Experimental and Analytical Studies of Wave Impact Forces on Ekofisk Platform Structures

Wave impact load occurring in a liquid storage tank during a sloshing motion is numerically simulated. Due to a violent sloshing, an excessive impact load can cause a critical damage to the tank structure. Recently this type of the accidents are reported and the problem becomes an important research topic in LNG (Liquefied Natural Gas) Tanker and FPSO (Floating Production Storage Offloading) design. To predict the sloshing impact load, Morison’s formula could be used for a practical reason. But using the Morison formula may provide directly an inaccurate estimation for the impact load because this formula is based on the linear model in the present nonlinear dominating phenomena. In this study, the wave impact load on the structure is obtained by imposing the exact nonlinear free surface conditions numerically and compared with that predicted by Morison’s formula. As a numerical method, a three-dimensional free surface flow in a tank is formulated in the scope of potential flow theory with the nonlinear free-surface conditions. A finite-element method based on Hamilton’s principle is employed as a numerical scheme. The problem is treated as an initial-value problem. The nonlinear problem is numerically solved through an iterative method at each time step.

To quantify the loading due to breaking waves, model tests are currently the option of choice. However, it is suspected that scale effects lead to an overestimation of the prototype loading. As such, a typical question in impact load assessment is how conservative load measurements are. To understand how realistic model testing is in this respect, it is necessary to quantify scale effects in the measured loading due to breaking waves as well as to investigate to which extent entrapped air in the wave and during the wave impact is involved. To do so, the BreaKin Joint Industry Project was started in 2016. The objective of the BreaKin JIP was to get more insight into scale effects involved in wave-in-deck model tests and to take first steps towards linking wave kinematics with measured impact loads. During this JIP wave-in-deck model tests were carried out in MARIN’s Depressurized Wave Basin (DWB) at two scales (25 and 50) in atmospheric and depressurized condition. In previous work, a summary overview of these model test results was presented [1]. The current paper provides more in-depth insights into possible sources of scale effects involved in wave-in-deck type of loading, based on further analysis of the existing model test data. It includes a detailed review of a.o. local loads in correlation to the variability of measured (global) impact loads between repeat measurements.

ABSTRACT. This paper reviews the planning, preparation and initial implementation of drilling operations for the latest phase of development of Phillips Petroleum's Ekofisk Field in the Norwegian Sector of the North Sea. This development includes the addition of 32 new production and injection wells to the existing t 80 well field. The unique aspect of the project was that, at the time of conception of the project, only 8 spare slots were available on existing structures, and preliminary economics proved the addition of a new structure unfavorable. The decision to utilize existing structures for the additional wells created many challenges for Phillips Petroleum Norway's engineers and this paper describes a number of the more interesting and unexpected of those challenges and their solutions. BACKGROUND. The Ekofisk field is the largest of the Phillips Norway Group's seven field " Greater Ekofisk" operation, located in the south west corner of the Norwegian sector of the North Sea. An overview of the " Greater Ekofisk" infrastructure is shown in fig.1. The Ekofisk field itself now comprises three production and two injection platforms, as well as eight other accommodation, process and transportation facility platforms. Production from the Ekofisk field peaked in 1980 with production figures of 618rO00 BOPD and 1,860 NMSCFD of gas. production decline, led to the evaluation and pilot study of a field water injection program. In 1986, the Ekofisk field water injection program commenced from the 2/4 Kilo injection platform (30 well slots capacity). Water injection into the Ekofisk reservoir reached 375rO00 BWPD in 1990 into a total of 20 wells. Positive results prompted Phillips to evaluate further development of water injection coverage, as well as adding producers to optimize recovery from the reservoir. The location of 2/4 Kilo, near the north end of the Ekofisk field, limits access to the center and south end of the field for injection purposes. Also, in 1986r nearly all 56 well slots located on the three Ekofisk production platforms, 2/4 Alpha (16 slots), 2/4 Bravo (24 slots) and 2/4 Charlie (16 slots) had been utilized. As a result, studies were initiated to evaluate the options for increasing the number of well slots at different locations in the field. Various scenarios were considered including new wellhead platforms, cantilever extensions to existing structures, and modifications to existing facilities. The decision was finally made a) to modify two of the existing production platforms (2/4 Alpha in the South of the field, and 2/4 Charlie near the field center) to add eight extra well slots on each structure, and b) to modify one of the bridge support tripods at the south end of the Ekofisk complex to accept a wellhead module for eight water injection wells. The location of the bridge support was ideal, in that it gave access to center and southern areas of the field for improved water injection coverage. The new injection platform was assigned the designation - 2/4 W.

High‐speed craft frequently experience large wave impact loads due to their large motions and accelerations. One solution to reduce the severity of motion and impact loadings is the installation of ride control systems. Part 1 of this study investigates the influence of control algorithms on the motions of a 112‐m highspeed catamaran using a 2.5‐m model fitted with a ride control system. The present study extends this to investigate the influence of control algorithms on the loads and internal forces acting on a hydroelastic segmented catamaran model. As in Part 1, the model active control system consisted of a center bow T‐Foil and two stern tabs. Six motion control feedback algorithms were used to activate the model‐scale ride control system and surfaces in a closed loop system: local motion, heave, and pitch control, each in a linear and nonlinear application. The loads were further determined with a passive ride control system and without control surfaces fitted for direct comparison. The model was segmented into seven parts, connected by flexible links that replicate the first two natural frequencies and mode shapes of the 112‐m INCAT vessel, enabling isolation and measurement of a center bow force and bending moments at two cross sections along the demi‐hulls. The model was tested in regular head seas at different wave heights and frequencies. From these tests, it was found that the pitch control mode was most effective and in 60‐mm model‐scale waves it significantly reduced the peak slam force by 90% and the average slam‐induced bending moment by 75% when compared with a bare hull without ride controls fitted. This clearly demonstrates the effectiveness of a ride control system in reducing wave impact loads acting on high‐speed catamaran vessels.

A fully nonlinear numerical wave tank (NWT) is set up for extreme wave modelling and wave impact simulation. As a first step, a focused wave train was model and the focal location where the steep focused wave is close to breaking was determined. Subsequently, an in-depth investigation is performed on the wave impact loads on the wave-ward side of a deck box of a semi-submersible subject to this extreme wave. The numerical results are also compared with the predictions in accordance to the DNVGL technical guide. It is concluded that the severe horizontal impact loads are caused by the “strike” action of the rushing water mass from near-breaking waves rather than the run-up jets. The predictions based on the technical guide are conservative and in general overestimate the impact loads. These findings could contribute to a more comprehensive understanding and a more accurate algorithm with respect to the horizontal impact loads.

To quantify the loading due to breaking waves, model tests are currently the option of choice. However, it is suspected that scale effects lead to an overestimation of the prototype loading. As such, a typical question in impact load assessment is how conservative load measurements are. To understand how realistic model testing is in this respect, it is necessary to quantify scale effects in the measured loading due to breaking waves as well as to investigate to which extent entrapped air in the wave and during the wave impact is involved. To do so, the BreaKin Joint Industry Project was started in 2016. The objective of the BreaKin JIP was to get more insight into scale effects involved in wave-in-deck model tests and to take first steps towards linking wave kinematics with measured impact loads. During this JIP wave-in-deck model tests were carried out in MARIN’s Depressurized Wave Basin (DWB) at two scales (25 and 50) in atmospheric and depressurized condition. This paper gives an overview of the results of the BreaKin JIP regarding loading processes involved in wave-in-deck type of impacts, effects of depressurization on measured loads and possible sources of scale effects.

Green water on the ship deck in rough sea conditions may induce extreme impulsive wave impacts on superstructures and result in severe structural damage. It is of great importance to consider green water loads in ship structure design. However, there are many challenges in predicting green water loads due to the strongly nonlinear wave–ship interactions and the multiphase, multi-scale nature of the wave impact phenomena. In this article, a three-dimensional hybrid Eulerian–Lagrangian approach is proposed for simulating green water loads on the ship deck. It is extended from an efficient and accurate two-dimensional method developed for fluid–structure interaction problems. In this method, the flow field is solved on a fixed regular Cartesian grid system in an Eulerian framework, whereas the solid body motion is tracked with a set of markers immersed in the fluid and solved in a Lagrangian framework. Two benchmark cases, green water on a fixed simplified Floating Production Storage and Offloading (FPSO) model and green water on ship, are simulated. Comparison between experimental data and numerical results shows that our method is a viable choice for predicting green water loads. Introduction In rough sea conditions, ships may undergo large-amplitude motions and suffer green water impacts due to the strongly nonlinear wave–body interaction. Large waves run up to the ship deck and impact on superstructures with huge volume of water impulsively. It can induce extreme impulsive wave impact loads on superstructures and result in severe structural damage. Thus, it is important to predict wave impact loads for ship structure design. However, there are still great challenges in predicting green water impact loads accurately because it is a flow phenomenon of high complexity and randomness due to its multiphase, highly turbulent nature with wave breaking, air entrainment, etc (Temarel et al. 2016).

Local and global loadings, which may cause the local damage and/or global failure and collapse of offshore structures and ships, are experimentally investigated in this study. The research question is how the elasticity of the structural section affects loading during severe environmental conditions. Two different experiments were undertaken in this study to try to answer this question: (i) vertical slamming impacts of a square flat plate, which represents a plate section of the bottom or bow of a ship structure, onto water surface with zero degree deadrise angle; (ii) wave impacts on a truncated vertical wall in water, where the wall represents a plate section of a hull. The plate and wall are constructed such that they can be either rigid or elastic by virtue of a specially designed spring system. The experiments were carried out in the University of Plymouth’s COAST Laboratory. For the cases considered here, elasticity of the impact plate and/or wall has an effect on the slamming and wave impact loads. Here the slamming impact loads (both pressure and force) were considerably reduced for the elastic plate compared to the rigid one, though only at high impact velocities. The total impact force on the elastic wall was found to reduce for the high aeration, flip-through and slightly breaking wave impacts. However, the impact pressure decreased on the elastic wall only under flip-through wave impact. Due to the elasticity of the plates, the impulse of the first positive phase of pressure and force decreases significantly for the vertical slamming impact tests. This significant effect of hydroelasticity is also found for the total force impulse on the vertical wall under wave impacts.Graphic abstractHydroelasticity effects on water-structure impacts: a impact pressures on dropped plates; b impact forces on dropped plates; c, d, e, f wave impact pressures on the vertical walls; g wave impact forces on the vertical walls; h wave force impulses on the vertical walls: elastic wall 1 vs. rigid wall (filled markers); elastic wall 2 vs. rigid wall (empty markers)

Purpose. Extreme situations at industrial facilities often lead to fires, in particular, fires occur in the event of drone attacks on oil storage facilities. In such cases, a significant amount of chemically hazardous substances (fuel combustion products) are released into the air, posing a risk of thermal injury to workers. Various means and structures are used to reduce the risk of thermal injury. When designing protective structures, it is necessary to determine in advance their effectiveness and the time of possible destruction. This requires the development and use of specialized mathematical models. This work is devoted to the development of numerical models that allow predicting thermal fields at an industrial site in case of fire and assessing the risk of destruction of a protective wall (barrier). Methodology. To numerically solve aerodynamic problems (determining the wind speed field), calculate areas of thermal pollution of the air at an industrial site, and determine the dynamics of temperature changes inside a protective barrier (wall), we use fundamental models of continuum mechanics. Finite difference schemes are used for the numerical solution of simulation equations. Findings. A computer program has been developed that makes it possible to solve a «coupled» problem of determining the zone of thermal pollution of air at an industrial site and the dynamics of heating of a protective barrier (wall). Using the proposed numerical model and computer program, it is possible to solve the problem of thermal conductivity and heat transfer in real time. Originality. A fast-calculation numerical model is presented for estimating the time of destruction of a protective barrier at an industrial site where a fire occurs. The analysis of thermal fields at an industrial site and inside the protective barrier was carried out by numerical integration of a multivariable heat transfer and heat conduction equation. The aerodynamics problem was solved based on a potential motion model. These numerical models require little computer time for practical implementation. Practical value. The proposed numerical models can be used for a preliminary assessment of the temperature field inside the protective barrier to determine its effectiveness in terms of protecting areas from thermal pollution at an industrial site. This info can be useful at the preliminary design stage of protective structures.

The primary purpose of our research efforts is to improve naval design and detection capabilities. Our current research efforts leverage high performance computing (HPC) resources to perform high-resolution numerical simulations with hundreds-of-millions to billions of unknowns to study wave breaking behind a transom stern, wave-impact loading, the generation of spray by high-speed planing craft, air entrainment by plunging breaking waves, forced-motion, and storm seas. This paper focuses on the air entrainment and free-surface turbulence in the flow behind a transom-stern and wave-impact loading on marine platforms. Two codes, Numerical Flow Analysis (NFA) and Boundary Data Immersion Method (BDIM), are used in this study. Both codes are Cartesian-based Large-Eddy Simulation (LES) formulations, and use either Volume-of-Fluid (VOF) (NFA) or conservative Volume-of-Fluid (cVOF) BDIM treatments to track the free-surface interface. The first project area discussed is the flow behind the transom stern. BDIM simulations are used to study the volume of entrained air behind the stern. The application of a Lagrangian bubble-extraction algorithm elucidates the location of air cavities in the wake and the bubble-size distribution for a flow that has over 10 percent void fraction. NFA simulations of the transom-stern flow are validated by comparing the numerical simulations to experiments performed at the Naval Surface Warfare Center, Carderock Division (NSWCCD), where good agreement between simulations and experiments is obtained for mean elevations and regions of white water in the wake. The second project area discussed is wave impact loading, a topic driven by recent structural failures of high-speed planing vessels and other advanced vehicles, as well as the devastation caused by Tsunamis impacting low-lying coastal areas. NFA simulations of wave breaking events are compared to the NSWCCD cube impact experiments and the Oregon State University, O.H. Hinsdale Wave Research Laboratories Tsunami experiments, and it is shown that NFA is able to accurately simulate the propagation of waves over long distances after which it also accurately predicts highly-energetic impact events.

Oil production from older producing basins onshore in western Europe continued to decline, but this was offset by increased production from the Ekofisk field in the Norwegian North Sea area, and from the first Danish offshore field to be commercially developed. Gas production in the British part of the southern North Sea increased to an annual average rate of 3.5 Bcf/d. In 1972 the most important exploration results continued to be in the North Sea basin. Large oil and gas discoveries were made in the British and Norwegian areas, and there were also favorable results from confirmation wells of 1971 discoveries and testing of wells suspended in 1971. New production licenses granted in the British offshore area in March included areas in the northern North Sea as well as west of the Shetland Islands and in the Celtic Sea. Work obligations connected with these grants guaranteed a high level of exploration in the British North Sea area until at least 1978. Interest In European offshore prospects spread to the entire Atlantic shelf and to the Mediterranean Sea. The 1971 gas discovery south of Ireland proved of questionable commercial importance. The Amposta Marino oil field off eastern Spain was developed. Gas and condensate were found in a well in the Aegean Sea. In the Soviet Union production of oil was 7,813,000 b/d and of gas 21.3 Bcf/d, both showing a lower growth rate than in recent years. A large-diameter pipeline provided a new outlet to the southeast for the large oil fields of the middle Ob’ Valley in the West Siberian basin, and the giant Medvezh’ye gas field of the northern West Siberian basin was put on production to a pipeline to Ural industrial centers. New oil fields were discovered in western Siberia, and gas was found in a previously nonproductive basin between the Aral Sea and Lake Balkhash. A second large offshore oil field was found in the eastern Caspian Sea. Large-scale gas production started in East Germany.

: The magnitude of wave impact loads varies greatly, depending upon whether the wave is breaking, as well as on the wave height, length, steepness, and the geometry and immersion of the impacted structure. This report describes an experiment that was performed to characterize the distribution of breaking and non-breaking wave impact loads over a surface, similar to those performed in 2005 with non-breaking wave impact loads and those performed in 2007 with breaking wave impact loads. In those experiments, the average loads were measured on a flat plate and a cylinder. In order to better understand the distribution of forces over a surface, the impact pressures in this experiment were measured on an instrumented test cube by using an array of slam panels and pressure gages. Plots of impact magnitude trends with wave height, wavelength, draft, and impact angle are presented. Overall, average impact pressures from the breaking waves are greater in magnitude than the impact pressures from the non-breaking waves and average impact pressures tend to increase with increased speed, though there was a dip in pressure at an intermediate speed for some panel locations.

Summary. The Greater Ekofisk complex consists of seven fields that produce into a central processing facility at Ekofisk Center, where gas and oil are separated. Gas is pipelined to Emden, West Germany, and oil is pipelined to Teesside, England. The complexity of the operations requires broad use of computers. This paper gives an overview of one of the most extensive systems of computer programs ever developed for operational assistance and reporting in offshore production of hydrocarbon reservoirs. Systems for the capture, concentration, storage, and retrieval of data are included, along with systems for simulating the surface facilities, for production allocation by component weights through C5+, for long-range production forecasting, and for operational and management reporting. Introduction Fields composing the Greater Ekofisk complex were discovered in the Norwegian Sector of the North Sea between 1968 and 1972. In order of discovery, these are Cod, Ekorisk, West Ekofisk, Tor, Eldfisk, Edda, and Albuskjell. The initial discoveries represented the first commercial oil fields found in the North Sea. Fig. 1 shows the Greater Ekofisk complex, Ekofisk and neighboring fields are located in the deepest part of the Tertiary basin in an area containing more than 1,350 ft [410 m] of Danian and Upper Paleocene sediments. About half of this interval consists of Danian and Upper Cretaceous chalk sections, and the other half is composed of Upper Paleocene clastics. The Danian and Upper Cretaceous chalks compose the primary hydrocarbon reservoirs in the Ekofisk, West Ekofisk, Tor, Edda, Eldfisk, and Albuskjell fields. In the Ekofisk area fields, an extensive fracture system extends both laterally and vertically in the chalk reservoirs. Oils are relatively volatile. with substantial variation in composition from field to field. Facilities for producing these fields were constructed in several stages. During the first phase, four Ekofisk wells were produced through separation facilities located on a jackup rig. Oil was loaded to tankers through a single-point mooring buoy. About 42,000 BOPD [6680 M 3 /d oil] was produced for 18 months in this manner. Associated gas was flared. The second phase consisted of installing three drilling platforms (two for production and one for gas reinjection and production), a field terminal platform, a four-story quarters platform, and a 1 × 106 -bbl [ 160 × 103-M3] concrete oil storage tank. Subsequent phases included construction of processing facilities at Ekofisk Center and gathering systems from the other fields as they were developed. A 36-in. [91.4-cm] pipeline 274 miles [441 km] long was constructed to transport gas to Emden, and a 34-in. [86.4-cm] pipeline 220 miles [354 km] long was constructed to transport oil to Teesside. A natural gas liquids (NGL) processing plant was built at Teesside to stabilize the oil and to extract ethane, commercial propane, isobutane, and commercial normal butane. The complexity of the operations made it obvious from a very early date that extensive use of computers would be required. A study was initiated in 1972 to determine computer hardware and software needed for Ekofisk operational reporting. The following major systems were found to be necessary.A system was needed to capture, to integrate, to store, and to retrieve data for metered flows. laboratory analyses, process conditions, valve status, well status, and other information required for day-today operating and reporting.A technical information system (TIS) was needed to produce operating and engineering reports from the data.A computer model simulating the surface facilities was needed to allow evaluation of changing field operations, changing process conditions, and proposed changes in operations.A means of allocating production from the various fields to the gas and oil product pipeline streams by components through C5+ was needed so that each field's light-component contribution to the gas and liquid streams would be properly credited.Long-range production forecasts for the Greater Ekorisk complex were needed, including compositional forecasts for each field and quantities and compositions of the Emden gas stream and the Teesside oil stream. These are not the only systems developed for use in operation of the Ekofisk complex. Many other programs, not discussed here, were later identified and developed. Capture and Initial Processing of Data Data are automatically obtained from a large number of points in the Ekofisk complex and at Teeside and Emden. JPT P. 809^

Peculiarly cylindrical legs on offshore platforms constantly are exerted vigorously to wave stresses. For their lifetime, a test on structural deformations and pressure measurements is taken. Experimental research on the wave forces affecting cylindrical structures is presented. To determine pressure distributions and deformations in the altered cylinder's surface properties by applying a hydrophobic coating. Hence, the angle at which water drops hit the surface has increased, thus, decreasing water adhesion to the surface. The geometric modification or additional weight coated on the cylinders' surfaces is not notable. Wave loads with various amplitudes and frequencies were applied repeatedly to cylinders with hydrophobic and hydrophilic surface materials. Three pressure sensors and a strain gauge were used to measure pressure distributions and the cylinders' surface deformations respectively. Two cylinders' fluctuation in pressure is carefully examined and analyzed. Cylinder's pressure response for hydrophobic surfaces varied based on the sensor's location by lowering the maximum pressure or the impact time. The structural deformations due to changing surface properties and strain readings were contrasted. The structural deformation changed, with the impacting wave distributed throughout the surface. Image processing demonstrates the variation in the water body's volume to which the surface is exposed, reinforcing this finding.

As part of their studies conducted regarding the wave impact loads acting on the ship bow and the structural responses thereto by a series of drop tests, the authors analyzed the scale effect, or the influence of the scale of bow models therein used on the results of the drop tests.It is often claimed that the estimated impulsive pressure determined by a model test varies with the scale of the model. To see if such a claim was well-founded or not, the authors carried out the drop test using a 1/15 scale model and compared the results with those obtained using the 1/3 scale model in the previous tests. The results of this comparative study may be summarized as follows.At first, the maximum impulsive pressure obtained using the 1/15 scale model was considerably smaller than that determined using the 1/3 scale model. This difference became small with large relative impact angle α. Such a difference must have come from the difference in relative size of the pressure gauges fitted to these two models. On the other hand, the equivalent static pressure Peq obtained using the 1/15 scale model and that obtained using the 1/3 scale model remained the same, if the size of the panel was proportional to the size of the model. That is, Peq didn't vary with the scale of the model.Further, investigation was made of the effects of the size and the location of the panel on Peq. Peq indicated a considerable change according to the size and location of the panel when the impact angle α was 0°, but the change became smaller with large α of, say, α=5°.