offshore platforms design overview

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OFFSHORE PLATFORMS DESIGN OVERVIEW

By Luis Manuel Luis

September, 2001

EXECUTIVE SUMMARY

Offshore structures are used worldwide for a variety of functions and in a variety of water depths, andenvironments.

The most commonly used offshore platforms in the Gulf of Mexico are made of steel, and are used for

oil/gas exploration and production.

The design and analyses of these offshore structures must be made in accordance with recommendations

published by the American Petroleum Institute (API).

The design and analysis of offshore platforms must be done taking into consideration many factors, including

the following important parameters:

Environmental (initial transportation, and in-place 100-year storm conditions)

Soil characteristics

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American Institute of Steel Construction (AISC) codes, and recommendations

Intensity level of consequences of failure

The entire design, installation, and operation must be approved by the Minerals Management Service

(MMS), a division of the US Department of the Interior. The MMS approval is contingent on a design and

analysis done in strict adherence to the API recommendations, and also on possible additional requirements

imposed by the MMS.


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INTRODUCTION

Offshore structures may be used for a variety of reasons:

Oil and gas exploration

Navigation aid towers

Bridges and causeways

Ship loading and unloading facilities

Offshore structures can be designed for installation in protected waters, such as lakes, rivers, and bays or in

the open sea, many kilometers from shorelines. The oil and gas exploration platforms are the best example

of offshore structures that can be placed in water depths of 2 kilometers or more. These structures may be

made of steel, reinforced concrete or a combination of both. In the United States these offshore oil and gas

platforms are generally made of various grades of steel, from mild steel (36,000 psi yield) to high strength

steel (50,000 to 52,000 psi yield)(240 MPa to 360 MPa). Although some of the older structures were made

of reinforced concrete, and even earlier ones were actually made of timber. However, for sake of modern

platform discussion we will address steel platforms only.

Within the category of steel platforms, there are various types of structures, depending on their use andprimarily on the water depth in which they will work.

TYPES OF OFFSHORE OIL/GAS EXPLORATION STRUCTURES

Offshore oil/gas exploration (and drilling) platforms can be of the following types.

Converted Jackup barges

Fixed tower structures

Tension Leg platforms (TLPs) Stationary floating SPARs

Each of these types is chosen primarily due to water depth considerations, and secondarily due to

the intended service and quantity of deck equipment necessary to perform its service.

The Converted jackup barges are the rarest, and may be used in remote areas with relatively

shallow water depths. Chevron (ZAGOC) uses some offshore Congo in the Lukami field, for

example.

The fixed tower structures are the most common offshore Louisiana and Texas coasts in the Gulf

of Mexico (GOM). These structures vary in size and height, and can be used in water depths upto about 300 meters, although most commonly in water depths less than 150 meters. Within this

category there are 4-leg, 6-leg, and 8-leg towers and also minimal structures whose decks are

supported by a single unbraced or pile-braced caisson. The minimal structures are used in water

depths less than 50 meters. The single caisson types of minimal structures are also used as

navigational aid towers in rivers, and bays, and are installed and maintained by the US Coast

Guard.

The Tension Leg Platforms are used in water depths greater than 300 meters. They consist of a

floating deck structure anchored to pile heads on the sea floor by means of long pipes which are

always kept in tension, and thus can be flexible without risk of a column buckling collapse failure

due to very high Kl/r ratios. ( The slenderness of columns is indicated by the Kl/r ratio; the

higher the ratio, the lower the compression allowable stress. )

The SPAR platforms are used in very deep water exploration, even in the Gulf of Mexico area,


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beyond the continental shelf. The SPAR is a vertical floating cylinder attached, by means of

cables, to anchors placed on the seafloor more than a kilometer away.

The engineering firms working in the offshore structural analysis field tackle each of these types

based on the size of company and the available engineering personnel resources to conduct the

analyses. Our experience derives from work doing structural analyses of fixed platforms for use

in less than 200 meter water depths.

ENVIRONMENTAL PARAMETERS

The design and analysis of offshore platforms in the US Gulf of Mexico is based on design

recommendations developed by the American Petroleum Institute (API) starting various decades

ago. The final design, installation and operation must also be checked and approved by the

Minerals Management Service (MMS) for all structures beyond the State land jurisdiction, which

means essentially that all offshore structures (and pipelines) must be approved by the MMS.

The design and analysis of fixed offshore platforms must thus be conducted in accordance with

the APIs Recommended Practice for Planning, Designing, and Constructing Fixed OffshorePlatforms Working Stress Design (API RP-2A) . The API recommendations have been

revised various times in the last 30 years and are compiled in a book entitled Recommended

Practice for Planning, Designing, and Constructing Fixed Offshore Platforms Working Stress

Design (API RP-2A-WSD). The latest revision accepted by the MMS is the 20thedition dated

July 1, 1993. Additionally, for re-assessment analyses, API has also issued a Supplement 1

edition dated February 1, 1997. API has actually published edition 21st

(on December 2000) of

its API RP-2A book, but this has not yet been accepted by the MMS.

The API RP-2A specifies minimum design criteria for a 100-year design storm.

Helicopter landing pads/decks on offshore platforms must conform to API RP-2L (latest edition

being the 4thedition, dated May 1996)

The Gulf of Mexico is subject yearly to hurricane storms that originate in the Atlantic Ocean and

may end up coming ashore in the Yucatan Peninsula or anywhere along the US Gulf Coast and/or

the Southeastern Atlantic coast of the states of Florida and the Carolinas, and even Virginia.

These hurricane storms help develop great waves and wind that can pound a structure for various

days.

Normally, for the analysis of offshore platforms, the environmental parameters include wave

heights of as much as 21 meters (depending on the water depth) and wind velocities of 170

km/hr, coupled with tides up to 4 m in shallow waters.

The most severe exposure region for extreme environmental loading is located offshore Texas

and Louisiana (between 86-98 Longitude W, and 27-31 Latitude N).

The API RP-2A also specifies that the lowest deck must maintain a minimum of 1.5 m (5 ft) air

gap between the bottom of the deck beams and the wave crest during the maximum expected

wave height. According to their plot of minimum deck elevation vs. Mean Lower Low Water

(MLLW) the highest values for minimum deck height exist for those structures located in waterdepths of around 30 meters. (At a water depth of 30 m, the 100-year wave height is about 17.4

meters, plus the tide which is about 1.6 meters). For this brief example, this means that the

minimum elevation of the bottom of the lowest deck should be about 16.2 meters. Note that the

wave displaces water and therefore the wave height is measured as the vertical distance between


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the wave trough and the wave crest, but the wave trough is below the MLLW.

The loads generated by these environmental conditions plus other loads generated by onboard

equipment must be reacted by the piles at the mudline and below.

GEOTECHNICAL DATA

Another essential part of the design of offshore structures is the soil investigation.

The soil investigation is vital to the design of any offshore structure, because it is the soil that

ultimately resists the enormous forces and moments present in the piling, at the bottom of the

ocean, created by the presence of the platform in the hostile ocean environment.

The soil can be clay, sand, silt, or a mixture of these.

Each project must acquire a site-specific soil report showing the soil stratification and its

characteristics for load bearing in tension and compression, shear resistance, and load-deflection

characteristics of axially and laterally loaded piles. This type of report is developed by doing soil

borings at the desired location, and then performing in-situ and laboratory tests in order todevelop data usable to the platform design engineer.

The soil report should show the calculated minimum axial capacities for piles of the same

diameter as the platform design piles. It should also show shear resistance values and pile tip end

bearing values. Pile axial capacity values are normally called T-Z values, shear values are

called P-Y values, and end bearing values are called Q-Z values.

These values, once provided to the engineer by the geotechnical engineers, will be input into the

structural analysis model (normally in StruCad or SACS software), and will determine minimum

pile penetrations and size. The minimum pile penetration must have a resistance capacity equal

to one and a half the maximum design loading on that pile, thus ensuring a factor of safety of

1.5. For operating loads, the FS must be 2.0 for piles. The ratio of the maximum combined

stresses to the maximum allowable stresses (Unity Checks) must not exceed 1.0, in the piles or

anywhere else in the platform.

Pile penetrations will vary depending on platform size and loads, and soil characteristics, but

normally range from about 30 meters to about 100 meters.

The soil characteristics are also used for a pile drivability analysis. Sandy soils are very desirable

for axial end bearing, but can be detrimental to pile driving when encountered near the surface.

Clay soils are easier to drive piles through but do not provide good support for end bearing,although they provide good resistance to laterally loaded piles.

STRUCTURAL ANALYSIS

To perform a structural analysis of a new or used platform we develop a mathematical model of the

structure using normally either of two common software packages developed for the offshore engineering:

SACS, or StruCAD.


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A model of the structure should include all principal members of the structure, appurtenances and major

equipment.

A typical offshore structure supported by piles will have a deck structure containing a Main Deck, a Cellar

Deck, and a Helideck. The deck structure is supported by deck legs connected to the top of the piles. The

piles extend from above the Mean Low Water through the mudline and into the soil for many tens of

meters. Underwater, the piles are contained inside the legs of a jacket structure which serves as bracing

for the piles against lateral loads. The jacket also serves as a template for the initial driving of the piles. (

The piles are driven through the inside of the legs of the jacket structure). The top of the jacket is placed

near the water level where a boat landing will be located for accessing the platform by boat.

The model definition file consists of:

Definition of the type of analysis, the mudline elevation and water depth.

Member sizes (member groups and sections).

Member joints definition.

Soil data (i.e. pile groups, T-Z, P-Y curve points).

Plate groups.

Joint coordinates.

Marine growth input.

Inertia and mass coefficients input.

Distributed load surface area definitions.

Wind area definitions.

Members and/or group overrides (i.e. overrides for marine growth for pile sections inside the


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jacket),

and finally followed by the load cases, which will include dead and live loading, environmental

loading, crane loads, etc.

Any analysis of offshore platforms must also include the equipment weights and/or a maximum deck live

loading (distributed area loading), dead loads in addition to the environmental loads mention above, and

wind loads. Underwater, the analysis must also include marine growth as a natural means of enlargement

of underwater projected areas subject to wave and current forces.

If cranes are included in the design, then the deck must be able to resist the cranes maximum overturning

moments coupled with corresponding maximum thrust loads for at least 8 positions of the crane boom

around a full 360 path.

The structural analysis will be a static linear analysis of the structure above the mudline combined with a

static non-linear analysis of the soil with the piles.

Additionally, checks will be made for all tubular joint connections to analyze the strength of tubular joints

against punching shear (tubular joint connections exist primarily in the jacket structure or between

members that will be submerged by the design wave). The punching shear analysis is colloquially referredto as joint can analysis. The UCs must not exceed 1.0. If joint can UCs exceed 1.0, these can be

remedied by the addition of doubler plates at joint between two pipe members. The doubler plate provides

a virtual increase in the chord pipes wall thickness preventing the brace pipe from puncturing through the

chord pipe member. Joint can overstress problems can also be fixed by increasing wall thickness of the

chord member involved, or increase the outside diameter of the brace/s.

All structural members will be chosen based on the results of the computer-aided in-place analysis. (Deck

stiffening members may be chosen due to maximum deck live load distribution or equipment loading). The

offshore platform designs normally use pipe or wide flange beams for all primary structural members.

After (or sometimes concurrently with) the structural analysis the design team will start the development

of construction drawings, which will incorporate all the dimensions and sizes optimized by the analyses

and will also add construction details for the field erection, transportation, and installation of the structure.

Of course, transportation and installation of the structure may require additional analyses.

NEW PLATFORMS

New platforms must be designed to adhere to stricter standards than older platforms. These

present day stricter environmental standards are a product of better engineering measurements

and lessons learned from past events.

New platforms must be designed to either the API RP-2A 19thor 20theditions criteria, which are

based on the 100-year hurricane storm loads.

These platforms must be structures capable of withstanding the most severe design loads and also

of surviving a design lifetime of fatigue loading.

The fatigue analysis is developed with input from a wave scatter diagram and from the natural

dynamic response of the platform, and the stiffness of the pile caps at the mudline. A detailed

fatigue analysis should be performed to assess cumulative fatigue damage. The analysis required

is a spectral fatigue analysis.

However, the API allows a simplified fatigue analysis if the platform:

Is in less than 122 m (400 ft) water depth.


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Is constructed of ductile steel.

Has redundant framing.

Has natural periods less than 3 seconds.

Single caisson platforms must be designed to have a maximum natural period of 3.0 seconds.

EXISTING PLATFORMS REASSESSMENT

Existing platforms can be analyzed using less stringent criteria, depending on their use and date of

initial construction, and most importantly life safety and consequence of failure.

Existing platforms are frequently re-analyzed due to upgrade plans for new (and heavier)

equipment, expanded production capacity, and addition of personnel living quarters.

The API RP-2A 20thedition, Supplement 1 defines three Exposure Category Levels for the

Gulf of Mexico: Level L-1, Level L-2, and Level L-3.

Level L-1 is defined as Full Population Hurricanes design.

Level L-2 is defined as Sudden Hurricanes design.

Level L-3 is defined as Winter Storms design.

All platforms that have oil storage facilities or serve as hubs for pipelines , and are thus

considered of high consequence in case of destruction must be re-assessed under L-1 Design

Level Analysis. High consequence refers to the environmental impact of oil spillage or the loss

of human life due to platform collapse. Normally, oil or gas storage facilities may fall in this

category.

If the platforms do not have significant onboard oil or gas storage facilities (even though they are

manned) will be considered Level L-2 Design. Their destruction will be of lower consequence

(since they are evacuated).

Finally, all lowest consequence structures (which are also never manned) can be analized in

accordance with the L-3 Design Level category.

Each of these categories will have specified minimum wave heights and periods, wind speeds,

and current speeds that are shown in figures in Section 17 of the Supplement 1.

The Minerals Management Service (MMS) must still approve the analysis and development plansprior to the platform owners being able to implement the changes. MMS permit applications for

facilities modifications must be submitted separately from those for structural modifications.

APPROVAL

MMS PERMITS

All offshore platform designs and/or modifications (whether structural or facilities) must be

approved by the Minerals Management Service (MMS).

The analysis results must demonstrate that the platforms have been designed (or modified) using

standard accepted methods and that the structures will be able to perform adequately in


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accordance within the maximum design parameters as prescribed by the API RP-2A and the

American Institute of Steel Construction (AISC) codes.

The permit application package must contain an analysis summary (and explanation of the

modifications, if applicable) and show the maximum foundation design loads, and unity checks.

It must have attached copies of the soil report, and the certified structural construction drawings.

It should also include a General and a Loads drawing showing two elevations of the complete

structure and pointing to the points where the highest unity check ratios occur (generally

UC>0.85).

The drawings and analysis summary are signed by the chartered engineer (licensed professional

engineer), but the complete package must be signed and submitted by the owner of the

platform/s.

The operation of the platform can not start until the MMS approval is granted and the platform

operator is notified.

FABRICATION AND INSTALLATION

MATERIALS

The API RP-2A lists the recommended material properties for structural steel plates, steel shapes

(i.e. channel, WF beams, angle bars, etc.), and structural steel pipes. Data can be found in Tables

8.1.4-1, 8.1.4-2, and 8.2.1-1 respectively.

At a minimum, steel plates and structural shapes must conform to the American Society for

Testing and Materials (ASTM) grade A36 (yield strength, 250 MPa). Structural shapes required

for higher strength applications must comply with ASTM specification A572, grade 50 (345MPa). Pipes must comply with API specification 5L, grade B (or ASTM A53, grade B), at a

minimum. For higher strength applications, pipe must conform to API 5L, grade X52.

Other materials also in the API tables may be used, but are less common in applications within

our field of experience.

TRANSPORTATION

The offshore structures are generally built onshore in fab yards for cost savings and to facilitateconstruction. Upon completion, these structures have to be transported offshore to the final

assembly site, onboard a vessel.

Therefore an offshore design and analysis of a new structure must include a transportation

analysis as well.

Care must be taken to ensure that the points of support of the structure can be reacted by a strong

section/s of the barge deck. This means that preferably the legs of the structure should be placed

on top of internal bulkheads or frames in the barge hull. If the dimensions of the structure can

not be arranged in a satisfactory manner to match the internal structure in the barge, then the use

of load spreaders may be necessary (depending on the weight of the structure).

The final loadout of the structure on the barge must include bracing to help counteract the forces

and overturning moments created by the motions of the barge in open waters. These motions are

roll, pitch, heave and yaw.


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To perform a transportation analysis, the engineer must have an environmental report showing

the worst seastate conditions during that time of the year throughout the course of the intended

route. Generally, it may assume a 20 degree angle of roll with a 10 second roll period, and a 10

degree angle of pitch with a 10 second period, plus a heave acceleration of 0.2 g. These

parameters must be converted, through hydrostatic calculations, to g-forces which will then be

applied to the structure along the respective horizontal axes (normally the X-axis for pitch, the

Y-axis for roll, and the Z-axis for heave).

The loadout and transportation plan must also be approved by the US Coast Guard, and verified

by a CVA (Certified Verification Agent).

ON-SITE INSTALLATION

All the structural sections of an offshore platform must also be designed to withstand the lifting

and installation stresses.

The jackets must be designed to be self supporting during installation. Consequently they musthave mudmats at the bottom horizontal brace level which will be resting on the mudline. The

mudmats are sections of the bottom of the jacket structure covered by stiffened plates to allow

the weight of the jacket to be supported by the top layer of the soil at the ocean floor (the

mudline). The mudmats are generally located adjacent to the jacket leg connections for obvious

structural reasons.

The piles must be designed to withstand the stresses during installation. The installation of the

piles is done above the waterline after the jacket has been lowered to the mudline. The piles are

installed in sections. The first section must be long enough to go from a few meters above the top

of the jacket leg to the mudline. The second section must be field welded to the first section at an

elevation slightly higher than the top of the jacket legs. At this stage the second section is

standing up to a height that is calculated depending on the size and weight of the pile driving

hammer (which is placed on top of the pile sections), because the pile section is behaving like a

cantilevered beam. All subsequent sections have to be designed as a cantilevered beam for the

same reason.

When all the piles have been driven to the required design penetration they will be trimmed at the

design top of pile elevation. The jacket will then be welded to the piles about 1.0 meters or less

below the top of the piles.

The deck structure, whose legs will have stabbing guides at the bottom, will be lowered to fit

on top of the piles, and will be welded to the piles.Any riser or other operational pipes will then be field installed onto the platform.


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Photograph of offshore platform in South Timbalier Block 21 G (offshore Louisiana)

ACKNOWLEDGEMENTS

Before I finish I must let everybody know the value of the help provided by some of my colleagues and

friends. I wish to thank Mrs. Yi Wang, and Mr. Jos Delagneau for their patience and help clarifying some

points in the text, thus contributing invaluably to the accuracy of this paper. Thanks also to all other friends

who offered to read the completed paper.


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REFERENCES

American Petroleum Institute (1220 L Street, Northwest, Washington DC 20005 USA) RP-2A 20th

edition,

and Supplement 1, dated December 1996

Minerals Management Service, Gulf of Mexico OCS (Outer Continental Shelf) Region (1201 Elmwood ParkBlvd., Harahan, LA 70123-2394, USA)

American Institute of Steel Construction (AISC), Inc. (1 East Wacker Drive, Suite 3100, Chicago, Illinois

60601, USA)

Atlas Engineering, Inc. (990 N. Corporate Drive, Suite 102, Harahan, LA 70123 USA)

American Society for Testing and Materials (1916 Race Street, Philadelphia, PA 19103-1187, USA)


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offshore platforms design overview

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