The Tyra field has played a crucial role in Denmark’s energy infrastructure for almost four decades. Located 225 kilometres off the west coast of Denmark, it is a vital part of the country’s gas industry. While it is temporarily closed due to natural subsidence in the seabed under the facilities, it is due to reopen in 2023. When the field resumes production, it will once again serve as the primary hub for gas production in the Danish part of the North Sea.
Before its closure, Maersk Oil operated the Tyra field, consisting of two production complexes: Tyra West and Tyra East. Following difficulties replacing riser guards on the Tyra East complex, Maersk Oil required assistance from an experienced engineering team. Expertise in offshore cutting operations, and a global track record of successful splash zone operations, was decisive in Maersk Oil’s decision to engage a team from OceanTech (Altrad Linjebygg)*.
Challenges associated with replacing riser guards
Riser guards protect risers from impact damage caused by vessel collisions, debris, or other moving objects in the splash zone. As risers transport highly flammable crude oil, a collision between vessels and risers can potentially cause environmental disasters and severe structural damage to the platform.
Conventional riser guards absorb and dissipate impact energy of up to 2.25 MJ. However, riser guards at platforms further north need to be far more robust due to the size of the supply vessels. For example, the Heidrun production and drilling platform in the Norwegian Sea must withstand impact energy of up to 28 MJ.
Riser guards are bulky, and the risers offer limited space for project personnel during installation. Replacing them is a challenging task requiring specialised equipment and advanced rigging and lifting techniques.
Other challenges include:
- The availability of offshore cranes on the platform
- The harsh and dynamic splash zone environment makes it difficult and dangerous for divers and workers to access the work area.
- Workers must complete the project in a timely manner, with minimal disruption to the production process. Downtime can result in significant financial losses.
- To ensure the safety of everyone involved, project personnel must cut the riser guard in a precise manner when replacing it.
Overall, these factors can make riser guard replacements in the splash zone a time-consuming task. It requires careful planning, customised solutions, and a tailor-made methodology to address the unique nature of the project.
Solving a complex engineering task
The Tyra East platform project demanded extensive planning to determine suitable cutting, rigging, and lifting methods.
The selection of cutting method, which includes options like waterjet cutting, plasma cutting, mechanical cutting and diamond wire saws, is contingent on the specific materials and conditions of the project. In harsh marine environments like the splash zone, diamond wire saws are considered the most efficient and safe as they allow for precision cutting and minimal damage to surrounding structures. Additionally, diamond wire saws can reduce the amount of debris generated during the cutting process, minimising environmental impact.
Another obstacle was determining the appropriate method for rigging and lifting. There was no crane coverage at the Tyra platform, and a 40-tonne load meant the weight had to be distributed along the bridge structures. Rigging and lifting operations must comply with strict safety and regulatory requirements while preserving the stability and integrity of the structures during the replacement process.
The custom diamond wire saw had been used in similar projects before, but the engineering team tested it specifically for this project at the OceanTech Subsea Test Center in Trondheim. We also undertook careful planning to identify the specific locations to cut and secure the equipment.
Our team successfully removed and replaced two riser guards using temporary rigging and lifting equipment. As certified and experienced welders had to carry out the welding in the splash zone, our team developed detailed welding procedures in advance, observing relevant offshore industry standards.
Experience is key to successful project completion
The successful completion of the Tyra project relied heavily on customised and innovative robotic solutions, as well as the experience of the engineering team. OceanTech employs various tools and methods, ranging from lightweight robotic solutions to heavy-duty solutions utilising work-class robotic arms.
OceanTech tools and methods are designed to eliminate the need for divers and remotely operated underwater vehicles (ROVs). They offer an adaptable solution unaffected by waves and strong currents, which are common problems in the splash zone. At the Tyra platform, ourteam operated the diamond wire saw with joysticks from a control unit on the topside of the platform. This provided complete oversight of the cutting process.
The OceanTech engineering team has a worldwide reputation for its field-tested technology, accumulated through years of successful deployment in various splash zone operations. The team possesses expertise in structural, mechanical, and electrical engineering, and it can assist clients with any design, calculation, or method planning challenges.
The combination of technology, the full range of the team’s expertise, and the ability to select the appropriate methods led to the success of the Tyra project. Several potential issues can arise in riser guard replacement projects. You may lose the load due to miscalculations in weight, potentially causing a disaster. The rigging and lifting operation is also complex, particularly when dealing with loads of 40 tonnes in harsh marine conditions. This requires careful planning and expertise in load calculations.
Ultimately, a project’s overall approach determines its success.
* OceanTech was established following a management buyout of the Splash Zone Division of Altrad Linjebygg in 2017.