Decreasing biodiversity above and below the sea

Top left: FIXED PLATFORM The jackup rig stands on extendible legs. It can be quickly moved to a new site, for instance to develop new natural gas fields.

Top right: FIXED PLATFORM Steel structures such as the American Bullwinkle platform are prefabricated on land and then towed out to sea. Source: World Ocean Review

Offshore oil production: locations and methods

The first offshore oil rigs were constructed at the end of the 19th century in California and were only around 15 meters deep. Since then, production has continuously moved to deeper waters, where drilling depth can be up to several thousand meters.

Rig types and structures

Speaking generally, there exist two key types of offshore oil rigs: floating and fixed rigs. Fixed platforms can be mobile (e.g. Jackup rigs) with concrete bodies (e.g. Norwegian Sea Troll) and Steel structures (e.g. American Bullwinkle). I have focused here on fixed steel platforms, since their decommissioning is more complex and they have a higher potential for usage in the interventions I have designed.

Decommissioning process

The construction of rigs usually takes place on land, after which the rig is carried to its destination at sea. The average lifespan of an oil rig is about twenty to thirty years.1 After this time, it is not economically profitable to maintain production on the rig and it must be decommissioned.

The laws on decommissioning vary from country to country. In some waters, oil rigs must be decommissioned within a fixed time frame, whereas in others a deadline is not clearly defined. Decommissioning is a very costly procedure (2)

1 2015. Planete Energies. [Online] 11. August 2015. https://www.planete-energies.com/en/medias/close/ life-cycle-oil-and-gas-fields. and involves a considerable amount of effort and time. Let us go through this process step by step.

1. Management

The first step involves reviewing existing contractual obligations and analyzing the process from a technical point of view. After the engineering analysis, operational planning can start. As the number of specific factors to the process is fairly high, several different contractors are necessary. There are different agencies which are specialized in each step of the process, for example for management, civil engineering, diving and cutting purposes.

2. Obtaining a permit

The next step is to prepare a decommission schedule and apply for a permit from the authorities. An execution plan describes the existing on-site environmental conditions, as well as activities and necessary equipment for implementing the withdrawal. When the plan is ready, it should be submitted to the government for approval. It is up to the authorities to decide whether the demolition can take place in the described manner or not. Since this process can take several years, companies are wise to start with it on time.

3. Platform preparation

Once decommissioning is permitted, preparation of the platform can start. This includes emptying tanks, dismantling structures into separate pieces, cutting pipes and cables between modules, preparing pieces for lifting and preparing the underwater sections of the structure for extraction.