More on Unfold/Fold Transformations of Normal Programs: Preservation of Fitting's Semantics

The unfold/fold transformation system defined by Tamaki and Sato was meant for definite programs. It transforms a program into an equivalent one in the sense of both the least Herbrand model semantics and the Computed Answer Substitution semantics. Seki extended the method to normal programs and specialized it in order to preserve also the finite failure set. The resulting system is correct wrt nearly all the declarative semantics for normal programs. An exception is Fitting's model semantics. In this paper we consider a slight variation of Seki's method and we study its correctness wrt Fitting's semantics. We define an applicability condition for the fold operation and we show that it ensures the preservation of the considered semantics through the transformation

Feasibility Study of Application of Residual Curvature Method in Deepwater Free Hanging Risers

Master's thesis in Offshore TechnologyThe Offshore Oil and Gas industry has been moving to deeper water and needs to manage production in more extreme conditions. Risers systems are essential components for the transportation of these hydrocarbons from producing subsea wells to topside facilities. To reduce costs and improve safety, it is vital to use the most proper and effective design for these conductor pipes. This thesis proposes a riser configuration to cope with high floater motions in deep water under extreme conditions such as hurricanes, producing from a typical subsea well. The idea was to modify the geometry of sections of free hanging steel catenary risers to improve its performance by partially decoupling the touch down point of this conductor pipe from floater motions. Residual Curvature Method (RCM) was applied to Steel Catenary Riser (SCR) to achieve this. This method has been applied to control thermal buckling during operation in subsea pipelines. The mentioned technique involves the creation of un-straightened segments or sections with residual curvature in the riser during installation. For ease, any SCR with sections with residual curvature applied will be named as RCSCR. This project focuses on the Residual Curvature Method applied to a high-pressure line steel catenary riser for 1500m water depth. It was determined that the RCM is a self-limiting process, where the section with residual curvature is able to absorb compressive forces, improve utilization and fatigue in the touch down area of the riser; however, the un-straightened section itself brings more bending moment, utilization and fatigue where it is located, limiting its application. The most optimum configuration for the section, which is made up of 4 subsections, must be applied close to the touch down area and is limited to a curvature of 0.009 m-1 for a 10 inch-internal diameter and 39 mm of wall thickness SCR; with this, obtaining a residual strain of 0.15% in the section. Likewise, it was found that the un-straightened section length that perform best was 180 m which is 12% of the water depth. A screening of the downward velocities at the hang-off point up in the platform was considered and analyzed, identifying the maximum values for buckling utilization, bending moment and compression, which are main critical responses. SCR with RCM, RCSCR was investigated in terms of the capabilities needed to handle the floater motions. The riser configuration was examined concerning strength, as well as fatigue performances to determine its limitations and merits. The strength assessment was achieved by using load cases with different sea states, including a typical 3h-winter storm and a hurricane occurring in the Gulf of Mexico. In this study, the sea states were established following JONSWAP wave spectra. The screening approach was based on the downward velocity or heave velocity at the hang-off point, which has been validated to be the principal design criteria for riser integrity. All the checks were performed in accordance with DNV codes. According to the results for extreme analysis, the SCR studied is restricted to a maximum downward velocity of 2.64 m/s, while the Steel Catenary Riser with Residual Curvature method (RCSCR) can cope with downward velocities up to 2.94 m/s at the hang-off point. Similarly, it was validated that the selected Weight Distributed SCR (WDSCR) configuration for this study can cope with a downward velocity of 3.2 m/s, while the combination of the Residual Curvature Method and WDSCR configuration can improve the coping of downward velocity at the hang-off point up to 4.01 m/s. On the other hand, the fatigue analysis of the riser configurations was carried out taking into account wave-induced fatigue. In general, the application of the un-straightened section through the Residual Curvature Method extended the life in fatigue of the SCR, increasing its value from 243 years to 599 years in the RCSCR configuration. The application of the Residual Curvature Method was found to be a viable solution to improve the performance for strength and fatigue of SCR and WDSCR configurations to cope with high motion of floaters. Investigation needs to be done for stability in riser-rotation while in operation, durability and optimization of the un-straightened section. This thesis work showed that, although deep water free hanging risers (SCR and WDSCR) can be limited for coping with large floater motions, economic and innovative solutions can be proven to increase their feasibility to handle higher heave motions in extreme conditions. All in all, even though it is a self-limiting technique, the application of the Residual Curvature Method reduces stress and fatigue loads in SCR and WDSCR configurations

The 31st Aerospace Mechanisms Symposium

The proceedings of the 31st Aerospace Mechanisms Symposium are reported. Topics covered include: robotics, deployment mechanisms, bearings, actuators, scanners, boom and antenna release, and test equipment. A major focus is the reporting of problems and solutions associated with the development and flight certification of new mechanisms

On the readability of machine checkable formal proofs

It is possible to implement mathematical proofs in a machine-readable language. Indeed, certain proofs, especially those deriving properties of safety-critical systems, are often required to be checked by machine in order to avoid human errors. However, machine checkable proofs are very hard to follow by a human reader. Because of their unreadability, such proofs are hard to implement, and more difficult still to maintain and modify. In this thesis we study the possibility of implementing machine checkable proofs in a more readable format. We design a declarative proof language, SPL, which is based on the Mizar language. We also implement a proof checker for SPL which derives theorems in the HOL system from SPL proof scripts. The language and its proof checker are extensible, in the sense that the user can modify and extend the syntax of the language and the deductive power of the proof checker during the mechanisation of a theory. A deductive database of trivial knowledge is used by the proof checker to derive facts which are considered trivial by the developer of mechanised theories so that the proofs of such facts can be omitted. We also introduce the notion of structured straightforward justifications, in which simple facts, or conclusions, are justified by a number of premises together with a number of inferences which are used in deriving the conclusion from the given premises. A tableau prover for first-order logic with equality is implemented as a HOL derived rule and used during the proof checking of SPL scripts. The work presented in this thesis also includes a case study involving the mechanisation of a number of results in group theory in SPL, in which the deductive power of the SPL proof checker is extended throughout the development of the theory