Design Philosophy for Offshore Structure

In this page I will talk about design philosophy for offshore structure. Following is a picture of different type of offshore platform:

There are different Types of Offshore Structures:

Fixed structures: Steel Jackets / Towers, Concrete Gravity Based Structures, Jack-ups, Compliant towers and Tension Leg Structures (TLP).
Sub-sea Structures: Drilling Production Units are located on sea-bed. These structures are economical for Marginal fields near existing installation.Please also note that sub-sea structures are Suitable for great water depths. It does not have direct vertical access to oil wells and no storage facility. Maintenance is also very difficult, but production time is early.
Floaters: Floating Production Storage Offloading unit (FPSO), Mooring Ship and Barge. Floaters does not have fixed vertical access to oil wells. But its popularity is increasing as it provides great storage facility and the cycle time is also comparatively less. Calm water environment is preferable for this type of structures.

Before starting the design , you need to check the followings:

Location of the platform - you need to find how far it is from sea coast
Function of the platform - Platforms have different functions. You will find out, this platform for what purpose.....drilling, production,or other..
Field development options - You need to check whether any existing infrastructure is available in and around
Ship impact factors -You will check whether there is any impact in existing shipping lanes.
Structure transportation and installation - You need to check with your project / construction engineering group about the location of fabrication and module yard.

Now you will follow the following steps to start the offshore structure and foundation design:

Step-1 : Basic design data collections

Maximum Water Depth Records (some data):

Following maximum water depth data are collected from different books and publications:

Exploration well : 2320m (Texaco, 1996)
Production well : 1675m (Mensa, Shell 1997)
FPSO : 1400m (South Marlim, Petrobras 1997)
TLP : 1160m (Ursa, Shell 1999)Fixed Platform : 412m (Bullwinkle, Shell 1988) (Jacket)

Clear height of the deck of the platform

Height of Bottom Of Deck should be Low water level plus the following:

10,000 year crest height or 1.3x100 year crest height ------available from environmental data
Depth of Mud mat penetration-------From soil data
Reservoir Compaction----------From soil data
Expected Increase in sea water level--------From environmental data

Different design data collections:

Obtain oceanographic data
Soil report
Geological factors (Environmental data...Earthquake, wind load, etc..)

Step-2 : Structural Description

Battered structure is more preferable. This structure has some advantages and disadvantages also:

Advantages:

Provides Wider base of jackets and reduces pile size requirement.
Provides greater mud-line area and gives better stability prior to piling.
Helps in transfer of some shear force to axial compression / tension in the structure columns / piles

Disadvantages:

Results in longer diagonal in the bottom bay of the structure, may increase material requirement
Induces bending in piles during driving into soil.

Please look into following pictures for different types of Vertical Bracing patterns:

Step-3 : Description of Loads:

Following Loads are acting on Offshore Structures:

DEAD LOAD
LIVE LOAD
ENVIRONMENTAL LOADS (Wind /Wave/Current/Water depth/Ice)
CONSTRUCTION LOADS
DYNAMIC LOADS: Vortex Induced Vibration, Wave forces, Diffraction and Wave slamming

Step-4 : Basic Structural design:

Initial member size selection

Members diameters are determined by stiffness, rather than strength.
Members are designed for axial loads (neglecting moment), keeping the allowable maximum stress ratio as 0.65
Guiding KL/r values are available for different areas. For preliminary design, consider K=0.75

Analysis and design of structure

Use Softwares like SACS for design and analysis of structure
Member sizes are justified with respect to the stipulations of the different codes.
All the strutural elements shall be checked for Fatigue, Soundness of connection between piles and jacket leg
You should also keep in mind about material used for Design, welding Procedure, accidental (Local) Stresses in members andcorrosion allowances

Design of Piles:

Following are basic considerations for pile design:

Pile Bearing Capacity
Pile Pull-out Capacity
Pile Lateral load capacity
Pile Drivability Analysis

I hope this page will be very helpful to you to understand the basic design of an offshore structure.

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Courtesy: Atanu Dutta, Civil/Structural Engineer

Copyright 2009. All rights reserved. Please do not print or copy of this page or any part of this page without written permission from Subhro Roy.
Disclaimer: This page is prepared based on experience on Civil Engineering Design. All definitions and most of the explanations are taken from different text books and international design codes, which are referenced in the contents. Any similarity of the content or part of with any company document is simply a coincidence. Subhro Roy is not responsible for that.