Remediating Platforms by Raising Topsides
Authors: Greeves John 
Published: June 2016

Technology Update

The design of fixed offshore platforms requires a minimum clearance from the mean water level to the topsides structure. This clearance is generally referred to as an air gap. The air gap is determined such that under the maximum design conditions, the crest of the wave passing through the platform does not make contact with or inundate the topsides.

Design codes such as API RP2A assess the air gap using the 1,000-year return period crest elevation combined with storm surge effects. Platform loads (shear and overturning moment) can increase by between 50% and 100% when the storm crest elevation exceeds the air gap and inundates the structure of the topsides.

The consequences of a storm wave impact on a platform’s topsides can be catastrophic. This was graphically shown by the collapse or near destruction of a number of offshore structures in the US Gulf of Mexico (GOM) during hurricanes Ivan in 2004 and Katrina and Rita in 2005. Approximately 250 significant structures, including eight-leg drilling and production platforms in water depths of up to 450 ft, were destroyed by these storms collectively.

Cleanup operations for the destroyed platforms remain ongoing. Costs for decommissioning a hurricane-destroyed platform are 10 times or greater the cost of decommissioning the same standing platform in a planned manner, excluding the value of lost or deferred oil and gas production.

Given that the destroyed platforms were designed according to approved standards and guidelines, why did the structures collapse?

Three primary reasons have been identified.

The platforms were in poor condition (suffering from corrosion and historical damage) and thus had design resistances less than planned for in the original design.

The maximum wave or waves experienced during these hurricanes were larger than assumed during the original design. Because wave loading on fixed offshore platforms varies with the square of the wave height, the actual loads experienced would be greater than those accounted for in the original design. And if the extreme wave crest exceeded the air gap, the design loads would increase exponentially and greatly exceed the original design case.

The air gap for certain platforms was less than at the original time of installation, which resulted in unexpected topsides inundation.

An Unacceptable Air Gap

In view of the critical nature of the air gap in the design and long-term viability of an offshore platform, what are the circumstances under which the air gap distance would no longer be acceptable? Three specific cases should be considered.

Actual design waves and storm surges are greater than those contemplated in the original design.

The consequence factor used for the design of the platform has changed, thereby requiring the use of new and more onerous conditions (larger design waves).

The platform air gap has decreased because of the offshore platform settling vertically relative to the mean sea level.

Research conducted after the major hurricanes in 2004 and 2005 showed that the numerical storm wave hindcast models, which add updated information to original forecast models, underpredicted extreme wave heights for the central portion of the GOM, compared with data measured during these storms.

The environmental criteria for the 100-year and 1,000-year return periods used for offshore platform design must be extrapolated from historical data recorded over much shorter time frames. Large variations in predicted values can arise from small errors made in fitting the historical data to complex statistical models. For example, the 100-year design wave in the GOM before these major storms was specified with a height of 72 ft.

The updated 100-year design wave is now 91 ft, an increase of more than 25%. This would imply that the air gap selected for platforms designed to handle the smaller design wave will now be too small for the larger design wave condition.