Offshore Structure

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YILDIZ TECHNICAL UNIVERSITY

NAVAL ARCHITECTURE AND MARITIME FACULTY

DEPARTMENT OF NAVAL ARCHITECTURE AND MARINE ENGINEERING

OFFSHORE STRUCTURE

110A2003

İSMAİL HAKKI TOPAL

BSc. RESEARCH PROJECT

ADVISER

ASSOC.PROF.DR. FUAT ALARÇİN

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FOREWORD

I would like to express my sincere thanks to Assoc.Prof.Dr. Fuat ALARÇİN who enables me to work on this topic. His guidance and motivation leads me to learn the present topic and finish the project on time.

June,2015

İsmail Hakkı Topal

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CONTENTS

Page

FOREWORD……………………………………………………………………………………………………,,,,,,,,………….2

CONTENTS……………………………………………………………………………………………………,,,,,,,,……………3

ABBREVIATION LIST………………………………………………………………………………………,,,,,,,……………5

FIGURE LIST……………………………………………………………………………………………………,,,,,,,,………….6

TABLE LIST…………………………………………………………………………………………………….….…,,,,,,,….…7

ABSTRACT………………………………………………………………………………………………………………..……….8

CHAPTER 1. INTRODUCTION ……………………………………………………………………………….………………..9

1.1 History……………………………………………………………………………………………….………………………9

1.2 Main Offshore Fields……………………………………………………………………………..…………………10

1.3 Challenges………………………………………………………….…………………………………………………….10

1.4 Effects On the Environment…………………………………………,,,,,,,,…………………………………..11

1.5 Offshore Disasters………………………………………………………………………..………,,,,,,,,…….……11

CHAPTER 2. FUNCTION OF OFFSHORE STRUCTURES…………………………….…………,,,,,………………13

2.1 Exploratory Drilling Structures……………………………………………………………………,,,,,,,……..13

2.2 Production Structures………………………………………………………………………………,,,,,,,……….14

2.3 Storage Structures……………………………………………………………,,,,,,,……………………………....15

2.4 Export Systems…………………………………………………….………………………………,,,,,,,……………17

CHAPTER 3. TYPE OF OFFSHORE STRUCTURES………………………………..………………,,,,……..……….19

3. Types of Offshore Structures……………………………………,,,,,,,,,………………………………………19

3.1 Fixed Platform……………………………………..……………………………………………..……………………20

3.2 Compliant Tower…………………………………………………………………………………….……………….20

3.3 Semi-Submersible……………………………………………………………………………………………..……..21

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CONTENTS

Page

3.4 Jackup Rig………………………………………………………………………………………………………………22

3.5 Drillships………………………………………………………………………………………………………………….23

3.6 Tension-Leg Platform………………………………………………………………………………………………24

3.7 Gravity-Based Structure………………………………………………………………………………………….25

3.8 Spar…………………………………………………………………………………………………………………………25

3.9 Normally Unmanned Installation (NUI)……………………………………………………………………26

3.10 Conductor Support System……………………………………………………………………………,,………27

3.11 Jacket………………………………………………………………………………………………………………………28

3.12 Floating Production Systems…………………………………………………………………………………..29

CHAPTER 4. DESIGN PHASE OF OFFSHORE STRUCTURES……………………………………..……..……….31

CHAPTER 5. ANALYSIS…………………………………………………….…………………..……………………..……….32

CHAPTER 6. CONCLUSION…………………………….………………..…………………..……………………..……….33

REFERENCES……………………………………………………………………………………………………………………34

CURRICULUM VITAE……………………………………………………………………………………………………….35

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ABBREVIATIONS LIST

BOP: Blowout Preventer

CT: Compliant Towe

ETLP: Extended Tension Leg Platporm

FLNG: Floating Liquefied Natural Gas

FPS: Floating Production System

FPSO: Floating Production, Storage, and Offloading System

FSO: Floating Storage and Offloading System

FSU: Floating Storage Unit

GBS: Gravity-Based Structures

IMO: International Maritime Organization

MODU: Mobile Offshore Drillings Units

NUI: Normally Unmanned Installation

SBM: Single Buoy Mooring

SPAR: Single Point Anchor Reservoir

SPM: Single Point Mooring

TLP: Tension Leg Platform

VLCC: Very Large Crude Carrier

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FIGURE LIST

Page

Figure 1.1 Accident of P-36 converted semi-submersible after

flooding in one column [Barusco,2002]……….………….………………………………………………….….12

Figure 2.1 500,000 barrel oil storage structure………………………………………………….……………17

Figure 2.2 Gullfaks C production platforms with 2 million barrel wet oil storage (Statoil)…………….…………………………….…………………..………………………………………………………….18

Figure 2.3 Types of Offshore Structures….……………………………………………………………………….19

Figure 2.4 Jackup Rig……………………………………………………………………………………………………...23

Figure 2.5 Tension-Leg Platform……………………………………………………………………………………..25

Figure 2.6 Jacket……………………………………………………………………………………………………………..29

Figure 2.7 FPSO Mystrasat at work off the shore of Nigeria…………………………………….………30

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TABLE LIST

Page

Table 2.1 Dirilling Rigs Classifications…………………………………………..……………..21

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ABSTRACT

OFFSHORE STRUCTURE

İsmail Hakkı TOPAL

Department of Naval Architecture and Marine Engineering

Advisor: Assoc. Prof. Dr. Fuat ALARÇİN

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CHAPTER 1

INTRODUCTION

1.1 History

Around 1891, the first submerged oil wells were drilled from platforms built on piles in the fresh waters of the Grand Lake St. Marys (a.k.a. Mercer County Reservoir) in Ohio. The wells were developed by small local companies such as Bryson, Riley Oil, German-American, and Banker’s Oil.

Around 1896, the first submerged oil wells in salt water were drilled in the portion of the Summerland field extending under the Santa Barbara Channel in California. The wells were drilled from piers extending from land out into the channel.

Other notable early submerged drilling activities occurred on the Canadian side of Lake Erie in the 1900s and Caddo Lake in Louisiana in the 1910s. Shortly thereafter wells were drilled in tidal zones along the Texas and Louisiana gulf coast. The Goose Creek Oil Field near Baytown, Texas is one such example. In the 1920’s drilling activities occurred from concrete platforms in Venezuela’s Lake Maracaibo.

One of the oldest subsea wells is the Bibi Eibat well, which came on stream in 1923 in Azerbaijan. The well was located on an artificial island in a shallow portion of the Caspian Sea. In the early 1930s, the Texas Co., later Texaco (now Chevron) developed the first mobile steel barges for drilling in the brackish coastal areas of the Gulf of Mexico.

In 1937, Pure Oil (now Chevron) and its partner Superior Oil (now ExxonMobil) used a fix platform to develop a field 1 mile offshore of Calcasieu Parish, Louisiana in 14 feet of water.

In 1946, Magnolia Petroleum (now ExxonMobil) drilled at a site 18 miles off the coast, erecting a platform in 18 feet of water off St. Mary Parish, Louisiana.

In early 1947, Superior Oil erected a drilling and production platform in 20 feet of water some 18 miles off Vermilion Parish, La. But it was Kerr-McGee Oil Industries (now Anadarko Petroleum), as operator for partners Phillips Petroleum (ConocoPhillips) and Stanolind Oil & Gas (BP) that completed its historic Ship Shoal Block 32 well in October 1947, months before Superior actually drilled a discovery from their Vermilion platform farther offshore. In any case, that made Kerr-McGee’s well the first oil discovery drilled out of sight of land.

When offshore drilling moved into deeper waters of up to 100 feet, fixed platform rigs were built, until demands for drilling equipment was needed in the 100- to 400-foot depth of

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INTRODUCTION

the Gulf of Mexico, the first jack-up rigs began appearing from specialized offshore drilling contractors such as ENSCO International.

The first semi-submersible appeared due to an accident in 1961. Blue Water Drilling Company owned and operated the four column submersible Blue Water Rig No.1 in the Gulf of Mexico for Shell Oil Company. As the pontoons were not sufficiently buoyant to support the weight of the rig and its consumables, it was towed between locations at a draught mid-way between the top of the pontoons and the underside of the deck. It was observed that the motions at this draught were very small and Blue Water Drilling and Shell jointly decided that the rig could be operated in the floating mode.

The first purpose built drilling semi-submersible Ocean Driller was launched in 1963. Since then, many semi-submersibles have been purpose-designed for the drilling industry mobile offshore fleet.

The first offshore drillship was the CUSS 1 developed for the Mohole project to drill into the earth’s crust.As of June, 2010, there were over 620 mobile offshore drilling rigs (Jackups, semisubs, drillships, barges) available for service in the competitive rig fleet.

The world’s deepest platform is currently the Perdido Spar in the Gulf of Mexico, floating in 2,438 meters of water. It is operated by Royal Dutch Shell and was built at a cost of $3 billion.

1.2 Main Offshore Fields

Notable offshore fields today are found in the North Sea, the Gulf of Mexico, the Campos and Santos Basins off the coasts of Brazil, Newfoundland and Nova Scotia, several fields off West Africa most notably west of Nigeria and Angola, as well as offshore fields in South East Asia and Sakhalin, Russia.

1.3 Challenges

Offshore oil and gas production is more challenging than land-based installations due to the remote and harsher environment. Much of the innovation in the offshore petroleum sector concerns overcoming these challenges, including the need to provide very large production facilities. Production and drilling facilities may be very large and a large investment, such as the Troll A platform standing on a depth of 300 meters.

Another type of offshore platform may float with a mooring system to maintain it on location. While a floating system may be lower cost in deeper waters than a fixed platform, the dynamic nature of the platforms introduces many challenges for the drilling and production facilities.

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INTRODUCTION

In both cases, the ocean adds several hundred meters to the fluid column in the drill string. The addition increases bottom hole pressure as well as the energy needed to lift sand and cuttings for oil-sand separation on the platform.

The trend today is to conduct more of the production subsea, by separating sand from oil and re-injecting sand before it is pumped up to the platform, or even pumping it onshore, with no installations visible above the sea. Subsea installations help to exploit resources at progressively deeper waters, locations which have been inaccessible, and overcome challenges posed by sea ice, such as in the Barents Sea.

Offshore manned facilities also present logistics and human resources challenges. An offshore oil platform is a small community in itself with cafeteria, sleeping quarters, management, and other support functions. In the North Sea, staff members are transported by helicopter for a two-week shift. They usually receive higher salary than other industry workers do. Supplies and waste are transported by ship, and the supply needs to be well planned because floor area on the platform is limited. Today, much effort goes into moving as much of the personnel as possible onshore, where management and technical experts are in touch with the platform by video conferencing. An onshore job is also more attractive for the aging workforce in the petroleum industry, at least in the western world. These efforts among others are contained in the established term integrated operations. The increased use of subsea facilities helps achieve the objective of moving more workers onshore. Subsea facilities are also easier to expand, with new separators or separate modules for different oil types, and are not limited by the fixed floor space of an offshore rig.

1.4 Effects on the Environment

Offshore oil production involves environmental risks, most notably oil spills from oil tankers or pipelines transporting oil from the platform to onshore facilities, and from leaks and accidents on the platform. Produced water is also generated, which is excess water from well drilling or production and includes varying amounts of oil, drilling fluid or other chemicals used in, or resulting from, oil production. According to the organization Culture Change, a Gulf of Mexico rig dumps about 90,000 tons of drilling fluid and metal cuttings over its lifetime, with its wells also contributing with heavy metals.

1.1 Offshore Disasters

Although most of the offshore structures constructed to date have withstood the test of time, there have been several catastrophic failures of offshore structures as well. Weather,

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INTRODUCTION

Figure 1.1 Accident of P-36 converted semi-submersible after flooding in one column [Barusco, 2002]

blowout, capsizing and human errors have resulted in the loss of a substantial number of fixed and floating structures. Between 1955 and 1968, nearly two dozen mobile drilling units have been destroyed. Within the two-year period between 1957 and 1959 alone, hurricanes Hilda and Betsy inflicted losses of hundreds of millions of dollars to drilling. production and pipeline facilities. Two semi-submersibles capsized and sank in the 1980s: Alexander Keilland, an accommodation vessel in the Norwegian north sea (1980), and Ocean Ranger offshore Hibernia, Canada (1982), resulted in the loss of hundreds of lives. The worst offshore disaster occurred when the Piper Alpha oil and gas platform caught fire in 1988. One hundred and sixty-seven lives were lost. In March, 2001, the world’s largest floating production system, the Petrobras P-36, sank in Campos basin (fig. 1.1) costing 10 lives [Barusco, 2002].

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FUNCTION OF OFFSHORE STRUCTURES

2.1 Exploratory Drilling Structures

Some of the desirable characteristics applicable to exploratory drilling units, such as limited structure motions and good station-keeping characteristics in relatively severe environment, are equally applicable to production units. MODUS must accommodate highly variable deck loads due to the different drilling requirements they will encounter. and they are usually designed for relatively high transit speeds to minimise mobilisation costs. Three of the most common forms of drilling structures are drillships, jack-up barges and semisubmersibles. Submersible gravity structures are also used for drilling in shallow water. These structures with buoyant legs and pontoons are set on seafloor by ballasting, thus allowing the structure to be deballasted, and moved to another location. Drillships are ship-shaped and self-propelled, which can accommodate the drilling equipment on board. They have the advantage of rapid transit between stations and can take up and leave stations quickly, especially if they are dynamically positioned instead of being moored in place. However, the large motions and thruster (or anchor) capacity limit the weather conditions in which they can drill.

The mobile semi-submersible drilling unit hulls typically consist of four or six columns connected with horizontal pontoons and support a large deck on top. Most of these structures do not have thrusters or dynamic positioning and are usually towed like the barges or transported on large purpose-built transport vessels. The semi-submersibles have good motion characteristics in severe environment and thus have the advantages of being able to stay in the drilling modes longer than a typical drillship.

The jack-up barges are usually buoyant during transit and are towed from station to station. Once they reach the drilling site, the legs of the jack-up (usually three in number) are set on the ocean bottom and the deck is jacked up above water level on these legs. During drilling the jack-ups act like fixed platforms. However. the water depth of about 150 m limits their operations to shallow-to-moderate water depths. Jack-up units have other constraints, such as being largely affected by the seafloor terrain and material characteristics and the time and environmental constraints associated with jacking operations.

The mooring systems for MODUs do not have to meet the same severe environmental requirements of production vessels. If severe weather is forecast, MODUs can disconnect the drilling riser and leave station, or slacken mooring lines to avoid damage. Permanent production facilities cannot afford this luxury and are required to remain within a safe watch circle under the most extreme weather conditions.

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Each of the three configurations discussed as exploratory structures in this section, with suitable modifications, is suitable for use as production structures. Many of the FPSs are converted drilling units with the drilling equipment being replaced by production equipment.

2.2 Production Structures

Production platforms are required to stay on station during its lifetime, which is usually from 20 to 30 years. In shallow waters, the most common type of production platforms is the fixed piled structures, commonly known as jackets in the offshore industry. These are tubular structures fixed to the seafloor by means of driven or drilled and grouted piles. The economic water depth limit for fixed platforms varies by environment. In the North Sea, the deepest fixed jacket platform, the BP Magnus platform, is in 610 ft (186 m) of water. The deepest concrete structure in the North Sea, the Shell Troll Gravity Base Structure, extends the fixed structure limit there to nearly 1000 ft (305 m) water depth. In the Gulf of Mexico, the Shell Bullwinkle platform holds the water depth record at 1352 ft (412 m) of water.

When the water depth exceeds these limits, compliant towers or floating production platforms become more attractive. Three compliant towers have been installed in the Gulf of Mexico. The deepest is the ChevronTexaco Petronius platform in 1754 ft (535 m) water depth. This is probably the economic limit for these types of structures.

Fixed and compliant platforms support conductor pipes, which are essentially extensions of the well casing from the seafloor. The conductors are supported along their length and are not free to move with the dynamics of the waves. The wellhead is at the deck of the platform and well operations are similar to land-based operations.

One of the most important requirements for floating production systems is their interface with risers. Production may originate from wellheads on the sea floor (wet trees), or from wellheads located on the structure (dry trees). The selection is driven by reservoir characteristics and has a significant impact on the selection of the structure. Dry tree risers are nearly vertical steel pipes, which must be designed to contain well pressure in all operational conditions. This places limits on the motions of the production platform. To date, only tension leg platforms and Spars have been used with these types of risers. Subsea production risers are typically composite flexible risers, which are more tolerant to vessel motions. These risers have been used with all types of floating platforms. Steel catenary risers (SCRs) have been employed from TLPs, Spars and semi-submersibles.

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In the same manner in which production may originate from wet or dry trees, drilling may be performed with a subsea blowout preventor (BOP) or a surface BOP. The BOP provides the safety shutoff capability in the case of an unexpected release of well pressure during drilling. Exploratory drilling units employ a subsea BOP and a low pressure drilling riser.

The drilling riser can be disconnected from the BOP in an emergency. Most floating production systems with drilling capability use a surface BOP. The drilling riser in this case must be designed to take the full well pressure and the vessel and mooring must be this riserthe harshest environmental conditions. Again, TLPs and Spars are the only floating production systems, which are currently performing drilling with a surface BOP.

There is an important distinction between the requirements of Gulf of Mexico and North Sea platform, which impact platform design. Extreme sea states in the Gulf of Mexico are associated with hurricanes. Platforms in the Gulf of Mexico shut down operations and are abandoned when a hurricane threat arises. North Sea platforms are subject to many fast moving weather fronts, which can create extreme sea states. They cannot be abandoned and operations continue through the weather conditions, which are as bad as or worse than the hurricanes in the Gulf of Mexico This distinction leads to differences in safety factors and design criteria for these locations. For example, North Sea floating production systems are currently designed to survive with two mooring lines missing. Gulf of Mexico standards only require designing for one mooring line missing. The environments in the other major offshore regions, Le. West Africa, Brazil and Southeast Asia are generally more benign than those in either the North Sea or the Gulf of Mexico.

2.3 Storage Structures

During the production of offshore oil, it may be desirable to store the crude temporarily at the offshore site before its transportation to the shore for processing. Storage capacity is dictated by the size of shuttle tankers and frequency of their trips. Historically, storage capacities have typically been between 15 and 25 days at peak production. These values are appropriate for FPSOs employed on remote, marginal fields. This was the original province of FPSOs. During the 1990s, FPSOs became more popular for large fields in more developed areas. Storage requirements and shuttle tanker specifications could be optimised. North Sea shuttle tankers, for example, are usually purpose-built vessels and the storage requirements can be optimised for a particular project. West African FPSOs are generally sized to store and load VLCCs for long voyages. Southeast Asian FPSOs typically offload to tankers of opportunity. Storage capacities for recent FPSO projects range from as low as three days

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production (BP Foinhaven in the North Atlantic) to as much as eleven days production (CNOC Liuhua in the South China Sea). The ship-shaped production platforms (Le. FPSOs) possess large enclosed volume, and are ideally suited for the combination of production and storage. Floating Storage and Offloading vessels, i.e. without processing, (FSOs) may also be used in conjunction with floating or fixed production platforms. The Shell Expro Brent Spar was used for this purpose for 20 years in the North Sea. FPSOs are the most prolific floating production systems. As of 2002, there were 91 installations [Offshore, 20021. There were also 63 fields developed with FSOs as of 1993.

Oil storage tanks are usually maintained at atmospheric pressure with an inert gas blanket. According to international regulations, water ballast tanks are required to be segregated from cargo tanks [IMO. 19781. Cargo and ballast management is an important aspect of FPSO operations.

Oil storage may also be accommodated at ambient pressure in tanks, which are open to the sea at the tank bottom. Oil, being lighter than water, may be pumped into the compartment displacing seawater out the bottom. The tank wall pressure is only a function of the height of the oil column and the difference in density between the water and the oil. A photograph of such a structure using this principle, called Khazzan storage tank, is shown in fig. 1.1 being towed to site. Three of these structures built in the sixties in the Persian Gulf in a water depth of about 150 ft (46 m) are still operating by Conoco. They are open at the bottom, having a storage capacity of 500,000 barrels and situated on the ocean floor by driven piles. These platforms do not perform any oil processing.

The same storage principle was also used on the floating structure Brent Spar, and on most of the large concrete gravity-base structures built in Norway in the 1980s and 90s (Condeeps). The Gullfaks C platform shown in fig. 2.2 is capable of storing almost 2 million barrels of oil in 712 ft (217 m) of water. The oil-over-water storage principle has been proposed for a drilling and production Spar [Bugno and Horton, 2000]

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Figure 2.1 500,000 barrel oil storage structure

2.4 Export Systems

The oil produced offshore requires transportation from the site to the shore. If the structure is located close to the shore or there is an existing infrastructure to tie-in, then underwater pipeline may be used for this purpose. The sequential development of fields from shallowto-moderate-to-deep water in the Gulf of Mexico has resulted in the development of a network of these subsea pipelines. These pipelines are supported on the floor of the ocean, connecting the production wells to the shore or other platforms.

However, for remote offshore locations, this method of transport is not economically feasible and other means of transportation is required. This transportation is usually accomplished by shuttle tankers. Sometimes, these tankers are moored directly to the storage or production structure. Often, however, special structures are required to moor the

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tankers. These structures take the form of floating buoys or articulated structures. The tankers are moored with the help of a floating hawser, which is retrieved from the ocean surface. The transfer of oil from the structure to the tanker is accomplished by a loading hose. Since a single line is used for the mooring, these special structures are called Single Point Mooring (SPM) or Single Buoy Mooring (SBM).

Figure 2.2 Gullfaks C production platforms with 2 million barrel wet oil storage (Statoil)

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TYPES OF OFFSHORE STRUCTURES

3. Types of Offshore Structures

Offshore structures may be defined as being either bottom-supported or floating. Bottomsupported structures are either “fixed” such as jackets and gravity base structures, or“compliant” such as the guyed tower and the compliant tower. Floating structuresare compliant by nature. They can be viewed either as “neutrally buoyant”. such as thesemi-submersible-based FPSs, ship-shaped FPSOs and monocolumn Spars, or “positivelybuoyant”, such as the Tension Leg Platforms.

Figure 2.3 Types of Offshore Structures

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3.1 Fixed Platform

These platforms are built on concrete or steel legs, or both, anchored directly onto the seabed, supporting a deck with space for drilling rigs, production facilities and crew quarters. Such platforms are, by virtue of their immobility, designed for very long term use (for

instance the Hibernia platform). Various types of structure are used: steel jacket, concrete caisson, floating steel, and even floating concrete. Steel jackets are vertical sections made of tubular steel members, and are usually piled into the seabed.

Concrete caisson structures, pioneered by the Condeep concept, often have in-built oil storage in tanks below the sea surface and these tanks were often used as a flotation capability, allowing them to be built close to shore (Norwegian fjords and Scottish firths are popular because they are sheltered and deep enough) and then floated to their final position where they are sunk to the seabed. Fixed platforms are economically feasible for installation in water depths up to about 520 m (1,710 ft).

3.2 Compliant Tower

A compliant tower (CT) is a fixed rig structure normally used for the offshore production of oil or gas. The rig consists of narrow, flexible (compliant) towers and a piled foundation supporting a conventional deck for drilling and production operations. Compliant towers are designed to sustain significant lateral deflections and forces, and are typically used in water depths ranging from 1,500 to 3,000 feet (450 to 900 m). At present the deepest is the Chevron Petronius tower in waters 623m deep.

With the use of flex elements such as flex legs or axial tubes, resonance is reduced and wave forces are de-amplified. This type of rig structure can be configured to adapt to existing fabrication and installation equipment. Compared with floating systems, such as tension-leg platforms and SPARs, the production risers are conventional and are subjected to less structural demands and flexing. However, because of cost, it becomes uneconomical to build compliant towers in depths greater than 1,000 meters. In such a case a floating production system is more appropriate, even with the increased cost of risers and mooring. Despite its flexibility, the compliant tower system is strong enough to withstand hurricane conditions.

The first tower emerged in the early 1980s with the installation of Exxon's Lena oil platform.

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3.3 Semi-Submersible

A semi-submersible (semisubmerged ship) is a specialised marine vessel used in a number of specific offshore roles such as offshore drilling rigs, safety vessels, oil production platforms, and heavy lift cranes. They are designed with good stability and seakeeping characteristics. Other terms include semisubmersible, semi-sub, or simply semi.

Offshore drilling in water depth greater than around 520 meters requires that operations be carried out from a floating vessel, as fixed structures are not practical. Initially in the early 1950s monohull ships were used like CUSS I, but these were found to have significant heave, pitch and yaw motions in large waves, and the industry needed more stable drilling platforms.

A semi-submersible obtains its buoyancy from ballasted, watertight pontoons located below the ocean surface and wave action. The operating deck can be located high above the sea level due to the good stability of the design, and therefore the operating deck is kept well away from the waves. Structural columns connect the pontoons and operating deck.

With its hull structure submerged at a deep draft, the semi-submersible is less affected by wave loadings than a normal ship. With a small water-plane area, however, the semi-submersible is sensitive to load changes, and therefore must be carefully trimmed to maintain stability. Unlike a submarine or submersible, during normal operations, a semi-submersible vessel is never entirely underwater.

A semi-submersible vessel is able to transform from a deep to a shallow draft by deballasting (removing ballast water from the hull), and thereby become a surface vessel. The heavy lift vessels use this capability to submerge the majority of their structure, locate beneath another floating vessel, and then deballast to pick up the other vessel as a cargo.

Drilling rig construction has historically occurred in boom periods and therefore "batches" of drilling rigs have been built. Offshore drilling rigs have been loosely classified in nominal "generations" depending upon the year built and water depth capability as follows:

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Table 2.1Drilling Rigs Classifications

3.4 Jackup Rig

A jackup rig or a self-elevating unit is a type of mobile platform that consists of a buoyant hull fitted with a number of movable legs, capable of raising its hull over the surface of the sea. The buoyant hull enables transportation of the unit and all attached machinery to a desired location. Once on location the hull is raised to the required elevation above the sea surface supported by the sea bed. The legs of such units may be designed to penetrate the sea bed, may be fitted with enlarged sections or footings, or may be attached to a bottom mat. Generally Jackup rigs are not self propelled and rely on tugs or heavy lift ships for transportation.

Jackup platforms are used as exploratory drilling platforms and offshore and wind farm service platforms. Jackup platforms have been the most popular and numerous of various mobile types in existence. The total number of Jackup 'Drilling' rigs in operation numbers about 540 at the end of 2013. The first one was designed by R. G. LeTourneau for Zapata Oil, owned by George H. W. Bush, who later became the 41st president of the United States.

A jackup is a floating barge fitted with long support legs that can be raised or lowered. The jackup is maneuvered (self-propelled or by towing) into location with its legs up and the hull floating on the water. Upon arrival at the work location, the legs are jacked down onto the seafloor. Then "preloading" takes place, where the weight of the barge and additional ballast water are used to drive the legs securely into the seabottom so they will not penetrate further while operations are carried out. After preloading, the jacking system is used to raise the entire barge above the water to a predetermined height or "air gap", so that wave, tidal and current loading acts only on the relatively slender legs and not on the barge hull.

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Modern jacking systems use a rack and pinion gear arrangement where the pinion gears are driven by hydraulic or electric motors and the rack is affixed to the legs.

Jackup rigs can only be placed in relatively shallow waters, generally less than 120 metres (390 ft) of water. However, a specialized class of jackup rigs known as premium or ultra-premium jackups are known to have operational capability in water depths ranging from 150 to 190 meters (500 to 625 feet).

Figure 2.4 Jackup Rig

3.5 Drillships

A drillship is a maritime vessel that has been fitted with drilling apparatus. It is most often used for exploratory drilling of new oil or gas wells in deep water but can also be used for scientific drilling. Early versions were built on a modified tanker hull, but purpose-built designs are used today. Most drillships are outfitted with a dynamic positioning system to maintain position over the well. They can drill in water depths up to 3,700 m (12,100 ft).

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3.6 Tension-Leg Platform

A tension-leg platform (TLP) or extended tension leg platform (ETLP) is a vertically moored floating structure normally used for the offshore production of oil or gas, and is particularly suited for water depths greater than 300 metres (about 1000 ft) and less than 1500 metres (about 4900 ft). Use of tension-leg platforms has also been proposed for wind turbines.

The platform is permanently moored by means of tethers or tendons grouped at each of the structure's corners. A group of tethers is called a tension leg. A feature of the design of the tethers is that they have relatively high axial stiffness (low elasticity), such that virtually all vertical motion of the platform is eliminated. This allows the platform to have the production wellheads on deck (connected directly to the subsea wells by rigid risers), instead of on the seafloor. This allows a simpler well completion and gives better control over the production from the oil or gas reservoir, and easier access for downhole intervention operations.

Larger TLPs will normally have a full drilling rig on the platform with which to drill and intervene on the wells. The smaller TLPs may have a workover rig, or in a few cases no production wellheads located on the platform at all.

The deepest (E)TLPs measured from the sea floor to the surface are:

4,674 ft (1,425 m) Magnolia ETLP. Its total height is some 5,000 feet (1,500 m).

4,300 ft (1,300 m) Marco Polo TLP

4,250 ft (1,300 m) Neptune TLP

3,863 ft (1,177 m) Kizomba A TLP

3,800 ft (1,200 m) Ursa TLP. Its height above surface is 485 ft (148 m) making a total height of 4,285 ft (1,306 m).

3,350 ft (1,020 m) Allegheny TLP

3,300 ft (1,000 m) W. Seno A TLP

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Figure 2.5 Tension-Leg Platform

3.7 Gravity-Based Structure

A gravity-based structure (GBS) is a support structure held in place by gravity. A common application for a GBS is an offshore oil platform. These structures are often constructed in fjords since their protected area and sufficient depth are very desirable for construction. A GBS intended for use as an offshore oil platform is constructed of steel reinforced concrete, often with tanks or cells which can be used to control the buoyancy of the finished GBS. When completed, a GBS is towed to its intended location and sunk. Prior to deployment, a study of the seabed will have been done in order to ensure it can withstand the vertical load exerted on it by that structure.

Gravity-based structures are also used for offshore wind power plants. By the end of 2010, 14 of the world's offshore wind farms were supported by gravity-based structures. The GBS are suited for water depths greater than 20 m. The deepest registered offshore wind farm with gravity-based structures is Thornton Bank 1, Belgium, with a depth up to 27.5 m. As offshore wind power plants are growing in size and moving towards deeper waters, the GBS is considered competitive in comparison with other support structures.

3.8 Spar

A spar is a type of floating oil platform typically used in very deep waters, and is named for logs used as buoys in shipping that are moored in place vertically. Spar production platforms have been developed as an alternative to conventional platforms. The deep draft

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design of spars makes them less affected by wind, wave and currents and allows for both dry tree and subsea production. Spars are most prevalent in the US Gulf of Mexico; however, there are also spars located offshore Malaysia and Norway.

A spar platform consists of a large-diameter, single vertical cylinder supporting a deck. The cylinder is weighted at the bottom by a chamber filled with a material that is more dense than water to lower the center of gravity of the platform and provide stability. Additionally, the spar hull is encircled by helical strakes to mitigate the effects of vortex-induced motion. Spars are permanently anchored to the seabed by way of a spread mooring system composed of either a chain-wire-chain or chain-polyester-chain configuration.

There are three primary types of spars; the classic spar, truss spar, and cell spar. The classic spar consists of the cylindrical hull noted above, with heavy ballast tanks located at the bottom of the cylinder.

A truss spar has a shorter cylindrical "hard tank" than a classic spar and has a truss structure connected to the bottom of the hard tank. This truss structure consists of four large orthogonal "leg" members with X-braces between each of the legs and heave plates at intermediate depths to provide damping. At the bottom of the truss structure, there is a relatively small keel, or soft tank, that houses the heavy ballasting material. Soft tanks are typically rectangular in shape but have also been round to accommodate specific construction concerns. The majority of spars are of this type.

A third type of spar, the cell spar, has a large central cylinder surrounded by smaller cylinders of alternating lengths. At the bottom of the longer cylinders is the soft tank housing the heavy ballasting material, similar to a truss spar. The cell spar design was only ever used for one platform, the Red Hawk spar, which was decommissioned in 2014 under the Bureau of Safety and Environmental Enforcement's "Rigs-to-Reefs" program. At the time of its decommissioning it was the deepest floating platform to ever be decommissioned.

3.1 Normally Unmanned Installation (NUI)

A Normally Unmanned Installation (NUI) is a type of automated offshore Oil/Gas platform designed to be primarily operated remotely, without the constant presence of personnel.

These generally were characterized by their small size, often consisting of just a well bay with a helipad on top. They are often a compromise of providing the convenience of surface wellheads, which are easier to build and maintain, while avoiding the high operating costs of a full production platform.

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They are generally only used in shallower water, where constructing many small NUIs is a relatively easy and cheap option as compared to the cost of using subsea wells.

This can be seen in the Southern North Sea where large numbers of wells are on smaller NUIs, compared with the more northern areas of the continental shelf where fewer larger platforms and subsea sites are the norm.

NUIs are commonly serviced from a nearby larger platform, e.g., Mungo serviced from Marnock. These installations will include an emergency shelter with essential food and water in order to provide a safe refuge in the event that weather or other considerations prevent a visiting crew from returning to base. Regular visits may be made for routine maintenance and for smaller well work such as wireline operations. Anything larger requires a drilling rig to be brought in, but this is still an advantage over subsea wells, which require a drilling rig or light intervention vessel for any well intervention.

In recent times NUI has become a phase within the decommissioning of previously manned offshore installations. Platforms would generally be positively isolated from hydrocarbons and flushed clean to suit environmental issues. Platforms would then only be visited infrequently for integrity checks and maintenance of temporary equipment left for power requirements and safety functionality. The platform would then remain in this phase until further decommissioning activities are carried out to remove the topsides in modules or piece small, then remove the jacket.

3.10 Conductor Support System

On offshore oil platforms, conductor support systems, also known as conductor supported systems or satellite platforms, are small unmanned installations consisting of little more than a well bay, and a small process plant. They are designed to operate in conjunction with a static production platform which is connected to the platform by flow lines and/or by Umbilical cable.

Traditionally, these jacket-type structures have been installed and used in shallow to medium water depths of up to 40 – 60 meters. The conductor supported system use its inherited strength of the well conductors to support both the wells and the topside structure.

The conductor supported system is particularly suited to areas with more benign environmental conditions. The well conductors act as both piles and conductors. These are drilled and installed using a drilling jackup rig using conventional drilling / lifting techniques.

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3.11 Jacket

Jacket refers in the oil and gas exploration and production to the steel frame supporting the deck and the topsides in a fixed offshore platform.

Most of the platforms are used in the shallow waters of the continental shelf, so 95% of the offshore platforms in the world are jacket designed

In these areas and where the water depth does not exceed 500 meters, these platforms may be anchored directly to the seabed.

These platforms are fixed and their deck is supported by a steel tubular structure having its feet on the seabed.

This steel tubular structure is called the jacket.

To fix the jacket onto the seabed, the jacket is equipped with thick steel piles of 2 meters diameter that can penetrate the sea floor up to 100 meters deep to ensure the stability of the whole platform.

The jacket may be hundreds meters high and weight thousands tonnes.

The height of the jacket is defined by the water depth plus about 15 meters above the sea level.

The tubular structure of a jacket is designed to support multiple constraints:

- Weight of the processing equipment (topsides) - Impact of the waves - Pressure of the wind on the topsides - Flow of the sea water streams and tides - Corrosion - Fatigue effect - Life cycle time

Acting as a cage, the jacket is protecting all the piping going through to the seabed.

This space tubular frame is also protecting these pipes from lateral load.

The deck structure is connected to the jacket by the deck legs transferring efforts both ways.

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The waves having a period of 14 to 20 seconds, the jacket is designed with a natural period of 2.5 seconds in order to prevent vibrations amplification under the waves effect.

As a result of all the requirements imposed to the jacket, its costs may represent up to 40% of the total platform capital expenditure.

Figure 2.6 Jacket

3.12 Floating Production Systems

The main types of floating production systems are FPSO (floating production, storage, and offloading system). FPSOs consist of large monohull structures, generally (but not always) shipshaped, equipped with processing facilities. These platforms are moored to a location for extended periods, and do not actually drill for oil or gas. Some variants of these applications, called FSO (floating storage and offloading system) or FSU (floating storage unit), are used exclusively for storage purposes, and host very little process equipment. This is one of the best sources for having floating production.

The world's first floating liquefied natural gas (FLNG) facility is currently under development.

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Figure FPSO Mystrasat at work off the shore of Nigeria

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DESIGN PHASE OF OFFSHORE STRUCTURE

The main purpose of design to produce safe, functional, economic, structures that can withstand loads structure.

The most important difference from other structures of offshore structures is different of operating and manufacturing zone, and structure is exposed to different loads at different stages. When designing offshore structure, structure or pieces of the structure must be examined individually. These phases;

Manufacturing

Transportation

Placement and Montage

Operation

Disassembly

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ANALYSIS

When designing offshore structure, should be studied environmental and structural loads and the response of structure interactively. Starting from environmental loads structurel loads and reactions are calculated. It can be summarized as the following.

Definition of wave climate around of structure depending on wave and current measurement or meteorological data

Determination of design, wave , current features

Calculation of wave and current forces (hydrodynamis forces) on the structure

Calculation of structure’s response

Account of structurel loads

Analysis of effects consisting of transport and immersion

Fatigue analysis of structure

Soil analysis

Piling analysis

Coupling analysis

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CONCLUSION

Offshore activities on the World increase rapidly. It has created an industry in itself. Oil platforms, storage tanks at sea, marine structures built on stilts, tanker loading platform, submarine pipeline is considered the group of marine structures. Also, offshore activities includes maintance and repair with search, design, machinery manifacturing, construction structure. All of them indicates the magnitude of the industry. UK is spending 2.9 billion pound per annum to oil and gas exploration and production activities in North sea. This is about twenty percent of the world's offshore industry. Just more than 2000 companies and more than 200000 staff are working in England. Thus, studies in this field have a great importance.

The aim of the researchs is to make existing structures more safe, proof, economic. Offshore activities carried out in deeper water and the more difficult natural conditions at the end of researches. UK is spending average 20 million pound per annum for researchs. offshore structures is not used yet widely in our country.

Oil exploration and production studies in our sea are on agenda. We must work to develop offshore structures, according to geological structure of our country, characteristics and needs of our seas.

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REFERENCES

[1] http://en.wikipedia.org/wiki/Oil_platform

[2] http://en.wikipedia.org/wiki/Floating_production_storage_and_offloading

[3] http://en.wikipedia.org/wiki/Conductor_support_system

[4] http://en.wikipedia.org/wiki/Normally_unmanned_installation

[5] http://en.wikipedia.org/wiki/Spar_(platform)

[6] http://en.wikipedia.org/wiki/Gravity-based_structure

[7] http://en.wikipedia.org/wiki/Tension-leg_platform

[8] http://en.wikipedia.org/wiki/Jackup_rig

[9] http://en.wikipedia.org/wiki/Semi-submersible

[10] http://en.wikipedia.org/wiki/Compliant_Tower

[11] http://www.2b1stconsulting.com/jacket/

[12] http://www.offshoreenergytoday.com/offshore-drilling-history-and-overview/

[13] http://www.marin.nl/web/Ships-Structures/Offshore-structures.htm

[14] http://nptel.ac.in/courses/114106011/

[15] http://www.4coffshore.com/windfarms/gravity-based-support-structures-aid274.html

[16] http://www.imo.org.tr/resimler/ekutuphane/pdf/1157.pdf

[17] Chakrabarti, S.K., (2005). Handbook of Ofshore Engineering ISBN-13: 978-0-08-044568-7

[18] Frieze, P.A., (2013). Ships and Offshore Structures, Offshore Structure Design And Construction

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CIRRICULUM VİTAE

PERSONAL INFORMATION

Name Surname : İsmail Hakkı TOPAL

Date of birth and place : 06/10/1992 / Bakırköy Foreign language : English E-mail : [email protected]

Offshore Structure

Education

Transcript of Offshore Structure