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Challenges for fixed/floating installations and pipelines

A number of major accidents involving loss of human life have occurred globally as a result of structural failure. A brief description of some challenges facing the industry is provided here.

Some of the challenges the industry is facing:

More and more facilities are either being used or planned for use beyond the scheduled design life. The maritime regulations set the design life at 20 years. With some exceptions, a period of 20 years was also the plan for a number of fixed facilities which were designed in the 1970s.

At present, there are few international industrial standards for evaluation of design life extension. In addition, the practice in this area is not uniform. The authorities have, in cooperation with the industry, initiated the development of better methods for inspection planning and criteria for scrapping of facilities.

When the seabed at an oilfield subsides, the facilities become more exposed to waves. This means that some facilities can no longer meet the strength requirements in connection with hurricanes. Compensating measures have therefore been implemented, for example through the fields being shut down and evacuated under certain weather conditions.

The PSA makes annual assessments of the risk posed by waves on the deck. This is presented in the RNNS report. The risk for the personnel on board is maintained at a constant level through the use of the evacuation procedures. However, the risk of material damage increases every year.

In recent years, the moorings of the facilities have been the underlying cause of a number of incidents. The problems have mainly been related to three areas: Mooring winches and their brake systems, the mooring line itself and the anchor. For example, brake failure was one of the main causes of the Ocean Vanguard incident in 2004.

There have also been cases where the holding power of the anchor itself has failed. This is a challenge, especially for the floating living quarters facilities. These are located next to or close by other facilities. A risk of collision arises when the anchor drags. Serious damage has also been caused to pipelines on the NCS as a result of collisions with anchors from large vessels.

Dynamic positioning
Dynamic positioning (DP systems) is the use of machine power and the propellers to hold facilities in place. DP systems represent a young technology compared to the use of anchors. The Petroleum Safety Authority Norway (PSA) is watching the industry's DP competence closely, audits the technical condition, and follows up to ensure that necessary maintenance is carried out.

Flexible risers
Flexible risers have statistically been shown to tolerate less strain than rigid risers, and may fail in new and unforeseen ways. There is no complete inspection method for mapping the condition of flexible risers. Inspection of technical condition must therefore be based on a comprehensive assessment of information from several inspection methods - which assesses parts of the riser separately. As the tools for necessary inspection are lacking, we also lack good opportunities for early warning of flaws compared to other types of risers.

Pipelines and risers
Pipelines and risers can be exposed to corrosion, unacceptable internal stresses and external damage from trawls and anchors.

Good opportunities are provided for monitoring general corrosion in ordinary steel through intelligent internal inspection. General corrosion would normally be expected to cause minor leaks. Other corrosion mechanisms involving such agents as carbon dioxide, hydrogen sulphide and hydrogen-induced stress can develop faster and less predictably.

Damage has recently been reported to field joints in pipelines lying on the seabed. Ever-larger and faster fishing vessels could be the cause of the increase in frequency. In the longer term, this type of damage could lead to increased consumption of cathodic anodes and the threat of external corrosion attacks.

Collision risk
A safety zone has been drawn around all petroleum facilities. In recent years, several measures have been implemented to reduce the number of collisions between facilities and vessels. The facility at the Gjøa field had to be moved westward during the planning and design phase - to avoid the main shipping lane and achieve an acceptable collision risk.

The industry itself has established systems for maintaining continuous overview of ship traffic which could threaten facilities on the Norwegian shelf.

Forces of nature
Wave height, sea currents, wind speed and earthquakes are factors which affect the petroleum facilities on the Norwegian shelf. The licensees are obligated to collect environmental data, including meteorological data, and in this manner have information available for the planning and implementation of activities. Usually, waves puts the largest strain on the facilities. Waves are also the largest risk in the event of erroneous determination of dimensional values.

Major accidents

The Alexander L. Kielland accident in 1980 has to a large degree influenced the work on structural safety in the Norwegian oil and gas activities. Crack formation in the load-bearing steel structure caused the flotel to capsize and 123 people died in the worst ever accident in the Norwegian oil industry

There have also been accidents on the Norwegian shelf with major accident potential. In 1989, West Gamma capsized and sank when the cargo on deck was displaced. Only luck prevented the loss of life when the Sleipner A substructure sank in Gandsfjorden in 1991.