bluuebunny/crossref_metadata_2025 huggingface dataset:
Bas in Oscillations In an Offshore Harbor
Authors: Ho Rong Tai , Bomze Herman Tai
Published: October 2008

A numerical method using an integral equation approach derived from Green's theorem has been developed for calculating the steady-state, wave-induced oscillations inside an offshore harbor of arbitrary shape surrounded by the open sea. It solves the outside diffracted wave field as well as the radiating wave field due to the interaction between the harbor and the ocean waves through the harbor entrances. The method was verified by checking against known solutions. It was then used to study the water surface oscillations inside a breakwater enclosure protecting a proposed floating nuclear power plant in the Atlantic Ocean. Results of the investigation are presented in this paper.

INTRODUCTION

Due to the rapid growth of energy demand and considerations of environmental impact, among others, a new concept has been developed for locating nuclear power plants offshore in the coastal zone. Public Service Electric and Gas Co. (PSE&G) of New Jersey has proposed to install two 1,150-mw nuclear power plants (FNP's on moored floating platforms of a standardized design. The plants Will be protected by a semicircular rubble-mound main breakwater facing the ocean and a straight closure breakwater facing the coast, located about 3 miles offshore slightly north of Atlantic City. The proposed project has been named the Atlantic Generating Station (AGS).

When a basin is connected to the open sea, the water surface inside the basin is subjected to the excitation of the incident waves propagating from the ocean into the basin entrance. A basin can be considered as a multiple-degree-of-freedom mechanical system with its appropriate inertial, dissipative, and restoring characteristics. The responding oscillation inside the basin is dependent upon the period of the incident wave and the basin and entrance configurations. This response may result in an amplification of wave heights inside the basin at certain incident wave frequencies, i.e., at the resonant frequencies of the basin system. It is essential to understand the characteristics of basin oscillations in order to determine the motions of the floating platforms and the required strength of their moorings.

Most existing numerical methods for basin oscillation analysis are developed for ordinary harbors inland of straight coastlines, so that the solution for the diffracted wave field outside the harbor is available immediately. Such harbors have only one entrance. In such a case, the problem reduces to a solution of the radiating wave due to the interaction between the harbor and the ocean wave. The AGS harbor presents a different problem since it lies offshore, requiring solution for the diffracted wave field, and since it has two openings to the sea.

LITERATURE REVIEW

Miles and Munk1 investigated the case for a rectangular basin connected directly to the open sea and obtained a balance at the harbor mouth between the ocean wave entering the basin and the wave radiating from the harbor mouth back into the open sea by means of an integral equation in terms of a Green's function.
A Probabilistic Investigation of Foundation Design for Offshore Gravity structures
Authors: Kraft Leland M. , Murff James D.
Published: October 2008

INTRODUCTION

Gravity structures play a prominent role today in North Sea oil development. These structures are not supported by piles as are most ocean structures but rather sit directly on the ocean bottom, and depend on their foundation geometries and large weights to resist severe environmental loadings. A number of structural and foundation configurations have been proposed; however, attention will be restricted in this discussion to a general configuration typical of the most prominent structures being constructed at present (1975). An example of a gravity structure is illustrated in Fig. 1. The structure foundation consists of a large caisson placed directly on the unprepared seabed surface. The deck is supported by large columns extending from the caisson. Various combinations of steel and reinforced concrete have been proposed, but the majority of structures are being constructed almost entirely of reinforced and pre-stressed concrete.

One of the primary engineering concerns with these structures is foundation design. Because of the variability associated with the environmental forces as well as the basic soil properties, this problem lends itself well to modern probabilistic procedures. Such procedures provide a rational, quantitative means for evaluating uncertainties affecting appropriate design, even though a of subjectivity will always remain in any such evaluation. The probabilistic method requires the engineer to formally and consistently recognize the variability of many of the important design parameters. The method gives management and others responsible for setting design criteria an opportunity to appraise cost-benefits of design levels required for given reliability safety levels. It also quantifies reliability to permit direct camparison with other options.

This paper presents a method for analyzing the reliability of gravity structure foundations in terms of simple loading and resistance models. The sources of variability in estimating resistance to loads are discussed with particular emphasis placed on the nature of soil property variability and uncertainty. These concepts are illustrated through an analysis of a typical gravity structure foundation.

THE RELIABILITY MODEL

Reliability as defined in this paper is a measure of the probability of survival. For example a perfectly reliable foundation corresponds to a zero probability of failure. Because of uncertainties in environmental loads and soil properties plus the ever present uncertainties due to simplifying assumptions in the analytical methods, the chance of failure is always present although acceptably small for proper designs.

Characterizing the structure's resistance as a function R and the load acting on a structure as a function L then the failure probability Pf is simply the probability that L is greater than R. For structures of the type presented in Fig. 1 we can reasonably consider the load and resistance to be statistically independent so that(Available In Full Paper)
Cathodic Protection Experience In Cook Inlet, Alaska
Authors: Barrett S.L. , Taylor J.M.
Published: October 2008

Amoco Production Company operates four of fourteen platforms in the Cook Inlet of Alaska. Amoco's ten years of underwater corrosion and cathodic protection experience in the Cook Inlet environment are reviewed. Tests show that 90-100 ma/sq.ft. are necessary to polarize the Cook Inlet structures where high tidal velocities are prevalent. Further, impressed current densities of 60 ma/sq.ft. maintain the cathodic protection as evidenced by the presence of a cathodic scale in the lower portions of the splash zone of the structure legs. An underwater corrosion coupon program was run to verify underwater protection and effects of structure shielding are described. Field data on protection are also confirmed by laboratory data.

Introduction:

The Cook Inlet is a marine estuary near Anchorage, Alaska. Amoco Production Company operates four of the fourteen offshore drilling and production platforms located in Cook Inlet. These four structures are jointly owned by Amoco, Atlantic Richfield, Phillips, Skelly and Standard of California. Platform Baker was installed in 1965, while Platforms Anna, Bruce and Dillon were installed in 1966. At the Amoco-operated platforms, water depths range from 65' to 110' (mean low water) with tidal ranges as high as 30' (-5' to +25' MLLW). Peak tidal currents are 10 FPS (7 MPH). The Cook Inlet is characterized in winter months by large floating pans of ice generally l' to 2' thick and as much as ½ mile across.

Seasonal variation in fresh water run-off dramatically affects the estuary turbidity and salinity. Glacial silt concentrations as high as 440 ppm usually make the water opaque. Salinity varies as much as 22% between summer and winter. Recognizing further that the water temperature varies from 28° F in the winter to 55° F in late summer, the observed resistivity of the Cook Inlet waters is from 0.43 ohm-meters at 28° Fin the winter to 0.29 ohm-meters at 55° F during periods of peak run-off in the summer. The waters of Cook Inlet are also oxygen-saturated to the ocean floor year-round.

Platform Construction:

Jackets of the four Amoco-operated platforms are of similar construction. As depicted in Figure 1, the structures are four-legged with cross-bracing located well below the water surface to minimize ice loading. The jackets are constructed of three types of material including Kaisaloy 50 MV® high strength steel in the submerged high stress areas and Sheffield Lo-Temp® steel in the high stress areas exposed to the atmosphere. Finally, the legs were provided with a ½" thick sacrificial corrosion wrap of A-36 steel in the tidal splash zone. This material is external to the structural portion of the leg. Average submerged unpitted structure surface areas vary-from 23,000 sq.ft. on Platform Bruce to over 50,000 sq.ft. on Platform Baker. All calculated current densities have been based upon unpitted surface areas although the actual surface areas can be expected to be significantly greater due to pitting.
Design and Construction of a Marine Terminal for North Sea Oil in Orkney, Scotland
Authors: Trainor Robert W. , Scott J.R. , Cairns W.J.
Published: October 2008

In the U.K. North Sea, Occidental of Britain, Inc., a subsidiary of Occidental Petroleum Corporation, is operator for a group consisting of Occidental Petroleum, 36.5 per cent; Getty Oil International (England) Ltd., 23.5 per cent; Allied Chemical (North Sea) Ltd. 20.0 per cent; Thomson Scottish Petroleum Ltd., 20 per cent. In March 1972 the U.K Government granted to the Occidental Group a petroleum production license for six blocks covering approximately 316,000 acres of the North Sea and exploration drilling started immediately. In January 1973 the first discovery was made in Block 15/17, 100 miles east of Wick, Scotland by Odeco's "Ocean Victory" and subsequent delineation drilling confirmed the commercial viability of the Piper Field (see Figure 1). The decision was taken to develop the Piper Field by means of a fixed steel platform and a pipeline to a shore" terminal. This paper describes the search for a suitable terminal site, the environmental studies that were made to satisfy the requirements of national and local government and thus obtain the necessary planning consents as well as a review of the engineering design and construction of the terminal itself. The selection of the terminal site on the island of Flotta in Orkney was made in May 1973; formal application for planning approval was made in July 1973 and planning consent was received in late JanuarY1g74 when site construction started. The first phase of the terminal development i.e. to handle the production of the Piper Field at 250,000 BPOD will be completed in June 1976 at a total cost of $164 millions.

INTRODUCTION

The initial problem facing Occidental was to select a suitable site for the terminal; obviously a prime consideration in selecting a terminal site was its proximity to the Piper Field; with marine pipeline costs running at approx. $1.1 millions per mile for the 30 inch mainline from the Piper Field this consideration was the determinant for the initial selection of the trial terminal sites. Nine locations were selected for examination; these included sites in the Shetland Islands, Orkney and on the Scottish mainland. Studies were made using all available maps, charts and weather records and site visits were made to enable estimates to be made of development costs, probable berth occupancy and operating costs. An analysis of these variables coupled with the differential costs of the pipeline for each case enabled a further screening process to be carried out and as a result, three sites were selected for detailed study; two of these sites were located in Orkney and one on the Scottish mainland.

Further detailed study of each site was made and discussions were held with the Scottish Development Department, the Highlands and Islands Development Board and the Orkney Islands Council.
Economic Models of Subsea Pipeline Connectors
Authors: Daley Gregory C.
Published: October 2008

Economic models analyze the sensitivity of some of the costs encountered in the eleven under sea pipeline connections on the 34" oil line from Ekofisk to Teesside for hydroball/ hydrocouples, hyperbaric chambers, and one-atmosphere chambers. The economic models are summarized by a graph showing the cost for eleven 34 inch connections as a function of the number of productive work days required per. connection. The results do not prove that one system is better than another, but do show the sensitivity to the day rates and productive work days required for each system.

INTRODUCTION

Economic models analyze one time costs and day rate charges for three systems. The first model involves hydroball/ hydrocouples installed by a lay barge, by the Arctic Surveyor, and by a vessel larger than the Arctic Surveyor. The eleven Ekofisk installations were made from lay barges and the Arctic Surveyor. This experience is utilized by the models. Other costs relate to equipment costs, protective covers, and inspection programs.

Costs for installing eleven connections with a hyperbaric system are developed into the second economic model. Costs for burying the splice piece are included.

The eleven Ekofiskpipeline connections are assumed to be riser connections to evaluate the one atmosphere chamber system. The equipment costs and other costs of the Lockheed System are modelled. At the time of this writing, the system is not applicable for mid line connections and therefore could only have been used on five of the eleven Ekofisk connections. This lessens the accuracy of the Ekofisk - Lockheed model.

THE EKOFISK OIL .AND GAS PIPELINES

The huge reserves of oil and gas discovered in the Ekofisk field by the Phillips Norway group in 1969; and later increased by the discovery of six adjoining fields, could only be economically transported by pipeline. This decision has resulted in the construction of two of the largest and longest marine pipelines ever to be built as shown in Figure 1. The large capacity of each line has also required the construction of two intermediate booster platforms on each of the pipelines, believed to be the first ever constructed. Phillips Petroleum Company Norway, operators of the Phillips Norway Group, acting on behalf of the Norpipe A/S, has completed the oil pipeline and is entering the final stages of completion of the gas pipeline. During 1973 and oil and gas pipeline; of this amount, 464 miles were marine pipeline. The 34" o.d. 220 mile oil pipeline from Ekofisk to Teesside, England has a capacity of one million barrels per day and the 36" o.d. 275 mile gas pipeline from Ekofisk to Emden, Germany, has a capacity of 2.28 billion cubic feet per day. Both the oil and gas pipelines are owned by Norpipe A/S a 50-50 joint venture of the Phillips Norway Croup and Statoil, the Norwegian national oil company.
Water Impact Loads And Dynamic Response Of Horizontal Circular Cylinders In Offshore Structures
Authors: Faltinsen O. , Kjærland O. , Nøttveit A. , Vinje T.
Published: October 2008

Model test results and numerical calculations of water impact loads on rigid horizontal circular cylinders are presented. The initial slamming force coefficient Cs is numerically estimated to be 3.1. Experiments give an average value of 5.3. The time history of the water induced load is examined. Dynamic response of three-dimensional elastic models of horizontal trusses of semisubmersibles and jackets are studied. Scaling of model test results result in significant stresses for realistic wave velocities in the North Sea.

INTRODUCTION

The wave-induced loads are an important factor in the design of marine structures for offshore operation in the hostile area of the North Sea. For some types of structures and wave induced loads the' loads can be well predicted. For other cases this is not true. In particular the calculation of wave loads on jackets is a difficult task. This paper will have particular application to the members of a jacket that are in the free surface zone. Ore to the wave notion those members will be in and out of the water. Cases of damages on this type of members have been reported from the North Sea. Similar problems can arise for the horizontal trusses of a semisubmersible.

In order to be able to deal with the problem we will consider a simplified problem. We will study a horizontal circular cylinder that is forced with constant velocity through an initially calm water surface and examine the force on the cylinder as it proceeds through the water. One of the first calculations of this type was done by van Kaman (1), who considered a wedgeshaped body. He used the free surface condition ? = 0, where ? is the velocity potential of the fluid notion. The force was written as(Mathematical equation available in full paper) where m' is vertical infinite frequency added mass coefficient for the body as a function of time. Von Kaman simplified the added mass calculations by using flat plate results. Fabula and Ruggles (2) did similar calculations for a cylinder with elliptical cross-section, but their added mass calculations were more sophisticated than von Karman' s calculations. Fabula' s method is only applicable up to the point that a circular cylinder bottom has proceeded a distance approximately a radius down in the fluid. In this paper we will present as method which is applicable for a rigid cylinder proceeding any distance down in the fluid. The method may be used for any cross-section and would hence be applicable in the study of whipping of ships. The theoretical calculations are compared with experimental results obtained by forcing rigid horizontal circular cylinders with constant velocity through an initially calm free surface.

In the second part we study the dynamic response of elastic horizontal circular cylinders that is forced with constant velocity through the water surface. A theoretical model is described. In the experimental part we are studying three-dimensional models of horizontal trusses of semisubmersibles and jackets.
Deep Ocean Mining - Technology Transfer From And To The Offshore Drilling Industry
Authors: Williams D.W. , McBride C.M. , Kinnaman S.C.
Published: October 2008

In late 1977, a dynamically stationed drillship, SEDCO 445, will be used in the North Pacific Ocean to deploy equipment for recovering 5000 short tons of manganese nodules. This pilot test will be conducted in 5300 meters of water for an international consortium and will utilize many proven designs and techniques of the offshore drilling industry. These proven techniques and some of the newly designed methods will be presented in this paper to emphasize the desirability of a continuation of technology transfer between this newly emerging mining industry and the existing offshore drilling industry.

INTRODUCTION

Deep ocean mining activity has taken a dramatic upward swing during the last 7 years. In 1970, Deepsea Ventures successfully conducted a manganese nodule mining test in 740 meters of water and became the first company to attract attention to the feasibility of hydraulically mining nodules. Increased metals prices, the need for potential stable metal sources, the talk of metal cartels, and improved technology have contributed to the formation of four active international consortiums. All companies are actively exploring the area between the Clarion and Clipperton fracture zones shown in Figure 1.

Ocean Management, Inc. located in Bellevue, Washington, is one of the four active. ocean mining groups. OMI represents and manages the activities of the four equal shareholders who are INCO (Canadian), DOMCO (Japanese), AMR (W. German), and SEDCO (U.S.)

During the last quarter of 1977, OMI will commence a pilot mining test using the dynamically stationed drillship, SEDCO 445 as illustrated in Figure 2. SEDCO is responsible for a major portion of the shipboard hardware design and operation during the planned 7 to 8 month test period. Many of the techniques to be used for operating the vessel and deploying equipment have been proven by the offshore drilling industry.

In contrast to the proven methods planned for this operation, some unique tools and procedures will be used. It is anticipated that some of the special mining equipment and procedures can be used by the offshore drilling industry in their search for fossil fuels in deep water.

CONTRIBUTION OF DRILLING INDUSTRY

Ship Design

With offshore operations extending into deeper water and more inhospitable environments, this decade has seen the development of the drillship progress from a tethered ship-shaped drilling platform into an efficiently designed, dynamically positioned mobile drilling vessel. Operational range and capability have been extended dramatically over the past decade. Similarly, the volume of engineering and operational experience necessary to produce these achievements has grown considerably. It is this wealth of knowledge of shipboard operations in a deep ocean environment that has provided the greatest of dividends to the newly emerging deep ocean mining industry.
Synopsis of Coast Guard Plans for Radio Aid to Navigation
Authors: Walker W.G. , Protzman W.G.
Published: October 2008

The U. S. Coast Guard is improving and expanding LORAN-C service to the U. S. Coastal Confluence Zone and to the Great Lakes. Subsequently, LORAN-C usage will be extended to Harbors and Harbor Entrance Areas for precise navigation. All new LORAN-C stations will provide time and frequency information. LORAN-A will be phased-out completely by mid-1980. Radio beacon service is being modestly expanded with improvements in availability and reliability and will continue to provide valuable service to recreational boaters and small commercial vessels for the foreseeable future. The OMEGA system, being installed by the U. S. Navy with Coast Guard and partner nation assistance, now provides useful navigation service on many areas of the high seas. Verification of accuracy and service is in process. Seven of eight planned permanent stations are now in operation. The U. S. Coast Guard will eventually assume the U. S. Government's responsibilities for the management of OMEGA.

LORAN-C

The following sections describe plans that complete the implementation of the Secretary of Transportation's decision to provide LORAN-C radio navigation service throughout the Coastal Confluence Zone (CCZ) of the United States. These plans include the expansion of LORAN-C service for the West Coast and Alaskan Waters in early 1977, and partial expansion of LORAN-C service for the northeast coast and southeast coast of the United States including the Gulf of Mexico by 1978. Complete new LORAN-C service on the East Coast will be available in July 1979 when the existing East Coast chain will be terminated. The expansion of service to cover all of the Great Lakes will follow in February 1980.

WEST COAST AND ALASKAN WATERS

The North Pacific LORAN-C chain has been providing service since 1960 in the Bering Sea and along the Aleutian Islands.

About March 1, 1977, a series of changes to the North Pacific chain began. One new secondary station located at Narrow Cape, Kodiak Island, Alaska, joined the chain and all stations began transmitting on a new rate. Operation will continue on this rate for the foreseeable future. About June 30, 1977, the secondary station at Sitkinak Island will stop transmitting permanently. This method of phasing-in the new station at Narrow Cape will permit the necessary changes with minimum disruption of service to existing users of LORAN-C and allow some time for the conversion of LORAN-C bottom obstruction data or fishing coordinates by the users.

Construction of six additional U. S. stations and one Canadian station is complete. Following calibration and final technical adjustment of the new stations three new LORAN-C chains will be operationally certified. It is anticipated that this new service will become available on the following schedule:U. S. West Coast LORAN-C Chain April 1977Canadian West Coast LORAN-C ChainMay 1977Gulf Alaska LORAN-C ChainJune 1977

When the station at Sitkinak Island stops transmitting, about June 30, 1977, the LORAN-C chains and coverage for the West Coast and Alaska will be as shown in Figure 1.
The World's Largest Single Point Mooring Terminals: Design And Construction Of The SALM System For 750,000 DWT Tankers
Authors: Gruy R.H. , Kiely W.L.
Published: October 2008

This paper will discuss the design, fabrication and installation of two (2) identical Single Anchor Leg Mooring (SALM) Terminals which serve as crude oil export terminals. Both are designed to safely moor and load tankers as large as 750,000 DWT in environmental conditions that will be occasionally severe.

The determination of mooring forces via calculation and model tests and the design and fabrication of the components which transfer these loads will be discussed. Of particular interest is the design and fabrication of the "heart" of the systems, the fluid swivel assemblies which include several innovative features with respect to operation and maintenance. These fluid swivels are the largest ever built for SPM applications and are designed to transfer crude oil and bunker fuel oil simultaneously and to absolutely preclude the possibility of commingling of the two fluids. Model tests that were conducted on the fluid swivel/loading hose system will be discussed.

The foundation design and the optimization of this design with respect to available offshore construction equipment will be outlined as will the installation methods employed.

INTRODUCTION

The safe, efficient and economical transportation of crude oil from typically remote producing areas to the consuming areas of the World presents several difficult problems. Solutions to any of these problems offer attractive cost savings for crude oil feed stocks.

A major step in optimizing crude oil transportation has been the development of the Very Large Crude Carrier (VLCC). Unfortunately, the development of the VLCC has threatened to outpace the development of terminals suitable for loading/unloading these giant, deep draft vessels. One solution to this problem that has gained wide acceptance in recent years is the Single Point Mooring (SPM) or "monobuoy" type of terminal.

The SPM terminal solves several problems that are inherent to other types of tanker loading/unloading facilities (harbors, jetties, sea islands, multiple buoys and fixed towers):It can be readily placed far offshore in the deep water necessary to accommodate deep draft tankers.It can operate efficiently in rough seas and is not sensitive to directional changes of wind, waves and currents. With the tanker moored via bow lines only to a single buoy, the tanker is free to "weathervane" about the buoy and stay head-on to the weather, thus reducing mooring forces and increasing terminal utilization.It reduces operational dependency on support vessels such as tugs.It is generally less expensive to construct and/or operate than the other types of loading/unloading facilities.

This paper describes the ARAMCO - Ju' Aymah SALM Terminals (Arabian Gulf) which are the largest export SPMs ever built, and discusses the unique design features that were incorporated into these systems to make them safer, more efficient and more economical to operate than previous SPM designs.
Interpretive Methods Of Stratigraphic Modeling
Authors: Frink Alan P. , Wittick Thomas R. , Dedman Emery V.
Published: October 2008

The technique described is of utmost importance in stratigraphic interpretation because it measures sonic velocities and densities and their lateral variations. It will work on layers as thin as 40 feet. These changes can be indicative of facies changes or changes in lithology. The relative amplitudes and stacking velocities of seismic data comprise the required input to the method, however, if sonic and density logs from the area are available, this information" can be incorporated leading to greater control of the final results. The procedure requires that the shape of the propagating wavelet in the data be known and that it be a symmetric, zero-phase wavelet.

The method is automated, and the end product is velocity and density information for a stacked seismic trace which is displayed similar to velocity and density logs. An iterative scheme minimizes the differences between the response of a seismic modeling system and the seismic data, based on matching amplitude ratios of the data. The method can be performed on as many stacked traces in an area as is necessary to achieve the objective of the explorationist.

The technique has been applied to marine data from several different geologic provinces. When the results were compared to. sonic and density logs in the proximity of the seismic data, the correlations were good.

INTRODUCTION

In order to effectively use seismic data to define and evaluate potential stratigraphic traps, we must be able to detect lateral changes in velocity or some lithology-related physical parameter such as acoustic impedance. We will demonstrate two modeling methods which use the amplitude information in the data to transform seismic data into velocity or acoustic impedance logs.

A related problem concerns the correlation of seismic data and well log information. The sonic and density logs give detailed information at the well which can be correlated to lithology; however, they can only measure velocity and density at the borehole. Conversely, seismic data, while lacking in the detail possessed by log data, can be used to detect and measure lateral variations in velocity. We will show methods which make seismic and well log data more correlative. We will further develop techniques for using the detailed velocity information available at the well to calibrate the logs derived from seismic data. These logs will then be used to measure velocities and their lateral variations away from the well.

GENERAL SEISMIC PROCESS

Figure 1 is a schematic representation of the seismic process and the role of the interpreter. In the center of the figure is a seismic trace which is the result of collecting seismic data over the subsurface depicted on the left of the figure. This operation can be expressed in terms of simple communications theory as the convolution of the earth reflection series with a propagating wavelet.
Rough Weather Drilling Vessel Simulation
Authors: Shell R.C. , Masso A.H.
Published: October 2008

The Gulf of Alaska may have the roughest wave climate ever faced by the offshore oil industry. The high cost of time lost due to these rough seas was a major factor in selecting a floating drilling rig. This paper describes a simulation method that was used to estimate down times of floating rigs in the Gulf of Alaska and can be applied to other roughweather areas.

The simulator has two parts: one that simulates operations and another that simulates weather interference with those operations. While the operations model "drills a well," a novel weather model generates both wave heights and periods. Wave heights and periods determine vessel motions, which are checked against operating limits. If motions exceed the limits, work is "slowed down" or "stopped." By "drilling" - simulating - a large number of wells, reliable statistics on the down time of a particular rig are developed.

The weather model in the simulator can be applied to other offshore areas provided enough wave data exist. This approach also can be used for simulating offshore tanker loading, pipelaying or platform installations. Furthermore, the simulator can assess the value of hardware improvements that permit operations to continue in rougher seas. Statistical tests on the weather model showed that it adequately simulated Gulf of Alaska sea conditions. Furthermore, rig down time calculated with the simulator proved to be more realistic than down times calculated with models using just wave height.

INTRODUCTION

While waiting for rough weather to subside, a floating drilling rig will still cost up to $100,000 per day even though no work is being accomplished. Down time is expensive. And weather down time is even more important in selecting rigs for rough weather areas. Minimizing this expense can be done best by selecting an optimum rig design, one with the least down time under given sea conditions. This is not an easy task, given the many different rig designs proposed for rough-weather areas like the Gulf of Alaska. This task was simplified by a drilling vessel simulator that calculates down time and pinpoints operations particularly sensitive to weather-caused interruptions.

The simulator used two interacting parts: one that simulates offshore drilling operations, and another that simulates weather interference with the operations. While the operational model "drills" a well, the weather model generates wave heights and periods. The wave heights and periods determine vessel motions that are checked against operating limits. If the motions exceed the limit, work is "slowed down" or stopped. By drilling many wells, the simulator develops reliable and relistic downtime statistics.

The novel weather model simulates both wave height and period, whereas previous weather simulators generate only wave height. This model also can be expanded to include additional parameters such as current and wind speeds. This advanced weather model was needed because motions of drilling vessels depend on both wave height and period.
Lateral·stability Of Submarine Pipelines
Authors: Karal Karel
Published: October 2008

The upper bound approach of the theory of perfect plasticity has been applied to develop a method for geotechnical stability analysis of unburied submarine pipelines. The coefficient of? lateral resistance of the soil and the corresponding displacements of the pipe depend on seven geometrical, loading and material parameters. Compared to the comfits based on Coulomb friction, the coefficients obtained by the present method are, in general, higher for pipes on sand and lower for pipes on clay.

INTRODUCTION

The complex problem of the interaction between water, pipe and soil for an unburied submarine pipeline is traditionally separated into two parts:the determination of wave and current action on the pipe in the form of lift, drag and inertia forces (in this paper termed as hydrokinetic forces); andthe investigation of lateral stability of the pipe exposed to known hydrodynamic forces.

Lateral stability is often the critical factor for determining the necessary weight of pipe coating, especially in Shallow waters (?100 - 120 m).

Appreciable attention has been devoted to the solution of part (a) of the problem,' including the pertinent gathering of environmental data and experimental results. If this effort is to gain the intended meaning, attention must be paid also to part (b) of the problem - transmission of the hydrodynamic forces into the soil.

This paper describes a method for-predicting the lateral soil resistance to a pipe. The method employs a limit analysis by upper bound technique 1, 2,3 afforded by the theory of perfect plasticity.

AVAILABLE METHODS OF ANALYSIS

The lateral resistance of soil to a submarine pipeline is commonly treated as Coulomb sliding friction, the lateral soil resistance Rl being written as:

(Mathematical equation available in full paper)

where f = friction coefficient and Wn net Submerged weight of the pipe defined as:

(Mathematical equation available in full paper)

Here Ws submerged weight of the pipe, and Pv = vertical component of hydrodynamic forces, taken as positive upwards

This approach is based on the assumptions that:the support is rigid,the pipe is sliding parallel to the surface of the support, andRl is independent of lateral displacement

These assumptions fail for most actual physical problems. The soil is not rigid, the sliding does not develop along the soil surface, and the soil resistance depends on the displacements of the pipe. Thus it should be expected that this approach will fail for the most actual loading - soil - pipe conditions. Reliable results, however, can be obtained for "light" pipelines supported by dense sand using the approach based on Coulomb friction.

Lyons7 has concluded that "Coulomb friction is not valid for lateral sliding on soft clay". For this case he has presented some results derived by a finite element procedure. These results alone, however, are not sufficient for determination of lateral resistance of pipes with other geometrical, loading and soil parameters.
Behavior Of The Bouchard #65 Oil Spill In The Ice-Covered Waters Of Buzzards Bay
Authors: Deslauriers Paul C. , Martin Seelye C.
Published: October 2008

Most of the present data on oil pollution in ice-covered waters has been obtained from spills in static ice conditions. Thus, the Buzzards Bay spill, which occurred in moving ice, was of particular interest. Initially, the strong tidal currents transported most of the oil leaking from the barge into the broken ice field and beneath the large ice floes. The oil then collected in the crack systems of the rafted ice, hummocks, pressure ridges, and leads, and occupied an area of about 0.1 sq km. Oil pools formed by the rafted ice contained up to 2,000 gal of pure oil. No. 2 oil that was incorporated in hummocks and ridges was not as concentrated. Some oil in concentrated areas spread onto the ice floe surface primarily by wind forces. The No. 2 oil weathered at different rates, ranging from 6 to 47 percent volume loss, depending upon ti1e degree of oil exposed to the air. On Feb. 5, 0.1 m of snow fell at Buzzards Bay covering most of the oil from view; oil in concentrated pools formed a slush-like mixture containing 30 percent oil by volume. This snow greatly hindered aerial surveillance, research efforts, and cleanup attempts. Several cleanup techniques were used; the most successful was direct suction from concentrated pools into vacuum trucks accounting for nearly 13,000 gal of recovered oil. The ice began to break up around Feb. 8, releasing the oil contained in the ice in the form of a thin sheen. In addition, oily ice floes were transported by the currents through Cape Cod Canal into Cape Cod Bay, where they melted. These processes continued until about Feb. 26, 1977, when Buzzards Bay was essentially free "of visible oil.

INTRODUCTION

Buzzards Bay, located on the Massachusetts Coast, is a shallow, nonestuarine bay, approximately 46 km long and 19 km wide, with an average depth of 11 m. Connecting Buzzards Bay with Cape Cod Bay to the north is the manmade Cape Cod Canal (Fig. 1). The winter of 1977 was unusually severe for most of the northeastern United States with unusual icing conditions reported for many parts of the New England coast from December through February. Buzzards Bay, which normally does not ice over, was 90 percent covered by Jan. 28.

The ice that formed was less saline than normal sea ice because the waters in northern Buzzards Bay are slightly less saline than the waters found in Cape Code Bay and the southern portion of Buzzards Bay. Also, during the month preceding the spill, the Buzzards Bay ice had gone through four periods of above-freezing temperatures and rain that lowered the salt content of the ice. This low-saline ice was not as porous or elastic as normal first-year sea ice.

Several types of sea-ice formations were found in Buzzards Bay. Shorefast ice, with a characteristic thickness of 0.3 m, covered the waters of the protected coves.
Foundation Design For Gravity Structures With Respect To Failure In Cyclic Loading
Authors: Foss Ivar , Dahlberg R. , Kvalstad T.
Published: October 2008

The behaviour of soils during cyclic loading is examined, and it is concluded that "Failure in cyclic loading" has to be considered as a separate failure mode for foundations of offshore gravity structures. This failure mode is discussed and distinguished from "Quasi-static stability failure" which is the other important failure mode in the ultimate limit state. Rational safety criteria for failure in cyclic loading, which are not explicitly given in any of the current regulations for offshore structures, are developed in the terminology of the limit state method. A number of practical problems in the analysis of this failure mode are discussed, such as: the effects of random loading, application of the spectral analysis method, the effect of storm duration, the effects of non-uniform soil conditions ox stress distributions, principles for improvement of foundation safety. The paper concludes that failure in cyclic loading is the critical failure mode for several cases of practical importance. Only simple cases may be analysed accurately within the present state-of-the-art, and further development of theoretical methods is encouraged. Centrifuge model tests are useful for investigation of more complicated failure modes.

1. INTRODUCTION

The effects of cyclic loading is one of the most important considerations in the design of foundations for offshore structures, notably gravity platforms. Such effects influence the safety of the foundations against ultimate failure as well as platform motions in storm and long-term effects such as settlements and horizontal displacements.

The purpose of the present paper is to propose a design procedure which introduces rational safety considerations for dynamically loaded foundations of gravity structures. The principles of the procedure are equally relevant for all types of soil, but in the paper examples are limited to clay soils.

The procedure is based on the limit state method. According to this method the criterion for an acceptable design is that design capacity ? design loading effect.

The design capacity is an expression for the strength of the structure (or the foundation) where the safety requirement is introduced through reduction of the strength by a materials factor ?m. Similarly, the design loading effects are based on loads increased by a load coefficienty?f. The formal requirement to safety is thus introduced through two partial safety factors. ?m shall mainly account for uncertainties related to the strength of the materials, whereas ?f shall account for uncertainties related to the loading effects.

The safety criterion is checked for several limit states. Total collapse or large inelastic deformations which may have similar consequence as a total failure are analysed in the ultimate limit state. Other limit states may consider serviceability requirements (deformations, durability, etc.), long-term effects (fatigue), or the risk of progressive collapse. In the following we shall concentrate on the ultimate limit state.
Field Test Of Aguyed Tower
Authors: Finn Lyle D. , Young Kenneth E.
Published: October 2008

A test guyed tower structure has been installed and maintained in 293 ft of water in the Gulf of Mexico. The experimental objectives, structural design, and tower construction are described in this paper. The test data presented illustrate that the wave induced dynamic response of the tower can be accurately simulated and that the spud can foundation performs satisfactorily. The field test results provide further confirmation that the guyed tower is technically feasible and a practical structure to serve as an offshore drilling and production platform.

INTRODUCTION

The guyed tower is a bottom founded offshore platform concept which differs in two major ways from a steel jacket platform. First, in contrast to a conventional jacket platform which uses pilings to transmit both the wave and gravity loads to the soil, the guyed tower uses a guyline and clump weight system to dissipate the wave energy and a spud can foundation to transfer the gravity loads to the soil. Second, since the sway period of the guyed tower is greater than the design wave period, the principal structure inertial forces always oppose the principal wave forces instead of adding to the total load as occurs on a conventional platform. Because of these two advantages the concept offers an economically attractive alternative to conventional platforms in water depths from 600 to at least 2000 ft (1).

In order to advance the development of the guyed tower, a one-fifth scale test structure was installed in the Gulf of Mexico during October of 1975 (2). Measurements of the structure's response to environmental forces corresponding to design level conditions were recorded and subsequently analyzed. This paper describes the main features of the design, fabrication, and installation of the test structure and presents highlights from the data obtained.

TEST OBJECTIVES

The main objectives of the test were to obtain data and information about the installation, dynamic response, guying system behavior, and spud can (foundation) behavior of a guyed tower structure in an actual offshore environment. The installation procedures that were tested included towing of the structure to location, upending, and setting on bottom by adding preload ballast to the spud can as well as deploying the guy lines, clump weights, and anchors, and tensioning the guylines to insure proper setting of the anchors.

The dynamic response of a guyed tower is more difficult to predict compared to fixed-bottom platforms because the tower motions are of sufficient magnitude to require consideration of the interaction between the structure velocity and the wave particle velocity. Further, preliminary analytical and model studies indicated that the response of the tower to waves is phased such that the principal inertial forces always oppose the principal wave forces. Thus the field test data served to substantiate the predicted dynamic response of the tower when subjected to actual ocean waves, wind, and current.