Floating Production Storage And Offloading Hull Research Articles

Collision Damage Analysis of FPSO Hull Caisson Protection Structure

The protection structures for the Floating Production Storage and Offloading (FPSO) caissons should be sufficiently strong to avoid contact with the caisson pipes even when the protection structure is damaged by the impact of the accompanying vessels. Collision events of protectors of appurtenances such as risers, mooring lines, and seawater lift caissons with supply vessel may cause structural damage to protection structures and even to the appurtenance structures and hull structures. This study introduces the collision impact analyses on three protective structures of FPSO against striking supply vessel whose displacement is 7,500 tons. The capacity of protection structures in view of strain energy has been assessed with simple beam FE models. The striking vessel has been modelled as a small rigid body, and impact simulation has been performed including material and geometric nonlinearities where ABAQUS Explicit tool, which is a commercial explicit code, has been used for non-linear collision analyses with protection structures. The results from the current work will be a guide to understanding the impact response of offshore structures and evaluation approaches, and will provide useful indications for the FPSO hull caisson protection design and operation. In addition, the findings obtained by the current study will be informative in the safe design of FPSO facilities.

FPSO collision local damage and ultimate longitudinal bending strength analyses

Abstract In offshore oil &gas production there is a large concentration of platforms in a limited area of the sea, with the consequent increase of vessel traffic in the region. Platform supply vessels (PSVs), shuttle tankers and maintenance and safety units operate very close to these production platforms, creating a propitious scenario for collisions. Thus, the risk of collision between vessels and platforms has significantly increased, causing concern from the point of view of life loss, material damage and marine environment degradation. It is important to adequately design the structures and to predict the effects of accidents on the involved vessels. In the structural analysis of collision, geometric and material nonlinearities must be considered, as well as the striking ship velocity, the vessels draft difference, among other variables. In this work, the collision of a platform supply vessel with a single-hull Floating Production Storage and Offloading (FPSO) platform is studied through numerical simulation using the ANSYS LS-DYNA computational system. Damage is locally characterized by the collision force on the FPSO hull and the depth of penetration (displacement). In addition, it is also shown the energy absorption capacity of each type of structural element for the collision scenarios studied and a criterion is proposed to determine which structural element group should receive more attention in the design phase in order to reduce the effects of a collision. Both the longitudinal strengths of the intact FPSO as well as the remainder after collision are evaluated by assessing the bending moment versus curvature curves. It is observed that a single-hull FPSO platform with the thicknesses of the structural elements maintained unchanged in the conversion of the original oil tanker (VLCC) shows a significant ultimate longitudinal strength even after collision of a larger support vessel at higher velocity than recommended by the IACS rule.

Ultimate Strength Analysis of FPSO Hull Girder Under Longitudinal Bending

Ship and offshore structures have become the key for the development of the offshore world. Production and processing equipment can be placed on a platform, ship and/or barge structure called FPSO (floating, production, storage, and offloading units). The FPSO’s hull works like a beam and deforms in the vertical plane. In this case, weight and bouyancy are not normally distributed and wave loading. The most important aspect of FPSO structure is the materials and structural strength used in the design. The critical condition of FPSO is when the structure is under the operation so that the structure needs to be analysed for the design requirement. In the present study, the ultimate strength of FPSO hull girder under longitudinal bending moment is analysed using numerical approach. The nonlinear finite element analysis is adopted to handle the calculation. For the simple case, the one-frame space and the fully cross section of FPSO are modeled. The quadrilateral shell element of the nonlinear method is used for meshing. By performing the boundary condition with the Multi Point Constraint (MPC) is applied where the neutral axis located. The vertical bending moment is also placed at this point (MPC) and it is connected to the all nodes at both sides of the cross sections. The ultimate strength of FPSO is calculated only for intact in hogging and sagging conditions. The ultimate strength analysis is represented in terms of the moment-curvature relationship for hogging and sagging. The result obtained the nonlinear finite element analysis is compared with Smith’s method including their collapse behavior. It is found that the comparison result of the ultimate strength is about 8.9% and 10.1% in hogging and sagging condition, respectively. It is shown that the nonlinear finite element analysis is in good aggrement with the analytical solution performed by Smith’s method.

Investigations of offshore breaking wave impacts on a large offshore structure

This paper describes numerical and laboratory investigations that have been carried out to gain a better understanding of the physical processes involved in offshore breaking wave impacts on a large offshore structure. The findings are relevant to offshore and coastal structures and to identifying the extreme loads, peak pressures and maximum run-up needed for their design. A truncated wall in a wave flume is used to represent a vertical section of an FPSO (Floating Production Storage and Offloading) hull, which is a typical large offshore structure. Four types of wave impact were identified in the tests, and are referred to as slightly-breaking, flip-through, large air pocket and broken wave impacts. Physical modelling was undertaken in Plymouth University’s COAST Laboratory and the open source Computational Fluid Dynamics (CFD) package-Open Field Operation and Manipulation (OpenFOAM) was adopted to study focused wave generation and wave impact on the hull. The method solves incompressible Unsteady Reynolds-averaged Navier–Stokes Equations (URANSE) using a finite volume method with two phase flows. A Volume of Fluid (VoF) interface capturing approach is used to model the free surface. A NewWave boundary condition is used to generate focused wave groups based on the first plus second-order (hereafter second-order) Stokes wave theory in the Numerical Wave Tank (NWT). By changing the focus location with respect to the wall, the wave impact type was altered in both the numerical and laboratory investigations.The results show that for the four wave impact types tested good agreement was achieved between numerical predictions and experimental measurements of surface elevation, run up and impact force. The peak pressures predicted by the simulation are lower than the experimentally measured results due to time step constraints, although the shape of the pressure time history is very similar. Four distinct wave impact types are identified for the vertical hull section and are found to be similar in character to those observed for a full depth vertical wall. The predicted force on the hull is found to be greatest for the large air pocket impact, and the highest run-up for the slightly-breaking wave impact. The pressure records show a high degree of spatial and temporal variation though the highest pressure recorded at any location was due to flip-through. This research has shown that different characteristic wave impact types are responsible for maximum load and greatest wave run-up and so need to be considered separately for design purposes.

Offshore oil production units: a new concept of oil production floating hull

The Floating, Production, Storage and Offloading (FPSO) production systems have become a popular choice and may produce fairly appropriate technical and economic benefits for oil fields located far away offshore. In such a condition, underwater pipeline networks would not certainly be adequate. An alternative solution would be the use of large capacity floating reservoir to store the processed oil on board and ship it to shore by smaller shuttle tankers. Thus, some desirable features for the FPSO system include large storage capacity, large deck-areas to accommodate separation plant and good hydrodynamic performance when acted by ocean waves, wind and sea currents. Until very recently, there were worldwide offers of old large tankers approaching life cycle limit. Such a market characteristic helped to increase the number of conversion of old large tankers onto stationary floating production units. The declining of that market associated with the fast increasing of the number of FPSO new projects to be installed mostly in the Southeastern Brazilian Coast, the Western Coast of Africa and, more recently due to relaxation in the legislation, in the Gulf of Mexico, has pressed for changes in the way the next generation of FPSO will be planned. It is clear now that most of the new FPSO hulls will need to be designed and built from scrap. On the other hand, the design of a ship-like hull form to work specifically as a stationary unit raises opportunities to improve its operational performance. There is a wide open field for potential innovations to be incorporated into the design of new FPSO hulls to increase their hydrodynamic performance and to achieve higher efficiency and significant savings derived from using less-expensive equipments and possibly increasing operational time. The motivation of this paper is deeply involved in the perspectives mentioned earlier. It discusses results related to a new particular concept for a FPSO hull. The main focus of the discussion points towards the hydrodynamic behaviour of the new hull in waves and tries to identify its potential effects on costs and operational revenue.

Floating protection system for FPSO

Several Floating Production, Storage and Offloading (FPSO) vessels have been converted from large oil tanker ships with single skin side hulls. Safety measures have been evaluated to avoid excessive risks due to collision with supply vessels. Possible environment damage due to oil spill could be prevented by the installation of physical barriers close to the storage compartments in order to avoid accidental contact between FPSO and supply vessel. A floating protection system for the collision of supply vessel on FPSO hull during offshore operations is proposed in this paper. It comprises a cylindrical floating body attached through pinned braces to the transverse bulkheads of the FPSO. The concept is analysed in relation to the structural behaviour under impact load, using experimental and numerical models. In addition, the absorbed impact energy of the system is investigated through its motion into water.

Time-Variant Reliability Assessment of FPSO Hull Girder With Long Cracks

An efficient time-variant reliability formulation for the safety assessment of an aging floating production storage and offloading (FPSO) vessels with the presence of through-thickness cracks (i.e., long cracks), is presented in this paper. Often in ship structures, cracks are detected by means of close visual inspection when they have already propagated through the thickness. The propagation of long cracks in stiffened panels is therefore considered, as they may be present in critical details of the deck and/or bottom plating of the vessel. Although it has been found that stiffened panels are tolerant to fatigue cracking, the safety of such structural components with the presence of long cracks may be threatened when exposed to overload extreme conditions, i.e., brittle or ductile fracture may occur. The probability of brittle fracture of an aging hull structure, i.e., a stiffened panel at the bottom plating with the presence of long cracks is studied in this paper. The mean stress effect due to the continuously varying still-water loading as well as residual stresses is explicitly accounted for in the crack growth calculation procedure presented herein. An analytical model is established for determining the equivalent long-term stress range including the mean stress effect. The continuously varying still-water load effects due to the operational nature of FPSOs introduce additional uncertainties in the estimation of fatigue damage as well as in the likelihood of fracture failure mode. In the present case study it is found that the time-invariant approach is a good approximation when dealing with the time-variant reliability problem. One of the main conclusions drawn from this study is that the still-water mean stress has a significant effect on the failure probabilities of stiffened panels with long cracks under brittle fracture mode.

Hull/mooring/riser coupled dynamic analysis and sensitivity study of a tanker-based FPSO

A computer program is developed for hull/mooring/riser coupled dynamic analysis of a tanker-based turret-moored FPSO (Floating Production Storage and Offloading) in waves, winds, and currents. In this computer program, the floating body is modeled as a rigid body with six degrees of freedom. The first- and second-order wave forces, added mass, and radiation damping at various yaw angles are calculated from the second-order diffraction/radiation panel program WAMIT. The wind and current forces for various yaw angles of FPSO are modeled following the empirical method suggested by OCIMF (Oil Company International Marine Forum). The mooring/riser dynamics are modeled using a rod theory and finite element method (FEM), with the governing equations described in a generalized coordinate system. The dynamics of hull, mooring lines, and risers are solved simultaneously at each time step in a combined matrix for the specified connection condition. For illustration, semi-taut chain-steel wire-chain mooring lines and steel catenary risers are employed and their effects on global FPSO hull motions are investigated. To better understand the physics related to the motion characteristics of a turret-moored FPSO, the role of various hydrodynamic contributions is analyzed and assessed including the effects of hull and mooring/riser viscous damping, second-order difference-frequency wave-force quadratic transfer functions, and yaw-angle dependent wave forces and hydrodynamic coefficients. To see the effects of hull and mooring/riser coupling and mooring/riser damping more clearly, the case with no drag forces on those slender members is also investigated. The numerical results are compared with MARIN's wave basin experiments.

TotalFinaElf awards Angolan work

A computer program is developed for hull/mooring/riser coupled dynamic analysis of a tanker-based turret-moored FPSO (Floating Production Storage and Offloading) in waves, winds, and currents. In this computer program, the floating body is modeled as a rigid body with six degrees of freedom. The first- and second-order wave forces, added mass, and radiation damping at various yaw angles are calculated from the second-order diffraction/radiation panel program WAMIT. The wind and current forces for various yaw angles of FPSO are modeled following the empirical method suggested by OCIMF (Oil Company International Marine Forum).The mooring/riser dynamics are modeled using a rod theory and finite element method (FEM), with the governing equations described in a generalized coordinate system. The dynamics of hull, mooring lines, and risers are solved simultaneously at each time step in a combined matrix for the specified connection condition. For illustration, semi-taut chain-steel wire-chain mooring lines and steel catenary risers are employed and their effects on global FPSO hull motions are investigated. To better understand the physics related to the motion characteristics of a turret-moored FPSO, the role of various hydrodynamic contributions is analyzed and assessed including the effects of hull and mooring/riser viscous damping, second-order difference-frequency wave-force quadratic transfer functions, and yaw-angle dependent wave forces and hydrodynamic coefficients. To see the effects of hull and mooring/riser coupling and mooring/riser damping more clearly, the case with no drag forces on those slender members is also investigated. The numerical results are compared with MARIN's wave basin experiments.

Time-variant reliability assessment of FPSO hull girders

Floating production, storage, and offloading (FPSO) systems have been used for the development of offshore oil and gas fields. This paper presents a methodology for the time-variant reliability assessment of FPSO hull girders subjected to degradations due to corrosion and fatigue. The corrosion defect is modeled as an exponential time function with a random corrosion rate. When corrosion occurs, the plate thickness is assumed to be uniformly reduced. The crack propagation is predicted based on a Paris–Erdogan equation. A modified Smith method and an ultimate strength equation are proposed to predict the progressive collapse and ultimate strength of the hull girder with an effective width of plating and beam-column formula. The modified Smith method and equation account for the interaction of axial compression, lateral pressure, transverse stress and shear stress. The Ferry–Borges method is applied to combine the stochastic processes of still water and wave-induced bending moments. The environmental severity factors are introduced in order to calculate the wave-induced bending moments of specific-site conditions. Failure is defined as the first outcrossing from the safe domain to failure domain for t>0. An efficient response surface approach is used to approximate the outcrossing rate with the independent excursions of high levels. The time-variant reliability and the parametric analysis are quantified. The COV of ultimate strength of hull girders is suggested for simple FORM. Finally, an example calculation is conducted to evaluate the effect of environmental severity factors and the ship's principal parameters on the required hull girder strength of FPSO.