Numerical and experimental studies on the flow field of a semi-submersible aquaculture platform

The paper presents the results of a series of experimental and numerical studies on the problem of bergy-bit/iceberg collision with semi-submersible/ gravity platforms. The numerical study on the collision of 500 tonne and 2,000 tonne mass bergy-bits on the main column of an 8-column semi-submersible concludes that unstrengthened columns of semi-submersibles will undergo permanent deformation and perhaps complete puncturing through when impacted by 500 tonne bergy-bits traveling with speeds more than 2.0 m/sec; but when the column is ice-strengthened, it resists the impact of 2,000 tonne bergy-bits traveling at 2.0 m/sec with very little deformation. Impact resistance tests were carried out on ten fibre reinforced and seven plain reinforced concrete shell panels. It was observed that the fibre-reinforced concrete panels are 1.5 times stronger than ordinary reinforced concrete panels. INTRODUCTION Concrete caisson-type gravity and steel floating semi-submersible platforms have been proposed for oil and gas exploration/production on the Canadian Eastern Sea Board. Drifting ice masses such as icebergs and bergy-bits pose serious impact threats to these offshore platforms. A detailed study carried out regarding the cost-effectiveness and suitability of the above-mentioned two types of platforms, taking into account the environmental conditions of the Grand Banks, seems to give a preferred rating for heavy production-type semi-submersible platforms; the gravity platform scores over the semi-submersible when the utilization of local manpower and material resources are considered. The possibility of large icebergs impacting a moored semi-submersible could be easily avoided by a proper ice surveillance program and quick-disconnect/re-entry procedures; but the possibility of impact with a medium-sized bergy-bit, which escapes radar detection during inclement weather conditions, should be considered. During a recent field study conducted by the International Ice Patrol in 1985 [1], the radar on the USCGC EVERGREEN detected only five of the seventeen bergy-bits drifting near the ship (the significant wave height was consistently lower than 2.0m). This paper presents the analytical and experimental study carried out on the response of a moored semi-submersible and concrete gravity platform panels to bergy-bit and other moving ice-mass impacts. REVIEW OF THE LITERATURE Very detailed design methods have been developed based on analytical, experimental and field investigations of the behaviour of offshore platforms under various types of environmental and operational loadings. However, not much work has yet been done in the area of protection of offshore platforms against collision with ships, supply boats or icebergs and their fragments. Most of the work in this area is mainly in the research and development stage. Most of the available studies deal with ship/supply boat collision with fixed/floating platforms. The actual interest in the problem of iceberg collision with gravity platforms, and bergy-bit impact on semi-submersibles started after the discovery of oil in the iceberg-frequented waters of the Grand Banks of Newfoundland.

A numerical and experimental study of internal solitary wave loads on semi-submersible platforms

In rough sea conditions, semi-submersible platform often suffers from extreme wave impact loads, which can result in structural damage. It is important to predict the wave impact loads on semi-submersible platform. Therefore, the purpose of this study is to investigate the wave impact loads on semi-submersible platform with numerical methods. A numerical method, based on a fixed regular Cartesian grid system, has been developed by the authors. In the method, the FDM (Finite Difference Method) is applied for solving flow field, and the THINC/SW (Tangent of Hyperbola for INterface Capturing with Slope Weighting) model, which is kind of VOF (Volume-of-Fluid) model, is adopted to capture the free surface. Some selected model test cases, form Exwave JIP project, will be used to validate the present numerical method and to analyze the wave impact loads on semi-submersible platform.

In this paper, the research of mean and slowly varying wave drift forces on a moored semi-submersible platform in irregular moderate waves are carried out. Both experimental and numerical studies are included. Experimental tests with a semi-submersible platform model scaled at 1:70 in an ocean basin were performed with focus on wave drift forces at low frequencies. Both head sea and beam sea conditions were tested. The measured results are compared with numerical simulation using the diffraction analysis program based on three-dimensional potential theory in time domain. Both head sea and beam sea tests are considered. It is found that the experimental and numerical results agree well with each other in the smaller waves for both head and beam sea tests, but not so well in the higher wave for head sea test.

The region over the pontoons, especially in the vicinity of columns, is typically a critical area in terms of upwell when analyzing the air gap of semisubmersible platforms. There is indication that numerical computations using potential flow theory may in some cases overestimate the free surface elevation in this region. To assess the possibility, experimental data is compared to numerical computations in three locations under the deck box: one location over the pontoons, one location in the vicinity of the pontoons and one location between the pontoons. The data was acquired in FORCE’s towing tank facility, in Lyngby, Denmark, by relative wave gauges fixed to the moored semisubmersible platform. The experimental data is treated in order to remove the global motions from the upwell signal. The resulting free surface elevation, which includes contributions from incident, diffracted and radiated wave fields, is compared to the disturbed free surface elevation calculated with linear diffraction-radiation theory. The study is initially conducted in irregular waves, where simulation statistics in 4 different sea states are compared to the experiments and the observed nonlinear effects are discussed. The extreme crest heights are compared with non-Gaussian models as defined in DNVGL-OTG-13 and as defined by Stansberg (2014). The study is then extended to regular waves. In a first stage we estimate the first harmonic components by removing all higher order effects, and compare the results to linear theory. For these band-pass filtered signals it is shown that results calculated with linear theory tend to overestimate free surface elevation in the location over the pontoons, but seem to correlate well with the experiments in the other locations. In a second stage the experimental crest heights are compared with non-linear models as defined in DNVGL-OTG-13 and as defined by Stansberg (2014). It is shown in this case study that the maximum free surface elevation over the pontoons in front of upwave columns can be severely overestimated if calculated with the current state of the art numerical models, which are based on linear diffraction-radiation theory. We explain the observed discrepancy in this case primarily by a very high linear predicted amplification induced by the shallow pontoon, with resulting high local steepness leading to local breaking and dissipation. Therefore, such pontoon effects should be addressed in semisubmersible platform air-gap analysis. The work also highlights the importance of having good experimental data available when preforming such analysis.

Numerical study on the flow field inside and around a semi-submersible aquaculture platform

An experimental study on electrokinetic improvement of the load-carrying capacity of offshore foundations embedded in soft clays is conducted. The paper presents a summary of method and results of two series of electrokinetic tests conducted on natural and simulated marine clays in small-scale and large-scale laboratory testing facilities. The results demonstrated that the load-carrying capacity of the foundation models increased up to three times after electrokinetic treatment. Introduction Recent developments in the offshore oil and gas industry have resulted in an increasing number of offshore construction projects that include different types of offshore platforms. If the seabed in the construction areas consists of soft marine clay deposits, these structures may encounter serious geotechnical problems. These soil deposits are widespread in the oceans throughout the world and are often characterized by the low shear strength and high compressibility that usually result in extensive settlement under structural loading. The construction and installation costs of the structures are enormous, and the consequences of failure can be catastrophic. To overcome the foundation problems caused by difficult soil conditions, there are two alternatives, i.e., increasing the size of the foundation or improving the soil. Electrokinetics (EK) may be used to strengthen the soil when the latter alternative is considered in design and construction. This study investigates the feasibility of electrokinetic strengthening of soft marine clays, and consequently increasing the load-carrying capacities of foundations embedded in such soils. Over the last decade, skirted foundations have been increasingly used to support large offshore structures such as gravity platform jackets, jackup rigs, semi-submersible platforms, floaters, tension leg platforms, sub-sea systems and other structures. Skirted foundations are used to resist both compressive forces from fixed jacket structures (e.g. Sleipner T platform) (Sparrevik, 1998) and tensile forces from floating or tension-lag platforms (Snorre TLP platform) (Fines et al., 1991). They have been used in the past in water depths that range from 70 m (e.g. Draupner E heavy jacket in the North Sea) (Tjelta, 1995) to about 1000 m (e.g. semi-submersible platform at Marlim Field offshore Brazil) (Mello et al., 1998). Skirted foundations are large, hollow, cylindrical foundation elements that are usually made of steel. Their capacity to carry loads depends on factors such as depth of skirt penetration, cylinder diameter, soil strength and the combination of horizontal, vertical and moment loads. When soft soils are encountered in a site, however, the load-carrying capacity is governed by an undrained shear failure in the soil. Therefore, the undrained strength of the soil becomes one of the major concerns in the design of skirted foundations. Skirted foundations are installed by penetrating the skirts into the seabed, first partially under self-weight, and then by creating an underpressure inside the cylinder (Andersen and Jostad, 1999). A thin zone of soil along the skirts will be remoulded during installation.

Experimental study of internal solitary wave loads on the semi-submersible platform

Experimental study on fire suppression performance of the high pressure water mist in the engine room of an offshore platform

The application of soil mechanics concepts to the problem of anchor holding power clearly illustrates the insufficiency of the concept of efficiency by itself, in that it is merely an empirical relationship between the weight of the anchor and its holding power. Laboratory and in-situ tests have been performed to analyze the behavior of special adjustable-configuration anchors as a function of the nature of the soil and to determine the influence of leading geometric and mechanical parameters on the kinematics. Holding power appears mainly linked to fluke surface area, depth of anchor burial and the mechanical properties of the soil. In-situ tests performed on small-sized anchors confirm the general analysis and provide bases for comparing the leading anchors used in offshore operations. INTRODUCTION The anchoring of floating supports (barges or semi submersible platforms) used during operations associated with the exploration for or production from offshore hydrocarbon fields raises specific problems that are essentially different from the anchoring problems encountered in traditional offshore operations:tensions on anchoring lines are especially great in excess of 100 tonnes for drilling platforms;the movements tolerated for the support are extremely limited so that anchoring points must be absolutely fixed (no dragging);anchoring time, which may vary with the type of operation, may be relatively long (several months in the case of drilling supports or several years for production supports). On the other hand, the mechanical properties of sea floor soil are generally known as the result of geophysical and geotechnical surveys. The rational calculation of an anchoring system involves:choosing a type of anchor especially suited to the problem at hand, i.e. taking the above-designed criteria into consideration;making an accurate estimate of the holding power of the type selected as calculated from soil data. Lack of information concerning anchor behavior and. the insufficiency of the concept of efficiency, as is lamented by most users, do not enable a satisfactory answer to be given to these two questions of prime importance. The joint research project undertaken in 1976 by the Institut Français du Pétrole (IFF) and the Centre National pour l'Exploitation des Oceans (CNEXO) had the following two main objectives :gaining better understanding of the phenomena characterizing anchor behavior and kinematics;improving ways of predicting anchor holding power as a function of the soil encountered. The complexity of the problem required the is plementation of highly diversified experimental and theorical methods :laboratory tests on scale models,in-situ tests of medium sized anchors,development of a computerized program for calculating holding power by finite elements,two dimensional analogical model testing to calibrate the numerical model. This paper reviews the leading experimental findings made during this project. The numerical findings will be published later on.

The air gap response is crucial for the safe design and operation of large-volume floating platforms such as semi-submersible and tension leg platforms. It is a complex task to perform numerical simulation on the air gap response considering the wave free surface elevation and the motions of the floating vessel. Therefore, the prediction of air gap response still relies heavily on model tests. This paper attempts to investigate the effects of the mooring system, especially the effects of the length of mooring lines, on the air gap response of semi-submersible platform based on model tests results. The scaled model of the semi-submersible platform is supported by a symmetric mooring system composed of 8 mooring lines. A set of model tests with different length of mooring lines was performed in the State Key Laboratory of Ocean Engineering basin at Shanghai Jiao Tong University, and the air gap responses of 15 locations were measured using wave probes. The results indicate that the mooring system plays an important role in the air gap response of semi-submersible platform.

Experimental study on vortex-induced motions of a semi-submersible platform with four square columns, part III: Effects of the collinear irregular and regular wave incidence and current

Experimental study on the effect of flare barriers on wave run-up and motion response of a semi-submersible platform

An improved time-domain response estimation method for floating structures based on rapid solution of a state-space model

Processing method and governing parameters for horizontal wave impact loads on a semi-submersible