To meet the challenges of severe weather conditions and ageing platforms, DNV is developing a new NORSOK standard to deal with platform lifetime extension.

Norway’s Oil Industry Association (OLF) has asked DNV to propose a new NORSOK standard encompassing rules and regulations for issues involved in lifetime extension, which have previously not been covered by newbuild standards such as NORSOK N-001. The new NORSOK N-006 will be an alternative to NORSOK N-001 for cases where structures are to be operated beyond original design requirements and structural resistance is not easily documented through ordinary design calculations.

The new standard is applicable to the assessment of complete structures, including substructures, topside structures, vessel hulls, foundations, mooring systems and subsea facilities.

The proposal for NORSOK N-006 has been close to a year in the making, and is now out for review in the industry. The process has been demanding, involving issues that have not been codified and regulated before. The final version is expected to be completed and ratified this autumn.

Operators must apply to the Petroleum Safety Authority to extend an operating license beyond the original design life – and must provide documentation that the platforms will be operated safely for people, for the environment and will also be profitable, with the efficient use of infrastructure.

Unique position
How should operators document the condition of the platform? What type of information should be sent to the authorities? How do you take existing rules and regulations and expand them for new conditions while maintaining the same level of safety?

DNV has an unique position to take on the task, due to our involvement in writing regulations for many years. We fully know the basis for the rules, and the possibilities for bringing them forward. Chief specialist for Offshore Structures at DNV, Gunnar Solland, has led the work with the new standard.

One of the main issues for maturing platforms on the Norwegian shelf is subsidence. In the most extreme cases, platforms have sunk by up to 10 meters. This level of subsidence is the most extreme in the world; and waves can now reach the platform decks in some instances.

Wave heights in the North Sea are estimated to be greater now than they were 30 years ago – perhaps due to climate change, or perhaps due to greater accuracy in measurement and more statistical data.

The combination of subsidence and high storm waves means that platforms endure much greater loads than they did at the beginning of their design lives. Some platforms no longer satisfy the original design requirements, or current NORSOK requirements for manned structures.

What are the criteria for shutting down and unmanning a platform? How big do the waves need to be? The proposed NORSOK N-006 standard – specifically written for Norwegian conditions – will provide the guidelines to these questions.

According to the current NORSOK standards, manned structures must withstand a 10,000-year wave, and people must be safe in such a condition. If this requirement is not fulfilled, the platforms must have unmanning plans for instances when waves are predicted to exceed a predefined safe level. The proposed new standard sets the criteria for safe operation in such cases.

Limiting evacuations
One of the subsiding fields is Ekofisk. It was partially evacuated due to storms just once last winter, with production shutting down on a third of the platforms. The number of evacuees to land was limited to approximately 500. The evacuation decisions were made based on data supplied by DNV. Optimally, knowledge and expertise will give full control, making it possible to limit evacuations and shutdowns to the minimum.

DNV is working with the Norwegian Meteorological Institute to define the height and forecast the type of wave that can cause damage. Today we have much better data on wave height than just a few years ago, and more reliable data means safer operations.

Improving methods
Solutions to reinforce ageing platforms are many. In the 1980s, DNV assisted in jacking up sinking platforms on the Ekofisk field. Due to cost increases, that procedure is less attractive today.

But, there are other options. If a mature jacket platform needs strengthening, one common method is to insert grout into the braces. New extra diagonal braces can also be mounted on the platform. If water is hitting the deck, it is possible to open the deck so the water goes straight through. Another option is to build a wall to protect the deck from waves.

Scour is another issue for ageing platforms. The seabed can be gradually washed away around the piles over the years. One area covered in the proposal is how to assess areas that are not possible to inspect, for example the piles.

Basic maintenance, such as the removal of marine growth, is also important. Generally there is a good safety culture in Norway, and operators take safety seriously. Conditions are changing, and the new NORSOK standard will take into consideration new weather and foundation challenges - which we increasingly will meet in the future.

Wave impact
Previously, there have been no established methods for determining the loads that deck structures are subjected to during wave impacts. DNV now performs extreme storm analyses run using advanced software such as COMFLOW, a computational fluid dynamics (CFD) programme. The most complicated analyses can take up to five days to process. Non-linear finite element (FE) analyses are then utilised to predict the effect of the loads – showing exactly what loads each joint and brace is subject to under a given storm condition. The FE software USFOS is used in this work.

In connection with writing the new NORSOK N-006 standards, DNV has pioneered research and published papers, such as ‘Prediction of Increased Jacket Substructure Loads Due to Wave in Deck.’

Insufficient air gap is an important factor in platform damages caused by waves. The original design requirement for positive air gap is no longer fulfilled for a number of jackettype structures still in production due to seabed subsidence and/or more onerous weather criteria.

The DNV study examines the effect of extreme wave crests hitting the deck of a jacket-type structure. The kinematics in the wave beneath the deck are strongly influenced by the deck itself. In this case, the increased fluid particle velocities must be accounted for when assessing the load on obstructions located in a zone below the deck, i.e. the upper part of the substructure and on under-deck attachments.

Usually the wave-in-deck loads and substructure loads are calculated separately by two different analysis programs. The undisturbed velocity field is then applied to the substructure. DNV researchers compared two methods, both based on computational fluid dynamics (CFD), for calculating the increased substructure loads due to disturbed fluid particle velocities during extreme wave-in-deck events.