OFFSHORE PLATFORMS
BY Hatef Hajian
August , 2008
INTRODUCTION
Offshore structures are used world wide for a variety of functions and in a variety of water depths , and environments.
The design and analysis of offshore platforms must be done taking to the consideration many factors , including the following important parameters :
- Environmental
- Soil characteristics
- Intensity of consequences of failure
Offshore platforms may be used for 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. Although some of the older structures were made of reinforced concrete , and even earlier ones were actually made of timber. However , foe sake of modern platform discussion we will address steel platforms only.
Within the category of steel platforms , there are various type s of structures , depending on their use and primarily 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
- Stationary floating SPARs
Each of these types is chosen primarily due to water depth consideration , and secondarily due to 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.
The fixed tower structures vary in size and height , and can be used in water depths up to about 300 meters , although most commonly in water depth 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 depth less than 50 meters. The single caisson types of minimal structures are also used as navigational aid towers in rivers , and bays.
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 a 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 ratio .( The slenderness of column 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 . 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 ach one 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 platform for use in less than 200 meters water depths.
ENVIRONMENTAL PARAMETERS
Normally, for the analysis of offshore platforms, the environmental parameters include wave heights 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 load generated by these environmental conditions plus other loads generated by onboard equipment must be reacted by the piles at the mudline and below.
GEOTHECHNICAL 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 presents 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 repot 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 to develop data usable to the platform design engineer.
The soil report should show the calculated minimum axial capacities for piles at 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 softwear ), and will determine minimum pile penetrations and size. The minimum pile penetration must have a resistance capacity equal to one and half the maximum design loading on that pile, thus ensuring the factor of safety of 1.5 . For operating loads, the factor of safety must be 2.0 piles. The ratio of 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.
STRUCTURAL ANALYSIS
To perform a structural analysis of a new or used platform we develop a mathematical model of the using normally either of tow common software packages developed for the offshore engineering : SACS, or StruCAD.
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 a 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 in placed near the water level where a boat landing will be located for accessing the platform by boat.
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 subjected to wave and current forces.
If cranes are included in the design, then the deck must be able to resist the crane's maximum overturning moments coupled with corresponding maximum thrust loads for at least 8 positions of the crane boom around a full 360 degrees 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 referred to 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 double plates at joint between two pipe members. The double plate provides a virtual increase in the chord pipe's wall thickness preventing the brace pipe form 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 currently with ) the structural analysis the design team will start the development of construction drawings, which will be incorporate all the dimensions and sizes optimized by the analyses and will also add construction detailed for the field erection, transportation, and installation of the structure.
Of course, transportation and installation of the structure may require additional analyses.
FABRICATIONS AND INSTALATIONS
TRANSPORTATION
The offshore structures are generally built onshore in " fab yards " for cost savings and to facilitate construction. 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 load out 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 water. These motions are roll, pitch, heave and yaw.
To perform the transportation analysis, the engineer must have an environmental report showing the worst sea state condition 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 sec 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 to the respective horizontal axes ( normally the X-axis for pitch, the Y-axis for roll, and the Z-axis for heave ).
ON-SITE INSTALATION
All the structural section for an offshore platform must also be designed to withstand the lifting and installation stresses.
The jacket must be design to be self supporting during installation. Consequently they must have " 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 a 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 form 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 stage 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 meter or less below the top of 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.
REFERENCES
Offshore oil and gas production platforms.
Planning and design of fixed offshore platforms.
Stochastic response of offshore platforms.
Offshore drilling platforms.
