Following the tragic and catastrophic accident in the Gulf of Mexico, all attentions are focused on offshore design of installations of oil platforms. For the safe production of oil, control of well pressures during drilling is a fundamental requirement. One of the pieces of equipment for offshore oil platforms is the riser which is a key component.
The structure systems that connect the platforms at the surface of the sea and the structures found on the seabed are called risers. Normally risers are made of steel or titanium pipe having a wall thickness of less than one inch with an outer diameter of less than thirty inches. The length of the risers depends on how deep the oil production is moving into the water. The deeper the water, the risers become longer and longer, some attaining up to 10,000 feet, but most of them are usually around 3,000 to 6,000 feet in length.
The main function of the riser system is to convey oil and appropriate fluids between the wells and the platforms. It is also designed to accommodate the drill bit and the drill stringand in order to deal with the movement of the drilling platform, flexibility in the riser is important. To avoid the subsea wells to be affected by the rolling and pitching of the drilling platforms, slip and ball joints are placed in strategic areas in the risers.
Several types of risers are available ranging from flexible riser, steel catenary riser (SCR), free standing riser to top tensioned rigid riser. However, the most popular riser used in deepwater is the Steel Catenary Riser which offers major advantages over the ‘conventional’ flexible or ‘hybrid’ freestanding risers.
The offshore oil riser system normally consists of one or several tubular conduits which are suspended in a vertical position from the floating platform towards the sea bed. At the bottom end of each conduit, a jumper connection and tensioning assembly is available for connecting the bottom ends of the conduit to a subsea oil well. An apparatus is also installed for stopping any horizontal movement of the bottom end of the conduits while on the other hand a vertical tension is applied in the conduits by the means of a weight.
Riser is therefore considered as a vital element for offshore oil platforms, as a failure in the riser will result in stoppage of oil production and can also lead to pollution and spillage. Eventually, risers which are normally slender,experience major fatigue damage like cracks along the pipes, under cyclic load when in deepwater due to the fact that they are prone to vibration under shifting winds, waves and water currents.
Consequently, the impact of a riser failure involves a high risk of human injury leading to death, a considerable amount of pollution in the environment as well as very high economic and political consequences.