bluuebunny/crossref_metadata_2025 huggingface dataset:
Designing the TLWP
Authors: Hunter A.F. , Zimmer R.A. , Wang W.J. , Bozeman J.D. , Adams C.J. , Rager B.L.
Published: October 2008

The Tension Leg Well Platform (TIWP) provides an efficient method for production oil and gas from deep waters in the Gulf of Mexico. It utilizes buoyancy to provide support for production facilities, well systems and a tension leg mooring system for stability am station keeping. The principal advantages of the TIWP include topside well completion and work over in deep water, ease of installation over a predrilled well template and an enhanced development schedule. Designing the TIWP is described in the following paper. This design was an interactive process which involved considerable innovation and balancing of various requirements.

INTRODUCTION

The Jolliet Field Tension Leg Well Platform (TIWP) has recently been installed in the Gulf of Mexico at a site near the southeast corner of Green Canyon Block 184. '!he site is approximately 170 miles southwest of New Orleans, Louisiana and located in 1,760 feet of water. The TD1P is comprised of four main elements: Drilling Template, Foundation Template, Tension Leg (or Tension Leg Mooring System), and the TD1P (Deck/ Hull) itself, as shown in Figure 1.

'!he Jolliet TIWP was installed by Conoco Inc. as the operator for a group of three production companies. Conoco's partners are oxy USA, Inc., a subsidiary of occidental Petroleum corporation, and Four star oil and Gas company, a subsidiary of Texaco Inc. An 0ITera11 review of the Jolliet Project can be found in Reference 1. The details of the TIWP design are reviewed in this paper. Detailed descriptions on fabrication, installation, am in-situ inspection aspects of the TIWP are Covered in References 2, 3, am 4, respectively. However, areas controlled by fabrication schemes, dictated by installation loadings, and impacted by inspection requirements are discussed in this paper.

The conceptual design was developed by Conoco' s Marine Division of the Production Engineering Research Department. Preliminary engineering was performed in-house supplemented by experienceddesign contractors. Final design of the 0ITera11 TI.WP hull, deck, facilities, mooring system and installation procedure were developed by a general design contractor with the support by selected design specialists. Periodic reviews were made with appropriate certified Verification Agents (CVAs) and regulatory agencies.

TLWP GLOBAL PERFORMANCE

Global performance for this project was defined as the behavior of the complete TLWP for both the permanent in place conditions and the temporary free-floating conditions. TLWP responses to weight and environmental loads were developed for each in place design iteration. In addition to platform movement and accelerations, consistent design loads on the hull and deck structure, riser system, tension leg mooring system and foundation template were provided. Draft and stability of the complete TLWP for the various free-floating operations and associated deck weights were also developed for each design iteration. As the design iterations proceeded, the optimum TLWP design emerged.
A Generalized In-Line Vortex-Induced Vibration Assessment Method
Authors: Reid D.L.
Published: October 2008

This paper identifies deficiencies and uncertainties in the present design procedures used for estimating inline (in the flow direction) vortex induced vibrations of circular cylinders in a near steady flow. A recently developed alternate design procedure which addresses some of these deficiencies, in particular non-uniform flow conditions, is discussed and compared with the existing methods.

INTRODUCTION

Vortex shedding from a bluff cylinder occurs in a wide range of flow conditions producing pressure fluctuations upon the cylinder. These fluctuations can excite structural elements with natural frequencies close to those of the pressure fluctuations and thus an assessment of this behavior is required in strength and fatigue evaluations during design.

This paper concentrates upon estimating the response of elements exposed to possible vortex induced excitation in the inline (with the flow) direction in a near steady current. During design possible vortex induced cross flow vibrations in both steady and unsteady (wave and current) flows must also be considered as well as wave induced inline motions.

Bronchi et al(1989)[ 1] indicates that inline vortex induced vibrations need only be considered when the peak wave induced velocity is less than 25% of the current velocity. Outside these conditions it is suggested that the unsteadiness of the flow inhibits the required vortex interaction from occurring and that the wave loading usually described by the Morison equation dominates the inline motions.

INLINE VORTEX INDUCED VIBRATIONS

For a detailed introduction to vortex shedding and its effects in near steady flows the reader is directed to King(I977)[2] and Griffin(I 981)[3].

The frequency of a vortex being shed from each side of a stationary cylinder in a Steady flow is described by the Strophes Number(St). This varies with Reynolds Number(Re) on a smooth stationary cylinder as presented in Figure I. The cross flow loading is dominated by this frequency while the inline loadings are around the frequency of shedding of individual vortices (? 2fv), In the zone 2xl05 < Re < 3xl06 the usually strong periodic vortex shedding is disrupted by the transition of the cylinder boundary layers from laminar to turbulent conditions. The location of this zone varies with surface roughness and turbulence levels. When a cylindrical structure vibrates in this zone, with surface roughness typical of marine structures, the usual narrow band vortex shedding is observed with a Strophes Number close to the lower bound of Figure I. For a particular structure, as St is nearly constant and (Vr)(St) = fv/fn then Vr describes the proximity of the system to a resonant condition.

Inline vortex induced excitation occurs, for a particular element, at lower flow speeds than cross-flow excitation, and thus although it produces much lower response amplitudes it may dominate fatigue assessments.
Burial of Vertically Loaded Offshore Pipelines in Weak Sediments
Authors: Dunlap W.A. , Bhojanala R.P. , Morris D.V.
Published: October 2008

Cyclic vertical loads were applied to a 5' long, OS diameter model pipe, in order to simulate the effect on pipeline stability of various lift forces including waves, in addition to self weight. Experiments were carried out in a large sediment -filled tank, applying controlled vertical loads or displacements to the pipe section with a servo-hydraulic system. The cyclic and incremental movement and forces were electronically recorded, to provide data for the behavior of similar full scale structures, and for analysis of the problem.

INTRODUCTION

Marine pipelines can be subjected to a variety of environmental loading conditions from waves, currents and even nearby platform movements. At many locations where pipelines support the offshore petroleum industry, the pipelines rest on or are embedded in very soft clay-rich sediments. Repeated loading may cause the pipelines to bury in these sediments. In some instances this may be desirable, but if a buried pipeline is subsequently subjected to transverse loads or vertical pullout, large bending stresses and pipe distress may result.

In an earlier paper (Morris et aI., 1988), the effect of repeated transverse loading on pipeline behavior was addressed. The present paper discusses vertical pipeline loading and can be considered a companion to the earlier paper. The investigative approach was the same for both studies: experiments were conducted using a model pipe subjected to loading in a soft sediment. By the use of appropriate non-dimensional relationships, results from the model test can be scaled to predict prototype pipe behavior.

EOUIPMENT AND TEST PROCEDURE

All tests for this study were conducted in a sediment filled tank which was 6' × 6' in plan view and 4' deep. A false bottom was placed in the tank to limit the sediment depth to 30". The tank was filled with a calcium betonies mixture which had 53% clay (0.002mm), 42% silt and 5% very fine sand. The liquid limit was 101 and the plasticity index was 62, which classified the sediment as a CH (Unified Soil Classification System). Water content of the sediment was continually monitored during the test program and adjusted to maintain an untrained shear strength range of 20-30 lbs/sq ft.

Vertical loading and/or displacement of the pipe was controlled with a closed loop, servo controlled hydraulic actuator. The actuator was mounted on a frame positioned alongside the test tank; a lever arm which pivoted on the edge of the tank transmitted motion from the actuator to the pipe. A load transducer mounted between the pipe and lever arm measured forces on the pipe and a linear variable differential transducer (LVDT) mounted on the pipe measured the vertical pipe movement. To ensure that the pipe moved only in the vertical direction, guide rods were attached to the ends of the pipe; these rods ran through linear ball bushings which were mounted on a beam spanning the test tank (Fig. 1).
Development of an Advanced Subsea Wellhead System Incorporating All Metal-to-Metal Sealing
Authors: Danner B.L. , Henderson H.O.
Published: October 2008

This paper describes the design, analysis, testing and the resources devoted to the development of an advanced sub sea wellhead system. This wellhead system significantly improves casing suspension/load capacities, annulus sealing and rig floor operations.

INTRODUCTION

Wells are now being drilled in water depths over 7,500 ft. with reservoir pressures approaching 15,000 psi, and temperatures approaching 180°C. A high-performance sub sea wellhead system providing simple rig operations, reliable sealing and improved load carrying capacity is an important element in safely and cost effectively drilling and developing deepwater fields.

In developing this system, major emphasis was placed on:passive load transfer.All metal SealingSimple weight set operation. These elements, combined with other specific system performance goals, provided the benefits necessary to warrant this project. The approach was to define a specification based on historical perspective and present day customer input, to design and analyze the elements using the most modern technique: available, to rigorously and thoroughly test the equipment under laboratory and field-simulated conditions, and to perform field trials. The wellhead resulting from this research is shown in Figure 1.

Specification

The detailed specification is a lengthy document covering all associated equipment from spud-in to tieback and sub sea completion interfaces. The basic requirements for the wellhead are:18-3/4 in. -15,000 psi rated working pressure.compliance with industry specifications: API 17D, NACE MR-01 75, and Certifiable for North Sea operations.Temperature range - 4°C thru + 120°C.Simple reliable installationWeight set operation.Passive casing suspension load carrying system.Single trip installation of hanger and seal assembly.Minimization of the number of trips and tools.Load carrying capacity -7.1 million lb.1 million lb. per hanger (13-3/8 in., 9-5/8 in., 7 in.).4.1 million lb. end-load due to 15 Ksi full bore test.Highly reliable all metal sealing.Maximized flow-by area and particle size.Compatibility with sub sea production systems as well a platform or TLP tieback systems.

DESIGN

Wellhead Housing

The external design of the housing is standard and can be fitted with a mandrel or a hub profile to interface with the BOP stack connector. The load transfer system from casing hanger to wellhead housing is completely passive as shown in Figure 2. Unlike previous systems, no mechanisms, shear pins or addition trips are required to provide over 7 million lb. of load carrying capacity. The high-strength load shoulder insert in the wellhead housing has bore clearance for a 17-1/2 in. bit. The pre-installed wert provides additional bearing area on the NACE-rated housing material while maintaining 360° of contact with the first casing anger. This hanger-to-housing interface allows the hanger to simply land upon a conventional angular shoulder while maintaining the ability to support a load-tested 8 million lbs.
Direct Determination of Sling Tensions in Heavy Rigging
Authors: Crull C.M.
Published: October 2008

Industry standards governing the design of heavy offshore lifts are limited in scope with regard to analytical techniques, particularly when sling length variation is considered. This paper presents approximate and exact methods for determining sling tensions, regardless of lift configuration. A parametric study examining critical features in lift geometry is presented.

INTRODUCTION

Throughout its history the offshore industry has moved to heavier and heavier offshore lifts in order to obtain the economies provided by minimizing offshore hookup time. At present there is at least one heavy lift vessel capable of lifting 14,000 metric tons operating. The economic risks associated with these heavier lifts make it imperative that engineers understand the implications of their lift analyses.

Current industry design standards, such as those of API and DnV, are adequate for situations which they cover. For situations where sling length variations exceed standard tolerances, which is common, or unusual sling configurations are used, guidelines provided put the burden on the engineer for analyzing and defining acceptance criteria for rigging.

To analyze this type of problem, this paper presents both approximate and "exact" techniques for analyzing sling configurations, either regular or unusual, which include slings with length variations. While the techniques presented are applicable to any size lift, the author originally developed them for lifts of 1,000 tons or more.

Exact analyses of rigging considering length variations in slings are non-linear. The approximate solution presented herein can be incorporated in an electronic spreadsheet for rapid solution of such problems. It provides results adequate for preliminary design and, under many circumstances, final design.

A technique is presented whereby the engineer can make an exact analysis of such non-linear systems with linear finite element analysis programs. Both solution methods were developed for single hook lifts. The exact solution principles can be extended to multi-hook lifts.

To quantify some of the variables in a lift analysis, a parametric study was performed. Results of this study can be used by engineers as an aid in recognizing which factors are more important in a lift analysis and which are less important. Help in determining when an approximate analysis is appropriate and when an exact analysis is justified is also provided by the parametric study. Brief comments on load factors for sling design and on in-plane deck bracing are included.

The primary aim of the techniques presented is to provide analytical methods for determining sling tensions, not stresses in the lifted structure. Once accurate sling tensions are available, analysis of the lifted structure is straightforward.
An External Scheme for Strengthening Offshore Platforms
Authors: Game W.J. , English K.G.
Published: October 2008

This paper will describe the method by which Esso Australia Ltd. has substantially upgraded four of its oldest producing Bass Strait Platforms. The method involves the installation of one or two pile struts on the West side of each platform and the implementation of associated On-Platform Strengthening.

This innovative strengthening scheme will be described in sufficient detail to allow its direct application to other jacket type structures which may require strengthening. The paper will also put emphasis on the engineering challenges that were associated with implementation of the strengthening of the platform jackets and placement and connection of the pile struts to the jacket and sea-bed.

INTRODUCTION/BACKGROUND

Esso, as operator of a joint venture with BHP Petroleum (Bass Strait) Pty. Ltd. (BHPP), currently has 18 oil and gas producing facilities in Bass Strait (Figure 1) situated off the South Eastern tip of the Australian mainland. The first five of these, Barracouta, Marlin, Halibut, Kingfish A and Kingfish B, known as the First Generation Platforms (FGPs), were designed and installed in the late sixties.

Important Producing Platforms

These FGPs continue to playa vital role in the Bass Strait oil and gas production system. Halibut is the most important platform to the Joint Venture as essentially all Bass Strait oil either flows through or is produced from this platform. In 1989 this amounted to an average of 3l0KBD and represented over 60% of Australia's crude oil production which equates to approximately 47% of Australia's crude consumption. Marlin, Barracouta and Snapper platforms produce all sales gas required for the State of Victoria.

Platform Evacuation Policy

As we came to a greater knowledge of Bass Strait sea states and behavior of calcareous soils, it was clear that safety margins on the FGP's in storm conditions were less than desirable. In the early seventies a staged evacuation of personnel was instituted when storm conditions were forecast.

Required Ultimate Platform Strength For comparison purposes the term "reserve strength ratio" (RSR) was used as a reference of platform global strengths. (MATHEMATICAL EQUATION AVAILABLE IN FULL PAPER) A minimum target RSR of 1.5 was adopted for the FGP platforms to be consistent with the intent of generally accepted design practices. Personnel storm evacuations would continue until this target was achieved.

Strengthening Options

In 1985 Esso/BHPP decided to structurally upgrade all FGPs to a minimum RSR of 1.5 and thereby enable continuous manning. The desired increase in calculated global strength could be achieved most cost effectively by marine growth control for Barracouta and pile struts (struts) for the other platforms - one each for Kingfish A, B and Halibut (Figure 2) and two for Marlin (Figure 3). Guy lines could also have achieved the desired strength increases but were deemed more expensive. A more detailed explanation of the strengthening approach is given in Reference 1 and summarized in Reference 2.
Jolliet TLWP Well Systems
Authors: Myers R.J. , Delgado J.H.
Published: October 2008

This paper cut lines the unique features of the Jolliet Project well Systems. same of these features are as a result of experience gained on the Hutton tension leg platform, while others came as a result of special condition relative to the Jolliet Project.

The paper describes the systems starting at the drilling template, moves up the production riser to the surface support facilities and finishes with the internal completion.

INTRODUCTION

The well systems on the Jolliet Project Tension leg well Platform (TLWP) comprise the components of the production riser system. Full size tension leg platforms may have tensioned production, export and drilling risers. In the case of the Jolliet Project, all wells were predrilled and work over?s will take place through a 9-5/8 inch casing. Cot1segUently, there is no need for a drilling riser as no new drilling, apart from sidetracks cut of a cased hole, is anticipated. The export risers are flexible pipeline risers to minimize weight am tie-ins on the TLWP.

The production riser system (Figure 1) consists of the sub sea wellhead which sits in the drilling template receptacle, lower riser connector, tapered joint, riser pipe, special top riser joint, wellhead, Christmas tree and tensioned. sitting inside the production riser is the internal tubing completion.

WELL SYSTEM Components

Long Wellhead Receptacles (Figure 2) one of the requirements of a TLP production riser system is to install the 30 inch wellhead vertically, as this in turn sets the verticality of the 18-3/4 inch high pressure housing. Anyl deviation from vertical in the high pressure housing has a detrimental effect on the fatigue life of the production riser.

The long wellhead receptacle (15 feet) has reduced diameters at the top and at the bottom that restricts the 30 inch casing angle to I' when it is being run am the 30 inch wellhead housing to 0.5' when installed. This system may be used with jetted or dynadrilled casing.

The receptacles form part of the well pods installed on the drilling template [1]. Because they do not serve as structural members, the receptacles were lightened with large, cut windows to reduce the in-place and lift weight of the well pods.

Guide Posts (Figure 3)

There are no permanent guideposts installed on the drilling template. The posts are removable and are installed each time a riser is run or drilling takes place. '1bey latch by means of their own weight into a receptacle and are released by a hydraulic tool that can be run on a FDV or on drill pipe from the surface.
An Appraisal of API RP2A Recommendations for Determining Skin Friction of Piles in Sand
Authors: Toolan F.E. , Lings M.L. , Mirza U.A.
Published: October 2008

API RP2A in its 14th edition (1984) recommends the use of an earth pressure coefficient of 0.8 for computing the tensile skin friction of an open ended pile driven into silica sands. In view of the impact of this recommendation on the design of North Sea piles driven into sands, a study was made of an expanded data base on the pull-out capacity of piles driven into sands. The expanded data base includes test data on relatively long North Sea conductors and in all 24 tension tests considered of reasonablyhigh quality were analysed. The new data base together with observations of pile behavior in sand has enabled the authors to develop two procedures for designing piles in sand. These two procedures differ from API in allowing skin friction to reduce at a given level as a pile is driven deeper. Comparisons of predicted and measured capacities of the test piles were made and compared to API (1984) predictions. It is concluded that the API Method is not reliable for loose or very dense sand sites. The two new methods provide much better predictions in these soil conditions.

INTRODUCTION

The Document API RP2A (Ref. 13) is updated regularly in the light of experience and research. From the foundations point of view, a major change was introduced in the 1984 issue (14th Edition) with regard to the assessment of skin friction of pilesdriven into sands. The 1984 and subsequent issues of API recommend using a coefficient of earth pressure, K, value of 0.8 for computing both compressive and tensile friction capacities of an open ended pile driven into silica sands. This recommendation may be compared to the pre 1984 practice where K values are taken as 0.5 and 0.7 for tensile and compressive loading, respectively of an open ended pile. One ingredient that has lead to the current API recommendations was the conclusions from the API supported study of Dennis and Olsen (Ref. 14) which collected and examined the data for 66 closed andopen ended driven piles, tested in compression and in tension.

The substantial increase of 60% in the K value for tensile loading has a major implication on the design of North Sea piles in sand. As a result, the Department of Energy and Fugro-McClelland supported a study in which the data base for pull-out tests on piles driven into sands was re-evaluated. This study which was reported by Lings (Ref. 15) consisted of re-interpreting the load test data and thesoil conditions for some 20 tests which were considered more reliable than others. The data base included proprietary data from a pull-out test on a 38m long conductor driven into dense to very dense North Sea sands. Lings I s conclusion was that the API (1984) method appears in general to give satisfactoryresults for piles up to 60 diameters penetration while at greater depths, the method seriously over predicts pile capacity. In a more recent work reported by Toolan and Ims (Ref. 16) the current API method for sand was also examined and shown
Developments in Fatigue Design Guidelines for Anchor Chains as Mooring Lines
Authors: Sincock P. , Lalani M.
Published: October 2008

This paper discusses the specification and design of moorings using anchor chain, with particular emphasis on long term mooring. A brief review of historical field data of offshore platform chain mooring lines is first presented. Current design methods for determining fatigue life and extreme loads for chain moorings are then discussed and compared.

With the numbers of floating production systems and other installations which use long term moorings growing rapidly, it is suggested that fatigue willbecome an increasingly important design parameter for all moorings. The majority of current Design Guidelines give only limited guidance on the design of chain moorings for long term position mooring. All test data on anchor chain fatigue life presently in the public domain is presented in both tabular and graphical formats. A detailed appraisal of this data is made.

It is concluded that there are a number of areas which warrant further study in the development of design guidelines for chain moorings, particularly with respect to fatigue life. This is currently the subject of an extensive joint industry research and development project in Europe, and is discussed in detail in this paper.

Introduction

Anchor chains are a widely used means of catenary mooring for both Mobile Drilling Units (MDUS) and, more recently, Floating Production Systems (FPSS). In addition other offshore structures, such as tanker single point moorings and spar loading buoys are frequently moored by means of chains. Ships almost universally employ chain anchor lines.

The design of anchor chain links, known as "common links", is over 100 years old and their dimensions follow a standard which is international (l), (2). Common links are made from a length of round steel bar, which is bent into a loop with straight sides, figure 1. The ends of the bar are flash butt welded, the join being positioned on one of the straight sides of the loop. A short length of rounded bar with shaped ends, known as a stud, is fitted between the straight sides of the loop. The primary purpose of the stud is not, as might be imagined, to increase the strength of the link, but to prevent the chain becoming tangled when slack. The stud is fixed in place by squeezing the sides of the loop inwards. For further security the end of the stud opposite to the flash weld may be welded to the loop.

Chain for ships is supplied in three grades, referred to as Grades 1, 2 and 3. For offshore use, chain of greater strength than these ship grades is normally specified. The chain grades most commonly used for offshore applications are K3 and ORQ (Offshore Rig Quality), which are essentially equivalent grades. Recently, however, two chain grades of higher strength than K3 (ORQ) have begun to gain in popularity for offshore use. These are 0RQ+20% and K4, which are approximately 20% and 30% stronger, respectively, than K3. A third grade, K5, which is stronger again than K4, is also likely to find increasing favour in the future.
Reliability of Techniques for Nondestructive Testing Over Coatings for MODU Inspection
Authors: Mahmoud S.E. , Abrams P.I. , Goldberg L.O.
Published: October 2008

A study was made of the reliability and sensitivity of nondestructive (NDT) techniques for locating flaws under coatings commonly found on offshore structures. Coating thicknesses ranged from 4-16 mils (0.1-0.4 mm). Ultrasonics were found to be more sensitive than magnetic-particle and eddy-current methods.

INTRODUCTION

The objective of this program was to determine the set of conditions required for the nondestructive detection of flaws in structures coated with paint or other coating systems. A successful inspection technique has the potential for substantial cost savings, particularly offshore. A number of instances have arisen in which coatings could not be removed, yet inspection of the underlying structure, and particularly welds, was required.

The inspection methods evaluated in this study were ultrasonics (UT), magnetic-particle (MT), and eddy-current (ET). These techniques are commonly used during the structural inspection of platforms and MODU's. Two types of coatings were selected for evaluation based upon their wide use on offshore structures. These were the epoxy-mastic type paint system and the urethane system (four-part coating system). The coating thicknesses varied from 4 to 16 thousands of an inch (0.10-0.40mm).

To simulate flaws sometimes found in welded connections, welds were made and purposely designed to produce shallow/tight cracks, deep/tight cracks, and shallow/wide cracks. In addition to these flaws, varieties of samples containing known defects were used in this investigation. All test samples were inspected with the various NDT techniques before and after application of coatings.

TEST PROCEDURES AND SAMPLES

Three types of test samples containing flaws were used in this investigation. The first type was in the form of steel plates 40 inches long and 12 inches wide (101.6 by 30.5 cm) containing defective welds. The second type comprised calibration samples containing known defects (depth and width of machined slits). The third type contained fabrication-type defects (toe cracks, laminations, and undercut) from real structures. Descriptions of test samples follow.

1. Artificially Produced Flaws in Plates

Shallow/very tight cracks were made by making short tack welds on high-strength steels with a cellulose-coated electrode (E70l0-Al) [1]. The accompanying undercut of these tack welds simulated shallow/wide cracks.

The deep/tight cracks were made by improperly welding the root-pass of a double butt weld joint. The improperly deposited downhill pass resulted in a center bead crack for the full length of the deposit.

2. Calibration Samples

Magnetic-field indicator with slit widths of 0.040 inch (1.0 mm). Burmah-Castrol magnetic-field indicators, Type I and Type III. Steel block with three EDM notches. The notches are one-inch long and 0.010 inch in width (25.4 by 0.25 mm). Depths of the three notches are 0.020 inch (0.5 mm), 0.040 inch (1 mm), and 0.080 inch (2 mm). This block is typically used in eddy-current testing to calibrate the equipment.
Jackup Performance and Foundation Fixity Under Developing Storm Conditions
Authors: Hambly E.C. , Imm G.R. , Stahl B.
Published: October 2008

The dynamics and foundation fixity of the Rowan Gorilla II jackup at Arbroath (308 ft water depth) in the North Sea have been investigated through a measurement program and an analytical study. The measurements, recorded during the winter of 1988/89, reveal the presence of a significant amount of foundation fixity during a storm which was almost an annual return period storm. A small reduction in foundation fixity does occur during this storm. The analytical study includes the effects of nonlinear hysteretic spudcan behavior, layered seabed conditions, and irregular wave loading, and has been calibrated using the results of the measurement program. The study compares static vs. dynamic analyses using different models of foundation fixity (none, linear and nonlinear). The study also gives a best estimate of the maximum jackup leg moment for an extreme event at Arbroath.

INTRODUCTION

Jackups have historically been analyzed using static analysis techniques assuming no foundation fixity. As jackups move into deeper waters and become more dynamically sensitive, the effects of dynamics and foundation fixity become more important and must be modelled if realistic results are to be achieved. Unfortunately, dynamic effects are difficult to model, and little full-scale foundation fixity data of jackups in storm conditions is available.

Amoco hired the Rowan Gorilla II jackup to drill at Arbroath in the Central North Sea during the winter of 1988/89. The water depth at this site is approximately 308 ft (94 m). Amoco saw this as a unique opportunity to investigate the effects of dynamics and foundation fixity in deeper water, and initiated the data collection and analytical effort described in this paper.

MEASUREMENT PROGRAM

A measurement program was conducted aboard the Rowan Gorilla II jackup at Arbroath from December 21, 1988, to January 16, 1989. The independent leg jackup is depicted in Figure 1 and described in Table 1. The measurement program consisted primarily of an infrared wave probe and a triaxial accelerometer, with some wind and current data also recorded. Figure Ib shows the locations of the wave probe and accelerometer, and the definitions of surge and sway directions. Measurements were recorded for 50 minutes of each hour.

The largest recorded storm was a 26 ft (8 m) significant wave height storm which occurred on December 24. Based upon the significant wave height, the individual maximum wave height is estimated to range from 44 to 49 ft (13.4 to 14.9 m), depending upon the assumed individual wave height distribution. The storm was slightly smaller than the expected annual return period storm (29 ft significant) for this site.

Jackup natural periods for different seastates were obtained from spectral density analyses of the acceleration records.
The Dry Transport of the Green Canyon Tension Leg Wellhead Platform by a Semisubmersible Heavy-Lift Ship
Authors: van Hoorn F. , Devoy S.D.
Published: October 2008

A Tension Leg Platform poses unique challenges with respect to its dry transportability. The engineering that is required before such a transport can be effected is given step by step. The development of a new type of cribbing block is described. The predicted extreme design environmental conditions and resulting extreme motions are compared with the actual monitored weather conditions and motions. From the correlation, conclusions are drawn.

INTRODUCTION

On May 26, 1989, the first TLWP (Tension Leg Wellhead Platform) was delivered in the U.S. Gulf by a self-propelled semisubmersible heavy-lift ship. This ship departed Singapore only 41 days earlier after loading the TLWP by means of the float-on method, see figure 1. The dry transport of this platform broke new ground in various areas. The unique nature of the TLWP required various unorthodox solutions. Conventional design methods were revised and new materials were developed.

The Green Canyon block 184 TLWP was built by a shipyard in Singapore, Republic of Singapore, which contracted with a HeavyLift Transporter for the dry transport of the TLWP to Pascagoula, Mississippi, where it was to be final outfitted before installation in the U.S. Gulf.

Dry transport by a self-propelled heavylift vessel offers the highest degree of safety in combination with a minimum of environmental loads on the cargo.

A wet tow was not a realistic option given the distance of over 12,000 miles and an anticipated tow speed of 2 - 3 knots. Transportation by tug/barge was considered, but the increased safety of the self-propelled option proved to be the most attractive.

The TLWP is constructed from stiffened cylindrical shells. The truss deck is carried by four columns of 12.2 meters in diameter, 46.2 meters in height, spaced 42.67 meters apart (center to center). Pontoons, 7 meters in diameter, connect the four columns near the base.

At the moment of load-out, the TLWP's displacement was 8,400 T with its center of gravity at 31.8 meters above its base. The draft measured 8.3 meters, leaving a freeboard on the pontoons of 1.2 meters. These pontoons were outfitted with bollards to facilitate the connection of tugboats and winch wires during on- and off-loading.

Given the relatively small footprints of the columns, in combination with the fact that these would mostly overhang the carrier's 40 meter wide deck, four sponsors were required to increase the support area. Much attention was paid to the footprint loads in order to obtain the optimum support configuration.
The Installation Design and Analysis of the Oseberg 'C' Jacket
Authors: Allen A.P. , Bjørke M. , Saetrevik T. , Auraaen E.G.
Published: October 2008

The Oseberg 'C' Jacket utilises for the first time a fully fixed mating system for jacket to template mating.

This paper presents a brief review of the available installation options for eight legged barge launched jackets and demonstrates the potential saving for each option. The design and analysis of the post launch installation phases of the Oseberg jacket is then described with particular emphasis on the development, design, analysis and model testing of the template mating system.

INTRODUCTION.

The installation in June 1988 of the Oseberg 'C' drilling template in the Norwegian Sector of the North Sea marked the beginning of the second phase of Norsk Hydro's Oseberg Project. The 14000 tonne Oseberg 'C' jacket, will be installed in the summer of 1990, by Heerema in 108 metres of water. Production start-up from the 35000 tonne topside scheduled for October 1991 will provide a further 100,000 bbl from the field bringing the total daily production up to 340,000 bbl.

Feasibility studies on a variety of alternative substructures led to the final selection of an eight legged barge launched jacket, together with a preinstalled subsea template as the most cost effective solution.

Provision of a drilling template to achieve early production has been a proven method of field development for some years, significantly reducing the lead time required and allowing increased oil production early in the field life.

The installation of a fixed jacket structure over a sub-sea template, having been first performed in the North Sea in 1979 (Fulmar wellhead jacket) has since been carried out on a number of occasions with a variety of jacket weights ranging from Fulmar at 2500 tonnes to the larger structures such as the Ekofisk 2/4 k water flood jacket, South East Forties and Oseberg 'B' at 18500 tonnes which was installed in 1987.

To date these structures have generally relied on the vertical docking technique in which the jacket is upended to a hover position 10 to 12 metres clear of the seabed and progressively lowered over the template engaging up to three previously installed docking piles of varying heights. Engagement is achieved, with either a passive or active tapered male/female docking sleeve system, which first engages with a large lateral gap between the sleeve and docking pile and reduces as the jacket or sleeve is lowered to provide accurate jacket/template location and thereby ensuring satisfactory conductor tieback.

The pile/sleeve gap is required for operational targeting reasons and to ensure against wave induced resonant jacket behaviour as the piles are engaged and the jacket stiffness properties change. In addition this gap must account for fabrication tolerances in the pile and sleeve, and installation tolerances in the pile.

The Oseberg 'C' jacket uses a new fixed mating system, patented by a Norwegian Company, which was developed with the following objectives;
A New Dimension in Offshore Emergency Evacuation
Authors: Withers M.H.
Published: October 2008

INTRODUCTION

It is ironic, if not tragic, that after some ±25 years in the offshore oil and-gas sector, in a real emergency one of the valid, possibly in some cases only, means of escape from an offshore platform is to jump.

To address this problem, much thought was applied around 1982/3 with the result that, today, there exists a viable and we feel, more effective means of escape from a dangerous offshore incident. While the basic thought behind the system is to provide an integral part of an overall safety philosophy the system may, in some cases, be considered as a first, rather khan "last ditch", means of escape.

The basic requirement was therefore to provide a safe, rapid means of mass transfer away from the danger area. The system called for–Maximum capacity and availability–Minimum risk to system users

The key design criteria for the system are those of:–Simplicity–Functionability–Flexibility

THE PRODUCT OVERVIEW

In terms Of translating conceptual requirements into hardware, the first consideration had to be given to the means transfer to sea level responding to the criteria and demands noted above. Through long and painful development the chute design was evolved of which more later. A relatively simple matter was to add the necessary interface to the platform, deployment and means to transport away from the area. In block form the final solution appears as shown in Fig 1.

Fig. 1 (Available in full paper)

The four integral components of the system may likewise be depicted as shown in figure 2

Fig. 2 (Available in full paper)

THE CHUTE

The heart of the system and the real invention is the patented chute consisting of a series of modular cells running from the proposed evacuation deck level to sea level. Each cell, one meter long, contains a speed retarding slide mounted in the opposite angle to that in the cell above and below it. The outer tube and internal slides are all made of Aramid Fibre netting. It will be seen that this enables the evacuee to control, to a highly accurate degree, his rate of descent. Ref. Fig. 3

Fig. 3 (Available in full paper)

Each cell is separated from the next by are aluminium ring set, as mentioned, at one meter stages. The foot of each contra-angle slide, stops short of the outer diameter of the ring thus forming a "D"-section through which the evacuee enters into the next cell. The bar of the "D" is, in fact, a stainless-steel spring which adjusts to the shape "D" the body as the evacuee passes to the next cell.
Behavior of Pile Plug in Sandy Soils During and After Driving
Authors: Brucy F. , Meunier J. , Nauroy J-F.
Published: October 2008

When open-ended piles are driven into the soil, a soil column is created inside. The advance of the soil column with respect to the pile penetration reflects the mode of pile penetration in the soil and influences the pile- soil interaction during and after installation. Formation of the soil co1umn inside open-ended model and large sca1e experimental piles was analysed at four sandy sites. The incremental filling ratio was found to be linked to the impact characteristics. The influence of the soil column on driving results as on static results was evaluated and discussed. In contrast to the driving results, the static loading result were not changed by the removal of the soil column. The plugging effect was explained by an exponential cumulation of stresses inside the piles.

INTRODUCTION

The majority of offshore platforms are founded on open ended tubular piles. When the piles are driven into the soi1, a soi1 column is created inside the pile. At the start of penetration, the pile cores a soi1 column the height of which is equal to the pile penetration. Then as penetration continues, the filling ratio may vary. At a certain stage, the soil column may act as a perfect plug, preventing any new soil intrusion. If that occurs, the pile penetrates the soil as a closed-ended pile. The advance of the soil column with respect to the pile penetration reflects the mode of pile penetration in the soil which influences the pile-soil interaction during and after installation. In practice the advance of the soil drivability methods use unit skin friction and unit toe resistance independent of the penetration mode of the pile. The contribution of the soil co1umn to the soil resistance to driving and to the static bearing capacity is taken as the lowest resistance of the two fo11owing mechanism. The plugged mechanism requires that the point resistance be calcu1ated over the full pile toe area, and the unplugged mechanism requires that an interna1 skin friction due to the soil co1umn be added to the point resistance of the pile annulus toe area. The same unit skin friction as the external on is used. This may be overly conservative for static design. In this paper, the advance of the soi1 co1umn inside experimental piles is reported -and analysed. The contributions of the soil column to the soil resistance to driving and to the static bearing capacity are evaluated and discussed. Open ended model piles 70 mm in diameter and 2,50m long and large scale piles 324 mm in diameter and 12 and 24 m long were tested at dense sandy sites. The experimental programme was carried out within CLAROM (a research association grouping French oil companies, contractors, one certification body and research institutes working on offshore operations).
A New, High-Frequency, Broadband Seismic Source
Authors: English J.M. , Inkpen S.T. , Guigne J.Y.
Published: October 2008

A new seismic source has been developed which emits a reproducible, high frequency, broadband acoustic signal. This device, suitable for marine sub-bottom profiling is a unique variation of the multi-electrode sparker source. Calibration experiments performed in "free field" conditions have shown that the output is tunable by varying the power supplied to the source. Typical output has a 4.2 kHz bandwidth and a 3.0 kHz centre frequency, measured 3 dB down, at an operating power of 480 Joules.

INTRODUCTION

Resolution of two seismic reflectors depends on the length of the acoustic pulse. Simply put, the shorter the pulse, the finer the temporal resolution. Taken to the extreme, the ideal pulse would be a spike with all the energy concentrated at a single point in time. Since bandwidth increases with decreasing pulse length, the desired pulse is broad band, containing high frequencies to permit resolution of thinly spaced reflectors, as well as low frequencies which achieve greater sub-surface penetration. [1]

For a given pulse length, the desired shape is 'front end loaded'; having its energy maximally concentrated at the onset. [2] Such a pulse is known as minimum phase. Digital statistical deconvolution processes assume that the source wavelet is minimum phase. The quality of the output from these deconvolution processes degrades as the pulse departs from minimum phase.

An acoustic source which delivered the same acoustic signature for an indefinite period of time would reduce the possibility of interpreting a deviation in source signature as a lithologic change in the subsurface. Finally, an ability to control and alter the length and shape of the emitted pulse would permit a tuning of the source to a particular seabed.

In summary, good quality seismic data depend, among other things, on the length, shape and consistency of the downgoing acoustic pulse, as well as on an ability to alter the pulse in a controlled manner. We have developed an acoustic source based on the common sparker source routinely used in sub-bottom profiling. The conventional multi-electrode sparker has many advantages including its ease of operation, versatility and low cost; however, it does suffer from a degradation in pulse shape through the course of a survey due to the deterioration of electrodes. [3] Our efforts have concentrated on engineering a multi-electrode sparker that emits a minimum phase, reproducible signal with a higher frequency and a broader bandwidth than the conventional sparker. This device was developed specifically for the unique requirements of a stationary high resolution geophysical site surveying tool. [4][5] However, calibration of the source in a towed mode is planned for the next phase of our research. This paper describes the source and shows examples of field data.

SPARKER THEORY

The term sparker is misleading, it tends to imply that the mechanism is the same as that of a spark plug. However, a spark plug is a device which includes a pair of electrodes with an insulator in the inter-electrode space.