Evaluation of Abrasive Wear of Polymeric Materials Using Single-Pass Pendulum Scratching

Scratching abrasion due to rubbing against the sediment layer is an important degradation mechanism of flexible cable in deep water oil and natural gas exploitation. The present study was initiated to gain relevant data on the wear behaviour of some commercial materials used to externally protect these cables. So, Comparison tests were carried out using the single-point scratching technique, which consists of a sharp point mounted at the extremity of a pendulum. The energy dissipated during the scratching is used to evaluate the relative scratch resistance. The results showed, that the contact geometry strongly affects the specific scratching energy. Using SEM imaging, it was found, that these changes were related to the operating wear mechanisms. The observed wear mechanisms are also compared with those observed on some cables in deep water operations.

An In-Depth Look at the Effectiveness of Smart Materials for Monitoring and Control of Composite Structural Panels

In recent years the field of smart structures, including sensing and control, has been growing at an extraordinary rate. As of today, however, there has been relatively little work performed in applying this technology for monitoring and control of offshore and marine structures. In offshore environment, structures are subjected to critical loading cycles (e.g., wave action, temperature changes, and heavily corrosive environment to name a few), unparalleled to those in any other environment. With the costs that are associated with the manufacturing of marine vessels and structures, and their day-to-day operations and maintenance cost, the selection of an effective method of monitoring their performance and integrity, as well as their control is of paramount importance. In this paper, we will present a brief, yet detailed description of the different smart materials that are available for structural monitoring and control, such as electrorheological fluids, shape memory alloys, fiber optics, piezoelectrics, and magnetostrictives. We will also discuss the applications of these materials, including their advantages and shortfalls. The outlined discussion will help the reader to select the most optimum smart material for a given application. The paper will further discuss an ongoing investigation of a piezoelectric system that is currently being developed for implementation into fiber-reinforced composite panels used in marine vessel applications.

Potential Dependence of Hydrogen Embrittlement Susceptibility of Ultra High Strength Low Alloy Steel

In order to investigate the susceptibility of the ultra high strength low alloy steel to hydrogen embrittlement, a slow strain rate tensile test was carried out in boric acid-borax buffer aqueous solutions of pH 10 at the potential range from corrosion potential to hydrogen gas evolution potential, including adsorbed hydrogen potential. Experimental results revealed that the susceptibility to hydrogen embrittlement was dependent on the applied potential and increased linearly with increasing applied cathodic potential in the adsorbed hydrogen potential region. On the other hand, in the hydrogen gas evolution potential region, the susceptibility was independent of the applied potential and showed almost no variation. Based on the results obtained, these changes in susceptibility to hydrogen embrittlement with applied potential have been discussed in terms of the variation in reduction behavior of oxide films on the specimen surface.

Remotely Operated Underwater Thermal Cutting Processes for the Decommissioning of Large North Sea Platforms

A study has demonstrated the automation of underwater thermal cutting processes for remote decommissioning operations. The first phase of cutting trials evaluated five cutting processes underwater (air plasma, oxy-hydrogen, flux-cored arc, oxy-petrol and Kerrie cable systems). In the second phase, three manual cutting systems (oxy-petrol, “Broco” type system and Kerrie cable) were adapted for operation by a manipulator arm of the type commonly used offshore on Remotely Operated Vehicles. This manipulator, which usually requires a human operator, was interfaced to computer simulation and control software. The results of underwater cuts on steel plate using these systems are discussed, with applicability to offshore decommissioning. Ongoing work using other novel thermal cutting processes is also described.

Experiences From Handling Mooring Line Polyester Fibre Ropes During Installation and Retrieval

The use of fibre ropes made by synthetic fibres have been used more frequent as the offshore industry is moving towards larger water depths. An important aspect is the effect of handling on the large, but delicate, ropes during installation offshore using tools and equipment that easily can destroy the load bearing capacities of the ropes. In order to get hands on experience in the field large polyester ropes have been used as inserts in catenary mooring lines for Mobile Offshore Units (MODU) working on depths ranging from 80 to 350 meters. The ropes have been integrated in the catenary chain mooring lines both in the suspended part and in the bottom part of the mooring leg thus having been exposed to seafloor clay. Subsea buoy have been attached to the ropes using smaller size fibre ropes in order to lift the mooring lines from the seafloor. The paper will detail how the large fibre ropes have been mobilised and demobilised repeatedly from/to storage drums to/from the installation vessel winch drum. During installation and retrieval the fibre ropes have been installed from the vessels winch drum using regular anchor handling equipment and vessels. When the MODU has been moved between locations some ropes have been retrieved to the vessels winch drums while the others have been used to tow and to keep the units station. Samples of the used ropes are taken and is subjected to a test program in the laboratory in order to document the effect of extensive use and handling and exposure to seabed clay.

Cost Effective Fabrication of Large Diameter High Strength Titanium Catenary Riser

A large diameter high strength titanium free-hanging catenary riser was evaluated by the Demo 2000 Ti-Rise project, from initiative of the Kristin Field development license. In order to reduce the uncertainties related to the schedule, cost, and special technical issues identified in the work related to a similar riser for future installation on the A˚sgard B semi-submersible platform, a fabrication qualification of a full scale riser in titanium was run. Several full-scale production girth welds were made in an in-situ fabrication environment. The welding was performed on extruded titanium grade 23 (ASTM) pipes with an ID of 25.5″) and wall thickness of 30 mm. The main challenge was to develop a highly productive TIG orbital welding procedure, which produced welds with as low pore content as possible. It is well known that sub-surface pores often are initiation sits for fatigue cracks in high strength titanium welds. This paper describes how a greatly improved productivity was obtained in combination with a high weld quality. NDT procedures were developed whit the main on the reliability to detect and locate possible sub-surface weld defects, volumetric defects such as pores and tungsten particles and planar defects such as lack of fusion. The results from the actual Non Destructive Testing (NDT), the mechanical testing, and the fatigue testing of the subjected welds are presented. The response of the catenary is optimised by varied distribution of weight coating along the riser’s length. A satisfactory weight coating with sufficient strength, bond strength, and wear properties was developed and qualified. The riser is planned to be fabricated from extruded titanium pipes, welded together onshore to one continuous piece. The field coating is added and the riser is loaded into the sea and towed offshore and installed.

Characteristic S-N Curves for Fatigue Design of Titanium Risers

A recommended practice for design of titanium risers is currently being developed as part of Det Norske Veritas’ series of standards and recommended practices for offshore structures. A recommendation is given herein for characteristic S-N curves for use in design of titanium risers against fatigue failure. As a basis for this recommendation, a set of statistical analyses of available fatigue test data have been carried out. Separate analyses have been carried out for base material and welds. The analysis results have been interpreted with respect to mean S-N curves as well as 97.7% lower tolerance bounds with 95% confidence. Characteristic S-N curves for base material and welds, which are not non-conservative with respect to these tolerance bounds, have been proposed. The paper presents the assumptions, the test data, the statistical analyses and their results, and the proposed characteristic S-N curves. The areas of application of the proposed curves are discussed with a particular view to stress range interval, material grade, weld position, temperature, and defect size.

A Reliability-Based Method for Determining Design Ice Loads on Offshore Structures

This paper describes a reliability-based methodology that has been developed at ExxonMobil Upstream Research Company (URC) for determining rational design ice loads on offshore structures. The URC methodology provides a systematic framework to account for Type I (aleatory) and Type II (epistemic) uncertainties in assessing global probabilistic ice hazards. Specifically, a logic-tree based approach is developed to model Type II uncertainties in the assessment of ice hazards. Although the method has general applicability, the present work considers a wide, vertical-sided, gravity-based structure (GBS) in a dynamic, annual ice environment. Both FORM/SORM methods and Monte Carlo simulation are used in the analyses. Results obtained from this reliability-based approach indicate that the modeling of Type II uncertainties plays a significant role in quantifying the ice hazards for determining the design ice load. Further, this effort may potentially reduce over-conservatism in typical deterministic ice load calculations. The probabilistic methodology developed in this study has broad applicability and can provide a rational framework for calculating design ice loads on other types of structures for arctic offshore development.

Polyamide 11: A High Tenacity Thermoplastic, Its Material Properties and the Influence of Ageing in Offshore Conditions

Polyamide 11 is a key material in the fabrication of offshore flexible pipes. It is mainly used as the flexible pressure layer assuring the impermeability of the fluid and gas carrying flexible pipe. A further important use is as outer sheath material where it protects efficiently the metal strip structure from sea water ingress even in highly dynamic applications. Given these important functions of polyamide 11 the knowledge of its precise material properties is essential for the design and the operating limits of flexible pipes. This paper aims to give a detailed understanding of the scope of the material properties such as fracture toughness, fatigue resistance and the mechanical response function. In a further step the influence of ageing on these properties is outlined with the aid of aged model specimen studies.

Fatigue and Ultimate Limit State of Grouted Tubular Joints

A joint industry project commenced in 1993 to develop a design manual for tubular joints, which are strengthened or repaired through chord grout filling. This project was carried out in two phases and was completed in the late 1990’s. The project comprised the conduct of over 200 SCF and ultimate load tests on large scale as-welded and grouted tubular joints. In addition to the testing programme, studies on offshore deployment were carried out to ensure that complete chord grout filling was achievable for all practical scenarios. The primary reason for carrying out this project is the industry-wide recognition that chord grout filling represents an extremely cost-effective and mechanically efficient method to strengthen or repair tubular joints. Further, it has been recognised that double-skin joints (e.g. pile through leg with annulus grout-filled) are often present in structures and the enhanced strength and fatigue characteristics as a result could be exploited to permit more efficient new platform designs or better estimation of joint performance for existing installations. Although API RP2A [1] and ISO [2] recommendations permit the use of grouted joints, little guidance is provided. The guidance that is provided is based on public domain data and engineering principles. This project was carried out to generate a substantial amount of new data/information, leading to the creation of a detail design manual for grouted joints [3,4].