Decommissioning of the Petrogas Halfweg Platform

The Petrogas Halfweg Platform, located in the Q field near Den Helder, was scheduled for decommissioning in 2017. However, a collision with a tanker disrupted the original plans, necessitating an alternative removal method. Iv was commissioned by Petrogas E&P to conduct a Front-End Engineering Design (FEED) study to assess the structural integrity of the platform during removal and transport. Originally designed to be both self-installing and self-removing, the platform’s damage meant it could no longer be dismantled as planned. Instead, a heavy lift vessel had to be used for decommissioning.

The collision caused deformations in the legs, crane foundation, and lower deck of the platform. Iv assessed the structural safety and residual strength using Finite Element Method (FEM) analyses, supported by data from 3D scans, drone inspections (carried out by Bluestream), and ROV surveys of the Gravity Base Structure (GBS) and platform legs.  These results formed the basis for the design of local reinforcements and the detailed decommissioning plan.

Due to the high risk of working beneath the platform, Iv designed reinforcements that could be installed entirely above deck. Calculations were also made to determine how the damaged structure would respond to wind, wave, and current loads during the lift. To safely detach the four legs, Iv defined the optimal cutting sequence and method, incorporating shear keys to prevent sudden stress release. The cutting process was validated through non-linear FEM modelling, ensuring the platform could be safely lifted.

Iv’s precise assessment of the platform’s structural integrity enabled the development of a revised and safe decommissioning plan. Early identification of potential damage and risks allowed the work to proceed in a controlled manner, minimising delays. Despite the challenging offshore conditions and structural damage, the platform was successfully removed.