Design of Offshore Structures

Over 3/4 the of the earth's surface is made up of water. So, with the insatiable needs of the of the ever-increasing population globally it reckons mankind to not just bank on this meager fourth area for our resources but also hone our capabilities into the wilderness of the oceans. The oceans as we know, are full of resources. So, effective tapping and exploitation of these resources on a long term basis can meet our pressing energy needs.Offshore structures are one of those installations and facilities in the deep ocean at strategic sites for the exploration, exploitation and large-scale production of natural resources like oil, gas, minerals, electricity etc.

Fig. 1: Infographic comparing sizes of a typical offshore structure with other man made megastructures. (Courtesy: cdn.theatlantic.com)

For the past several decades, the increase in the quest for exploitation of petroleum resources has lead to an extensive spatter of these offshore resources all over the world.

Offshore Structures: A Detailed Synopsis

In simple definitive terms, offshore structures are man-made installations in the resource-rich sites in oceans for the purpose of exploration, exploitation and production of petroleum, oil, gas and other power and energy resources. Generally, all offshore platforms have: 

• A superficial structure, mostly above the water level consisting of operations,  machinery, accommodation, control systems, piping systems, etc.

• An intrinsic underlying  system consisting of drilling equipments:  drill bit, drill collar, risers and conduits, wellheads for the purpose of extraction of oil from the sea bed

Parts of an Offshore Platform

An offshore platform deals with topside facilities as well as the underlying drilling facilities. The topside facilities lying above the sea level involves an optimization between  space as well weight yet involves all the neccesities for the oil extraction purposes. The ambient components of any offshore platform involves derricks, drilling rigs, oil storage facilities and tanks, injection compressors, gas compressors , gas turbine generators, HVAC,  Piping, Instrumentation, Basic machinery like primary and auxillary power generators, cooling system, pressure regulation etc. Also worth mentioning are control wheelhouses for operating personnel, suitable accommodation and helipads. Cranes and lifting system for loading and unloading operations are also there.

In the undersea extraction systems, some of the terms worth mentioning are drill collars, drill bits, wellheads, conduits, risers, BOPs, etc. Though not complicating stuff about the details and description of each of these, drill bits and collars involve drilling into the sea bed to create the oil well for extraction. On the other hand, wellheads provide structural and pressure containing interface at the opening to any oil or gas well for drilling and production.

Risers are components synonymous with any drilling platform inherently. This is basically a conduit that provides an extension from wellhead subsea to the drilling system above. They are basically of two types: marine drilling risers for floating platforms or tie-back drilling risers for fixed platforms. Risers have the catalytic role of conducting the crude oil or gas from the wellhead to the drilling rig by the virtue of a piston like mechanism which in turn is sent to the above rig for processing. The design of the riser depends on the filed layout, vessel interface, fluid properties and environmental conditions. Risers maybe flexible or rigid. Also  they remain stable and in tension due to self weight. They may have profiles to reduce excess load and nonviscous fluid flow.

Mooring and Anchoring

Given the varying sea conditions and the environmental vagaries, for proper positioning, ease of operation and safety , mooring facilities have to be initiated. Mooring may be by the virtue of steel or synthetic mooring lines. Steel ropes having high strength and rigidity have catenary nature and may use more length. On the other hand synthetic mooring lines are elastic, taut -shape and uses lesser length.

Fig. 2: Different parts of an offshore structure with piling arrangements.  (Courtesy: www2.southeastern.edu)

Also corrosion is a big setback for stell ropes as compared to the synthetic ones. The platform may also be under certain conditions be anchored or held fixed to a jetty or pier if possible. Drillships, FPSOs or sometimes semi-submersibles mostly use DPS (Dynamic Positioning Systems) systems for positioning. 

Loads on any offshore Structure

Any offshore structure can be subject to various loads like:

• Permanent Loads
• Operating Loads 
• Environmental Loads    
• Construction
• Accidental Loads

Permanent loads involve the construction loads like the weight of the entire structure, ballast systems, weight of machinery, accomodation and other equipment. For members below the waterline it involves the hydrodynamic forces and also the hydrostatic pressure forces like the buoyancy and pressure loads.

On the other hand, operating loads involve the loads subservient under the ongoing operations like  loading and unloading, drilling, mooring operations, additional loads generated by cranes and derricks.They take into account impact, momentum, vibration, slosh dynamics, material fatigue etc.  Also involves weight of manpower,equipment, storage like the crude oil which has been extracted, life-support systems and so on.

Environmental loads may be subdivided into the various categories like

1. Wind Loads : These loads act on the above portion of the platform by virtue of the blowing winds of varying extremities.For instance during regular sea breeze, this aspect is not to be taken care of but where the situation is graver like in case of gale storms ruffling the seas hither and tither, the consequent aftermath on the structure is a pressing concern. Especially when height to width ratio is more than 5, the cyclic wave loads due to the induced vortex flow has to be taken seriously into account.Structural members have to be designed such that they can withstand longitudinal loads with greater endurance than transverse for higher slenderness ratios.Snowfall or frost heaving which is a phenomenon of abnormal swelling of soil during cold temperature may pose a potential threat to the structure.
2. Temperature and Atmospheric Pressure loads cause thermal expansion, contraction and sometimes deformation. Suppose a platform amidst the cold northern Atlantic during winters have ti endure high amount of low pressure and temperature vagaries while somewhere near the Tropics during summer, it may be subjugated to high temperatures, pressures, humidity or warm underwater currents sometimes leading to expansion. 
3. Seismic Activities often take place in the underwater oceanic plates where they drift, vibrate, intimate or recede away from each other. These oceanic seismologic behaviour which often leads to climate change and variance in the ocean wave mannerisms have a detrimental impact. Sometimes these oceanic activities pave way for catastrophies like the Tsunami which may be fatal. 
4. Lateral Loads from soil , groundwater or seabed may endanger their piling or foundations hence often aggravated by seafloor scour. 
5. Wave loads are the most prominent terror that poses threat to all offshore structures. The ceaseless oceanic activities at the surface from light ripples to gigantic waves are a matter of vast study which I do not include in this article. Only I would mention that the waves are basically gravity contact forces energy transfer phenomenon that occurs due to wind pressure on the water surface with a given force or speed, duration and fetch(distance over which the wind blows). Waves may be of internal or external type or maybe normal sea waves, swells or breaking waves.Technically, the total wave forces on larger, broader structures maybe through diffraction theory while that of slender bodies with D/L ratio greater than 0.2 and considered hydrodynamically transparent use Morrison's equation for calculating wave impact. Thus the preliminary design of any offshore structure takes care of its resilience to the waves it faces. 
6. Marine growth accumulates on the submerged surfaces, increasing the wave forces through higher drag forces and surface roughness.

                                             

                                                 

 Fig. 3: Wave disturbances in water surface (Courtesy: www.pbslearningmedia.org)

                           

Installation Loads

These are temporary loads that arise during the fabrication or erection of the platform or its components. During fabrication erection lifts of various structural components generate lifting forces, while in installation phase, forces are generated during platform load out, transportation to the site, launching and upending, as well as lifts related to installation.

Fig. 4: Offshore Structure receiving minor repairs from shore supply vessels.  (Courtesy: www.theneweconomy.com)

All lifted system loads may be suited to withstand the loads of the material as well as sustain the static equilibrium of the lifted component, the slings in tension and the entire system. Other forces involve the drag forces to a jacket when transported from the fabrication yard to the site or tugging a completed semi-sub by a Heavy lift ship. Also another crucial aspect taken into account is the final "fixation" of the structure like in case of a jack-up plunging its legs to the sea-bed, or while mooring a semi-submersible or while merely clamping the legs of a jacket platform to the sea-bed through piling foundations. These activities often generate terrible heave motion which unrestrained may lead to operational failure or even collapse of the structure. 

Accidental Loads

To err is human. If we go by these words in our day-to-day lives, such multi-purpose projects are not out of the bag. According to DNV, accidental loads are loads which may occur as a result of pure accident or exceptional circumstances. So, the worst instances may be collision, breakage, flooding of buoyant parts, fire or explosion. Since the majority of offshore platforms deal with oil, gas , petroleum which are inflammable storage and productions needs to be with umpteen care. Evacuation measures need to be implemented in case of the unwarranted.

Fig. 5: Firefighting operations in progress in the vent of a major fire onboard the structure.  (Courtesy: abc.net.au)

Fire extinguishers and proper recovery equipment should be handy to combat any hazard and revert to the recovery process as fast as possible. 

This was all about design considerations for offshore structures which every offshore engineer is always aware of. The next time we will take a look at the different types of offshore structures in existence. Oh, and before leaving, do not forget to follow the youtube link for an exciting documentary on these 'Megastructures'.LSD

 

Article By: Subhodeep Ghosh