Comparison Between Offshore Structural Engineering Design and Building Design

Comparison Between Offshore Structural Engineering Design and Building Design

Introduction to offshore structural engineering design
Platform diagram
Offshore structural engineering is a relatively new field of engineering concerned, particular mainly in the design and installation of fixed offshore platforms for various purposes. It was in year 1947, when the first steel structure was installed in the middle of ocean of the Gulf of Mexico. It was differ from the conventional structural engineering (building structural engineering) mainly in the problems which have to be considered in the transportation phase of the structure to the offshore construction site, installation method of the structure, the ability of the structure to withstand the specific design loading, mainly the severe environmental loading during its intended life. The primary driving force behind this technology was come from the oil and gas industry, which was one of the largest energy resources industries in the world. This industry needed fixed platform for exploitation of the extensive hydrocarbon which deposited deep below the sea bed. However, the offshore engineering not limited solely to the oil and gas industry, but also for the important applications for the military and navigational purposes.

An offshore structure is usually specified simply as a requirement at a designated offshore site for an operational deck having a prescribed minimum working area and carrying a prescribed minimum weight loading. Normally in oil and gas industry, offshore structures can be categorized into three types based on its function, which are oil drilling platform, processing platform, and living quarter platform. Oil drilling platform usually is the largest and most complicated platform among the three. It contains the machineries and power supply plants needed to perform oil drilling. Processing platform is mainly dealing with the processing of the raw oil and gas materials come from the oil drilling platform, while the living quarter platform provides shell and living facilities for the workers working for the offshore oil drilling. Living quarter platform normally been built nearby the oil drilling platform for the convenience of the mobility of the workers. Despite, sometimes there will be bridges connecting living quarter platform to the oil drilling platform.

For the offshore structural engineering design, environmental loading is one of the most critical issues in compare to building structure. Unlike building structures, offshore structures have to withstand the very strong wind force (wind force in the middle of ocean is much stronger than the wind force acted on the onshore building structures), current and waves, possibly with the hazards from floating ice and earthquakes, and the changes of tides. Besides environmental loading, dead imposed loads of the platform have to be considered in detail during the design phase. The operating load is very critical to be considered particular in designing oil drilling platform. The oil drilling machine will produce a very large impact on the platform during its operation. In addition, the design process has to consider the installation method and transportation of the structure to the offshore construction site. Due to the location of the construction site, which is in the middle of the ocean, there are many factors needed to be considered in the platform design, for example, method of transporting the platform and jacket to the construction site, weather condition, wave and current, method of installation, and method of conducting piling foundation.

Loading factor
Before proposed offshore structure design can be analyzed, it is necessary to have quantitative estimates of all significant loadings that the structure is likely to be experienced and withstand mainly environmental loadings and dead loads.
During the storm conditions, overwater wind is significant in the design of offshore structures because of the very large forces it induced on the upper exposed parts of the structures, which is the platform. Wind speed during the hurricane conditions can exceed 100mph, which can cause 100kips or more of horizontal forces on a platform structure.

Surface waves during storm condition are also of major importance in the design of offshore structures because of the large forces produced by the water motion on the submerged parts of the structure, which are the jacket legs. Wave height is the difference between maximum and minimum elevations of the water surface. During storm condition, the wave height can exceed 50ft, with associated water motion, it can induce horizontal forces of few hundred kips or more on an offshore structure.

Currents can contribute significant impact to the total forces exerted on the submerged parts of the structure. Currents generally refer to the motion of water that arises from sources other than surface waves. There are 4 types of currents:
1. Tidal currents = Currents which arise from astronomical forces exerted on the water surface.
2. Wind drift currents = Currents from the drag of the local wind on the water surface.
3. River currents = Currents from the discharge of the rivers
4. Ocean currents = Currents from the drag of large-scale wind systems on the ocean
During the storm conditions, currents of the ocean at the water surface can exceed 2ft/sec or more, which can contribute rise to the horizontal structural forces about 10% or more, of the wave induced forces.

Wind force
The wind force exerted on a structure depends on the size and shape of the structural members in the path of the wind, but more importantly, it depends on the speed of the wind blowing. The greatest wind speed at a particular site can be estimated from the analysis of local daily weather records. However, because of the fluctuations of wind speed over any measuring time, such records necessarily consider averaged wind speed measurement over a finite of time. For general structural design purposes, 50-year recurrence interval is used, but for offshore structures, to avoid the possibility of unusually high loss of life and property in extreme condition, 100-year interval is normally been used. Table below shows that 100-year return period of wind speed assumed (referenced to 10.0m above MSL) in construction of BLB wellhead platform which located in the Malaysia-Thailand Joint Development Area (MTJDA).
Speed
(m/s) Direction
N NE E SE S SW W NW ALL
1-hr mean 20.0 19.8 19.0 14.5 18.4 17.4 17.0 16.0 20.2
1-min mean 23.6 23.3 22.4 17.1 21.7 20.5 20.0 18.8 23.8
3-sec gust 26.6 26.4 25.3 19.4 24.5 23.1 22.6 21.3 26.9

The wind speed assumed for offshore structures design is normally about 10% greater than those nearby coastal stations, which act as a station to measure the wind speed of the particular area, for conservative purpose.
However, wind speed varies with height above MSL. Therefore, it is necessarily to calculate the wind speed in different height. For BLB wellhead platform design, the formula for calculating the wind speed at different height is based on the American Petroleum Institute (API) standard.
VH = VR x [ 1 + C x ln (H/HR) ]
C = 5.73 x 10-2 x ( 1 + 0.0457 x VR )1/2
Where:
VH = Wind velocity at specific height (m/s)
VR = Wind velocity at reference height usually 10m above MSL (m/s)
HR = Reference height, usually 10m above MSL (m)
H = Height above MSL (m)
Wind force