Limiting operational wave criterion for spool installation lift - with emphasis on analysis and wind-wave modeling

Abstract

Spools are rigid pipe sections which are parts of the infrastructure for transporting produced hydrocarbons and injection fluids subsea. Installing them includes a subsea lifting operation commonly carried out by use of the crane on an offshore construction vessel. Such operations are highly sensitive to waves, and usually limited by conditions such as excessive pendulum motions of the lifted structure and occurrence of slack lifting slings during transition through the wave zone. The industry practice is to perform software analyses of vessel motions and hydrodynamic loading acting on the spool(s) when deployed and lifted through the wave zone, in order to establish a limiting operational wave criterion. That is to determine acceptable sea states for such a lifting operation to be safely carried out.

A new Offshore Standard was recently issued, the DNV-OS-H206 “Loadout, transport and installation of subsea objects (VMO Standard – Part 2-6)”. The new standard distinguishes between characteristic vessel motions generated by wind seas and the once generated by swell. A new requirement is introduced demanding that the wind sea is regarded as short crested when analyzing vessel response for operations that are independent of vessel heading. In addition, a minimum requirement to consider the situation where the wind sea and swell is acting with 90° degrees difference in propagation direction is introduced for subsea lifting operations.

This report addresses the problem of whether or not including spreading when describing the wind sea is more conservative for spool installation lifting as compared to earlier recommended practice where waves could be assumed being long crested. Furthermore, the question about potential benefits of doing more detailed assessments of the combination of the wind sea and swell than the minimum required by DNV-OS-H206 is raised. Both aspects are referring to the resulting limiting operational wave criterion, where conditions in the North Sea and Norwegian Sea are of interest.

A case study including a thorough process of establishing a model in the software package OrcaFlex, of a state of the art spool installation lift has been the basis for the investigations carried out. The model consists of an installation vessel, lifting arrangement and spools with properties modeled at a level of detail making it representative for the real world system. A range of dynamic time domain analyses have then been carried out where the system is subjected to sea states relevant for the problem defined. The methodology is, however, similar for all analyses carried out. Simulating the lowering from approximately 2 meters above deck level down to the sea surface identifies potential excessive pendulum motions, whereas as the wave zone crossing is assessed by running simulations for selected positions through the wave zone, ensuring that loads from the irregular sea is transferred to the system. Vessel motions are described by detailed RAO values and all relevant wave induced hydrodynamic loads experienced by the lifted spools are accounted for.

Analyses in a wind sea comparison study showed that modeling the wind sea as short crested waves described by the JONSWAP spectrum introduces significantly higher roll motions to the installation vessel. This subsequently leads to both excessive pendulum motions for a wide range of wave peak periods and large hydrodynamic loading on the spools because of increased crane tip motions, slamming loads in particular. The acceptable significant wave height for carrying out the lifting operation reduces. Considering this particular spool installation lift as representative also for other similar operations one can in general conclude that the limiting operational wave criterion for deployment and lifting through the wave zone for spool installations is more conservative as a result of these regulations being implemented.

A combined wind sea and swell study revealed that the situation where wind sea and swell is acting with 90° difference in propagation direction and where the swell approaches the vessel as beam sea with periods coinciding with the natural period of the vessel’s roll motion and/or the horizontal motion of the lifted spools, as the most critical wave situation one can encounter. This study also showed that there are several benefits of doing analyses that are more refined where the wind sea and swell are modeled as separate wave trains. First, it allows one to identify a range of sea states characterized by other possible directions of the wind sea and swell than the worst case scenario, for which the operation is considered safe to carry out. Another profound merit is the opportunity to account for the vessel’s heading relative to the wind sea and swell directions. This allows one to benefit from performing analyses based on conditions more similar to the actual offshore operation, where the vessel will be able to obtain an optimal heading relative to the wind sea and the swell. This advantage is particularly evident for situations of swell dominated sea states due to the essential assumption that it is reasonable to model wind sea and swell as separate wave trains, where the swell is assumed regular and not prone to the requirement of analyzing response for directions ±15° of the assumed vessel heading, as is the requirement for wind sea.

The new standard’s distinction between characteristic vessel motions generated by wind seas and the once generated by swell should be seen as an encouragement to establish a new practice where these consistently also are analyzed separately. This is further supported by the fact that weather forecasts providing information about wave conditions at an installation site, which the decision to initiate an operation is based upon, can now provide information on a level much more detailed than what is currently utilized for establishing the actual limiting operational wave criterion for an operation. That is, information about height, period and direction of wind sea and swell, separately. The draft of a possible future practice where such detailed analyses, performed during transit or waiting on weather is presented in this report.

An interesting continuation of the work in this report would be to investigate the new regulations’ level of conservatism. This would require a comparison of analyses results and actual measured vessel motions. Also the issue of dynamic positioning accuracy and hence level of uncertainty related to the vessel’s ability to maintain heading throughout an operation should be included in such a study. The usefulness of the presented possible future practice of establishing limiting operational criteria and initiating operations offshore should be further assessed by applying it in practice for an actual spool installation lift operation. This includes preparing a methodology where also uncertainty in forecasted wave period can be accounted for, as an extension of todays ∝-factor which accounts only for uncertainty in the forecasted significant wave height.