Factors Influencing the Design and Construction of LNG Storage Tanks

A number of LNG storage tank designs have been implemented on past and current projects. Such designs include full containment tanks with a 9%Ni inner liner for primary containment and storage of LNG at cryogenic temperature, a carbon steel liner for secondary liquid containment, thermal corner protection and vapor barriers on the inner tank and a concrete outer tank for resistance to external loads. LNG storage tanks are a key component of an LNG liquefaction or regasification facility and there is only a handful of specialized contractors that possess the tank technology and track record of the materials involved and the construction know how required for successful execution. This paper will focus on factors influencing LNG storage tanks design selection and construction execution strategies which can be a function of functional requirements as well as project location.

Modeling the Effect of Reduced Liquid-Wetting on Spontaneous Imbibition for Condensate Blocking Applications

In this paper, we present our interpretation for some of the unexpected experimental results that we obtained during 3D spontaneous imbibition experiments. We treated carbonate rocks with flourochemical polymers where we altered the wettability towards reduced liquid-wetting. The expected result is a reduced imbibition rate as a result of reduced capillary forces. Although the early imbibition rate decreased, we observed an increase in rate at late imbibition time resulting in a higher liquid saturation in the altered rock. We used numerical modeling to interpret the result and show that this observation actually represents the target wettability state we should be seeking in some applications.

The Mooring Bay Concept for LNG Loading in Harsh and Ice Conditions

The demand for natural gas from offshore fields is continuously increasing. Especially future production from Arctic waters comes into focus in context with global warming effects leading to the development of a dedicated technology. Relevant approaches work with floating turret moored production terminals (FLNG) receiving gas via flexible risers from subsea or onshore fields. These terminals provide on-board gas treatment and liquefaction facilities as well as huge storage capabilities for LNG (Liquefied Natural Gas), LPG (Liquefied Petrol Gases) and condensate. Products are transferred to periodically operating shuttle tankers for onshore supply reducing the need for local onshore processing plants providing increased production flexibility (future movability or adaptation of capacity). Nevertheless, in case of harsh environmental conditions or ice coverage the offshore transfer of cryogenic liquids between the terminal and the tankers becomes a major challenge. In the framework of the joint research project MPLS20 ([1]), an innovative offshore mooring and cargo transfer system has been developed and analyzed. MPLS20 is developed by the project partners Nexans ([2]) and Brugg ([3]), leading manufacturers of vacuum insulated, flexible cryogenic transfer pipes, IMPaC ([4]), an innovative engineering company that has been involved in many projects for the international oil and gas industry for more than 25 years and the Technical University (TU) Berlin, Department of Land- and Sea Transportation Systems (NAOE, [5]), with great expertise in numerical analyses and model tests. The overall system is based on IMPaC’s patented and certified offshore ‘Mooring Bay’ concept allowing mooring of the vessels in tandem configuration and simultaneous handling and operation of up to six flexible transfer pipes in full aerial mode. The concept is outlined to operate with flexible transfer lines with 16-inch inner diameter like the newly designed and certified corrugated pipes from Nexans and Brugg. The mooring concept and its major subsystems have proven their operability by means of extensive numerical analysis, model tests and a professional ship handling simulator resulting in an overall transfer solution suitable to be used especially under Arctic conditions like addressed by the EU joint research project ACCESS (http://access-eu.org/). The paper introduces the new offshore LNG transfer system and focuses especially on its safe and reliable operability in the Arctic — with ice coverage as well as in open water conditions.

Iceberg Shape Sensitivity in Ship Impact Assessment in View of Existing Material Models

Large natural resources in the Arctic region will in the coming years require significant shipping activity within and through the Arctic region. When operating in Arctic open water, there is a significant risk of high-energy encounters with smaller ice masses like bergy bits and growlers. Consequently, there is a need to assess the structural response to high energy encounters in ice-infested waters. Experimental data of high energy ice impact are scarce, and numerical models could be used as a tool to provide insight into the possible physical processes and to their structural implications. This paper focuses on impact with small icebergs and bergy bits. In order to rely on the numerical results, it is necessary to have a good understanding of the physical parameters describing the iceberg interaction. Icebergs are in general inhomogeneous with properties dependent among other on temperature, grain size, strain rate, shape and imperfections. Ice crushing is a complicated process involving fracture, melting, high confinement and high pressures. This necessitates significant simplifications in the material modeling. For engineering purposes a representative load model is applied rather than a physically correct ice material model. The local shape dependency of iceberg interaction is investigated by existing representative load material models. For blunt objects and moderate deformations the models agree well, and show a similar range of energy vs. hull deformation. For sharper objects the material models disagree quite strongly. The material model from Liu et.al (2011) crush the ice easily, whereas the models from Gagnon (2007) and Gagnon (2011) both penetrate the hull. From a physical perspective, a sharp ice edge should crush initially until sufficient force is mobilized to deform the vessel hull. Which ice features that will crush or penetrate is important to know in order to efficiently design against iceberg impact. Further work is needed to assess the energy dissipation in ice during crushing, especially for sharp features. This will enable the material models to be calibrated towards an energy criterion, and yield more coherent results. At the moment it is difficult to conclude if any of the ice models behave in a physically acceptable manner based on the structural deformation. Consequently, it is premature to conclude in a design situation as to which local ice shapes are important to design against.

Challenges About Testing, Welding and NDT of CRA Pipelines in Brazilian Pre-Salt

Some new pre-salt fields at Santos Basin in Brazil are located in water depths as deep as 2200m and about 300 km away from the coast. There is variable level of contaminants in the produced fluid, mainly CO2 that affects the material selection of the infield flowlines and risers. Based on these constraints, Petrobras has selected UNS N06625 clad or lined steel linepipes to develop the first fields in Pre-Salt area and also the module 3 of Roncador a post-salt field in Campos Basin. Several challenges have arisen during design, construction and installation of these facilities related to pipeline welding procedures, NDT inspection and Engineering Critical Assessment (ECA). Firstly weld overmatching condition may not be fully achieved due to differences in mechanical properties between UNS 06625 and API X65, and concern increases when reel-lay installation method is chosen. Another welding issue is the maximum interpass temperature of nickel alloys (DNV and PETROBRAS standards limit that to 100°C) and this impacts pipeline installation productivity. Thirdly, back purging characteristics and number of passes protected with purging gases affects the possibility of root oxidation. Also, the inspection of the weld overlay in the pipe end of lined linepipes is other point of great concern. As defect sizing is mandatory for ECA, lined pipes have been designed with a weld overlay length which allows the inspection of the final girth weld by AUT. However, AUT solutions are normally more efficient in rolled or extruded materials than in weld overlaid ones. Additionally, the ECA methodology for both girth welds and weld overlay has complexities that are not usually addressed in a regular ECA for carbon steel pipelines (e.g. internal misalignment of girth welds in risers has stringent requirements because of its effects on fatigue performance and, consequently, the ECA girth weld criteria). Also CRA clad/lined pipelines and risers qualification program may include additional testing when compared with usual carbon steel welding qualification process (namely pitting and intergranular corrosion, full scale fatigue, spooling trials of lined linepipes, segment testing for ECA, and others). Finally, the contribution of clad/lined layer in pipeline design strength is also discussed. This work presents challenges PETROBRAS has faced at design and construction phases of on-going Guaré and Lula-NE pre-salt fields and Roncador field projects, as well as the solutions proposed by the project team in order to overcome the issues raised during project execution.

Aspects of Importance Related to Arctic DP Operations

International operators are seeking, investigating and pursuing new business opportunities in the Arctic. While operating in the Arctic, there will be a considerable need for vessels to keep their position during various operations which may include lifting, installation, crew change, evacuation, and maybe drilling. Opposed to open water, the drifting ice poses severe limitations as to how stationkeeping operations may be carried out. Dynamic positioning systems are currently developed aiding stationkeeping without mooring systems. There is a considerable need to enhance the open water DP systems for use in a new forcing environment. Essentially a new technology has to be developed with time. For that reason, considerable knowledge is required concerning current limitations and boundary conditions. This paper addresses some of the generic challenges related to DP operations in ice together with relevant learnings which are employed in mentioned DP enhancements.

Surveillance Models of Large-Scale ESP With High Viscosity Fluids and Gas

The paper reports on developmental research on the effects of viscosity and two phases, liquid–gas fluids on ESPs which are multi stage centrifugal pumps for deep bore holes. Multiphase viscous performance in a full-scale Electrical Submersible Pump (ESP) system at Shell’s Gasmer facility has been studied experimentally and theoretically. The main objectives is to predict the operational conditions that cause degradations for high viscosity fluids when operating in high Gas Liquid Radio (GLR) wells to support operation in Shell major Projects. The system studied was a 1025 series tandem WJE 1000. The test was performed using this configuration with ten or more pump stages moving fluids with viscosity from 2 to 200 cP at various speed, intake pressure and Gas Void Fractions (GVF). For safety considerations the injected gas was restricted to nitrogen or air. The ESP system is a central artificial lift method commonly used for medium to high flow rate wells. Multiphase flow and viscous fluids causes problems in pump applications. Viscous fluids and free gas inside an ESP can cause head degradation and gas locking. Substantial attempts have been made to model centrifugal pump performance under gas-liquid viscous applications, however due to the complexity this is still a uncertain problem. The determination of the two-phase flow performance in these harmful conditions in the ESP is fundamental aspects in the surveillance operation. The testing at Shell’s Gasmer facility revealed that the ESP system performed as theoretical over the range of single flowrates and light viscosity oils up to Gas Volume Fractions (GVF) around 25%. The developed correlations predict GVF at the pump intake based on the operational parameters. ESP performance degrades at viscosity higher than 100cp as compared to light oil applications, gas lock condition is observed at gas fraction higher than 45%. Pump flowrate can be obtained from electrical current and boost for all range of GVF and speed. The main technical contributions are the analysis of pump head degradation under two important variables, high viscosity and two-phase flow inside the ESP.

Establishment of Testing Methodology for Forge Welded Tubular Products

A state-of-the-art small-scale solid state forge welding machine has been fabricated for checking weldability by Shielded Active Gas Forge Welding (SAG-FW) of tubular products applicable predominantly for, but not limited to offshore Industries. Effective, fast and inexpensive welding and testing of joints make this small-scale method suitable for evaluating weldability of a material before starting regular qualification and fabrication in a full-scale welding machine normally located in spool base or offshore. The small-scale machine provides a complete package for pre-qualification studies, including assessment of welding conditions, material flow behavior, heat treatment options. However, there are considerable challenges relating to application of international standards of testing as well as interpretation and use of results in the context of large-scale welding. In this paper results from small-scale welding and weld characterization of an API 5L X65 quality are presented. First, a detailed test plan for analyzing the weld is outlined. This procedure is subsequently applied for checking the welds to be produced in the full-scale machine. Short-comings in using the small-scale process as well as the possible remedies are discussed in detail.

Planning for Construction Work in Cold Climate Regions

With increased interests in oil and gas exploration in cold climate regions, it is not realistic that all construction activities can take place during the short summer and work will continue into the early fall and possibly later. The offshore contractors must, therefore, be ready to participate in construction work in these regions during an extended season, i.e. outside the summer season with milder weather conditions. It is also important that some key work-intensive activities (e.g. pipe laying) can start as early as possible in the season. This paper will discuss the challenges associated with construction work in cold climate regions with emphasis on the physical conditions, in particular with reference to Polar Low Pressures and the potential for icing, as well as the logistics of working long distances from established supply bases. Large uncertainties in weather forecasts call for proper management decisions accounting for the specifics of the area. Long periods of “waiting on weather” might result and management must have the patience to wait until safe operations can commence. Emphasis will be on the Barents Sea where recent hydrocarbon findings have proven very encouraging and where a huge area soon will be opened for exploration, following the agreement on the border between Norway and Russia, potentially calling for joint Norwegian–Russian construction projects (Bulakh et al., 2011).

Non Parametric Probabilistic Approach of Ice Load Peaks on Ship Hulls

Due to its complex phenomenon, ice-induced load process could not be physically possible to be modeled by a specific well established probabilistic model. Therefore, a non parametric approximate probabilistic approach should be performed based on the available data. This paper describes the procedure and also the extrapolation to get the short term and long term extreme values. Comparison with the classical approach, where the initial distribution of ice-induced load is assumed to be one of the well established probabilistic models, was made. The comparisons discussed in this paper were explored by using the same available data of full scale measurement on board a coastal guard vessel KV Svalbard during the winter 2007. There was a tendency that the non parametric approach produces more conservative results.