Offshore Engineering

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Offshore Engineering

P Vannucci

To cite this version:

P Vannucci. Offshore Engineering. Master. Université de Versailles Saint Quentin en Yvelines, France. 2008. �cel-01529018�

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Master SPI DSME - Dimensionnement

des Structures Mécaniques dans leur

Environnement

2ème_Année

Cours Mécanique pour l’Industrie du Pétrole Part 2: Offshore Engineering

P. Vannucci 2 Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr

Foreword

This course on Offshore Engineering is a basic course serving as an

introduction to the problems concerning design and construction of offshore platforms, normally used in oil industry.

So, some topics will be considered in this course, namely the

principal ones that concern the structural design of an offshore platform; some other topics, like for instance marine installation procedures, corrosion protection, facilities and plants engineering will not be considered here.

The course is addressed to undergraduate students having already

a good knowledge in structural engineering and fluid mechanics.

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3 Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr Chapter 1: Introduction 7 General considerations

Platform functions and types Historical background

Design process

Standards and regulations

Chapter 2: Design actions 65

Types of loads Operational loadings Deformation loads Accidental loads Environmental loadings Storm selection Wind forces Ice forces Snow loads Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr

Content

Current forces Marine growth Earthquake actions

Design load combinations

Chapter 3: Hydrodynamics of wave forces 140

The actions of a fluid on an immerged body Inertial force: the added mass

The drag force

Wave theories

Slender and large bodies

First order wave action on a slender body

The case of a moving body

Second order wave action on a slender body

First order wave action on a large body

The Mc Camy and Fuchs solution

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5 Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr

Content

The simplified Garrett solution The Ogilvie solution

Second order wave action on large bodies

Something about the case of multiple bodies Oscillating large bodies

Wave slamming

Vortex shedding

Numerical methods for the wave action

The spectral method in wave forces calculation

Chapter 4: Dynamical considerations 373

Introduction

Response to regular waves

Response to an impulsive force

Response to irregular waves: the spectral analysis

The wave force spectrum

6 Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr Bibliography

The most part of information, figures and diagrams of this course,

where not explicitly indicated, have been taken from the following sources.

Books

T. Sarpkaya & Isaacson: Mechanics of wave forces on offshore

structures. Van Nostrand, 1981.

B. Mc Clelland & M. D. Reifel (Eds.): Planning and design of fixed

offshore platforms. Van Nostrand, 1986.

O. C. Zienkiewicz, R. W. Lewis & K. G. Stagg (Eds.): Numerical

methods in offshore engineering. J. Wiley, 1978.

M. G. Hallam, N. J. Heaf & L. R. Wotton (Eds.): Dynamics of marine

structures. Report UR8 – CIRIA, 1978.

Internet sites

http://www.kuleuven.ac.be/bwk/materials/Teaching/master/wg15a/l0200 .htm

http://www.nts.no/norsok http://www.offshore.tudelft.nl/

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Introduction

Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr

Content

General considerations 9

Platform functions and types 10

Historical background 48

Design process 52

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General considerations

Offshore engineering is a branch of civil engineering involved with

the construction of sea structures, far form the coast.

Such structures are normally called platforms. They are usually

employed in oil industry, but there are also platforms for broadcasting, navigational lighting, radar surveillance, space operations, oceanographic research and so on.

The general design requirements for an offshore platform are similar

to any industrial structure; the first step in the design is to develop a concept of the structure based on its functional requirements, environmental restraints and construction method. The function of an offshore platform is to provide a secure working support, so the platform must be structurally adequate to withstand both operational and environmental loading; in addition, it must be practical to construct and economically feasible.

Nevertheless, due to their characteristics, to environmental and

geographic aspects, to the construction procedures, offshore platforms are very peculiar structures, and several unusual aspects must be taken into account in its design phase: environmental loadings, construction phases, economic aspects, linked to the type, construction procedure and date and to the installation site.

Platform functions and types

During the last five decades, several types of offshore platforms

haves been designed and used; this variety of platform types is due to different factors: technological and scientific progress, economical factors, need to exploit deeper natural reservoirs, ecological constraints.

A possible, of course incomplete, classification is the following one.

jack up

semi-submersible drilling ship Mobile offshore platforms

TLP

free standing tower guyed tower spar tower compliant concrete gravity steel gravity steel jacket steel tower rigid Fixed offshore platforms

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Fixed offshore platforms are normally used for production, while

mobile platforms are almost exclusively used for exploration and drilling phases.

The difference between fixed and compliant platforms is in the way

they face environmental (namely wind and wave) lateral actions.

As its name clearly indicates, a fixed platform is a traditional

structure, in the sense that its deformation under lateral loads is small, but it is located into the sea water.

Unlike fixed, compliant platforms are designed to move under lateral

forces, so that the effects of these forces are mitigated. The trade-off in a compliant platform is between excursion amplitude and restraining force.

Compliant platforms are used in deep water, where the stiffness of a

fixed platform decreases while its cost increases, and they are the only technical solution in very deep waters (> 500 m).

The fluid-structure interaction is a capital aspect in platform design,

but it assumes a biggest role in the case of a compliant platform.

Platform functions and types

General scheme of offshore platforms in relation with water depth.

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Platform functions and types

Steel jacket: it is the most

classical and widely used offshore platform. A typical jacket is shown in the figure.

It is composed of three

principal parts: the deck, carrying the topsides (living quarters, drilling derrick, consumables, facilities, helideck, flare etc.), the jacket itself and the foundation piles.

Steel jackets are normally

used in shallow to moderate deep waters (from 20 to 100 m), but they have been used up to 500 m of water. 14 Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr

Platform functions and types

If not too big, jackets are built in a

dock and then charged onto a barge by a crane. Large jackets are built on one side, directly on a barge or on rails to be the skidded onto a barge.

An important characteristic of a

jacket is a small floatability: in fact, legs are not plugged, as they are the templates for the piles, and the braces are normally too small to ensure the necessary buoyancy. So, a barge is needed to carry them to the field, where the jacket is put into water by a crane, for small jackets, or directly skidded or rolled off the barge (launching

operation) for large jackets. ww

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In the first case, the design phase

must take into consideration the lifting phase, while in the second cases a complete analysis of the launching phase must be done, in order to asses the transient stress distribution and to control the actual behaviour of the jacket during the transportation and the launching phases, namely if an additional buoyancy is needed and if the jacket touch the sea bottom during launching.

Numerical investigations are normally

used to simulate these phases, which considerably condition the design of a steel jacket. In the next pages,

these phases are outlined. ww

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Platform functions and types

Launching phases of a steel jacket.

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Platform functions and types

Two pictures of a jacket installed by a crane and of the mating of a

deck and a jacket.

ww w. of fs hor e-te chnol ogy .c om ww w. of fs hor e-te chnol ogy .c om 18 Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr

Platform functions and types

The construction phases of a jacket. 1: construction in a coast yard;

2: transportation by barge on the oil field;

3: launching of the jacket.

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Another characteristic of steel jackets is the foundation system: as

previously said, piles are directly inserted into the legs and they directly support the deck. Sometimes, especially for larger platforms, additional skirt piles are necessary. Once the piles driven, they are grouted into the legs to join them to the platform.

Once the piles installed and grouted, the deck is placed at the top of

the jacket by a crane. Normally, all the installments and facilities are already installed onto the deck before its mating with the deck.

ww w. st ru ct ur ae. de ww w. ri g zone. co m Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr

Platform functions and types

Steel tower: it is a large jacket where

the piles cannot be inserted in the legs mainly for economical reasons.

In fact, too long piles are too

expensive. So, when the platform is located in deep waters, the jacket becomes very heavy and the piles cannot be as long as the legs. They become skirt piles inserted in sleeves around the outside of the legs.

In this way, the legs are plugged and

normally sufficient to ensure the buoyancy: the jacket does not need a barge to be carried on the site, as it floats (eventually with auxiliary buoyancy) and can be towed. This is very convenient both for economical and construction aspects.

They exist tower structures installed in

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Platform functions and types

Some relevant achievements of jacketed structures.

Platform functions and types

Steel gravity platforms: this type of structure, rarely used, uses its

own weight to counter the lateral actions due to wind and waves that tend to overturn the platform: the weight is used as a stabilizing force.

Nevertheless, the real reason for using gravity platforms is the

nature of the soil: when it is of solid rock, it is impossible to drive piles into it, so the gravity solution is the only possible one.

Normally, gravity platforms are concrete platforms, but in some

cases a steel solution can be adopted, in relation with several factors, mainly economic considerations.

Normally, the structure has a certain number of large tanks, flooded

by water or by crude oil, to ballast the platform and provide the necessary weight to counter overturning lateral forces.

These tanks, in the transportation phase, provide the necessary

buoyancy.

An important feature of all the gravity platforms is that they can be

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The Maureen Alpha platform is a steel

gravity platform with a weight of 112000 t, height of 241 m.

It has been installed in 1983 in the

North Sea; in 2001 it has been removed and replaced on another oil field.

ww w. par os ci ent ifi c. com ww w. te cnomar e. it ww w. raeng. or g. uk Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr

Platform functions and types

Loanga platform

(Nigeria): it is a steel gravity platform, with inclined risers to optimize the exploitation of the field.

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Platform functions and types

Concrete gravity platforms: they are the hugest and most

impressive structures ever built.

In this platforms, the steel structure supporting the deck is totally or

partially replaced by a concrete structure of large dimensions.

ww w. ogp. or g. uk ww w. ogp. or g. uk 26 Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr

Platform functions and types

Concrete gravity platforms are used when some particular

circumstances are present:

economical factors: in some cases, the construction of a very large

concrete structure can be cheaper than the construction of a steel structure;

ecological factors: a concrete platform can be very huge, so as to

concentrate onboard some industrial treatments of the crude and to allow a great stocking capacity in the ballast cells;

construction conditions: the pile driving operation for a steel jacket

needs usually 5 to 10 days; in the North Sea it is rare to have such a period of fine weather; the installation in the oil field of a concrete gravity platform, complete with its deck, requires a shorter period (1 to 2 days);

decommissioning aspects: concrete gravity platforms can be

decommissioned and eventually re-used;

soil conditions: when the soil is made of rock it is impossible to drive

piles into it: the gravity solution is then the only one possible;

geographical conditions: the presence of calm and deep waters not far

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27 Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr ww w. ogp. or g. uk

The above factors have often been determinant in the choice of this

kind of platforms in the North Sea.

Nevertheless, rather recently concrete gravity platforms have been

commissioned in other parts of the world (East Russia, Philippines and so on).

These structures can reach a height of 400 m and weigh more than

800000 t. Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr

Platform functions and types

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Platform functions and types

Source: www.ogp.org.uk 30 Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr

Platform functions and types

It is important to understand the construction phases of this kind of

platforms: the fundamental aspect is the Archimedes' force.

In fact, the concrete substructure is built onshore, in a dock under

the sea level.

Once the base is ready, the dock is flooded and the base floats; it is

then towed in deeper but calm waters, where the construction of the substructure continues on the floating base.

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Once the substructure ready, it is towed in sufficiently deep and

calm waters (a fiord gives the optimal conditions) for the mating operation: the deck, carried by two barges with all the topsides, is mated to the concrete substructure just by an operation of ballasting and deballasting the concrete substructure with water.

All these operations are outlined in the following scheme.

Platform functions and types

Malampaya concrete gravity platform (Philippines; the first concrete

gravity platform in Asia): all the construction phases.

1. construction in a dock under the sea level

2. construction of the towers 3. flooding of the dock: the platforms is ready

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Platform functions and types

Source: www.arup.com

4. towing of the concrete substructure

ww w. mal a mpa ya. co m 6. mating

5. ballasting of the concrete substructure

7. the final platform

Platform functions and types

The disaster of Sleipner A: this platform produces oil and gas in

the North Sea in a water depth of 82 m. The first concrete base structure for Sleipner A sprang a leak and sank under a controlled ballasting operation during preparation for deck mating in Gandsfjorden outside Stavanger, Norway on 23 August 1991.

The loss was caused by a failure in a cell wall, resulting in a serious

crack and a leakage that the pumps were not able to cope with. The wall failed as a result of a combination of a serious error in the

finite element analysis and insufficient anchorage of the reinforcement in a critical zone.

A better idea of what was involved can be obtained from the photos

in the following page. The top deck weighs 57,000 t, and provides accommodation for about 200 people and support for drilling equipment weighing about 40,000 t.

When the first model sank in August 1991, the crash caused a

seismic event registering 3° on the Richter scale, and left nothing but a pile of debris at 220 m of depth. The failure involved a total economic loss of about $700 million.

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The post accident investigation traced the error to inaccurate finite

element approximation of the linear elastic model of the cells (using the popular finite element program NASTRAN). The shear stresses were underestimated by 47%, leading to insufficient design. In particular, certain concrete walls were not thick enough. More careful finite element analysis, made after the accident, predicted that failure would occur with this design at a depth of 62 m, which matches well with the actual occurrence at 65 m.

Source: http://www.ima.umn.edu/~arnold/disasters/sleipner.html ww w. im a. umn. edu ww w. im a. umn. edu Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr

Platform functions and types

Free standing towers: they are classical towers but so slender that

their structural behavior is that of a compliant structure: large sway displacements and high oscillating period.

Baldpate: the highest, freestanding compliant structure in the world.

Characteristics:

water depth: 501 m;

sway response cycle: 30 s; lateral displacement: 3 m;

cross section: 42.6 x 42.6 m (bottom),

27.4 x 27.4 m (top);

weight of the tower:

28900 t;

weight of deck and

topsides: 2700 t; foundation: 12 piles driven for 130 m. ww w. of fs hor e-te chnol ogy .c om w. of fs hor e-te chnol ogy .c om

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Platform functions and types

Guyed towers: these compliant platforms are composed by a

slender jacket, normally pin-joined at its base, whose vertical stable position is ensured by the buoyancy of the structure itself and by a series of mooring catenary lines.

The structure can oscillate under the lateral actions, the restoring

force being provided by the buoyancy and the mooring lines. The clump weights provide additional restraining forces in case of storm, when they are lifted off the seafloor.

These platforms are used for water depth in the range 200-600 m,

and they can be re-used.

Platform functions and types

SPAR towers: these platforms are composed by a large steel tube

as substructure directly supporting the deck and topsides.

The tube is ballasted so as its floating stable equilibrium position is

vertical (including topsides), and moored by tensioned risers and by mooring lines (catenaries).

On the lateral surface of the large vertical cylinder there are

helicoids, installed to counter vortex-shedding.

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39 Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr www.offshore-technology.com

TLP (Tension Leg Platforms): they are floating structures

anchored to the seafloor by a series of vertical tendons (tethers) pre-tensioned by extra-buoyancy. The tethers are made by steel pipes.

A TLP is composed by 4 principal parts: the foundation template, the

tethers, the hull and the deck.

Some TLPs (e. g. Heidrun) have a concrete hull.

TLP are very large structures, able to host

great payloads. So, they are used for great fields and can host some refining processes and have a good storage capacity.

TLP can be used from 150 m of water depth

on, and theoretically there is no limit of water depth for their use.

The restoring force is given by extra

buoyancy; this is obtained deballasting the TLP hull once the tethers installed.

TLPs can be re-used. Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr

Platform functions and types

The constructing phases of a TLP are similar to those of a concrete

gravity platform, and are sketched in the figure.

a: construction of the hull in a dock; b: towing the hull to the mating site; c: towing the deck to the mating site; d: mating;

e: tethers positioning;

f: deballasting for tensioning the tethers.

ww w. of fs hor e-te chnol ogy .c om www.rigzone.com

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Platform functions and types

TLPs have known a wide diffusion all over

the world in the last decade. The water depth, in 20 years, has been multiplied by a factor of ten. ww w. of fs hor e-te chnol ogy .c om 88450 290610 66225 106000 16602 displacement (t) 26018 35380 30000 4170 topsides weight (t) 20321 21772 43700 1950 hull's weight (t) 110 x 110 87,6 x 103,1 100 x 100 55 x55 dimension (m) 54 column height (m) 26 24 22,6 24,4 12,2 diam. col. (m) 1158 345 872 395 536 water depth (m) Ursa Heidrun Auger Snorre Jolliet 88450 290610 66225 106000 16602 displacement (t) 26018 35380 30000 4170 topsides weight (t) 20321 21772 43700 1950 hull's weight (t) 110 x 110 87,6 x 103,1 100 x 100 55 x55 dimension (m) 54 column height (m) 26 24 22,6 24,4 12,2 diam. col. (m) 1158 345 872 395 536 water depth (m) Ursa Heidrun Auger Snorre Jolliet 42 Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr

Platform functions and types

Some pictures of TLPs (source: Atlantia Offshore LTD).

Snorre Magnolia

Marlin Heidrun

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Recently, a new TLP concept has been developed: it is that known

as mini TLP.

In this solution, only one column is present, just as in the case of

spare towers.

The column is anchored to the

seafloor by pretensioned tethers that are fixed at the end of three pontoons at the bottom of the cylinder.

These TLP have a less payload

capacity, and are normally used for deep water small fields.

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Platform functions and types

Drilling ships: like all the mobile systems, drilling ships are used

mostly for the drilling phase, but they can be used, at least temporarily, also as FPS (Floating Production System).

A drilling ship is, as its name indicates, a common ship equipped

with a drilling system (a derrick tower).

It is maintained in its position by a system of mooring catenaries,

eventually assisted by servo-motors and GPS positioning.

ww w. of fs hor e-te chnol ogy .c om

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Platform functions and types

Semi-submersibles: as their name indicates, these are special

ships, normally composed of two pontoons, some columns and a deck. The deck is equipped for all the drilling operations.

A semi-submersible is a complete platform, that can navigate as it is

furnished of motors. Once in place, its positioning is provided by a system of catenaries normally controlled by a GPS system.

Recently a concrete semi-submersible

has been constructed.

ww w. dor is -eng ineer ing. co m ww w. of fs hor e-te chnol ogy .c om www.doris-engineering.com www.doris-engineering.com 46 Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr

Platform functions and types

Jack-ups: these are special mobile platforms,

normally used for the drilling operations.

They are triangular barges, completely

equipped for the drilling operations and disposing of three truss legs.

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These legs can be lifted

or lowered by motors. When the legs are lifted, the jack-up can navigate just as a common ship. Once arrived on the field, the jack-up lowers the legs so as to be fixed in the drilling place and it lifts itself at the right height above the sea level.

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In this course, only fixed platforms will be considered, as the case of

mobile platforms concerns much more naval engineering.

Nevertheless, several considerations which will be made in the

following, such as dynamic response, wind and wave force analysis and so on, concern as well mobile platforms, namely jack ups.

ww w. of fs hor e-te chnol ogy .c om Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr

Historical background

1896. Summerland, California: the first oil and gas operations over

water, with wells drilled from piers extending from shore.

1909 or 1910. Ferry Lake, Louisiana: wells drilled using a wood

deck erected on a platform supported by cypress trunks driven as piling.

1924 and after. Lake Maracaibo, Venezuela: wells drilled from

wooden platforms supported on timber pilings.

1930s: the oil industry moves into the marsh and swamplands of

South Louisiana, and then, as a natural extension, into shallow waters of the Gulf of Mexico, using existing technology for timber structures.

1937. Gulf of Mexico: the first platform close to shore; it is a

traditional timber-pile structure.

1945. Gulf of Mexico: Magnolia Petroleum Company builds a

wooden structure in about 6 m of water and drills the first offshore well remote from shore.

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Historical background

1947. Gulf of Mexico: the first steel platform is installed out of sight

of land, in 6 m of water by Superior Oil Company, and another in 5.4 m of water by the Kerr McGee-Phillips-Stanolind group. These early steel platforms were fabricated entirely offshore. They were supported by a large number of small steel pilings (40-60) driven in varying directions and to varying depths. After the pilings were driven, horizontal pipe braces were laid out on the construction barge and cut to fit.

1948. The first prefabricated substructure sections assembled on

the site.

The first steel platform (1947).

1950. The first onshore fabrication

of unitized substructures, referred to as templates or jackets. The steel jacket was placed on the ocean floor, where it acted as a template for the steel piles that were driven through its tubular legs. 50 Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr

Historical background

1955 to 1978. The water depth for steel jacket platform construction

passes from about 30 to 311.8 m: a progression of ten times in 20 years.

1973. North sea: the first concrete gravity platform, Ekofisk, is

installed in 70 m of water.

1983. Gulf of Mexico: Exxon installs the first guyed tower, Lena, in

304 m of water.

1984. North Sea: Conoco installs Hutton, the first TLP, in 147 m of

water, used as a drilling and production platform.

1991. Bullwinkle, the highest rigid steel jacket is installed in 412 m of

water.

1995. The highest concrete platform is installed in 330 m of water. 1998. Baldpate, the highest steel jacket, is installed in 501 m of

water: it is a compliant free standing platform.

1999. A TLP is installed for the first time in more than 1000 m of

water: Ursa, in 1158 m of water, in the Gulf of Mexico.

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51 Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr 1998; 501 1984; 147 1991; 412 1978; 311,8 1965; 68,5 1955; 30 1947; 6 1976; 258,6 1975; 144 1997; 980 1994; 872 1989; 536 1996; 894 1999; 1158 2004; 1311 2005; 1425 1977; 151 1993; 250 1989; 217 1973; 70 1995; 330 0 200 400 600 800 1000 1200 1400 1600 1940 1950 1960 1970 1980 1990 2000 2010 Year W a te r dept h ( m ) Jackets TLP Concrete Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr

Design process

The design of an offshore platform is a very complex process, as

several different aspects must be taken into account.

Normally, in a preliminary phase data concerning the industrial

activity, the construction facilities, the environmental conditions must be collected.

These data concern the payload, that is the total weight, surface and

distribution of the topsides facilities, installments and plants; all this, of course, depends on the kind of industrial activity of the platform (see hereafter).

Then, the environmental and geographic conditions must be known;

the design storm must be selected and the characteristic wave determined, which is fundamental to assess the wave lateral forces on the platform.

The wind characteristics must also be determined, in order to

evaluate the maximum horizontal force acting on the superstructure.

The soil stratigraphy must also be known, which is capital for

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Design process

In fact, the number and length of piles, which are in connection with

soil properties and stratigraphy but also with the platform dimensions and with the amount of horizontal actions, can affect greatly the design of the platform. Moreover, the existence of bad soil layers or of solid rock can suggest, and eventually impose, the choice of a gravity platform.

The construction phases must always be taken into account during

the design process. In fact, small jackets and decks are lifted and installed by cranes, and so these phases must be considered and submitted to structural calculation, as very different from the normal conditions the jacket and the deck are designed for.

Greater jackets are skidded onto barges and then launched into the

sea, eventually with the aid of a crane or of floating units. Also these phases must be carefully considered and analyzed, as very different from the usual situation of the jacket.

Design process

Finally, the transitory phase of pile driving must be studied: during

this phase, the jacket must not overturn under the action of moderate waves.

In the design phase, the corrosion aspects and the marine growth

must be considered: for the first problem, an additional thickness of the steel members is used (normally 5 mm), and also the use of sacrificial anodes, while for the latter an extra thickness (about 50 mm) is taken into account to calculate the wave action and the total immersed weight.

Another important factor is time: it is always important to construct

the platform in as less time as possible, for evident economical reasons, and this can affect considerably the structural choices and the construction phases.

For rigid structures installed in water depths to about 100 m, static

analyses are normally adequate, because these structures are sufficiently stiff, so that dynamic effect can be safely ignored.

(29)

Unlike this case, that is in deeper water and anyway for any situation

in which the fundamental period of the structure exceeds 3 sec (compliant structures), the effect of cyclic platform motion caused by wave action becomes an important factor and must be carefully analyzed: the fluid-structure interaction becomes an important problem and must be properly taken into account in the design phase.

Usually, a preliminary design is done, taken into consideration a

cardinal rule in offshore engineering: the onshore work must be maximized and the offshore work minimized, and this for economical reasons.

This preliminary design is useful to obtain the overall dimensions of

the platform, the number of piles and so on.

Once this preliminary design done and approved, a detailed design

is done, in which all the structural parts are studied and calculated in detail. Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr

Design process

In this phase, also the exact construction

sequence is attentively considered and taken into consideration in the structural calculations.

The knowledge of the climatic and geographic

data allows to determine some fundamental dimensions of the structure.

In fact, once the LAT (Lowest Astronomical

Tide) and the design wave known, the height h of the jacket can be determined:

Here, a is the wave amplitude, t_max is the

maximum tide and ag is the so-called air-gap, a safety height, normally 0.5 m.

Actually, no point of the deck must be

positioned under the height h, and this to put all the installments off the wave action.

ag a t LAT h= + _max + + ag a LAT tmax

(30)

Design process

The most important parameter, however, in the design phase is the

economical aspect: this point conditions very much the choice of the structure type.

In the figure, it is shown a comparison of the relative cost trends for

different platform types, for mild-sea (e.g. the Gulf of Mexico) and for extreme-sea conditions (e.g. the North Sea).

It is apparent that the cost of bottom-supported platforms increases

rather quickly with the water depth: beyond a certain water depth, these are no more cost-effective.

Standards and regulations

The offshore installments must be safe to protect the human,

marine, and coastal environments, and to preserve the very considerable investments of time and resources involved.

It is the purpose of this section to identify the methods by which

responsible parties seek assurance of the structural integrity of offshore platforms.

Until the late 1960s, structural integrity in the U.S. offshore industry

was largely the responsibility of the designers, who worked to a variety of standards drawn from coastal and onshore engineering experience.

The first published design standard for fixed offshore platforms was

issued in 1969 by the American Petroleum Institute (API).

Actually, the structural integrity can be checked through one of three

similar, but different, procedures known as verification, certification, and classification, having the characteristic that each is carried out by specialist organizations independent of both the owner and the designer.

(31)

The object of quality assurance procedures is to verify that the best

available technical and environmental knowledge has been applied during the phases of design, construction, installation, and operation of an offshore platform.

Classification is a quality assurance service provided by one of the

private organizations known as classification societies, and begins during the design phase of a structure.

Criteria for classification are the society’s published standards,

known as rules or guides. The procedure begins with submittal of engineering calculations, specifications and fabrication drawings so that the society can verify compliance of the design with the rules.

During the subsequent phases of construction, installation, and

operation, surveys are conducted as necessary to ensure complete and continuing adherence to standards.

The classification procedure includes periodic inspections and

special damage surveys to ensure that integrity and serviceability are maintained throughout the life of the installation.

Standards and regulations

Well-known classification societies include the American Bureau of

Shipping, Det Norske Veritas (Norway) and Lloyd’s Register of Shipping (U.K.).

These and other societies have expanded their scope of services

beyond the traditional classification of vessels of commerce to encompass many other types of marine structures, including offshore structures.

Certification is a quality assurance procedure under which the

owner or a government mandates adherence to specified standards or rules for design or construction and requires verification of compliance by one of a limited number of authorized certification agents.

While an authorized certification agent may apply the standards of

its own organization in the process of evaluation, the standards specified by the owner or government having jurisdiction prevail in cases of conflict.

(32)

Standards and regulations

Examples of assuring structural integrity by certification are the

procedures adopted by the U. K. and by Norway.

In the U.K. regulations stipulate that a fixed offshore structure must

be certified by American Bureau of Shipping, Det Norske Veritas (Norway), Lloyd’s Register of Shipping (U.K.), Bureau Veritas (France), Germanischer Lloyd (Germany) or Halcrow Ewbank and Associates Certification Group (U. K.).

The U.K. regulations make it clear that the owner has the

responsibility for sound design and construction as well as adequate maintenance of the offshore installation.

Norwegian regulations specify only the Norwegian Petroleum

Directorate (NPD) as the approval agency, but NPD customarily contracts with independent review agencies to confirm compliance with its rules. The owner carries responsibility for assuring structural integrity. 62 Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr

Standards and regulations

Verification is the procedure used in the U.S. to assure structural

integrity of offshore platforms. It is administered by the government under the Platform Verification Program.

In the USA, verification is mandatory for a new platform and for

major modifications to an existing platform if any of the following conditions will exist:

it is installed in water deeper than 120 m; it has a natural period greater than 3 s;

it is installed in an area having unstable bottom conditions; it is installed in a frontier area;

it uses an unusual design concept in comparison to typical installations

in the region.

In 1979, a study conducted in the USA revealed that in the Gulf of

Mexico, over 32000 platform years of service had been accumulated with 37 platforms lost.

(33)

Of these, 34 losses were due to overloading from hurricane waves,

and 3 losses occurred because of hurricane wave loading combined with wave-induced motion of unstable sea-floor deposits. This represents a failure rate of 0.1 percent annually.

By 1984, five years after the period covered by the above statistics,

approximately 14000 platform years of service had been added without further losses.

Present offshore technology, standards, and regulations provide

adequate safety of fixed offshore structures against general collapse.

The general knowledge about this topic has spread rapidly all over

the world and at present all the countries have similar and equivalent rules. Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr

Nevertheless, there are some rules that are considered as reference

norms:

API RP 2A - Recommended Practice for planning, designing,

constructing fixed offshore platforms. In the following of this course, if not specified differently, API norms are intended to be used (abridged in API);

Det Norske Veritas – Rules for the design, construction and inspection

of off-shore structures (abridged in the following with DNV);

British Standard 6235 – Code of practice for fixed offshore structures

(abridged in the following with BS);

DOE – Offshore installations: guidance on design and construction;

German norms.

NTS: Norsok Standard – Actions and action effects (abridged in the

following with NTS – Norwegian Technology Standards, or Norsok Standard).

(34)

Chapter 2

Design actions

Content

Types of loads 67 Operational loadings 68 Deformation loads 77 Accidental loads 79 Environmental loadings 81 Storm selection 82 Wind forces 99 Ice forces 114 Snow loads 127 Current forces 128 Marine growth 131 Earthquake actions 133

(35)

As for any other type of structures, loads acting upon an offshore

platform can be divided into two different main classes:

permanent (dead) loads;

variables (live) loads.

In turn, these two classes can be divided into several subclasses of

loads:

Permanent loads are composed of structural loads, operational dead

loads, deformation and hydrostatic permanent loads.

Variable loads are composed of operational live loads, environmental

loads, accidental loads, variable hydrostatic and deformation loads.

Structural load: it is the self weight of the structure constituting the

platform; hence, it is a result of the design process.

Hydrostatic loads: they are the buoyancy of some submerged

members, buoyancy that, for a part of it, can be also variable as a function of the immersion (depending upon tides and waves). It must be remarked that hydrostatic loads are essential in floating structures, as they give the global stiffness of the structure.

Operational loadings

Operational loadings: they are all the loadings which are inherent

to the proper use and functions of the platform.

So, each technical equipment onboard the platform (production and

process moduli, but also the appurtenances, i.e. the boat landing, the helideck, the cranes, the flare boom and so on) and each bulk material and technical pieces (e.g. drilling tubes) used onboard for different purposes are operational loads.

Usually, operational loadings are divided into:

topsides;

consumables;

human weights.

Topsides: it is the whole system hosted by the platform, normally by

the deck: process units, pumps, generators, drilling units, cranes, helideck, flare, living quarters, bridges, boat landing and so on. Topsides are permanent loads.

(36)

Operational loadings

Consumables: it is the weight of all the items and liquids that must

be stored onboard for production, treatment and so on: fuel (diesel or gas condensate), potable water, waste treatment, corrosion inhibitors, lubricants, drilling tubulars, drilling mud, chemicals for various treatments and so on.

Human weights: it is the weight of the personnel onboard; it is often

already considered in the weight of the living quarters.

In the design process, all these weights are input data.

In fact, once the production capabilities decided, the designers are

able to decide what goes onboard and its dimensions.

Normally, topsides facilities, like for instance living quarters or

process units, are pre-assembled and standardized items.

So, once decided the number and type of items to be used, the

designer is able to know the topsides weights and dimensions, and following the norms concerning mutual distances of units, to decide the deck: arrangement and dimensions.

Operational loadings

It must be recalled that pre-fabricated units or pre-designed items

(like a helideck) apply generally speaking a complex load to the platform structure.

For instance, the action applied by a flare to the deck of the platform

consists not only of vertical loads, but of all the loads applied by the truss of the flare, which can have also horizontal components.

In addition, the loads given by a modulus are not, in general,

uniformly distributed, but they are applied in some singular precise points, which support the topside weights: of course, this affects the deck design.

However, in a first design phase, some average uniform loads can

be used for pre-dimensioning: from 2 t/m² for usual platforms up to 4 t/m² for North sea platforms.

A more rigorous method is to utilize the following figures to obtain

topside weights and deck areas. The data in these figures are from installed platforms.

(37)

Topsides weight as a function of production rate.

Deck area as a function of production rate.

(38)

Actually, once the deck arrangement chosen, all operating loads are

usually given by the operator and the equipment manufacturers.

An example of detailed operational loads is given in the following

table.

Operational loadings

Loads to be taken into account (kN/m2₎ _{For portions of the structure} _{For the} structure as a

whole Zone considered Flooring and

joints components Other

Process zone (around wells and large-scale machines) 5 5 2.5

Drilling zone 5 5 2.5

Catwalks and walking platforms (except emergency exits) 3 2.5 1

Stairways (except emergency exits) 4 3 0

Module roofing 2 1.5 1

Emergency exits 5 5 0

STORAGE Storage floors - heavy

Storage floors - light

18 9 12 6 8 4 Delivery zone 10 10 5 Non-attributed area 6 4 3 74 Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr Notes:

In the process and drilling zone, for loads on a portion of the structure,

the load must be accumulated with a point load equal to the weight of the heaviest part likely to be removed, with a minimum value of 5 kN.

Point loads are assumed as being applied to a 0,3m x 0,3m surface.

In the absence of specific data from the manufacturer, and at an

early design stage, the following values are recommended in BS norms:

crew quarters and passageways: 3,2 KN/m2_;

working areas: 8,5 KN/m2; storage areas: γ H KN/m2;

with

γ is the specific weight of stored materials, not to be taken less than

6,87KN/m3_;

H is the storage height (m).

(39)

The Norwegian NTS in Norsok Standard gives the following average

accidental (i.e. variable) loads for the deck design:

The design is intended to be divided into three parts:

Local design: design of deck plates and stiffeners;

Primary design: design of deck beams and beam-columns;

Global design: design of deck main structure (and substructure).

Notes:

Wheel actions to be added to distributed actions where relevant. (Wheel

actions can normally be considered acting on an area of 0.3 x 0.3 m).

Point actions to be applied on an area 0.1 x 0.1 m, and at the most

severe position, but not added to wheel actions or distributed actions.

The distributed load q is to be evaluated for each case. Storage areas

for cement, wet or dry mud should be the maximum of 13 kN/m² and

ρgH, where H is the storage height in m. Laydown areas are not

normally to be designed for less than 15 kN/m².

The factor f is the min of 1.0 and (0.5 + 3/A0.5_{), where A is the action}

area in m².

Global action cases should be established based upon “worst case”,

representative variable action combinations, complying with the limiting global criteria to the structure.

(40)

Deformation loads

Deformation loads: they are all the loads produced by an imposed

state of strain on the structure.

Such imposed states of strain can be produced by temperature

differences, ground settlements, pre-stressed states.

In the case of temperature differences, they are typical variable

deformation loads, with different periods (daily, annual).

In hyperstatic structures, temperature differences produce stress

states that can be very important.

To be recalled that the submerged part of the platform is at an

almost constant temperature, while the emerged part can be subjected to very important temperature excursions, generally varying with time and place.

In addition, some structural parts are subjected to temperature

differences produced by the production processes: the most representative case is that of risers, with the external temperature equal to that of the sea and the internal one typical of oil produced, normally rather high.

Another example is that of submerged tanks in gravity platform: the

tank can be filled in with water or with oil or gas, and they have different temperatures.

Pre-stressed states are used in concrete platforms for preventing

from concrete cracks.

In TLP, the tethers are highly pre-stressed by the extra buoyancy of

the hull, which gives the global stiffness of the platform.

Of course, this pre-stress affects also the stress distribution in the

hull.

Ground settlements often occur during reservoir exploitation.

If they are not uniform, they induce stresses in hyperstatic

structures.

(41)

Accidental loads: they are caused by wrong operations or by

events not considered in the normal operations of the platforms, such as explosions, impacts, fires and so on.

The most important accidental loads for a platform are those caused

by impact, that produce impulsive forces.

These impulsive forces can arise during some transient operations,

and the most important are those that arise during the landing of helicopters and during the mooring of vessels to the platform.

BS norms consider two situations for helicopter landing: heavy and

emergency.

For the first case, the impact load is equal to 1.5 times the maximum

take-off weight, whilst for the second case this factor is 2.5. In both cases, a horizontal load applied at the points of impact and equal to half the maximum take-off weight must be taken into account.

NTS give some more detailed conditions for the analysis of impact

during helicopter landing, making a distinction between the local forces and the global forces.

For vessel mooring, design impact forces, to be applied at the boat

landing but also to other less favorable parts, should be computed for the largest ship foreseen for the platform, at a velocity corresponding to operational speed.

BS norms, for instance, consider for the minimum design impact that

generated by a vessel of 2500 tons with a velocity of 0.5 m/s.

The added mass must be considered (i.e. DNV norms prescribe an

added mass of 40% for sideways collisions and of 10% for stern or bow impacts).

All the kinetic energy of the ship must be absorbed by the fender, or,

for the zones out of the boat landing, the platform must be able to withstand without major damages (only local damages are allowed) to the same impacts.

When deeper investigations are not conducted, the impact zone

goes from 10 m under the LAT to 13 m above the LAT.

Other accidental loads often considered are those produced by the

fall of objects.

(42)

Environmental loadings

Environment gives different kinds of actions on offshore platforms:

wave forces; wind forces; ice forces; snow loads; current forces; marine growth; earthquake actions.

We will briefly consider in the following these loadings, leaving to the

next chapter the hydrodynamics of wave forces, the most important environmental action on offshore structures.

The most part of the environmental loadings depends upon

meteorological conditions.

So, before considering the different actions, we deal in the next

section with the procedure to select a design storm.

Storm selection

There is a relation between the frequency and the intensity of the

events that a structure may experience. Such a relation is qualitatively sketched in the figure.

The structure is designed to withstand some extreme event S_D,

which has a frequency f_D. The choice of the event is made so that

(43)

during the lifetime L of the structure is reasonably small. This is called the encounter probability and represents one of several ways to characterize environmental risk.

Usually a designer should consider three types of events:

those causing fatigue effects;

those that may interfere with normal operations;

those that may produce the failure or a severe damage of the structure.

Each category requires a different type of data and statistical

analysis for resolution of the associated problems.

Considered as random process, a storm must be statistically

described.

Actually, the physical elements of a storm (e.g. the wave height or

the wind velocity) are described, using the recorded meteorological data, by some distribution laws, particularly suited to describe extreme events, such the occurrence of a storm of great intensity.

The random wave variables, usually recorded and processed, are

defined in the following figure.

Storm selection

(44)

If a suitable convention is adopted, the random sea shown in the

previous figure can be described in terms of wave height (H) and period (T).

In engineering applications the usual definition of wave height is the

maximum surface elevation difference between successive up-crossings of still water level.

This leads to the definition of zero-crossing period T_z.

Longuet-Higgins (1952) has shown that, for certain conditions the

cumulative distribution of wave height is approximated by the Rayleigh distribution:

σ_η=: standard deviation of η(t) (see previous figure).

Storm selection

.

1 )

(

2 2 8ση H

e

H

P

−

=

→

=

− 2 2 8 2

4 )

(

ση η

σ

H

e

H

p

A measure often used of the random wave field is the significant

wave height, H_S.

H_sis taken as H_1/3, the expected value of the highest one-third of the

waves in a sample.

For the case of the Rayleigh distribution, it can be shown that:

H_1/3= 4σ_η.

The expected value of the maximum wave height H_max in a sample

of N waves is given by:

Usually, designers switch at this point from a probabilistic to a

deterministic approach: they introduce a regular train of waves

where the wave height is taken as equal to H_sor H_max.

Storm selection

. ln 2 2 3 1 N H Hmax =

(45)

So, in this deterministic approach the statistics of waves is used only

to obtain a wave height for a regular sea state.

However, in this approach there is the problem of the selection of a

wave period to be associated with the wave height.

This point, concerning the choice of the wave period, will be treated

after.

The probabilistic models developed thus far relate to short-term

stationary conditions.

They must be developed for the prediction of the expected

maximum wave height, the design wave, over a long-term period such as 50 or 100 years (the centennial wave).

The long-term wave height distribution is, usually, based on

one-year’s data and its extrapolation to a return-period of 50 or 100 years is made using the Weibull or Gumbel distribution, which are typical probability laws of extreme values.

The Weibull probability law is

α: scale parameter, α>0; γ: shape parameter, γ>0;

η: location parameter, H≥η;

The values of the mode µ, mean m, median M and standard

deviation σ of the distribution are given by:

Γ is the gamma function:

.

1 )

(

)

(

1 γ γ α η α η γ

α

η

α

γ

      − −       − − −

−

=

→











 −

=

H H

e

H

P

e

H

p

Storm selection

( )

ln2 , 1 2 1 1 . , 1 1 , 1 1 2 1 1       ₊ Γ −       ₊ Γ = + =       ₊ Γ + =       ₋ + =

γ

α

σ

α

η

γ

α

η

γ

α

η

µ

γ γ M m . ) ( 0 1

∫

∞ − − = Γ x e ttx dt

(46)

The Gumbel probability law is

µ : location parameter; it corresponds to the mode of the distribution;

β : scale parameter:

σ : standard deviation of the distribution.

The values of the mean m and of the median M are given

respectively by

Both the Weibull and the Gumbel distribution of the maximum wave

height can be derived from an histogram of the recorded daily highest waves in one year.

.

)

(

1 )

(

β µ β µ β µ

β

− − − − − − − −

=

→

=

H H e e H

e

H

P

e

H

p

Storm selection

; 7797 . 0 6_σ _σ π β = ≅ . 2858 . 0 , 45 . 0

σ

µ

σ

µ

+ = + = M m 90 Cop yri gh t P . Vannucci – U VSQ pao lo .v annucci@ m ec a. uv sq .fr

In the figure, the probability density function and the cumulate

probability function for the case of Weibull (for η=0) and Gumbel

distribution sharing the same mode (µ= 5 m) and median (M= 5.73

m); the curves do not fit the same recorded histogram.

p(H) P(H) H(m) H(m) Gumbel Gumbel Weibull Weibull

Storm selection

(47)

The probability distribution used to fit the wave height data is used to

select the design wave.

This is usually done choosing a return period T_R: it is the average

time interval between successive events of the design wave being equalled or exceeded: