Kjell Kåre Fjelde

On a rough AUSM scheme for a one-dimensional two-phase model

Abstract We are interested in exploring Advection Upstream Splitting Method (AUSM) schemes for hyperbolic systems of conservation laws which do not allow any analytical calculation of the Jacobian. For this purpose, we consider a two-phase model which has been used for modeling of unsteady compressible flow of oil and gas in pipes. The model consists of two mass conservation equations, one for each phase, and a common momentum equation. Since no analytical Jacobian can be obtained it is more difficult to use classical schemes such as Roe- and Godunov-type schemes. We propose an AUSM scheme for the current two-phase model obtained through natural generalizations of ideas described in M.-S. Liou [J. Comput Phys 129(2) (1996) 364]. A main feature of AUSM is simplicity and efficiency since it does not require the Jacobian. In particular, we prove that the proposed AUSM type scheme preserves the positivity of scalar quantities such as pressure, fluid densities and volume fractions. This guarantees that the scheme can handle the important and delicate case of transition from two-phase to single-phase flow without introducing negative masses. Many numerical results are included to confirm the accuracy and robustness of the proposed AUSM scheme. In particular, it is demonstrated that the AUSM scheme gives low numerical dissipation at volume fraction contact discontinuities and is able to produce stable and non-oscillatory solutions, also when more complex slip relations are used, that is, when the relative motion of one phase with respect to the other is more or less complicated. This makes the scheme suitable for simulations of many important two-phase flow processes.

Relaxation schemes for the calculation of two-phase flow in pipes

In this paper, we are interested in some basic investigations of properties of the relaxation schemes first introduced by Jin and Xin [1]. The main advantages of these schemes are that they neither require the use of Riemann solvers nor the computation of nonlinear flux Jacobians. This can be an important advantage when more complex models are considered where it is not possible to perform analytical calculations of Jacobians and/or when considering fluids with nonstandard equation of state. We apply the schemes (relaxing and relaxed) to a certain two-phase model where Jacobians cannot in general be calculated analytically. We first demonstrate that the original relaxation schemes of Jin and Xin produce a poor approximation for a typical mass transport example which involves transition from two-phase flow to single-phase flow. However, by introducing a slight modification of the original relaxation model by splitting the momentum flux into a mass and pressure part, we obtain some flux splitting relaxation schemes which for typical two-phase flow cases yield a more accurate and robust approximation.

A Study of Possible Solutions for Cost Efficient Subsea Well Abandonment

There is a large number of subsea production wells offshore Norway approaching the end of their lifetime. Considering high spread rate of semisubmersible rigs, abandonment operations of these wells will be quite expensive. Moreover, Plug and abandonment (P&A) can easily contribute with 25% of the total costs of drilling for exploration wells offshore Norway. Hence it is of great importance to seek approaches and solutions to reduce the P&A cost. This paper reviews some possible new ways and also alternative technologies as the solutions to cut down the P&A expenses. Some of these technologies are now being used offshore Norway.In the first section of this paper, challenges of performing P&A operations offshore Norway together with the main cost drivers are discussed. It is then briefly argued how to consider issues such as barriers setting depth, cementing depth and logging in the design and well construction phases to ease or avoid future P&A challenges. For hydrocarbon exploitation in the Barents Sea and Arctic regions it is important to take into account the P&A phase in the early stage of planning and development.Light well intervention vessels as alternatives to semisubmersible rigs are recognized of being the largest contributor to cost saving. It will then be discussed to what extent vessel technologies can cut down the expenses for subsea abandonment. New ways of performing P&A can be another contributor to cost saving. It is shown how research and testing can assure the operators of new ways for performing P&A. Retrieval of production tubing is a challenging suboperation such that it imposes significant cost to subsea well abandonment. There have been performed studies on how P&A could be performed with tubing left in hole and it is of interest to pursue this further. We will investigate how the abandonment operations can be simplified and be more cost efficient if the production tubing can be left inside the well where the basic assumptions for being able to do it is accounted for.In addition, some complexities in abandonment operations can cause additional cost. An example of such complexities can be the need to establish two permanent barriers for potential permeable zones in overburden. It will be demonstrated how much can be saved with respect to cost if the regulations allow to ease some parts of abandonment operations.In this study, a probabilistic approach as a systematic tool to produce unbiased results is applied to quantify cost savings of new alternatives compared to the traditional ones.© 2015 ASME