Petrobras has been successfully dealing with deep water floating production systems using flexible pipes since 1977. During the completion of Marlim South 3 well in 1977, Petrobras was surprised by the occurrence of two birdcage type failures. At that time, Marlim South 3, in a water depth of 1709 m was the deepest offshore production well in operation. Since then, Petrobras has been testing flexible pipes using a field test known as DIP test. In a DIP test, an empty end capped sample of a flexible pipe, about 150m long, is partially supported by the sea bottom and connected to a lay vessel by a pipe follower or a wire rope. The flexible pipe has to withstand a 4 hour period of cyclic bending due to the motions of the lay vessel. The DIP test has provided Petrobras with information on a new failure mode: lateral buckling in the armor wire. Although a birdcage failure is equally undesirable, lateral buckling of the armor wires implies more danger because it can go unnoticed. In 2001, a research project was set up by the Research and Development Center of Petrobras that was aimed at reproducing the flexible pipe failure modes under laboratory conditions. The purpose was to obtain a better understanding of the failure process, as well as to develop testing alternatives to avoid the significant costs related to DIP tests. In order to assess the effect of cyclic bending as a major factor in degrading the longitudinal compressive strength of flexible pipes 15 destructive tests were performed on 4 inch diameter flexible pipe samples. Two test rigs that accommodated three types of test and a number of test procedures were developed in the project. The number of bending cycles to failure for each sample was determined when subjected to compressive action corresponding to its operational depth. Tests to evaluate the effect of pre-existing damage were also conducted. Special attention was devoted to the effect of layer arrangement on compressive failure. The test results clearly identified the basic failure modes under investigation (i. e. birdcaging and lateral buckling of the armor wires). Suggestions regarding simplified testing procedures and corresponding performance criteria are also presented.
A procedure is presented to determine the vibration frequencies and mode shapes of submerged structures. Hamilton's principle is used to formulate the problem, and general sequences of trial functions are introduced to permit the direct extremization of the corresponding energy functional. The fluid-structure interaction is accounted for by restricting the trial functions of the fluid to represent the flow around the immersed body imposed by each trial function of the structure. The trial functions for the fluid are built up from the contributions of triangular constant source elements distributed over the wetted surface of the structure, the accuracy of which in representing a flow is tested on an accelerated sphere. The structure is modeled with finite elements. The accuracy of the hydroelastic procedure is accessed on two examples as compared to experimental data: a cantilever plate and a freely floating ship.
The conversion of ageing tankers is a tempting alternative to newbuildings when FPSO units are planned. The low cost of an already classified hull and an existing pumping facility compensate for the repair and updating costs a conversion may demand. Newbuildings on the other hand allow for improved production plant and storage tank lay-out. The trade offs between conversions and newbuildings are analyzed from the point of view of strength and general arrangement. Converted tankers carry a serious strength drawback since the emptied engine space enhances the high buoyancy region at the stern which significantly increases bending moment amidships. In general one or more cargo tanks must be kept at partial loading to compensate for the increased bending moment. A new building lay-out is analyzed in which the tanker engine space is completely eliminated. Utility systems are resized and placed on the main deck aft of the production plant. Accommodation spaces are also resized and rearranged in order to fit into the reduced deck space available. The gains in strength are quantified and the lay-out implications are discussed in detail. A comparative study is presented between a conversion of a hypothetical tanker and an equivalent new building.
A floating structure is proposed to act as a dry wellhead completion support for a mono-column design developed by Petrobras: the MONOBR FPSO. After the discussion of the design issues involved, the MONOBR design is introduced and the peculiarities affecting the insertion of a floating structure into its moonpool are analyzed. In what follows, the main characteristics of the proposed floating structure layout are presented in the context of its purpose of allowing for the effortless vertical motion relatively to the housing mono-column unit, followed by the considerations pertinent to the ballast system devised to allowing for the adequate operation, by the characteristics of the roller mechanisms proposed to ensure the aligned motion desired and by the analysis of the structural issues and the proposed structural layout.
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