Semi-submersible Drilling Unit Research Articles

E&P Notes (July 2021)

E&P Notes (July 2021)

Position Loss Risk Analysis of Dynamic Positioning Systems of Semi-submersible Drilling Units by Considering Time-varying Failure

Bai, X.; Zan, Y.F.; Luo, X.F.; Huang, K., and Guo, R.N., 2020. Position loss risk analysis of dynamic positioning systems of semi-submersible drilling units by considering time-varying failure. In: Guido Aldana, P.A. and Kantamaneni, K. (eds.), Advances in Water Resources, Coastal Management, and Marine Science Technology. Journal of Coastal Research, Special Issue No. 104, pp. 160–165. Coconut Creek (Florida), ISSN 0749-0208.Position loss risk analysis of dynamic positioning (DP) systems is essential for the operation of semi-submersible drilling units. Traditional risk analysis methods ignore the probability of equipment failure with time-varying, which may result in inaccurate analysis. In this study, fault tree analysis was combined with the continuous-time Bayesian method based on the D-vine copula model to analyze loss of position (LOP). The D-vine model decomposition method was used to calculate multiple two-dimensional copula correlation measures for determining the system reliability with time and for calculating the effect on the occurrence of LOP. The key aspects of LOP were analyzed from the perspective of the variability of equipment at the time of failure. The results can provide a crucial reference for the maintenance of the DP system.

Investigation of jack-up leg extension for deep water operations

Since the first jack-up was built, jackups have become the most popular type of mobile offshore drilling unit (MODU) for offshore exploration and development purposes in shallow water. The most pivotal component of the jack-up unit is the leg, which can directly affect the global performance of the unit. In this paper, an investigation into extending the length of the jack up leg is carried out in order to study the enhancement of the rig capability to drill in deeper water approaching the range of the Semisubmersible Drilling Unit (SSDU) (300-1000ft). A study of the performance of a deep-water jack-up unit is performed with different leg lengths. Typical leg scantling dimensions and identical external loads are assigned, and then a detailed Finite Element Analysis (FEA) model is created in order to simulate the jack-up leg unit’s structural behavior. A Multi-point Constraint (MPC) element together with the spring element is used to deal with the boundary conditions. Finally, a comparative analysis for five leg lengths is carried out to illustrate their performance, including the ultimate static strength, and weight.

Beneath the horizon

The sinking of the Deepwater Horizon is a disaster for the personnel who were killed, for the environment, and for deep-water drilling. However, an array of underwater vehicle technology has benefited. The subsea activities being undertaken in response to the tragic incidents aboard the Deepwater Horizon semisubmersible mobile drilling unit (MODU) have been performed by remotely operated vehicles (ROVs). ROVs are underwater robots powered and controlled from a surface vessel or platform via an umbilical. The ROVs used are equipped with dual manipulators and are of a grouping known as 'work-class' (WROV) as they perform the work that would have historically been undertaken by divers in shallow waters.

The motion performance and mooring system of deepwater semi-submersible drilling unit

This paper analyzes the motion performance and mooring system of deepwater semi-submersible drilling unit in the district of the South China Sea using the MOSES procedure system. After the 3-D panel model of the unit was built, the 3-D diffraction-radiation theory was used to obtain the hydrodynamic loads on the wet surfaces and the response amplitude operators (RAO) of the unit. According to the environmental data, the short-term motion response to motion performance of the unit is predicted by the spectral method. Then a time-domain calculation was done to analyze the motion of the unit with its mooring system. The research results can be a reference for the model test of unit

Total-System Approach Through Wired-Drillpipe Technology

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 112742, "A Step Change in Total-System Approach Through Wired-Drillpipe Technology," by Vebjorn Nygaard, SPE, Mohammad Jahangir, SPE, Trond Gravem, SPE, and Eugene Nathan, SPE, Inteq; John Evans, SPE, Hughes Christensen; Mike Reeves, SPE, IntelliServ; and Henrik Wolter, SPE, and Sigve Hovda, SPE, StatoilHydro, prepared for the 2008 IADC/SPE Drilling Conference, Orlando, Florida, 4–6 March. The paper has not been peer reviewed. Increased oil and gas recovery requires drilling complex extended-reach wells with optimized reservoir exposure for improved production and minimum overall production costs. Introduction of wired-drillpipe technology has facilitated a step change in two-way data communication, resulting in high-speed data transmission providing greater volume, resolution, and quality of formation-evaluation data and drilling-dynamics data. Direct control of rotary-steerable tools has been enhanced to allow instantaneous programming changes and better use of dynamics data to enhance the decision-making process to address drilling-abnormality challenges, hole quality, gross rate of penetration, and bottomhole-assembly (BHA) reliability. The high-band-width technology was used while drilling two laterals in the Troll field in the Norwegian North Sea in 2007. Introduction The broadband network used in this telemetry drillstring enables an ultra-high-speed alternative to mud-pulse- and electromagnetic-telemetry methods. The network uses individually modified drilling tubulars to provide bidirectional, real-time, drillstring telemetry at speeds up to 57,000 bit/sec. This bandwidth, compared with existing technology, makes it possible to obtain large volumes of data from downhole tools (and other measurement nodes along the drillstring) instantaneously, expanding the quantity and quality of information available while drilling. The network uses a high-strength coaxial cable and low-loss inductive coils embedded in double-shouldered connections in each tubular joint to convey information. Currently available telemetry tubulars include various sizes of Range-2 and -3 drillpipe, heavyweight drillpipe, drill collars, and many BHA components. The network's bidirectional architecture allows simultaneous high-speed transmission of downhole data to the surface and commands from the surface to downhole devices, such as rotary-steerable systems (RSSs). Inserting a physical and electrical interface to the telemetry drillstring (an interface subassembly) makes existing measurement-while-drilling (MWD), logging-while-drilling (LWD) RSS tools fully compatible with the network, allowing high-bandwidth communication between all connected tools and a surface acquisition/control system. During 2006, the operator selected the West Venture rig, a semisubmersible drilling unit operating in the Troll field, for their first use of this drillstring-telemetry technology. The selected project offered significant drilling challenges and presented an opportunity to use multiple MWD, LWD, and RSS tools in combination with the network. Drilling activity with these technologies began in 2007.

Planning the First Deepwater Well Offshore Norway

Summary BP Norge was awarded acreage in the Norwegian Fifteenth Licensing Round, with water depths up to 1420 meters. Three-dimensional seismic studies revealed a number of significant structures in these licenses, and the decision was made to evaluate the most interesting of these at the earliest opportunity. As a consequence, the well engineering team proceeded to plan the first exploration well, which was to be drilled in 1275 meters of water in the Nyk High area off northern Norway. The team faced two significant challenges. The rig market for fourth generation semisubmersible drilling units capable of drilling in these water depths was such that there were no Norwegian approved vessels available. Hence, they had to plan for the importation of an appropriate rig and introduce a new drilling contractor to the Norwegian sector. Additionally, the water depth was indicative of a dynamically positioned drilling operation, and the Norwegian authorities needed to be convinced that this unit and the associated drilling program were appropriate for the task at hand. Consequently, there were a number of key processes that were central to the success of this exercise. These included detailed project planning, wide ranging risk assessments, and accessing the knowledge and skills of groups inside BP Exploration with experience in these operations. Visible compliance with our internal technical and Health, Safety, and Environment (HSE) systems and policies was a central feature of this project, along with compliance with the Norwegian requirements. As the first well of this nature in the Norwegian sector, these processes were all key to final approval to proceed with the drilling activity. We describe how these aims were achieved by detailing the well engineering team project management, the risk assessment procedures that were implemented, the approach to regulatory compliance, and the verification processes that were put in place. The detailed engineering studies and the performance of the first well will be reported separately in later papers.

Offshore Application of a Novel Technology for Drilling Vertical Boreholes

Summary A new concept for automatically drilling vertical boreholes was recently implemented by Elf Enterprise Caledonia - the Vertical Drilling System. The VDS was used to drill the 16" hole section of a North Sea exploration well. This was the first time this technology had been used offshore, drilling from a semi-submersible drilling unit. The VDS was shown to have an application in penetrating a drilling target that required a near vertical wellbore. Technical functioning of the tool and field experience is reported along with performance comparisons to offset wells.

Regulatory Considerations in the Design of Tension-Leg Platforms

Summary The tension-leg platform (TLP) is an accepted development option indeepwater Gulf of Mexico projects. The first TLP installed in projects. Thefirst TLP installed in U.S. coastal waters is now in operation. Thecertification of TLP's under current U.S. regulations is a new experience forthe platform operators accustomed to operating in the Gulf of Mexico and forthe regulatory bodies with platform-design jurisdiction. Introduction Operators in the Gulf of Mexico introduced two unique deepwater floatingproduction developments in 1987 and 1988. The Placid Green Canyon floatingproduction unit is based on a converted semisubmersible drilling unit in1,450-ft [440-m] -deep water. Conoco's Jolliet tension-leg well platform(TLWP), in 1,760-ft [535-m] -deep water, is based on the original TLP conceptdeveloped in the North Sea (Fig. 1). Other operators have proposed TLP's forGulf of Mexico sites because TLP technology is now established and the economicadvantages of TLP's are comparable to those of fixed or floating platforms. Of particular interest to operators are the hidden costs of design, construction, and installation for these new deepwater developmentalternatives. This paper addresses a fairly narrow aspect of the overalleconomics of the TLP concept: the design certification process and its impacton costs. Factors unique to TLP certification are discussed. This paper alsobriefly explains the background of TLP regulations in U.S. coastal waters andcompares the basic philosophies of maritime and oilfield philosophies ofmaritime and oilfield practices on which current regulations are based.practices on which current regulations are based. OCS Regulatory Approach Regulatory requirements for TLP's evolved from existing regulations forfixed platforms and mobile offshore drilling units (MODU's). The currentregulatory approach is the result of combining fixed-production platformrequirements developed by platform requirements developed by the MineralsManagement Service (MMS) and MODU requirements developed by the U.S. CoastGuard. Ref. 1 provides background on the evolution of the rules; Refs. 2through 4 give information on actual regulations. Two main areas of interest to TLP operators are the division of authoritybetween the Coast Guard and the MMS concerning process facilities and technicalstandards to process facilities and technical standards to be applied for theremaining TLP systems. Specific technical requirements for TLP's do not existtoday within Coast Guard or MMS regulations. This paper explains thesignificant differences in philosophy between these two agencies. Background of Coast Guard Regulations. The Coast Guard's main interest ispreserving the safety of life at sea. Its requirements derive from 150 years'experience in the marine environment. The Coast Guard's extensive regulations, found in the Code of Federal Regulations, cover all types of vessels, includingpassenger, cargo, and fishing vessels and MODU'S. While based largely onconventional merchant-vessel practices, Coast Guard requirements for MODU'S, developed with the assistance of the oil industry, are "in tune" withthe offshore workplace environment and are supported by the offshoreindustry. In formulating its strategy for reviewing TLP designs, the Coast Guarddetermined that MODU regulations would form a good basis for TLP safetyregulations. It recognized the need to remain flexible in setting specificrequirements for the TLP. Rather than generating specific requirements forTLP's, the Coast Guard chose to work with the industry and the API indeveloping design standards for TLP'S. API RP2T provides general guidance forthe design, provides general guidance for the design, construction, andoperation of TLP'S, liberally incorporating the practices and requirements ofthe Coast Guard and the American Bureau of Shipping. The initial application ofRP2T was on the Jolliet TLWP project. Background of MMS Regulations. The MMS is responsible for E and P on theU.S. Outer Continental Shelf (OCS). MMS requirements are based onfixed-platform practices in the Gulf of Mexico, southern practices in the Gulfof Mexico, southern California, and Atlantic coast. MMS' interest is preserving and protecting the environment and geology inareas protecting the environment and geology in areas of petroleum E and P. Itis interested mainly in drilling and production systems, including well-baysafety and fire protection, hazardous-area classification, blowout prevention, subsea equipment design, prevention, subsea equipment design, emergencyshutdown procedures, and platform fire protection. JPT June 1990 P. 764

Operational Aspects of Extended High-Pressure Well Tests from a Floater

Summary During Summer 1985, an extended high-pressure gas-condensate well test was conducted from a semisubmersible drilling unit in Block 29/5a. This paper is a case history discussing the planning and implementation of the production-test operation. The Well 29/5a-3 production test is believed to have been the most extensive high-pressure test conducted in the U.K. North Sea from a floating drilling unit. In the past, because of equipment and/or environmental constraints, high-pressure testing has been limited to short flow and buildup periods. Also, the majority of "expected" high-pressure tests conducted with 15,000-psi [103-MPa] equipment either have rapidly depleted (high-pressure, low-volume zone) or have been terminated prematurely because of mechanical problems. In contrast, the Well 29/5a-3 production test experienced no major equipment failures. In addition, surface pressure readout gauges were successfully installed under extreme pressure and temperature conditions. Proper operational planning and equipment selection proved vital in implementing the production test and achieving the main objectives.

Local Buckling of Bracing Members in Semi-submersible Drilling Unit (2 nd Report)

An Idealized Structural Unit model of a tubular bracing member is developed considering the influence of local buckling which takes place after the ultimate strength is attained. That is, (1) The simplified method which was proposed in the 1 st report for a tubular member subjected to eccentric axial thrust is expanded so that the ultimate strength can be evaluated under general end loads and distributed lateral load. This method is applied to determine the ultimate strength of a member in the framework of the Idealized Structural Unit Method (ISUM) analysis.(2) An approximate method is proposed to determine the internal forces at a cross section after the ultimate strength is attained. Using these internal forces, strain distribution is determined. The occurence of local buckling is checked based on this strain at every load increments.(3) After the local buckling has started, its influence is reflected upon the full plastic strength interaction relationship through plastic nodal displacements of the element. A formulation to derive the elastic-plastic stiffness matrix is described considering the influence of local buckling.Several examples are analyzed applying the newly developed element. The calculated results are compared with those by the finite element method analysis and experiment, and its validity and usefulness are demonstrated.

Elastic-Plastic Behaviour of Structural Members and Systems with Crack Damage (1st Report)

Crack damages are sometimes detected in offshore structures for petroleum activities. In this paper, influences of cracks on the performance of structural members and systems are theoretically investigated. A computer program SYSTAN 2 based on the Idealized Structural Unit Method is developed for an elastic-plastic analysis of two-dimensional structural systems containing cracked members. The influence of cracks on the full plastic strength interaction relationships is considered.Example calculations are carried out with SYSTAN 2. First, the ultimate strength of a both ends simply supported tubular member with a circumferential through-thickness crack is evaluated under axial load. It has been found that compressive ultimate strength depends on the slenderness ratio of a member and the magnitude of initial deflection as well as the crack size. On the other hand, tensile ultimate strength depends only on the size of a crack.A quasi-dynamical analysis is also performed for the transverse section of a semi-submersible drilling unit with assumed crack damages in the lower horizontal brace. The following observations have been made : (1) The intact structure behaves elastically under the extreme cyclic wave loading.(2) With a moderate crack damage in the brace, the cumulated plastic deformation at every wave cycle levels off after some wave cycles.(3) With a more significant crack damage, the progressive plastic collapse of the transverse section takes place.Further work is needed to account for the elastic-plastic crack tip behaviour and the crack growth effect.

Local Buckling of Bracing Members in Semi-submersible Drilling Unit (1st Report)

The local buckling of tubular members after ultimate strength is attained was investigated from both experimental and theoretical viewpoints. First, axial compression tests were carried out on three tubular specimens similar to the bracing member in an existing semisubmersible drilling unit. It was known that : (1) After the ultimate strength was attained, local buckling of a cosine mode first takes place at the compression side of bending. The buckling wave spreads about one half circle in the circumferential direction, but has short wave length in the axial direction.(2) At the same time, load carrying capacity suddenly decreases.(3) Then, the formations of local denting deformation follow at the foot of initial cosine buckling wave.A simplified method was proposed to analyze the elastic-plastic behaviour of a tubular member under axial compression. Two models were newly proposed which simulate the post-local buckling behaviour of a tubular member based on the observed results in the experiments. They are COS model and DENT model.Combining these models with the proposed simplified method, a series of analysis was performed on the test specimens newly carried out and previously done. The results of calculations were compared with the experimental ones, and the validity and the usefulness of the proposed method of analysis were demonstrated.

Semisubmersible Design Considerations—Some New Developments

This paper reviews the basic stability considerations applied in the design of semisubmersible drilling units. Rules and regulations governing the stability of mobile offshore drilling units (MODU's) were first introduced in the late 1960's. There has been considerable debate over the level of safety achieved in the present generation of MODU's based on existing rules. The assumptions implied in the early rules are discussed and the merits of a "yardstick" approach to the rules are presented. The paper summarizes the evolution of stability rules for mobile offshore drilling units and describes how the rules have changed due to public reaction to several major disasters.

Interaction of Ice Floes With Columns of a Semi-Submersible Platform

A preliminary research project has been carried out to determine the effect of small ice floes on a semi-submersible drilling unit. Physical model tests have been conducted with two main objectives: first, to determine the ability of the columns to prevent ice from passing between them (arching) and thus minimizing the chance of riser damage, and second, to determine the total ice load on the semi-submersible during interaction with ice field concentrations. Three models were used, representing one half of a four, six or eight-legged semi-submersible platform. The dimensions and spacing of the columns were such that the heave and pitch responses were kept constant. The tests were conducted at a model scale of 1:30 using synthetic ice. Test results showed the maximum load measured on a four-legged semi-submersible model, at 100 percent ice floe concentration, was on the order of half of that measured on six or eight-legged semi-submersible models. Also the ice loads on a four-legged semi-submersible model at lower ice floe concentration were substantially less than those for six or eight-legged semi-submersible models. The total ice load on semi-submersible models is found to be a function of ice floe concentration. Up to about 75 percent ice concentration, ice loads varied linearly. Beyond that, the loads increased exponentially for all semi-submersible models.

An Analysis of Tanker-Based Floating Production Systems for Small Offshore Fields

Summary A historical review of floating production storage and offloading (FPSO) systems for marginal fields is presented. The areas in which such systems are presented. The areas in which such systems are operating, ranges of model tests, and current applicability of mooring systems are reviewed to present capabilities of these concepts. Limitations are reviewed to assess the applicability of these concepts to similar sites.Consideration of processing operations on floating vessels are identified. Particularly, the effects of vessel motion on the processing equipment, gas disposal, and crude offloading are addressed. It will be shown that operations are becoming well understood and routine. Future problem areas and solutions under development are identified. Specific examples are cited and design refinements are noted.While various floating production systems have been in operation for only a few years, their potential for solving the problems of producing small offshore fields and for achieving early production is impressive. Introduction Oil discoveries have been made in offshore areas the world over, which, even at today's crude prices, are considered uneconomical to develop with the conventional fixed platform approach. The type of discovery considered marginal varies according to location, water depth, and recoverable reserves. Several innovations to produce such marginal fields economically have been produce such marginal fields economically have been implemented. Among these are tankers moored on site, taking production through risers from seafloor wells and with processing equipment located on board the tanker. This paper discusses the applicability of such tanker-based floating production systems in two diverse areas-offshore Southeast Asia and the North Sea. The discussion makes use of the results of studies conducted by Marathon Oil Co. over the past 5 years. Historical Development of FPSO Systems Early floating production systems used converted semisubmersible drilling units. Argyll gained prominence as the first floating production system using prominence as the first floating production system using the Transworld 58. ENIEPSA, Spain's national oil company, moored a SEDCO 135 class semisubmersible over a three-well cluster as a production platform in 305 ft (93 m) of water in the Dorado field in the Spanish Mediterranean. Development of several Petrobras fields offshore Brazil, Chevron's Casablanca field offshore Spain, and BP's Buchan in the North Sea also used semisubmersibles.In some cases, the semisubmersibles were selected primarily because they were immediately available. But primarily because they were immediately available. But high day rates, conversion time and costs, and low availability of semisubmersibles have made tankers a more attractive component of floating production systems. A tanker surplus in recent years has made it possible for operators to purchase them at a relatively possible for operators to purchase them at a relatively low cost, thus reducing field development costs significantly by unifying the tanker with subsea production systems. production systems. The use of moored tankers as permanent floating storage at offshore oil fields started in the mid-1960's. However, the first FPSO system went into service in 1977, in Shell Oil Co.'s Castellon field. The development began with production from a single well, with provisions for an increase to three wells. The time from provisions for an increase to three wells. The time from go-ahead to production was only 11 months, primarily because the production system was a variation of several relatively simple systems already in use in the offshore industry. Even though the mooring system was new, design and fabrication were carried out within this limited time. The basic simplicity, combined with relatively low capital and operating costs, permits economical development of offshore fields with small recoverable reserves such as Castellon. The production equipment aboard the Castellon tanker is designed to handle 20,000 B/D (130 m3/h) oil. The associated gas is used as fuel in the ship's boilers, and the excess is disposed of in a pair of incinerators. JPT p. 1871