This thesis has proposed a slender well concept for exploration drilling with 15000 psi pressure rating based on enabling technologies. The main findings relates to the use of expandable liner hangers to reduce the required radial clearance between consecutive casing sections. Finite element analysis of the liner hanger expansion is performed in Ansys Workbench, a platform for advanced engineering simulations. The background for the thesis relates to the high cost of constructing offshore wells. There is a potential for considerable cost reduction by starting the well with a substantially smaller diameter, without compromising the final pipe size across the zone of interest. The topic builds on ongoing research in SBBU – Centre for drilling and wells for improved recovery, a joint project between NTNU, Sintef, University in Stavanger and IRIS. The slender well concept renders the possibility to use modified 3rd or 4th generation semi-submersible rigs. These rigs have significantly lower day-rates compared to new 5th and 6th generation rigs. Cost reduction is also expected with respect to consumption of steel for casing, drilling fluids and cement. Additional savings in steel is obtained by basing the casing program mainly on liners. The expandable liner hanger of choice is based on the XPak liner hanger developed by TIW. Finite element analysis indicated that a pressure rating of 15000 psi is feasible with the proposed liner hanger system. To avoid reduction in burst and collapse rating, the expansion mandrel is retained in the liner hanger after expansion. The mandrel is designed such that it creates an internal flush design with the liner string. It is recommended to use metal-to-metal sealing to avoid communication around the liner top. The slender well is constructed with limited radial clearance between consecutive casing sections. The problem of high surge pressures during running in hole is overcome by introducing a surge protection system with an artificial inner annulus to displace drilling fluids. The concept of pre-installing a liner string in the surface casing is introduced to render the possibility of an additional casing section and reduction in riser ID. The concept is untested and further evaluation is recommended. For further work on the topic proper field testing is recommended to validate the reliability of the concept. An assessment of slender well production drilling is also recommended to fully exploit the potential in slender well design. More detailed analysis and testing is necessary to qualify the expandable liner hanger for 15000 psi.

Duplex stainless steel flanges are normally forged to form, as required by the ASTM A182/A182M standard, but may also potentially be machined directly from forged stainless steel bar. In order to evaluate the integrity of such flanges, axisymmetric elastic-plastic finite element models have been developed, considering static effects such as bolt load and internal pressure. Additionally, tensile testing of a sample forged bar (UNS S31803) has been conducted. The stress distribution in a flange during gasket seating and operating conditions has been determined, as well as the degree of plastic strain caused by the bolt loads. The maximum stresses have been found to be around the same values as the minimum yield strength requirement of the studied material (UNS S31803), and the location of the maximum stress concentrations have been identified as the gasket groove. The tensile tests of the forged bar (UNS S31803) have shown that the yield and tensile strength properties are considerably higher than the standardized minimum requirements. The elastic modulus of the forged bar has also been determined, and was found to be lower than anticipated in the axisymmetric models. Some specimens have also been found to exhibit highly non-linear elastic properties.

Well integrity is an important topic in oil and gas well operations. The intention is to control the reservoir fluids and well pressures, by technical, operational and organizational barriers. Inadequate barriers can lead to unwanted influx and cause a kick, which can escalate further into a blowout. The Deepwater Horizon accident in the Gulf of Mexico in 2010 raised concerns about the safety of offshore deepwater drilling. This rig was considered to be an efficient and safe drilling unit, until the fatal blowout occurred leading to the loss of eleven lives, and the worst environmental disaster in US history.The main objective of a well barrier is to prevent leakage from the wellbore to the external environment during the various well operations. Well barriers are classified as primary or secondary barriers. The primary barriers are the barriers closest to the hydrocarbons and are the first obstacle to unwanted flow of formation fluid. The secondary barrier acts as a backup barrier. Barriers can further be classified as static or dynamic. The static barriers apply for the production phase, meaning that they are present over a longer period of time. Dynamic barriers occur during drilling and well intervention, and these are the once that are most difficult to assess because of the constantly changing parameters involved.Various drilling types such as overbalanced (conventional) and underbalanced drilling are used in the search for oil and gas. The main difference between these types, is how the pressure in the wellbore is maintained, and for this different barriers are needed.Laws and regulations dictate the requirements the industry has to follow. The Petroleum Safety Authority governs the regulations in Norway, and gives requirements to well barriers. In their regulations, they also refer to other guidelines and standards for more detailed requirements. NORSOK D-010 from 2013, “Well integrity in drilling and well operations”, is a widely used standard which defines requirements and guidelines relating to well integrity in drilling and well activities. Ways to regulate is found to vary between countries, such as for Norway and the US. During drilling, a dynamic barrier situation applies, where the activation of the barrier depends on the current situation, and there is no pre-made sequence of barrier activation. Overall, this makes the assessment a difficult task. Various approaches have been suggested for the assessment of dynamic barriers over the years, with contributions to the topic from oil-companies, standards and research papers. Most of what has been suggested has some kind of weaknesses, and no common approach has been accepted and put into use by the industry. As a contribution a new approach has been suggested in this thesis. An overbalanced drilling operation in deep waters initiated by a kick is selected as case study. The approach is based on using two different types of analysis, in combination. An event tree is used to illustrate the dynamics of the event, and various fault trees are used to analyze the barriers involved. To understand what can cause a kick, the possibility of avoiding it, and what to do if it happens are given attention. From the suggested approach, it emerges that the human factor plays an important role in the kick evaluation and to maintain the well integrity. If the primary barrier is lost, there will be a kick. For this to escalate into a blowout, the secondary barrier must also be lost. If the control equipment functions as intended, lack of detection and understanding, together with insufficient training and competence of personnel can make this happen.To prevent major accidents and hazardous events in the future, it is important to learn and gain experience from previous incidents and learn from each other, and the ability to work together, also between companies, on these difficult topics.