigue design, the state of the art approach for such floaters is spectral fatigue analysis.
The calculations of the fatigue damage using a full spectral direct calculation approach is labor intensive, especially when design iterations are needed. The complete assessment procedure comprises hydrodynamic analysis to compute wave-induced loads; structural analysis for both global coarse mesh and local fine mesh finite element models; statistical analysis to calculate the short-term stress response based on the environmental conditions on site; and lastly fatigue strength evaluation using the applicable fatigue S-N curve. This analysis approach can be rather time consuming in cases where many structural details are assessed or where multiple iterations are needed to reach a satisfactory design.
In recent years, there has been several cases of mooring line failures on various floating offshore units. In several of those cases, the failures were identified months or years after they initially occurred. Most assets being designed for single line failure only, this means that the risk of a catastrophic failure of the whole mooring system is quite high if the failure of a single line cannot be detected reliably. To mitigate that risk, class societies such as DNV GL have introduced requirements for the use of lines tensions monitoring systems as part of the mooring class notations (POSMOOR). However, most of the line tension monitoring systems available on the market today have proven unable to remain functional after more than 2 years in operation, due to the harsh conditions and loads they are exposed to. For that reason, an alternative system for line failure detection is needed.
In this paper, a system is developed to detect reliably a single line failure based solely on GPS and motions sensors data installed on the asset. The GPS and motion time series are used to train a neural network which can then reproduce any motion signal as a function of the others, capturing all the complex nonlinear correlations between the wave frequency and drift motions of the asset along its 6 degrees of freedom. Any change in the mooring system properties such as a line break has an impact on those correlations, this change is captured by the neural network, therefore enabling it to detect a line failure. The accuracy of the system is demonstrated using numerical simulations for an FPSO in various sea states, where a line break occurs at one time instant during the simulation.
A moored floater is a 6-DOFs system where the loads are stochastic environmental loads, wind waves and current. The behavior of the system in a given sea state is governed by its inertia, damping and stiffness properties. The failure of one or several mooring lines has an obvious effect on the stiffness, but also on the damping of the whole system since the drag on the lines is one of the damping contributions. The floater displacement in the same sea state will therefore be different for an intact or damaged system. Those differences might not be easily detectable since depending on the mooring system configuration, and the sea state, a line failure might not change significantly the basic statistics of the motions such as mean and standard deviation. In those cases, changes in the higher order statistics, as well as in the correlations between different degrees of freedom motions need to be detected to identify a line failure.
Assuming perfect knowledge of soil strength and prescribing load factors for loads, we calibrate the necessary requirement for the material factor on characteristic soil strength by tuning a reliability analysis to meet a prescribed target failure probability. Keeping this calibrated material factor unchanged, the reliability analysis is repeated with the stochastic model for soil strength altered to include statistical uncertainty owing to limited soil data. A reduced "cautious" value of the characteristic soil strength is determined such that the failure probability resulting from the analysis is maintained equal to the target. Based on this reduced value, the minimum confidence level needed for characteristic value estimation is interpreted. Using soil strength for prediction of axial pile capacity as an example, this paper outlines a procedure for reliability-based calibration of the minimum confidence needed when estimating characteristic soil strength, defined as the mean value, with confidence for use in offshore pile design.
Although Trinidad and Tobago has an abundant supply of relatively pure CO2 and more than 1 billion barrels of heavy oil deposits there are no active enhanced oil recovery (EOR) projects using carbon dioxide (CO2).
In this paper, we have performed black oil simulation studies to evaluate several injection strategies with carbonated water, varying the salinity and viscosity of injected water. The salinity was varied by 1,000 and 35,000 ppm. The viscosity was increased by adding 0.1 weight percent polymer to injected water. The investigation was carried out using a commercial reservoir simulator. The simulation grid represents the properties of a quarter five-spot of the Lower Forest sand of the Forest Reserve Field. The reservoir simulation components used are water, polymer, H, Na, Cl-, dead oil, solution gas and CO2. The Stone #1 three-phase relative permeability model was used to calculate the three-phase relative permeabilities from two-phase data. In addition, a factorial experimental design was utilized and twelve simulation runs were done along with nine benchmark runs for comparison to other EOR methods.
From the results obtained the following was concluded: water salinity has no effect on either oil recovery or carbon dioxide storage; polymer injection increases oil recovery and carbon dioxide storage. We found the optimal injection strategy to be a cycling of carbonated water alternating with polymer injection.
This paper aims to numerically simulate the loading process when a moored ship is intruded by an ice ridge. Ice force caused by ice keel is calculated based on suggestions from ISO while the ice force due to consolidated layer is taken as level ice and simulated with circumferential crack method. The equation of motion is solved at each time step. A case study is given to show main features during the moored ship and ice ridge interaction. The result shows that the present numerical simulation is promising to be used in the design for moored structures in ice ridge.
In the Arctic, there exist many different types of features such as pure level ice, brash ice, ice rubble and ridges, ridge fields and icebergs, all with different structural and mechanical properties and behavior. For ships and offshore structures, first year ice ridge is a key consideration due to the extreme ice loads acting on the structures. It is crucial to determine the design load levels for offshore structures in ice-infested waters, can also bring a threat to shipping and navigation activities.
Typically, an ice ridge is formed when ice sheets are compressed against each other due to environmental factors, such as wind, current in the sea, thermal expansion etc. From geometry aspect of ice ridge, it is composed of three parts: sail, consolidated layer and keel. The above water part, called the sail, has pores filled with air and snow. The underwater part, called the keel, has pores filled with water and air pockets can exist. The ridge keel is further separated into an upper refrozen layer called the consolidated layer and a lower unconsolidated part. The consolidated layer grows through the ridge lifetime as a function of the surrounding meteorological and oceanographic conditions, air and water temperature, snow depth and the velocity of the wind, and surrounding currents are of principal importance. There was a wide variation in the shapes of the first-year sea ice ridges (Timco & Burden, 1997).
By developing general constitutive laws for ice ridge, Heinonen (2004) and Serré (2011) used finite element software to simulate the ice ridge load. At present, moored ships are often used to oil exploration and exploitation in ice-infested waters. For example, starting in the mid-1970s to the late 1980s, Dome Petroleum deployed floating drill-ships named Canmar during the summer months. In some water, the ice ridge action should be taken into consideration. A sketch of the moored ship in ice ridge is shown in Figure 1.
Offshore pipelines operating in sour environment require that the line pipe material have sound fatigue and fracture properties. The fatigue and fracture of X65 steel in sour environment is a hydrogen assisted cracking mechanism. Hydrogen is generated as a result of the corrosion of the steel in sour environments and its uptake in steel is facilitated in the presence of sulfide ion as the hydrogen recombination poison agent. It is essential to understand the interaction of hydrogen with the steel to help understand the fatigue and fracture performance of X65 steel in sour environments. This is typically done by performing hydrogen flux measurement on line pipe steel samples in sour environment. In this paper, hydrogen flux tests were performed on as manufactured X65 pipe (parent and weld) and strained and aged pipe in a sour environment. The analysis of the measured hydrogen flux in sour environment to determine the effective diffusivity was demonstrated. The hydrogen flux results were analyzed to characterize the trapping of hydrogen in the steel. The obtained results were compared to those obtained with thermal desorption analysis.
Offshore pipeline operations require understanding the fatigue crack growth rate and fracture toughness property of the pipe steel in sour or sweet environment. Previous work has been performed to understand the fatigue and fracture toughness performance of C-Mn steels in dynamic applications like risers and flowlines in sour environments (Buitrago, Weir, Kan, Hudak and McMaster, 2004; Thodla, Gui, Robin and Xia, 2010). Generally, the cracking of carbon steel in sour environments is a hydrogen driven mechanism. The presence of H2S typically will enhance the crack growth by promoting hydrogen generation and ingress into the steel (McMaster, Bowman, Thompson, Zhang and Kinyon, 2008; Gangloff, 2009). It has been found that in moderate to severely sour environments, bulk charged hydrogen from the corrosion processes as opposed to the hydrogen generated at the crack tip during plastic straining plays a dominant role in enhancing the FCGR of line pipe steels in sour environments (Gui, Ramgopal and Muller, 2012).
This paper presents the application of a reliability based design methodology for interference between trawl gear and pipelines, and the associated cost savings in subsea rock installation.
As part of Wintershall Norge's Maria development project, a 26 km long 14″ production flowline with DEH (Direct Electrical Heating) and a 46 km (43 km + 3 km infield) long 12″ water injection pipeline has been installed and left exposed on the seabed. The seabed topography in the area is un-even and scarred by iceberg ploughmarks, leading to a significant number of large free spans. Furthermore, fishing activity must be taken into account in the design of these pipelines. According to DNV-RP-F111, if the trawl gear hits in a free span, the pull-over load will be significantly higher than if it hits a section in contact with the seabed.
Preliminary analysis indicated that by using the LRFD (Load and Resistance Factor Design) approach as per DNV-RP-F111, a large number of free spans would require rock infill in order to limit the trawl pull-over load. In order to optimize the design and potentially reduce the requirement for free span infill, an optimised methodology based on SRA (Structural Reliability Analysis) was proposed by Wintershall, developed by DNV GL and implemented by Subsea 7 during detail design phase.
The optimised methodology involves FE analyses of the sensitivity to various parameters and Monte Carlo simulations, in order to quantitatively assess the probability of failure. Specific FE models analysing the bending moment capacity and response were established. The various input parameters were assessed and included as distributions if deemed required from the sensitivity analysis. Finally, a Monte Carlo simulation was performed to calculate the probability of failure.
It was demonstrated that the target safety levels defined by DNV-OS-F101 were reached without free span infill, and hence significant savings in subsea rock installation could be achieved without any deviation from the DNV-OS-F101 code. Cost saving due to reduced subsea rock installation scope is estimated to 7.6 mill Euro. Wintershall facilitated a close dialogue between DNV GL and Subsea 7 throughout the work progress. This resulted in a constructive and open approach, ensuring efficient delivery of the detailed design in line with project schedule.
The work carried out has demonstrated two relevant examples where performing a SRA, as opposed to a LRFD, has resulted in a significant reduction of seabed intervention requirements and an associated cost saving, while still being in compliance with DNV-OS-F101 target safety levels. To our knowledge this is the first time the SRA method has been applied for trawl interference design on a live project, and on a CRA (Corrosion Resistant Alloy) lined pipeline with DEH system attached. The success was made possible by close collaboration between the parties involved; Wintershall, DNV GL and Subsea 7.
Modeling of long marine risers subjected to VIV is a challenging problem to solve using 3D Computational Fluid Dynamics (CFD) due to the high length-to-diameter ratio and usage of different VIV suppression devices. The scale of the problem can also get extremely large as the risers may have lengths up to thousands of meters, which makes it hard to use 3D CFD simulations. However, CFD is unique to capture complex flow around bluff structures and able to model risers with and without suppression devices and involve effects of nonlinear structural response inherently.
ExxonMobil's high quality data on the behavior of high length-to-diameter ratio (L/D) risers subjected to vortex induced vibrations (VIV) in uniform and sheared flow (
Ellingsen, H. P. (DNV GL) | Lim, Y. K. (DNV GL) | Shin, H. C. (DNV GL) | Singh, K. (American Bureau of Shipping) | Kang, S. Y. (Hyundai Heavy Industries) | Kim, Y. H. (Samsung Heavy Industries) | Cho, S. (Daewoo Shipbuilding and Marine Engineering) | Ok, D. (DNV GL)
This paper describes measures to reduce the cost and delay of large offshore projects by use of new standard specifications for bulk items developed through a Joint Industry Project (JIP) initiated in 2015. The JIP has established several new standards specifications for structure, piping and Electrical and Instrumentation (E&I) with broad participation from fabricators, class societies, operators and engineering companies. The main objective of the JIP has been to reduce cost and increase predictability without compromising quality and safety by adopting industrial agreed standard components and equipment as well as procedures.
This paper outlines examples how and why the new standard specifications will reduce Capital Expenditure (CAPEX) and delay related to bulk items and the process applied in the JIP of aligning different standards and project specifications. A gap analysis method was applied to compare overlap and differences amongst standards and criteria for selection of standards were defined to select the most appropriate requirements and specifications. Business case templates have been established, documenting many examples causing delays and misunderstandings which would be addressed and eliminated through application of the new standard specifications.
The results cover the most common bulk items such as structural steel, outfitting, piping, cables, cable trays, instrument tubing, piping insulation and monorails. In some cases, the standard specifications bridge the gaps between existing international standards such as from International Organization for Standardization (ISO) and The American Petroleum Institute (API). In other cases, new requirements and specifications fill gaps in areas with lack of details in international standards or where no international standards and codes exists. The bulk standards will remove obstacles and streamline the fabrication process to the benefit for all stakeholders in engineering, procurement and construction. Although the new standard specifications are considered a quick win to save cost on bulk items in the short term, long term efforts are required to bring alignment amongst international standards such as ISO and API.
Following the design and material standardization, the next major step to be taken by the JIP and the industry is to create common coding and data formats of design and material. This step has the potential to make a big impact for CAPEX reduction and reduced time to first oil by creating a seamless interface between engineering and fabrication.