Installation Analysis
Installation analysis is a type of calculation for the validation of lifting capacity, strength capacity for lifting objects and vessel structures, etc. Installation analysis can be divided into two categories based on different phases of installation engineering:
(1) preliminary installation analysis of the front-end engineering design (FEED) for determining the installation method, installation vessels and equipment, as well as the relevant installation duration and cost estimations, and
(2) detailed installation analysis of the engineering design with the goal of developing and installation procedure and relevant installation drawings. The preliminary installation analysis is just used for some critical installation activities such as SPS splash zone lowering and pipeline installation at maximum water depth to prove the feasibility/ capability of the vessel and equipment, whereas the detailed installation analysis is designed to provide a step-by-step analysis of the SPS installation.
The subsea installation can be categorized into three types of installation events:
(1) subsea equipment deployment installation,
(2) pipeline/riser installation, and
(3) umbilical installation. The minimum required data for an installation analysis normally include:
- Environment and geotechnical data;
- Vessel motion characteristics;
- Subsea production system data;
- Rigging system data.
Contents
Subsea Structure Installation Analysis
The subsea structure installation procedure normally includes the following:
- Load-out and sea-fastening;
- Transportation;
- Site survey;
- Deployment, typically including overboarding, splash zone lowering, midwater lowering, landing, and positioning and setting;
- As-built survey.
This article describe only the structure deployment analysis, which may be the most critical analysis for subsea installation, while other analyses such as barge strength verification for transportation and lifting analysis for loadout, are not included in this article. Analysis of subsea structure deployment provides the maximum allowable sea states and maximum expected cable tensions and the motions of installed equipment during installation. Finite element software (e.g.,Orcaflex) is used for the installation analysis. The objective of the detailed installation analysis is to provide a step-by-step analysis to aid in the generation of the equipment deployment procedure. The installation analysis can be divided into two stages: static analysis without any environmental loading, and dynamic analysis with environmental loading such as current and wave. The static analysis determines the relationship between vessel position, wire payouts, and tensions for the system in a static state. The dynamic analysis is performed for the system under environmental loads in order to determine the maximum allowable installation sea states and the maximum tension required. The analyses are carried out for a range of wave heights and wave periods. The analysis model comprises the installation vessel associated with its RAOs, drilling pipe with running tools or a crane with its winch wires, and rigging systems and equipment.
Pipeline/Riser Installation Analysis
The four normal installation methods for pipelines/risers are as follows:
- S-laying method, in which the pipe is laid from a near-horizontal position on a lay barge using a combination of horizontal tension and a stinger (bend-limiting support).
- J-laying method, in which the pipe is laid from an elevated tower on a lay barge using longitudinal tension without an overbend configuration at the sea surface. Load-out and transportation of pipe joints will be performed by the transportation barge at the same time pipe is being laid by the pipe-laying vessel for the S-laying and J-laying method.
- Reel-laying method, in which the pipe is made up at some remote location onshore, spooled onto a large radius reel aboard a reel-lay vessel, and then unreeled, straightened, and laid down to the seabed at the offshore installation location.
- Towing lay method, in which the pipe is made up at some remote location onshore, transported to the offshore installation site by towing, and laid down. The towing is either on the water surface (surface towing), at a controlled depth below the surface (control depth towing method [CDTM]), or on the sea bottom (bottom towing), where the different water depths are used mainly to reduce the fatigue damage due to wave action. A subsea riser or pipeline is exposed to different loads during installation from a laying vessel depending on the installation methods. The loads include hydrostatic pressure, axial tension, and bending. The normal failure modes may be local buckling and buckling propagation due to external pressure and bending moments for pipeline/riser installation. The installation analysis includes two parts: static analysis and dynamic analysis. The stress or strain criteria used for the installation calculation are different based on the project. In most cases, the strain criteria are used for the analysis. However, in some projects, stress criteria are also used. For example, the stress criteria are 72% SMYS and 96% SMYS for pipeline sag-bend area and pipeline overbend area, respectively, according to DNV OS F101 2007. The stress analysis during the pipeline installation procedure should be carried out in detail to check the stress criteria. The analysis provides the maximum allowable sea states and maximum expected tension loads and stress/strain distribution during installation procedure. The pipeline/riser installation analysis is normally carried out by using OFFPIPE or Orcaflex software.
Umbilical Installation Analysis
Umbilicals are laid using one of the following typical methods:
- The umbilical is initiated at the manifold with a stab and hinge-over connection or a pull-in/connection method and terminated near the subsea well with a second end lay down sled (i.e., infield umbilical connection from manifold to satellite well). The connection between the umbilical and the subsea well is later made using a combination of the following tie-in methods: (1) rigid or a flexible jumper, (2) junction plates, and (3) flying leads.
- The umbilical is initiated at the manifold with a stab and hinge-over connection or a pull-in/connection method. It is laid in the direction to the fixed or floating production system and pulled through an I/J tube or cross-hauled from the laying vessel to the floating production vessel.
- The umbilical can also be initiated at the fixed or floating production system and terminated near the subsea structure with a second end umbilical termination assembly (i.e., termination head, lay down sled, umbilical termination unit). A pull-in/connection tool operated by an ROV may be used to connect the umbilical to the subsea structure. The objective of the umbilical installation analysis is to provide a stepby- step analysis that can aid in the generation of the umbilical installation procedure. The analysis also provides the maximum allowable sea states, maximum expected loads, and guidelines for vessel offsets. The analysis is performed in two stages: static analysis without any environmental loading on the model and dynamic analysis with environmental loading such as current and wave. Dynamic analysis is performed by selecting the worst cases (based on minimum bending radius and tension) from the static analysis and then applying environmental loading such as current action, wave action, peak period, and directionality to the model. The directions of wave and current may be conservatively assumed to be the same. The design criteria for an umbilical installation are as follows:
- Minimum bending radius (MBR) of umbilical;
- Maximum allowable tension and compression loads;
- Maximum allowable crushing loads, which can be translated to check the maximum allowable top tension;
- Lateral stability on the seabed, which can be translated to check the maximum allowable tension force of the touchdown point. The analysis model for a normal umbilical lay includes the installation vessel with its RAOs and umbilical. The umbilical for lay initiation analysis is modeled additionally with the umbilical termination head (UTH). The umbilical for laydown analysis is modeled in Orcaflex additionally with a wire attached to the end of the umbilical, together with the end termination/ bend stiffener/pulling head assembly.
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