Lessons Learnt from Lifting Operations and Towing of Heavy Structures in North Sea
Authors: Aarset Kenneth , Sarkar Arunjyoti , Karunakaran Daniel N. 
Published: May 2011

Abstract
The challenges in the installation of subsea structures are increasing very rapidly in the recent years. Larger and heavier subsea structures are being installed in harsh environments and in deep waters. Sophisticated lifting analysis is needed to establish safe installation criteria. Also, novel installation methods need to be devised to install heavy structures using small vessels.
In this paper, state-of-the-art installation analysis based on DnV-RP-H103 with a new approach to model the complex hydrodynamic loads at splash zone is described. Through recent installation work, this procedure is exemplified and the lessons learnt from the installation of subsea structures and long spools are discussed. A novel method by sub-surface tow, to install heavy subsea structure using a small construction vessel is out lined and the lessons learnt from installation of four structures are discussed.
The project experience showed that DNV-RP-H103 provides an excellent basis for the lifting analysis. In the recent North Sea installation campaign, by applying the new approach, a safe installation was performed at higher sea state. The sub-surface tow method proposed is robust and safe method to install heavy structures using small construction vessels and the lessons learnt from these installation campaigns clearly indicate that this method is easy, safe and robust in harsh environment.

Introduction
The challenges in the installation of subsea structures are increasing very rapidly in the recent years. Larger and heavier subsea structures are being installed in harsh environments and in deep waters. Novel installation methods need to be devised to install heavy structures using small vessels. Also, sophisticated lifting analysis is needed to establish safe installation criteria.
This paper presents the outlined offshore lifting installation operation and a novel tow and installation method. It includes a description of associated equipment with specifications, analysis, verification and experiences.

Installation of subsea structure, lifting operations
Subsea structures have increased in weight and size in the recent years. Larger monohull construction vessel with a high capacity cranes have entered the marked to be able to take the challenge to install large structures in the harsh environment such as in the North Sea. The Skarv project where 24 lift were performed with use of the construction vessel Seven Seas with various types of structures of extreme weight combined with large outer dimension was performed over campaigns in 2009 and 2010. The Subsea Template Structure was designed to be installed as three modules to reduce the overall weight. The modules consisted of a foundation structure, manifold and a protection structure.
Structures installed by Seven Seas as follows: Five Subsea Templates Structures (4 slots and 6 slots) divided in three modules, Template Foundation structures, Template Manifolds and Template Protection Structures. Also one Gas Injection Riser Base, four Umbilical Riser Base, two Pipe line End manifold and two Export Riser Bases Even with 400 Te capacity mast crane these structure became a challenge to install giving an acceptable and safe installation criteria. State-of-the-art installation analysis based on DnV-RP-H103 with a new approach to model the complex hydrodynamic loads at splash zone was needed to establish that the lift would be possible based on expected weather at Skarv Field.

Wet tow and installation of subsea templates
Traditionally massive Subsea templates as the Tyrihans-templates are installed using heavy-lift vessels. Due to limited availability of heavy-lift vessels, a challenge to develop an alternative concept was introduced. The challenge was to come up with a safe, cost-effective and flexible system with reduced requirements to the installation vessel.