Displacing 10,000t of Water to Install 2,500t of Steel Buoy at 250m Below Sea Level
Authors: van Diemen Johannes G. , Saint-Marcoux Jean-François , Otani Lucas , Sahonero Daniel , Trovoado Leandro Cerqueira 
Published: April 2015

Abstract

Objectives/Scope
The successful installation of the four BSR (Buoy Supporting Risers) systems in Brazil's Sapinhoá and Lula NE fields provides the final breakthrough for this new riser design concept, in which SCR's motions are uncoupled from FPSO motions by providing an intermediary support through a vertically tethered submerged buoy at 250m below the sea surface.
The installation of the systems' foundations (16 suction anchors & 64 counterweights), tethers (32 spiral strand wires complete with top and bottom connectors, chain, chain tensioners and other special links) and the four buoys (2800T steel weight, 10000T gross buoyancy each) did not happen without overcoming serious challenges in operational constraints and events.

Methods, Procedures, Process
Considerable installation engineering efforts led to solutions for the logistical problems associated with the sheer quantity, volume, size and weight of the foundations and top connector structures as well as for the actual installation in waters of 2140m depth and in seas often characterised by bi-directional swells.
In buoy design, solutions were found for the conflicting requirements for permanent and temporary weight-buoyancy distribution and pressure differentials, further complicated by the need to accommodate a considerable amount of temporary ballasting, pressurisation, survey and other equipment with associated ROV access envelopes in an area already congested with the structural provisions for the permanent SCRs, flexibles, umbilicals and general monitoring equipment.

Results, Observations, Conclusions
The required ballast configurations and detailed installation procedures for buoy tow-out and lowering and tether hook-up to the foundations were determined following the development of a buoy/tether numerical analysis model addressing weight, stability, ballast distribution, internal pressure and hydrodynamic issues. This was backed-up by model tests and thorough sensitivity analysis on all relevant parameters to ensure the system's integrity in the most rigorous risk and contingency scenarios would not be compromised.
During the offshore phase, the robustness of the installation concept, the ballasting, pressurisation, chain tensioning, bottom connector and other equipment, in conjunction with the developed installation analysis tool, provided sufficient flexibility to resolve the operational set-backs related to equipment damage caused by extensive weather exposure.
Other operational improvements were made continuously, taking advantage of the repetitive nature of the installation scope for foundations, tethers and buoys by evaluating all installation steps systematically and applying the assimilated lessons learned rapidly after thorough Management-of-Change risk assessments.

Novel/Additive information
Numerical modelling for dynamic analysis, prototype tank tests and step-by-step 3D simulation methods were applied to ensure reliable multi-vessel operations offshore. The field results obtained during the first time installation of such a large system demonstrated the BSR behaved as predicted.
The first four BSR systems were installed in more than 2100m water depth, bringing to life a novel riser system for ultra-deep water field developments.